It’s important to understand each call setup protocol in a VoIP system, because each different call type can involve different components and different protocols.
First, there is On-Net calling, which takes place between two IP phones on the same logical network. In this scenario, the phones use setup protocols like SIP or SCCP to interact with a call server to set-up and take down each phone call. These types of calls are entirely IP based – they’re carried on the network and do not have to go out to the PTSN.
The simplest On-Net scenario, Intracluster Calling, means that both phones are on the same call server or cluster of call servers.
Using Cisco IP phones a long-lived TCP connection is required to “register” with the call server. The connection between the phone and call server requires an exchange of messages every 30 seconds or so, with the call server in a keep-alive connection. If network connectivity between phone and call server is lost or intermittent, than the connection may be broken, and the “keep-alive” messages may not be received.
One potential hazard here is that if the network links are going up and down, you could have phones constantly trying to re-register with the call server. Phones not registered with the call server cannot make or receive phone calls. When the handset is first picked up, the phone sends a message to the call server. The call server responds with a message to the phone to play the dial tone. (If the call server is busy or the network round trip time is high, the message could be delayed.)
After the dial tone, the user dials the number. To the end-user, this should be transparent, but it’s actually a four-step process where the calling phone sends the dialed digits to the call server, the call server determines where on the network the called phone is, the call server sends setup messages to the called phone which tells it to ring, and the call server sends setup messages back to the calling phone which tells it that the called phone is ringing.
Problems can occur at this stage – if the call server is busy it’ll take too long to look up the called phone information, and if the round trip time between calling/called phone and call server is too long, the overall delay will be high.
The next level of complexity is with an intercluster call. An intercluster call takes place between 2 IP phones that are registered with different call server clusters. The call server clusters use H.323 or SIP to communicate with each other – in this scenario, there’s an extra “call setup” that occurs between the two clusters when a call is made from a phone on one cluster to a phone on the other.
Next, we’ll talk about Off-Net calling, which travels from the IP network to the PSTN through, typically, a voice gateway. Now the call server is communicating with the gateway which is now in turn communicating with a separate network, the PTSN. And of course, the greater amount of connections needed to set up a call, the greater the potential amount of delay.
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