BSSAP (Base Station Subsystem Application Part)

The Base Station Subsystem Application Part (BSSAP) is a protocol that is used in cellular networks to support communication between a Base Station Subsystem (BSS) and a Mobile Switching Center (MSC). The BSSAP protocol is a part of the Signaling System 7 (SS7) protocol suite, which is used for the management of signaling messages in the Public Switched Telephone Network (PSTN).

In this article, we will discuss the BSSAP protocol in detail, including its architecture, functions, and messages.

BSSAP Architecture

The BSSAP protocol operates at the Network layer of the OSI model and provides functions for the exchange of signaling messages between the BSS and MSC. The BSSAP protocol is divided into three layers: the BSSMAP layer, the DTAP layer, and the SCCP layer.

The BSSMAP (Base Station System Management Application Part) layer is responsible for managing the BSS-related functions, such as the establishment and release of radio channels, handover management, and paging. The BSSMAP layer is used to exchange signaling messages between the BSS and MSC.

The DTAP (Direct Transfer Application Part) layer is responsible for managing the call-related functions, such as call setup, call clearing, and call forwarding. The DTAP layer is used to exchange signaling messages between the MSC and Mobile Station (MS).

The SCCP (Signaling Connection Control Part) layer provides the connection-oriented and connectionless services for the BSSAP protocol. The SCCP layer is responsible for the addressing, routing, and management of signaling messages in the SS7 network.

BSSAP Functions

The BSSAP protocol provides various functions that are necessary for the operation of the cellular network. Some of the key functions of the BSSAP protocol are described below.

Radio Resource Management (RRM)

The BSSAP protocol is responsible for managing the radio resources in the BSS. The BSSAP protocol is used to establish, maintain, and release the radio channels between the BSS and MS. The BSSAP protocol is also used to manage the handover of the MS between different cells in the BSS.

Call Control

The BSSAP protocol is responsible for the control of the calls between the MS and MSC. The BSSAP protocol is used to initiate and terminate the calls, and to manage the call-related events such as call forwarding, call waiting, and call hold.

Location Management

The BSSAP protocol is responsible for managing the location of the MS in the BSS. The BSSAP protocol is used to update the location of the MS when it moves from one cell to another within the BSS. The BSSAP protocol is also used to manage the paging of the MS when it is in idle mode.

Bearer Management

The BSSAP protocol is responsible for managing the bearer channels between the MS and MSC. The BSSAP protocol is used to establish, maintain, and release the bearer channels for the voice and data communication between the MS and MSC.

BSSAP Messages

The BSSAP protocol uses various messages for the exchange of signaling information between the BSS and MSC. Some of the key messages of the BSSAP protocol are described below.

Location Update Request (LUR)

The Location Update Request message is used by the MS to update its location in the BSS. The LUR message contains the current location of the MS, including the Cell ID and the Location Area Identity (LAI).

Location Update Accept (LUA)

The Location Update Accept message is sent by the MSC to the MS in response to the LUR message. The LUA message acknowledges the receipt of the LUR message and updates the location of the MS in the MSC.

Call Setup (CST)

The Call Setup message is used by the MS to initiate a call to another MS or to a PSTN user. The CST message contains the called party number, the type of call (voice or data), and other call-related information.

Call Proceeding (CPG)

The Call Proceeding message is sent by the MSC to the MS in response to the CST message. The CPG message indicates that the call setup process is in progress and provides the necessary information for the MS to proceed with the call setup.

Call Connect (COT)

The Call Connect message is sent by the MSC to the MS to indicate that the call has been connected. The COT message contains the necessary information for the MS to start the voice or data communication.

Handover Request (HO-REQ)

The Handover Request message is used by the BSS to request the MSC to perform a handover of the MS to a neighboring cell. The HO-REQ message contains the necessary information for the MSC to perform the handover, such as the target cell ID and the reason for the handover.

Handover Required (HO-REQD)

The Handover Required message is sent by the MSC to the BSS in response to the HO-REQ message. The HO-REQD message indicates that the handover is required and provides the necessary information for the BSS to perform the handover.

Handover Command (HO-CMD)

The Handover Command message is used by the MSC to command the BSS to perform the handover of the MS to the target cell. The HO-CMD message contains the necessary information for the BSS to perform the handover, such as the timing advance and the handover direction.

Conclusion

The BSSAP protocol is an important part of the cellular network architecture, and it provides various functions for the management of signaling messages between the BSS and MSC. The BSSAP protocol is responsible for managing the radio resources, call control, location management, and bearer management in the cellular network. The BSSAP protocol uses various messages for the exchange of signaling information between the BSS and MSC, and these messages are used for the establishment and release of radio channels, handover management, and paging. The BSSAP protocol plays a crucial role in the operation of the cellular network, and it ensures the reliable and efficient communication between the BSS and MSC.