The GSM Mobile Telephony System

This folio discusses the GSM mobile telecom system that is progressively fashionable and established throughout the planet. The term Global System for Mobile communication sometimes suggests that the GSM common place and protocols within the frequency spectrum around 900MHz there\'s conjointly DCS1800 - GSM protocols however at completely different air frequencies around 1800 megacycle - and within the u.  s., wherever spectrum for private Communication Services (PCS) was auctioned at around 1900MHz, operator’s victimization the aptly-named GSM1900 is competitive against a in or dinateness of different standards. As a result of this, the first and most widely-used GSM frequency implementation is additionally turning into referred to as GSM900 and DCS1800 is additionally referred to as GSM1800. However, though the physical frequencies used take issue, the protocols and design stay constant.

This page isn't associate degree introduction to the fundamentals of cellular telecom. (That article covers variety of standards fashionable within the u.  S.likewise as describing cells and frequency use.)

A detailed description of the GSM system may be found in [MOU92] and in [BJU95], additionally to the Global System for Mobile communication recommendations. Associate degree index to GSM data is accessible. This summary Of the GSM System and Protocol design is value reading, as is another summary.

The following sections can in short describe the purposeful entities, the logical, physical channel structure and the radio interface waving procedure and therefore the TDMA structure supported GSM.

Some GSM system parameters are listed within the table below:

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 One(1):  below shows the GSM system design that consists of the change system, the bottom station system and therefore the user instrumentality.

The purposeful entities ar in short explained as follows:

MS=Mobile Station. The MS is that the physical instrumentality employed by a subscriber, most frequently a standard hand-held cellphone.

BTS=Base Transceiver Station. The BTS includes the radio transmission and reception devices, and conjointly manages the signal process associated with the air interface.

TRAU=The Transcoder Rate adapter Unit. The TRAU (not shown within the higher than figure) functionally belongs to the BTS. The TRAU permits the employment of lower rates (32, sixteen or eight kbps) over the A-bis interface rather than the sixty four kbps ISDN rate that the Master of Science is intended. The TRAU may be placed at the BTS, the BSC, or (immediately before of) the Master of Science.

BSC=Base Station Controller. The BSC manages the radio interface, in the main through the allocation, unleash and relinquishing of radio channels.

BSS=Base Station System. The BSS consists of a BSC and one or additional BTSs.

MSC=Mobile change Centre. The Master of Science is essentially associate degree ISDN-switch, coordinating  and fixing calls to and from MSs. associate degree Inter-Working operate (IWF) is also needed to adapt GSM specific rates thereto employed in a selected PSTN/ PLMN.

VLR=Visitor Location Register. The VLR contains all the subscriber knowledge, each permanent and temporary, that are necessary to regulate a MS within the MSCs coverage space. The VLR is often accomplished as associate degree integral a part of the Master of Science, instead of a separate entity.

AuC=Authentication Centre. The Auto defenses Undies de Colombia information contains the subscriber authentication keys and therefore the algorithmic rule needed to calculate the authentication parameters to be transferred to the HLR.

HLR=Home Location Register. The HLR information is employed to store permanent and semi-permanent subscriber data; in and of itself, the HLR can perpetually grasp within which location space the MS is (assuming the MS is in an exceedingly coverage area), associate degreed this knowledge is employed to find an MS within the event of a MS terminating decision set-up.

EIR=Equipment Identity Register. The EIR information contains data on the MS and its capabilities. The IMEI (International Mobile Subscriber Identity) is employed to interrogate the EIR.

GMSC=Gateway Mobile change Centre. The GMSC is that the purpose to that a MS terminating decision is at first routed, with none information of the MS\'s location. The GMSC is therefore to blame of getting the MSRN (Mobile Station Roaming Number) from the HLR supported the MSISDN (Mobile Station ISDN range, the \"directory number\" of a MS) and routing the decision to the proper visited Master of Science. The \"MSC\" a part of the term GMSC is dishonest, since the entry operation doesn’t/ need any linking to a Master of Science.

SMS-G=This is that the term employed by [MOU92] to conjointly describe the 2 Short Message Services Gateways delineates within the GSM recommendations. The SMS-GMSC (Short Message Service entry Mobile change Centre) is for mobile terminating short messages, and SMS-IWMSC (Short Message Service Inter-Working Mobile change Centre) for mobile originating short messages. The SMS-GMSC role is analogous thereto of the GMSC, whereas the SMS-IWMSC provides a set access purpose to the Short Message Service Centre.

Interfaces.The previous figure conjointly shows the GSM interfaces; they\'re in short explained below.

Um=The air interface is employed for exchanges between a MS and a BSS. LAPDm, a changed version of the ISDN LAPD, is employed for signaling.

Abis=This could be a BSS internal interface linking the BSC and a BTS, and it\'s not been standardized. The Abis interface permits management of the radio instrumentality and frequency allocation within the BTS.

A=The A interface is between the BSS and therefore the Master of Science. The A interface manages the allocation of appropriate radio resources to the MSs and quality management.

B=The B interface between the Master of Science and therefore the VLR uses the MAP/B protocol. Most MSCs are related to a VLR, creating the B interface \"internal\". Whenever the Master of Science desires access to knowledge concerning a MS placed in its space, it interrogates the VLR victimization the MAP/B protocol over the B interface.

C=The C interface is between the HLR and a GMSC or a SMS-G. every decision originating outside of GSM (i.e., a MS terminating decision from the PSTN) has got to bear a entry to get the routing data needed to finish the decision, and therefore the MAP/C protocol over the C interface is employed for this purpose. Also, the Master of Science might optionally forward charge data to the HLR once decision clearing.

D=The D interface is the information associated with the situation of the MS and to the management of the subscriber.

E=The E interface interconnects 2 MSCs. The E interface exchanges knowledge associated with relinquishing between the anchor and relay MSCs victimization the MAP/E protocol.

F=The F interface connects the Master of Science to the EIR, and uses the MAP/F protocol to verify the standing of the IMEI that the Master of Science has retrieved from the MS.

G=The G interface interconnects 2 VLRs of various MSCs and uses the MAP/G protocol to transfer subscriber data, during e.g. a location update procedure.

H=The H interface is between the Master of Science and therefore 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 that the interface between the Master of Science and therefore the MS. Messages changed over the I interface ar relayed transparently through the BSS.

Protocols over the A, A-Bis and Um interfaces:

Figure Two(2): below shows the signaling,

The CM, metric linear unit and RR layers along correspond to layer 3 within the ISO OSI protocol suite, and layer 2 consists of LAPD and LAPDm. Usually, the lower 3 layers terminate within the same node. Not therefore in GSM, wherever the practicality is meet distinct purposeful entities with standardized interfaces between them. For example, the RR a part of layer 3 is meet the MS, BTS, BSC, and MSC.

CM=The Communication Management (CM) layer consists of fixing calls at the users' request. Its functions square measure divided in three: decision management, which manages the circuit bound services; Supplementary services management, that permits modifications and checking of the supplementary services configuration; Short Message Services, that provides point-to-point short message services.

MM=The quality Management (MM) layer is to blame of maintaining the placement knowledge, additionally to the authentication and ciphering procedures.

RR=The Radio Resource (RR) Management layer is to blame of creating and maintaining a stable uninterrupted communications path between the MS and MS over that waving and user knowledge will be sent. Handovers square measure a part of the RR layers responsibility. Most of the functions square measure controlled by the BSC, BTS, and MS, the' some square measure performed by the MS (in explicit for inter-MSC handovers).

RR=The RR' layer is that the part of the RR practicality that is managed by the BTS.

LAPDm=The layer 2 protocol is provided for by LAPDm over the air-interface. This protocol could be a changed version of the LAPD (Link Access Protocol for the ISDN D-channel) protocol. the most modifications square measure thanks to the tight synchronization needed in TDMA and bit error protection mechanism needed over the air-interface (and in GSM handled by layer 1), creating the corresponding practicality of the LAPD protocol redundant (and so wasteful over the air-interface). The LAPD frame flags square measure replaced by a length indicator, and also the FEC field is removed.

BTSM=The Base Transceiver Station Management (BTSM) is to blame for transferring the RR info (not provided for within the BTS by the RR' protocol) to the BSC.

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

BSSAP=The Base Station System Application half (BSSAP) is split into 2 elements, the BSSMAP and also the DTAP (not shown within the on top of figure). The message exchanges square measure handled by SS7. Messages that aren't clear to the BSC square measure carried by the bottom Station System Management Application half (BSSMAP), that supports all of the procedures between the MS and also the BSS that need interpretation and process of knowledge associated with only cries, and source administration. The communications concerning the MS and MS that square measure clear to the BSC (MM and CM messages) square measure catered for by the Direct Transfer Application half (DTAP).

SCCP=The Signaling association management half (SCCP) from SS7.

MTP=The Message Transport half (MTP) of SS7.

Logical and physical channels:

GSM distinguishes between physical channels (the timeslot) and logical channels (the info carried by the physical channels). Many continual timeslots on a carrier represent a physical channel, that square measure employed by totally different logical channels to transfer info - each user knowledge and signaling. The GSM traffic and associated management channels square measure illustrated in Figure seven below.

Common channels

The forward common channels square measure used for broadcasting bulletin board info, paging and response to channel requests. The come common channel could be a slotted ciao sort random access channel employed by the MS to request channel resources before temporal arrangement info is sent by the BSS, and uses a burst with an extended guard amount.

Dedicated point-to-point channels.

The dedicated point-to-point channels square measure divided into 2 main teams, the dedicated gesturing channels and also the traffic channels. The dedicated motioning channels square measure accustomed set-up the association, and also the traffic channel of a spread of rates is employed to convey the user info once the session is established. Each channel sorts have in-band signaling: SACCH for e.g. link watching and FACCH for time-critical signing throughout e.g. a handover. The FACCH "steals" the whole traffic burst for waving.

Figure Three (3): These logical channels square measure outlined in GSM:

TCHf= Full rate traffic channel.

TCH h= Half rate traffic channel.

BCCH= Broadcast Network info, e.g. for describing this management channel structure. The BCCH could be a point-to-multipoint channel (BSS-to-MS).

SCH=Synchronization of the MSs.

FCH=MS frequency correction.

AGCH=Acknowledge channel requests from MS and allot a SDCCH.

PCH=MS terminating decision announcement.

RACH=MS access requests, response to decision announcement, location update, etc.

FACCHt=For time important beckoning over the TCH (e.g. for relinquishing signaling). Traffic burst is purloined for a full signaling burst.

SACCHt=TCH in-band indicating, e.g. for link watching.

SDCCH=For signaling exchanges, e.g. throughout decision setup, registration / location updates.

FACCHs=FACCH for the SDCCH. The SDCCH burst is purloined for a full gesticulating burst. perform not clear within the gift version of GSM (could be used for e.g. relinquishing of AN eight-rate channel, i.e. employing a "SDCCH-like" channel for alternative functions than nodding).

SACCHs=SDCCH in-band signaling, e.g. for link watching TDMA structure and throughputs

Figure Four (4): below shows a simplified diagram of the GSM TDMA format and also the structure of the conventional burst. 

The Normal burst includes a outturn when secret writing of twenty-two.8 kbps, and offers full rate voice at a web bitrate of thirteen kbps and knowledge at up to nine.6 kbps. GSM has additionally such a half-rate service by time-multiplexing 2 users onto the TDMA structure. This service offers a gross bitrate of eleven.4 kbps, and knowledge at four.8 kbps.