Because GSM was originally designed for circuit-switched service, the intro-
duction of packet-switched transmission requires some significant functional
and operational changes. GPRS introduces two new network nodes, GPRS
support nodes (GSN) to support end-to-end packet transfer. They are serving
GPRS support node (SGSN) and gateway GPRS support node (GGSN), which
are shown in the simplified network architecture in Figure 5.16. To keep the
figure simple, many GSM network elements, such as EIR and SMSC, have
been excluded. Circuit-switched calls are routed from the BSC via the MSC
to PSTN.
For GPRS operation the HLR is enhanced with GPRS subscriber data
and routing information. The HLR is to be updated to include GPRS register
(GR), which stores packet user related data, such as IP address of the present
SGSN. The GR stores routing information (SGSN address) and maps IMSI
to one or more Packet Data Protocol (PDP) addresses if addresses are perma-
nently assigned to subscribers. Typically, an Internet Protocol (IP) address is
assigned for a subscriber on demand, that is, when she attaches GPRS.
Dynamic address is released in GPRS detach, when the MS is disconnected
from the GPRS network. The major upgrades in the BS subsystem are new
channel coders in BTSs and packet control units (PCUs) in BSCs. PCUs take
care of the packet transmission between MSs and SGSN.
