A Interface Protocol Stack
The A interface interconnects a BSC with an MSC and is considered to lie within the GSM core
network. Unlike, the GSM-specific Um and A-bis interfaces, the core network A interface
operates using subsets of the SS7 Signalling standard. These protocols are defined in the 08-
series of the ETSI GSM Recommendations.
network. Unlike, the GSM-specific Um and A-bis interfaces, the core network A interface
operates using subsets of the SS7 Signalling standard. These protocols are defined in the 08-
series of the ETSI GSM Recommendations.
5.8.1 A INTERFACE - LAYER 1
The physical medium within layer 1 is similar to that described for the A-bis interface in
terms of ISDN connections.
terms of ISDN connections.
5.8.2 A INTERFACE - LAYER 1&2 SIGNALLING
A interface signalling is based on the SS7 protocol which essentially covers both layer 1 and 2
of the reference model. Message transport in the SS7 network is realised through the Message
Transfer Part (MTP). MTP provides for both routing and transport of signalling messages.
A slightly modified version of the MTP, MTP’, has been defined for the protected transport of
signalling messages over the A interface.
of the reference model. Message transport in the SS7 network is realised through the Message
Transfer Part (MTP). MTP provides for both routing and transport of signalling messages.
A slightly modified version of the MTP, MTP’, has been defined for the protected transport of
signalling messages over the A interface.
5.8.3 A INTERFACE - LAYER 3
Layer 3 utilises a number of protocols, each providing different functionality. For GSMspecific
signalling between the BSC and the MSC the Base Station Subsystem Application Part
(BSSAP) has been defined which itself is divided into the Direct Transfer Application Part
(DTAP) and the Base Station Subsystem Mobile Application Part (BSSMAP). In addition,
layer 3 messages are transported using the Signalling Connection Control Part (SCCP)
signalling between the BSC and the MSC the Base Station Subsystem Application Part
(BSSAP) has been defined which itself is divided into the Direct Transfer Application Part
(DTAP) and the Base Station Subsystem Mobile Application Part (BSSMAP). In addition,
layer 3 messages are transported using the Signalling Connection Control Part (SCCP)
5.8.3.1 DTAP
The DTAP is used to convey messages between the MS and the MSC i.e. the Call
Control (CC) and Mobility Management (MM) messages. At the A interface they are
transmitted from the MSC with DTAP and then passed transparently through the
BSS, across the A-bis interface to the MS without BTS interpretation.
Control (CC) and Mobility Management (MM) messages. At the A interface they are
transmitted from the MSC with DTAP and then passed transparently through the
BSS, across the A-bis interface to the MS without BTS interpretation.
5.8.3.2 BSSMAP
The BSSMAP is the protocol responsible for all the administration of the BSS radio
resources. However, some of these functions require involvement of the MSC, for
example, some handover situations and release of connections or channels. Such
actions are initiated and controlled by the MSC. This control is the responsibility of
the BSSMAP RR messages are mapped and converted within the BSC into BSSMAP
procedures and messages and vice versa.
resources. However, some of these functions require involvement of the MSC, for
example, some handover situations and release of connections or channels. Such
actions are initiated and controlled by the MSC. This control is the responsibility of
the BSSMAP RR messages are mapped and converted within the BSC into BSSMAP
procedures and messages and vice versa.
5.8.3.3 SCCP
For signalling transactions between the MSC and the MS (i.e. CM and MM) it is
necessary to establish and identify distinct logical connections. The SCCP is used for
this purpose.
necessary to establish and identify distinct logical connections. The SCCP is used for
this purpose.
Summary
• What are Protocols
• OSI 7 Layer Model
• GSM Protocol Overview
• GSM Transmission Protocols
• GSM Signalling Protocols
• Air, A-bis and A Interface Protocols
This Section has covered the following topics:
• OSI 7 Layer Model
• GSM Protocol Overview
• GSM Transmission Protocols
• GSM Signalling Protocols
• Air, A-bis and A Interface Protocols
This Section has covered the following topics:
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