Objectives

Upon completion of this chapter, you should be able to

• Understand the way the mobile station communicates with the SGSN via LLC.

• Discuss the ways the data protocols work at Layer 2 and 3.

• Describe the use of SNDCP protocols.

• Understand the services that the SGSN offers to the mobile station.

• Describe how the protocols stack up.

• Describe the use of the TLLI.

Logical Link Control (LLC) Layer

The Logical Link Control (LLC) layer provides a reliable logical link between the 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 connection is released and a new logical link connection is established (this is done by the attach operation and the Packet Datagram Protocol [PDP] context 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...