otocol data units (SN-PDUs) between the SNDCP layer and the LLC layer Procedures for transferring LLC protocol data units (LLC-PDUs) between the mobile station and the SGSN Procedures for unacknowledged point-to-point delivery of LLC-PDUs between the mobile station and the SGSN Procedures for acknowledged point-to-point delivery of LLC-PDUs between the mobile station and the SGSN Procedures for point-to-multipoint delivery of LLC-PDUs from the SGSN to the mobile station Procedures for detecting and recovering from lost or corrupted LLC-PDUs Procedures for flow control of LLC-PDUs between the mobile station and the SGSN Procedures for ciphering the LLC-PDUs (applies to both unacknowledged and acknowledged LLC-PDU delivery) 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. Mobile Station to SGSN Interface Mobile Station to SGSN Interface The layer functions are organized SO that the ciphering resides immedi- ately above the Radio Link Control/Medium Access Control (RLC/MAC) layer in the mobile station and immediately above the BSS GPRS Protocol (BSSGP) layer in the SGSN. LLC Service Access Point Identifiers (SAPIs) The LLC provides six Service Access Points (SAPs) to the upper-layer pro- tocols. Think of these as tunnels between the layers. The tunnels enable the passage of data between the Layer 2 and Layer 3 entities, as shown in Fig- ure 8-2 and described as follows: Four SAPs are dedicated to the SNDCP that manages data packet transmission. This protocol only deals with transmission. Currently, only one SAP exists for each quality of service (QoS). One SAP is dedicated to GMM. One SAP is dedicated to the SMS. A Service Access Point Identifier (SAPI) identifies each SAP. Figure 8-2 Upper Layer The LLC SAPI Service for Service for Layer for Network data unit signaling short message functions as an SNDCP GMM/SM access to the Layer Service Acces