Objectives

Upon completion of this chapter, you should be able to

• Describe the PCUSN interface.

• Discuss the way that Frame Relay works.

• Understand the way GPRS uses the Frame Relay services at the G _b interface.

• Combine the benefits of Frame Relay and the advantages of GPRS.

• Understand why ETSI chose Frame Relay as the transport for the PCUSN.

9.2 High-Level Characteristics of the G _bInterface

In contrast to the A interface, where a single user has sole use of a dedicated physical resource throughout the lifetime of a call irrespective of information flow, the G _b interface enables many users to be multiplexed over a common physical resource. GPRS signaling and user data may be sent on the same physical resources. Access rates per user may vary from zero data to the maximum possible bandwidth (for example, the available bit rate of an E1).

9.3 Position of BSSGP Within the Protocol Stack on the G _bInterface

The following peer protocols have been identified across the G _b interface: the Base Station Subsystem GPRS Protocol (BSSGP) and the underlying network service (NS). The NS transports BSSGP packet data units (PDUs) between a BSS and a Serving GPRS Support Node (SGSN). The primary functions of the BSSGP include

• In the downlink, the provision by an SGSN to a BSS of radio-related information used by the Radio Link Control/ Medium Access Control (RLC/MAC) function