GSM Protocols and Interface

What are the Protocols mapped over GSM interface?

 Air interface Um:


Um interface is the air interface in GSM network.It is used to communicate MS and BTS. As the frequency spectrum is limited resource so time- and frequency-division multiple access (TDMA/FDMA) modulation process was chosen. The FDMA part involves the division by frequency of the (maximum) 25 MHz allocated bandwidth into 124 carrier frequencies spaced 200 kHz apart. One or more carrier frequencies are assigned to each base station. Each of these carrier frequencies is then divided in time, using a TDMA scheme. The fundamental unit of time in this TDMA scheme is called a burst period and it lasts approx. 0.577 ms. Eight burst periods are grouped into a TDMA frame (approx. 4.615 ms), which forms the basic unit for the definition of logical channels. One physical channel is one burst period per TDMA frame.

The International Telecommunication Union (ITU) manages international allocation of radio spectrum (among many other functions), has allocated the following bands:


Uplink: 890±915 MHz (= mobile station to base station)

Downlink: 935±960 MHz (= base station to mobile station).

GSM1800 (previously: DCS-1800):

Uplink: 1710±1785 MHz

Downlink: 1805±1880 MHz

GSM1900 (previously: PCS-1900):

Uplink: 1850±1910 MHz

Downlink: 1930±1990 MHz

Protocols on the air interface:

Layer 1:

Enables physical transmission (TDMA, FDMA, etc.)

Except on the air interface, PCM 30 or ISDN links are used

Layer 2:

 LAP-Dm protocol is used in this layer

 Layer 3: Contains the following sub layers which are as below

Radio resource management (RR)

Mobility management (MM)

Connection management (CM)


Abis interface:

The Abis interface lies within the base station subsystem (BSS) and represents the dividing line between the BSC function and the BTS. The BSC and BTS can be connected using leased lines, radio links or metropolitan area networks (MANs).

Basically, two channel types exist between the BSC and BTS:

  • Traffic channels (TCH): Can be configured in 8, 16 and 64 kbit/s formats and transport user data
  • Signalling channels: Can be configured in 16, 32, 56 and 64 kbit/s formats and are used for signalling purposes between the BTS and BSC.

Protocols on the Abis interface: The following protocols are used:

  • Layer 1 : 048 Mbit/s (ITU-T: E1) or 1.544 Mbit/s (ANSI: T1) PCM facility with 64/32/16 kbit/s signalling channels and16 kbit/s traffic channels (4 per timeslot)
  • Layer 2 : Here, the LAP-D protocol is used as the transport mechanism for data messaging between the BTS and BSC.Within GSM the SAPI refers to the link identifier transmitted in the LAPD protocol that was inherited from ISDN.
  • Layer 3: BTS management (BTSM) works mainly in this layer. BTSM distinguishes three logical signalling connections with the SAPI (Service Access Point Identifier). SAPI 0 is used by all messages coming from or going to the radio interface. SAPI 62 provides 0&M message transport between the BTS and BSC. SAPI 63 is used for dynamic management of TEIs as well as for layer 2 management functions.

A interface:

The A interface lies between the BSC and MSC. If the BSC contains the transcoder equipment (TCE), a traffic channel (TCH) occupies a complete 64 kbit/s timeslot in the 2 Mbit/s or 1.544 Mbit/s PCM link. Out of 32 available timeslots on the PCM link, a maximum of 30 traffic channels can be operated simultaneously,since at least 2 timeslots are needed for control and signaling purposes(TS0 for FAS/NFAS and another TS for signalling, usually TS16) on PCM facilities.

One signaling channel supports many 64 kbit/s PCM facilities between one BSC and the MSC. Normally two active 64 kbit/s timeslots are used for this purpose.If the MSC is equipped with a TCE, the TCHs are converted from 64 kbit/s to 16 kbit/s in the transcoder equipment. If the BCS does not contain a TCE, then the TCHs are 16 kbit/s on the A interface. Between the BSC and MSC, the TCHs are recorded from 64 kbit/s to16 kbit/s in the transcoder equipment (TCE).

Protocols on the A interface:

 The signalling protocol (layer 2+3) between the BSC and MSC is based on the SS7 standard, but is transmitted along with the user data within the PCM facility. Normally timeslot 16 (TS16) of the 64 kbit/s frame is used.

The following protocols are used in A interface:

Layer 1: 2.048 Mbit/s (ITU-T: E1) or 1.544 Mbit/s (ANSI: T1) PCM link.

Layer 2: SS7-based protocols are used for layer 2; the message transfer part (MTP) protocol is responsible for transmission security between the BSC and MSC and the signaling connection control part (SCCP) protocol allows global addressing of network elements which is Global Address (GT) Translation. Depending on which traffic routing is happened to other operator network

MTP and SCCP also perform layer 3 functions. SCCP is used to transport DTAP and base station management application part (BSSMAP) messages on the A interface, ensuring both connectionless and connection-oriented message flows. The connections can be related to a specific MS or radio channel.

An SCCP connection can involve the following protocols:

  • From the MS:

MM: CM service request

RR: Paging response

MM: Location updating request

MM: CM re-establishment request.

  • From the MSC: Initiation of an external handover (BSSMAP: handover request).

The MSC always manages an SCCP connection.

Layer 3: Contains the base station system application part (BSSAP) protocol. This layer has multiple parts on the MSC end:

  • The base station management application part (BSSMAP) protocol is the counterpart to the RR protocol on the air interface.
  • The direct transfer application part (DTAP) protocol transmits CC and MM messages and is transmitted transparently through the BTS and BSC.


MSC protocols:

Message Transfer Part (MTP):

The Message Transfer Part (MTP) layer of the SS7 protocol provides the routing and network interface capabilities that support SCCP, TCAP, and ISUP. Message Transfer part (MTP) is divided into three levels.

MTP Level 1 (Physical layer):

It defines the physical, electrical, and functional characteristics of the digital signalling link. Physical interfaces defined include E-1 (2048 kb/s; 32 64 kb/s channels), DS-1 (1544 kb/s; 24 64 kp/s channels), V.35 (64 kb/s), DS-0 (64 kb/s), and DS-0A (56 kb/s).

MTP Level 2(Data Link) :

It provides the reliability aspects of MTP including error monitoring and recovery. (MTP-2) is a signalling link which together with MTP-3 provides reliable transfer of signalling messages between two directly connected signalling points.

MTP Level 3(Network Layer):

It provides the link, route, and traffic management aspects of MTP. MTP 3, thus ensures reliable transfer of the signalling messages, even in the case of the failure of the signalling links and signalling transfer points. The protocol therefore includes the appropriate functions and procedures necessary both to inform the remote parts of the signalling network of the consequences of a fault, and appropriately reconfigure the routing of messages through the signalling network.

MAP (Mobile Application Part):

It used to control queries to the different databases in the mobile radio network (HLR, VLR and EIR). MAP responsibilities include access and location management (e.g. where is the called subscriber location?), MSC-MSC handover, security functions, O&M, SMS and supplementary services.

TCAP (Transaction Capabilities Application Part):

It provides universal calls and functions for handling requests to distributed application processes.

ISUP (ISDN User Part):

It controls interworking (e.g. call setup/takedown) between PLMNs and other networks, and provides the same basic functionalities as TUP.

INAP (Intelligent Network Application Part):

It implements intelligent supplementary services (e.g. free call, time-dependent routing functions in a central service center).

TUP (Telephone User Part):

It implements interworking between PLMNs and other networks. TUP is normally used to provide international connections and is slowly being replaced by ISUP.

Signalling Connection Control Part (SCCP):

The Signalling Connection Control Part (SCCP) layer of the SS7 stack provides provides connectionless and connection-oriented network services and global title translation (GTT) capabilities above MTP Level 3. SCCP is used as the transport layer for TCAP-based services.

For GT translation, STP (Signalling Transfer Point) is used in GSM network as an external entity. In performing GTT, an STP does not need to know the exact final destination DPC(Destination Point Code) of a message. It can, instead, perform intermediate GTT, in which it uses its tables to find another STP further along the route to the destination. That STP, in turn, can perform final GTT, routing the message to its actual destination.

Ref: GSM-pocket Guide book

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