Temporary Logical Link Identifier

Description

The Temporary Logical Link Identifier (TLLI) is a 32-bit identifier that plays a crucial role in GPRS and EDGE network operation. It uniquely identifies the logical link connection between a Mobile Station (MS) and the Serving GPRS Support Node (SGSN) at the LLC (Logical Link Control) sublayer. The TLLI is derived from, and is used in conjunction with, the Packet-Temporary Mobile Subscriber Identity (P-TMSI). There are several types of TLLI, defined by their derivation method: Local TLLI (derived from the P-TMSI assigned by the current SGSN), Foreign TLLI (derived from a P-TMSI assigned by a previous SGSN), Random TLLI (chosen randomly by the MS if it has no valid P-TMSI), and Auxiliary TLLI (used in specific procedures). The MS includes a TLLI in the header of every LLC frame it sends uplink. The network (SGSN) uses this TLLI to associate the frame with the correct MS context, which contains session parameters, subscription data, and location information. Conversely, when the SGSN sends data downlink to the MS, it addresses the LLC frame using the MS's TLLI. The TLLI's scope is local to a routing area; if an MS moves to a new routing area served by a different SGSN, a new TLLI is derived from a new P-TMSI. This local scope provides a layer of subscriber identity privacy, as the true IMSI is rarely signaled over the radio. The TLLI is essential for multiplexing—the network infrastructure uses it to route LLC Protocol Data Units (PDUs) between the correct SGSN and the correct Base Station System (BSS) for the target MS, managing the shared radio resources among multiple simultaneous users.

Purpose & Motivation

The TLLI was introduced with GPRS to provide an efficient and secure method for identifying mobile data sessions at the logical link layer. Prior circuit-switched GSM used the Temporary Mobile Subscriber Identity (TMSI) for signaling but did not have an equivalent persistent identifier for packet data flows. The design goals for TLLI were severalfold: First, to avoid frequent transmission of the permanent subscriber identity (IMSI) over the air interface, enhancing privacy and security. Second, to provide a handle for the network to manage the state of a logical link connection, including flow control and error recovery at the LLC layer. Third, to enable efficient resource management in the BSS and SGSN, as these nodes can use the compact TLLI to quickly look up the extensive MS context needed for processing each data packet. By being temporary and locally significant, it simplifies context transfer during inter-SGSN routing area updates. The TLLI solved the problem of how to support multiple concurrent packet data sessions (logical links) to and from a single MS and across a shared radio channel, establishing the foundational addressing mechanism for 2G/3G packet-switched mobility.