23.0603GPPGeneral Packet Radio Service (GPRS)Release 17Service descriptionStage 2TS
GPRS transparently transports PDP PDUs between packet data networks and MSs. All PDP PDUs are encapsulated and decapsulated for routeing purposes. Encapsulation functionality exists at the MS, at the RNC, at the Iu mode BSC, at the SGSN, at the S‑GW, and at the GGSN/P‑GW. Encapsulation allows PDP PDUs to be delivered to and associated with the correct PDP context in the MS, the SGSN, or the GGSN/P‑GW. Two different encapsulation schemes are used; one for the backbone network between two GSNs, between SGSNs and S‑GWs, and between an SGSN and an RNC, and one for the A/Gb mode connection between the SGSN and the MS or for the Iu mode RRC connection between the RAN and the MS.
Encapsulation requires that the MS is attached to GPRS, and that the PDP Context Activation procedure has been executed. If the GPRS Attach or PDP Context Activation procedures cannot be successfully executed, then uplink PDP PDUs are discarded in the MS. If these procedures have not been executed when a downlink PDP PDU arrives in the GGSN /P‑GW, then the downlink PDP PDU shall be discarded, rejected, or the Network-Requested PDP Context Activation procedure shall be initiated. Network-Requested PDP Context Activation is not supported for connectivity over S4.
9.6.1 Encapsulation Between Core Network Nodes
Core network nodes encapsulate a PDP PDU with a GPRS Tunnelling Protocol header, and insert this GTP PDU in a UDP PDU that again is inserted in an IP PDU. The IP and GTP PDU headers contain the core network node addresses and tunnel endpoint identifier necessary to uniquely address a PDP context.
For connectivity between SGSNs and between SGSNs and GGSNs based on Gn/Gp, the GTP encapsulation header is defined in TS 29.060 . For connectivity between SGSNs and between SGSNs and S‑GWs based on S16 and S4, respectively, the GTP encapsulation header is defined in TS 29.274 .
9.6.2 Encapsulation Between SGSN and RAN in Iu mode
On the Iu interface, a PDP PDU is encapsulated with a GPRS Tunnelling Protocol header as specified in TS 29.060 .
9.6.3 Encapsulation Between SGSN and MS in A/Gb mode
Between an SGSN and an MS in A/Gb mode, an SGSN or MS PDP context is uniquely addressed with a temporary logical link identity and a network layer service access point identifier pair. TLLI is derived from the P‑TMSI. An NSAPI is assigned when the MS initiates the PDP Context Activation function. The relationship between TLLI / NSAPI and LLC / SNDCP is illustrated in Figure 94. TLLI and NSAPI are described in clause "NSAPI and TLLI for A/Gb mode".
9.6.4 Encapsulation Between RAN and MS in Iu mode
On the Uu interface, a PDP PDU is encapsulated with PDCP.