Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 113

In the previous sections, we have discussed the encoding and decoding of signaling messages between a UE/MS and their respective RAN of the GSM, UMTS, LTE, and 5G NR systems over their respective air interface. To perform certain functions and procedures between the RAN and the CN, they also exchange signaling or control plane messages over the respective logical interface. The IEs of a signaling or control plane message exchanged over the concerned logical interface between the RAN and CN are packed and unpacked using a particular encoding and decoding method, which are described below:

 Between GSM BSC and MSC: A‐Interface

The logical A‐interface is used to exchange signaling messages between the GSM BSC and the MSC. Over the A‐interface, the following types of messages are exchanged between the BSC and MSC.

 Base Station Management Application Part (BSSMAP)

Signaling messages for various functions and procedures performed between the BSC and MSC are classified into BSSMAP types. BSSMAP messages, between BSC and MSC, are also encoded/decoded and are described in a tabular format similar to the air interface Layer 3 messages. However, unlike the Layer 3 messages, BSSMAP messages do not contain a message header. Each BSSAMP message starts with its message type, followed by the associated IEs.

 Direct Transfer Application Part (DTAP)

DTAP messages are exchanged between the UE and MSC only. All the air interface Layer 3 CC and MM signaling messages that are transparently forwarded by the BSC, received from the MS to the MSC without processing by BSC, are classified into DTAP types. DTAP messages are the air interface Layer 3 messages with a header containing protocol discriminator information in the header of every message that is exchanged between the MS and MSC. Because of this, the DTAP message is encoded and decoded as described in Section 4.1.1.