mobile station (MS) and the Serving GPRS Support Node (SGSN). A Temporary Logical Link Identifier (TLLI) is used for addressing at the LLC layer. The LLC is independent of the underlying radio interface protocols. The LLC provides services necessary to maintain a ciphered data link between a mobile station and an SGSN. The logical link is maintained as the mobile station moves between cells served by the same SGSN. When the mobile station moves to a cell being served by a different SGSN and performs a new attach, the existing con- nection is released and a new logical link connection is established (this is done by the attach operation and the Packet Datagram Protocol [PDP] con- text activation). The LLC provides for acknowledged and unacknowledged point-to-point delivery of LLC protocol data units (PDUs) between the mobile station and the SGSN and point-to-multipoint delivery of packets from the SGSN to the mobile station. The LLC layer also provides procedures for detecting errors from cor- rupted PDUs. It does this by checking the frame check sequence (FCS) in the LLC frame format. The FCS contains the value of the Cyclic Redundancy Check (CRC) calculation that is performed over the entire contents of the header and the information fields. The unacknowledged mode of transfer has no error recovery. For the acknowledged mode of transfer, the LLC may request retransmission of the frames of data for which an acknowledgment has not been received. 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 What LLC Does This layer provides a highly reliable ciphered logical link. A logical link between a mobile station and its SGSN is identified by a TLLI. The LLC is independent of the underlying radio interface protocols in order to accept the introduction of alternative GPRS radio solution