The GGSN GPRS Gateway Support Node

The GGSN is the interface between the GPRS backbone and the external packet data
networks. It converts GPRS packets from the SGSN into the appropriate packet
data protocol (PDP) format (IP or X25). Incoming data packets have their PDP addresses
converted to the GSM address of the destination user, and re-addressed packets are sent
to the responsible SGSN. GSN to GSN interconnection is via an IP based GPRS backhaul.
Within the backbone, PDN packets are encapsulated and transmitted using GPRS
tunneling protocol.
A SGSN may need to route its packets over different GGSNs to reach different
packet data networks.
Figure 12.7 shows the intra- and inter-PLMN (Public Land Mobile Network) interconnection.
The intra-PLMN backbone connects GSNs of the same PLMN, that is, a
private IP-based network specific to the GPRS network provider. Inter-PLMN backbones
connect the GSNs of different operators (supported by an appropriate service
level agreement).
The Gn and Gp interfaces allow the SGSNs to exchange user profiles when a user
moves from one SGSN to another. The HLR stores the user profile, current SGSN
address, and PDP for each GPRS user in the PLMN. The Gr interface is used to
exchange information between the HLR and the SGSN. The Gs interface interconnects
the SGSN and MSC/VLR database. The Gd interface interconnects the SMS gateway
with the SGSN.

GPRS bearer services include point-to-point (PTP) services, which can be connectionless
or connection-oriented (for example, X25), or point-to multipoint (PTM) services,
for example, supporting multicasting within a geographical area—traditionally
referred to in PMR as open channel working or group calling.
GPRS QoS is based on simple service precedence, reliability, delay, and throughput.
Service precedence is either high, normal, or low. Table 12.2 shows the three classes of
reliability.
GPRS QoS also has four classes of delay, listed in Table 12.3.
Delay is defined as end-to-end transfer time between two communicating handsets
or between a handset and the Gi interface to the external PDN. It includes delay for the
request and assignment of radio resources and transit delay in the GPRS backbone.
Transfer delays outside the GPRS network are not included; as presently specified,
GPRS does not support end-to-end guaranteed QoS. Throughput is specified as maximum,
peak bit rate, and mean bit rate. 292