GSM overview

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[ Preface | Architecture | Interfaces | Protocols | Channels | TDMA ]


This page discusses the GSM mobile telephony system, which is increasingly popular and established throughout the world. The term GSM usually means the GSM standard and protocols in the frequency spectrum around 900MHz. There is also DCS1800 - GSM protocols but at different air frequencies around 1800 MHz - and in the United States, where spectrum for Personal Communication Services (PCS) was auctioned at around 1900MHz, operators using the aptly-named GSM1900 are competing against a plethora of other standards. As a result of this, the original and most widely-used GSM frequency implementation is also becoming known as GSM900, and DCS1800 is also known as GSM1800. However, although the physical frequencies used differ, the protocols and architecture remain the same.

This page is not an introduction to the basics of cellular telephony. (That article covers a number of standards popular in the United States, as well as describing cells and frequency reuse.)

A detailed description of the GSM system can be found in [MOU92] and in [BJU95], in addition to the GSM recommendations. An index to GSM information is available. This Overview Of The GSM System and Protocol Architecture is worth reading, as is another overview.

The following sections will briefly describe the functional entities, the radio interface signalling protocol, the logical and physical channel structure and the TDMA structure based on GSM.

Some GSM system parameters are listed in the table below. Wikipedia gives a summary of GSM frequency use.

Multiple Access Method TDMA / FDMA
base station to mobile frequencies (MHz) 935-960 (basic GSM)
mobile to base station frequencies (MHz) 890-915 (basic GSM)
Duplexing FDD
Channel spacing, kHz 200
Modulation GMSK
Portable TX power, maximum / average (mW) 1000 / 125
Power control, handset and BSS Yes
Speech coding and rate (kbps) RPE-LTP / 13
Speech Channels per RF channel: 8
Channel rate (kbps) 270.833
Channel coding Rate 1/2 convolutional
Frame duration (ms) 4.615

System architecture

Figure 4 below shows the GSM system architecture, which consists of the switching system, the base station system and the user equipment.

GSM system architecture

The functional entities are briefly explained as follows:

MS Mobile Station. The MS is the physical equipment used by a subscriber, most often a normal hand-held cellular telephone.
BTS Base Transceiver Station. The BTS comprises the radio transmission and reception devices, and also manages the signal processing related to the air interface.
TRAU The Transcoder Rate Adaptor Unit. The TRAU (not shown in the above figure) functionally belongs to the BTS. The TRAU enables the use of lower rates (32, 16 or 8 kbps) over the A-bis interface instead of the 64 kbps ISDN rate for which the MSC is designed. The TRAU can be located at the BTS, the BSC, or (immediately in front of) the MSC.
BSC Base Station Controller. The BSC manages the radio interface, mainly through the allocation, release and handover of radio channels.
BSS Base Station System. The BSS consists of a BSC and one or more BTSs.
MSC Mobile Switching Centre. The MSC is basically an ISDN-switch, coordinating and setting up calls to and from MSs. An Inter-Working Function (IWF) may be required to adapt GSM specific rates to that used in a particular PSTN/ PLMN.
VLR Visitor Location Register. The VLR contains all the subscriber data, both permanent and temporary, which are necessary to control a MS in the MSCs coverage area. The VLR is commonly realised as an integral part of the MSC, rather than a separate entity.
AuC Authentication Centre. The AuC database contains the subscriber authentication keys and the algorithm required to calculate the authentication parameters to be transferred to the HLR.
HLR Home Location Register. The HLR database is used to store permanent and semi-permanent subscriber data; as such, the HLR will always know in which location area the MS is (assuming the MS is in a coverage area), and this data is used to locate an MS in the event of a MS terminating call set-up.
EIR Equipment Identity Register. The EIR database contains information on the MS and its capabilities. The IMEI (International Mobile Subscriber Identity) is used to interrogate the EIR.
GMSC Gateway Mobile Switching Centre. The GMSC is the point to which a MS terminating call is initially routed, without any knowledge of the MS's location. The GMSC is thus in charge of obtaining the MSRN (Mobile Station Roaming Number) from the HLR based on the MSISDN (Mobile Station ISDN number, the "directory number" of a MS) and routing the call to the correct visited MSC. The "MSC" part of the term GMSC is misleading, since the gateway operation does not require any linking to a MSC.
This is the term used by [MOU92] to collectively describe the two Short Message Services Gateways described in the GSM recommendations. The SMS-GMSC (Short Message Service Gateway Mobile Switching Centre) is for mobile terminating short messages, and SMS-IWMSC (Short Message Service Inter-Working Mobile Switching Centre) for mobile originating short messages. The SMS-GMSC role is similar to that of the GMSC, whereas the SMS-IWMSC provides a fixed access point to the Short Message Service Centre.


The previous figure also shows the GSM interfaces; they are briefly explained below.
Um The air interface is used for exchanges between a MS and a BSS. LAPDm, a modified version of the ISDN LAPD, is used for signalling.
Abis This is a BSS internal interface linking the BSC and a BTS, and it has not been standardised. The Abis interface allows control of the radio equipment and radio frequency allocation in the BTS.
A The A interface is between the BSS and the MSC. The A interface manages the allocation of suitable radio resources to the MSs and mobility management.
B The B interface between the MSC and the VLR uses the MAP/B protocol. Most MSCs are associated with a VLR, making the B interface "internal". Whenever the MSC needs access to data regarding a MS located in its area, it interrogates the VLR using the MAP/B protocol over the B interface.
C The C interface is between the HLR and a GMSC or a SMS-G. Each call originating outside of GSM (i.e., a MS terminating call from the PSTN) has to go through a Gateway to obtain the routing information required to complete the call, and the MAP/C protocol over the C interface is used for this purpose. Also, the MSC may optionally forward billing information to the HLR after call clearing.
D The D interface is between the VLR and HLR, and uses the MAP/D protocol to exchange the data related to the location of the MS and to the management of the subscriber.
E The E interface interconnects two MSCs. The E interface exchanges data related to handover between the anchor and relay MSCs using the MAP/E protocol.
F The F interface connects the MSC to the EIR, and uses the MAP/F protocol to verify the status of the IMEI that the MSC has retrieved from the MS.
G The G interface interconnects two VLRs of different MSCs and uses the MAP/G protocol to transfer subscriber information, during e.g. a location update procedure.
H The H interface is between the MSC and the SMS-G, and uses the MAP/H protocol to support the transfer of short messages.
I The I interface (not shown in Figure 1) is the interface between the MSC and the MS. Messages exchanged over the I interface are relayed transparently through the BSS.

Protocols over the A, A-Bis and Um interfaces

Figure 6 below shows the signalling protocols between the MS and BTS, between the BTS and BSC, and between the BSC and the MSC.


The CM, MM and RR layers together correspond to layer three in the ISO OSI protocol suite, and layer two is composed of LAPD and LAPDm. Customarily, the lower three layers terminate in the same node. Not so in GSM, where the functionality is spread over distinct functional entities with standardised interfaces between them. For instance, the RR 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 manages the circuit oriented services; Supplementary services management, which allows modifications and checking of the supplementary services configuration; Short Message Services, which provides point-to-point short message services.
MM The Mobility Management (MM) layer is in charge of maintaining the location data, in addition to the authentication and ciphering procedures.
RR The Radio Resource (RR) Management layer is in charge of establishing and maintaining a stable uninterrupted communications path between the MSC and MS over which signalling 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.).
RR' The RR' layer is the part of the RR functionality which is managed by the BTS.
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 Protocol for the ISDN D-channel) protocol. The main modifications are due to the tight synchronisation required in TDMA and bit error protection mechanism required over the air-interface (and in GSM handled by layer 1), making the corresponding functionality 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.
BTSM The Base Transceiver Station Management (BTSM) is responsible for transferring the RR information (not provided for in the BTS by the RR' protocol) to the BSC.
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 two parts, the BSSMAP and the DTAP (not shown in the above figure). The message exchanges are handled by SS7. Messages which are not transparent to the BSC are carried by the Base Station System Management Application Part (BSSMAP), which supports all of the procedures between the MSC and the BSS that require interpretation and processing of information related to single calls, and resource management. The messages between the MSC and MS which are transparent to the BSC (MM and CM messages) are catered for by the Direct Transfer Application Part (DTAP).
SCCP The Signalling Connection Control Part (SCCP) from SS7.
MTP The Message Transport Part (MTP) of SS7.

Logical and physical channels

GSM distinguishes between physical channels (the timeslot) and logical channels (the information carried by the physical channels). Several recurring timeslots on a carrier constitute a physical channel, which are used by different logical channels to transfer information - both user data and signalling. The GSM traffic and associated control channels are illustrated in Figure 7 below.

GSM channels and timeslots

Common channels
The forward common channels are used for broadcasting bulletin board information, paging and response to channel requests. The return common channel is a slotted Aloha type random access channel used by the MS to request channel resources before timing information is conveyed by the BSS, and uses a burst with an extended guard period.

Dedicated point-to-point channels.
The dedicated point-to-point channels are divided into two main groups, the dedicated signalling channels and the traffic channels. The dedicated signalling channels are used to set-up the connection, and the traffic channel of a variety of rates is used to convey the user information once the session is established. Both channel types have in-band signalling: SACCH for e.g. link monitoring, and FACCH for time-critical signalling during e.g. a handover. The FACCH "steals" the entire traffic burst for signalling.

These logical channels are defined in GSM:

TCHf Full rate traffic channel.
TCH h Half rate traffic channel.
BCCH Broadcast Network information, e.g. for describing the current control channel structure. The BCCH is a point-to-multipoint channel (BSS-to-MS).
SCH Synchronisation of the MSs.
FCH MS frequency correction.
AGCH Acknowledge channel requests from MS and allocate a SDCCH.
PCH MS terminating call announcement.
RACH MS access requests, response to call announcement, location update, etc.
FACCHt For time critical signalling over the TCH (e.g. for handover signalling). Traffic burst is stolen for a full signalling burst.
SACCHt TCH in-band signalling, e.g. for link monitoring.
SDCCH For signalling exchanges, e.g. during call setup, registration / location updates.
FACCHs FACCH for the SDCCH. The SDCCH burst is stolen for a full signalling burst. Function not clear in the present version of GSM (could be used for e.g. handover of an eight-rate channel, i.e. using a "SDCCH-like" channel for other purposes than signalling).
SACCHs SDCCH in-band signalling, e.g. for link monitoring

TDMA structure and throughputs

Figure 5 below shows a simplified diagram of the GSM TDMA format and the structure of the Normal burst.

TDMA structure

The Normal burst has a throughput after coding of 22.8 kbps, and offers full rate voice at a net bitrate of 13 kbps and data at up to 9.6 kbps. GSM has also specified a half-rate service by time-multiplexing two users onto the TDMA structure. This service offers a gross bitrate of 11.4 kbps, and data at 4.8 kbps.
Lloyd Wood (
last updated 24 April 2007