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When designing a VoIP network that will run over a DSL network, what should the sustained bandwidth

I am in the process of designing a VoIP network that will run over a DSL network. The IPPBX's will be Avaya IP office. The codec to be used between sites is g.729a. The DSL tail circuits will be pppoATM. I am a bit confused in trying to estimate the bandwidth required on the links.

  • 3 x VoIP calls at 24kbps (excludes Layer2 overhead)
  • Data will take about 128kbps.


    1. What should be the sustained bandwidth I should plan for?
    2. What Layer 2 overhead does one assume for pppoATM?
    3. The VPN is a private IP network implemented over MPLS and therefore the core network would run over ATM?
  • To determine the sustained bandwidth you should look at the calling pattern between the various sites. If you say the data will be constant at about 128 kb/s, then the bandwidth required will depend upon the number of simultaneous calls you expect to have.

    PPPoA is defined in RFC 2364. Many DSL IADs support both methods:

    RFC 2364 PPPoA VCMUX
    RFC 2364 PPPoA LLC

    A good reference for comparing the overhead of the different types of encapsulation is found here.

    Another thing to consider is that the way that packets are divided up and inserted into the ATM cells can change the overhead.

    A decent thread discussing overhead of protocols running over DSL can be found here.

    Dig Deeper on Network Planning and Testing for IP Telephony

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