Layer 3 Message in GSM

GSM Layer III Messages

Signaling Protocol In GSM

In  telecommunication system, signaling is required to coordinate the distributed entities of the network.

The signaling protocol in GSM is structured into three general layers as shown in the following figure.

•        Layer 1 (Physical layer)

 It uses the data channel structures over the air interface

•        Layer 2 (Data link layer)

It provides error-free transmission between adjacent entities, based on the ISDN’s LAPD protocol for the Um and Abis interfaces, and on SS7’s Message Transfer Protocol (MTP) for the other Layer interfaces

•       Layer 3 (Networking or messaging layer)

Responsible for the communication of network resources, mobility, code format and call-related management messages between various network entities

 Layer III is the GSM signaling protocol, which is divided into 3 sub-layers:

Radio Resource Management to control the setup, maintenance, and termination of radio and fixed channels, including handovers;

Mobility Management to manage the location updating and registration procedures, as well as security and authentication; and

Connection Management to handles general call control and manages Supplementary Services and the Short Message Service.

System Information Type 1

When MS need to know which frequency band to use and which frequencies within the band it should use during frequency hopping

The ARFCN for all frequencies used in the cell.

RACH control parameters

Information is also provided instructing the MS how to perform access to the system. This information is provided in the RACH control parameters information element 

Cell Bar access (CBA, CELL_BAR_ACCESS)

Cell barred for access is denoted by using one bit only (yes/no).

Call reestablish permitted (RE)

MAXRET (Max retransmissions)

Tx-Interger

Access control class ( ACC) is described using a bit map with 16 bits. All MSs are spread out on ACC 0-9. Priority group MSs use ACC 11-15. A bit set to 1 indicates that the class is barred for access.

Emergency calls EC=0 means that all MSs are allowed to make emergency calls, while EC=1 means that only  MSs belonging to priority classes 11-15 are allowed to make emergency calls. 

System Information Type 2

The neighbor cells description information element denotes the frequencies of the BCCH carriers to be monitored by the MSs in the cell.

BA-IND

This is a BCCH allocation sequence number indication. BAIND can have the value 0 or 1. When the operator changes the frequency list, the BA-IND changes from 1 to 0 or vice versa.

BA ARFCN

These are absolute RF channels on which the MSs should signal strength  perform measurements.

System Information Type 3

SI3 msg are showing the information of CGI, Cell selection and basic idle mode parameter.

Location Area Identification (LAI)

MCC Mobile Country Code (3 digits)

MNC Mobile Network Code (2-3 digits)

LAC Location Area Code (2 octets: 0-65535 different LACs)

Cell Selection Parameters

Maximum power level of control channel (MS_TXPWR_MAX_CCH)

Minimum receiving level of MS permitted to access (RXLEV_ACCESS_MIN)

Additional reselection parameter indicator (ACS)

Half-rate supported(NECI)

Power control indication (PWRC): The power control indicator equal to 1 means set

Discontinuous transmission (DTX)

Radio link timeout (RLT)

ATT Attach/Detach allowed 

AGBLK (BS_AG_BLKS_RES) This is the number of CCCH blocks reserved for the AGCH

CCCH_CONFThis is the number of Basic Physical Channels used for the CCCH

CCCH-CONF : (0) 1 basic physical channel used for CCCH, not combined with SDCCHs

MFRMS (BS_PA_MFRMS) This is the number of 51 multiframe periods for transmission of Paging Request messages in the same paging group (2-9).

T3212 This is the time-out value for MS periodic location updating. T3212 has values from 0 to 255.  (Time for periodic LU = 6min * T3212 value)

System Information Type 4

In system information type 4, MSs are informed if the cell broadcast function is used in this cell and on what frequency the CBCH is found.

The LAI, the cell selection parameters, the RACH control parameters and rest octets are also included in the type 4 message.

CBCH Channel Description (Optional)

In this information element, the channel used for SMS cell broadcast is described.

CHANNEL TYPE

This is the channel combination and sub-channel used for CBCH. For example, SDCCH/4 + SACCH/C4 or CBCH,  sub channel 2

TN : This represents the time slot number

TSC : This is the training sequence code. BCC part of BSIC is recommended.

H : This is the hopping parameter, where H=0 means single RF channel and H=1 means RF hopping channel.

type 3 rest octets. These are PI, CBQ, CELL_RESELECT_OFFSET, TEMPORARY_OFFSET, and PENALTY_TIME

System Information Type 5

It describes the neighbor cell BCCH frequencies(BA2 list). It is mandatory and is transmitted on SACCH. MS can read the frequency described in SYS INFO 5 in dedicated mode, report the relevant information of neighbor cells in measurement report, and use it as the basis for handover.

Neighbor Cells Description

The neighbor cells description information element denotes the frequencies of the BCCH carriers to be monitored by the MSs in the cell.

System Information Type 6

When in active mode, the MS needs to know if the LAI changes. In case the LAI changes the MS must perform location updating when the call is released.

 If the MS changes between cells (within the location area) where RDLINKOUT or DTX conditions differ the new cell options must be reported to the MS.

NCC permitted information element is also included

System Information Type 7 & 8

System information type 7 & 8 are optionally sent on BCCH extended if system information type 4 does not contain all information needed for cell re-selection. 

Measurement Report

The mobile unit always scans BCCH of up to 16 neighboring cells, and forms a list of the six best candidates for possible handover. This information is transmitted to current Base Station at least once per second. BSC and MSC use this information for handover algorithm.