GSM ARCHITECTURE

GSM Architecture Overview GSM Architecture Overview

Section 1 – GSM Architecture Overview

IntroductionIntroduction

It provides an overview of the GSM network architecture. This includes a brief explanation of the different network subsystems and a description of the functionality of the elements within each of the subsystems. Topics include:

• General architecture overview

• The Mobile Station (MS) Subsystem and Elements

• The Base Station Subsystem (BSS) and Elements

• The Network Subsystem (NSS) and Elements

• Introduction to network interfaces


A GSM network is made up of three subsystems:

• The Mobile Station (MS)

• The Base Station Sub-system (BSS) – comprising a BSC and several BTSs

• The Network and Switching Sub-system (NSS) – comprising an MSC and associated registers

The interfaces defined between each of these sub systems include:

• 'A' interface between NSS and BSS

• 'Abis' interface between BSC and BTS (within the BSS)

• 'Um' air interface between the BSS and the MS


Abbreviations:

MSC – Mobile Switching Center

BSS – Base Station Sub-system

BSC – Base Station Controller

HLR – Home Location Register

BTS – Base Transceiver Station

VLR – Visitor Location Register

TRX – Transceiver

AuC – Authentication Center

MS – Mobile Station

EIR – Equipment Identity Register

OMC – Operations and Maintenance Center

PSTN – Public Switched Telephone Network


Mobile StationMobile StationThe Mobile Station (MS) consists of the physical equipment

used by a PLMN subscriber to connect to the network. It comprises the Mobile Equipment (ME) and the Subscriber Identity Module (SIM). The ME forms part of the Mobile Termination (MT) which, depending on the application and services, may also include various types of Terminal Equipment (TE) and associated Terminal Adapter (TA).


• The IMSI identifies the subscriber within the GSM network while

the MS ISDN is the actual telephone number a caller (possibly in another network) uses to reach that person.

• Security is provided by the use of an authentication key and by the transmission of a temporary subscriber identity (TMSI) across the radio interface where possible to avoid using the permanent IMSI identity.

• The IMEI may be used to block certain types of equipment from accessing the network if they are unsuitable and also to check for stolen equipment.


MS and SIMMS and SIM


The mobile station consists of :

• mobile equipment (ME)

• subscriber identity module (SIM)

The SIM stores permanent and temporary data about the mobile, the subscriber and the network, including :

• The International Mobile Subscribers Identity (IMSI)

• MS ISDN number of subscriber

• Authentication key (Ki) and algorithms for authentication check

The mobile equipment has a unique International Mobile Equipment Identity (IMEI), which is used by the EIR


Base Station Subsystem (BSS)Base Station Subsystem (BSS)


The BSS comprises:

• Base Station Controller (BSC)

• One or more Base Transceiver Stations (BTSs)

The purpose of the BTS is to:

• provide radio access to the mobile stations

• manage the radio access aspects of the system

BTS contains:

• Radio Transmitter/Receiver (TRX)

• Signal processing and control equipment

• Antennas and feeder cables


The BSC:

• allocates a channel for the duration of a call

• maintains the call:

monitors quality

controls the power transmitted by the BTS or MS

generates a handover to another cell when required


Network Switching System (NSS)Network Switching System (NSS)

The NSS combines the call routing switches (MSCs and GMSC) with database registers required to keep track of subscribers’ movements and use of the system. Call routing between MSCs is taken via existing PSTN or ISDN networks. Signaling between the registers uses Signaling System No. 7 protocol.


Functions of the MSC:

• Switching calls, controlling calls and logging calls

• Interface with PSTN, ISDN, PSPDN

• Mobility management over the radio network and other networks

• Radio Resource management - handovers between BSCs

• Billing Information


InterfacesInterfaces

BSC

VLR

MSC

Um

AbisA

ISDN, TUP


ExerciseExercise

Q1. Name the interfaces used between

Mobile and BTS

BTS and BSC

BSC and MSC


Access Network Access Network

Section 2 – Access Network

ObjectiveObjective

The Trainee will be able to understand:

• Different BTS configuration commonly used in the network

• Advantages of the configuration and optimal use of the trunks

• Abis mapping



Access network is a connection between MS and NSS, comprise of BTSs & BSCs. It is responsible for radio management.

BSC looks towards MSC through single A-interface as being the entity responsible for communicating with Mobile Stations in a certain area. The radio equipment of a BSS may support one or more cells.

A BSS may consist of one or more base stations, where an A-bis-interface is implemented.


BSS ConfigurationBSS Configuration

• Collocated BTS

• Remote BTS

• Daisy Chain BTS

• Star Configuration

• Loop Configuration


Collocated BTS: BTS is situated along with BSC or the MSC and no

additional E1 link is required.

BSC

BTS


Remote BTS : BTS is situated in a stand alone position and additional E1

links are required to connect to BSC.

BSC

BTS


Daisy ChainDaisy Chain

MSC

BSC

BTS 1

BTS 2

BTS 3

BTS 4


Star ConfigurationStar Configuration

MSC

BSC

BTS 1

BTS 2

BTS 3

BTS 4

BSC

BTS 3


Loop ConfigurationLoop Configuration

MSC

BSC

BTS 1

BTS 2

BTS 3

BTS 4


Comparison of Different ConfigurationsComparison of Different Configurations

• Daisy Chain: Easy to implement, effective utilization of transmission links but if one of the link fails, all the BTSs connected in the chain will went off.

• Star Configuration: Easy to implement but poor utilization of links. Each BTS require one E1 to connect to BSC. But if link goes down only individual BTS will be affected.

• Loop Configuration: Slightly difficult to implement but effective utilization of E1 links. Even if one link goes off BTS will continue to communicate with the network from the other side.


BSS InterfacesBSS Interfaces

• Air Interface: Radio Interface between the BTS and Mobile the supports frequency hopping and diversity.

• A Interface: Interface carried by a 2-Mb link between NSS and BSS. At this interface level,

transcoding takes place.

• OMC Interface: X25 Link.


Abis Interface (BTS - BSC)Abis Interface (BTS - BSC)

If the BTS and BSC are not combined, A-bis interface will be used. Otherwise, BS interface will be used. Several frame unit channels are multiplexed on the same PCM support and BSC and BTS can be remote from each other. Its main functions are:

• Conversion of 260 – bit encoded blocks (corresponding to 160x8 – bit samples for 20ms)

• Encoded block synchronization

• Vocal activity detection

• Alarm dispatch to BSC via PCM

• Test loop back operation


TRX 1

TRX 2


ExerciseExercise

Q1. In How many ways BTSs can be connected and which configuration gives the optimal solution?

Q2. What is a difference between BS interface and Abis interface?

Q3. How many time slots are occupied by 1TRX on a PCM frame?


NSS Topology NSS Topology

Section 3 – NSS Topology

ObjectiveObjective


• Terminology used in Network Sub System

• Protocols and Interfaces inside NSS

• Call routing and circuit groups

• Switching modules

• Stand alone and integrated HLR

• Echo canceller and TRAU location

• Authentication, Ciphering, OMC, Billing center

• Transit Switch



Network Sub System can be considered as a heart of the GSM Network. All the major activities like switching of calls, routing, security functions, call handling, charging, operation & maintenance, handover decisions, takes place within the entities of NSS.

Various kinds of interfaces are used to communicate between the different entities. Different methods are used to optimize and provide the quality network with the minimum operating cost.


Network Switching System (NSS)Network Switching System (NSS)

Key elements of the NSS:

• Mobile Switching Center (MSC)

• Visitor Location Register (VLR)

• Home Location Register (HLR)

• Authentication Center (AuC)

• Equipment Identity Register (EIR)

• Gateway MSC (GMSC)

These elements are interconnected by means of an SS7 network


NSS IdentifierNSS Identifier

IMEI – International Mobile Equipment Identifier.

The IMEI is an internationally-unique serial number allocated to the MS hardware at the time of manufacture. It is registered by the network operator and (optionally) stored in the AuC for validation purposes.

IMEI = TAC + FAC + SNR +sp

TAC = Type Approval Code by central GSM body

FAC = Final Assembly Code, identifies the manufacturer

SNR = Serial Number, unique six digit number

sp = spare for future use


IMSI – International Mobile Subscriber Identifier

When a subscriber registers with a network operator, a unique subscriber IMSI identifier is issued and stored in the SIM of the MS as well as in the HLR . An MS can only function fully if it is operated with a valid SIM inserted into an MS with a valid IMEI. IMSI consist of three parts:

IMSI = MCC + MNC + MSIN

MCC = Mobile Country Code

MNC = Mobile Network Code

MSIN = Mobile Station Identification Number


TMSI –Temporary Mobile Subscriber Identity

A TMSI is used to protect the true identity (IMSI) of a subscriber. It is issued by and stored within a VLR (not in the HLR) when an IMSI attach takes place or a Location Area (LA) update takes place. At the MS it is stored in the MS’s SIM. The issued TMSI only has validity within a specific LA.

Since TMSI has local significance, the structure may be chosen by the administration. It should not be more than four octets.


MSISDN – Mobile Station ISDN Number

The MSISDN represents the ‘true’ or ‘dialled’ number associated with the subscriber. It is assigned to the subscriber by the network operator at registration and is stored in the SIM.

According to the CCITT recommendations, it is composed in the following way:

MSISDN = CC + NDC + SN

CC = Country Code

NDC = National Destination Code

SN = Subscriber Number


MSRN – Mobile Station Roaming Number

The MSRN is a temporary, location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility. It is stored in the VLR and associated HLR but not in the MS. The MSRN is used by the VLR associated MSC for call routing within the MSC/VLR service area.


LAI – Location Area Identity

Each Location Area within the PLMN has an associated internationally unique identifier (LAI). The LAI is broadcast regularly by BTSs on the Broadcast Control channel (BCCH), thus uniquely identifying each cell with

an associated LA.

LAI = MCC + MNC + LAC

MCC = Mobile Country Code, same as in IMSI

MNC = Mobile Network Code, same as in IMSI

LAC = Location Area Code, identifies a location area within a GSM PLMN network. Maximum length of LAC is 16 bits.


Mobile Switching Center (MSC)Mobile Switching Center (MSC)

The Mobile services Switching Center (MSC) performs the telephony switching functions of the system. It also controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN) and Public Land Mobile Network (PLMN).

Difference between a MSC and an exchange in a fixed network is - MSC has to take into account the impact of the allocation of radio resources and the mobile nature of the subscribers and has to perform in addition, at least the following procedures:


• required for location registration

• procedures required for handover

An MSC can be connected to only one VLR. Therefore, all mobile stations that move around under base stations connected to the MSC are always managed by the same VLR.

An MSC would communicate typically with one EIR. While it is possible for an MSC to communicate to multiple EIRs, this is highly unlikely since the EIR provides a centralized and geographic independent function.


The MSC consults an HLR to determine how a call should be routed to a given mobile station:

• For incoming calls to a mobile station, the MSC would typically consult one HLR.

• For mobile-to-mobile calls in larger networks, a MSC could consult HLRs of other systems to help minimize the trunk paths to the other mobile station.

A given MSC can be interconnected to other MSCs to support inter-MSC handovers


The following are typical MSC functions in a cellular system:

• Provide switched connections with PSTN

• Provide switched connections between mobile subscribers

• Provide coordination over signaling with mobiles

• Coordinate the location and handover process

• Provide custom services to mobile users

• Collect billing data


ProtocolsProtocols

MSC/BSC MSC/HLR

MSC/VLR

MSC/EIR

MSC/GMSC

VLR/VLR

VLR/HLR

MSC/MSC

OMC/MSC

OMC/HLR

OMC/VLR

OMC/BSS

MSC/Fixed Network

MSC/Voice messaging

BSSMAP TCAP+MAP X.225 R2, ISUP other Signaling

SCCP SCCP X.224

MTP MTP X.25 MTP

SS7 SS7


Switching In MSCSwitching In MSC

Signaling network is separated from the speech network and consists of

• signaling Links (SL)

• signaling Point (SP)

• signaling Transfer Part (STP).


Telephony system contains:

• Group Switch to switch the calls,

• ST to perform signaling in accordance with SS7

• Trunk interfaces for interfacing the PCM.

Group switch provides a semi permanent connection between time slot (PCM) and ST.


Signaling Point (SP)Signaling Point (SP)

SP provides the functions of signaling and transmit and receive messages to and from different nodes. Each SP in the network will have an identification code termed as signaling Point Code (SPC).


Signaling Transfer Point (STP)Signaling Transfer Point (STP)

Signaling Transfer Part is signaling point that only transfers messages from one signaling point (SP) to another.

SP (SPC)

SP (SPC)

STP

STP


Signaling Link (SL)Signaling Link (SL)

Signaling Link is the 64kbps link interconnecting two signaling Points and provides the functions of message error control and message sequencing. Each signaling Link has an SLC (signaling Link Code), which identifies the signaling Link with in the signaling Link Set.


Service Switching Point (SSP)Service Switching Point (SSP)

The MSC contains:

• The Service Switching Point

• One or more radio control point

SSP handles the usual switching function and can be connected via 2Mbps PCM link with:

• Other exchanges of fixed PSTN or mobile PLMN,

• Points on the SS7 signaling network,

• X.25 network

Continued…..


• The OA&M network,

• The Intelligent network,

• PSTN via user data channels and signaling channels using ISUP and R2 protocols,

• Other elements of the GSM


Switching Function of SSP:

• Main control,

• Switching matrix,

• PCM multiplex connection,

• Service circuits

• Operation and maintenance

• Establishing and releasing section of the links from and to mobiles,

• Finding circuits to the BSS; special circuit groups are created. SSP selects an incoming and outgoing circuit.


Call RoutingCall Routing

• If a number received is a national or international number, the address information is passed to the SSP.

• If the number received is an HPLMN (Home PLMN), the RCP asks the HLR for a roaming number (MSRN). This MSRN is passed to the SSP for routing.

• If the number received is an emergency service number, the originating geographic area is attached to it and the combined information passed to the SSP.

Continued…..


In the SSP the number received from RCP follow the standard

translation process:

• Preliminary analysis: Selection of a translator (national, international),

• Translation: Determination of a routing depend on the first digits dialled,

• Routing: Determination of an outing circuit group.


Circuit GroupsCircuit Groups

Call routes from the MSC through circuit groups. Different circuit groups are created inside it:

• Group for the PSTN (according to the exchange)

• Group for the BSCs

• Group for the Supplementary services

• Group for the IWF


MSC

CG1

CG2

CGn

CGa

CGx

CG

CG

BSC1

BSC2

BSCn

PSTN1

PSTNx

Supplementary Services

IWF


InterfacesInterfaces


A-Interface (MSC – BSC)A-Interface (MSC – BSC)

The interface between the MSC and its BSS is specified in the 08-series of GSM Technical Specifications. The BSS-MSC interface is used to carry information concerning:

• BSS management;

• call handling;

• mobility management.


B-Interface (MSC – VLR)B-Interface (MSC – VLR)

The VLR is the location and management data base for the mobile subscribers roaming in the area controlled by the associated MSC(s). Whenever the MSC needs data related to a given mobile station currently located in its area, it interrogates the VLR. When a mobile station initiates a location updating procedure with an MSC, the MSC informs its VLR which stores the relevant information. This procedure occurs whenever an MS roams to another location area. Also, when a subscriber activates a specific supplementary service or modifies some data attached to a service, the MSC informs (via the VLR) the HLR which stores these modifications and updates the VLR if required.


C-Interface (HLR - MSC)C-Interface (HLR - MSC)

The Gateway MSC must interrogate the HLR of the required subscriber to obtain routing information for a call or a short message directed to that subscriber.


D-Interface (HLR - VLR)D-Interface (HLR - VLR)

This interface is used to exchange the data related to the location of the mobile station and to the management of the subscriber. The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area. To support this, the location registers have to exchange data. The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station.

The HLR sends to the VLR all the data needed to support the service to the mobile subscriber. The HLR then instructs the previous VLR to cancel the location registration of this subscriber. Exchanges of data may occur when the mobile subscriber requires a particular service, when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means


E-Interface (MSC - MSC)E-Interface (MSC - MSC)

When a mobile station moves from one MSC area to another during a call, a handover procedure has to be performed in order to continue the communication. For that purpose the MSCs have to exchange data to initiate and then to realize the operation. After the handover operation has been completed, the MSCs will exchange information to transfer A-interface signaling as necessary. When a short message is to be transferred between a Mobile Station and Short Message Service Centre (SC), in either direction, this interface is used to transfer the message between the MSC serving the Mobile Station and the MSC which acts as the interface to the SC.


F-Interface (MSC - EIR)F-Interface (MSC - EIR)

This interface is used between MSC and EIR to exchange data, in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station.


G-Interface (VLR - VLR)G-Interface (VLR - VLR)

When a mobile subscriber moves from a VLR area to another Location Registration procedure will happen. This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR.


H-Interface (HLR - AUC)H-Interface (HLR - AUC)

When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data, the HLR requests the data from the AuC. The protocol used to transfer the data over this interface is not standardized.


Switch ModulesSwitch Modules

Switch has three major types of equipment modules:

• Switching module (SM)

• Communication module (CM)

• Administrative module (AM)


Switching Module (SM):

All external lines, trunks, and special services circuits are terminated at the switching module. The analog and digital signals are converted to the digital format used inside the switch. The SM performs almost 95% of the call processing and maintenance functions including:

• Line and trunk scanning

• Tone generation

• Announcements

• Call progress supervision

• Routine maintenance and self-maintenance.


The SM also provides subscriber calling features including:

— call waiting

— abbreviated dialing

— call diversion

— conference calls.

SM further has two components:

1. Control units - Control all activities within the SM, such as call processing and maintenance functions.

2. Peripheral units - Perform testing functions and provide customers and other exchanges access to the switch.


Communication Module (CM):

The CM serves as the hub (focal point) for all inter module communication in a switch. The CM has four main functions:

1. Call switching - The CM interconnects the paths between modules to complete telephone calls and to relay data.

2. Message switching - The CM provides paths to send information between processors to process calls, maintain records, and perform system tasks.

Continued…..


3. Network timing - The CM provides accurate timing and

synchronization for the switch.

4. Fast pump - The CM provides resources to quickly download (pump) an SM’s software if needed.


Administrative Module (AM):

The AM controls the CM and communicates with all the SMs (through the CM). The AM monitors itself and the CM for malfunctions. If there are any problems, they are reported to maintenance personnel.

The AM performs resource allocation and processing functions that are done more efficiently on a centralized basis such as:

• Call routing for inter module and intra module calls

• Administrative data processing/billing data

Continued…..


• Traffic measurement reports/system performance reports

• Memory management

• System maintenance

• Maintaining file records of changes to the system Software Release.

• Personnel interface/system monitoring

• Allocating trunks for call processing.


Switch

SM AM CM

Control Unit

Peripheral Unit

MSGS TMS

Control Unit

I/O Processor

Disk Unit

Tape Unit

MCC


Home Location RegisterHome Location Register

HLR is a database that stores subscription and set of functions needed to manage subscriber data in one PLMN area. Any administrative action by the service provider or changes made by subscriber is first carried out on the HLR and then update the VLR. Following are the subscriber data which frequently changes:

- Identification number MSISDN & IMSI

- Service restriction

- Teleservices

- Bearer services

- Supplementary services


Beside the permanent data it also include dynamic data of home

subscriber including VLR address, call forward number and call barring numbers.

Triplets are also stored in the HLR for the authentication purpose.

The HLR communicates with other nodes: VLR, AUC, GMSC & SMS – SC via MAP (Mobile Access Protocol). To support this communication HLR needs MTP and SCCP


MAP (Mobile Application Protocol)MAP (Mobile Application Protocol)

The only way via which HLR communicates with other GSM nodes is Mobile Access Protocol. Number of functional blocks exist to support different MAP operations eg HLCAP is used for location cancellation or HLUAP is required for location updating. Other functions defined on the MAP are:

- Inter MSC Handover and subsequent handover

- Update HLR and VLR

- Fault Recovery

- Management and handling of supplementary services.

Continued…..


- Support of Short Message Services.

- Call establishment / delivery

- Security related data.

- Retrieval of subscriber data during call setup.

HLR also needs to communicate with GMSC, VLR, AUC and SMS-SC, for which MTP and SCCP is essential.


SCCP (Signaling Connection Control Point)SCCP (Signaling Connection Control Point)

All MAP messaging use SCCP to analyze the GT (Global Title) of incoming information. If GT belongs to anther node, then SCCP will use the services of MTP (Message Transfer Part) to reroute the message.

SCCP must have the GT analysis to terminate and route MAP

messages from all nodes it communicates with.

To find out the DPC, SCCP looks in a routing case translation

table. The information about the DPC is then sent to MTP which sends the message to the appropriate SP.


MTP (Message Transfer Part)MTP (Message Transfer Part)

MTP must be defined to allow the nodes to communicate with each other.

The MTP provides the means for reliable transport and delivery of UP (User Part) information across the No. 7 network eg ISDN User part (ISUP), the Telephone User Part (TUP), Signaling Connection Control Part (SCCP), Interworking function User Part (IWUP) and Data User Part (DUP)

Continued…..


MTP has the ability to react to system and network failure that affect the user information.

MTP further has three functional levels:

1. MTP Level 1 – Signaling data link

2. MTP Level 2 – Signaling link

3. MTP Level 3 – Signaling network


HLR connects with MSC via C interface, VLR via D interface


HLR can be configured in two ways:

1. Integrated with MSC


1. Hs

2. Stand Alone HLR (External Database)


Integrated Vs Stand Alone HLRIntegrated Vs Stand Alone HLR

The Integrated HLR is accessed by other MSC’s/ VLR’s via MAP, and the switch can use MAP to query other off switch HLRs. The main advantages with an integrated HLR solution at this early stage are:

• Efficient use of HW and lower HW investments

• Fewer physical connections required due to fewer physical nodes

• Less capacity required in No. 7 network as major part of HLR signaling is internal within MSC/VLR/HLR


• A single fault will affect a smaller number of subscribers than if

standalone HLR is used

Major drawbacks are:

• Less processing capacity available for MSC/VLR.

Additional Switching capacity will be required earlier

• Migration to standalone HLR (which is to be preferred in a mature larger network) will induce major changes in the network

• Administration of subscriptions

• Operation and maintenance for HLR geographically distributed


In Stand Alone HLR, call processing activities are not performed by the switch. Only HLR queries are handled via the GSM standard MAP messages coming over signaling links from other Mobile Switching Centers (MSCs) in the wireless network.


Benefits:

• All HLR data is centralized, thus simplifying its ongoing maintenance and operation

• High HLR Capacity

• High processing capacity

• On going enhancement

There are some drawbacks with standalone HLR

A fault in a HLR will affect many subscribers

A fault in a HLR will increase the signaling substantially in

the whole signaling network


HLR is responsible for:HLR is responsible for:

• Connection of mobile subscribers and definition of

corresponding subscriber data.

• Subscription to basic services.

• Registration/deletion of supplementary services.

• Activation/deactivation of supplementary services.

• Interrogation of supplementary services status.

Continued…..


• Functions for analysis of mobile subscriber numbers

(MSISDN, IMSI, additional MSISDN) and other types of

addresses.

• Statistical functions for collecting data regarding the

performance of the system.

• Functions for communication with GMSC and VLR using

the No. 7 signaling system and MAP

• Handling of authentication and ciphering data for mobile

subscribers including communication with an authentication center.

Continue…..


• Get Password/Register Password

• Alert Service Center

• Provide Roaming Number

• Send Routing Information for SMS

• Send Routing Information for GMSC

• Set Message Waiting Data


Visitor Location RegisterVisitor Location Register

It is a subscriber database containing the information about all the MS currently located in the MSC service area. VLR can be considered as a distributed HLR in the case of a roaming subscriber. If MS moves into a new service area (MSC), VLR requests the HLR to provide the relevant data and store it, for making the calls for that MS.

VLR is always integrated with MSC to avoid the signaling load on the system.

It can also be viewed as a subset of a HLR.


VLR connects with MSC via B interface, HLR via D interface and with

another VLR via G interface.


G

VLR is responsible for

• Setting up and controlling calls along with supplementary services.

• Continuity of speech (Handover)

• Location updating and registration

• Updating the mobile subscriber data.

• Maintain the security of the subscriber by allocating TMSI

Continued…..


• Receiving and delivering short messages

• Handling signaling to and from

- BSC and MSs using BSSMAP

- other networks eg PSTN, ISDN using TUP

• IMEI check

• Retrieve data from HLR like authentication data, IMSI,

ciphering key

Continued…..


• Retrieve information for incoming calls.

• Retrieve information for outgoing calls.

•Attach/Detach IMSI

• Search for mobile subscriber, paging and complete the call.


Security FeatureSecurity Feature

Both the users and the network operator must be protected against undesirable intrusion of third party. As a consequence, a security feature is implemented in the telecommunication services. The following parts of the system have been reinforced and provide the various security features:

1. Access to the network authentication

2. Radio part ciphering

3. Mobile equipment equipment identification

4. IMSI temporary identity


Authentication Center (AUC)Authentication Center (AUC)

AUC is always integrated with HLR for the purpose of the authentication. At subscription time, the Subscriber Authentication Key (Ki) is allocated to the subscriber, together with the IMSI. The Ki is stored in the AUC and used to provide the triplets, same Ki is also stored in the SIM.

AUC stores the following information for each subscriber

1. The IMSI number,

2. The individual authentication key Ki,

3. A version of A3 and A8 algorithm.

Continued…..


Authentication is required at each registration, at each call setup

attempt (mobile originated or terminated), at the time of location updating, before supplementary service activation, de-activation , registration.

HLR uses the IMSI to communicate with AUC, triplets are requested in sets of five.

Continued…..


In AUC following steps are used to produce one triplet:

1. A non- predictable random number, RAND, is produced

2. RAND & Ki are used to calculate the Signed Response (SRES) and the Ciphering Key (Kc)

3. RAND, SRES and Kc are delivered together to HLR as one triplet.

HLR delivers these triplets to MSC/VLR on request in such a way that VLR always has at least one triplet.


Authentication Procedure:

The MSC/VLR transmits the RAND (128 bits) to the mobile. The MS computes the SRES (32 bits) using RAND, subscriber authentication key Ki (128 bits) and algorithm A3. MS sends back this SERS to AUC and is tested for validity.


MS BTS MSC/VLR HLR AUC OMC

A4

A4IMSI Ki

A3 A8 Triplets

Generation

KiRAND

RAND SERS

Kc

A2

Triplets

Ciphering Function

A5

Kc

RAND

=?SERS

IMSI

Ki

A3

A8

Ciphering Function

A5

Kc

SIM Card


CipheringCiphering

The user data and signaling data passes over the radio interface are ciphered to prevent intrusion. The ciphered key (Kc) previously computed by the AUC is sent from the VLR to the BSS after the mobile has been authenticated. The Kc is also computed in the MS and in the way both ends of the radio link (MS and BSS) possess the same key.


Procedure:

For the authentication procedure, when SRES is being calculated, the Ciphering Key (Kc), is calculating too, using the algorithm A8.

The Kc is used by the MS and the BTS in order to cipher and decipher the bit stream that is sent on the radio path.


AUC

Ki

A3

A8A8

Ciphering/Deciphering

Speech, data,sig

A5

Kc

A3

Choice of random no RAND (128 bits)

=?

OK

SIM

Ki

A3

A8

A5Speech, data,sig

A8

Kc (64 bits)

A3

SERS

RAND

SERS

Ciphering Command

Ciphered Data


Subscriber ConfidentialitySubscriber Confidentiality

The subscriber identity (IMSI), since is considered sensitive information, is not normally transmitted on the radio channel. A local, temporary identity is used for all interchanges. The identity (TMSI) is assigned after each change of authenticated location. For other cases:

• Call set-up

• Use of supplementary services

• Use of SMS

Continued…..


A TMSI is allocated when the one supplied by the MS is considered out of

date or when the MS does not provide the TMSI.

Transmission of the TMSI over the traffic channel is ciphered.


Equipment Identification Register (EIR)Equipment Identification Register (EIR)

Purpose of this feature is to make sure that no stolen or unauthorized mobile equipment is used in the network.

EIR is a database that stores a unique International Mobile Equipment Identity (IMEI) number for each item of mobile equipment.


Procedure:

• The MSC/VLR requests the IMEI from the MS and sends it to a EIR.

• On request of IMEI, the EIR makes use of three possible defined lists:

- A white list: containing all number of all equipment identities that have been allocated in the different participating countries.

- A black list: containing all equipment identities that are considered to be barred.

- A grey list: containing (operator’s decision) faulty or non-approved mobile equipment.

• Result is sent to MSC/VLR and influences the decision about access to the system.


EIR MSC/VLR MS

Storage of all number series mobile equipment that have been allocated in the different GSM -

countries

Storage of all grey/black – listed mobile equipment

Storage of the equipment

identity IMEI

Call Setup

IMEI Request

Sends IMEI

Check IMEI

Access/ barring info

Continues/Stops call setup procedure


Echo CancellerEcho Canceller

In order to eliminate echo effects (noticeable by the mobile subscribers while in conversation with PSTN subscribers) caused by the time delay due to coding and decoding of signal processing, group of echo cancellers are installed even for local calls.

This is rarely a problem when communicating between two MSs. However, when connecting to a PSTN telephone, the signal must pass through a 4-wire to 2-wire hybrid transformer.

Continued…..


The function of this transformer is - some of the energy at the 4-

wire receive side from the mobile is coupled back to the 4-wire transmit side and thus speech is retransmitted back to the mobile.

As a result, all calls on to the PSTN must pass through an echo canceller to remove what would otherwise be a noticeable and annoying echo.

Continued…..


The process of canceling echo involves two steps:

• First, as the call is set up, the echo canceller employs a digital adaptive filter to set up a model or characterization of the voice signal and echo passing through the echo canceller. As a voice path passes back through the cancellation system, the echo canceller compares the signal and the model to dynamically cancel existing echo. It removes more than 80 to 90 percent of the echo across the network.

• The second process utilizes a non-linear processor (NLP) to eliminate the remaining residual echo by attenuating the signal below the noise floor.


Transcoder and Rate Adaptor Unit (TRAU)Transcoder and Rate Adaptor Unit (TRAU)

The primary function of the TRAU is to convert 16kps (inc signaling) GSM speech channels to 64kbps PCM channels in the uplink direction and the reverse in the downlink direction. The reason this process is necessary is because MSCs only switch at the 64kbps channel level.


TRAU LocationsTRAU Locations

TRAU can be physically located in the BTS, BSC or MSC and hence leads to a variety of installation configurations.


Advantages of Different ConfigurationsAdvantages of Different Configurations

Case 1, TRAU at BTS: If the TRAU is installed at the BTS, each 16kbps GSM channel would need to be mapped to its own 64kbps PCM channel. This results in 75% of the transmission bandwidth being wasted across both the Abis (BTS-BSC) and A (BSC-MSC) interface.

Case 2, TRAU at BSC: If the TRAU is installed at the BSC, 16kbps GSM channel mapped to 64kbps at the A (BSC-MSC) interface, which increases the requirement of the Transmission trunks.


Case 3, TRAU at MSC: If the TRAU is placed at the MSC, as is generally

the case in current networks, a multiplexer can be placed at the BTS which enables 4 x 16kbps GSM channels to be multiplexed onto one 64kbps PCM channel, using 4 x 16kbps ISDN D-channels. In this configuration, only at arrival at the MSC is the 16-64kbps channel conversion necessary, thereby maximizing the efficient usage of the transmission medium by increasing the GSM channel throughput per PCM 2048 bearer from 30 to 120 channels.


Operation And Maintenance Center (OMC)Operation And Maintenance Center (OMC)

The OMC centralizes all operations and maintenance activities for the MSCs and BSSs using remote software control. It provides remote testing, operations, and maintenance capabilities for the entire system from one central location. Each BSS, MSC, HLR, VLR, EIR, and AUC can be monitored and controlled from the OMC.


OMC Functional ArchitectureOMC Functional Architecture

Operating System

Communications Handler

Database

MMI

Event/ Alarm Management Security

Management

Fault Management

Performance Management

Configuration Management


The OMC supports the following network management functions:

• Event Management - General functions of the OMC include operator input and output messages, application input commands, and application output reports.

• Fault Management - The OMC provides fault management such as diagnostics and alarms for the MSC and BSS. It provides the means to isolate and minimize the effects of faults in the network thereby enabling the network to operate in efficient manner.

Continued…..


• Security Management – It provides an extensive range of features to ensure that access to the OMC functions is restricted to relevant personnel.

The security features are as follows:

Password Authentication of OMC operator

Logging of OMC access attempt

Configurable user access restrictions

Automatic logoff


• Configuration Management - Configuration management for the

BSS consists of generic download, non-volatile memory download, database administration, and translations download. For the MSC, software release updates, database administration (route analysis, IMSI analysis table), and subscriber administration (connect/disconnect) are supported.

• Performance Management - Performance management supports data collection (such as traffic data, handovers, statistics, plant measurements, and volume data) and basic reporting.


Billing CenterBilling Center

Charging analysis is the process of analysing the Charging Case and then ultimately generating the TT (Toll Ticketing) record so that an itemised bill can be produced and then sent on to the customer.

The tariff structure consists of two parts:

• The network access component

• The network utilization component


The network utilization component is registered on a per call basis.

Charging starts at the moment the subscriber answers, or on connection to an answering machine internally in the network.

The main elements are:

• Use of GSM PLMNs

• Use of national / international PSTNs

• Use of connection between different networks

• Use of the signaling system no.7


Depending on the type of call, one or more call tickets can be generated:

• Outgoing call to fixed network: a call tickets is generated by the originating MSC.

• Incoming call from the fixed network: two call tickets are created: one in the GMSC and another in the destination MSC. If a call forwarding supplementary service is in operation, other call tickets are generated in the MSC and the GMSC.

Continued…..


• Outgoing call from a mobile subscriber to another mobile

subscriber belonging to same PLMN: three call tickets are created: one in the originating MSC, one in the GMSC (which is in this case is the originating MSC) and another in the destination MSC.

Call tickets mainly register the following information:

1. IMSI

2. Identity (MSISDN) and type (MSC or GMSC)

3. Mobile subscriber location identity


4. Other party’s identity

5. Call type (incoming, outgoing, forwarded etc)

6. Call status

7. Teleservices and bearer service

8. Date and time

9. Call duration


Call Detail Record (CDRs)Call Detail Record (CDRs)

• Each call within the PLMN creates one or more call records

• These records is generated by the MSC/GMSC originating the call

• The records are known as a ‘Call Detail Records’ (CDRs)

• CDRs contain the following information:

- Subscriber Identity

- Number called

- Call Length

- Route of call

• Often referred to as ‘Toll Tickets’


Call Charge ProcedureCall Charge Procedure

• Network supplies originating MS with CAI details

• MS calculates AOC record using CAI details

• This record acts as a ‘toll ticket’ which tracks the call on its route through various networks

• Each call component can generate a separate CDR

• The record passes along the backbone to the home network

• Billing computer generates bills based on cumulative CDRs

• HPLMN collects the charges

• HPLM reimburses VPLMN using TAPs in accordance with roaming agreement


The Transferred Account Procedure (TAP) is the mechanism by which operators exchange roaming billing information. This is how roaming partners are able to bill each other for the use of networks and services through a standard process.


Gateway MSC (GMSC)Gateway MSC (GMSC)

Gateway MSC (GMSC) connects the PLMN with other networks and the entry point for the mobile subscriber calls having the interrogation facility. It has the function to obtain the information from the HLR about the subscriber’s current location and reroute the calls accordingly.

In case of the network having only on MSC, the same MSC work as the GMSC, while in the case having more than one MSC, one dedicated MSC works as GMSC.


Roaming NumberRoaming Number

A MSRN is used during the call setup phase for mobile terminating calls. Each mobile terminating call enters the GMSC in the PLMN. The call is then re-routed by the GMSC, to the MSC where the called mobile subscriber is located. For this purpose, a unique number (MSRN) is allocated by the MSC and provided to the GMSC.


Call SetupCall Setup


1. GMSC receives a signaling message "Initial Address

Message" for the incoming call (MSISDN).

2. GMSC sends a signaling message to the HLR where the subscriber data is stored (MSISDN).

3. The VLR address that corresponds to the subscriber location and the IMSI are retrieved. HLR sends a signaling message using the VLR address as the destination (IMSI).

4. VLR having received the message, requests MSC to seize an idle MSRN and to associate it with the IMSI received. VLR sends back the result to the HLR (MSRN).


5. HLR sends back the result to the GMSC (MSRN).

6. GMSC uses MSRN to re-route the call to the MSC. MSC performs digit analysis on the received MSRN and find the association with IMSI. The MSRN is released and the IMSI is used for the final establishment of the call.


Transit SwitchTransit Switch

When planning the trunk network architecture, it is important to take into consideration the future expansion of the network.

Some factors that influence the trunk network configuration are:

• Number of MSCs

• Transmission costs

• Traffic distribution

• Traffic volume

• PSTN tariffs


In case of a medium networks (having 5 - 10 MSCs), some of the

MSCs are used as transits for the others and the number of direct links between the MSCs are restricted.

In case of large networks (having more than 10 MSCs), separate transit exchanges are used. These are connected to all MSCs and are working with load sharing.

Transit functionality is used for passing on calls to another node. This provides a hierarchical structured network.


High Usage trunk


Traffic between MSCs and from MSCs to other networks is routed over two

MSCs in a similar way as is used for the small network. The TGMSCs are used as interconnecting exchanges, since they have trunks to all MSCs in the operators PLMN.

MSCs located in the same city area or in close cities are likely to be interconnected by high usage routes, while traffic between distant MSCs is likely more economically routed over the TMSCs.


ADVANTAGES OF USING TRANSIT EXCHANGESADVANTAGES OF USING TRANSIT EXCHANGES

The use of transit exchanges implies a more stable network structure and some of the most important benefits are:

• increased flexibility

• enhanced reliability

• easily expandable network

• platform for functional development

• lower handling costs

• improved signaling network


Value Added ServicesValue Added Services

Value Added Services includes the following:

• Point-to-Point Short Message Services

• Cell Broadcast Short Message Service

• Voice/Fax Mail

• Pre-Paid SIM

The products associated with each of these services as well as the required interfaces into the core network elements are defined as:


Short Message Services (SMS)Short Message Services (SMS)

The Point-to-Point and Cell Broadcast Short Message Services are implemented using the Short Message Service Center (SMSC) and Cell Broadcast Center (CBC).

SMSC is built around proven Open Systems Platforms from the UNIX based computer platform to the MSC/HLR/VLR interfaces utilizing SS7.


Following are the services and functions for which SMSC is

capable of:

• Alerting services to indicate call or message waiting

• Paging interfaces providing full industry standard TAP interworking

• Information services - subscription to financial, weather, traffic, etc. services

• DTMF message entry via interactive voice prompts

• E-mail

• Network administration including bill reminders, statements on demand, network

• service information and handset reprogramming.


The CBC product is based on the same Open Systems Platforms with an

X.25 interface to BSC components. It offers a wide range of applications, which include advertising, general and specialist information distribution services along with other non-mobile terminal applications. The services and functionality that the CBC can provide includes:

• Customer care information

• Weather and traffic reports

• Free advertising

• Variable re-transmission rates

• Distributed network interface units to handle varying network loads

• Local and remote message submission facility.


SMS Network ComponentsSMS Network Components


Callers which cannot reach the MS are given the option (by the VMS) to

leave either a short message or a voice mail message. Message waiting notification will be delivered to the MS when the MS is reachable. The VMS (voice mail system) communicates with the SMS SC via TCP/IP or X.25.

The VMS has a trunk and signaling interface to the PSTN (e.g., R2, ISUP signaling). The VMS has a trunk and signaling interface to the MSC for mobile subscriber to access his/her voice mail.


SMS ApplicationsSMS Applications

• SMS up to 160 alphanumeric characters.

• Alert services (MT-SMS)

— Voice Message Alert

— FAX/Telex Message Alert

— E-mail System Alert

— Paging Bureau Emulation Services.

• Information Services

— Financial Services (stock market queries and alerts)

— Weather or traffic information (e.g., from TV/radio station data feeds)


• Network Administration

— Bill reminders (MT-SMS), bill payment

— Statements on demand (MO and MT-SMS)

— Handset re-programming and much more.


VMSVMS

It supports a wide range of innovative applications including:

• Call answering

• Voice and fax bulletin boards

• Information on demand

• One number services

• Voice and fax messaging

• Interactive voice response

• Prepaid calling cards

• Voice activated dialing


Pre Paid SIMPre Paid SIM

The functionality of the Pre-Paid SIM feature includes:

• Provision of pre-defined limits based on air time or talk time

• Service provisioning including various provisioning options (point of sale, service providers, etc.) and definitions of pre-paid categories (throw away, top up, etc.)

• Service execution for air and talk time credit usage

• GSM MAP services, teleservice, bearer services and supplementary services will all be available to the Pre-Paid SIM subscriber, with possible limitations, as required by the network operator.


Supplementary ServicesSupplementary Services

Wide range available in GSM standard and Operators can also define their own

In GSM it is possible for the subscribers to check and modify

the parameters and status of their Supplementary Services


Some of the Supplementary Services are:

• Calling Line Identification/Restriction

• Connected Line Identification/Restriction

• Call Forwarding

• Call Waiting

• Call Hold

• Conference Calling

• Conference Calling

• Advice of charge

• Call barring


ExerciseExercise

Q1. Write a full form of following : IMEI, TMSI, MSRN, LAI, ST, STP, SSP

Q2. How many circuit groups are required for 3 BSCs and 10 PSTN?

Q3. List down the three functions of each HLR & VLR.

Q4. Fill in the following:

E interface is used between ------

H interface is used between-------


Algorithm A8 is used for ----------

Algorithm A3 is used for ----------

Transit exchanges are used to reduce the ---------

Q5. List down the different locations of TRAU and explain the best position.

Q6. What information is contained in the CDRs?

Q7. 2 advantages of transit switch.

Q8. Name some of supplementary services.


GSM Signaling GSM Signaling

Section 4 – GSM Signaling

ObjectiveObjective


• signaling between MSC/VLR and BSS

• Concept of DTAP

• Concept of BSSMAP

• signaling between BSC and BTS

• Functions of LAPDm

• Functions of LAPD

• Frame structure of LAPDm And LAPD



There are two different types of communication channels:

• Traffic channel at 64 Kbps, carrying speech or data for radio channels.

• signaling channels at 64Kbps, carrying signaling information.

In PCM one time slot is reserved for signaling and remaining are used for transmitting speech or data. As the entire siganlling is done on 64Kbps , there should be special function converting the information to 64Kbps format and back at the receiving end.


Protocols in GSM NetworksProtocols in GSM Networks

VLR

VLR

MSC

AUC

HLR EIR

BSC

BTS

BSSAP

LAPD

MAP MAP

MAP MAPMAP

ISDN

GMSC

MSC

PSTN

ISUP

ISUP

MAP

TUP

MS LAPDm

Switching System

Base Station System


GSM Signaling MatrixGSM Signaling Matrix

LAPDm

MS BTS MSC

DTAP

RR

RIL3

RIL3 RSM

LAPDm LAPD

BSC

RSM

BSSMAP

BSSAP

LAPD

SCCP

MTP2 &3

MTP1 MTP1

MTP2 & 3

SCCP

BSSAP

DTAP BSSMAP

MAP

TCAP

ISUP

Database


• MSC uses ISUP/TUP protocols for PSTN signaling.

• MAP siganlling for database applications like HLR, VLR, EIR, AUC, SMS-SC, GMSC.

• GSM specific protocol as BSSAP, which comprises of DTAP and BSSMAP.

• The BSC on layer 2 uses LAPD protocol, which is an ISDN.

• BTS has LAPDm as layer 2 protocol.

• Mobile has DTAP for MSC and RR for Radio Resource signaling.


MAP (Mobile Application Part)MAP (Mobile Application Part)

MAP is a protocol specially designed for GSM requirement. It is installed in MSC, VLR, HLR, EIR and communicates in case of:

• Location registration

• Location cancellation

• Handling/management/ retrieval of subscriber data.

• Handover

• Transfer of security/ authentication data.


BSS Application Part (BSSAP)BSS Application Part (BSSAP)

BSSAP is used for signaling between MSC/VLR and BSS. Three groups of signals belong to BSSAP

1. DTAP

2. BSSMAP

3. Initial MS messages


MSC

MS

BSC/BTS

DTAP

Initial MS Message

BSSMAP LAPDm

Transparent to BSS


Direct Transfer Application Part (DTAP)Direct Transfer Application Part (DTAP)

DTAP is a messages between the MSC and MS, passes through the BSS transparently. These are call control and mobility management messages directed towards a specific mobile.

3 main type of DTAP messages are:

• Messages for mobility management like location update, authentication, identity request

• Messages for circuit mode connections call control

• Messages for supplementary services


BSSMAPBSSMAP

BSS management messages (BSSMAP) between MSC and BSS (BSC/ BTS), which are necessary for resource management, handover control, paging order etc. The BSSMAP messages can either be connection less or connection oriented.


Initial MS MessagesInitial MS Messages

These messages are passed unchanged through BSS, but BSS analyses part of the messages and is not transparent like DTAP messages.

Between BSS and MSC, the initial MS message is transferred in the layer 3 information in the BSSMAP.

The Initial MS messages are:

• CM Request

• Location update request

• Paging response


LAPDmLAPDm

Link Access Procedures on the Dm channel (LAPDm) is the layer 2 protocol used to convey signaling information between layer 3 entities across the radio interface. Dm channel refers to the control channels, independent of the type including broadcast, common or dedicated control channels.

LAPDm is based on the ISDN protocol LAPD, used on the Abis interface. Due to the radio environment, the LAPD protocol can not be used in its original form. Therefore, LAPDm segments the message into a number of shorter messages.


Data exchanged between the data link layer and the physical layer is 23 octets for BCCH, CCCH, SDCCH and FACCH. For SACCH only, 21octets are sent from layer 2 to layer 1.

LAPDm functions include:

• LAPDm provides one or more data link connections on a

Dm channel. Data Link Connection Identifier (DLCI) is used for discriminating between data link connections.

• It allows layer 3 message units be delivered transparently between layer 3 entities.

• It provides sequence control to maintain the sequential order of frames across the data link connections.


LAPDm Frame StructureLAPDm Frame Structure

info length command address

N(R) P/F N(S) 0 0 0 1 SAPI CR 1


Sequence Number: N(S) send sequence number of the transmitted frame. N(R) is receive sequence number.

P/F : All frames contain the Poll/Final bit. In command frames, the P/F bit is referred to as the P bit. In response frames, the P/F bit is referred to as the F bit.

Service Access Point Identifier: Service Access Points (SAPs) of a layer are defined as gates through which services are offered to an adjacent higher layer.SAP is identified with the Service Access Point Identifier (SAPI).

SAPI = 0 for normal signaling of DTAP & RR

SAPI = 3 for short message services


LAPDm has no error detection and correction. It is used in two modes:

• Acknowledge &

• Unacknowledged

and having a different structure for both.


LAPDLAPD

All signaling messages on the Abis interface use the Link Access Procedures on the D-channel. (LAPD protocol). LAPD provides two kinds of signaling:

• unacknowledged information

• acknowledged information

LAPD link handling is a basic function to provide data links on the 64 kbps physical connections between BSC and BTS.


Links are provided for operation and maintenance (O&M) of the

links, for O&M of the BTS equipment and for transmission of layer 3 Abis messages.

Each physical connection can support a number of data links

(logical connections). On each physical connection each data

link is identified by a unique TEI/SAPI


LAPD has three sub signaling channels

1. RSL (Radio signaling Link), deals with traffic management, TRX signaling.

2. OML (Operation & Maintenance Link), serves for maintenance related info and transmission of traffic statistics.

3. L2M (Layer 2 Management), used for management of the different signaling on the same time slot.


LAPD Frame StructureLAPD Frame Structure

Flag FCS info length command address Flag

N(R ) P/F N(S) 0 TEI 1 SAPI CR 0


LAPD Frame structure is made up of:

Flag: Indicates the beginning and end of each frame unit. Flag has a pattern of 01111110.

FCS: Frame Check Sequence, provides the error checking for the frame. If error is found frame will be retransmitted.

Command: It has two types of structure, in acknowledge mode it has N(S) and N(R ). N(S) is a sequence number of frame sent and N(R ) is the sequence number of the frame expected to receive next.


C/R: This bit indicates whether it is command or response.

P/F: In command frames, the P/F bit is referred to as the P bit and the other end transmits the response by setting this bit to F.

TEI: Terminal Endpoint Identifier, is a unique identification of each physical entity on either side like each TRX within a BTS have a unique TEI.


SAPI: Service Access Point Identifier, used to identify the type of link.

SAPI = 0 for RSL

SAPI = 62 for OML

SAPI = 63 for L2ML

Each LAPD link is identify by SAPI/TEI pair.


ExerciseExercise

Q1. Name the protocol which is transparent to BSS and what information is used to transfer on this protocol?

Q2. Name the protocols used between

Mobile and BTS

BTS and BSC

BSC to MSC

MSC to PSTN


Call Handling Call Handling

Section 5 – Call Handling

ObjectiveObjective


• Basic call concepts

• Location Area concepts

• Call setup in different scenarios

• SMS routing

• Intra and Inter MSC handovers



Call setup is required to establish communication between a Mobile Station and Network Subsystem (NSS). The NSS is responsible for establishing a connection with the corresponded. Different types of calls require different teleservices.

For the optimum utilization of the network, different location areas will be defined to reduce the paging load on the system.


Basic Types of CallsBasic Types of Calls

There are three basic types of call:

1. Mobility Management calls: Such as Location update. These are used to collect information about the MS and only signaling channels are used.

2. Service calls: Such as SMS. These calls passes very small information, therefore signaling channels are used.

3. User traffic calls: Such as speech or data. Large amount of data is exchanged hence traffic channels are used.


Basic Call SetupBasic Call Setup


Subscriber on switch A places a call to a Subscriber on switch B:

1. Switch A analyzes the dialed digits and determines that it needs to send the call to switch B.

2. Switch A selects an idle trunk between itself and switch B

and formulate IAM

3. STP W receives a message, inspects its routing label, and determines that it is to be routed to switch B.

4. Switch B receives the message. On analyzing the message, it determines that it serves the called number and that the called number is idle.

5. Switch B formulates an address complete message (ACM), which indicates that the IAM has reached its proper destination.


6. Switch B picks one of its links and transmits the ACM over the

link for routing to switch A.

7. STP X receives the message, inspects its routing label, and determines that it is to be routed to switch A.

8. On receiving the ACM, switch A connects the calling subscriber

9. When and/or if the called subscriber picks up the phone, switch B formulates an answer message (ANM),

10. Switch B selects the same link it used to transmit the ACM

11. STP X recognizes that the ANM is addressed to switch A and forwards it over link


12. Switch A ensures that the calling subscriber is connected and conversation can take place.

13. If the calling subscriber hangs up first switch A will generate a release message (REL) addressed to switch B.

14. STP W receives the REL

15. Switch B receives the REL, disconnects the trunk from the subscriber line, returns the trunk to idle status.

16. STP X receives the RLC, determines that it is addressed to switch A.

17. On receiving the RLC, switch A idles the identified trunk.


Location RegistrationLocation Registration

When the mobile is turned on first time in the network, it has no indications in its data about an old Location Area Identity. MS immediately inform the network and request for the Location Update to the MSC/ VLR. After registration MSC/ VLR will consider the MS as active and marked the MS as “attached”.


Location UpdateLocation Update

When the MS moves from one LA to another, it has to register. This registration is performed when the MS detects another LAI than the one stored. This is called location updating. This function provides mobile subscribers with uninterrupted service throughout the GSM coverage area so that they can:

• Be called on a permanent directory number irrespective of their location at the time of call.

• Access the network whatever their position


There are four different types of location updating:

Normal

IMSI detach

IMSI attach

Periodic registration


Normal UpdateNormal Update

• The Base Transceiver Station (BTS) of every cell continually transmits the Location Area Identity (LAI) on BCCH.

• If MS detects LAI is different from the one stored in the SIM-card, it is forced to do a location update.

• If the mobile subscriber is unknown in the MSC/VLR (new subscriber) then the new MSC/VLR must be updated, from the HLR, with subscriber information.

• It also consider the case of the location update timer runs out.


1. The MS requests a location update to be carried out in the new MSC/VLR. The IMSI is used to identify the MS.

2. In the new MSC/VLR, an analysis of the IMSI number is carried out. The result of this analysis is a modification of the IMSI to a Mobile Global Title (MGT)

3. The new MSC/VLR requests the subscriber information for the MS from the HLR.

4. The HLR stores the address of the new MSC/VLR and sends the subscriber data to the new MSC/VLR.


5. The HLR also orders the old serving MSC/VLR to cancel all information about the subscriber since the MS is now served by another MSC/VLR.

6. When the new MSC/VLR receives the information from the HLR, it will send a location updating confirmation message to the MS.


IMSI DetachIMSI Detach

The MS must inform the network when it is entering an

inactive state (detach).

1. At power off or when the SIM card is taken out, the MS asks for a signaling channel

2. The MS uses this signaling channel to send the IMSI detach message to the MSC/VLR.

3. In the VLR, an IMSI detach flag is set for the subscriber which is used to reject incoming calls to the MS.

The detach will not be acknowledged.


Only the VLR is updated with the “detached” information.Only the VLR is updated with the “detached” information.


IMSI AttachIMSI Attach

The attach procedure is performed only when the MS is turned on and is in the same LA as it was when it sent the detach message. If the MS changes location area while being switched off, it is forced to do a normal location update. The procedure is as follows

1. The MS requesting a signaling channel.

2. The MSC/VLR receives the IMSI attach message from the MS.

3. The MSC/VLR sets the IMSI attach in the VLR, that is, the mobile is ready for normal call handling.

4. The VLR returns an acknowledgment to the MS.


Periodic Location UpdatePeriodic Location Update

To avoid unnecessary paging of the MS in case the MSC never got the IMSI detach message, there is another type of location updating called periodic registration.

The procedure is controlled by timers both in the MS and in the MSC.

If the MS does not register within the determined interval plus a guard time, then the scanning function in the MSC detects this and the MS will be marked detached.


PagingPaging

A call to MS is routed to the MSC/ VLR and send a paging message to the MS. This message is broadcast all over the Location Area (LA), which means that all BTSs with in the LA will send a paging message to the mobile. The MS, moving in the LA and listening to the CCCH information, will hear the paging message and answer it immediately.


Paging CapacityPaging Capacity

Paging capacity is the number of mobiles that can be paged per second

This depends on:• CCCH configuration• AGCH blocks reservation• Type of paging message used

• Paging message takes 4 bursts (1 CCCH block)

• This can page up to 4 mobiles depending on the message type used


Paging Message TypesPaging Message Types

Type 1: can address up to two mobiles using either IMSI or TMSI

Type 2: can address up to 3 mobiles, one by IMSI and other 2 by TMSI.

Type 3: can address up to 4 mobiles using the TMSI only.

If the network does not use TMSI then only type 1 is used in the network.


Calculation Of Paging CapacityCalculation Of Paging Capacity

X = number of mobiles paged per paging message (1 to 4)

Y = number of possible paging messages per multiframe

Duration of channel multiframe = 0.235 seconds (235 ms)

• X depends on paging message type

• Y depends on CCCH configuration in the multiframe (e.g. 3 or 9) and the number of AGCH blocks reserved


PCH DimensioningPCH Dimensioning

Paging channel requirement in blocks per multiframe is given by:

Calls = Number of calls predicted for the location area during busy hour

MT = Fraction of calls which are mobile terminated

PF = Paging Factor = number of pages required per call

M = safety margin

PMF = Paging Message Factor = number of pages per message

Number of control channel multiframes per second = 4.25


ExampleExample

A particular location area contains 50 000 subscribers. It is predicted that 30% of these will receive a call during the busy hour. On average 2 pages are needed per call and only type 3 paging messages (TMSI) are used.

This gives the following data:Calls = 50 000MT = 0.3PF = 2PMF = 4


A typical safety margin for peak variations in number of calls is 1.2

• 1 PCH block per multiframe will be adequate


Paging ControlPaging Control

The MSC has to initiate the paging procedure, as it holds the information on the last MS location update.

MSC sends a paging message to BSC and sets a timer for response from the MS, which is send as a part of service request message. The paging message from the MSC contains a cell list identifier, identifying the cells in which paging message is to be transmitted.


Call From MS (Mobile to PSTN)Call From MS (Mobile to PSTN)

cb

VLR

MSCExch PSTN

PLMN


Call From MS OverviewCall From MS Overview

• Mobile is active and idle, wants to set up a call

• User dial the number and press send, at first moment it sends on RACH

• MSC/VLR assigns a dedicated channel

• If the calling MS is allowed to make a call MSC/VLR acknowledges the access request

• Depending on whether a fixed or a mobile subscriber is called, number is analyzed directly in the MSC/VLR.

• Call setup message is acknowledged as soon as the link is ready.

• MS is also assigned to move to a dedicated traffic channel TCH.


Signaling InterfacesSignaling Interfaces

BSC

VLR

MSC

LAPDm

LAPD

DTAP

BSSMAP

PLMNISDN/PSTN

ISUP/TUP


Point Of Interconnect (POI) Location Point Of Interconnect (POI) Location

In case of long distance mobile to PSTN call, circuits define to route a call in the switch should be such that, call can travel maximum distance on the airtime and minimum on the land line to enhance the revenue.

Call should handover to the POI as near as possible to the subscriber location.


Call to MS (PSTN to Mobile)Call to MS (PSTN to Mobile)

GMSC

GSM/PLMN

PSTNMSISDN

Link is setup from local exchange to the GMSC


GMSC

GSM/PLMN

PSTN

HLR

signaling No.7: Interrogation function used by GMSC

MSISDN + MSRN request


GMSC

GSM/PLMN

PSTN

HLR

VLR

MSC

signaling No7: Request for MSRN to VLR

IMSI


GMSC

GSM/PLMN

PSTN

HLR

VLR

MSC

IMSI

MSRN in VLR. signaling No 7: MSRN send to GMSC

MSRN request + IMSIMSRN


GSM/PLMN

PSTN

VLR

MSCGMSC

Link is setup from GMSC to MSC/VLR

HLR


GSM/PLMN

PSTN

VLR

MSCGMSC HLRBSC

signaling No7: Paging message is sent to the BSS


GSM/PLMN

PSTN

VLR

MSCGMSC

HLR

BSC

Air path signaling: Paging message is sent over the air path to MS. The MS answers.


GSM/PLMN

PSTN

VLR

MSCGMSC

HLR

BSC

The link is setup from the MSC/VLR to the MS, completing the connection from subscriber to subscriber


Signaling InterfacesSignaling Interfaces

BSC

VLR

MSC

LAPDm

LAPD

DTAP

BSSMAP

PLMNISDN/PSTN

ISUP/TUP

HLR

GMSC

MAP

ISUP/TUP


Mobile to Mobile (Mobile Originated)Mobile to Mobile (Mobile Originated)

MS BTS BSC MSCChannel Request

rn Channel Request

rn+fn+TA

SDDCH Allocation

TA+SDDCH+power

Channel Activation

SCCP Connection Req

Immediate assign (AGCH)Immediate assign commd

Switch to SDDCH

rn+fn+TA+ SDCCH

Channel Activation Ack

SABM

Establish Indication

SCCP Connection Confirm

Service Request

Service Request

UA

Cm+Service Request


MS BTS BSC MSC

Setup (SDCCH)

Tele/bearer service called party no.

Layer 3CCLayer 3CC

Layer 3CCLayer 3CCCall proceeding

Assignment request

Channel type+cmTCH allocation

Physical context request

Physical context confirm

Power+TAChannel activation

TCH+TA+cipher+DTX+power

SACCH

TA+power updates

Channel activation ack

Assignment command (SDCCH)

Release SDCCH


MS BTS BSC MSC

SABM (FACCH)

Establish indication

UA (FACCH)Set transcoder

Assignment complete (FACCH)

Set switching path

alerting

connect

Layer 3CC

Layer 3CC

Layer 3CC

Layer 3CC

Layer 3CC

Connect ack

Initiate SDCCH release


Mobile to Mobile (Mobile Terminated)Mobile to Mobile (Mobile Terminated)

MS BTS BSC MSC

Paging

TMSI/IMSI+cell listPaging command

TMSI/IMSI paging group+ channel noTMSI/IMSI

Paging request (PCH)

Paging request (RACH)

Channel required

Radio and Link Establishment Procedure


MS BTS BSC MSC

Layer 3CC Layer 3CCSetup

Tele/bearer service

Layer 3CCLayer 3CC

Call confirmed (SDCCH)bearer service

Normal Assignment Procedure for TCH

alertingLayer 3CC

Layer 3CC

Layer 3CCLayer 3CC

connect

Layer 3CC Layer 3CCConnect acknowledge

Ring tone

User answer


SMS Point to PointSMS Point to Point

The Short Message Service, SMS, provides means of sending text messages, to and from GSM mobile station. SMS makes use of service centre, which acts a store and forward center for short messages.


Mobile Terminated SMSMobile Terminated SMS

SMS - C SMS - GMSC MSC/VLR

HLR


SMS –C has the capability to transfer the short messages and also

provides the information about the delivery.

1. A user sends a message to an SMS – C

2. SMS – C sends the message to the SMS – GMSC

3. SMS – GMSC interrogates the HLR for routing information

4. HLR interrogates MSC/VLR for a roaming number

5. MSC/VLR returns a MSRN to the SMS – GMSC via HLR

6. SMS – GMSC reroutes the message to MSC/VLR

Continued…..


7. MS is paged and a connection is setup between MS and the

network.

8. If authentication was successful the MSC/VLR delivers the message to the MS. It is transmitted on the allocated SDCCH

9. If the delivery was successful a delivery report is sent from MSC/VLR to the SMS – C.

In the case of an unsuccessful delivery the service messages waiting will provide the HLR and VLR with the information that there is a message in the originating SMS – C waiting to be delivered to the MS.


Mobile Originated SMSMobile Originated SMS

MSC/VLR SMS - C


1. MS establishes a connection to the network, as in the case of normal

call setup. (This step is not performed if the MS is in busy mode, since there already exists a connection)

2. If the authentication was successful MS sends the short message to the SMS – C via MSC/VLR. The SMS – C in turn forward the short message to its destination. This could be MS or a terminal in the fixed network.


HandoverHandover

Changing to a new traffic channel during call setup or busy state is called Handover. The network makes the decision about the change. After receiving the information about the signal strength and quality the BSC ranks the neighboring BTSs using the information.

After a evaluation of the situation and the decision to start the handover procedure, the network is responsible for the setup of a link to the new BTS.


Intra MSC HandoverIntra MSC Handover

BSC

BSCVLR

MSC

New link

Old link


Intra MSC handover: Handover within the same MSC/VLR service

area but different BSCs.

• The BSC request for a handover from MSC/VLR

• New link (MSC/VLR to new BSC to new BTS) is setup and if a free TCH is available, it must be reserved.

• MS receives the order to change to the new frequency and the new TCH.

• If the BTS change has also change of location area, the MS sends a request for location update after the call.


FlowchartFlowchart

BSC2 MSC BSC1 MS

H.O. RequestH.O. Required

H.O. Request Ack

H.O. CommandH.O. Command

Measurement Report

H.O. Completed

H.O. CompletedClear Command

Clear Completed


Inter MSC HandoverInter MSC Handover

BSC

VLR

MSC

VLR

MSC BSC

New link

Old link


Inter MSC handover: handover between the two BSCs controlled by two

different MScs. Lot of signals exchanges are required before the handover can take place.

• The serving exchange sends a handover request to the target exchange

• Target exchange will take over the responsibility for preparing the connection to the new BTS.

• After the setup of a link between the two exchanges, the serving exchange will send a handover command to the MS.


FlowchartFlowchart

VLR (MSC-B)

BSC2 MSC-B MSC-A BSC1 MSMeasurement

ReportH.O. Required

Perform H.O.Allocate H.O.Number

Send H.O. Report

H.O. Request

H.O. Request Ack

RAD CH Ack

I AM (ISUP)

ACM (ISUP) H.O. Command H.O. Command

H.O. Complete

H.O. Complete

Send End Signal

Clear Command

Clear CompleteANS (ISUP)


ExerciseExercise

Q1. Name the types of the location updates exists in the mobile network?

Q2. Describe the different kinds of paging messages?

Q3. Calculate the paging capacity (mobiles paged per second) for the following operator setting:

paging message type = 3

blocks reserved for CCCH and AGCH = 3


Q4. Calculate the PCH requirement for following:

Number of subscriber = 75,000

Busy hour calls = 40%

Assume on average 2 pages required per call

Safety margin for peak variation in number of calls =1.2

Paging message of type 2

Q5. Which part of the network allocates the MSRN to the call?


SS7 SS7

Section 6 – SS7

CONTENTSCONTENTS

• Introduction

• Signaling Modes

• CCS 7 Vs. CAS

• CCS 7 Link Types

• CCS 7 Signaling Network

• Signaling Network Components

• CCS 7 Architecture

• CCS 7 Functional Blocks

• MTP• Signaling Data link (Level 1)• Signaling Link Functions (Level 2)• Signaling Network Functions (Level 3)

• MTP User Functions (Level 4)

Section 6 – SS7

• Functions of Signaling Link (Level 2)

• Organization of signaling Information

• Signal Units

• Signal Unit Delimitation/ Flag Imitation Prevention

• Error Detection

• Error Correction

• Basic Method

• Positive Ack

• Negative Ack

• Preventive Cyclic Re-Transmission

• Error Rate Monitoring

• Signal Unit Error Rate Monitor

• Alignment Error Rate Monitor

Section 6 – SS7

• Signaling Network Functions (level 3)

Service Information Octet

• Routing Label

• Signaling Message Handling

• Message Discrimination

• Message Distribution

• Message Routing• Signaling link Management

• Link activation

• Link restoration• Flow Control

Section 6 – SS7


Common Channel Signaling System No. 7 (i.e., SS7 or C7 ) is a global standard for telecommunications defined by the International Telecommunication Union (ITU)Telecommunication Standardization Sector (ITU-T). The standard defines the procedures and protocol by which network elements in the public switched telephone network (PSTN) exchange information over a digital signaling network to effect wireless (cellular) and wire line call setup, routing and control.

Section 6 – SS7

The SS7 network and protocol are used for:

• basic call setup, management, and tear down

• wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication

• local number portability (LNP)

• toll-free (800/888) and toll (900) wireline services

• enhanced call features such as call forwarding, calling party name/number display, and three-way calling

• efficient and secure worldwide telecommunications

Section 6 – SS7

Signaling TypesSignaling Types

There are two types of Signaling :

1. Channel Associated Signaling (CAS)

2. Common Channel Signaling (CCS7)

Channel Associated Signaling: signaling is always sent on the same connection as that of speech.The Signaling is associated with speech.

Section 6 – SS7

Common Channel Signaling: signaling network is separated from the speech network.Every signaling information will have a label which indicates to which speech connection this signaling information belongs to.The signaling channel has no specific position (timeslot).The same signaling channel carries information for all speech circuits as and when required basis.

Section 6 – SS7

Advantage Of CCS7 Over CASAdvantage Of CCS7 Over CAS

• A dedicated signaling link required for each speech channel in CAS e.g. 3 channels in 3 PCMs : CCS 7 uses only 1 channel for a number of PCMs

• CAS is slow, so longer call setup times : CCS 7 - 64kbps fast & efficient.

• In CAS, no possibility of signaling during the “talking phase” : CCS 7 signaling is independent of speech.

• CAS supports limited set of signals : CCS 7 supports signal units of variable length max. 279 octets - so much more signaling info can be exchanged than is possible with CAS.

Section 6 – SS7

• Usage of messages instead of pre-defined bit patterns enables to transfer call related signaling info (call establishment) as well as non call related call info ( location update , handover , short messages etc.)

• CCS 7 - modular ; easy introduction of new & advanced services.

Section 6 – SS7

SS7 Signaling Link TypesSS7 Signaling Link Types

Section 6 – SS7

Section 6 – SS7

C7 Signaling NetworkC7 Signaling Network

SP(SPC)

SP(SPC)

1 2 3 4 5 16

31

30

STP

SL(SLC)

SL(SLC)SLSCIC •SP: Signaling Point

•SPC: Signaling Point Code

•STP: Signaling Transfer Point

•SL: Signaling Link

•SLC: Signaling Link Code

•SLS: Signaling Link Set

•CIC: Circuit Identity code

Section 6 – SS7

signaling Network Componentssignaling Network Components

• Signaling Points• logically separate entities from a signaling network point of

view.

• Origination Point Code• A signaling point at which a message is generated, i.e. the

location of the source User Part function, is the originating point of that message.

Section 6 – SS7

• Destination Point Code• A signaling point to which a message is destined, i.e. the

location of the receiving User Part function, is the destination point of that message.

• Signal Transfer Point• A signaling point at which a message is received on one

signaling link and is transferred to another link, i.e. neither the location of the source nor the receiving User Part function, is a Signal Transfer Point (STP).

For a particular signaling relation, the two signaling points thus function as originating and destination points for the messages exchanged in the two directions between them.

Section 6 – SS7

• Signaling Links• The common channel carrying signaling information is called

Signaling link.

• Link Set• A number of signaling links that directly interconnect two

signaling points constitute a signaling link-set.

• Signaling Routes• The pre-determined path, consisting of a succession of

SPs/STPs and the interconnecting signaling links, that a message takes through the signaling network between the origination point and the destination point is the signaling route for that signaling relation

Section 6 – SS7

• Signaling Modes

• The term “signaling mode” refers to the association between the path taken by a signaling message and the signaling relation to which the message refers.

Section 6 – SS7

TCAP ISUP TUP

SCCP

Signaling Network

Signaling link

Signaling data link

MTP

Level 1

Level 2

Level 3

Level 4 : User Parts

Layer 3

Layer 1

Layer 2

Layers 4 to 7

CCS 7 ArchitectureCCS 7 Architecture1

Section 6 – SS7

Message Transfer Part (MTP)Message Transfer Part (MTP)

• Function:• to provide a reliable transfer and delivery of signaling

information across the signaling network and to have the ability to react and take necessary actions in response to system and network failures to ensure that reliable transfer is maintained.

• Includes the functions of layers 1 to 3 of the OSI reference model.

• User functions in CCS 7 MTP terms are:– the ISDN User Part (ISUP)– the Telephone User Part (TUP)

Section 6 – SS7

the signaling Connection Control Part (SCCP)– the Data User Part (DUP)

• The SCCP also has Users. These are:– the ISDN User Part (ISUP)– Transaction Capabilities (TC)– Operations Maintenance and Administration Part (OMAP)

Section 6 – SS7

Functions of MTPFunctions of MTP

Message handling

signaling link

signaling datalink

Networkmanagement

Level 1

Level 2

Level 3

f

Section 6 – SS7

Signaling Data Link (MTP Level 1 )Signaling Data Link (MTP Level 1 )

• Defines the physical, electrical and functional characteristics and the physical interface towards the transmission medium (PCM30)

• signaling Data Link is a bi-directional transmission path for signaling consisting of two data channels operating together in opposite directions at the same data rate.

• Digital : 64 kbps channels. For PCM30 HDB3 coding is used

- Minimum allowed bit rate for telephone call control application : 4.8kbps

Section 6 – SS7

Signaling Link Functions (MTP Level 2)Signaling Link Functions (MTP Level 2)

• Together with signaling data link, the signaling link functions provide a signaling link for the reliable transfer of signaling messages between two adjacent signaling points.

• Messages are transferred over signaling link in variable length messages called signal Units which contain additional information to guarantee a secure transmission.

Section 6 – SS7

Functions:

• Delimitation of signaling units by means of Flags.• Flag limitation prevention by bit stuffing.• Error detection by means of Check bits included in each

signaling unit.• Error control by re-transmission and signaling unit sequence

control by means of sequence numbers and continuous ACKs

• Signaling link failure detection by signaling unit error rate monitoring and signaling link recovery by special procedures.

Section 6 – SS7

Signaling Network Functions (MTP Level 3)Signaling Network Functions (MTP Level 3)

• Level 3 in principle defines those transport functions and procedures that are common to and independent of the operation of individual signaling links.

These functions fall into two major categories:

Signaling message handling functions – These transfer the message to the proper signaling link or User Part.The main functions are:-

• Message discrimination function• Message distribution function• Message routing function

Section 6 – SS7

signaling network management functions – These control the current

message routing and configuration of the signaling network facilities and in the case of signaling network failures, control the reconfigurations and other actions to preserve or restore the normal message transfer capability. Contains signaling link management, traffic management and route management.The main functions are:-

• Signaling link management• Signaling traffic management• Signaling route management

Section 6 – SS7

MTP User functions (Level 4)MTP User functions (Level 4)

• User Parts defines the functions and procedures of the signaling system that are particular to a certain type of user of the system. The following entities are defined as User Parts in CCS 7.

• Telephone User Part (TUP)• The TUP Recommendations define the international

telephone call control signaling functions for use over CCS 7.

• Data User Part (DUP)• The Data User Part defines the protocol to control

interexchange circuits used on data calls, and data call facility registration and cancellation.

Section 6 – SS7

• ISDN User Part (ISUP)

• The ISUP encompasses signaling functions required to provide switched services and user facilities for voice and non-voice applications in the ISDN.

• Signaling Connection Control Part (SCCP)• The SCCP provides additional functions to the Message

Transfer Part to provide connectionless and connection-oriented network services to transfer circuit-related, and non-circuit-related signaling information.

• Key Enhancements by SCCP

Section 6 – SS7

• Enhanced Addressing Capability

• upto 255 users can be addressed by the use of Subsystem Numbers (SSN)

• SCCP provides a routing function which allows signaling messages to be routed to a signaling point based on, for example, dialled digits. This capability involves a translation function which translates the global title (e.g. dialled digits) into a signaling point code and a sub-system number.

• Connectionless and Connection-Oriented Services

• Class 0 : basic connectionless service

• Class 1 : sequenced connectionless service

• Class 2 : basic connection-oriented service

• Class 3 : flow control connection-oriented service

Section 6 – SS7

TCAPTCAP

• TCAP provides services for non-circuit related services.TCAP receives messages from SCCP and routes it to the user.TCAP makes it possible to have several transactions running simultaneously.

• TCAP consists of component sub-layer and the transaction sub-layer.The component layer provides information exchange between two layers by the means of dialogues. A dialogue will contain several components like action , response etc.The transaction identifier gives each transaction a unique identity which is also known as transaction identifier.

Section 6 – SS7

• TCAP acts as a secretary to a manager who has several engineers

reporting to it. The secretary handles all the transactions from the manager and sends it across the appropriate engineer and also keeps track of each transactions by having identified files for each engineers transaction.

Section 6 – SS7

Global TitleGlobal Title

Global title is the address of the Signaling Point which does not clearly mention the destination address for routing. It is translated by SCCP to get the destination address.e.g. the dialled digits.On an incoming call,GMSC uses the Global title to determine the destination.

A MAP message entering or originating from an exchange must either be a terminating message or a message to be routed to another exchange.

Section 6 – SS7

By analyzing the global title(GT) of the called address,the SCCP will either route the message to another node with the help of global title routing case (GTRC) or terminate the message in the node.

In the terminating node the message will be distributed to the correct user with the help of the subsystem number (SSN).

Section 6 – SS7

Organization of Signaling InformationOrganization of Signaling Information

• Signal Unit : - A group of bits forming a separately transferable entity used to convey information on a signaling link.

• Are of variable length; maximum length : 280 bytes (including 272 signaling information bytes)

• Three types of signal units, differentiated by the length indicator field contained in each.

Section 6 – SS7

• {length limitation is imposed to control the delays one message can cause to others due to their emission time}

• Fill-in signal unit (FISU) ; LI = 0• Link status signal unit (LSSU) ; LI = 1or 2• Message signal unit (MSU) ; LI = 3 to 63

Section 6 – SS7

Signal UnitsSignal Units• MSU:• convey the signaling information between the user parts

(level 4) of the adjacent signaling points. E.g. IAM , ACM , REL.

• LSSU:• a signal unit which contains status information about the

signaling link.• FISU :• a signal unit containing only error control and delimitation

information which is transmitted when there are no MSUs or LSSUs to be transmitted.

This is done to allow for a consistent error monitoring so that faulty links can be quickly detected and removed from service even when traffic is low.

Section 6 – SS7

Signal UnitsSignal Units

F8

CK 16

SIF8n,n>=2

SIO 8

LI 6

FIB 1

FSN 7

BIB 1

BSN 7

F8

F8

CK 16

LI 6

FIB 1

FSN 7

BIB 1

BSN 7

F8 FISU

F8

CK 16

SF8 or 16

LI 6

FIB 1

FSN 7

BIB 1

BSN 7

F8

LSSU

MSU

2

2

2

1

Section 6 – SS7

SU Delimitation / Flag imitation PreventionSU Delimitation / Flag imitation Prevention

• Signal Unit Delimitation :• A unique pattern on the signaling data link is used to delimit a signal

unit :- 0111 1110.

01111110 01111110Main part of Message

•Flag imitation Prevention :

>> to ensure that no false flags are produced in the signal units, only five consecutive one’s are allowed inside the signal unit. If more than five one’s occur consecutively, a zero is inserted after the fifth one and is removed again in the receiving signal terminal. This is called “bit stuffing”.

Section 6 – SS7

Error DetectionError Detection

• Error Detection :

-each signal unit has standard CCITT 16 bit cyclic redundancy check (CRC) checksum to enable the receiving terminal to check that all bits have been received correctly.

• CK generated by transmitting SP on all fields except the Flag.

• Receiving SP calculates CK and compares with CK in the signal unit.

• Mismatch interpreted as error in received signal unit & error correction procedures are invoked.

Section 6 – SS7

Error CorrectionError Correction

• Two forms of error correction methods are used :• Basic method• Preventive cyclic re-transmission (PCR)

• Basic Method:• re-transmission occurs only when transmitting SP is informed by

receiving SP about the signal units received in error• is a positive / negative ACK re-transmission error correction system

Section 6 – SS7

• For sequence control, each signaling unit is assigned forward &

backward sequence numbers and forward & backward indicator bits.• Sequence Numbering is performed independently at the two SPs

interconnecting the link.

The sequence numbers are 7 bits long, meaning that at most 127 messages can be transmitted without receiving a positive ACK.

Section 6 – SS7

Positive AcknowledgmentPositive Acknowledgment

FSN=125,FIB=BIB=1

FSN=126,FIB=BIB=1

BSN=126,FIB=BIB=1

FSN=35,FIB=BIB=1

MSU saved in RTB

MSU saved in RTB

Both MSU deleted fm RTB

Correctly received

MSU with positive ack,FSN=34

MSU,BSN remains 126

Correctly received

1

Section 6 – SS7

Negative AcknowledgmentNegative Acknowledgment

• Errored MSU is discarded and not delivered to level 3 of MTP

• SP sends a negative ack in the next SU• BSN retains the FSN of last correctly received MSU• BIB is inverted

• All messages with FSN > received BSN sent one by one by fetching from RTB

• FIB value inverted in all retransmitted messages

• Until all messages in the RTB are retransmitted, no fresh MSUs are sent.

Section 6 – SS7

Preventive Cyclic Re-transmissionPreventive Cyclic Re-transmission

• Preventive Cyclic Retransmission:• Retransmission takes place for signal units whose correct reception

is not confirmed by the receiving SP• is a positive ACK cyclic re-transmission forward error correction

system.• A copy of the transmitted MSU is retained at the transmitting

terminal unit until a positive ACK for that MSU is received.

Section 6 – SS7

• Re transmission Rules :

• when there are no new MSUs to be sent, all MSUs not positively acknowledged are retransmitted cyclically.

• If new signal units are available, the retransmission cycle (if any) is interrupted and the signal units transmitted with first priority.

• Under normal conditions, with no MSUs to be transmitted or cyclically re-transmitted, FISUs are sent continuously.

Section 6 – SS7

Basic Versus PCRBasic Versus PCR

• In both methods, only errored MSUs and LSSUs are corrected.• Errors in FISUs are detected but not corrected

• Both methods are designed to avoid out of sequence and duplicated messages when error correction takes place.

• PCR method is used when the propagation delay is large (satellite transmission).

Section 6 – SS7

• With large propagation delays, the basic method becomes

inappropriate because NACK system causes message delays to be too long for the erroneous MSUs

• CCITT recommendations : PCR should be used when one way propagation delay exceeds 15ms.

• Drawback of PCR : inefficient bandwidth utilization

• I.e. the maximum load level a link can be engineered for is much less with PCR.

Section 6 – SS7

Error Rate MonitoringError Rate Monitoring

• Level 2 functions detect a failure in the following circumstances:

High error rate on the signaling units.

Excessive re-alignment period.

Excessive ACK delay.

Signaling terminal failure.

Reception of continuous FISUs.• Two types of signaling error rate monitor is provided

signaling Unit Error Rate Monitor (SUERM). Alignment Error Rate Monitor(AERM).

Section 6 – SS7

Signaling Unit Error Rate MonitorSignaling Unit Error Rate Monitor

• Is used while a signaling link is In Service. It provides the criteria for taking a signaling link OOS due to excessive error rate.

• Is based on a signaling unit error count (including FISUs) , incremented & decremented using the “leaky bucket” algorithm.

Section 6 – SS7

• For each errored signaling unit , the count is incremented by

one and for each 256 signaling units received (whether errored or not), a positive count is decremented by one (a zero count is left at zero). When the count reaches 64, an excessive error rate indication is sent to Level 3 and the signaling link is put OOS.

• The error rate on signaling units should not exceed

• 64 consecutive erroneous signaling units or • 1 erroneous signaling unit out of every 256 on an

average.

Section 6 – SS7

Alignment Error Rate MonitorAlignment Error Rate Monitor

• Is used while a signaling link is in the proving state of the initial alignment procedure.

• Provides a criteria for rejecting a signaling link for service during the initial alignment due to an excessive error rate.

Section 6 – SS7

• The Alignment error rate monitor is a linear counter which is started at zero at the start of the proving period and the count is incremented by one for each LSSU unit received in error. A proving period is aborted if the threshold for the alignment error rate monitor count is exceeded before the proving period timer expires.

Parameter Value

Tin 5

Tie 1

M 5

Section 6 – SS7

Message Label types (SIF)Message Label types (SIF)

T11 56 110 -93/d 06

SLC

SLS

SLS

SLS

Circuit ID code

CircuitID code

Originatingpoint code




Destinationpoint code




Managem ent information

Signalling information



MTP management messages: Label type A

TUP messages: Label type B

ISUP mess ages: Label type C

SCCP mes sages: Label type D

R outing label

FIG U RE 7 /Q .7 00

SS N o. 7 m essag e lab e l typ e s

Section 6 – SS7

Message LabelMessage Label

• CIC• identity of the physical circuit that carries the call for which

the signaling information is meant.

• SLS• signaling link selection is used for load sharing between

signaling links.

• SLC• signaling link code identifies the signaling link connecting the

origination and destination SPs

For implementation of level 3 functions, the required fields are :

Service Information Octet (SIO)

Routing Label

Section 6 – SS7

Service Information OctetService Information Octet

• Includes :-• service indicator (SI- 4-bits)• sub service indicator or network indicator (NI- 2-bits)

• The SI will determine the “User”, e.g. TUP, SCCP, ISUP and the NI will determine which network is concerned, e.g. international or national.

• Subservice Field Codes (NI)

D C B A Spare

0 0 International network

0 1 Spare (for international use only)

1 0 National network

1 1 Reserved for national use

Section 6 – SS7

Service Indicator CodesService Indicator Codes

D C B A

0 0 0 0 Signaling network management messages

0 0 0 1 Signaling network testing and maintenance messages

0 0 1 0 Spare

0 0 1 1 SCCP

0 1 0 0 Telephone User Part

0 1 0 1 ISDN User Part

0 1 1 0 Data User Part (call and circuit-related messages)

0 1 1 1 Data User Part(facility registration & cancellation messages)

1 0 0 0 Reserved for MTP Testing User Part

1 0 0 1 Broadband ISDN User Part

1 0 1 0 Satellite ISDN User Part

1 0 1 1 )

to

1 1 1 1 ) Spare

Section 6 – SS7

Routing LabelRouting Label

• 32 bits , consists of :• Origination Point Code - 14 bits• Destination Point Code - 14 bits• Signaling link selection - 4 bits

• The NI, together with 14-bit point code, allows for four signaling networks each with up to 16,384 point codes.

SLS Originating Point Code Destination Point Code

Section 6 – SS7

Signaling Message HandlingSignaling Message Handling

• Discrimination :• discrimination function compares the DPC in the routing label with

the point code of own SP

• If DPC = own SP ; message meant for this SP

• If DPC <> own SP ; further processing performed by routing function

• Distribution :• distribution function examines Service Indicator to deliver the

message to the desired user part

Section 6 – SS7

• Routing :

• routing function determines the signaling link on which the message is to be sent

• concerned with OG signaling messages• routing table is examined along with DPC in the message to

determine the OG SLS available to route the message.

Section 6 – SS7

Signaling Link ManagementSignaling Link Management

• Controls the links connected to the SP to maintain certain minimum capability of carrying signaling traffic under normal operation & in the event of failures

» Link activation

• process of making a signaling link ready to carry signaling traffic

» Link restoration

• procedure to bring a previously failed link back into service

Section 6 – SS7

Flow ControlFlow Control

• CCS 7, in common with other transport mechanisms, needs to limit the input of data when congestion onset is detected. The nature of CCS 7 will lead to SP/STP overload congestion being spread through the signaling network if no action is taken. This will result in impaired signaling performance and message loss. In addition to signaling network congestion within a node, congestion will also require action to prevent signaling performance from deteriorating. There is thus a need for flow control within the signaling system to maintain the required signaling performance.

Section 6 – SS7

ExerciseExercise

Q1. Name the two different kind of signaling types and compare the two.

Q2. Name the users of the TCAP.

Q3. How many types of connections occur in SCCP?

Q4. Out of following, which is used for monitoring the status of link MSU, LSSU, FISU

Section 6 – SS7

Q5. How many consecutive 1s are allowed in signaling units and why?

Section 6 – SS7

Dimensioning Dimensioning

Section 7 – Dimensioning

ObjectiveObjective


• Mapping on the air interface

• Microwave planning concepts

• signaling link dimensioning and load sharing

• Routing strategies

• Erlang B, Erlang C

• Numbering plan used in mobile networks

• GPRS concepts



In a traditional telephony - signaling means the passing of information from one point to another for setting up and supervision of telephone calls.

• subscriber – exchange signaling (signaling between subscriber and the local exchange)

• inter-exchange signaling (signaling between exchanges).

With the development of the CCITT Signaling System No. 7 the capabilities have been enhanced to be able to handle non-call related data. End user data can be transferred, as with the Short Message Service.


Abis MappingAbis Mapping

Besides the traffic channels, the Abis interface also carries the required signaling information in 64 Kbit/s channels. One signaling channel is normally provided for each transceiver within a BTS for controlling upto 8 subscribers per carrier frequency.


Sig TRX 2

Sig TRX 1

TS 0

BSC

TRX 1

TRX 2

1 2 3

4 5 6 7

0

1 2 3

4 5 6 7

0


TS Arrangement on PCM Link :

1 Sector occupies 2TS for TCH (64 Kbps)

1TS for signaling

Total number of Time slot in one PCM 32

Out of which 1 is used as FAS and other for internal signaling.

TS available for carrying the information 30

Therefore total number of TRXs that can be cater on one PCM

= 30/3 = 10


Example:

Assuming that network has BTSs of 2 TRX in each sector, then max number of BTSs that can share the 1PCm link is:

1 Sector occupy 5TS

Therefore, one BTS occupy 15TS

Hence, totoal number of BTSs are = 30/15

= 2



TS BTS 1 BTS 2 0 PCM Management Information 1 TRX 1 2 TRX 1 3 TRX1 4 TRX1 5 TRX 2 6 TRX 2 7 TRX 2 8 TRX 2 9 TRX 3 10 TRX 3 11 TRX 3 12 TRX 3 13 TRX 4 14 TRX 4 15 TRX 4 16 TRX 4 17 TRX 5 18 TRX 5 19 TRX 5 20 TRX 5 21 TRX 6 22 TRX 6 23 TRX 6 24 TRX 6 25 Signalling BTS1, Sector1 26 Signalling BTS1, Sector2 27 Signalling BTS1, Sector3 28 Signalling BTS2, Sector1 29 Signalling BTS2, Sector2 30 Signalling BTS2, Sector3 31 Control Ring

Microwave LinksMicrowave Links

A Telecom Network has two main constituent

1. Access Network and

2. Connectivity which is the backbone connectivity.

Optical fiber is most popular for high–capacity routes in Network however microwave radio used in lower capacity routes, in difficult terrain, in private and military communication where the advantage of flexibility, security and speed of installation offered by radio are particularly valuable.


Cellular Network Application Cellular Network Application

MSC BSC

BTS

BTS


Microwave Hop: It is a bi-directional transmission system

containing 2 DMR one at each end of connecting elements.

The information could be on 2MB or higher interface. The microwave frequency bands and the radio channel spacing in these bands have been all standardized by CCIR.

Some typical frequency bands are 2, 4, 6,7,8, 11 & 14 GHz. Above 11GHz rain attenuation becomes a greater problem and hence restrict to short haul (shorter hop length). Each band is further divided into several blocks of channels which is a pair of frequencies, f & f’ for transmission and reception.


PropagationPropagation

Microwave beam passes through the part of the atmosphere, which is in close proximity of surface of the earth. Radio waves, like light waves are also electromagnetic waves, though of lesser frequency, also have the properties of light waves like attenuation, refraction, diffraction, scattering and polarization. While designing the system and engineering link, the effect of all these are to be taken into consideration.

The loss between the transmitting and receiving antenna with


transmission medium as vacuum is termed as Free Space Loss.

Lfs = 92.4 + 20 log d + 20 log f

d = distance in Kms

f = frequency in Ghz


Refraction K-factorRefraction K-factor

It is the scaling factor that helps to quantify the curvature of the radio beam

K = effective earth radius / true earth radius

True earth radius = 6370 km

The angle of curvature by refraction is denoted by the k-factor, defined as the ratio of the effective earth radius (radius of earth which allow the beam to draw as a straight line) to the true earth radius.


Path Clearance ProcessPath Clearance Process

• Microwave Link is based on LOS

• Microwave Path curvature is based on Refraction (K)

• Microwave Path should also have Fresnel Zone clearance to avoid diffraction

Fresnel Zone: The area around the line of sight path which results into a reflection of 180° (half wave length) at the receiver is termed as First Fresnel Zone. The area which results in 2 and 3 half wave lengths are Second Fresnel Zone.


Fn = 17.3 Sqrt ( nd1d2/f D)

Fn = Radius of Fresnel Zone (center point at path)

d1 = distance from one end of path to reflection point (km)

d2 = distance from other end of path to reflection point (km)

D = d1 + d2

f = frequency (GHz)

n = number of Fresnel Zone


Path ProfilePath Profile

Linear Method

• Microwave beam is drawn as a straight line

• The effective earth curvature height (h) is calculated for a desired k-factor

h= (d1d2) / 12.75 k• Fresnel Zone clearance is then calculated for the same k value

Earth Bulge = Effective earth curvature height + Fresnel Zone clearance


CountermeasuresCountermeasures

Flat Fading:• Link Overbuilding (Antenna gains, improved receiver

performance, power)

• Shorten distance between sites

• Path diversity

Selective Fading:

• Space diversity

• Frequency diversity

Equipment Reliability:Hot- Standby arrangement


Space DiversitySpace Diversity


Frequency DiversityFrequency Diversity

Tx 1

Tx 2

Rx 1

Rx 2


Over Reach InterferenceOver Reach Interference

f1

f1’

f2

f2’

f1

f1’


Signaling Planning ObjectiveSignaling Planning Objective

The main planning objectives are:

• Reliability - disturbances in the signaling should be avoided.

• Robustness - a fault in one part of the network should not affect other parts.

• Simple Network Architecture - the structure of the network should be easy to understand.

• Short Delay Times - to cater for high quality of service.


Purpose: to dimension the correct amount of hardware to meet the requirements.

• Over dimension > inefficiency

• Under dimension > congestion

• Input data: - subscriber data

- network data

- GoS

- equipment limitations

Signaling Link Dimensioning


Simplicity is achieved by introducing hierarchical levels. Hierarchical

networks are flexible and allow fast expansion of the PLMN. Hierarchical networks are also easy to operate and manage.

Major part of signaling network delay is induced in intermediate nodes and not so much on the links (in a properly dimensioned network). Hierarchical network structures are therefore also to be preferred from his point of view.


Definition of Traffic

A =BHCA x MHT

3600

Where: A is the traffic expressed in Erlang (E)

BHCA = Busy Hour Call Attempts

MHT is the average holding time (s)

3600 is the number of seconds per hour


When designing the network, redundancy is of major importance. There

are cases though when separation of the connections on different routes is not plausible. One should then at least consider hardware redundancy.


Traffic Link RedundancyTraffic Link Redundancy

80% of the traffic saved if one link goes down

2 separated routes 3 separated routes

The redundancy factor becomes 1.6 and 1.2 respectively

eg 10E per link then: 80*(10+10)=16E

80*(10+10+10)/2=16E


C7 Signaling Concept in the GSM NetworkC7 Signaling Concept in the GSM Network

Maximum signaling load per signaling link

30 % under normal conditions

60 % under overload conditions

64 kbit/s = 8000 octets/s (1 octet = 8 bits)

Normal load = 0.3 x 64 = 19.2 kbit/s or

0.3 x 8000 = 2400 octets/s

Overload = 0.6 x 64 = 38.4 kbit/s or

0.6 x 8000 = 4800 octets/s


A widely used dimension rule, based on No. 7 signaling link dimensioning for plain PSTN with TUP, is to allow 30% load on links in normal operation and 60% in failure situations.

In GSM networks 20% load in normal operation is often used. With MAP MSUs instead of TUP the same signaling volume is generated by fewer and longer MSUs that implies a more bursty load requiring more margin to achieve the same quality.


Signaling VolumesSignaling VolumesSignaling is required not only for setting up of call connections, but also for

finding and upgrading the present location of the subscriber. Enhanced security including both authentication and equipment identity control require No. 7 signaling.

Estimates of the signaling generated by different events in the network can be used to calculate the approximate signaling load.


Signaling Calculation Model:

The main input parameters are:

• Traffic per subscriber

• Mean Call holding Time

• Percentage MT traffic

• Location Updates per subscriber and hour

• Inter MSC handovers per call

• IMSI attach per subscriber and hour

• Number of authentication triplets fetched at a time

• short messages per subscriber and hour


signaling Volume Examplesignaling Volume Example

Model 1 Model 2

Traffic per sub 0.030E 0.025E

Mean holding time 100s 120s

MT Percentage 33% 25%

Location Updates new VLR / 1.1 0.45sub&hour

Inter MSC Ho/call 0.10 0.05

SM / sub&hour 0.5 0.1

MSC - HLR kb/s per ksub 1.55 0.65

MSC - MSC kb/s per ksub 0.35 0.15

MSC -EIR kb/s per ksub 0.20 0.10


There is a different possibilities for the operator to influence the signaling

volumes per subscriber:

• Placing of MSC borders as well as LA borders impact the mobility experienced in the network. (it reduces the Location Area update signaling)

• Parameter settings in the AUC for use of selective authentication

• Parameter settings in the EIR for IMEI check


C7 Routing StrategiesC7 Routing Strategies

In order to meet the need for extended services, i.e. communication with databases without speech connections, the SCCP is used. SCCP maintains connection oriented (CO), connectionless (CL) network services, circuit related and non-circuit related signaling.

• Connection-oriented signaling: used when many messages to transfer between two signaling points (SP) and when messages are so long that segmenting is needed.


• Connectionless signaling is used for MAP. In connectionless signaling all message signaling units contain all information required to route each message unit to the correct destination.

• Circuit related signaling is signaling related to a specific speech or data connection

• Non circuit related signaling is signaling not connected to any speech or data connection, i.e. roaming signaling in mobile application.

SCCP make possible routing of the message on a higher level (Global Title Translation (GTT), SCCP rerouting), i.e. handle the logical signaling connection, and MTP is responsible for transporting the message through the network in a reliable manner.


SCCP RoutingSCCP Routing


The SSN indicates the subsystem so the message is distributed to the right

software in the terminating node. SSN points out MAP HLR, MAP VLR, MAP MSC/GMSC, BSSAP, MAP EIR, MAP AUC, MAP SC, and ISUP.


MTP RoutingMTP Routing

The routing procedure as well as the load sharing between link sets and within link sets is done using:

• Network Indicator (NI),

• Destination Point Code,

• an Originating Point Code (OPC) and

• a four bit signaling Link Selection code (SLS).

NI identifies a No.7 Network. DPC and OPC are the signaling Point Code (SPC) that uniquely defines a signaling Point (SP) in the No.7 signaling network.


MTP signaling route could either be one signaling link set or load sharing over signaling link sets.


Signaling route alternatives with different priorities can be defined

and the routing alternative with lower priority will not be set into action until the alternative with the higher priority is totally blocked.

Signaling routing in the GSM can be understand by the example of the network having three HLRs in three different zones along with STPs.


Routing principles for No. 7 signaling:

• Western MSC load-share signaling to HLRs over Western STP to East HLR and East STP to East HLR. Second choice, if both link sets are out of order, signaling is routed over Central STP to East HLR.

• Similar is the case for other two HLRs.

• HLRs are connected to all three STP. Routing of signaling depends on destined MSC group:

• signaling towards western MSCs is routed in load-share over W E and E E. Second choice, if both link sets are out of order, signaling is routed over C E.


• signaling towards central MSCs routed in load-share over W E and C E.

Second choice, if both link sets are out of order, signaling is routed over E Tr.

signaling towards eastern MSCs routed in load-share over C E and E E. Second choice, if both link sets are out of order, signaling is routed over C E.


Signaling Load SharingSignaling Load Sharing

For load sharing both between link-sets and between the links on the link-sets the signaling Link Selection code is used. This is a four-bit code that is set by the MTP user. Which bit to be used as the load sharing bit for load sharing between the link sets is set by the LSHB-parameter (Load sharing Bit) in the exchange data.

If all links get the same number of SLS codes they will all carry the same load, i.e. the load is evenly distributed. If all the links do not get the same number of SLS codes then the load will not be evenly distributed.


The maximum load on the link set is limited by the signaling links carrying

most of the signaling load


C is the maximum load in normal operation for one link. For example, if we allow 30% maximum load on each 64kb/s link and we have 8 signaling links in a link-set. Then, assuming that we do not load share with another link-set (i.e. four bit load share within the link-set) the capacity of link set is 8*30%*64kb/s=153.6 Kbps.


MTP Changeover in case of link failureMTP Changeover in case of link failure


Traffic ModelsTraffic ModelsTwo commonly used models are Erlang B and Erlang C:


Erlang BErlang B

This is a loss model, in that blocked calls are simply lost rather than being held in some form of queuing system.

It assumes that call arrivals follow a Poisson process, that the number of users is much greater than the number of channels.

From the Erlang-B table, 7 channels and a GoS of 0.02 (2%) corresponds to A= 2.9354 Erl of offered traffic.


Therefore, carried traffic = A (1- GoS)

= 2.9354 (1- 0.02)

= 2.87669Erl

Channel Utilization: This is the ratio of carried traffic to number of channels

Therefore,

Channel Utilization = 2.87669/7

= 0.41 or 41%


Calls that cannot be handled are put in a queue until a channel becomes available. The queuing delay is a function of the offered packet traffic, the maximum number of links available and the mean holding time of each call. The Erlang C formulas are used to determine the probability of a delay occurring, the probability of such a delay being larger than a certain time and also the mean delay.

Erlang CErlang C


Example: As compared with circuit switched traffic with a blocking

probability of 2% 17.5 Erlangs corresponds to 22 Erlang in C table.

This suggests that there is a gain in trunking efficiency offered by tolerating a 10 ms delay in transmission.

Mean delay depends on the mean holding time, which in turn is proportional to the packet size. Packet size can be reduce in order to reduce the holding time but it increase the signaling overheads.


Processor LoadProcessor Load


DefinitionsDefinitions

The processor load is the proportion of time that the processor executes instructions having real time requirements. It is normally expressed in percentage of its full capacity.

It has following components:

Idle load: This component depends on the functionality and to some extent on the size of the exchange. The idle load is not dependent on the traffic or other external activities but varies from processor to processor.

Continued…..


Usage load: This component is caused by operation and

maintenance activities such as data dumps, commands, traffic measurements and printout of statistics.

Traffic load: This component is used for traffic handling.

Loadability: The loadability is the upper limit for the allowed processor load. It depends on the processor but also on the job lengths and delay requirements.

Continued…..


Load per call: This is the amount of execution time that the

processor has to spend in setting up and disconnecting a call. Load per call is normally expressed in milliseconds (ms), but is sometimes expressed as the number of ASA (assembler) instructions necessary to fulfill the task.

Traffic peak margin: Is sometimes referred to as Safety margin. The traffic peak margin is normally 20-35% of the available traffic load. This is needed to allow for unpredictable traffic peaks.


CapacityCapacity

Traffic capacity, (e g 2,500 Erlang), tells how many simultaneous calls a unit can handle. One Erlang corresponds to one busy line. If a subscriber calls 25 mErlang during busy hour, he is in average calling 25/1000 of the hour (=25*60*60/1000 = 90 seconds).

Erlang can be limited by for example the group switch, available speech trunks, transcoders etc. But this does not give any idea about the processor loading as well as nor about non call activities.

Continued…..


Call capacity, (e g 100,000 BHCA), tells how many call attempts a

unit can handle during busy hour. This figure is a better measure of processor capacity but still, this measure does not take into account non-call related activities.

Subscriber capacity, (e g 60,000 subscribers), tells how many subscribers that can be served by a unit. This figure is strongly depending on subscriber behavior.

Continued…..


Addressing capacity, (e g 1020 TRXs), tells how many HW or SW

devices that can be connected / defined. This is also known as system limits. Here, no considerations to real-time processing needs or amount of traffic are made.


Traffic Load DistributionTraffic Load Distribution


In the default traffic load distribution for a GMSC/MSC/HLR the call part

takes about 70% of the capacity of the traffic load, the location updating part about 25%, the SMS part 3% and supplementary services approximately 2%.

If one look into the traffic part (70% of traffic load) the actual basic load part is 53% of the usage load, a gate way load part is 7.5%, a charging part 5%, a handover part 3% and a part used for authentication about 1%


Numbering PlanNumbering Plan

The MSISDN is a number which uniquely identifies a mobile telephone subscription in the public switched telephone network numbering plan. These are the digits dialed when calling a mobile subscriber.

In GSM 900/GSM 1800, the MSISDN consists of the following:

MSISDN = CC + NDC + SN


CC = Country Code

NDC = National Destination Code



International Prefix

Country Code

National Destination Code

Subscriber Number

0091 98 113 23448

The digits ‘113’ identify the GSM 900/GSM 1800 PLMN area code.

The digits ‘23448’ define the five digits, which identify the

mobile subscriber.


A NDC is allocated to each PLMN. In some countries, more than

one NDC may be required for each PLMN.

The international MSISDN number may be of variable length.The maximum length is 15 digits, prefixes not included.

Example: Singapore PSTN subscriber is calling to an Indian GSM PLMN subscriber

Continued…..


International Mobile Subscriber Identity (IMSI)International Mobile Subscriber Identity (IMSI)

The IMSI is the information which uniquely identifies a sub in a GSM PLMN. It is used in all the signaling in the PLMN.

It will be stored in the in the Subscriber Identity Module (SIM), as well as in the HLR and in the serving VLR.

It consists of three different parts


IMSI = MCC + MNC + MSIN

MCC = Mobile Country Code (3 digits)

MNC = Mobile Network Code (2 digits)

MSIN= Mobile Station Identification Number

All network related subscriber information is connected to the IMSI.


In GSM 1900, the MSISDN consists of the following:

MSISDN = CC + NPA + SN

CC = Country Code

NPA = Number Planning Area



The NPA is allocated to each GSM 1900 PLMN. The length of MSISDN

depends on the structure and operating plan of each operator. The maximum length is 15 digits, prefixes not included.


Examples:

xyz = operator code

abcde = Subscriber number

STD code = PSTN area code (11 for delhi)

• Call from PSTN to PLMN

Local Call 98 xyz abcde

Outside area call 0 98 xyz abcde

• Call from PLMN to PSTN

Local Call 0+STD code+SN

Outside area call 0+STD code+SN


GPRS Core Network PlanningGPRS Core Network Planning


Circuit Vs Packet DataCircuit Vs Packet Data

Circuit Switched Service:

• 2G system (primarily voice and data on circuit switched air interface)

• Call charging based on channel holding time.

• Maximum number of users per TDMA channel is 8

• Suitable for constant bit rate applications

• Resource allocation is done such that UL and DL are paired.


Packet Switched Service:

• Several users can share the same channel.

• Charges based on channel usage (actual usage of byte transferred).

• Well suited for bursty traffic.

• Resource allocation done independently on UL and DL (good for applications with asymmetrical bit rate)

• Dynamic allocation of resources

• Can multiplex traffic (voice, data, video).


2

O f f e r e d G P R S T r a f f i c0

1

2

3

4

5

6

7

8

9

1 0

T C H

C i r c u i t S w i t c h e d T r a f f i c0

2

4

6

8

1 0

1 2

1 4

T C H

G S M

c a p a c i t y

6

8

1 0

1 2

1 4

0

2

4

S p e e c h tr a ffi c le a v e s s o m e c a p a c i ty fo r p a c k e t d a ta


GPRS System featureGPRS System feature

• Variable quality of service.

• Independent packet routing.

• Protocol transparent (encapsulation & tunneling)

• Slotted ALOHA for random access procedure

• Provides IP connectivity to mobile subscriber.

• Build on existing GSM infrastructure with added nodes for supporting packets.

Serving GPRS Support Node (SGSN)

Gateway GPRS Support Node (GGSN)


Conceptual View on GPRSConceptual View on GPRS

Shared GSM and GPRS Infrastructure

InternetCorporate Intranet

InternetCorporate IntranetGPRS CoreGPRS Core

BSCBSCBTSBTS

GSM Voice

Access Point

GPRS Infrastructure IP World


Air Interface - Mobile TerminalAir Interface - Mobile Terminal

• Type C GPRS only (or manually switched between GPRS and speech modes)

• Type B GPRS and Speech (not at same time) (Automatically switches between GPRS and speech modes)

• Type A GPRS and Speech at the same time BSC

BTS


GPRS Attach / DetachGPRS Attach / Detach

• Attach

Performed when the MS indicates its presence to PLMN for the purpose of using GPRS service

Carried out between MS and SGSN

MS identifies itself with its GSM identity

GPRS subscription necessary for successful attach

• Detach

Performed when the MS indicates to the PLMN that it no longer be using GPRS services

MS identifies itself with its GSM identity


System ArchitectureSystem Architecture

BTS

BTS

BTS

BSC SGSN GGSN

Data Networks

Um Abis

HLR

Gb Gn

Gr Gc

Gi


SGSNSGSN

• Responsible for delivery of packets to mobile subscribers in its service area.

• Mobility Management

• Logical link management, authentication

• GPRS user- related data needed by SGSN to perform routing and transfer functionality stored in GPRS Register eg current cell, current VLR, user profile including IMSI and its address in PDN.

• Interface point between core and Radio networks


• Acts as an interface between GPRS network and external PDNs

• Mainly responsible for packet routing, transfer and mobility management

Converts packets from SGSN into appropriate PDP format and sends them out to corresponding PDN

PDP addresses of incoming data packets from PDN are converted to IMSI of the destination user and sent to the responsible SGSN.

Tunneling

GGSNGGSN


• Circuit Switched traffic has priority

• In each cell Circuit Switched & Packet Switched territories are defined

• Territories consist of consecutive timeslots

TRX 1

TRX 2

CCCH TS TS TS TS TS TS TS

TS TS TS TS TS TS TSTS

Circuit Switched Territory

Packet Switched Territory

Territory border moves Dynamically based on Circuit

Switched traffic load

Default GPRS

Capacity

Dedicated GPRS

Capacity

TS TS

Additional GPRS

Capacity

TS TS

GPRS and GSM Resource sharing


Capacity ManagementCapacity Management

• Dedicated GPRS Capacity

TCHs reserved exclusively for GPRS use.

• Default GPRS Capacity

TCHs always allocated to the GPRS when circuit switched load permits.

Keeps GPRS timeslots consecutive (important for multislot operation)


PDP Context Activation - 1PDP Context Activation - 1Accessing the HLRAccessing the HLR

BTS BSC

SGSN

GGSN

Intranet

GPRS Backbone IP Network

SS7

HLR

DNS

• Access Point Name = Reference to an external packet data network the user wants to connect to

Internet

APN="Intranet.Ltd.com"

1.

2.

AccessPoint

• (1) MS sends "Activate PDP Context Request" to SGSN

– Access Point Name

– PDP Type (IP)

– PDP Address (empty == dynamic)

– QoS & other options

(2) SGSN checks against HLRAccess Point NameDynamic / static IP addressQoS


PDP Context Activation - 2PDP Context Activation - 2Finding the GGSNFinding the GGSN

BTS BSC

SGSN

GGSN


DNS

• DNS = Domain Name System = mechanism to map logical names to IP addresses

Intranet

1.

2.

AccessPoint

• (1) SGSN gets the GGSN IP address from DNS

– APN maps to the GGSN IP address

(2) SGSN sends "Create PDP Context Request" to GGSN

PDP Type (IP)PDP Address (if empty=> dynamic address) Access Point NameQoS


PDP Context Activation - 3PDP Context Activation - 3Access Point SelectionAccess Point Selection

BTS BSC

SGSN

GGSN


DNS

Intranet

Internet

APN="Intranet.Ltd.com"

• Access Point Name refers to the external network the subscriber wants to use

–Physical/logical interface in GGSN

• Access Point configuration in GGSN defines where to connect the user

• If dynamic address, allocated by GGSN


PDP Context Activation - 4PDP Context Activation - 4Context ActivatedContext Activated

BTS BSC

SGSN

GGSN


Intranet

Internet

1.

2.

• (1) GGSN sends "Create PDP Context Response" back to SGSN

• (2) SGSN sends "Activate PDP Context Accept" to the MS

• SGSN now ready to route user traffic between MS and GGSN


ExerciseExercise

Q1. How many PCMs are required for one BTS with 2,1,2 and other with 3,2,1 configuration?

Q2. Calculate the free space loss for 20Km distance at 15GHz frequency?

Q3. Calculate the 2nd Fresnel Zone for total distance of 20Km at a distance of 10Km from one end. Frequency used is 15GHz.

Q4. What precaution has to be taken to avoid the over reach problem in the microwave links?


Optimisation Optimisation

Section 8 – Optimisation

ObjectiveObjective


• signaling delay in the network

• Effect on the network while introducing the new releases

• Impacts of subscriber behavior

• TCP/IP concepts



The goal of optimization is to ensure the network is operating at optimum efficiency and within the defined quality of service constraints. This is achieved by implementing corrective action and procedures to rectify network problems identified though analysis of performance management monitoring parameters.

Vendors are continually seeking ways of maximizing revenue generation with minimum additional investment. One way of achieving this is to identify areas where the network is not operating at peek efficiency and making adjustments for improvement.


Optimization is a Cyclic ProcessOptimization is a Cyclic Process


Signaling DelaySignaling Delay

The signaling network delay depends on a variety of parameters, among others: bit error rate, signaling link propagation and processing time, average link load, mean MSU length on link, mean MSU length of transmitted signal, number of signaling links in signaling path, number of STPs in signaling link path, buffering and queuing times in STP etc.

Key parameters that are varied are mean MSU-length, mean signaling link load, and number of STPs and signaling links in path.


Typical values used for calculating the delay:

Bit Error Rate on link 8.3x10-4

Mean MSU lengths a) 23 oct

b) 74 oct

STP delay 20ms

Signaling link propagation 10ms

and processing


For a constant bit error rate of 8.3x10-4 and basic error correction, the

waiting times (Tw) on the outgoing side are shown in table below for mean MSU length 23 octets and for mean MSU lengths of 74 octets.


STP Delay (TSTP): In CCITT Blue Book, a cross STP delay of 20ms is estimated for 0.2 link load.

Propagation and Processing Time (TL): This includes transmission time on link and processing time of message. The overall main part of TL is the transmission time. For ground-installed links for which basic error correction is used, TL should be less than 15ms.


Signaling Network Delay Example: Consider two cases

1. the signal passes one intermediate STP before reaching its destination

2. the signal passes two intermediate STPs before reaching its destination


Signaling Network Delay with one intermediate STP.

The signaling link delay, SLD is derived from:SLD = 2x(TW + TL) + TSTP

Signaling Network Delay with two intermediate STPs.

The signaling link delay, SLD is derived from:

SLD = 3x(TW + TL) + 2xTSTP


It is to be mentioned that dependence between the MSU lengths and the

delay times is not necessarily linear.


Impacts On CapacityImpacts On Capacity

• When introducing a new release

New releases typically mean a drop of 10-15% of system capacity. The BSC decrease is often less than for MSC. The reason is that new BSC releases often contain more O&M improvements than traffical ones.

• Subscriber Behavior

The call type affects the capacity required per call, e.g., the load per call is different depending on type of call. Load per call is defined as the execution time of a call. This is the time necessary to execute the program code for a call in the CP (Central Processor). By a call is meant a call setup, call release and information sent in connection with the call.


Call attempts have the highest impact on capacity. One call setup

plus clear consumes about 25 ms execution time. SMS point-to-point takes about 2/3 of call execution in the BSC (2/3 of 25 ms). Most SMS/ptp are mobile terminated, and need paging as well.

Registrations take roughly 1/3 of call execution in the BSC. Due to the big number of them, the total CP load from registrations is often higher than for calls.


• Network Configuration

The number of BSCs per MSC can have a major impact on the system capacity due to the shift of intra-BSC handovers to the inter-BSC handovers, which will increase in case of a higher number of BSCs. An increase of the number of inter-BSC handovers with a factor of 10 will take 7% more of the capacity.

A MSC configuration with stand alone HLR will increase the capacity of the MSC with 15% compared to a MSC with integrated HLR (worth mention that this 15% figure has been derived from comparing the total MSC/HLR capacity with the maximum capacity of a MSC without HLR).


The BSC covering areas should generally be chosen so that the

boundaries as far as possible are located in areas with low handover intensity. The reason is that high handover frequency decreases MSC and BSC capacity. Consequently, boundaries through city kernels and areas close to highways should, if possible, be avoided.

The value that the periodic location update is set to affects the capacity. The period can be set between 6 and 1530 minutes in steps of 6 minutes. The minimum period sustainable by the system depends on the number of subscribers and their traffic behavior.


The number of periodic location updates has a significant impact

on the MSC capacity, therefore it is advisable to set the periodic location update timer very carefully. Most operators choose a short period for the forced registration, caused by the fear of loosing track of the subscribers. In case of system recovery after a large restart the periodic location update rate will impact the recovery time severely. Therefore the recommendation is to use 120 minutes for the timer value. It is worth mention that the positive effect on the MSC may impact the BSC performance negatively due to a higher number of pagings.


Number of Location Areas (LAs) has impact on BSC load. If there are

many cells per area, the local page attempts will be quite heavy. If increasing the number of LAs, the paging load will go down. On the other hand: If high movability for mobiles, the load from location updates will increase. When finding the optimal point, also load in MSC must be looked into.


• Adding New Applications

The following table presents the CP capacity impacts on an average node

AUC (Authentication Center) -0.4%

FNR (Flexible Numbering) -2.5%

SCF (Service Control Function) -2.0%

(Based on 10% IN calls)

SSF (Service Switching Function) -10%

(Based on 10% IN calls)

PRA (Primary Rate Access 30B + D) -19%

(Based on 10k BHCA PRA traffic)


Capacity GainsCapacity Gains

• IMEI Check on Location Update

It is possible to switch off the IMEI check function for location update, which increases the capacity with 2%.

• Usage of Toll Ticket

Output only those call data records that are needed, where possible accounting should be used instead. For instance switching off the Land to Land call data record increases the capacity with 3.2%.


• TMSI Reallocation

Switching off the TMSI reallocation at location update, change of LAI, intra-MSC function will result in 2% more capacity.

• Authentication at Location Update

Switching off authentication at location update, change of LAI, intra-MSC will result in an increase of the capacity with 1%.


• Selective IMEI Check

It is advisable to use the selective IMEI check for all access types, which results in a gain of capacity of 4%. To be able to decrease the system recovery time it is recommended to switch off IMEI checking for the access type location update.

• Selective Authentication

The usage of selective authentication for all access types is strongly recommended from a capacity point of view. In case of the activation of selective authentication instead of authentication for each access, the increase of capacity is equal to 6.2%.


ConclusionConclusion

A better network and cell planning will result in some cases in more capacity, when less location updates and handovers are needed. Moreover the number of small nodes in a network may decrease the overall network capacity, since they may introduce more inter-MSC handovers, more new registrations and a higher amount of transit traffic compared to a network with several big nodes. Furthermore the split of GMSC and MSC allows a better maintainable network and more capacity in the separate entities, also the usage of different processors for each entity will be possible. Stand-alone HLR will also increase the total capacity in the network.


GPRS TCP/IP StrategiesGPRS TCP/IP Strategies

Datagram: It is a technical term for a packet of data and composed of many components. The most basic is:

010001010100101010100100101111010100101010010101010010101001010100101010101001010101001011100001111101001001000101010001000000011110010010100100010101001010101001001011110101001010100101010100101010010101001010101010010101010010111000111110100100100010101000100000001111001001010010001010100101010100100101111010100101010010101010010101001010100101010101001010101001011100001111

1010010010001010100010000000111100100101001000100

To: 129.23.88.12

From: 136.24.87.23Header

Data


IP Datagram ComponentsIP Datagram Components

Options (and padding)

Data

Destination Address

Source Address

Time to Live Header ChecksumProtocol

Identification Flags Fragmentation Offset

Version IHL Type of Service Total Length


What’s in a DatagramWhat’s in a Datagram

• Version: Version of IP (example: IPv4, IPv6)

• IP Header Length: The datagram’s header size in 32 bit words.

• Type of Service: Indicates “priority” of the packet. This is determined by the type of data in the packet. (QoS - Quality of Service)

• Total length: Size of the IP packet (in bytes).

• Identification: An integer number identifying the datagram.


• Flags: A 3-bit field of which the low-order 2 bits control

fragmentation. One bit specifies whether the packet can be fragmented; the second bit specifies whether the packet is the last fragment in a series of fragmented packets.

• Fragmentation Offset: A sequence number for the bytes in this packet when reassembling.

• Time-to-live: A counter that discards the datagram when it reaches a limited. This prevents the packet from looping endlessly on the network.

• Protocol: Indicates which upper-layer protocol receives

incoming packets after IP processing is complete.


• Header Checksum: Helps ensure IP header integrity.

• Source Address: Specifies the sending node.

• Destination Address: Specifies the receiving node.

• Options: Allows IP to support various options, such as security. • Data: Information payload.


• TCP/IP is the Packet Data

technology used by the Internet.

• GPRS will also be using the TCP/IP standard.


Physical

Link

Network

Transport

TC

P/IP

7-L

aye

r S

tack

(O

SI R

efe

ren

ce M

ode

l)

TCP

Fiber cable, Microwave link

IP

Network Interface Card

WWW, e-mail, data services

Session

Presentation

Application


TCP CharacteristicsTCP Characteristics

• Concerned only with the origin and destination on the network.

• Adapts to congestion

• Provides virtual connection


IP AddressingIP Addressing

• For example: • 150.215.17.9 (Octets 0-255)• In binary form, it looks like:

10010110.11010111.00010001.00001001

• “IP number” is like an address

136.20.2.3136.20.2.2136.20.2.1


• An IP address consists of two parts

• Identifies the network• Identifies the node or host

• These two parts specifies the class where the node belongs..


Address ClassesAddress Classes

• There are 5 different address classes.

• The first byte of the first octet determines the class of the address. • Class A addresses start with 0. • Class B addresses start with 10. • Class C addresses start with 110. • Class D addresses start with 1110. • Class E addresses start with 1111


5 Classes of IP Address5 Classes of IP Address

1

125

31

63

15

15

Quantity of Domains (Networks) in eachClass

Class A: 1-126

Class B: 128-191

Class C: 192-223

127: Reserved (loopback)


Finding an IP’s Network AddressFinding an IP’s Network Address

• When a node receives a packet, it needs to determine the Network Address of the network where the destination node belongs.

• This is done by using the network subnet mask.

• Subtracting the subnet mask to an IP address results in the identification of the network and node sections of an the IP address

10010110.11010111.00010001.00001001 150.215.017.009

- 11111111.11111111.00000000.00000000 255.255.000.000

10010110.11010111.00000000.00000000 150.215.000.000


Transmission MethodsTransmission Methods

• Transmission is the supporting layer under TCP/IP.

• Types of transmission• Frame Relay• ATM (Asynchronous Transfer Mode)


ATMATM

Asynchronous Transfer Mode - A high speed, low delay, multiplexing and switching technology that can support any type of traffic including voice, data, and video applications. ATM is ideally suited to applications that cannot tolerate time delay, as well as for transporting frame relay and IP traffic that are characterized as “bursty”.


Other Packet-Based NetworksOther Packet-Based Networks

• X.25 --- A popular standard for packet-switching networks.

• CLNP --- (Connection-Less Network Protocol) derived from IP.


GSM ARCHITECTURE

Technology

Transcript of GSM ARCHITECTURE