The PDSN therefore serves the same
role as the GGSN, acting as the gateway into other networks. The PDSN interfaces with
the media gateway within the IMS domain if there is voice; otherwise, for packet data
the interface is to packet entities within the packet domain. The session is controlled
by SIP, so the SIP signaling is routed to the P-CSCF within the IMS.
Figure 2.6 UMTS network
Other
network MSC GGSN
MSC
BSC RNC
Node
A
Node
B
Node
A
Node
B
RNC
SGSN
VLR
HLR
BTS BTS BTS
UMTS network
54 Chapter 2
From IMS to IMS
In a perfect IMS universe, there is no need for access networks to support anything
other than IP and SIP. Of course, we are not in a perfect IMS world (at least not yet),
and there is a significant amount of network still in service that is not packet. For this
reason, we will not realize the full benefits of a total SIP network for a long time.
When I say total SIP network, I am referring to a network where all of the devices
are equipped with SIP user agents and are capable of supporting IMS SIP. This would
allow the devices to interface with the CSCF within the IMS domain directly without
any conversions or signaling gateways (see Figure 2.8). MGCFs would not be required
since the devices are already SIP capable.
These same devices would be able to transmit everything in packet mode, eliminating
the need for media gateways and other support devices in the network.
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