The interworking between other networks is
critical to ensuring that legacy networks can be left in place while IMS is implemented.
Again, only the call control is sent through the MGC function. In fact, the MGC communicates
with media gateways (MGs) to control the actual voice transmission. The
MG supports the bearer traffic under the control of the MGC.
Locating the P-CSCF
As already mentioned, a device connects first to the access network. Once this connection
is made, the device needs to communicate its location (address) to the network so
that it can begin receiving e-mail, voice mail, phone calls, etc. This requires connecting
to the IMS.
108 Chapter 5
Access to the IMS network is achieved through the Proxy-Call Session Control
Function (P-CSCF). The device must find the P-CSCF within its present domain.
P-CSCF discovery can be coupled with the IP address assignment (whereas the IP address
and the P-CSCF location are delivered together), or through a query to a Dynamic
Host Configuration Protocol (DHCP) server. The DHCP should return the domain name
for the appropriate P-CSCF serving the area the device is located in (or serving the
domain the device is located in).
For example, if I power up my PC at home and connect to the network, the network
will provide me the address of the P-CSCF that is serving my area. The address consists
of a domain name only.
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