The Radio Network Controller
(RNC) in the UMTS portion of the network interfaces with the SGSN within the same
network, which in turn interfaces with the GGSN.
Figure 2.5 GPRS network architecture
Other
network MSC GGSN
MSC
BSC
SGSN
VLR
HLR
BTS BTS BTS
GPRS network
Moving from Legacy to Convergence 53
It is the GGSN that then provides the connectivity back into the IMS P-CSCF. The
P-CSCF is always the first point of access in the IMS, regardless of the access method.
Voice traffic does not necessarily go through this same route, because (as you??™ll recall)
the voice must go through media gateway, under the control of the MGCF. SIP signaling
is the only data that is routed through the CSCF functions of the IMS.
This is the same then as in wireline VoIP, where the voice is sent to a media gateway
(MG) for converting to packet, and the signaling is sent to the MGCF for conversion to
SIP. Once the signaling has been converted to SIP, the MGCF is then able to communicate
directly with the IMS P-CSCF.
CDMA is a little different in that the architecture does not include the SGSN/GGSN
(see Figure 2.7). However, there is an equivalent function: the Packet Control Function
(PCF) interfacing with the Base Station Controller (BSC), which is responsible for the
routing of packet data to the packet network within CDMA.
The Packet Data Service Node (PDSN) then provides connectivity to packet services
such as the Internet, or other packet networks.
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