OMF001006 GSM Signaling System-BSSAP(BSS) ISSUE2.0

Distribution Sub-layerTo discriminate DTAP and BSSMAP messages To distribute the DTAP messages to radio link L2 access point To converge the DTAP messages received from radio link L2

OMF001006 GSM Signaling System - BSSAP(BSS)

ISSUE 2.0

References

 31033202-BSS Signaling Analysis Manual

 GSM Specification 0808

 GSM Specification 0408

 Distinguish BSSMAP and DTAP messages

 Know some BSSMAP and DTAP message

elements

1 BSSAP Functional Structure

2 Typical Messages

3 BSC Signaling and Interface Tracing

BSSAP Functional Structure

 BSSAP ： BSS Application Part

 BSSMAP ： BSS Management Application Part

 DTAP ： Direct Transfer Part

 CM

 MM

 SCCP ： Signaling Connection Control Part

 MTP ： Message Transfer Part

BSSAPSCCPMTP-3MTP-2MTP-1

BSSAP Functional Structure

Interface

Correspondin

g BSC Processing Modules

Distribution Function

Corresponding MSC Application and Control

Modules

Distribution Sub-layer

To discriminate DTAP and BSSMAP messages

To distribute the DTAP messages to radio link L2 access point

To converge the DTAP messages received from radio link L2 to

A-interface signaling link

Functions of BSSAP Distribution Sub-layer

Components of Distribution Sub-layer

Octet 1 Octet 2

L3 Messages

Distribution Data Unit

DTAP takes charge of transferring transparent L3 messages from MS

to MSC or from MSC to MS which BSS will not analyze It is transferred between BSS and MSC by using SCCP service class 2 (basic connection-oriented class)

III L3 Information

User data

I Distribution Data Unit

II Length Indication

DTAP Distribution Data Unit

Distribution Data Unit has two parameters:

 Discriminator : 00000000

 Data Link Connection Identity ： DLCI ： :

 To indicate the data link type that should be used on the radio interface for the messages sent from MSC to BSS

 To indicate the data link type on the radio interface that reports the message which is sent by BSS to MSC.

Discriminator:

Bit: 8 7 6 5 4 3 2 1

0 0 0 0 0 0 0 D D=1 stands for transparent transmission (DTAP)

 The transmission of BSSMAP messages upon SCCP is for the BSSMAP functional entities of both MSC and BSS to exchange information.

 The distribution data unit of BSSMAP messages only has the discriminator

‘D’ should be equal to 0 which stands for non-transparent transmission.

Discriminator:

Bit: 8 7 6 5 4 3 2 1

0 0 0 0 0 0 0 D D=0 stands for non-transparent transmission (BSSAP)

1 BSSAP Functional Structure

2 Typical Messages

3 BSC Signaling and Interface Tracing

2 Typical Messages

Section 1 BSSMAP Message

Section 2 DTAP Message

Assignment Request

 This message is sent from the MSC to the BSS via the relevant SCCP connection in order to request the BSS to assign radio resource(s), the attributes of which are defined within the message

 The message may also include the terrestrial circuit to be used

INFORMATION ELEMENT REFERENCE DIRECTION TYPE LEN

Assignment Failure

The ASSIGNMENT FAILURE message is sent from the BSS to the MSC via the relevant SCCP connection It indicates that there has been a failure in the assignment process at the BSS and that the assignment procedure has been aborted

INFORMATION ELEMENT REFERENCE DIRECTION TYPE LEN

Handover Required

 This message is sent from the BSS to the MSC to indicate that for a given MS which already has dedicated radio resource(s) assigned, a handover is required for the reason given by the cause element

 The message is sent via the BSSAP SCCP connection associated with the dedicated resource(s)

Handover Request Acknowledge

 This message is sent from the BSS to the MSC and indicates that the request to support a handover at the target BSS can be supported by the BSS, and also to which radio channel(s) the MS should be directed

 The message is sent via the BSSAP SCCP connection associated with the dedicated resource

Handover Required Reject

This message is sent from the MSC to the BSS via the relevant SCCP connection It indicates to the BSS that the HANDOVER REQUIRED message has not resulted in handover

Handover Complete

 This message is sent from the BSS to the MSC via the relevant SCCP connection

 It indicates that the correct MS has successfully accessed the target cell

Handover Failure

This message is sent from the BSS to the MSC via the relevant SCCP connection It indicates to the MSC that there has been a failure in the resource allocation process

on handover, and that the handover has been aborted

 This message is sent from the MSC to the BSS and contains sufficient information to allow the paging message to be transmitted by the correct cells at the correct time

 This message is sent as a connectionless SCCP message

Complete Layer 3 Information

 The message is sent from the BSS to the MSC on receipt of the initial layer 3 message on a dedicated channel, e.g PAGING RESPONSE, LOCATION UPDATING REQUEST, CM REESTABLISHMENT REQUEST, CM SERVICE REQUEST, IMSI DETACH

 The BSS does not analyse the contents of the initial layer 3 message, other than the Classmark information

 The message is sent via the BSSAP SCCP connection established for the associated dedicated resource(s)

2 Typical Messages

Section 1 BSSMAP Message

Section 2 DTAP Message

Location Updating Accept

This message is sent by the network to the mobile station to indicate that updating or IMSI attach in the network has been completed

IEI Information element Type / Reference Presence Format Length

Mobility management

protocol discriminator Protocol discriminator/10.2 M V 1/2

Location Updating Accept

Location area

identification Location area identification/10.5.1.3 M V 5

Location Updating Reject

This message is sent by the network to the mobile station to indicate that updating or IMSI attach has failed

IEI Information element Type / Reference Presence Format Length

Mobility management

protocol discriminator Protocol discriminator10.2 M V 1/2

Skip Indicator Skip Indicator

Location Updating

Disconnect (network to mobile station direction)

This message is sent by the network to indicate that the end-to-end connection is cleared

IEI Information element Type / Reference Presence Format Length

Disconnect (mobile station to network direction)

Disconnect

This message is sent by the mobile station to request the network to clear an end-to-end connection.

The message contents are similar to those of

“Disconnect (network to mobile station direction) “

Setup (mobile originating call

Setup

1 BSSAP Functional Structure

2 Typical Messages

3 BSC Signaling and Interface Tracing

Means of BSC Signaling and Interface Tracing

 GSM Interface Tracing Review

 NO7 Message Tracing

 NO7 Message Tracing Review

Comparison of different means

 Signaling Analyzer

 Expensive, additional investment needed.

 Can trace all the signaling of A-interface and Abis-interface from the phy sical layer.

 GSM Interface Trace

 No additional equipment and investment needed.

 A-interface trace – to trace BSSAP and SCCP in BSC.

 A-bis interface trace – to trace A-bis interface L3 messages.

 Um interface trace – to trace Um interface L3 messages.

Relevant GSM Specifications

 GM 0808 ： A interface

 GM 0858 ： A-bis interface

 GM 0408 ： Um interface