Mobile Network A Interface

The interface between a BSC (Base Station Controller) and an MSC (Mobile Switching Center) is called the A interface. Running on the standard connection of 64Kbits/s or 2048 Kbits/s (or 1544 Kbits/s in North America), SS7 MTP and SCCP transporation protocols are used to deliver the Base Station Application Part (BSSAP) between the BSC and MSC.

Base Station Application Part (BSSAP) can further be devided into

1. Direct Transfer Application Part (DTAP)
2. Base Station System Operation and Management Part (BSSOMAP)

The DTAP is used to transfer call control and mobility management messages between the MSC and MS. The DTAP information in these messages is not interrepted by the BSC. The majority of radio interface messages are transferred across the A interface by DTAP, except for messages belonging to the Radio Resource (RR) management protocol.

The DTAP message header is as shown in the following figure.

The protocol discriminator distinguishes the payload protocol of the messages as follows:

• 0000 Group call control
• 0001 Broadcast call control
• 0010 PDSS1
• 0011 Call control, call relate SS messages
• 0100 PDSS2
• 0101 Mobility Management messages
• 0110 Radio resource meanagement messages
• 1001 SMS messages
• 1011 Non-call related SS messages
• 1110 Extension of the PD to one octet length
• 1111 Tests procedures

The message type field defines the different messages in different protocols.

The BSSOMAP is used to convey the base station operation and management messages.