1

ABSTRACTThe rapid pace at which mobile communication has grown is quite astonishing. Mobile

communication is now viewed as a necessity and is one of the fastest growing and most demanding

technologies. There were different generations in the development of a mobile system. The First

Generation (1G) was analog with limited network services and no roaming facility. The Second

Generation (2G) brought significant advancements to the mobile technology in terms of service

sophistication, capacity and quality as they made the communication system digital. GSM is a 2G

technology. The introduction of wireless access to the internet enhanced the 2G system to 2.5G.

GPRS, IN features, SMS are some of the features of 2.5G mobiles. There is a concept of Third

Generation systems as well (3G) which will allow communication, information and entertainment

services to be delivered via wireless terminals. An example of 3G system is Unified Mobile

Telephony System (UMTS).

The key advantage of GSM systems has been higher digital voice quality and low cost alternatives

to making calls such as text messaging. The advantage for network operators has been the ability to

deploy equipment from different, vendors because the open standard allows easy inter-operability.

Like other cellular standards GSM allows network operators to offer roaming services which mean

subscribers can use their phones all over the world.

In this report, we try to understand the basic concepts of GSM technology which is the most

extensively used and universally accepted technology till date across the world for mobile phone

communication. We will have a look at the GSM network implementation, network components,

basic scenarios while mobile station is active, idle or switched off, traffic channels and day to day

activities of the network engineers trying to keep the network up and running.

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Chapter 1

INTRODUCTION TO IDEA CELLULAR LIMITED

Idea Cellular is a publicly listed company, having listed on the Bombay Stock Exchange (BSE) and

the National Stock Exchange (NSE) in March 2007.

As India’s leading GSM Mobile Services operator, Idea Cellular has licenses to operate in all 22

Service Areas. Presently, operations exist in 11 Service Areas covering Delhi, Maharashtra, Goa,

Gujarat, Andhra Pradesh, Madhya Pradesh, Chattisgarh, Uttaranchal, Haryana, UP-West, Himachal

Pradesh, UP-East, Rajasthan and Kerala. With a customer base of over 26 million, Idea Cellular’s

footprint currently covers approximately 60% of India’s telecom population.

Idea Cellular was incorporated as Birla Communications on March 14, 1995 and later on

rechristened and given the current name on May 1, 2002. The company is a part of Aditya Birla

Group which has a holding of around 98.3 percent share in the company. Mr Kumar Mangalam Birla

is the Chairman of the company whereas Mr Himanshu Kapania is the Managing Director.

A frontrunner in introducing revolutionary tariff plans, Idea Cellular has the distinction of offering

the most customer friendly and competitive Pre Paid offerings, for the first time in India in an

increasingly segmented market. Customer Service and Innovation are the drivers of this Cellular

Brand. A brand known for their many firsts, Idea is only operator to launch GPRS and EDGE in the

country. Idea has received international recognition for its path-breaking innovations when it won the

GSM Association Award for “Best Billing and Customer Care Solution” for 2 consecutive years.

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Chapter 2

INTRODUCTION AND HISTORY OF MOBILE TELECOMMUNICATIONS

(GSM)During the early1980s, analog cellular telephone systems were experiencing rapid growth in

Europe, particularly in Scandinavia and the United Kingdom, but also in France and

Germany. Each country developed its own system, which was incompatible with everyone

else's in equipment and operation. This was an undesirable situation, because not only was

the mobile equipment limited to operation within national boundaries, which in a unified

Europe were increasingly unimportant, but there was a very limited market for each type of

equipment, so economies of scale , and the subsequent savings ,could not be realized. The

Europeans realized this early on, and in 1982 the Conference of European Posts and

Telegraphs(CEPT) formed a study group called the Groupe Spécial Mobile (GSM) to study

and develop a an European public land mobile system. The proposed system had to meet

certain criteria:

good subjective speech quality,

low terminal and service cost,

support for international roaming,

ability to support handheld terminals,

support for range of new services and facilities,

spectral efficiency, and

ISDN compatibility.

GSM P h as e s : In the late 1980s, the groups involved in developing the GSM standard realized that within

the given time-frame they could not complete the specifications for the entire range of GSM

services and features as originally planned. Because of this, it was decided that GSM would

released in phases with phase1 consisting of a limited set of services and features.

Phase 1 - Phase1 contains the most common services including:

Voice telephony

International roaming

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Basic fax/data services(up to 9.6Kbits/s)

Call forwarding

Call barring

Short Message Service(SMS)

Phase 2

Advice of charge

Calling line identification

Call waiting

Call hold

Conference calling

Closed user groups

Additional data communications capabilities

Phase 2+

The standardization groups have already defined the next phase, 2+. This program covers

multiple subscriber numbers and a variety of business oriented features. Some of the

enhancements offered byPhase2+include:

Multiple service profiles

Private numbering plans

Access to Centrex services

Interworking with GSM 1800, GSM 1900 and the Digital Enhanced Cordless

Telecommunications (DECT) standard Priorities and time schedules for new features and

functions depend primarily on the interest shown by operating companies and

manufacturers and technical developments in related areas.

Phase 2 + +

Enhanced Data rates for Global Evolution (EDGE), a new modulation method which

increases capacity on the air interface.

Customized Application for Mobile Enhanced Logic (CAMEL), standard, governing IN

service access while roaming internationally.

High Speed Circuit Switched Data (HSCSD), a method of delivering higher data rates

per subscriber by allocating an increased number of time-slots per call.

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6

Chapter 3

GSM NETWORK ARCHITECTUREThe various components of a GSM network are here. If we go from subscriber side, we have the MS

that is the handheld radio terminal, the Base transceiver station that communicated directly with the

MS on air. Then we have the base station controller that controls a group of BTSs the interface

between BTS and sBSC is called Abis and is largely proprietary. The BSC is connected to the

Mobile services switching center or simply the switch. The switch is connected to terrestrial

networks. There are databases for managing subscriber services and mobility, namely the Home

Location Register and Visitor Location Register. Most importantly we have the billing center

connected to the switch. There is also an OMC for monitoring all the components of the network.

SW ITCHING SYSTEM(SS ) COMPONENTS

M obile se r v i ce s S w i t c hing Cen t e r ( M SC ) : The MSC performs the telephony switching functions

for the Mobile network. It controls calls to and from other telephony and data systems, such as the

Public Switched Telephone Network (PSTN), Integrated Services Digital Network (ISDN),public

data networks, private networks and other mobile networks.

#Gateway Functionality:Gateway functionality enables an MSC to interrogate a network's HLR

in order to route a call to a Mobile Station (MS). Such an MSC is called a Gateway MSC (GMSC).

7

For example, if a person connected to the PSTN wants to make a call to a GSM mobile

subscriber ,then the PSTN exchange will access the GSM network by first connecting the call to a

GMSC. The same is true of a call from an MS to another MS.

Any MSC in the mobile network can function as a gateway by integration of the appropriate

software.

Home Loc a tion R e g i s ter( H LR): The HLR is a centralized network database that stores and

manages all mobile subscriptions belonging to a specific operator.It acts as a permanent store for a

person's subscription information until that subscription is canceled. The information stored

includes:

Subscriber identity

Subscriber supplementary services

Subscriber location information

Subscriber authentication information

The HLR can be implemented in the same network node as the MSC or as a stand-alone database. If

the capacity of the HLR is exceeded, additional HLRs may be added.

Vi s itor Lo c a tion R e g i s ter(VL R ) : The VLR database contains information about all the mobile

subscribers currently located in an MSC service area. Thus, there is one VLR for each MSC in a

network. The VLR temporarily stores subscription information so that the MSC can service all the

subscribers currently visiting that MSC service area. The VLR can be regarded as a distributed HLR

as it holds a copy of the HLR information stored about the subscriber. When a subscriber roams into

a new MSC service area, the VLR connected to that MSC requests information about the subscriber

from the subscriber's HLR. The HLR sends a copy of the information to the VLR and updates its

own location information. When the subscriber makes a call, the VLR will already have sthe

information required for callset-up.

A u t henti ca tion C e n t e r ( A UC ) : The main function of the AUC is to authenticate the subscribers

attempting to use a network. In this way, it is used to protect network operators against fraud. The

AUC is a database connected to the HLR which provides it with the authentication parameters and

ciphering keys used to ensure network security.

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Equipm e nt I denti t y R e gi s ter (EIR ) : The EIR is a database containing mobile equipment identity

information which helps to block calls from stolen, unauthorized, or defective MSs. It should be

noted that due to subscriber-equipment separation in GSM, the barring of MS equipment does not

result in automatic barring of a subscriber.

SHORT MESSAGE SERVICE CENTRE(SMSC): SMS is a service that allows a mobile to

exchange text messages with other entities. Messages can be upto 160 characters. SMSC serves to

store and relay the messages from different entities to MS. SMSC communicates with MSC on

MAP for relaying messages.

VOICE MAIL SERVICE CENTRE (VMSC): Voice mail service is like an answering machine

service offered to mobile subscribers. VMS contains the subscription data for all voice mail

subscribers. VMS also stores voice messages for subscribers. VMS is connected to the MSC. VMS

service is used by operators to increase the call completion rate and to raise additional revenue from

calls that could otherwise have not completed. Each subscriber is provided with a certain memory

corresponding to minutes of speech. Calls can be diverted to the voice mail on busy, usually an SMS

alert is sent to the subscriber when a message has been deposited in the box.

B A SEST A T I ON SYS T E M( B SS) C O M PO N E NT S

Ba s e S t a tion Con t rol le r (BSC): The BSC manages all the radio-related functions of a GSM

network.It is a high capacity switch that provides functions such as MS handover, radio channel

assignment and the collection of cell configuration data. A number of BSCs maybe controlled by

each MSC.

Ba s e T r a ns c e i v e r S tation ( B TS ) : The BTS controls the radio interface to the MS. The BTS

comprises the radio equipment such as transceivers and antennas which are needed to serve each cell

in the network. A group of BTSs are controlled by a BSC.

NETWORK M ON I T OR I NG CENTERS

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Op e r a tion and Ma int e nan c eC e n t e r(OM C ) : An OMC is a computerized monitoring center which

is connected to other network components such as MSCs and BSCs viaX.25 data network links.In

the OMC, staff are presented with information about the status of the network and can monitor and

control a variety of system parameters. There maybe one or several OMCs within a network

depending on the network size.

Ne t w ork Man a ge m e n t Cen t e r (N M C): Centralized control of a network is done at a Network

Management Center (NMC).Only one NMC is required for a network and this controls the . The

advantage of this hierarchical approach is that staff at the NMC can concentrate on long term

system-wide issues, whereas local personnel at each OMC can concentrate on short term, regional

issues.OMC and NMC functionality can be combined in the same physical network node or

implemented at different locations.

M O B I L E S T A T I ON ( M S)

An MS is used by a mobile subscriber to communicate with the mobile network. Several types of

MSs exist, each allowing the subscriber to make and receive calls. Manufacturers of MSs offer a

variety of designs and features to meet the needs of different markets. The range or coverage area of

an MS depends on the output power of the MS. Different types of MSs have different output power

capabilities and consequently different ranges. For example, hand-held MSs have a lower output

power and shorter range than car-installed MSs with a roof mounted antenna.

GSM MSs consist of:

MOBILE TERMINAL

The ME consists of radio processing functions and an interface to the user and other terminal

equipment.

SIM – SUBSCRIBER IDENTITY MODULE

SIM card as defined by GSM is a removable module which can be inserted inside the Mobile Phone

when the user wants to use the MS. It is available in two sizes the Credit Card size; known as ISO

SIM or the stamp size; known as Pug in SIM. SIM card has a unique information on it which is its

identity the IMSI. This identity is unique to each SIM used in any of the GSM Networks. This

identity is used by the Network to accomplish Signaling with the MS and also to do signaling with

other GSM networks for information transfer related to a particular connection.

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SIM also has a two level protection to prevent misuse. SIM can be optionally by the user be set for a

PIN code. Whenever the MS is turned ON or the SIM is inserted, the Mobile will ask the user to

enter the PIN code. If three false entries PIN are entered then the SIM is blocked. Then the user

needs to enter a Pin Unblocking Key (PUK). Ten false entries of PUK will permanently disable the

SIM card permanently.

Memory in the SIM :

ROM ( 6 – 16 kbytes) : Contains the OS and Security algorithms A3 and A8

RAM (128-256 bytes) : Buffer and for execution of algorithms

EEPROM (2-8 kbytes) : IMSI, Ki, Short Messages, Abbreviated dialing

SIM brings the advantages of security and portability for subscribers. For example, subscriberA's

mobile terminal may have been stolen. However, subscriber A's own SIM can be used in another

person's mobile terminal and the calls will be charged to subscribers A.

GSM INTERFACES

Abis A E B

Air F E

GsGf

MS

BTS BSC MSC MSC

VLR GMSC

EIR

SGSN

GGSN

11

Gr

Gn

Packet switched Circuit (voice)interfaces Switched(data ) Interfaces

GSM GEOGR APHI C AL NETWORK STRUCT URE

HLR

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GSM network architecture

Every telephone network needs a specific structure to route incoming calls to the correct

exchange and then on to the subscriber. In a mobile network, this structure is very

important because the subscribers are mobile. As subscribers move through the network,

these structures are used to monitor their location.

CELL: A cell is the basic unit of a cellular system and is defined as the area of radio

coverage given by one BS antenna system. Each cell is assigned a unique number called

Cell Global Identity(CGI).Ina complete network covering an entire country, the number of

cells can be quite high.

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LO C A TION A R E A (L A ) : A Location Area(LA)is defined as a group of cells. Within the

network a subscriber‟s location is linked to the LA in which they are currently located. The

identity of the current LA is stored in the VLR. When an MS crosses the boundary between

two cells belonging to different LA‟s, it must report its new Location Area to the network.

If it crosses a cell boundary within a LA, it does not report its new cell location to the

network. When there is a call for an MS, a paging message is broadcast within all the cells

belonging to the relevant LA.

M SC S E RVICE A R E A :An MSC service area is made up of a number of LAs and

represents the geographical part of the network controlled by one MSC. Inorder to be able

to route a call to an MS, the subscriber's MSC service area is also recorded and monitored.

The subscriber's MSC service area is stored in the HLR.

PL M N S E RVICE A R E A : A Public Land Mobile Network(PLMN) service area is the

entire set of cells served by one network operator and is defined as the area in which an

operator offers radio cover age and access to its network. In any one country there may

be several PLMN service areas, one for each mobile operator's network.

G S M S ER V ICE A R E A : The GSM service area is the entire geographical area in which

a subscriber can gain access to a GSM network. The GSM service area increases as more

operators sign contracts agreeing to work together. Currently, the GSM service area spans

dozens of countries across the world from Ireland to Australia, South Africa and the

14

Americas. International roaming is the term applied when an MS moves from one PLMN

to another when abroad.

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Chapter 4GSM IDENTITIES

To switch a call to a mobile subscriber, the right identifying codes must be used.

A mobile subscriber can make, receive or forward calls from any location within

the GSM Public Land Mobile Network (PLMN) with a high degree of security.

The GSM network uses more than one addressing and numbering plan to identify

different networks. The identities used in a GSM PLMN network are described

in the text that follows next.

MOBILE STATION ISDN NUMBER (MSISDN)

The MSISDN is a number that uniquely identifies a mobile telephone subscription

within the Public Switched Telephony Network (PSTN) numbering plan.

In CME 20 the MSISDN is composed of:

MSISDN = CC + NDC + SN

• CC = Country Code

• NDC = National Destination Code

• SN = Subscriber Number

INTERNATIONAL MOBILE SUBSCRIBER IDENTITY (IMSI)

The IMSI is a unique identifying code allocated to each subscriber allowing

correct identification over the radio path and through the GSM PLMN network.

It is used for all identification signalling in the PLMN and all network related

subscriber information is connected to it. The IMSI is stored in the Subscriber

Identity Module (SIM), as well as in the HLR and the VLR. It consists of three

different parts:

IMSI = MCC + MNC + MSIN

• MCC = Mobile Country Code

• MNC = Mobile Network Code

• MSIN = Mobile Subscriber Identification Number

According to the GSM specifications, IMSI can have a

maximum length of 15 digits.

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TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI)

The TMSI can be used to keep subscriber information confidential on the air interface. It

also increases the paging capacity as the length of the TMSI is shorter than the length of the

IMSI.The TMSI is relevant on the local MSC/VLR level only and is changed at certain

events or time intervals. Each local operator can define their own TMSI structure. The

TMSI should not consist of more than four octets when used within a Location Area (LA),

for example, for paging.

When a cell within a new Location Area (LA) is entered, the Location Area Identity (LAI)

must be added to the four octets toper form a location update.

MOBILE STATION ROAMING NUMBER (MSRN)

When a mobile terminating call is to be set up, the HLR of the called subscriber requests

the current MSC/VLR to allocate a MSRN to the called subscriber. This MSRN is returned

via the HLR to the GMSC. The GMSC routes the call to the MSC/VLR exchange where

the called subscriber is currently registered. The routing is done using the MSRN. When

the routing is completed, the MSRN is released. The interrogation call routing function

(request for MSRN) is part of MAP.

All data exchanged between GMSC-HLR-MSC/VLR for the purpose of interrogation is

sent over S7 signaling. The MSRN is built up like an MSISDN.

In GSM, the MSRN is composed of the following:

MSRN = CC + NDC + SN

• CC = Country Code

• NDC = National Destination Code

• SN = Subscriber Number

In some particular markets the MSRN is composed of the

following:

MSRN = CC + NPA + SN

• CC = Country Code

• NPA = Number Planning Area

• SN = Subscriber Number

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INTERNATIONAL MOBILE EQUIPMENT IDENTITY (IMEI)

The IMEI uniquely identifies a Mobile Station (MS) as a piece or assembly of equipment. If a stolen or not type-approved IMEI is used, mobiles causing severe malfunctions can be barred. TheIMEI consists of 15 digits. The IMEI consists of the following:IMEI = TAC + FAC + SNR + sp• TAC = Type Approval CodeDetermined by a central WCDMA/UMTS body andidentifies the type of equipment.• FAC = Final Assembly CodeThe FAC identifies the manufacturer of the equipment• SNR = serial no.The SNR is an individual serial number of six digits which uniquely identifies all equipment within each TAC and FAC.• sp = spare part for future use; this digit should always be zero when it is transmitted by the UEThe IMEI has a total length of 15 digits.

LOCATION AREA IDENTITY (LAI)The LAI, used for paging, indicates to the MSC in which location area the MS is operating. It is also used for location updating of mobile subscribers.The LAI contains the following :LAI = MCC + MNC + LAC• MCC = Mobile Country CodeIdentical to IMSI MCC• MNC = Mobile Network Code

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CELL GLOBAL IDENTITY (CGI)The CGI is used for cell identification within a location area.The CGI contains the same information as the LAI and also includes a Cell Identity (CI). The CI has a maximum length of16 bits.CGI consists of:CGI = MCC + MNC + LAC + CI

ADDRESSING THE SWITCHING SYSTEM ENTITIES

GLOBAL TITLE (GT)

A Global Title (GT) is an identifying code, such as dialed digits ,which does not explicitly contain information that allows routing in the signaling network. This requires the Signaling Connection Control Part (SCCP) translation function.The GT is used for addressing signaling information.Different numbering plans are used to distinguish different networks.• E.164 is the numbering plan for PSTN/ISDN• E.212 is the numbering plan for GSM PLMNEach network entity is identified by its international PSTN/ISDN number, that is, its own command defined address which has the following structure:

19

Example: E.164: CC + NDC(or NPA) + SNThe CC, NDC, and SN identify the node within the whole GSM, as well as the entity. Entities include the HLR, MSC, VLR, EIR and AUC.See the SCCP chapter for more information.During an incoming call to a mobile subscriber, the GMSC analyzes the MSISDN to locate the appropriate HLR. The digit sin the Subscriber Number (MSISDN) are used for the signal routing to the HLR.

MOBILE GLOBAL TITLE (MGT)

When an MS is powered on in a PLMN, the VLR must communicate with the MS’s HLR to perform location updating.

The only data available in the MSC/VLR for the SCCP addressing of the HLR is the IMSI number. However, for signaling in the international PSTN/ISDN network, IMSI cannot be used. Thus, it is necessary to convert the IMSI number in the MSC/VLR into a Global Title (GT), which enables routing of the S7 signaling to the proper HLR.

Structure of MGT

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CHAPTER 5GSM FREQUENCY BANDS AND PROTOCOLS

As GSM has grown worldwide, it has expanded to operate at four main frequency bands:

900, 1800,1900 and 800.

GSM 900:The original frequency band specified for GSM was 900 MHz. Most GSM

networks worldwide use this band .In some countries and extended version of GSM 900

can be used, which provides extra network capacity. This extended version of GSM is

called E- GSM, while the primary version is called P-GSM.

GSM 1800: In 1990, in order to increase competition between operators, the United

Kingdom requested the start of a new version of GSM adapted to the 1800 MHz

frequency band. Licenses have been issued in several countries and networks are in

full operation.

By granting licenses for GSM 1800 in addition to GSM 900, a country can increase the

number of operators. In this way, due to increased competition, the service to

subscribers is improved.

GSM 1900:In 1995, the Personal Communications Services (PCS) concept was specified

in the United States. The basic idea is to enable"person-to-person"communication rather

than "station-to station". PCS does not require that such services be implemented using

cellular technology, but this has proven to be the most effective method. The frequencies

available for PCS area round1900 MHz. As GSM 900 could not be used in North

America due to prior allocation of the 900 MHz frequencies, GSM 1900MHzis seen as an

opportunity to bridge this gap. The main difference between the American GSM 1900

standard and GSM 900 is that it supports ANSI signaling.

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GSM 800:The multiband support in Ericsson‟s GSM system is now enhanced to include

support for the GSM 800MHzband, thus increasing capacity for operators with a licence

for this frequency. This frequency band was traditionally usedby TDMA in USA.

The following figure shows the various GSM frequency bands:

The GSM networks use a certain protocol for signaling. This protocol can be the tried and

tested SS7 or Signaling System 7 or the upcoming SIGTRAN or the new technologies like

MGW, MSS, etc. Here we will give an introduction to SS7 and take up SIGTRAN in a little

bit of detail. This is because SS& is used widely today in GSM networks so we will talk

about it in detail later on.

SS7 : SS7 is a signaling method in which a signaling channel conveys, by means of labeled

messages, signaling information relating to call setup, control, network management, and network

maintenance. Common Channel Signaling System No. 7 (i.e., SS7 or C7) is a global standard for

telecommunications defined by ITU-T. The standard defines the procedures and protocol by which

network elements in PSTN exchange information over a digital signaling network to cellular and

wire line call setup, routing and control. The ITU definition of SS7 allows for national variants

such as ANSI and Bellcore standards used in North America and ETSI standard used in Europe.

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) wire line services,

Enhanced call features such as call forwarding, calling party name/number display,

and three-way calling,

Efficient and secure worldwide telecommunications.

SIGTRAN

SS7 over IP is the general term to describe how SS7 based application protocols can send

their message over an IP based transport network. SS7 over IP is specified by IETF

(Internet Engineering Task Force). The responsible group within the IETF is called

22

SIGTRAN (Signaling Transport). The initial problem triggering the creation of SIGTRAN

was to enable classical SCN (Switched Circuit Networks) networks to be connected to IP

networks for voice transport. To enable voice transmission over general transport systems a

new network design approach is necessary. This new concept is generally described by

separation of concern. This term means, that the network is divided into levels dealing

with a very restricted task. Three levels are typically required:

• Transport Network Plane : The transport network plane deals with the transport of bits

only. Special tasks of the transport plane are to handle access related things. The transport

network can be homogeneous (one transport technology) or heterogeneous (multiple

transport technologies).

• Network Control Plane : The network control plane has the responsibility to control the

transport network. In other words the network control plane shall handle the logical set up

of transport bearer services within the transport network plane for circuit switched calls or

packet sessions. The classical call control and session management is implemented here.

The control of the transport network should be done in a bearer independent fashion, to

enable easy exchange of the transport technologies.

• Application & Service Plane : The services offered by the network control plane

together with the transport network plane are very basic (call/session). To enable

sophisticated user services possibly with user specific components or provider specific

features additional functionality is needed. This is what the application & service plane

gives us. These functionalities shall be combined here. Examples for entities in this plane

are SCP (Service Control Point) for intelligent network IN, CSE (CAMEL Service

Environment) for mobile IN, HLR (Home Location Register) for GSM/UMTS subscriber

data.

In this architecture there is signaling between the planes and within the planes. SIGTRAN

especially deals with the signaling exchanged within the network control plane and

between network control and transport network plane.

23

24

The functional model of SIGTRAN reflects the network model described above, but considers

network control and transport network plane only. The functional model is composed out of

three basic elements:

• Signaling Gateway (SG) : The signaling gateway has the task to terminate one signaling

system and to relay the higher layer signaling message to another signaling system. In case of

SIGTRAN a SG will typically terminate either classical or broadband SS7 and map it to SS7

over IP. The signaling can be forwarded from a SG either towards other SGs or to a MGC

(Media Gateway Controller).

• Media Gateway (MG) : A MG is a pure transport network entity. It usually has three tasks.

First a MG can relay from one transport technology to another (for example from PCM to

ATM or IP, etc.). Second a MG can make a media format conversion. This can be for

instance a codec conversion (e.g. from A-law codec to AMR codec of UMTS/GSM). Third a

MG performs switching/routing functionality. MGs next to pure routers and switches form the

transport network plane.

• Media Gateway Controller (MGC) : The MG alone will of course not know which type of

transport service is needed for a certain application. To configure the media stream bearer

service in the MG is the task of the MGC. The MGC is in the network control plane and can be

considered as the logical part of a switch, MSC or SGSN. In UMTS Release 4 also the name

MSC Server and SGSN Server is applied for the MGC. So the MGC performs call control and

session management. To map the logical control into physical bearer services there is signaling

between MGC and MG, but also between MGC and MGC. If the all/session signaling is

initially coming from a SCN network, the signaling for call control/session management is also

coming or going to the SG.

SIGTRAN Protocol Architecture

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SIGTRAN is the name of the IETF Workgroup responsible for SS7 over IP. The SIGTRAN

protocol architecture is designed for maximal variability and extensibility. The basic

architecture is formed by three general layers:

• Transport Layer: The transport layer shall be a standard transport protocol that is used in

unmodified way. The requirements are very low level. To be exact, what standard IP offers is

enough. So the transport layer must provide an unreliable datagram service, no sequence

control and no reliability is requested.

• Common Signaling Transport Layer: The common signaling transport layer uses the raw

transport layer services and enhances it with reliability and sequence order control. In principle

TCP could fulfill this, but when multiple signaling links between the same

endpoints are required TCP is not sufficient any more. Instead SIGTRAN defines a new

reliable transport protocols named SCTP (Stream Control Transmission Protocol). SCTP

serves signaling protocols with transmission capabilities in a very general way.

• SCN Adaptation Module: The users of SS7 over IP are the classical SS7 MTP user

protocols like ISUP, SCCP, BICC, etc. Of course nobody wants to make a complete redesign

of these protocols because of the modified transport technology. Therefore user adaptation

layers are placed on top of SCTP. These adaptation modules will offer to the higher layer an

emulated interface of a basic transmission protocol. In the figure shown are two of them.

M3UA (MTP level 3 User Adaptation) emulates MTP level 3 for MTP users. In contrast to that

SUA( SCCP User Adaptation) offers the interface of SCCP to SCCP subsystems. In this way

one could implement emulations for arbitrary transport protocols.

On top of the SIGTRAN protocol stack the user protocols shall have the feeling to be in a

classical SS7 environment.

SS7 Protocol

Signaling System #7 (SS7) has become one of the most important assets within any carrier‟s

network. Already deemed important for interconnecting calls from one network to the next,

SS7 has also become a network rich in user data. SS7 is really a control protocol, used to

provide instructions to the various elements within a telephony network. These instructions

may be how to route a call through the network, what features a caller has subscribed to, or, in

the case of number portability, which carrier will be used to handle the call. In order to provide

26

this level of instruction, a great deal of information must be sent from one element to another.

Everything from the caller‟s telephone number to his or her calling card number, as well as

other pertinent data, is sent through the network to the various network elements involved in

connecting the caller to his or her destination. If there were a means of trapping all this

information and storing it for analysis (which, of course, there is), carriers would find a rich

resource for identifying the users of their network.

The data can be used for determining new marketing campaigns, the success of new feature

offerings, and much more. In fact, I often refer to this data as the “three W‟s:” who is using the

network, when they are using the network, and why they are using the network. This data is

crucial to the success of any business to ensure they are meeting the needs of their customers.

Many carriers are just now realizing the benefits of mining the data traversing the SS7 network

and are utilizing this data to maintain revenue assurance in all aspects of the business. SS7 has

even become an important revenue source for carriers who have learned to tap its links and

interface to back- office billing systems.

What was once an obscure, little-known technology has become one of the industry‟s most

prized possessions. However, SS7 will not live forever. Particular aspects of SS7 will continue

to thrive throughout the signaling networks, but already the lower layers of the SS7 protocols

are being replaced by protocols based on the Transmission Control Protocol/Internet Protocol

(TCP/IP). Although the applications (circuit connection and database access) will not change,

the transport mechanism is already changing. Still, SS7 is a long way from obsolescence and is

extremely important to understand if one plans on making a career out of telecommunications.

To fully understand how and why SS7 is used, one first must understand the basics of

telephony as well as basic signaling.

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) wire line services

Enhanced call features such as call forwarding, calling party name/number display, and

three-way calling

Efficient and secure worldwide telecommunications

27

Signaling Points

Each signaling point in the SS7 network is uniquely identified by a numeric point code. Point

codes are carried in signaling messages exchanged between signaling points to identify the

source and destination of each message. Each signaling point uses a routing table to select the

appropriate signaling path for each message.

There are three kinds of signaling points in the SS7 network (Fig. 1):

SSP (Service Switching Point)

STP (Signal Transfer Point)

SCP (Service Control Point)

SS7 Signaling Points

SSPs are switches that originate, terminate, or tandem calls. An SSP sends signaling messages

to other SSPs to setup, manage, and release voice circuits required to complete a call. An SSP

may also send a query message to a centralized database (an SCP) to determine how to route a

call (e.g., a toll-free 1-800/888 call in North America). An SCP sends a response to the

originating SSP containing the routing number(s) associated with the dialed number. An

alternate routing number may be used by the SSP if the primary number is busy or the call is

unanswered within a specified time. Actual call features vary from network to network and

from service to service.

Network traffic between signaling points may be routed via a packet switch called an STP. An

STP routes each incoming message to an outgoing signaling link based on routing information

contained in the SS7 message. Because it acts as a network hub, an STP provides improved

utilization of the SS7 network by eliminating the need for direct links between signaling points.

An STP may perform global title translation, a procedure by which the destination signaling

28

point is determined from digits present in the signaling message (e.g., the dialed 800 number,

calling card number, or mobile subscriber identification number). An STP can also act as a

"firewall" to screen SS7 messages exchanged with other networks.

Because the SS7 network is critical to call processing, SCPs and STPs are usually deployed in

mated pair configurations in separate physical locations to ensure network-wide service in the

event of an isolated failure. Links between signaling points are also provisioned in pairs.

Traffic is shared across all links in the linkset. If one of the links fails, the signaling traffic is

rerouted over another link in the linkset. The SS7 protocol provides both error correction and

retransmission capabilities to allow continued service in the event of signaling point or link

failures.

SS7 Signaling Link Types

A Link: An "A" (access) link connects a signaling end point (e.g., an SCP or SSP) to

an STP. Only messages originating from or destined to the signaling end point are

transmitted on an "A" link.

B Link: A "B" (bridge) link connects an STP to another STP. Typically, a quad of "B"

links interconnect peer (or primary) STPs (e.g., the STPs from one network to the STPs

of another network). The distinction between a "B" link and a "D" link is rather

arbitrary. For this reason, such links may be referred to as "B/D" links.

29

C Link: A "C" (cross) link connects STPs performing identical functions into a mated

pair. A "C" link is used only when an STP has no other route available to a destination

signaling point due to link failure(s). Note that SCPs may also be deployed in pairs to

improve reliability; unlike STPs, however, mated SCPs are not interconnected by

signaling links.

D Link: A "D" (diagonal) link connects a secondary (e.g., local or regional) STP pair to

a primary (e.g., inter-network gateway) STP pair in a quad-link configuration.

Secondary STPs within the same network are connected via a quad of "D" links. The

distinction between a "B" link and a "D" link is rather arbitrary. For this reason, such

links may be referred to as "B/D" links.

E Link: An "E" (extended) link connects an SSP to an alternate STP. "E" links provide

an alternate signaling path if an SSP's "home" STP cannot be reached via an "A" link.

"E" links are not usually provisioned unless the benefit of a marginally higher degree of

reliability justifies the added expense.

F Link: An "F" (fully associated) link connects two signaling end points (i.e., SSPs and

SCPs). "F" links are not usually used in networks with STPs. In networks without

STPs, "F" links directly connect signaling points.

ACCESS SIGNALING

There are many types of access signaling, for example, PSTN analog subscriber line signaling,

ISDN Digital Subscriber Signaling System (DSS1), and signaling between the MS and the

network in the GSM system.

Signaling on the analog subscriber line between a telephony subscriber and the Local

Exchange (LE) is performed by means of on/off hook signals, dialed digits, information tones

(dial tone, busy tone, etc.), recorded announcements, and ringing signals.

The dialed digits can be sent in two different ways: as decadic pulses (used for old-type rotary-

dial telephones), or as a combination of two tones (used for modern pushbutton

telephones). The latter system is known as the Dual Tone Multi Frequency (DTMF).

The information tones (dial tone, ringing tone, busy tone, etc.) are audio signals used to keep

the calling party (the A-subscriber) informed about what is going on in the network during the

setup of a call.

Digital Subscriber Signaling System No. 1 (DSS1) is the standard access signaling system used

in ISDN. It is also called a D-channel signalling.

30

TRUNK SIGNALING The Inter-exchange Signaling information is usually transported on

of the time slots in a PCM link, either in association with the speech channel or independently.

There are two commonly used methods for Inter Exchange Signaling:

CHANNEL ASSOCIATED SIGNALING (CAS)

Channel Associated Signaling (CAS) means that the signaling is always sent on the same

connection (PCM link) as the traffic.

•Line Signals

Line Signals are used during the whole duration of a call to monitor the status of the

connection and traffic circuit.

Example: Seizure, Answer signals.

•Register Signals

The Register Signals are used during the set-up phase of a

call to transfer address and category information. Example:

dialed B-number.

COMMON CHANNEL SIGNALING (CCS)

In CCS, signaling messages (or data packets) are transmitted over time slots in a PCM link

reserved for the purpose of signaling, instead of Line Signals and Register Signals (which do

not exist in CCS).

OPEN SYSTEM INTERCONNECT(OSI)

The hardware and software functions of the SS7 protocol are divided into functional

abstractions called "levels". These levels map loosely to the Open Systems Interconnect

(OSI) 7-layer model.

For each layer in the Reference Model the standardization is split

into two main parts:

•Service definition defines the functions that each layer should contain and with which

services the layer should provide the user or the layer closest above.

•Protocol definition specifies how the functions within a layer in one system co-operate with

the corresponding functions in another system.

31

The advantages of this well-structured model are that a protocol within one layer can be

exchanged without affecting the other layers and the implementation of the functions within a

layer is free.

COMMUNICATION PROCESS

Each layer has its own specified functions and provides specific services for the layers above.

It is important to define the interfaces between different layers and the functions within each

layer. The way a function is realized within a layer is not predicted. Logically, the

communication between functions always takes place on the same level according to the

protocols for that level.

Only functions on the same level can “talk to each other”. In the transmitting system, the

protocol for each layer adds information to the data received from the layer above. The

addition usually consists of a header and/or a trailer.

In the receiving system, the additions are used, for example, to identify bits or data fields

carrying information for that specific layer only. These fields are decoded by layer

functionality and are removed when delivering the message to the applications or layers

above. When the data reaches the application layer on the receiving side, it consists of only the

data that originated in the application layer of the sending system.

Logically, each layer communicates with the corresponding layer in the other system. This

communication is called Peer-to-Peer communication and is controlled by the layer’s protocol.

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DESCRIPTION OF LAYERS

APPLICATION LAYER

This layer provides services for support of the user’s applicationprocess and for control of all

communication between applications.

Examples of layer 7 functions are file transfer, message handling,directory services, and

operation and maintenance.

PRESENTATION LAYER

This layer defines how data is to be represented, that is, the syntax.

The presentation layer transforms the syntax used in the applicationinto the common syntax

needed for the communication between applications. Layer 6 contains data compression.

SESSION LAYER

This layer establishes connections between presentation layers in different systems. It also

controls the connection, the synchronization and the disconnection of the dialog. It allows the

presentation layer to determine checkpoints, from which the retransmission will start when the

data transmission has been interrupted.

TRANSPORT LAYER

This layer guarantees that the bearer service has the quality required by the application in

question. Examples of functions are error detection and correction (end-to-end), and flow

control. The transport layer optimizes the data communication, for example by

multiplexing or splitting data streams before they reach the network.

NETWORK LAYER

The basic network layer service is to provide a transparent channel. This means that the

application requesting a channel ignores network problems and the related signal exchange

because that is the task of the lower levels. It just requires an open channel, transparent for the

transmission of data, between transport layers in different systems. The Network Layer

establishes, maintains, and releases connections between the nodes in the network and handles

addressing and routing of circuits.

DATA LINK LAYER

This layer provides an essentially error-free point-to-point circuit between network layers. The

layer contains resources for error detection, error correction, flow control, and retransmission.

PHYSICAL LAYER

This layer provides mechanical, electrical, functional, and procedural resources for activating,

maintaining, and blocking physical circuits for the transmission of bits between data link

33

layers. The physical layer contains functions for converting data into signals compatible with

the transmission medium. For the communication between only two exchanges, layers 1 and

2 are sufficient. For the communication between all exchanges in the network, layer 3 must be

added because it provides addressing.

Characteristics of SS7

A major characteristic of the signaling system is that its structure ensures flexibility and

modularity for different applications.

Other characteristics of SS No.7 include these features:

High flexibility

Many different types of telecommunication services can use SS7. It is used to set up

and release connections in traditional telephony and data communication, in mobile

telephony and

data communication, for the provision of ISDN services, and many other applications.

SS7 is also used for the interchange of data between databases (for example,

VLR↔HLR in GSM

network).

High capacity

A single signaling link can support several thousand traffic circuits.

High speed

Setting up a call through a number of exchanges takes less than a second.

High reliability

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The system contains powerful functions for elimination of disturbances in the signaling

network. One example is the possibility of choosing alternative links for signaling.

Economical

The fact that a wide range of telecommunication services and connections can use one and the

same signaling system is an important economical aspect. Besides, SS7 is much simpler and

requires much less hardware than older signaling systems. SS7 can handle both so-called

circuit related signaling and noncircuit related signaling. The term circuit in this context

means traffic channel. Thus, circuit related signaling means signals sent to support a call

establishment or release procedure.

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Chapter - 6MESSAGE TRANSFER PART (MTP)

The Message Transfer Part (MTP) is divided into three levels.

MTP Level 1, is equivalent to the OSI Physical Layer. MTP Level 1 defines the physical,

electrical, and functional characteristics of the digital signaling link. Physical interfaces defined

include E-1 (2048 kb/s; 32 64 kb/s channels), DS-1 (1544 kb/s; 2464kb/s channels), V.35 (64

kb/s), DS-0 (64 kb/s), and DS-0A (56 kb/s).

MTP Level 2 ensures accurate end-to-end transmission of a message across a signaling link.

Level 2 implements flow control, message sequence validation, and error checking. When an

error occurs on a signaling link, the message (or set of messages) is retransmitted. MTP Level

2 is equivalent to the OSI Data Link Layer.

MTP Level 3 provides message routing between signaling points in the SS7 network. MTP

Level 3 re-routes traffic away from failed links and signaling points and controls traffic when

congestion occurs. MTP Level 3 is equivalent to the OSI Network Layer.

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ISDN User Part (ISUP)

The ISDN User Part (ISUP) defines the protocol used to set-up, manage, and release trunk

circuits that carry voice and data between terminating line exchanges (e.g., between a calling

party and a called party). ISUP is used for both ISDN and non- ISDN calls. However, calls that

originate and terminate at the same switch do not use ISUP signaling.

General overview of ISUP

Telephone User Part (TUP)

In some parts of the world (e.g., China, Brazil), the Telephone User Part (TUP) is used to

support basic call setup and tear-down. TUP handles analog circuits only. In many countries,

ISUP has replaced TUP for call management.

Signaling Connection Control Part (SCCP)

SCCP provides connectionless and connection-oriented network services and global title

translation (GTT) capabilities above MTP Level 3. A global title is an address which is

translated by SCCP into a destination point code and subsystem number. SCCP is used as the

transport layer for TCAP-based services.

37

Transaction Capabilities Applications Part (TCAP)

TCAP supports the exchange of non-circuit related data between applications across the SS7

network using the SCCP connectionless service. Queries and responses sent between SSPs and

SCPs are carried in TCAP messages. For example, an SSP sends a TCAP query to determine

the routing number associated with a dialed 800/888 number and to to check the personal

identification number (PIN) of a calling card user. In mobile networks (IS-41 and GSM),

TCAP carries Mobile Application Part (MAP) messages sent between mobile switches and

databases to support user authentication, equipment identification, and roaming.

TCAP in GSM

Operations, Maintenance and Administration Part (OMAP) and ASE

OMAP and ASE are areas for future definition. Presently, OMAP services may be used to

verify network routing databases and to diagnose link problems.

Other Applications of SS7 in GSM Networks

In GSM networks, signaling is not as simple as in the PSTN. There are extra signaling

requirements in GSM due to the different architecture of the network which requires a large

amount of non-call-related signaling. In the first instance the subscriber is mobile, unlike the

PSTN telephone which is always in one place. Therefore, a continuous tracking of the mobile

station is required which results in what is known as the Location Update procedure. This

procedure is an example of non-call-related signaling, where the mobile phone and the network

38

are communicating but no call is taking place. This requires additional sets of standard

messages to fulfill the signaling requirements of GSM networks.

These additional protocol layers are

Base Station Subsystem Application Part (BSSAP): The first of these additional protocol

layers, which are specific to GSM networks, is the Base Station Subsystem Application Part

(BSSAP). This layer is used when an MSC communicates with the BSC and the mobile station.

Since the mobile station and MSC have to communicate via the BSC, there must be a virtual

connection, therefore the service of SCCP is also needed. The authentication verification

procedure and assigning a new TMSI all take place with the standard sets of messages of

BSSAP.

Communication between MSC and BSC also uses the BSSAP protocol layer. Therefore,

BSSAP serves two purposes:

MSC-BSC signaling

MSC-MS signaling.

Mobile Application Part (MAP): The example of a location update procedure mentioned

previously is not confined only to the MSC-BSC section, it spans multiple PLMNs. In case of a

first time location update by an international roaming subscriber (where he is not in his home

network), the VLR has to get the data from the subscriber‟s HLR via the gateway MSC of the

subscriber‟s home network. While a mobile terminated call is being handled, the MSRN has

to be requested from the HLR without routing the call to HLR. Therefore, for these cases another

protocol layer was added to the SS7 called Mobile Application Part (MAP). MAP is used for

signaling communication between NSS elements.

Structure of MAP

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Chapter 7

SIGNALING LINK

When you lift the receiver alight at the exchange starts to glow which is an

Indication that you need service.

The lady asks you who you want to talk with and you tell her the person‟s name.

indication that service will be provided with a further request for address, which is

provided.

She makes the connection to the called party and the phone starts ringing. Connection

of the speech path is completed and the called party is alerted.

The called party answers, and you are connected. Answer and conversation.

The lady at the exchange monitors your call during the conversation. Supervision.

When you hangup, a light glows to indicate this and she pulls out the plugs. An

indication of disconnection and clearing of thecall.

The signaling data link and the signaling link functions together constitute the whole Signaling

Link (SL) that is used for reliable transmission of signaling messages between two signaling

points. In AXE 10, signaling link functions are implemented in the signaling terminal (ST).

The main requirements of the SL functions are error-free transfer of signaling messages,

correct sequence, and no loss or duplication. These requirements imply different link functions

on level 2.

Different types of link control information must be interchanged on level 2 between two

adjacent SPs. Level 2 adds this control information for outgoing signal units and interprets it

40

for incoming signal units. The information is transferred as predefined parameter fields in each

signal unit. Furthermore, different types of link control information must be interchanged

on level 2 between two signaling terminals.

In addition, level 2 handles the link management functions required to provide continuous

monitoring and supervision under both normal and abnormal transfer conditions, for

example when disturbances or faults affect the performance of a

signaling link or a link set.

A signaling message, delivered by the higher levels, is transferred over the signaling link by

means of a variable length signal unit.

SIGNAL UNITS

Each frame must contain all information to reach the destination point and to invoke the

desired reaction, which is theactual reason for sending the signaling message. A signal unit

consists of several fields. Each field contains a certain number of bits representing specific

information. The user signalling message is carried in a data field called the Signaling

Information Field (SIF) within the signal unit frame.All information that must be transferred is

transmitted using oneof the signaling message formats engaging the abilities of MTP.

Signal Unit Types

There are three types of signal units:

41

FISU Fill-In Signal Unit (FISU) is used for error supervision of the link and to keep

the link running when there are no MSUs to be sent.

LSSU The LSSU is sent in response to each status change on the SL informing the

remote side about this new status, for example, for starting up a signaling link. A Status

Field (SF) contains one

or two octets and is generated by the signalling terminal.

MSU The Message Signal Unit (MSU) is used to carry the signal information (or data

units) between user parts. The MSU is retransmitted when an error is detected. In

addition, there are

MSUs that are used for signalling network management and signalling network testing and

maintenance. However, these are not sent to a user part, but stay in the MTP level 3.

Fields in Signal Units

Flag - Each signal unit is enclosed between two flags, an opening flag

and a closing flag. The closing flag of the signaling unit is

normally the opening flag of the next signal unit. Both are coded

as “01111110”.To ensure that the bit pattern cannot be imitated elsewhere in the

signal unit, bit stuffing is used. Bit stuffing means adding an extra zero after five consecutive

ones in the message.

Check Bits- The error detection function is performed by means of 16 check

42

bits (CK), provided at the end of each signal unit. The checksum (check bits) is generated by

the transmitting signalling terminal by means of a specified algorithm. On the

receiving signalling terminal the same algorithm is used to calculate the checksum. This

checksum is then compared with the received checksum.

Error correction fields - The error correction field contains 16 bits. It consists of the

following:

Backward Sequence Number (BSN) 7 bits reserved for sequence numbers from 0 to 127.

These are used to acknowledge the correct transmission of a signal unit.

Backward Indicator Bit (BIB)The BIB (1 bit) marks the signal unit as:

Positive acknowledged if the logical value of the BIB bit is the same as that received in the

latest signal unit. Negative acknowledged if the value of BIB is not equal to the value in the

latest received signal unit.

Forward Sequence Number (FSN) FSN is the 7-bit field for sequence numbers from 0 to

127.

The FSN is used to recognize the signal units, which have been received “out of sequence”.

Forward Indicator Bit (FIB) If the logical value of the FIB is equal to the one in the

previous signal unit, the receiver is informed that the signal unit is sent for the first time. If the

logical value of FIB is not equal to the one in the previous signal unit, the receiver

is informed that it is a repetition of a previously sent signal unit.

Length Indicator: The Length Indicator (LI) indicates the number of octets in the

fields between the LI field and the CK field. This makes it possible to differentiate between the

three types of signal units. The following shows the LI of the different signal units.

LI>2 MSU

LI=1 or LI=2 LSSU

LI=0 FISU

For Message Signal Units, in which Signalling Information Field

has 62 or more octets, LI is set to 63.

CHAPTER - 8

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IN PROTOCOLS

The Intelligent Network (IN) is a set of resources that facilitates the speedy introduction of

customized, competitive, advanced telecom services by operators without significant additional

investment in the existing telephony infrastructure. IN resources include hardware and

software. IN enables telecom operators to implement new services in reaction to and

anticipation of competing services from other providers. IN separates the physical network

from the systems used to delivevalue-added services. Note that IN services were first designed

for, and introduced in fixed telephony networks.

IN SERVICES

A few examples of IN services are very briefly described below:

Account Card Calling (ACC)

This service allows the user to use public telephones for making his/her calls, and instead of

using coins or phone cards, the call will automatically be charged to the account card.

Freephone (FPH)

Freephone was the first type of IN service offered to users. It is used by companies that want to

advertise their services by offering their customers the convenience of calling free of charge.

In other words, freephone means that call charges are reversed and paid by the receiving

company.

Premium Rate (PRM)

Premium Rate means that calls are charged at a rate that is higher than that of regular calls.

Premium Rate is normally used in connection with information and entertainment services

offered by service providers.

Universal Access Number (UAN)

A company with operating units in several places can use a Universal Access Number

throughout its organization. Customers who dial the Universal Access Number are

automatically routed to the nearest open office or to an office with free lines.

Televoting (VOT)

The televoting service can be used during a radio or television program where listeners or

viewers are asked to call in and register their opinion on an issue, answering for

example “Yes” or “No”.

Virtual Private Network (VPN) and Mobile VPN

44

With the Virtual Private Network product a company's fixed and mobile phones are integrated

in one network with one numbering plan.

Personal Number (PN)

Call handling is simplified by the possibility of having only one telephone number for all

incoming calls. The calls will be sent in accordance to the personal choice of destination,

which could be one or many phones, including mobile, fixed, or pager. The service supports

advanced functionality, such as routing profiles including both sequential and parallel alert

options.

Information and Business (I&B)

This is a multi-purpose service with a flexible and modular design. The flexible structure

allows the service provider to choose which features to include in a subscription in order to

satisfy the service subscriber best. The Information & Business service can be customized to

operate as a Freephone, Premium Rate, or Universal Access Number service.

Prepaid Service (PPS)

“Prepaid” implies pre-payment of subscriptions that is, payment for a set amount of services in

advance. Users can refill their “account” using scratch cards.

Prepaid Service Light (PPL)

This is similar to PPS, but functionalities are limited and the service platform is less complex.

The discussion of the service details and implementation is beyond the scope of this book. This

chapter will focus on signaling used in the mobile IN environment.

In general, two groups of IN services can be distinguished:

Terminating

The IN functionalities are triggered for the terminating part of the call. In GSM this will be

detected in the HLR when there is an interrogation from the GMSC. Personal Number is an

example of a service belonging to this group.

45

Originating - The IN functionalities are triggered when the subscriber originates a call, as

special handling may be required for all or some of the calls. In GSM, this is detected by the

MSC/VLR serving the subscriber. Virtual Private Network is an example of a service from this

group.

IN NETWORK STRUCTURE

Generally, the network architecture constitutes three levels:

Switching layer

This layer consists of the network elements responsible for routing and switching calls,

including the capability to route IN calls.

Intelligent layer

This layer consists of nodes with subscriber and service data responsible for handling the IN

calls.

Management layer

The service management environment function is, in Ericsson implementations, implemented

on a UNIX platform with Service Management Application System (SMAS) software. There

are SMAS interfaces to all mobile IN nodes performing either the Service Control Function

(SCF) or the Service Data Function (SDF).

The IN-specific nodes are briefly described below:

SSF or gsm SSF (Service Switching Function)

This function may “trigger” IN call control in the SCF by an initial operation sent to the SCF.

The SSF executes operations requested by the SCF. The SSF also contains

capabilities to communicate with other entities containing an SCF function and to respond to

instructions from the SCF. The operations mainly operate on the basic call process. The

46

SSF also provides users with access to the network and performs any necessary switching

functionalities.

SCF or gsm SCF (Service Control Function)

The SCF keeps and executes the IN service logic and data.The execution results in operations

that should be performed by other functional entities.

SRF or gsm SRF (Service Resource Function)

The SRF provides the specialized resources required for the execution of IN provided services,

for example, Dual Tone Multi-Frequency (DTMF) receivers, announcements. The SRF is

either part of the SSF or is contained in a stand-alone node called the Intelligent Peripheral

(IP).

SDF (Service Data Function)

This function executes operations requested by the SCF. The operations mainly operate on a

database, which holds data that can be used by Service Logic Programs in the SCF to

provide individualized services. This functional entity maybe a commercial database

containing credit card numbers, etc. If the functional entity is a stand-alone node, it is called a

point, for example, the Service Switching Point (SSP) is a stand-alone node performing

Service Switching Functions (SSF), the Service Control Point (SCP) is a stand-alone node

performing Service Control Functions (SCF). If both SSP and SCP are integrated in a stand-

alone node, it is called Service Switching and Control Point (SSCP).

Connect the Call

The SSF serves as a “slave” to the SCF, performing the “orders” received from the SCF,

reporting events, and sending notifications to the SCF.

Chapter 9

GENERAL CALL FLOW

47

MOBILE ORGINATING AND MOBILE TERMINATING

A simple call in a GSM network can be very confusing and complex to understand. But to

understand the processes that are necessary to establish a complete connection in a GSM

network we take an example to describes a call from a fixed network subscriber to a mobile

subscriber in a GSM network:

1. The incoming call is passed from the fixed network to the gateway

MSC (GMSC).

2. Then, based on the IMSI numbers of the called party, its HLR is determined.

3.The HLR checks for the existence of the called number. Then the relevant VLR is requested

to provide a mobile station roaming number (MSRN).

4. This is transmitted back to the GMSC.

5. Then the connection is switched through to the responsible MSC.

6.Now the VLR is queried for the location range and reachability status of the mobile subscriber.

7. If the MS is marked reachable, a radio call is enabled

8. And executed in all radio zones assigned to the VLR.

9.When the mobile subscriber telephone responds to the page request from the current radio cell,

10. All necessary security procedures are executed.

11. If this is successful, the VLR indicates to the MSC

12. that the call can be completed.

This is a simple example but an actual call is a complex one to understand. It consist of a

MOC(Mobile Originating Call) and MTC(Mobile Terminating Call).

MOBILE-TERMINATED SMS

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The mobile-terminated SMS has the capability to transfer a short message from the SC to a MS/UE. In addition, it provides information about the delivery of the message through a delivery report, which confirms the delivery of the short message, or through a failure report, which informs the originator that the short message has not been delivered and the reason why. If the short message is not delivered, a specific procedure for later delivery is used. The mobile-terminated short message can be input to the SC from a variety of sources, for example, speech, internet, facsimile, or other MS/UEs.MSC/VLR

1. A terminating short message to a mobile subscriber is always routed from the SC to the SMS-GMSC. This is carried out using the Forward Mobile Terminating Short Message.2. The SMS-GMSC requests routing information from the HLR through the signal Send Routing Information For Short Message. The MSISDN, received from the SC, is used toaddress the HLR. The message contains the called subscriber’s MSISDN, the priority, and the SC address3. The HLR checks subscriber data related to the received MSISDN number. In the case of an error, the error code is sent back to the SMS-GMSC. The error checks are performed in thefollowing order:– Unknown subscriber (if the MSISDN is not allocated)– Teleservices not provisioned (if SMS transfer is not provisioned for the mobile subscriber)– Call barred (if status of the service ‘barring of incoming calls” is active)– Absent subscriber (if location is unknown for the mobile subscriber)

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If none of the above errors occurs, the following routing\ information is sent back:MSC-identity (a signaling address, not a roaming number)

IMSI MWD-set (optional)

MWD-set is a parameter that indicates whether or not the SC address has previously been stored in the Message Waiting Data (MWD) list. The SC is included in the MWD list in thefollowing cases:

Location is unknown or restricted. Location information is available, but the MWD list already contains

another SC address, and the priority of this message is low.Up to eight SC addresses may be stored per MS/UE.4. The MS/UE address, received from the HLR, is used for addressing the signal Forward Short Message to the MSC where the MS/UE is currently located. In the MSC/VLR, the identity of the mobile subscriber is derived from the IMSI received in this message.

The mobile subscriber is paged using the Temporary Mobile Subscriber Identity (TMSI) or the IMSI, if the TMSI is unavailable. After the mobile subscriber has responded to the paging, the procedure for security-related functions, such as authentication, cipher mode setting, and IMEI check, is invoked, if carried out in the network. If the SC has more than one SMS for the same subscriber, it is noted in the MAP message. Thus the MAP dialogue is kept open saving the paging and authentication procedure in the MSC for the following SMS. Signaling towards the HLR is not needed for the subsequent SMS. The short message is transferred to the MS/UE on a signalling channel.

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5. If a short message is successfully delivered, a result message is sent to the SMS-GMSC. In the case of failure, an error message is returned .6. The delivery report is sent to the SC

MOBILE-ORIGINATED SMS

The mobile-originated SMS provides the means to transfer a short message from a mobile to an SC. This can be carried out either when the mobile is idle, or when a connection (such asspeech or fax) already exists. For both successful and unsuccessful deliveries, the mobile receives a delivery report.1. The MS/UE sends a Connection Management request to the MSC/VLR to set up a signaling connection.2. In the case where the MS/UE is idle, the MSC assigns a signaling channel and may start authentication and ciphering. Otherwise, parallel transaction is performed.An equipment identity check may be performed.For short message sending, two protocols are used:

Connection Management Protocol for the air interface Relay Protocol for the relaying of short messages

The principle structure of Relay Protocol messages is: Destination address Originating address User information or reason code

3. Once the connection is set up, the SMS data is sent from the MS/UE to the MSC. The MSC sends back to the MS/UE an “Acknowledge” message when it has received the completeSMS data.4. The short message is transferred from the MSC/VLR to the SMS-IWMSC.

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5. From there, the message Forward Mobile Originated Short Message is forwarded to the SC.6. A delivery report is sent back to the SMS-IWMSC.7. This delivery report is forwarded back to the current MSC/VLR.8. The MS/UE is informed and releases the connection by sending a “CP-Ack” message to the MSC/VLR. If the MS/UE has several SMS to send, instead of sending the final “CP-Ack”message, the MS/UE sends a new “CM-Service-Request” message. When it is received by the MSC, the Iu interface is ordered to maintain open and the authentication procedure is avoided for the subsequent SMS transactions. However the MAP dialogue is released for every MO-SMS in the multiple transactions.

Chapter 10

MONOLITHIC AND SPLIT ARCHITECTURE

Many of the nodes within the pre R10 release GSM/GPRS/EDGE architecture perform the

roles of connection control and bearer control. For example, the Mobile Switching Center

(MSC) performs call control functions such as B-number analysis and IMSI analysis to

determine user plane and control plane routing, as well as bearer control, comprising tasks

such as physically switching user plane connections and managing bearer (route) resources. A

similar example may be made of the SGSN for GPRS. This is fundamentally incompatible

with the horizontally integrated network model specified for 3G networks. As a consequence,

there is a need to split nodes into their respective functions. For Circuit Mode services,

Ericsson has chosen to “split” the MSC as a two-step process. Firstly, the MSC internal

software structure will be modified such that call-control and bearer-control are logically

separated within the same node. This will happen in Ericsson implementation CN1.5 (see

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Error! Reference source not found.). The second step allows physical separation into server

and MGW as illustrated by This will happen in Ericsson implementation CN2.0 onwards. For

packet mode services, Ericsson had planned to split the SGSN in release CN4.0, however,

3GPP (the UMTS standardization body) have removed the SGSN architecture split as a

requirement, such that it is unlikely to happen.

Monolithic v/s split architecture

MONOLITHIC ARCHITECTURE

One concrete way of offering a smooth migration to layered architecture is the support of

server functionality in MSC/VLR and MSC/MGW. The solution makes it possible to use a

monolithic node, MSC/VLR or MSC/MGw, to additionally act as a MSC server.The benefit

with this solution is increased flexibility in how the network is migrated to the Layered

Architecture at the same time As the existing monolithic infrastructure is re-used. The need for

HW investments is thus reduced. Existing over capacity or capacity freed up in the monolithic

node processors when traffic is moved to the layered architecture, could be re-used for the

server function in the same node. To make this migration solution possible, a migration feature

in the MSC has been developed. It decides for each new call if the MSC should act as a

monolithic node or a server node. Decision criteria for a mobile originated call could be which

RNC or BSC the call is originating from or which type of access that is used, WCDMA or

GSM. It is also possible to use a statistical factor that could direct a certain percentage of the

traffic to the M-MGW, allowing for a stepwise migration to the layered architecture.

A WCDMA network is added to an existing GSM network.The RNCs and BSCs are connected

to both the monolithic MS Cand the M-MGW. The M-MGW is controlled by the existing

monolithic MSC that is acting as a MSC Server as well. By having both the WCDMA and the

GSM networks connected to both the monolithic MSC as well as the M-MGW, handover

between the systems can be achieved. This feature can be used in a number of other

configurations, including pure GSM networks.

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Server feature in monolithic MSC

SPLIT ARCHITECTURE

The split architecture is where the MSC server and the M-MGW’s are separated.

Chapter 11

ERICSSON HARDWARE

AXE PLATFORM

The access network connects users, for instance telephone subscribers, to a unit, switch. When two subscribers connected to for example different exchanges, speak to each other the speech has to be transmitted between them. This transmission is done in digital form and that means that the analog signal from a subscriber has to be converted to a digital one. The conversionFrom analog to digital or the opposite can be done in different ways using different methods.

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Switching means that incoming digital information is forwarded to a specific outgoing connection. Different subsystems in AXE can perform switching. The Group switch Subsystem, GSS, performs for example switching between access network and trunk network. The main switching function in a local exchange is to interconnect timeslots to and from the subscriber access network and the trunk (transport) network. AXE 810 performs switching according to T-S method, compared to previous AXE versions that use T-S-T method. The structure of the new GS890 in AXE 810 and the new clocksCL890 are described and the new sub racks required by AXE810 are mentioned. When different exchanges will cooperate it is necessary to synchronize them.The procedure to update GS from previous AXE versions toGS890 is presented.

Access to AXE :The connections to an AXE local exchange can either be directfrom the individual subscriber or through some intermediate equipment. The direct connections to the AXE local exchange from an ordinary PSTN subscriber is normally on an analogue subscriber line.

Another connection from a subscriber could be on a 2B+D digital subscriber connection for ISDN Basic Access. This access gives the subscriber two 64 kbps channels (Bchannels) and one 16 kbps channel (D-channel). They can beused simultaneously from the subscribers network access point. The two B-channels can be used for telephony, fax, datacommunication etc. The D-channel is used for signaling or forpacket switched services.

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Switching functions in AXE

The switching functions in AXE are performed by the following subsystems :· Group Switching Subsystem (GSS)· Subscriber Switching Subsystem (SSS)· Extended Switching Subsystem (ESS)

GSS functions are used for selecting, connecting anddisconnecting paths through the group switch.SSS functions are used in the access network to connectsubscribers to the local exchangeESS functions are used for announcement of recorded messagesand for simultaneous connection of more than two subscribers.

Digital switching

The two principles of digital switching are:· Time switching· Space switchingTime switching is based on time division multiplexing (TDM)systems for example pulse code modulation (PCM).A PCM link can be shared in time by a number of speechchannels. Each channel's share of this time is known as a timeslot, and each time slot carries a speech sample.

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Basic time switch operation

A simple time switch is made up of:· A speech store for temporary storage of the speech samples· A control store which controls the reading out from thespeech store

Space switching is used to switch timeslots from an incoming PCM system to an outgoing PCM system.The space switch is composed of a matrix of cross points(electronic gates). To connect a timeslot in an incoming PCM system to a timeslot in an outgoing PCM system an appropriate across point of the space switch is operated for a defined period(an internal timeslot).

GROUP SWITCH (APT) :In all AXE systems that connect two or more subscribers to oneanother, the Group Switch is the dominant feature, and is generally seen as the core around which the system is built. The Group Switching Subsystem, GSS, has the following basic functions:· Selection, connection and disconnection of speech or signal paths passing through the Group Switch.· Supervision of hardware in the subsystem by continuous, periodic and traffic-dependent supervision, for example through-connection tests.· Supervision of DL, Digital Link, interfaces connected to the switch.AXE Survey· To maintain a stable and accurate clock frequency for network synchronization purposes.

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Different versions of Group Switch are available dependent on the hardware in use. The older version of Group Switch is based on BYB202 hardware, still available in many places around the world. In BYB501 structure, the hardware was minimized and it was possible to put more functions in one board. As it was named in he previous chapter the APT is used to describe hardware and software related to telephony.The first version of BYB501 hardware was used for APT 1.3 and1.4The latest version of BYB501 is used with APT1.5 in AXE 810.

APT/APZ

At, System Level 2, the AXE system is divided into two parts:· APT, which is the switching part. For example, APT provides the switching functions needed to implement a PSTN local exchange or node.· APZ, which is the control part. APZ is the computer system that runs the software programs controlling the operation of the switching part.APT and APZ are in turn divided into subsystems, each of which has a specific function. The name of each subsystem reflects its function. For example, the group switching subsystem (GSS) is the central switching part of the AXE system.APT and APZ are implemented Switch Distributed Board (XDB) AXE hierarchical structure

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I/O SYSTEM

RP bus

RP – regional processor

CP – every device is controlled by it, hence, we always redundancy, i.e., CP-A and CP-

B.

#one CP can support 32 RP buses

#one RP bus is of 10mbps serial

#one RP contains 32 RPs

#hence, one CP supports 1024 RPs

#in one RPBIS , there are 4 ports for connection to GS and 2 for cross connection to

redundant RPHM

Since, there should not be much cabling in the CP, an interface ,namely, RPHM (regional

processor handler magazine) is introduced.

#each RPHM has 8 RPBIS posts

#one RPBIS post can handle 4RP buses

** group switch also have redundancy , i.e. , GS A and GS B ; but they are independent to each

other.

CP

RP RP RP

devices devices devices devices devices devices

Group Switch

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Chapter 12

GENERIC ERICSSON MAGAZINE ( GEM )

Another far-reaching improvement is the GEM, a high-capacity, flexible, and scalablemagazine (sub rack) that anticipates future developments (Figure 6). GEM-based nodeswill be smaller, dissipate less power, and have greater maximum capacity. Their implementation will dramatically improve the cost of ownership and cut time-to-customer

for AXE. In previous versions of AXE, each function was located in a separate magazine, and advances in technology to modify the architecture rather than merely shrink the hardware. One advantage of this change is that considerably fewer magazine and board types are needed in each node. A second ad-vantage is that many internal AXE interfaces have been moved to the GEM back-plane, reducing the need for cables.

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A GEM can house two SCB-RPs, providing an Ethernet switch, control function, maintenance

support, and power distribution; two switch boards (XDB), providing a 16 K plane duplicated group switch; up to 22 device boards with 15 mm spac- ing, such as ET155, ECP, or TRA; pairs of DLEBs, providing multiplexing functionality for DL3 cable

interfaces, placed in device slots; and CL890 clock modules placed in deviceslots. The GEM, which has been developed for use in the BYB 501 equipment

practice, provides several infrastructure functions via the backplane for the boards

housed in the magazine: duplicated power distribution; duplicated group switch connection; duplicated serial RP bus; maintenance bus for hardware inventory and fault indication; duplicated Ethernet, 10 or 100 Mbit/s; extremely robust backplane connectors.

TRA - transcoder and rate adaptor ( only in BSC) ECP - echo cancellor and pool (only in monolithic MSC)

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ET1551 – common in all nodes SCB-RP – support correcting board RP 1 GEM capacity = 16k 1 AXE 810 = 32 GEM magazines RP–regional processor interface

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Chapter 12

CONCLUSION

GSM technology which is the most extensively used and universally accepted technology till

date across the world for mobile phone communication. We looked at the GSM network

implementation, network components, and basic scenarios while mobile station is active, idle or

switched off, traffic channels and day to day activities of the network engineers trying to keep

the network up and running.

As is evident from the chapters in the books, the complex world of telecommunications is and

will continue to be full of challenges. We will very likely have a large number of different

networks in GSM operation for a long time to come - networks that are positioned to be able

to provide a many-faceted range of user-friendly services, be it individually or in cooperation

between networks.

The variety of possible diagrams showing GSM architecture and its details of working are

described in the report which gives the clear description of its working and its protocols.

The knowledge of hardware of Ericsson is helpful in understanding the general structure of

the hardware of the telecommunication industries and their vendors. It gives us the overview

in terms of installation of hardware , economical and technological.

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Chapter13

REFERNCES

GSM System Survey, Ericsson

Google.com

Wikipedia.com

GSM Introduction, Bell Labs Innovation , Lucent Technologies

GSM Pocket Guide, Waldeland Goltermann

GSM SwitchingServicesand Protocols, WileyPublications