Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 121

 Packet TMSI (P‐TMSI)

A P‐TMSI is allocated by the GPRS PS domain SGSN to protect a mobile user's identity while communicating over the air interface. A P‐TMSI is used as the identity of an MS during the GPRS air interface Layer 3 mobility management‐related procedures such as the routing area update, paging request, and an attach request.

A P‐TMSI has an end‐to‐end significance and has 32 bits in length. The first two Most Significant Bit (MSB) bits (31, 30) of a P‐TMSI are always set to 1's. A new P‐TMSI is allocated to a mobile device on entering a new routing area under a different SGSN where the MS performs a routing area update procedure with the CN. The SGSN maintains an association between the IMSI of the mobile device and the allocated P‐TMSI.

 Network Resource Identifier (NRI)

An NRI is a CN‐level identifier and is used to identify a GSM MSC or SGSN or an LTE/EPS MME. Several MSCs or SGSNs or MMEs can be configured in a pool to serve mobile users in a load‐balanced way. An NRI is assigned and identifies uniquely an individual CN element, i.e. MSC, SGSN, or an MME, out of all the CN elements configured in a pool.

An NRI has an end‐to‐end significance and has a length from 0 to 10 bits; NRI having length 0 means the feature is not used at the CN end. More about the CN element pool and their selection are described later in Chapter 7.

 Temporary Logical Link Identity (TLLI)

A TLLI identifies a logical link at the Logical Link Control (LLC) layer between the MS and the SGSN of a GPRS system. At the GPRS LLC layer level, an MS is uniquely identified by a temporary logical link identifier (TLLI). A TLLI is constructed from a P‐TMSI that is allocated by the SGSN which has an end‐to‐end significance and is used across the MS, BSC, and the SGSN.