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You can’t measure the quality of a VoIP call without having a set of metrics. While traditional TCP application performance metrics – like transaction time and throughput – are important, they may be difficult to relate to the user experience with the VoIP phone system. There are some user experience metrics that relate directly with call setup performance, however.
First, there’s the delay to the dial tone. In a VoIP system, the phone has to send a message to the call server letting the server know that the phone is off the hook. The call server then sends a message back to the phone telling it to play the dial tone. The time it takes for the message to be sent and the response received is the delay to dial tone.
Here, the main components involved in delay to dial tone are network round trip time (RTT) and the server processing time required to receive the off-hook message and send the response.
How much delay is too much when waiting for a dial tone? About 4 seconds – that’s when the users will begin to think that the phone system is broken and hang up.
(Incidentally, PTSN phone dial tones are near instantaneous, so this only applies for the outbound bound gateway call leg. Of course, you don’t get a dial tone anyway when you’re answering a ringing phone, so this is kind of moot.)
Second, there’s the post-dial delay, the time between when the user dials a phone number and the audible presence of a ringing or busy tone. There are some typical times – a local connection should take about three seconds, a toll connection should take about five seconds, an international connection should take about eight seconds, and interplanetary connections should take about 12 hours.*
*(Based on E.T.’s “Phone Home.”)
These delay metrics are different for every different type of call setup protocol.
Third, there’s call setup failures that don’t even allow the call to connect. You probably know these as the dreaded “fast busy” signal. You place a call and instead of ringing or a normal busy signal, you get a fast busy signal. This indicates that a call setup failure has occurred. This could be caused by any number of issues such as: not enough bandwidth, not enough channels to the PSTN, or some type of network error. If you get enough of these call failures, then from the user perspective, the phone system is not available.
