gsm layer3

7/29/2019 GSM Layer3

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GSM Call Processing&

Layer-3 Messages

By: Syed Farhan Raza NaqviIn-House Training RF Central 3


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Following are the highlights of todays training session.

GSM Signaling Matrix.

Layer-3 Messages Classification.

Radio Interface Messages.

Mobile Phone Modes during a Call.

Mobile Originating Call Flow.

Mobile Terminating Call Flow.

Handover Flows for Intra/Inter Cell & Inter MSC HOs.

SMS & Location Update Procedures.

Introduction


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GSM Signalling Matrix


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The fig above gives an overview of GSM Protocol Matrix.

The MSC uses ISUP/TUP protocols for PSTN signaling. It has MAP forsignaling to database applications like HLR, VLR, EIR, AUC, SMSSC, and GMSC.

On the A interface it has GSM specific protocol as BSSAP. The BSSAP

comprises of DTAP and BSSMAP.

DTAP signaling is direct signaling between mobile and MSC, whereas

BSSMAP is mobile related signaling between MSC and BSC.

The BSC also uses the BSSAP but only the BSSMAP part of it for MSC.

On the BTS side GSM has defined its own set of protocols in 08.58 which

have no official name but we term it as RSM (Radio Subsystem Management.

The BSC on layer 2 uses LAPD (Link Access Procedures on D - Channel)

which is an ISDN protocol and includes functions for error correction and

detection and frame delimitation by insertion of flags.



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The BTS on the mobile has again set of GSM protocols for physical layer

which is the Radio Air Interface and uses LAPDm for Layer 2 signaling. This isa modified LAPD protocol with one difference being some LAPD functions

like error correction are removed and used at Layer 1, and second major

difference being the LAPDm messages cannot be lengthy due limitation of

burst length.

The BTS on the Mobile user part uses RIL3 which is again a GSM protocol

for Radio Resource Management. The Mobile on the other hand in RIL3 has

DTAP for MSC signaling and RR for Radio Resource signaling



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MAP (Mobile Application Part)

MAP messages are between Switching and Database interfaces.

MAP messages are not present on the Air Interface.

Not much significance related to Drive Testing.

Can be useful in analyzing some Protocol failures experienced on the

air interface during drive testing.


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BSSAP (BSS Application Part)

BSSAP is used between BSC and MSC on A interface.

It uses the services of MTP and SCCP and hence it is based on C7.

BSSAP deals with signaling between MSC and BSC and also MSC and

Mobile.


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BSSAP has two subsets the DTAP and the BSSMAP.

DTAP deals with the signaling messages directly between the MSC and

Mobile. The BSC and/or BTS just routes theses messages. DTAP messagesare all connection oriented messaged.

BSSMAP messages are MS related messages but originate and

terminate at the BSC, from where BSC modifies the message and send it

across in different message (RSM, 08.58 messages).

BSSMAP:

Between BSC and MSC

Linked to certain messages on the Air Interface

Useful in troubleshooting certain Blocked Calls and Dropped Calls

problems in conjunction with Drive Testing

DTAP (Direct Transfer Application Part):

Transparent Messages between MSC and MS

Further divided into sub-categories ( MM ,CC,SMS,SS)


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BSSMAP Messages

Assignment Complete

Assignment Failure

Assignment Request

Candidate Response

Cipher Mode Command

Cipher Mode Complete

Classmark Update

Complete Layer 3 Information

Handover Candidate EnquiryHandover Command

Handover Complete

Handover Detect

Handover Failure

Handover Performed

Handover RequestHandover Request Acknowledge

Handover Required Reject

Trace Invocation

Queuing Indication


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DTAP Messages

Types of DTAP Messages:

CC : Call control and call related SS Messages

Related to call setup and activation/deactivation of SS between. MM : Mobility Management Messages

Location updates, security procedures and connection management.

SS : Non-Call related Supplementary Messages

Like call barring, call forwarding etc.

SMS : Short Message Service Messages

Related of transmission and reception of SMS

DTAP Messages:

MM Message

Authentication RequestAuthentication Response

CM Re-Establishment Request

CM Service Accept

CM Service Reject

CM Service Request

Identity RequestIdentity Response

IMSI DetachLocation Updating Accept

Location Updating Reject

Location Updating Request

TMSI Reallocation Command

TMSI Reallocation Complete


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DTAP Messages

CC Messages

Alerting

Call Confirmed

Call Proceeding

Connect

Connect Acknowledge

Disconnect

Emergency Setup

FacilityHold

Hold Acknowledge

Modify

Modify Complete

Notify

ProgressRelease

Release Complete

Retrieve

Retrieve Acknowledge

Setup

SS Messages

Hold

Hold Acknowledge

Hold Reject

Retrieve

Retrieve Acknowledge

Retrieve Reject

Facility

Register

SMS Messages

CP-Ack

CP-DataCP-Error

RP-Ack

RP-Data

RP-Error

All types of DTAP messages described

are RIL3 (Radio Interface layer 3)

messages.


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Radio Interface

Radio Interface (Layer 1):

Following are the functions of layer 1:

Mapping of Logical Channels to Physical Channel

Channel Coding

Ciphering

Cell Selection and Reselection

Connection Establishment on Logical Channels

Measurement of serving and neighboring cells signal level Setting of Timing Advance

Transmit Power Control as commanded by BTS.

Radio Interface (Layer 3) Messages Categories:

DTAP Messages: Transparent to BSC, CC, MM, SS, SMS as described earlier. RR Messages: Radio Resource messages specific to BTS and MS.


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Radio Interface

RR Messages:

Assignment Command

Assignment CompleteAssignment Failure

Channel Mode Modify

Channel Mode Modify Acknowledge

Channel Release

Channel Request

Ciphering Mode CommandCiphering Mode Complete

Classmark Change

Frequency Redefinition

Handover Access

Handover Command

Handover CompleteHandover Failure

Immediate Assignment

Immediate Assignment Extended

Immediate Assignment Reject

Measurement Report

Paging Request Type 1Paging Request Type 2

Paging Request Type 3

System Information Type 1



System Information Type 4System Information Type 5


System Information Type 2bis


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Mobile Phone Mode


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Layer-2 LAPDm

LAPDm signaling is done using Frames.

LAPDm frames contain Address, Control and L3 information.

LAPDm has a window size of 1 (every frame sent must be acknowledged before next

is sent, in acknowledge mode)

Lapdm has two SAPI (Service Access Point Identifier). SAPI 0 for RR and DTAP

signaling & SAPI 3 for SMS.


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Mobile Originating Call


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