What IP telephony architecture
is all about. The hard facts on softswitches.
<<<...So, you'll find that your DHCP suddenly needs to cater for many more devices than before. It's recommended that your phones go into different VLANs from your PCs - this helps management and QoS enforcement but it also lets you use different ranges of IP addresses that won't conflict with or use up valuable IP addressing space. Typically, RFC1918 addresses are used for phones. The phone will register with a softswitch and often with a backup one too, using a TCP connection to ensure it can tell if its primary softswitch fails and it needs to fail over to use the secondary.
When a phone wants to make a call, it will send a call request to the softswitch. This will figure out the IP address of the device being called, another handset or a gateway if it's an external call, say, and send a call setup request to the destination. Once the call is answered direct communication, using RTP, is set up between the endpoints and the softswitch disengages itself from the process until the handsets signal the end of the call, or need to use another service, such as call transfer. Inline power from the LAN switches is a big selling point for IPT, since you don't need power bricks.
But this really only gives any financial saving in a brand new building, where you can save on wiring. Similarly for the LAN connection, as most IP phones have a small internal switch (or sometimes hub) that lets you plug the LAN switch into the phone, and a PC into the phone too. That way you don't need separate ports for all your phones as well as the PC connections (although you can have them if you want).
Of course, not all switches are capable of providing power, or of coping with having two devices hanging off one port and treating them differently in terms of VLAN allocation and QoS, so an IPT deployment may involve some major infrastructure upgrades (see So you think you have a voice-ready network?). The more expensive phones support XML, which opens up a whole other subject on the things you can do with your phones, apart from making calls, but that's for another day - as is the complex world of unified messaging (voicemail but with email and fax support too). We'll look at these topics in a future article.