gsm techinal notes

8/14/2019 GSM Techinal Notes

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IMR_TEC_GSM Version 1.0Technical Note on G

INDIA MOBILITY RESEARCH

TECHNICAL NOTE ON GSM


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The information contained in this document is the proprietary and exclusive property of India MobilityResearch Pvt. Ltd. except as otherwise indicated. No part of this document, in whole or in part, may bereproduced, stored, transmitted, or used for design purposes without the prior written permission of IndiaMobility Research Pvt. Ltd. .

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Version History

Document history

Version 1.0 Aug, 2005 First Version.

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Table of Contents

1 Overview of GSM .............................................................................................................81.1 History .......................................................................................................................81.2 Analog to Digital transmission ...................................................................................91.3 Cellular Systems ........................................................................................................9

1.3.1 Types of Cells ....................................................................................................112 GSM Services ...............................................................................................................12

2.1 Phase 1 .....................................................................................................................122.2 Phase 2 .....................................................................................................................142.3 Phase 2+ ..................................................................................................................16

3 GSM Architecture ...........................................................................................................183.1 GSM Network Elements ..........................................................................................19

3.1.1 Mobile Station ...................................................................................................193.1.2 The Base Station System (BSS) ........................................................................193.1.3 Network Switching System ...............................................................................19

3.2 GSM Interfaces ........................................................................................................23

3.2.1 A ........................................................................................................................243.2.2 Abis ...................................................................................................................243.2.3 B ........................................................................................................................243.2.4 C ........................................................................................................................243.2.5 D ........................................................................................................................243.2.6 E ........................................................................................................................253.2.7 F ........................................................................................................................253.2.8 G ........................................................................................................................263.2.9 H ........................................................................................................................26

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3.2.10 Um ..................................................................................................................263.3 GSM Protocol Stack ................................................................................................273.4 Channels ...................................................................................................................29

3.4.1 Logical Channels ..............................................................................................303.4.2 Physical Channels .............................................................................................33

4 GSM System parameters ................................................................................................35

1 Overview of GSM1.1 HistoryIn the early 1980s, analog cellular telephone systems were experiencing rapid growth inEurope, particularly in Scandinavia and the United Kingdom, but also in France andGermany. Each country developed its own system, which was incompatible witheveryone else's in equipment and operation. This was an undesirable situation, becausenot only was the mobile equipment limited to operation within national boundaries, butthere was also a very limited market for each type of equipment, so economies of scaleand the subsequent savings could not be realized.

To solve this problem, in 1982 the Conference of European Posts and Telegraphs (CEPT)formed a study group called the Groupe Spcial Mobile (GSM) to study and developsecond generation digital mobile telephone systems. In 1989, GSM responsibility wastransferred to the European Telecommunication Standards Institute (ETSI), and phase I ofthe GSM specifications were published in 1990. The standardized system had to meetcertain criterias:

Spectrum efficiency International roaming

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Low mobile and base stations costs Good subjective voice quality Compatibility with other systems such as ISDN (Integrated Services Digital

Network) Ability to support new services

The first GSM network based on this standard started its operation in Finland in 1991.Since then GSM had become popular and today GSM is the fastest growingcommunication technology of all time and it accounts for the 75% of the worlds digitalmobile market.

1.2 Analog to Digital transmissionThe move from 1st generation to the 2nd generation essentially signifies the switch fromthe analog to digital systems. Digital transmission has a number of advantages overanalog transmission:

It economizes bandwidth. Signal manipulation is possible by signal processing It can maintain superior quality of voice transmission over long distances. It requires lower average transmitter power. It enables smaller and less expensive individual receivers and transmitters. It offers voice privacy. It can be easily adapted to the transmission of data as well as voice

communication.

The 1st

generation mobile systems were based on FDMA (Analog) and the secondgeneration systems were based on TDMA (Digital). GSM incorporates a FDMA-TDMAhybrid.

1.3 Cellular SystemsThe operation of a cellular system relies on two fundamental resources

Power

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Frequency spectrum.

For a viable cellular system efficient use of these resources is required.

Cellular systems in general are interference limited systems. To cover a large geographic

area with a limited amount of spectrum, reuse of frequency is required. Frequency reuseis a concept whereby large number of low powered transmitters sharing the samefrequency spectrum is used to cover a cellular coverage area.

To implement this concept the entire coverage area is divided into cells and thetransmitters are placed at the center of the cell. The cells are modeled in a hexagonalshape. The assumed shape is purely for theoretical study. The frequency band allocatedto the system is distributed over a group of cells and this distribution is repeated over thecovering area. Frequencies used in a cell will be reused several cells away. The distancebetween the cells using the same frequency must be sufficient to avoid interference. Thisdistance is called reuse distance (D). It is given by D=R(3K) where R is the radius ofthe cell and K is the cluster size. A cluster is the group of cells which divide thefrequency spectrum among themselves and cluster size is the number of cells in a cluster.It follows the formula K= (i+j)2 ij , where i,j= 1,2,3,n. A typical cluster can contain4,7,12 or 21 cells.

The concept of frequency reuse facilitates high capacity, high quality of service and lowcost.

Figure 1: Representation of Cells

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Ideal cells Fictitious cells


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1.3.1 Types of Cells

The cell size determines the number of cells available to cover geographic area and thetotal capacity available to all users. Based on the size of cells, the cells are classified as

Macro cells Micro cells Pico cells Umbrella cells.

The coverage area of each cell is different in different environments. Macro cells can beregarded as cells where the base station antenna is installed in a mast or a building aboveaverage roof top level. Micro cells are cells whose antenna height is under average rooftop level; they are typically used in urban areas. Pico cells are small cells whose diameteris a few dozen meters; they are mainly used indoors. On the other hand, umbrella cellsare used to cover shadowed regions of smaller cells and fill in gaps in coverage between

those cells.

Advantages of cell structures: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally

Disadvantages: fixed network needed for the base stations

handover (changing from one cell to another) necessary interference with other cells

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2 GSM ServicesGSM services can be classified as teleservices, bearer services, and supplementaryservices.

Teleservices: These are services that people use directly like talking on the phone. Bearer services: These are services that become useful only if some kind of

contraption of software is added on the phone. Internet surfing is an example ofan application that uses bearer services.

Supplementary services: This is an inherent feature of a network to which a phoneis connected. Putting a call on hold is an example of supplementary services.

The standardization of GSM took place in phases: Phase 1 (1991), Phase 2 (October1995), and Phase 2+.

2.1 Phase 1More features were specified in Phase 1 than actually being implemented. Phase 1concentrated on providing more basic services like voice communication and Short

Message Service (SMS). List of features available in Phase 1 is given in Table 1.Table 1: Features covered by Phase 1

Service Category Service Comment

Teleservices Telephony Full-rate speech at 13 KbpsEmergency Calls 112 defined as a GSM

wide number to a localemergency service

Short message service Alphanumeric message

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(SMS) - point to point exchange between twoindividual users via adedicated service center

Short message service(SMS) Cell broadcast

Alphanumeric informationto all mobile stations within

one cell or area but notduring a speech.Videotext accessTeletext transmissionFacsimile alternate speechand facsimile Group 3Automatic facsimileGroup 3

Bearer Services Asynchronous data 300, 1200, 1200/75, 2400,4800, 9600 bps transparent/

nontransparentSynchronous data 2400, 4800, 9600 bps,transparent

Asynchronous PAD access 300, 1200, 1200/75, 2400,4800, 9600 bps transparent/nontransparent

Synchronous packet dataduplex 2400 bps T/NT

2400, 4800, 9600 bps,transparent / nontransparent

Alternate speech/dataSpeech followed by data

Supplementary Services Call forwarding (CF) All calls, calls when asubscriber is busy, notreachable, or not available

Call barring (CB) All outgoing calls, alloutgoing international calls,all outgoing internationalcalls except those to thehome country, all incomingcalls, all incoming calls

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when roaming

2.2 Phase 2As the industry caught up with the Phase 1 specification, the appetite and demand for

advanced features grew. Phase 2 added the Half-rate and Enhanced full rate speech codec.Phase 2 also added new mechanisms to the network infrastructure to handle the radioresource, location management, call control and higher level functions throughout thenetwork. The Phase 2 enhancement is presented in Table 2.

Table 2: Features covered by GSM Phase 2


Teleservices Half-rate speech codec(HR)

Optional implementation,implying the use of dual

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rate (half and full rate) inone phone

Enhanced full-rate speechcodec (EFR)

Optional implementation,implying the use of dualrate (enhanced full rate and

full rate) in one phone

Supplementary Services Calling line identification(CLI)

Presentation and restrictionof displaying the callers ID

Connected lineidentification (COL)

Presentation and restrictionof displaying the called ID

Call waiting (CW) During an active call, thesubscriber will be informedabout another incoming call(offered together with callhold)

Call hold (CH) Put one call on hold in order to answer/originate anothercall (offered together withcall waiting)

Multiparty communication(MPTY)

Up to five ongoing calls can be joined to a multipartycommunication

Closed user group (CUG) Similar to a feature knownfrom trunking services

Advice of charge (AoC) On-line charge informationUnstructured supplementary

services data (USSD)

Allows operator-defined

individual servicesOperator-determinedbarring (ODB)

Enables the operator torestrict certain features fromindividual subscribers

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2.3 Phase 2+It includes all additional features specified for GSM after October 1995 (the freezing ofPhase 2 standard). The first release in Phase 2+ is the Release 96. It included specialadditions like communications applications and GSM for railway operations. SMSsupport for Arabian and Asian languages were introduced. The data rates were alsoenhanced to 64 kbps with the addition of High Speed Circuit Switched Data (HSCSD).The features introduced in Release 96 are presented in Table 3.

Table 3: Features covered by Release 96 of GSM Phase 2+


Teleservices Voice group service (VGS) Voice group call service(VGCS) and voice broadcast services (VBS), both defined for UIC

applicationsExtension to the SMSalphabet

Allowing UCS2 with 16-bitcharacter representation

Second SMS cell broadcastchannel

Increasing capacity for cellbroadcast services (CBS)

Bearer Services High-speed circuit-switcheddata (HSCSD)

Combination of up to eighttime slots for a single link

Packet data on signalingchannels (PDS)

Reusing signaling resourcesfor packet data traffic

14.4 Kbps user data rate

Supplementary Services Explicit call transfer (ECT)

Enhanced multilevelprecedence and preemptionservice (eMLPP)- Phase 1

Priority for certain callsdefined for UICapplications

SIM Features Barred dialing numbers(BDN)

Bars certain numbers frombeing called

ME personalization Allows restriction of phone

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usage to particular SIMcards (subsidy protection)

Service dialing numbers(SDN)

Preprogrammed servicenumbers on the SIM, protected from erasure by

subscriberSIM application toolkit Vehicle for operators todefine their own valueadded services

Network Features Network identity and timezone (NITZ)

Simple download ofnetwork relatedinformation and data to theMS

CAMEL Phase 1 IN features for roamingGSM subscribers

Radio local loop (RLL) GSM extension for fixednetworksSupport of optimal routing(SOR)

Reduced charging due tooptimized routing

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3 GSM Architecture

Access Network Core Network

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Figure 2: GSM Architecture

3.1 GSM Network Elements

The GSM network elements can be broadly classified as

MS (Mobile Station) BSS (Base Station System) NSS (Network Switching System)

3.1.1 Mobile Station

The mobile station consists of the physical equipment used by a PLMN subscriber; itcomprises the Mobile Equipment (ME) and the Subscriber Identity Module (SIM). TheME comprises the Mobile Termination (MT) which, depending on the application and

services, may support various combinations of Terminal Adapter (TA) and TerminalEquipment (TE) functional groups.

3.1.2 The Base Station System (BSS)

The Base Station System (BSS) is the system of base station equipments (transceivers,controllers, etc...) which is viewed by the MSC through a single A-interface as being theentity responsible for communicating with Mobile Stations in a certain area. Similarly, inPLMNs supporting GPRS, the BSS also has an interface to an SGSN. The radioequipment of a BSS may support one or more cells. A BSS may consist of one or morebase stations. Where an Abis-interface is implemented. The BSS consists of one Base

Station Controller (BSC) and one or more Base Transceiver Station (BTS). A BaseStation Controller (BSC) is a network component in the PLMN with the functions forcontrol of one or more BTS. A Base Transceiver Station (BTS) is a network componentwhich serves one cell.

3.1.3 Network Switching System

The Network Switching System (NSS) is also called as the core network. It comprises of MSC (Mobile Switching Center)

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GMSC (Gateway Mobile Switching Center) HLR (Home Location Register) VLR (Visitor Location Register) AuC (Authentication Center) EIR (Equipment Identity Register) IWF (Internetworking Function)

3.1.3.1 The Mobile-services Switching Centre (MSC)

The Mobile-services Switching Centre is an exchange which performs all the switchingand signaling functions for mobile stations located in a geographical area designated asthe MSC area. The main difference between a MSC and an exchange in a fixed networkis that the MSC has to take into account the impact of the allocation of radio resourcesand the mobile nature of the subscribers and in addition has to perform locationregistration and handover.

3.1.3.2 The Gateway MSC (GMSC)If a network, delivering a call to the PLMN cannot interrogate the HLR, the call is routedto an MSC. This MSC will interrogate the appropriate HLR and then route the call to theMSC where the mobile station is located. The MSC which performs the routing functionto the actual location of the MS is called the Gateway MSC (GMSC). The acceptance ofan interrogation to an HLR is the decision of the operator. The choice of which MSCs canact as Gateway MSCs is for the operator to decide (i.e. all MSCs or some designatedMSCs).

3.1.3.3 The Home Location Register (HLR)

This functional entity is a data base in charge of the management of mobile subscribers. A

PLMN may contain one or several HLRs: it depends on the number of mobilesubscribers, on the capacity of the equipment and on the organization of the network. TheHLR contains the subscription information and some location information enabling thecharging and routing of calls towards the MSC where the MS is registered (e.g. the MSRoaming Number, the VLR Number, the MSC Number, the Local MS Identity). If GPRSis supported HLR additionally contains location information enabling the charging androuting of messages in the SGSN where the MS is currently registered (e.g. the SGSNNumber). Different types of identity are attached to each mobile subscription and arestored in the HLR. If GPRS is not supported the following identities are stored:

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the International Mobile Station Identity (IMSI)

one or more Mobile Station International ISDN number(s) (MSISDN)

if GPRS is supported, the following identities are stored

the International Mobile Station Identity (IMSI) one or more Mobile Station International ISDN number(s) (MSISDN)

zero or more Packet Data Protocol (PDP) address(es).

There is always at least one identity, apart from the IMSI, attached to each mobilesubscription and stored in the HLR. The IMSI or, the MSISDN may be used as a key toaccess the information in the database for a mobile subscription. The data base containsother information such as:

teleservices and bearer services subscription information

service restrictions (e.g. roaming limitation) - a list of all the group IDs aservice subscriber is entitled to use to establish voice group or broadcast calls

supplementary services; the HLR contains the parameters attached to these services

and, if GPRS is supported

information about if a GGSN is allowed to dynamically allocate PDP addresses fora subscriber.

3.1.3.4 The Visitor Location Register (VLR)

A mobile station roaming in an MSC area is controlled by the Visitor Location Register incharge of this area. When a Mobile Station (MS) enters a new location area it starts aregistration procedure. The MSC in charge of that area notices this registration andtransfers to the Visitor Location Register, the identity of the location area where the MS issituated. If this MS is no yet registered, the VLR and the HLR exchange information toallow the proper handling of calls involving the MS. A VLR may be in charge of one orseveral MSC areas. The VLR contains also the information needed to handle the calls set-up or received by the MSs registered in its data base (for some supplementary services

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the VLR may have to obtain additional information from the HLR) the followingelements are included:

- the International Mobile Subscriber Identity (IMSI);

- the Mobile Station International ISDN number (MSISDN);

- the Mobile Station Roaming Number (MSRN)

- the Temporary Mobile Station Identity (TMSI), if applicable;

- the Local Mobile Station Identity (LMSI), if used;

- the location area where the mobile station has been registered.

the identity of the SGSN where the MS has been registered. Only applicable toPLMNs supporting GPRS and which have a Gs interface between MSC/VLR andSGSN

- the last known location and the initial location of the MS;

The VLR also contains supplementary service parameters attached to the mobilesubscriber and received from the HLR.

3.1.3.5 The Authentication Centre (AuC)

The Authentication Centre (AuC) is associated with an HLR, and stores an identity keyfor each mobile subscriber registered with the associated HLR. This key is used togenerate:

data which are used to authenticate the International Mobile Subscriber

Identity (IMSI)

a key used to cipher communication over the radio path between the mobilestation and the network.

3.1.3.6 The Equipment Identity Register (EIR)

This functional entity contains one or several databases which store(s) the IMEIs used inthe GSM system. The mobile equipment may be classified as "white listed", "grey listed"and "black listed" and therefore may be stored in three separate lists. An IMEI may also

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be unknown to the EIR. An EIR shall as a minimum contain a "white list" (Equipmentclassified as "white listed").

3.1.3.7 Internetworking Function (IWF)

Internetworking function is used to interconnect the PLMN (Public Land MobileNetwork) with the PSDN (Packet Switched Data Network) and PSTN (Public SwitchedTelephone Network). This element is not part of the GSM architecture.

3.2 GSM Interfaces

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Figure 3: GSM Interfaces

3.2.1 A

The A interface between the MSC and its BSS. It is used to carry information concerning,BSS management, call handling, mobility management.

3.2.2 Abis

When the BSS consists of a Base Station Controller (BSC) and one or more BaseTransceiver Stations (BTS), this interface is used between the BSC and BTS to supportthe services offered to the GSM users and subscribers. The interface also allows controlof the radio equipment and radio frequency allocation in the BTS.

3.2.3 B

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

3.2.4 C

The Gateway MSC must interrogate the HLR of the required subscriber to obtain routinginformation for a call or a short message directed to that subscriber. Signaling on thisinterface uses the Mobile Application Part (MAP), which in turn uses the services ofTransaction Capabilities.

3.2.5 D

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3.2.8 G

When a mobile subscriber moves from a VLR area to another Location Registrationprocedure will happen. This procedure may include the retrieval of the IMSI andauthentication parameters from the old VLR. Signaling on this interface uses the Mobile

Application Part (MAP), which in turn uses the services of Transaction Capabilities.

3.2.9 H

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

3.2.10 Um

Um is the air interface used for exchanges between a MS and a BSS. LAPDm, a modified

version of the ISDN LAPD, is used for signaling.

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3.3 GSM Protocol Stack

Figure 4: GSM Protocol Stack

The signaling protocol in GSM is structured into three general layers depending on theinterface, as shown in Figure 4. Layer 1 is the physical layer, which uses the channelstructures discussed above over the air interface. Layer 2 is the data link layer. Across theUm interface, the data link layer is a modified version of the LAPD protocol used inISDN, called LAPDm. Across the A interface, the Message Transfer Part layer 2 ofSignaling System Number 7 is used. Layer 3 of the GSM signaling protocol is itselfdivided into 3 sub layers Radio Resources (RR), Mobility Management (MM),Connection Management (CM).

The CM, MM and RR layers together correspond to layer three in the ISO OSI protocolsuite. The layer two is composed of LAPD and LAPDm. Customarily, the lower three

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layers terminate in the same node. Not so in GSM, where the functionality is spread overdistinct functional entities with standardized interfaces between them. For instance, theRR part of layer three is spread over the MS, BTS, BSC, and MSC.

CM: The Communication Management (CM) layer consists of setting up calls at

the users request. Its functions are divided in three: Call control, which managesthe circuit oriented services; Supplementary services management, which allowsmodifications and checking of the supplementary services configuration; ShortMessage Services, which provides point-to-point short message services.

MM: The Mobility Management (MM) layer is in charge of maintaining thelocation data, in addition to the authentication and ciphering procedures.

RR: The Radio Resource (RR) Management layer is in charge of establishing andmaintaining a stable uninterrupted communications path between the MSC andMS over which signaling and user data can be conveyed. Handovers are part of

the RR layers responsibility. Most of the functions are controlled by the BSC,BTS, and MS, though some are performed by the MSC (in particular for inter-MSC handovers.).

LAPDm: The layer two protocol is provided for by LAPDm over the air-interface. This protocol is a modified version of the LAPD (Link Access Protocolfor the ISDN D-channel) protocol. The main modifications are due to the tightsynchronisation required in TDMA and bit error protection mechanism requiredover the air-interface (and in GSM handled by layer 1), making the correspondingfunctionality of the LAPD protocol redundant (and thus wasteful over the air-interface). The LAPD frame flags are replaced by a length indicator, and the FEC

field is removed.

LAPD: This is the ISDN LAPD protocol (Link Access Protocol for the ISDN D-channel) providing error-free transmission between the BSC and MSC.

BSSAP: The Base Station System Application Part (BSSAP) is split into twoparts, the BSSMAP and the DTAP (not shown in the above figure). The messageexchanges are handled by SS7. Messages which are not transparent to the BSCare carried by the Base Station System Management Application Part (BSSMAP),

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which supports all of the procedures between the MSC and the BSS that requireinterpretation and processing of information related to single calls, and resourcemanagement. The messages between the MSC and MS which are transparent tothe BSC (MM and CM messages) are catered for by the Direct TransferApplication Part (DTAP).

SCCP: The Signaling Connection Control Part (SCCP) from SS7.

MTP: The Message Transport Part (MTP) of SS7

3.4 ChannelsTwo types of channels are involved in GSM Physical channels and Logical Channels.GSM distinguishes between physical channels (the timeslot) and logical channels (theinformation carried by the physical channels). Several recurring timeslots on a carrierconstitute a physical channel, which are used by different logical channels to transferinformation - both user data and signaling.

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LOGICAL CHANNELS

TRAFFIC SIGNALLING

FULL RATE

Bm 22.8 Kb/S

HALF RATE

Lm 11.4 Kb/S

BROADCAST COMMON CONTROL DEDICATED CONTROL

FCCH SCH B CCH

PCHRACH

AGCH

SDCCH SACCH FACCH

FCCH -- FREQUENCY CORRECTION CHANNEL

SCH -- SYNCHRONISATION CHANNEL

BCC H -- BROADCAST CONTROL CHANNEL

PCH -- PAGING CHANNEL

RACH -- RANDOM ACCESS CHANNEL

AGC H -- ACCESS GRANTED CHANNEL

SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL

SACCH -- SLOW ASSOCIATED CONTROL CHANNEL

FACCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLY

UPLINK ONLY

BOTH UP &

DOWNLINKS

Figure 5: Logical Channels

3.4.1 Logical Channels

Logical channels include traffic channels and signaling channel. The traffic channel isused for transfer of voice or data and the control channel is used to register units, set upcalls and monitor call progress. GSM can use two types of traffic channels

TCH/FS (full rate) TCH/HS (half rate)

GSM has three types of control channels BCH (Broadcast Channel), CCCH (CommonControl Channel), DCCH (Dedicated Control Channel).

BCH enables a subscriber unit to synchronize with the network (only downlink).

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BCCH: Broadcast control channel (BCCH) is a base to mobilechannel which provides general information about the network, thecell in which the mobile is currently located and the adjacent cells

SCH: Synchronization channel (SCH) is a base to mobile channel

which carries information for frame synchronization and identificationof the base station transceiver

FCCH: Frequency correction channel (FCCH) is a base to mobilechannel which provides information for carrier synchronization

CCCH is used for subscriber unit registration and call set up. (both up link anddownlink).

RACH: Random Access Channel is used by the MS to accessrequests, response to call announcement, location update, etc.

AGCH: Acknowledge channel requests from MS and allocate aSDCCH.

PCH: Paging channel is used by the MS terminating callannouncement.

DCCH takes over one subscriber unit which have been assigned a dedicatedchannel. They transfer control messages during the conversation (both up link anddown link).

SDCCH (Stand- alone dedicated control channel): Exchange ofsignaling information between the MS and BTS when no TCH isactive

SACCH (Slow Associated control channel): Transmission ofsignaling data during a connection (One SACCH TS every 120ms)

FACCH (Fast Associated control channel): Transmission ofsignaling data during a connection (used only if necessary)

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The logical channels used in the process of location update and call establishment isgiven in Figure 6 and Figure 7.

Figure 6: Location update from the mobile

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Figure 7: Mobile terminating call

3.4.2 Physical Channels

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1 2 3 4 5 6 7 8

higher GSM frame structures

935-960 MHz

124 channels (200 kHz)

downlink

890-915 MHz124 channels (200 kHz)

uplink

freq

uency

time

GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 s577 s

guard

space

guard

spacet ail user dat a TrainingS S user data tail

3 bits 57 bits 26 bits 57 bits1 1 3

GSM - TDMA/FDMA

Figure 8 : GSM Physical Channel

Physical channels are all the available TSs of a BTS, whereas every TS corresponds to aphysical channel. Logical channels are piggybacked on the physical channels

In GSM, a 25 Mhz frequency band is divided, using a FDMA scheme, into 124 carrierfrequencies spaced one from each other by a 200 khz frequency band. Normally a 25Mhz frequency band can provide 125 carrier frequencies but the first carrier frequency isused as a guard band between GSM and other services working on lower frequencies.Each carrier frequency is then divided in time using a TDMA scheme. This scheme splitsthe radio channel, with a width of 200 khz, into 8 bursts. A burst is the unit of time in aTDMA system, and it lasts approximately 0.577 ms. A TDMA frame is formed with 8bursts and lasts, consequently, 4.615 ms. Each of the eight bursts, that form a TDMAframe, are then assigned to a single user.

To overcome the effect of multipath fading and alleviate channel eves dropping GSMuses the concept of frequency hopping. The frequency hopping used in GSM is slow

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frequency hopping whereby a frequency is changed for every TDMA frame. Figure 9shows the GSM frame hierarchy and how the logical channels are mapped onto a frame.

0 1 2 3 4 5 6 2043 2044 2045 2046 2047

0 1 2 3 4 48 49 50

0 1 2 24 25

0 1 2 3 24 25

0 1 2 3 4 48 49 50

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

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

1 HYPER FRAME = 2048 SUPERFRAMES = 2 715 648 TDMA FRAMES ( 3 H 28 MIN 53 S 760 MS )

1 SUPER FRAME = 1326 TDMA FRAMES ( 6.12 S )

LEFT (OR) RIGHT

1 MULTI FRAME = 51 TDMA FRAMES (235 .4 ms )

1 SUPER FRAME = 26 MULTI FRAMES

1 SUPER FRAME = 51 MULTI FRAMES

1 MULTIFRAME = 26 TDMA FRAMES ( 120 ms )

TDMA FRAME NO.

0 1

0 1

HIERARCHY OF FRAMES

1 2 3 4 155 156

1 TIME SLOT = 156.25 BITS

( 0.577 ms)

(4.615ms)

(4.615 ms)

1 bit =36.9 micro sec

TRAFFIC CHANNELS

SIGNALLING CHANNELS

Figure 9: GSM frame hierarchy

4 GSM System parametersA GSM network can operate in anyone of these frequency bands - 800, 900, 1800, 1900MHz. For the operation two frequency bands, of 25 Mhz each is needed. One for uplinkand the other for downlink. The 25 Mhz bandwidth is divided into 125 channels of200khz each. With this breakup the maximum achievable data rate on a single GSM datachannel is 9.6 Kbps. However, the data rates can be enhanced using HSCSD or with theimplementation of EDGE. Some important system parameters of a typical GSM networkare presented in Table 4.

Multiple Access Method TDMA / FDMAUplink frequencies (MHz) 933-960 (basic GSM)Downlink frequencies (MHz) 890-915 (basic GSM)Duplexing FDD

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Channel spacing, kHz 200Modulation GMSKPortable TX power, maximum / average (mW) 1000 / 125Power control, handset and BSS YesSpeech coding and rate (kbps) RPE-LTP / 13Speech Channels per RF channel: 8Channel rate (kbps) 270.833Channel coding Rate 1/2 convolutionalFrame duration (ms) 4.615

Table 4 : GSM parameters

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gsm techinal notes

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