International mobile subscriber identity (IMSI) is a unique identifier for a GSM/GPRS subscriber in a PLMN.Likewise, in GPRS, the MS is assigned temporary identities. These are used at different interfaces and serve specific purposes.
P-TMSI
A packet temporary mobile subscriber identity (P-TMSI) is assigned to a GPRS MS at the time of GPRS attach. Like TMSI, P-TMSI is used to avoid transmitting IMSI over the air interface. P-TMSI is of local significance and is applicable in the area served by an SGSN. If the MS moves out to a new SGSN area, the current serving SGSN assigns a new P-TMSI to the MS.
TLLI
The temporary logical link identity (TLLI) is a temporary identity used during a PDP session over the Um and Gb interfaces. The MS or the SGSN derives the TLLI, using the P-TMSI. The TLLI can be derived in one of the three ways:
1. A local TLLI is built by using the P-TMSI assigned by the SGSN.
2. A foreign TLLI is derived from a P-TMSI allocated in a different routing area.
3. The MS generates a random TLLI in the absence of a valid P-TMSI.
The network can assign a new P-TMSI any time. The MS then derives the value of TLLI by using the new P-TMSI.
NSAPI
The network layer service access point identifier (NSAPI) is used with TLLI for network layer routing. The NSAPI acts as an index for the appropriate packet data protocol (PDP) that is using the services of SNDCP. When an IP packet is received at the MS for a particular IP address at a service access point, the IP packet is encapsulated and the NSAPI (from the previous activation of PDP context) value is attached. TLLI is set to the MS’s TLLI before the encapsulated IP packet is passed to the SNDCP layer. The SGSN, on receiving the IP PDU, analyzes the TLLI and NSAPI and forwards the IP PDU to the right GGSN. The SGSN maintains a table with information similar to that shown in Table 4-3 to make the routing decision.
TABLE 4-3 Example SGSN Network Layer Routing Data
|
MS1 |
TLLI = 1 NSAPI 8 |
TEID 2 |
GGSN IP |
|
MS2 |
TLLI = 2 NSAPI 6 |
TEID 5 |
GGSN IP |
|
MS3 |
TLLI = 3 NSAPI 3 |
TEID 6 |
GGSN IP |
TEID
A tunnel end point identifier (TEID) is used by the GTP protocol between GSNs to identify a tunnel end point in the receiving GTP-C or GTP-U protocol entity and to identify a PDP context. As shown in Table 4-3, each PDP context has a one-to-one relationship between the TEID and the NSAPI/IMSI (IMSI and TLLI have a one-to-one relationship). The receiving-end side of a GTP-U tunnel locally assigns the TEID value. The TEID value is then made known to the transmitting side by GTP-C protocol.
