Major Stages of Voice Processing in VoIP
For voice transmission over an IP network, the voice wavelength must be sampled, quantized, encoded, optionally compressed, and then encapsulated in a VoIP packet.
Gatekeeper: Cisco gatekeepers are used to group gateways into logical zones and perform call routing between them. Gateways are responsible for edge routing decisions between the Public Switched Telephone Network (PSTN) and the H.323 network. Cisco gatekeepers handle the core call routing among devices in the H.323 network and provide centralized dial plan administration. Without a Cisco gatekeeper, explicit IP addresses for each terminating gateway would have to be configured at the originating gateway and matched to a Voice over IP (VoIP) dial-peer. With a Cisco gatekeeper, gateways query the gatekeeper when trying to establish VoIP calls with remote VoIP gateways.
In GNS3 I have been using a 3725 with c3725-adventerprisek9-mz.124-18. For soft phones I have been using Cisco IP Communicator and this: http://www.ipblue.com/download.asp
Those two are the only softphones I could find supporting SCCP. There are a TON that support SIP. I am using VMWare workstation 10.0, but vmware player works fine too. I am running CUCM 8.6 in vmware.
I’d suggest getting comfortable with loopback adapters in GNS3. This is how you can use the softphones. There are some really good step by step CCNA Voice virtual labs on youtube…like this one: http://www.youtube.com/watch?v=ou_OM5taXpw