As mentioned in the previous question I am looking at sending voice communications over a Metro Ethernet connection shared also by data connections. In addition to the previous question I have this one involving QoS guarantees and TDMoE. Voice communications are expected to use a little over 25-30% of MAN link capacity (basically running a PRI over TDMoE, which is 15% plus significant encapsulation overhead.

My current plan is just to ensure that the TDMoE is properly classed re: CoS and use QoS management on the switches to ensure that it is transmitted first. This seems simple, straight-forward, and hopefully ensures reliable voice transmission rates.

Is this good enough? Is there anything QoS-wise that I am missing that I need to be doing?

2 Answers 2


Can you provide some more information on the equipment used and how much bandwidth you have? Will your voice equipment mark packets itself or do you need to take care of it? Usually CoS 5 and/or DSCP EF is used to carry voice payload. Voice should be sent in some form of LLQ (Low Latency Queue).

If you give us some more specifics we could help with providing a policy for you.


You're going to want a CIR that matches closely with what the bit rate of your "TDM circuit" will be, and a very small CBS (if any at all). A substantial CBS has the potential to introduce FDV because there's some overhead/ability to burst - you want to avoid this. You normally don't have bursts with TDM traffic, but you do with Ethernet, and you don't want that option to be there.

If you're up to it, check out the MEF 8 technical spec/implementation agreement for emulating PDH circuits over metro (now "Carrier") Ethernet. Direct link here:


The good bits on CoS start on page 26.

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