GPRS Tunnelling Protocol (GTP)

is the defining IP protocol of the GPRS core network. Primarily it is the protocol which allows end users of a GSM or WCDMA network to move from place to place whilst continuing to connect to the internet as if from one location at the Gateway GPRS Support Node (GGSN). It does this by carrying the subscriber's data from the subscriber's current Serving GPRS Support Node (SGSN) to the GGSN which is handling the subscriber's session. Three forms of GTP are used by the GPRS core network.

* GTP-U: for transfer of user data in separated tunnels for each PDP context
* GTP-C: for control reasons including:
o setup and deletion of PDP contexts
o verification of GSN reachability
o updates, e.g. as subscribers move from one SGSN to another.
* GTP' : for transfer of charging data from GSNs to the charging function.

GGSNs and SGSNs (collectively known as GSNs) listen for GTP-C messages on UDP port 2123 and for GTP-U messages on port 2152. This communication happens within a single network or may, in the case of international roaming, happen internationally, probably across a GPRS Roaming Exchange (GRX).

The "Charging Gateway Function" (CGF) listens to GTP' messages sent from the GSNs on UDP port 3386. The core network sends charging information to the CGF, typically including PDP context activation times and the quantity of data which the end user has transferred. However, this communication which occurs within one network is less standardised and may, depending on the vendor and configuration options, use proprietary encoding or even an entirely proprietary system.