To VOIP QOS or not to VOIP QOS?

April 1st, 2009

In telephony, IP QOS is somewhere between a science and an art. Setting up VOIP QOS on your network is essential for toll quality voice from end point to end point, especially across a WAN. Historically, in ShoreTel, IP packets were marked with the DSCP value set in the Call Control Options page. Generally this is generally set as a value of 184 or Precedence Level 5, what CISCO would call Express Forwarding or EF. This value is represented as 184 (10111000 or 46) but as a TOS/Differential Service Control Point marking it is only applied to the IP layer and has no impact on your LAN. Additionally, IP packets were only marked on the media stream between IP phones, not between the switches or between the phones and the switches. Version 9 of ShoreTel, now reports “system wide” TOS/DSCP support, which represents a significant improvement in your ability to control VOIP QOS. At the LAN level, it is important to know that you are working with Ethernet frames and for this reason the only QOS marking available to you is a VLAN tag. Inside the VLAN tag, three bits have been set aside for precedence markings and are named COS for “class of service”. If you are NOT running SIP on your network, you have another QOS tool available to you. ShoreTel media streams in other than SIP environments run on UDP port 5004 enabling you to prioritize voice over data at the transport level. ShoreTel also provides the opportunity for you to establish “admission bandwidth control” per site, to assure that the next phone call does not exceed the limits you have set with this parameter. Beware that this parameter exists only within the ShoreTel architecture and has no real knowledge about the actual bandwidth utilization of your network. Establishing this threshold is left entirely to the engineer designing the network. In large part IP QOS is best determined at the IP level and is heavily dependent on establishing queue in your routers that service latency sensitive traffic, voice and video, over less sensitive best efforts traffic. Knowing about these different QOS markings is the science. Knowing how to pass markings from one level to the next is the art of QOS!

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