UMTS Networks : Architecture, Mobility and Services

316 UMTS Networks
GTP-U is a transport network protocol supporting user-plane data transfer in the
UMTS network. GTP-U belongs to the PS domain in the UMTS network and is
located in the RNC at the UTRAN end, as well as in the SGSN and GGSN at the
CN end.
GTP-U operates on three different interfaces: Gn, Gp and Iu-PS. The Gn interface is
between GSNs (i.e., SGSNs and GGSNs) belonging to the same UMTS packet domain
network. The Gp interface is used for network interworking between two SGSNs or
between an SGSN and GGSN belonging to different UMTS packet domain networks.
The interface between the RNC and SGSN is the Iu-PS.
Over all interfaces GTP-U operates on top of the UDP/IP protocol family.
UDP provides connectionless message transfer between two IP network nodes. UDP
addressing is used to identify GTP-U end points within source and destination network
elements. UDP also provides a checksum mechanism to detect transmission errors
inside data packets. As is generally known, the main service of the IP is the routing
of messages between source and destination network elements. This routing is based on
IP network addresses (both IPv4 and IPv6 can be used within the UMTS transport
network).
GTP-U provides connectionless data transfer services for upper layers and allows
multi-protocol user data packets to be tunnelled across the Iu-PS, Gn and Gp interfaces.
Because of encapsulation and tunnelling mechanisms it is possible to transfer user
data packets, although different routing protocols are used in the packet domain IP
backbone network. In addition, UMTS network elements and protocols transferring
user data packets between the UTRAN and GGSN have no need of being aware of
different addressing mechanisms at the IP layer to be able to associate user data packets
with specific PDP contexts.
Since the main purpose of GTP-U is to transfer user data, it has been optimised
to that specific task. GTP-U multiplexes packets received from one interface (e.g., Gi
at the GGSN) and addressed to several destinations (different UEs). It receives the
user data packet from the external packet network, interprets the destination of
the packet and passes the packet onto the next node along the path. Tasks for
setting up the tunnels between GTP-U end points have been excluded from
GTP-U, and control-plane protocols are used for these purposes. On the Gn interface,
GTP-C (GTP for the control plane) and, on the Iu-PS interface, Radio Access
Network Application Part (RANAP) protocols are used to control the set-up of
GTP-U tunnels.
GTP-U entities have to interwork with other protocol entities as the data packets are
transported. In the GGSN a GTP-U entity communicates with the packet relay functionality
located at the edge of the UMTS network (i.e., the Gi interface). At the Iu-PS
end of an SGSN a GTP-U entity interworks with another GTP-U entity at the Gn end.
Within the RNC a GTP-U entity terminates the tunnel and forwards the packets to the
frame protocol entity at the radio network layer.
Interworking between GTP-U and the neighbouring protocol entities is quite simple.
After all, each parameter related to user data packet transfer has been negotiated in
advance by the control plane. User data packets are simply passed to the next entity,
which is then able to handle the packet according to its predefined PDP context
parameters.