The user plane consists of a layered protocol structure providing user information transfer, along with associated information transfer control procedures (e.g. flow control, error detection, error correction and error recovery). The user plane independence of the Network Subsystem (NSS) platform from the underlying radio interface is preserved via the Gb interface. The following user plane is used in A/Gb mode.
GPRS Tunnelling Protocol for the user plane (GTP U): This protocol tunnels user data between core network nodes in the backbone network. The GPRS Tunnelling Protocol shall encapsulate all PDP PDUs. GTP is specified in TS 29.060, or TS 29.274.
UDP carries GTP PDUs for protocols that do not need a reliable data link (e.g. IP), and provides protection against corrupted GTP PDUs. UDP is defined in RFC 768 .
IP: This is the backbone network protocol used for routeing user data and control signalling. The backbone network may initially be based on the IPv4. Ultimately, IPv6 shall be used. When IPv6 is used in the backbone, then IPv4 shall also be supported. IPv4 is defined in RFC 791  and IPv6 is defined in RFC 2460 .
Subnetwork Dependent Convergence Protocol (SNDCP): This transmission functionality maps network-level characteristics onto the characteristics of the underlying network. SNDCP is specified in TS 44.065.
Logical Link Control (LLC): This layer provides a highly reliable ciphered logical link. LLC shall be independent of the underlying radio interface protocols in order to allow introduction of alternative GPRS radio solutions with minimum changes to the NSS. LLC is specified in TS 44.064.
Relay: In the BSS, this function relays LLC PDUs between the Um and Gb interfaces. In the SGSN, this function relays PDP PDUs either between the Gb and Gn interfaces or between the Gb and S4 interfaces.
Base Station System GPRS Protocol (BSSGP): This layer conveys routeing- and QoS-related information between the BSS and the SGSN. BSSGP does not perform error correction. BSSGP is specified in TS 48.018.
Network Service (NS): This layer transports BSSGP PDUs. NS is specified in TS 48.016.
RLC/MAC: This layer contains two functions: The Radio Link Control function provides a radio-solution-dependent reliable link. The Medium Access Control function controls the access signalling (request and grant) procedures for the radio channel, and the mapping of LLC frames onto the GSM physical channel. RLC/MAC is defined in TS 44.060.
GSM RF: As defined in the TS 45.xxx series of specifications.
GPRS Tunnelling Protocol for the user plane (GTP U): This protocol tunnels user data between SGSNs and GGSNs (Gn or Gp), between SGSNs and S-GWs (S4), between S-GWs and P-GWs (S5 or S8) and between SGSNs in the backbone network (Gn or S16).
User Datagram Protocol (UDP): This protocol transfers user data between GSNs. UDP is defined in RFC 768.