* What the carriers don't want you to know about MPLS and voice
Over the past few weeks we’ve been discussing a number of topics surrounding queuing, dropped packets, and, to a certain extent, voice. This week we’re especially please to have a guest reply to these discussions from our longtime friend and associate, Michael Finneran, president and principal consultant at dBrn Associates.
Michael wrote, “After a month of persistent research and nagging with AT&T and Verizon Business, I was able to determine how excess real-time traffic is actually handled in their MPLS VPN network services – it’s dropped.
“That’s right, excess real time traffic is not downgraded, marked ‘discard eligible’, or forced to stand in the corner for 30 minutes, it’s dropped at the edge router. That comes from Alan Giorkaris, AT&T’s product manager for Enhanced VPN Product Features and from Mike Marcellin, Verizon Business’ director of IP and Ethernet Networking Services. By the way, excess traffic in the other (i.e. non-real time) categories will be forwarded if there is capacity available, but it is marked ‘out of contract’ and hence has a higher probability of discard.
“Given the difficulty I had in eliciting this kernel of information, I got the distinct impression that this was not something that anyone was in a big hurry to share. It took about a month to set up interviews after submitting the questions in writing, and even then, it took from a few hours to several days to get the final answers.
“Like Jack Nicholson’s Col. Jessup character in ‘A Few Good Men,’ maybe the carriers think we can’t handle the truth, but in fact, this makes the configuration for voice services wonderfully unambiguous. The network designer’s job is always easier if we know precisely how things work. We have to determine the number of voice trunks we will require from each site, identify the number of bits per second required for each trunk, and multiply those numbers together to determine the requirement for MPLS real time capacity (AT&T calls it ‘CoS1’ and Verizon calls it ‘Expedited Forwarding’).
“Of course, if you screw up the configuration, the result will be ugly. The way packet dropping works, if you provision 15 voice trunks over a real time service that only has the capacity for 10, you don’t get 10 good trunks and five bad ones – you get 15 bad ones! In short, we have to understand the capability of the network service, compute the capacity required, utilize call access control to limit the number of trunks, and ensure that the voice packets are marked (i.e. Diff Serv Control Point settings) to assign them the real-time traffic class.”
Many thanks to Michael, for sharing the above. Michael has kindly offered a much more extensive version of his findings, available at Webtorials. In this expanded version, he discusses the whole story along with the actual SLAs for delay, jitter, and packet loss, a chart for determining the bit rate requirements for VoIP trunks with different voice coding and packet sizes, and a great summary of the service level guarantees for AT&T and Verizon Business.
