ocols have been identified across the Gb 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 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PCUSN-to-SGSN Interface (Gb) PCUSN-to-SGSN Interface (Gb) In the uplink, the provision by a BSS to an SGSN of radio-related information derived from the RLC/MAC function The provision of functionality to enable two physically distinct nodes, an SGSN and a BSS, to operate node management control functions The Protocol Stack for Gb Interface The Gb interface is located between the packet control unit (PCU) and SGSN, as shown in Figure 9-1. On each node, we retrieve the following: Physical layer This defines the characteristics of the medium being used. NS This defines the Layer 2 protocol that is being used (Frame Relay will be used initially) and some specific procedures. BSSGP This mainly manages buffers for flow control between the PCU and SGSN. It provides services for the upper-layer entities. Network Management (NM) This local entity manages the buffers and virtual circuits between the two nodes. GPRS Mobility Management (GMM) The GMM part that is located just above the BSSGP part deals only with mobility messages between SGSN and PCU (such as a paging procedure). Figure 9-1 The Gb protocol Relay stack. BSSGP BSSGP Physical Physical Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. PCUSN-t