VoIP Call Setup Performance

Chapter 2 - VoIP: Do You See What I’m Saying? eBook
By NetQoS

Published 2007, Posted February 2008

Abstract:

Your experience with any phone system begins when you pick up the handset. You may then press a button to talk or take some other step to get a dial tone. Or you may skip the dial tone altogether and go directly to dialing the phone number you want to call. As new unified communications dashboard applications roll out, you may just click on an icon to place a call to another person. But regardless of the actions you take to place a call, the quality of experience that you have with that phone system is largely shaped by your perception of the availability and call quality that the system provides. From a user’s perspective, phone system availability can be summarized with the following basic criteria:

If you pick up the phone and don’t receive a dial tone within several seconds, then you’ll likely hang up, thinking the phone system is down. If you dial a phone number and don’t hear ringing or a busy signal within several seconds, then your perception is that you can’t make calls. If the call fails to connect and you hear a fast busy signal, then you’ll likely think that the call did not go through and that you have to try again. All of these experiences – getting a dial tone, connecting the call, ringing the other party – are dependent on the performance of the call setup protocols in a VoIP system.

The PSTN has done a good job of shaping our expectations for call setup performance. In fact, over the years, call setup time has steadily decreased. Eyers and Schulzrinne point out that call setup time has dropped steadily over the last 80 years, from a high of 4 minutes in 1923, to 1.2 minutes in 1928, down to 10.9 seconds in 1978, and to less than 2.5 seconds in 1998. So as we’ve come to expect high performance for call setup from the PSTN, what happens on a VoIP network? Can this same level of call setup performance be obtained? Should it even be a goal? The answer is yes, but it takes a certain amount of both attention and management.

In the previous chapter, we introduced some of the key call setup protocols like SIP, MGCP, H.323, and SCCP. If you are not familiar with the basics of these protocols, we encourage you to take a look at Chapter 1. In the present chapter, we will discuss the performance characteristics of the call setup protocols and how you can tune them to ensure optimal user experience.

First, it’s important to understand that in any VoIP system, there are a couple of different call types that involve different components and protocols. The interaction of calls of a certain type with your components and the protocols they "speak" can affect the call setup performance that users experience, for better or worse.

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