Siemens Gsm Overwiew

D900/D1800/D1900 Overview Siemens

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Contents

1 General Terminology 3

1.1 Global System for Mobile Communications (GSM) 4

1.2 Public Land Mobile Network (PLMN) 5

1.3 Radio Cell 5

1.4 Mobile Station (MS) 6

1.5 Subscriber Identity Module (SIM) 6

1.6 Location Registration 8

2 Architecture of the D900/D1800/D1900 PLMN 9

2.1 Network Components of the Radio Subsystem (RSS) 11

2.2 Network Components of the Switching Subsystem (SSS) 15

2.3 Network Components of the Operation and Maintenance Subsystem(OMS) 26

3 Connections between the PLMN Elements 29

4 Radio Interface (Um) 31

4.1 GSM900/GSM1800/GSM1900 Frequency Bands 33

4.2 Radio Frequency Carriers (RFC) 34

4.3 Physical Channels 36

4.4 Signaling Channels 38

4.5 Time Organization of the Physical Channels 44

4.6 Bursts/Timeslot Structure 46

5 GSM Key Functions 49

5.1 Frequency Hopping Management 50

5.2 Antenna Diversity 56

D900/D1800/D1900 Overview

Siemens D900/D1800/D1900 Overview

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5.3 Power Control 57

5.4 Handover Management 58

5.5 GSM-R 62

5.6 Multiband Operation 65

5.7 Hierarchical Cell Structure 66

5.8 Concentric Cells 69

5.9 High Speed Circuit Switched Data HSCSD 71

5.10 General Packet Radio Service 74

6 Exercises 91


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1 General Terminology


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1.1 Global System for Mobile Communications (GSM)

The Global System for Mobile Communications (GSM) is a standard for mobilecommunications. This standard was developed by the EuropeanTelecommunications Standards Institute (ETSI).

Within ETSI, special work groups called Special Mobile Groups (SMG), areresponsible for the GSM standard.

SMG

ETSI

Fig. 1 European telecommunications standards institute


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1.2 Public Land Mobile Network (PLMN)

A Public Land Mobile Network (PLMN) is a network, established and operated by itslicensed operator(s), for the specific purpose of providing land mobile communicationservices to the public.

1.3 Radio Cell

A radio cell is the smallest service area in a PLMN. The term “cell” comes from thehoneycomb shape of the areas into which the PLMN coverage area is divided. A cellconsists of a base station transmitting over a small geographic area that isrepresented as a hexagon. The whole public land mobile network (PLMN) area iscovered by a great number of radio cells.

In the nomenclature used in Siemens documentation, a cell is also called BaseTransceiver Station (BTS).

The max size of a cell may be up to 35 km radius for the D900 system and up to 8km radius for the D1800/D1900 system.

Radio cell or cell or BTS

Fig. 2 Radio cell


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1.4 Mobile Station (MS)

The Mobile Station (MS) is the radio equipment needed by a subscriber to access theservices provided by the PLMN. The MS may be a fixed station (e.g. installed invehicles) or a portable handheld station.

1.5 Subscriber Identity Module (SIM)

The Subscriber Identity Module (SIM) provides the mobile equipment (ME) with anidentity. The mobile equipment is not operational (except for emergency calls) withouta SIM. The SIM must be inserted into the mobile equipment before it can be used.

The SIM is a smart card with a microprocessor and memory. The SIM may be thesize of a credit card or even smaller for portable phones.


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Mobile Subscriber Base Station Mobile Subscriber

Fig. 3 Mobile subscriber

ME + SIM = MS

+ =

Fig. 4 Mobile station


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1.6 Location Registration

After inserting the SIM card into the mobile equipment and switching on the mobileequipment, the mobile station will initiate a series of signaling activities. When thesignaling activities are completed, the current location will be stored on the SIM cardand the network will be aware of the subscriber’s current location.

Short messages received from the network may also be stored on the SIM card.

To protect the SIM from unauthorized use, the subscriber must enter a four digitpersonal identification number (PIN). The PIN is stored on the SIM; if the wrong PINis entered three consecutive times, the card blocks itself, and may be unblocked onlywith an eight digit code that is also stored on the SIM.

ME + SIM = MS

+ =

Fig. 5 Location registration


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2 Architecture of theD900/D1800/D1900 PLMN


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The major subsystems of a Public Land Mobile Network (PLMN) are:

� Radio Subsystem (RSS)

� Switching Subsystem (SSS)

� Operation and Maintenance Subsystem (OMS)

D900/D1800/D1900 System

Operation & Maintainance Subsystem

Switching Subsystem Radio Subsystem

RSSSSS

OMS

SIEMENS

N IXDORF

Fig. 6 Major subsystems of a PLMN


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2.1 Network Components of the RadioSubsystem (RSS)

The Radio Subsystem (RSS) consists of the following:

� Mobile Station (MS)

� Base Station System (BSS)

� Radio Interface (Um)

The Mobile Station and the Base Station System communicate across the Uminterface, also known as the air interface or radio link.

Radio Subsystem (RSS)

BSS

To/from

SSS

To/from

OMS

Mobile

Station

Um

Fig. 7 Components of the RSS


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2.1.1 Network Components of the Base Station System (BSS)

The BSS includes the following:

� Base Station Controller (BSC)

� Base Transceiver Station Equipment (BTSE)

� Transcoder and Rate Adapter Unit (TRAU)

One Base Station Controller can control several BTSE and several TRAU.

The interface between the BSC and the BTSE is called the Abis interface, theinterface between BSC and TRAU is the Asub interface. The interface between theBase Station System and the Switching Subsystem (or the TRAU and the MSCrespectively) is called the A interface.

Base

Transceiver

Station

Equipment

BTSE

BTSE

BTSE

Transcoder

and Rate

Adapter Unit

TRAU

Base Station

Controller

BSC

Abis

Asub A

Base Station System (BSS)

Fig. 8 Network components of the BSS


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2.1.1.1 The Network Element BSC

The Base Station Controller (BSC) is responsible for the intelligent functions in theBase Station System (BSS). The BSC assigns traffic channel connections from theSSS to the BTSE. Furthermore, it controls the whole Base Station System.

2.1.1.2 The Network Element BTSE

The Base Transceiver Station Equipment (BTSE) comprises the radio transmissionand reception equipment, including the antennas, and also the signaling processingspecific to the radio interface. The BTSE contains one or more transceivers (TRX)and serves up to three cells.

2.1.1.3 The Network Element TRAU

The Transcoder and Rate Adapter Unit (TRAU) is the equipment in which coding anddecoding is carried out as well as rate adaptation.

The two main functional units of the TRAU are:

� Transcoder (TC) for speech coding/compression

� Rate Adapter (RA) for data rate adaptation


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Each Base Station System (BSS) is connected to a Mobile Services Switching Center(MSC) which belongs to the SSS and is responsible for routing traffic channelconnections. The MSC itself are connected to each other and to the fixed network(i.e. PSTN).

BSS

MSC

MSC

BSS

BSS

BSS

BSS

BSS

MSC

Fixed Network

MSC

Fig. 9 BSS and MSC


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2.2 Network Components of the SwitchingSubsystem (SSS)

The network components of the Switching Subsystem (SSS) are:

� Mobile Services Switching Center (MSC)

� Home Location Register (HLR)

� Visitor Location Register (VLR)

� Authentication Center (AC)

� Equipment Identification Register (EIR)

Switching Subsystem (SSS)

HLR AC EIR

MSC

VLR

MSC

VLR

Fig. 10 Network components of the SSS


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2.2.1 The Network Element MSC

The Mobile Services Switching Center (MSC) is responsible for establishing trafficchannel connections

� to the BSS

� to other MSC

� to other networks (e.g., public switched telephone network (PSTN)).

The database of a MSC contains information for the routing of traffic channelconnections and handling of the basic and supplementary services. The MSC alsoperforms administration of cells and location areas.

BSS Switching Subsystem (SSS)

BTSEBSC

BTSETRAU

MSC MSC

PSTN

Fig. 11 Mobile services switching center


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2.2.2 The Network Elements HLR and VLR

In a D900/D1800/D1900 PLMN, the subscriber administration is not performed by theexchanges because the mobile subscriber is not permanently connected to a MSC.Instead, the mobile subscriber’s current location determines which MSC isresponsible for the mobile subscriber at that moment.

Therefore, the PLMN contains a network component called Home Location Register(HLR) that administers the subscriber’s data.

The HLR is a database where the mobile subscribers are created, deleted, andbarred by the operator. It contains all permanent subscriber identities, as well as theservices that a mobile subscriber is authorized to use.

The Visitor Location Register (VLR) contains the relevant data of all mobilesubscribers currently located in the service area of a MSC. The permanent data arethe same as the data found in the HLR. Locating the data in the VLR, as well as inthe HLR, reduces the data traffic to the HLR because it is not necessary to ask forthis data every time it is needed.

If a mobile station is located in its own MSC, it still uses the two different registers,even though the VLR seems redundant. The consistency simplifies the procedures.


BTSEBSC

BTSETRAU

MSC MSC

PSTN

HLR

Restriction

None

Barred

None

None

Service

Tele

Tele

Tele/Fax

Tele/Fax

Subscriber

234-8000

234-8200

234-2340

234-2256

Fig. 12 Home location register


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BTSEBSC

BTSETRAU

MSC MSC

PSTN

VLR

Restriction

None

Service

Tele

Subscriber

234-8000

VLR

HLRSubscriber Data

Fig. 13 Visitor location register


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During a location update (when the subscriber moves to a different VLR servicearea), the new (current) VLR requests subscriber data from the HLR.


BTSEBSC

BTSETRAU

MSC MSC

VLR VLR

HLRSubscriber Data

Subscriber DataRequest

PSTN

Fig. 14 Visitor location register: data request


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2.2.3 The Network Element AC

The purpose of the authentication is to protect the network against unauthorizedusers. The authentication feature ensures that the user (mobile subscriber) is who heclaims to be. Subscriber authentication is performed at each registration and at eachcall set-up attempt (mobile originating or terminating).

If a mobile subscriber wants to access the network, the VLR verifies whether its SIMcard is approved via the authentication procedure.

For this procedure, the VLR uses authentication parameters, called triples, that aregenerated continuously for each subscriber by the Authentication Center (AC). Thetriples consist of RAND (Random Number), SRES (Signed Response) and Kc(Cipher Key).

The network element AC is associated with the HLR. Upon request from the VLR, theAC provides the VLR with these triples.


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BTSEBSC

BTSETRAU

MSC MSC

VLR VLR

HLR

PSTN

ACRAND SRES Kc

RAND

SRES

Fig. 15 Authentication

For the authentication, the RAND is sent to the MS. The MS uses the RAND with thealgorithm A3 and the Authentication Key stored on the SIM to also calculate theSigned Response.

The MS sends the SRES to the MSC/VLR where it is compared with the valuecalculated by the MSC/VLR.

If the SRES sent from the MS matches the SRES calculated by the MSC/VLR,access to the network is granted and the Cipher Key Kc is sent to the BTSE for usein encrypting and decrypting messages to and from the MS.

If the SRES do not match, the MS is denied access to the network.


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2.2.4 The Network Element EIR

The Equipment Identification Register (EIR) is a database that stores theInternational Mobile Equipment Identity (IMEI) numbers for all registered MobileEquipment (ME). The IMEI uniquely identifies all registered ME. There is generallyone EIR per PLMN. It interfaces to the various HLRs in the PLMN.


BTSEBSC

BTSETRAU

MSC MSC

VLR VLR

HLRAC

PSTN

IMEIEIR

Fig. 16 Equipment identification register


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The structure of the IMEI is as follows:

TAC is the type approval code.

This 6-character code allows approved types to be distinguished from non-approvedtypes. The TAC is administered centrally.

FAC is the final assembly code.

This two-character code identifies the place of manufacture and designates the finalassembly. The FAC is administered by the manufacturer.

SNR is the serial number.

This 6-character code is an individual code allocated to all equipment with the sameTAC and FAC. The SNR is issued by the manufacturer of the mobile station.

The Spare Bit (SP) is always zero.

IMEI

Type

Approval

Code

(TAC)

Final

Assembly

Code

(FAC)

Serial

Number

(SNR)

Spare

Bit

(BT)

15 Digits

Fig. 17 International mobile equipment identity


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There are three classes of ME that are stored in the database, and each group hasdifferent characteristics:

� the black list for barred mobile stations (e.g., mobiles that have been reportedstolen).

� the gray list for mobile stations to be observed

� the white list for approved mobile stations


BTSEBSC

BTSETRAU

MSC MSC

VLR VLR

HLRAC

PSTN

EIR

Black List

Unapproved

equipment,

because it is

stolen or

defective.

Grey List

Approved but

suspicious

equipment.

Stolen?

Defective?

White List

Approved,

error-free,

unsuspicious

equipment.

EIR

? ? ?

IMEI

Fig. 18 Black, gray and white list


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The EIR checks whether the IMEI fits into one of the three categories and passes theresult to the MSC. If, for example the ME fits into the black list, the call would beblocked.


BTSEBSC

BTSETRAU

MSC MSC

VLR VLR

HLRAC

PSTN

EIR

whitegreyblack

IMEI

Fig. 19 Mobile equipment of black list


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2.3 Network Components of the Operation andMaintenance Subsystem (OMS)

Monitoring of the network components of the SSS and the BSS can be performedcentrally by the Operation and Maintenance Subsystem (OMS).

The OMS is realized in operation and maintenance centers (OMC), which consists ofan OMC-B for the administration of BSS network elements and an OMC-S for theadministration of SSS network elements within the PLMN.

The operation and maintenance for SSS and BSS are independent of each other.The OMC-B and OMC-S may be combined in the same location.

D900/D1800/D1900 System


Radio Subsystem Switching Subsystem

BTSEHLR

OMC

EIRVLR

MSC

AC

BTSE RAU

VLR

MSC

BTSE BSC

SI EMENS

NIXDORF

Fig. 20 Operation and maintenance subsystem


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There are two different possibilities to connect the OMC-B to the BSC:

� Either via dedicated line

� Or via A-interface connection (nailed-up connection)

OMC-BSS OMC-SSS

OMS

BTSE BSC TRAU MSC

VLR HLR

AC

EIR

BSS SSS

dedicated

line

A-interface connection

Fig. 21 Connection of OMC-B to BSC


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3 Connections between the PLMN Elements


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All the network elements of the Base Station System and the Switching Subsystemare connected via PCM 30 (or PCM24) lines.

These PCM lines may be normal cables, coaxial cables, or e.g. microwave links.

D900/D1800/D1900 System


Radio Subsystem Switching Subsystem

BTSEHLR

OMC

EIRVLR

MSC

AC

BTSE TRAU

VLR

MSC

BTSE BSC

Fig. 22 Connections between the network elements of BSS and SSS


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4 Radio Interface (Um)


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The Um-interface is the radio interface between the BTSE (antenna) and the mobilestation.

Mobile Station

Uplink

Downlink

Um

BTSE

Fig. 23 Um-interface


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4.1 GSM900/GSM1800/GSM1900 Frequency Bands

A specific frequency range is assigned to GSM900/GSM1800/GSM1900 systems.

This frequency range is divided into two sub-bands:

1. UPLINK (UL) for the radio transmission between the MS and the BTSE

2. DOWNLINK (DL) for the radio transmission between the BTSE and the MS

The frequencies for GSM900/GSM1800/GSM1900 (also calledGSM900/DCS1800/PCS1900) are listed below.

EGSM 900

Uplink Downlink

DCS 1800

Uplink Downlink

PCS 1900

Uplink Downlink

With DCS 1800 the UPLINK

ranges from 1710 MHz to

1785 MHz and the

DOWNLINK from 1805 MHz

to 1880 MHz.

With the Extended GSM 900,

the UPLINK ranges from 880

MHz to 915 MHz and the

DOWNLINK from 925 MHz to

960 MHz.

With PCS 1900 the UPLINK

ranges from 1850 MHz to

1910 MHz and the

DOWNLINK from 1930 MHz

to 1990 MHz.

1850 199019301910

MHz

1710 188018051785

MHz

880 960925915

MHz

Fig. 24 Frequency ranges for GSM900, DCS1800 and PCS1900


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4.2 Radio Frequency Carriers (RFC)

Both sub-bands (UPLINK and DOWNLINK) are divided into Carriers (C) or RadioFrequency Carriers (RFC) with a bandwidth of 200 kHz. The division of both thesesubbands into Radio Frequency Carriers is called Frequency Division MultipleAccess (FDMA).

The differences between GSM900, DCS1800, and PCS1900 relate to the following:

� Operating frequency

� Bandwidth of the sub-bands

� Number of Radio Frequency Carriers (RFC) provided.

Extended

GSM 900

Uplink Downlink

DCS

1800

PCS

1900

880

MHz915

MHz

925

MHz

960

MHz

1710

MHz1785

MHz

1805

MHz

1880

MHz

1850

MHz1910

MHz

1930

MHz

1990

MHz

C C C C. . . CCCC . . .

200

kHz

200

kHz

Fig. 25 Radio frequency carriers for GSM900, DCS1800 and PCS1900


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Depending on the traffic volume, every radio cell uses one or more RFC. Since thenumber of RFC is limited, the same RFC must be used several times. To avoid co-channel interference, a safe distance is required between the BTSE using the sameRFC. This safe distance is called reuse distance.

The size of a single cell depends on the topology of the surrounding and on the trafficvolume. In hilly regions or in densely populated areas with high traffic volume the cellradius is kept small. Such a small cell radius can be achieved by reducing the outputpower of the base station

RFC 24

RFC 32

RFC 8

RFC 5

RFC 2

RFC 17

RFC 13

RFC 47

RFC 40RFC 24

RFC 32

RFC 11

RFC 47

RFC 8

RFC 5

RFC 2

reuse distance

Fig. 26 Radio frequency carriers and reuse distance


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4.3 Physical Channels

A physical channel on the air interface uses Time Division Multiple Access (TDMA)similar to PCM 30/PCM24.

One RFC consists of eight TDMA channels.

RFC x

RFC yRFC x

RFC x

RFC y

TDM

A Fra

me

(4.6

15 m

s)

200 kHz 200 kHz

0 15 31

TDMA Frame

(125 �s)

PCM 30

RFC x

Fig. 27 Time division multiple access

RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

101

2

3

4

5

6

7RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

201

2

3

4

5

6

7RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

301

2

3

4

5

6

7 RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

1RFC

174

RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3RFC

174

RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3

RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3RFC

2

RFC

3RFC

3RFC

3RFC

3RFC

3RFC

3RFC

1

Uplink Downlink

FDMA FDMA

f

t

TDMATDMA

Fig. 28 Frequency division multiple access and time division multiple (t=time, f=frequency)


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A physical channel is defined by a specific carrier (RFC) in the uplink band, thecorresponding carrier in the downlink band and by the timeslot number in the TDMAframe.

A physical channel can function as a traffic channel (for transmission of speech anddata information) or as a signaling channel.

RFC RFC

Physical Channel

Uplink Downlink

corresponding

Traffic

Channel

Traffic

Channel

Fig. 29 Physical channel


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4.4 Signaling Channels

Three types of signaling channels can be distinguished:

� Broadcast Control Channels

� Common Control Channels

� Dedicated Control Channels

CHANNELS

TRAFFIC

CHANNELS

FULL

RATE

HALF

RATE

COMMON

CONTROL

CHANNLES

DOWNLINK UPLINK FAST

DOWNLINK

&UPLINK

TCH/F TCH/H BCCH FCCH SCH PCH AGCH RACH SACCH SDCCH FACCH

SIGNALLING

CHANNELS

BROADCAST

CONTROL

CHANNLES

DEDICATED

CONTROL

CHANNELS

DOWNLINK SLOW

Fig. 30 Signaling channels


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4.4.1 Broadcast Control Channels

The broadcast control channels are defined for the BTSE to MS direction (downlink)only and are subdivided into:

1. the Broadcast Control Channel (BCCH) which is defined on a per cell basis andinforms the mobile station about the various parameters to be used in the cell forreceiving the service. Information sent over this channel includes country code,network code, local area code, PLMN code, RF channels used within the cellwhere the mobile is located, surrounding cells, and frequency hopping sequencenumber.

2. the Frequency Correction Channel (FCCH) which carries information forfrequency correction of the MS downlink. This will allow a MS to accurately tuneto a BS.

3. the Synchronization Channel (SCH) which gives the mobile station informationabout the frame number and BTS identification code (BSIC).

Example of Broadcast Channel

The BTSE continuously transmits data, such as cell identity via the BroadcastChannel. The MS uses this information to determine their current cell location.

BTSE

BCCH

cell idendity: 262041422

Fig. 31 Example of a broadcast channel information


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4.4.2 Common Control Channels

Common Control Channels are specified as unidirectional channels, either on thedownlink or the uplink. The following sub-channels are distinguished:

1. the Paging Channel (PCH) which is used in downlink direction to page mobilestations,

2. the Access Grant Channel (AGCH), also used in downlink direction, to assign adedicated channel to a mobile station,

3. the Random Access Channel (RACH) which is an uplink channel to indicate amobile station’s request for a dedicated channel.

Example for the use of Common Control Channels

Before a Mobile Subscriber can send the dialing number to the BTSE, it must firstrequest a Stand alone Dedicated Control Channel (SDCCH) via the Random AccessChannel (RACH).

BTSE

RACH

Request of a SDCCH

12142341234

Fig. 32 Example for the need of a random access channel


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The BSC assigns a SDCCH to the MS via the Access Grant Channel (AGCH).

BTSE

AGCH

SDCCH provided

12142341234

Fig. 33 Example for the need of an access grant channel


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All of the signaling information (i.e., dialed digits, authentication, traffic channelassignment) for call setup is exchanged over the SDCCH.

BTSE

SDCCH

Control Information

Fig. 34 Example for the need of a stand alone dedicated control channel


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4.4.3 Dedicated Control Channels

Dedicated Control Channels are full duplex (bi-directional) channels. They aresubdivided into:

1. the Slow Associated Control Channel (SACCH) which is a duplex channel and isalways associated with a TCH or SDCCH (embedded within the same framestructure). The SACCH is used for the transmission of radio link measurementdata.

2. the Fast Associated Control Channel (FACCH) which is a duplex channel alwaysassociated with a TCH and is used for the transmission of signaling data, afterthe set up of the call, when the SDCCH has been already released. The FACCHdata are inserted into the TCH burst instead of traffic data (bit stealing). Thisinsertion is indicated by a "stealing Flag" (i.e., the FACCH may contain handoverinformation).

3. the Stand alone Dedicated Control Channel (SDCCH) which is a duplex channelnormally assigned after the MS request and is used for signaling purposes, forthe set up of the calls or other network procedures.

4.4.4 Channel Combinations

All the above mentioned channels are logical channels. Physical channels are alwayscombinations of different logical channels.

The following combinations of logical channel types are allowed for the radiotimeslots and specified by GSM (Rec. 05.02):

a) TCH/F + FACCH/F + SACCH/TF

b) TCH/H + FACCH/H + SACCH/TH

c) SDCCH/8 + SACCH/C8

d) FCCH + SCH + BCCH + CCCH

e) FCCH + SCH + BCCH + CCCH + SDCCH/4 + SACCH/C4

f) BCCH + CCCH

g) SDCCH/8 + SACCH/8 + CBCH

h) FCCH + SCH + BCCH + CCCH + SDCCH/4 + SACCH/C4 + CBCH

A "+" indicates that the channels are used simultaneously. Channel combinations "d","e" , and "h" can be used only for timeslot 0.


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4.5 Time Organization of the Physical Channels

All of the control channel types are "logical channels", which are appropriatelygrouped and assigned to a single physical channel (i.e., to a single timeslot on theradio interface).

4.5.1 Structure of a User Channel Multiframe

A TCH is always allocated with its associated slow-rate channel (SACCH). Twenty-six TDMA frames (=120 ms) are grouped together to form one multiframe forspeech/data. TCH are sent on 24 timeslots; one slot is used for SACCH signalinginformation and one slot remains unused.

Not only subscriber information (speech/data) can be transmitted in a traffic channelTCH. If the signaling requirement increases (e.g. in case of a handover), signalinginformation FACCH can also be transmitted instead of TCH. This is indicated by a so-called stealing flag in the normal burst.

T

7

0

1

TT T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

TT

0 1 62 3 4 7 8 9 10 11 12 135 15 16 17 18 19 20 21 22 23 24 2514

SSS

S

ST T

T

8 bursts per cycle time

T TCH

S SACCH

unused

Fig. 35 Structure of a user channel multiframe


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4.5.2 Structure of a Signaling Channel Multiframe

In a control or signaling channel multiframe fifty-one TDMA frames (=235.4 ms) aregrouped together to form one multiframe.

As an example the following figure shows the basic combination including (downlinkdirection) a FCCH, SCH, BCCH, and AGCH/PCH all on the same timeslot (i.e., 0).The uplink direction contains a RACH.

The BCCH with the AGCH/PCH uses 40 slots per multiframe. These 40 timeslots aregrouped together as 10 groups of four. The BCCH use the first four timeslots of thefirst group of 10. The remaining timeslots are used by the AGCH/PCH.

The FCCH is sent on timeslot 0 in frame 0, 10, 20 ,30 and 40, while the SCH is sentin timeslot 0 on frame 1, 11, 21, 31 and 41.

T

7

0

1

8 B P c y c le t im e

0 1 0 2 0 3 0 4 01

621 1

1 22 1

2 23 1

3 24 1

4 2

F C C H

S C H

B C C H

A G C H /P C Hd o w n lin k

R A C HU P L IN KUPLINK

DOWNLINK

Fig. 36 Structure of the basic combination including FCCH, SCH, BCCH and AGCH/PCH


MN1780EU08MN_000146

4.6 Bursts/Timeslot Structure

A burst represents the physical content of a timeslot. Timeslots may carry differentkinds of bursts:

� Normal bursts

� Access burst

� Frequency correction burst

� Synchronization burst

� Dummy burst

A burst is a period of the radio frequency carrier that is modulated by a data stream.The modulation is applied for the useful duration of the burst. In general, the usefulduration of the burst is the duration equivalent of 148 bits excluding the access burstthat has a useful duration equivalent of 88 bits. To minimize interference, the mobilestation is required during the guard periods (GP) to attenuate its transmissionamplitude and to adjust possible time shifts and amplitudes of the bursts.

The normal burst is used to carry information on traffic channels and on signalingchannels, except for the Random Access Channel (RACH), Synchronization Channel(SCH), and Frequency Correction Channel (FCCH). Its structure is shown below.

� T = Tail Bits: This burst section consists of three bits (always coded with `000’) andis needed for synchronization at the receive side.

� Coded (Encrypted) bits: There are two burst sections which contain the coded andencrypted traffic information (speech or data) in 2 x (57 +1) bits. The 1 bit is calledstealing flag and indicates whether the 57 bits are really user data or FACCHsignaling information.

� Training Sequence: This burst section contains 26 bits used for synchronization.Eight different training sequences have been defined by GSM.

� GP = Guard Period: These 8.25 bits serve to guard phase deviations (due to themovement of the MS) and to reduce the transmission power.

The access burst has an extended guard period that helps to control the time lag ofthe signals due to the initial distance between mobile station and BTSE. Once thetime lag has been corrected (timing advance), the additional time lag resulting fromthe alteration of distance of a moving mobile station is controlled with the aid of thenormal guard periods of 8.25 bit duration.

Frequency correction bursts are sent by the BTSE and are used by the mobilestation to adjust its receiver and transmitter frequencies (frequency synchronization).

Synchronization bursts are used to establish an initial bit and framesynchronization (time synchronization).


MN1780EU08MN_000147

Dummy bursts are inserted if TCH timeslots on the BCCH carrier are not filled withuser data to reach a constant level of the BCCH carrier. This is necessary becausethe level of the BCCH carrier is evaluated for handover decisions and also for thedecision which cell shall be the serving cell during call set up.

7 0 1 2 3 4 5 6 7 0 1 . . .

T

3

Coded Bits

58

Training Sequence

26

Coded Bits

58

T

3

GP

8.25

Normal Burst

. . .

Fig. 37 Normal burst


MN1780EU08MN_000148

Um

Interface

Burst Formats

Burst

3

T

57

Encrypted

1

S

26

Training

3

T

57

Encrypted

3

T

8.25

GP

8

T

41

Synch. Sequence

36

Encrypted

3

T

68,25

GP

3

T

142

Fixed Bits

3

T

8.25

GP

3

T

39

Encrypted

64 Extended

Training Sequence

39

Encrypted

3

T

8.25

GP

3

T

58

Mixed Bits

26

Training

58

Mixed Bits

3

T

8.25

GP

Normal Burst

Access Burst

Frequency Correction Burst

Synchronization Burst

Dummy Burst

577 �s

156.25 Bits (577 �s)

3.693 �s per Bit

Fig. 38 Burst formats


MN1780EU08MN_000149

5 GSM Key Functions


MN1780EU08MN_000150

A short description of GSM key functions is provided hereafter.

Among the GSM basic functions there are the following:

� Frequency Hopping Management

� Antenna Diversity

� RF Power Control

� Handover Management

� GSM-R

� Multiband Operation

� Hierarchical Cell Structure

� Concentric Cells

5.1 Frequency Hopping Management

Basics:

The radio signal at the Air- Interface is affected by so called multipath propagation.

The signal, transmitted by the BTSE, reaches the MS via several ways:

� the direct way between the BTSE and the MS (indicated by 1 in the followingpicture).

� the delayed way due to reflections at different obstacles, like buildings, rocks etc.(indicated by 2 in the following picture)


MN1780EU08MN_000151

SIEMENS

MS

BTSE

1

2

2

1 direct way2 delayed way due to reflections

Fig. 39 Multipath propagation

The interaction of the different signals (positive or negative interference) results in anincrimination or decrimintation of the received signal level at the Mobile Station. Theactual signal level strongly depends on the position of the MS as shown in the figurebelow.

In the worst case the fading can result in a call drop, when the level of the receiversignal falls below the threshold of the required Rx- sensitivity.

Up to now only the downlink direction (BTS --> MS) was regarded. Of course, thesame problems also occur in the uplink direction.


MN1780EU08MN_000152

Rx- Sensitivity

Distance

Receiver Level approx. 17 cm at 900 Mhz

Fading Dips

Fig. 40 Resulting Rx level due to multipath propagation (fading)

One mean to reduce the losses due to multipath propagation and fading is theFrequency Hopping. It is applied in the uplink- and downlink direction simultaneously.

Frequency Hopping is performed by assigning a sequence of different frequencies toa physical radiochannel at the Um Interface.

Subsequent timeslots in a generic frame follow a specified hopping sequence (cyclicor random).

At the counterside the assigned radiochannel is seen as "changing" or "hopping"from one frequency to the next.


MN1780EU08MN_000153

frame 0 frame 1 frame 2 frame 3 frame 4 frame 5

RFC 1

RFC 2

RFC 3

RFC 4

RFC 5

TDMA frame

4,615 ms

Timeslot

0,577 ms

Fig. 41 Frequency hopping via 5 frequencies

The benefit of Frequency Hopping is a kind of equalization of the link quality byaveraging of short term fading.

Realization

There are two kinds of Frequency Hopping:

� Baseband Hopping

� RF Hopping or Synthesizer Hopping.

With Baseband Hopping each physical TRX transmits at is own ARFCN, its outputfrequency is not changed.

The switching of the correct channel to the correct TRX is performed in the baseband parts of the BTS.


MN1780EU08MN_000154

The advantage of the Baseband Frequency Hopping is that all RF Parts operate atfixed frequencies, so all types of combiners

� Hybridcombiner

� Duplexcombiner

� Filtercombiner

can be used.

The number of frequencies used for hopping is limited to the number of TRX.

With Synthesizer Frequency Hopping the RF part of each TRX switches between thedefined ARFCN.

An advantage is that more ARFCN can be used than TRX are present.

On the other hand, no filter combiner can be used in this case.

With the help of frequency hopping an improvement of

� the S/N ratio in noise limited systems (rural areas)

� the C/I ratio in interference limited systems (urban areas)

can be achieved.

The logical channels are divided in two groups regarding the frequency hopping:

� non hopping channels, which don't change their frequency (BCCH, CCCH)

� hopping channels, which may change their frequency (TCH, SDCCH)


MN1780EU08MN_000155

B

U

S

1

TRX 0

TRX 1

BBSIG 3

BBSIG 2

BBSIG 1

BBSIG 0

BTSEcontrol

Generation of the logical channels

BTSE RF Part

-operating at fixed

frequencies

TRX 2

TRX 3

executionof the

switching

Fig. 42 Realization of the baseband hopping


MN1780EU08MN_000156

5.2 Antenna Diversity

The purpose of Antenna Diversity is to improve the receiver performance and thusthe uplink quality under fading conditions.

Antenna Diversity is applied in the uplink direction only, in the receiver path of theBTS.

To implement Antenna Diversity two spatially separated receiving antennas have tobe installed at the BTS site. Each antenna is connected to a separate receive path ofthe TRX.

Diversity Combining of the two received signals is performed within the Transceiverand Processing Unit (TPU) of the TRX.

Diversity Cobining

(TPU in the BTS)

Rx Path Rx Path

Receive Signal

Fig. 43 Antenna diversity


MN1780EU08MN_000157

5.3 Power Control

The purpose of RF Power Control is to dynamically adapt the RF output power in theMS and in the BTS according to the propagation losses on the radio path.

It is aimed to use the lowest possible transmission power which yields the requiredcommunication quality. Therefore,

� the power consumption of the Mobile Station and

� the total interference level in the radio system

are minimized.

BTSE

SIEMENS

Distance D2

Distance D1

Power Out 1

Power Out 2

Fig. 44 RF power control


MN1780EU08MN_000158

In the SBS dynamic RF Power Control is provided for the MS and for the BTS.

Downlink Power Control, being optional in the GSM, may be enabled or disabled byan O&M Command.

RF Power Control for the MS is based on uplink measurements, gathered by theBTS.

Power Control for the BTS is based on downlink measurements, gathered by the MS.

The criteria for power control decisions and actions are

� the received signal strength

� the received signal quality.

The decision whether a RF power increase or decrease is required derives fromthreshold comparisons which are applied to the respective measurement results.

The threshold comparison is preceded by a measurement averaging process.

The measurement averaging process and the power control decision algorithm foreach call in progress are located in the BTS.

5.4 Handover Management

Handover (HO) means that a call in progress is switched over from one radio channelto another one. Handover, in general, is required to hold a call in progress when theMobile Station passes from one cell coverage area to another one.

Handover takes place between radio channels which may belong

� to the same BTS (intracell handover) or

� to different BTS (intercell handover).

If the handover (HO) is controlled by the BSC, it is called "internal handover", if it iscontrolled by the MSC, it is called "external handover".

The following picture gives an overview about possible handover types.

The SBS supports intercell and intracell handovers.


MN1780EU08MN_000159

BSC 2

1

2

3

BSC 1

MSC1

1 intracell handover2 intercell handover - internal3 intercell handover - external

Fig. 45 Types of handovers

The handover due to radio criterion is based on

� uplink measurement results (gathered by the BTS) and

� downlink measurement results (gathered by the MS)

Criteria for handover decision are:

a) received signal strength (uplink and downlink)

b) signal quality (uplink and downlink)

c) MS - BTS distance (timing advance)

d) radio link power budget for adjacent cells

e) interference, i.e. strong signal and poor quality (uplink and downlink)


MN1780EU08MN_000160

The radio link power budget for adjacent cells relative to the current cell derives fromsignal strength measurements carried out by the MS on the BCCH carriersbroadcasted by the adjacent BTSs.

The first four criteria are used to determine whether an intercell handover is requiredas the MS leaves the current cell coverage area. The respective handover causesare

� signal strength

� signal quality

� distance

� power budget

Fig. 46 Handover due to the radio criterion


MN1780EU08MN_000161

The fifth criterion indicates the necessity of an intracell handover due to excessiveco-channel or adjacent cell interferences on the current radio path.

The decision whether a Handover is required for a call in progress derives fromthreshold comparisons which are applied to the respective measurement results.

The threshold comparisons are preceded by a measurement averaging process.

If an intercell handover is to be initiated, the power budget criterion is utilized to setup a list of preferred handover target cells in a decreasing order of priority.

This target cell list forms the basis for the final handover decisions to be made in theBSC or the MSC.

In the SBS the BTS determines whether a handover due to the radio criteria isrequired.

The measurement averaging processes, the threshold comparison processes as wellas the target cell list compilation for each call in progress are located in the BTS. Thefinal handover decision and handover execution are performed in the BSC or MSC,see also the following picture.

Inter-cell HOI: target cell list

1. neighbor cell

2. neighbor cell

3. neighbor cell

...

4. neighbor cell

5. neighbor cell

...

...

decreasing

order of

priority

Fig. 47 Target cell list


MN1780EU08MN_000162

5.5 GSM-R

Railway companies in Europe have a lot of different communication requirements foroperation and maintenance of their railroad network.

For these different tasks several incompatible systems are used in parallel. Toovercome this coexistence and to simplify the communication a solution based on theGSM standard can be used: the GSM-R.

The structure of a BSS GSM-R network is similar to the structure of a conventionalGSM900 network, see also the following picture:

BTS BTS BTS

BSC

Controller

A

Controller

B

Network

Interface

MSC

Cell 1 Cell 2 Cell 3

Fig. 48 Structure of a GSM-R network


MN1780EU08MN_000163

In comparison with the traditional GSM900 networks GSM-R solution shows thefollowing differences:

� The frequency range in the Up- and Downlink is located below the frequencyrange of the extended GSM band.

� The receiver in the BTSE must be able to handle the so called doppler frequencyshift, because of a moving MS. Since the speed of the trains can be up to 250 -400 km/h this shift is bigger than in the conventional GSM system.

� For reliability reasons the BTSE should be connected to the BSC in a "loop"configuration.


MN1780EU08MN_000164

The following picture gives an overview about the complete network structure ofGSM-R:

S IEMENS

SIEM ENS

SIEM ENS

BSC

MSC/ VLR

HLR

Railway PBA

NetworkATC Center

OMC-S OMC-B

BTSE

BTSE

BTSE

TRAU

Fig. 49 Complete structure of a GSM-R network


MN1780EU08MN_000165

5.6 Multiband Operation

Multiband Operation is the possibility to offer the GSM services in both GSM900 andGSM1800 frequency bands by one network operator which has got the license forboth frequency bands.

Therefore, the operation of "dual band" MS is supported as well as handoverbetween cells belonging to different bands is also supported, with an appropriatemanagement of the target cell list.

The transceiver equipment for both frequency bands can be implemented in thesame BTSE. This allows a network operator to use an already existing networkstructure when expanding an GSM900 network with GSM1800 equipment and viceversa.

The following picture presents a network with a mixed configuration GSM900/GSM1800 cells.

The feature Multiband Operation allows the network operator to overcome the limitedradio resources and to extend the network capacity especially in "hot spot" areas.

BTS-5

GSM1800

BSC

BTS-4

GSM 1800

BTS-3

GSM 900

BTS-2

GSM 900BTS-1

GSM 900

BTS-0

GSM 900

Fig. 50 Network with mixed cell configuration GSM900/ GSM1800


MN1780EU08MN_000166

5.7 Hierarchical Cell Structure

With the feature "Hierarchical Cell Structure" the network operator is able to organizea multiple layer network.

When considering a typical network evolution, the network organization is driven firstby coverage requirements and later on by capacity requirements.

In the first stage coverage of the network area can be achieved easily by usingumbrella cells and macro cells.

In the course of the network evolution the capacity can be increased by theimplementation of small cells, i.e. micro cells and pico cells.

Fig. 51 Network area coverage by using umbrella and macro cells


MN1780EU08MN_000167

Fig. 52 Increasing the network capacity by implementation of small cells


MN1780EU08MN_000168

The following picture represents a scenario of a multiple layer network.

For the management of such a multiple layer network in the SBS the cells arecharacterized with settable priority levels. Up to 16 different priority levels can beassigned to the different layers and cells respectively.

Umbrella Cell

GSM 900

Layer 5

Macro Cell

GSM 900

Layer 4

Small Cell

GSM 900

Layer 3

Pico Cell

GSM 900/1800

Layer 1

Micro Cell

GSM 900

Layer 2

Fig. 53 Multiple layer network


MN1780EU08MN_000169

5.8 Concentric Cells

The Base Station Subsystem provides a "concentric cell" structure, sometimesreferred to as "overlay" - "underlay" cells.

Such a cell is subdivided in two areas:

� inner area

� complete area.

The main concept of this configuration is to have two different sets of TRX within thesame cell.

The output power of the TRX which serve the inner area is strongly reduced. Thisresults in a smaller cell radius of the inner area.

f5

f2

f6

f3

f4

f1

BTSE

complete areainner area

Fig. 54 Sectored concentric cells with inner and complete area


MN1780EU08MN_000170

A particular type of intracell handover is given, between inner and outer area.

The concentric cell feature can be used to introduce an additional frequency re-usepattern for the frequencies in the inner area with a shorter re-use distance. Thisresults in an increased network capacity.


MN1780EU08MN_000171

5.9 High Speed Circuit Switched Data HSCSD

HSCSD (High Speed Circuit Switched Data) provides circuit switched datatransmission with higher data rates by using max. 8 TCH simultaneously. HSCSD isparticularly suited for real-time applications e.g. video communication.

High Speed Circuit Switched Data provides the following kinds of services:

Transparent / Non-transparent services:

For transparent HSCSD connections the BSC is not allowed to change the user datarate, but the BSC may modify the number of TCH used by the connection (in thiscase the data rate per TCH changes).

For non-transparent HSCSD connections the BSC is also allowed to change the userdata rate (data compression on the air i/f, on fixed network side -to and from modemof internet provider - no compression at data rates 19.2 kbit/s, 38.4 kbit/s or 64 kbit/sin later versions).

4 x 14.4 kbit/s

57,6 kbit/s

Fig. 55 High speed circuit switched data HSCSD


MN1780EU08MN_000172

Symmetric and pseudo-asymmetric services:

In symmetric service the timeslot allocation for downlink and uplink is symmetric andthe same data rates are used in down- and uplink direction.

In pseudo-asymmetric service the timeslot allocation for downlink and uplink issymmetric but the data rate used in uplink direction is lower than in downlinkdirection.

9.6 kbit/s /

14.4 kbit/s

28.8 kbit/s

57.6 kbit/s

High-Speed Circuit switched Data HSCSD

Main Features: - connection-oriented

- continuous datastream

- no new network elements

- only software modification

- minor hardware changes

Fig. 56 High-speed circuit switched data - main features


MN1780EU08MN_000173

The following conditions characterize an HSCSD connection:

� All n radio timeslots are located on the same TRX.

� The same frequency hopping sequence and training sequence is used.

� Only TCH/F are used.

� In symmetric configuration individual signal level and quality reporting for eachchannel is applied.

� For an asymmetric HSCSD configuration individual signal level and qualityreporting is used for those channels which have uplink SACCH associated withthem.

� The quality measurements reported on the main channel are based on the worstquality measured on the main and the unidirectional downlink timeslots used.

� In both symmetric and asymmetric HSCSD configuration the neighbor cellmeasurements are reported on each uplink channel used.

Handovers:

All TCH used in an HSCSD connection are handed over simultaneously. The BSCmay modify the number of timeslots used for the connection and the channel codingwhen handing over the connection to the new channels.

All kinds of handovers are supported.

Flexible air resource allocation:

The BSC is responsible for flexible air resource allocation. It may modify the numberof TCH/F as well as the channel coding used for the connection. Reasons for thechange of the resource allocation may be either a lack of radio resources, handoverand/or maintenance of the service quality.The change of the air resource allocation is done by the BSC using new service levelupgrading and downgrading procedures.

Characteristics of the Siemens implementation

The implementation of HSCSD in the SBS uses at most 4 TCH/F for one HSCSDcall.

� HSCSD is a fast solution to introduce high speed data services in the network

� no hardware impacts on the system

� no introduction of new interfaces required

� no changes to the lower layers for all interfaces between network elements


MN1780EU08MN_000174

5.10 General Packet Radio Service

5.10.1 Data Transmission in GSM Phase 2+

To increase the data transmission rates, in GSM phase 2+ new bearer services withrates comparable to or higher as ISDN are developed:

� HSCSD (High Speed Circuit Switched Data)

� GPRS (General Packet Radio Services)

� EDGE (Enhanced Data Rates for the GSM Evolution)

High Speed Circuit Switched Data HSCSD

HSCSD (Rec. 02.34) is a circuit switched data service (only point-to-point) forapplications with higher band width demands and continuous data stream, e.g.motion pictures or video telephony. The higher band width is achieved by combining1-8 physical channels for one subscriber. Additionally, the data transmission codecwas changed such that a maximum of 14.4 kbit/s instead of 9.6 kbit/s can betransmitted per physical channel. In this way, HSCSD theoretically enablestransmission rates up to 115.2 kbit/s. In order to implement HSCSD merely the GSM-PLMN software must be modified. More problematic is the high volume of resourcesneeded.

General Packet Radio Service GPRS

With GPRS it is possible to combine 1-8 physical channel for one user, just as withHSCSD. Various new coding schemes with transmission rates of up to 21.4 kbit/s perphysical channel enable theoretical transmission rates up to 171.2 kbit/s. Opposite toHSCSD, GPRS is a packet-switched bearer service, meaning that the same physicalchannel can be used for different subscribers. GPRS is resource efficient forapplications with a short-term need for high data rates (e.g. surfing the Internet, E-mail, ...). GPRS also enables point-to-multipoint transmission and volume dependentcharging. Extensions of the GSM network and protocol architecture are necessary forGPRS implementation.

Enhanced Data rates for the GSM Evolution EDGE

EDGE (Release`99) is able to realize up to 69.2 kbit/s per physical channel thoughthe change of the GSM modulation procedure (8PSK instead of GMSK).Theoretically, transmission rates of up to 553.6 kbit/s (meeting 3G requirements)would be possible by combining up to 8 channels. A combination of GPRS andEDGE could offer optimum usage of Inter- and Intranet, ensuring highest economy infrequency resource utilization at the same time.


MN1780EU08MN_000175

Data transmission

in GSM Phase 2+

TS 1 2 3 4 •••• 8

Phase 1/2:

1 TS with max. 9,6 kbit/s

HSCSD, GPRS, EDGE:

combining 1 - 8 TS

HSCSD: Circuit Switched (CS); 14,4 kbps / TS

no HW-changes; but: high demands to radio resources

GPRS: Packet Switched (PS);

flexible data rates up to 21,4 kbps / TS; HW- & protocol changes;

resource efficient

EDGE: Rel`99

8PSK instead of GMSK � modulation rate 813 kbps instead of 270 kbpsmax. 69.2 kbps / TS

Fig. 57 Data transmission in Phase 2+


MN1780EU08MN_000176

5.10.2 Architecture

For introducing GPRS, the logical GSM architecture is extended by two functionalunits:

� The Serving GPRS Support Node SGSN is on the same hierarchic level as MSCand has functions comparable to those of a Visited MSC (VMSC).

� The Gateway GPRS Support Node GGSN has functions comparable with thoseof a Gateway MSC (GMSC) and offers interworking functions for establishingcontact between the GSM/GPRS-PLMN and external packet data networks PDN

� A GPRS Support Node GSN includes the central functions required to supportthe GPRS. One PLMN can contain one or more GSNs.

In addition to GSN, extensions of functions in other GSM functional units arenecessary:

� In the BSS a Packet Control Unit PCU ensures the reception/adaptation ofpacket data from SGSN into BSS and vice versa.

� GPRS subscriber data are added to the HLR. On the following pages of this scriptthis extension will be termed GPRS Register GR.


MN1780EU08MN_000177

Channel Codec Unit CCUin BTS

for channel coding

Mobile

DTE

SGSNServing GPRS

Support Node

PSTN

Internet

Intranet

X.25

GGSNGateway GPRS

Support Node

VMSC /

VLRGMSC

HLR

New network entities:• SGSN• GGSN

zu PDN

GPRS - Architecture

ISDNPCU

BSS

GPRS subscription data(GPRS Register GR)

Packet Control Unit PCU

for protocol conversion &

radio resource

management

Fig. 58 Architecture


MN1780EU08MN_000178

5.10.2.1 Interfaces

Integration of functions GGSN and SGSN (which are necessary for GPRS) into aGSM-PLMN makes it necessary to provide names for a series of new interfaces inaddition to interfaces A-G already defined in the GSM-PLMN:

� Gb - between an SGSN and a BSS; Gb allows the exchange of signaling and userdata: Unlike the A-interface, in which a user is assigned a certain physicalresource for the entire/full duration of a connection, on Gb a resource is onlyassigned in case of activity (i. e. when data are being transmitted/received). Alarge number of subscribers use the same physical resources. The same holds forinterfaces Gi, Gn and Gp.

� Gc - between a GGSN and an HLR

� Gd - between an SMS-GMSC / SMS-IWMSC and an SGSN

� Gf - between an SGSN and an EIR

� Gi - between GPRS and an external packet data network PDN

� Gn - between two GPRS support nodes GSN within the same PLMN

� Gp - between two GSN located in different PLMNs. The Gp interface allows thesupporting of GPRS services over an area of cooperating GPRS PLMNs.

� Gr - between an SGSN and an HLR

� Gs - between an SGSN and an MSC/VLR; serves to support an MS using bothGPRS and circuit switched services (e.g. update of location information).


MN1780EU08MN_000179

MS BSS SGSN GGSN PDN TE

GGSN

other PLMN

SMS-GMSC

SMS-IWMSC

MSC/VLR HLR/(GR)

SM-SC

EIRSGSN

Gp Gf

Gi

GrGc

Gn

GbUm

Gs

Gd

E C

A

D

GPRS - Architecture:

Interfaces Signalling only

Signalling &

Data transmission

Gn

Fig. 59 Interfaces


MN1780EU08MN_000180

5.10.2.2 Serving GPRS Support Node (SGSN)

SGSN realizes a large number of functions for performing GPRS services.

SGSN is on the same hierarchic level as an MSC and handles many functionscomparable to a Visited MSC (VMSC).

SGSN

� is the node serving GPRS mobile stations in a region assigned to it;

� traces the location of the respective GPRS MSs (Mobility Management functions);

� is responsible for the paging of MS;

� performs security functions and access control (authentication/cipher settingprocedures,...); procedures are based on the same algorithm, ciphers and criteriaas in the former GSM. Ciphering algorithms have been optimized for thetransmission of packet data;

� has routing/traffic-management functions;

� collects data connected with fees/charges;

� realizes the interfaces to GGSN (Gn), PCU (Gb), other PLMNs (Gp). Furtherinterfaces may be added (Gr, Gs, Gf).

5.10.2.3 Gateway GPRS Support Node (GGSN)

GGSN realizes functions comparable to those of a gateway MSC.

GGSN

� is the node allowing contact/interworking between a GSM PLMN and a packetdata network PDN (realization Gi-interface);

� contains the routing information for GPRS subscribers available in the PLMN.Routing information serves to contact the respective SGSN in the providing area ofwhich an MS is momentarily located;

� has a screening function;

� can inquire about location informations from the HLR via the optional Gc interface.


MN1780EU08MN_000181

GGSN

SGSN & GGSN

SGSN

Serving GPRS Support Node SGSN� serves MSs in SGSN area

� Mobility Management functions, e.g

� Update Location, Attach, Paging,..

� Security and access control:

� Authentication, Cipher setting, IMEI Check...

� New cipher algorithm

� Routing / Traffic-Management

� collecting charging data

� realises Interfaces: Gn, Gb, Gd, Gp, Gr, Gs, Gf

Gateway GPRS Support Node SGSN� Gi-,Gn-Interface: Interworking PLMN � PDN

� Routing Information for attached GPRS user

� Screening / Filtering

� collecting charging data

� optional Gc interface

Fig. 60 SGSN and GGSN


MN1780EU08MN_000182

5.10.2.4 Packet Control Unit PCU

In the BSS, the PCU serves

� for the management of GPRS radio channels (Radio Channel Managementfunctions), e.g. power control, congestion control, broadcast control information

� for the temporal organization of the packet data transfer for uplink and downlink

� It has channel access control functions, e.g. access request and grants

� it serves for converting protocols from the Gb interface to the radio interface Um.

Three options for positioning the PCU are provided in Rec. 03.60:

� Option A: In the BTS

� Option B: in the BSC

� Option C: In spatial connection with the SGSN

5.10.2.5 Channel Codec Unit CCU

The CCU contains the following functions:

� Channel coding, including forward error correction FEC and interleaving

� Radio channel measurements, including received quality and signal level, timingadvance measurements

5.10.2.6 GPRS Mobile Stations MS

A GPRS MS can work in three different operational modes. The operational modedepends on the service an MS is attached to (GPRS or GPRS and other GSMservices) and on the mobile station’s capacity of simultaneously handling GPRS andother GSM services.

� „Class A“ operational mode: The MS is attached to GPRS and other GSM servicesand the MS supports the simultaneous handling of GPRS and other GSMservices.

� „Class B“ operational mode: The MS is attached to GPRS and other GSMservices, but the MS cannot handle them simultaneously.

� „Class C“ operational mode: The MS is attached exclusively to GPRS services.


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CCU

CCUPCU

BTS BSC site GSN site

CCU

CCU


CCU

CCU


PCU

PCU

A

B

C

optional:

PCU-locationPCU, CCU, GPRS - MS

Um Abis

Gb

GPRS-MS:(operational mode)

• Class-A: MS attached to GPRS & non-GPRS simultaneous handling

• Class-B: as A, not simultaneous• Class-C: GPRS only

MS

MS

MS

Packet Control Unit PCU• Channel Access Control functions• Radio Channel Management functions

(Power Control, Congestion Control,...)• scheduling data transmission (UL/DL)

• protocol conversion (Gb � Um)

Gb

Channel Codec Unit CCU• Channel Coding (FEC, Interleaving,..)

• Radio Channel Measurement funcions (received quality & signal level, TA,..)

Fig. 61 PCU, CCU, MS


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5.10.3 Radio Interface

GSM RF:

GPRS Layer 1 (Um)

L1-

tasks

Transmission

of user &

signalling data

determinate &

actualise

Timing Advance

Cell SelectionMeasure

signal strength

Power Control

functions Resource optimisation:1 physical channel to be used

by many MSs simultaneously !!

asymmetrical trafficUL / DL possible !!

High data rate trafficup to 171.2 kbit/s:

combining 1..8 PDCH for 1 MS !!

Allocation of physical channel(Packet Data Channel PDCH)

dynamically: 1 or 4 Radio Blocks

(1 Radio Block = 4 Normal Burst

in 4 consecutive TDMA-frames)

�� User & signalling data of several MSs

statistically to be multiplexed into 1 PDCH

(also fixed allocation possible)

Fig. 62 Radio Interface

The tasks of layer 1 radio interface relate to the transmission of user and signalingdata as well as to the measuring of receiver performance, cell selection,determination and updating of the delayed MS transmission (timing advance TA),power control PC and channel coding.

In the GPRS, a decisive difference to the realization of the connection-orientedservices (circuit-switched services) relates to the fact that a physical channel and aso-called packet data channel can be used by several mobile stations at the sametime. One packet data channel is allocated per radio block, i.e. for four consecutiveTDMA frames and not for a specific time interval. This means that signaling and thepacket data traffic of several mobile stations can be statistically multiplexed into onepacket data channel. Furthermore, the packet data channel can be seizedasymmetrically.

On the other hand it is also possible for a mobile station to use more than one packetdata channel at the same time, i.e. to combine several physical channels of one radiocarrier. In principle, up to 8 packet data channels can be seized simultaneously.


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5.10.3.1 Management of Radio Resources/ Coding Schemes

In a GPRS-supported cell , one or several physical channels can be allocated toGPRS transmission. These physical channels (Packet Data Channels PDCHs) areshared by GPRS mobile stations and are taken from the common/shared pool of allavailable physical channels of the cell.

Distribution of the physical channels for various logical packet data channels is basedon blocks of 4 normal bursts each. Uplink (UL) and downlink (DL) for GPRS packetdata are assigned separately (consideration of asymmetrical traffic peaks). Allocationof circuit switched services and GPRS is achieved dynamically, depending on whatcapacities are required („capacity on demand“). PDCHs need not be allocatedpermanently; however, it is possible for the operator to permanently or temporarilyreserve a number of physical channels for GPRS traffic.

New GPRS coding schemes (CS) - CS1 - CS4 - have been defined for thetransmission of packet data traffic channel PDTCH (Rec. 03.64). Coding schemescan be assigned as a function of the quality of the radio interface. Normally, groups of4 burst blocks each are coded together.

CS-1 makes use of the same coding scheme as has been specified for SDCCH inGSM Rec. 05.03. It consists of a half rate convolutional code for forward errorcorrection FEC. CS-1 corresponds to a data rate of 9.05 kbit/s.

CS-4 has no redundancy in transmission (no FEC) and corresponds to a data rate of21.4 kbit/s.

CS-2 and CS-3 represent punctured versions of the same half rate convolutionalcode as CS-1.

CS-2 corresponds to a rate of 13.4 kbit/s, while CS-3 corresponds to a data rate of15.6 kbit/s.

In principle, 1 to 8 time slots TS of a TDMA frame can be combined dynamically for auser for the transmission of GPRS packet data. Theoretically it is thus possible toachieve peak performances of up to 171.2 kbit/s (8x21.4 kbit/s) with GPRS.


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9,05 kbit/s

13,4 kbit/s

15,6 kbit/s

21,4 kbit/s

CS-1

CS-2

CS-3

CS-4

Coding

Schemes

different

redundancy (FEC) �

“Um transmission quality”

Radio Resource Management / Coding Schemes

CS & PS (GPRS):

“capacity on demand”

Physical channel of one cell

GPRS-MSs:

sharing physical channel

GPRS-MSs:

combining 1-8 TS

Common coding of 4 Bursts(Radio Block)

Up to

171,2 kbit/s(theoretically)

1 - 8

channel

GPRS-MSs:

asymmetric UL / DL

Fig. 63 Coding Schemes


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5.10.3.2 Logical GPRS Radio Channels

In addition to the nine existing logical radio channels used for signaling (BCCH, SCH,FCCH, PCH, RACH, AGCH as well as SDCCH, SACCH and FACCH) and the TrafficChannel (TCH) for circuit switched user information, a new set of logical channelswas defined for GPRS.

Packet traffic is realized by means of the Packet Traffic Channel (PTCH) whichincludes the following :

� Packet Data Traffic Channel PDTCH.

� Packet Associated Control Channel PACCH

� Packet Timing Advance Control Channel PTCCH

The PDTCH is temporarily assigned to the mobile stations MS. Via the PDTCH, userdata (point-to-point or point-to-multipoint) or GPRS mobility management and sessionmanagement GMM/SM information is transmitted.

The PACCH was defined for the transmission of signaling (low level signaling) to adedicated GPRS-MS. It carries information relating to data confirmation, resourceallocation and exchange of power control information.

The Packet Common Control Channel PCCCH has been newly defined. It consistsof a set of logical channels which are used for common control signaling to start theconnection set-up:

� Packet Random Access Channel PRACH

� Packet Paging Channel PPCH

� Packet Access Grant Channel PAGCH

� Packet Notification Channel PNCH

PRACH and PAGCH fulfil GPRS-MS functions which are analog to the “classical”logical channels RACH and AGCH for non-GPRS-users. The PNCH is used for theinitiation of point-to-multipoint multicast (PtM multicast).

For the transmission of system information to the GPRS mobile stations, the

� Packet Broadcast Control Channel PBCCH

was defined analog to the “classical” BCCH.

In a physical channel all different types of logical channels can be contained (noseparation into traffic and signaling channels respectively as is done in conventionalGSM).

The allowed channel combinations on one time slot are:

� PBCCH + PCCCH + PDTCH + PACCH + PTCCH

� PCCCH + PDTCH + PACCH + PTCCH

� PDTCH + PACCH + PTCCH

where PCCCH=PPCH + PRACH + PAGCH + PNCH.


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New logical channels for GPRS

Packet-

Signalling

Packet

Traffic Channel

PTCH

DL

UL

UL & DL

Packet

Common

Channels

PCCCH

DL

System information

for GPRS-MS

PBCCH

Packet Broadcast

Control Channel

PPCH

Packet Paging Channel

PAGCH

Packet Access

Grant Channel

PNCH

Packet Notification Channel

Paging GPRS-MS

Resource Allocation

for Setup of

Packet-Transfer

Notification

for GPRS-MS in

PtM Multicast

PRACH

Packet Random

Access Channel

access request for UL

packet data transmission

Transmission of user packet data

Multislot operation: one MS

many PDTCH simultaneously

PTCCHPacket Timing Advance

Control Channel

PACCHPacket Associated

Control Channel

Dedicated signalling MS-network

e.g.: Acknowledgements, Power

Control, Resource (Re-)Assignment

Estimation and Updates of

Timing advance for one MS

PDTCHPacket Data Traffic Chan.

Control

Fig. 64 Logical channels for GPRS


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5.10.3.3 Multiframes in GPRS

The GPRS packet data traffic is arranged in 52-type multiframes (GSM Rec. 03.64).52 TDMA frames in each case are combined to form one GPRS traffic channelmultiframe which is subdivided into 12 blocks with 4 TDMA frames each. One block(B0-B11) contains one radio block in each case (4 normal bursts, which are related toeach other by means of convolutional coding). Every thirteenth TDMA frame is idle.The idle frames are used by the MS to be able to determine the various base stationidentity codes BSIC, to carry out timing advance updates procedures or interferencemeasurements for the realization of power control.

For packet common control channels PCCH, conventional 51-type multiframes canbe used for signaling or 52-type multiframes.

iB0 B1 B2 B3 B4 B5 i B6 B7 B8 i B9 B10B11 i

52 TDMA Frames = PDCH Multiframe

4 Frames 1 Frame

New multiframe

for GPRS

• PDCH follows 52 multiframe structure

• 52 Multiframe: 12 Blocks à 4 TDMA-frames

• PCCCHs: „classical“ 51er Multiframes

or 52er Multiframes

B0 - B11 = Radio Blocks (Data / Signalling)

i = Idle frame

• BCCH indicates PDCH with PBCCH (in B0)

• DL: this PDCH bears PDCCH & PBCCH

PBCCH in B0 (+ max. 3 further blocks; indicated in B0)

PBCCH indicates PCCCH blocks & further PDCHs with PCCCH

• UL: PDCH with PCCCH: all blocks to be used for PRACH, PDTCH, PACCH

PDCH without PCCCH: PDTCH & PACCH only

Idle frame:• Identification of BSICs• Timing Advance Update Procedure

• Interference measurements for Power Control

Fig. 65 Multiframes


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6 Exercises

� What means GSM?

� What means PLMN?

� What are the major subsystems in a PLMN?

� List the major functions of the network components of the BSS and SSS.

� Monitoring of the BSS and SSS is performed by which subsystem?

� What is a radio cell?

� What influences the size of a radio cell and what is the maximum size of a radiocell in GSM900 ?

� Try to find out a correlation between the guard period of an access burst and themaximum size of a GSM cell.

� What is the frequency range for GSM900 (extended band)?

� How many radio frequency carriers do you have in GSM900?

� Can a cell have more than one RFC?

� What is meant with FDMA?

� What is meant with TDMA?

� What type of burst is used for traffic channels?

� Can the FCCH, SCH, BCCH, AGCH and PCH be assigned to the same timeslot?

� What does HSCSD stand for?

� What are the differences between GPRS and HSCSD?

� What is the (theoretical) maximum data rate for GPRS?

� How many logical channels are used in GPRS?


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Siemens Gsm Overwiew

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