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layer 2 header

Viewing 6 posts - 1 through 6 (of 6 total)
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  • #31508 Reply
    riz
    Guest

    As i have seen the VoIP calculator offered by this web site, lines to VoIP bandwidth calculator and Earlan to VoIP calculator, and read the tutorials on site. the thing i am not able to get is that why layer 2 header is not being added while calculating the bandwidth of headers being added(RTP,UDP,IP)as this will not give the actuall bandwidth required (and any packet has to pass through layer 1 via datalink layer). and in eg. for G.711 the 16kbps added(if we take 50 samples per sec) for headers for every 64kbps will not sufficient bandwidth if we omit layer 2 header. although the layer 2 is changeable and used according to requirement.

    Regards
    Riz khan

    #31509 Reply
    Webmaster
    Guest

    It’s true that the free online calculators do not take the datalink protocol into account. The datalink protocol could be PPP or ATM etc. and would add to the overall badwidth requirement.

    But, the protocols you mention are not layer 2. IP is a layer 3 protocol (network). UDP is a layer 4 protocol (transport) and RTP is a layer 5 protocol (session). These three protocols are taken into account by the calculators.

    For more information about the layered approach, you can study a document we published online.

    Downloadable versions which also take layer 2 into account are available for purchase.

    #31510 Reply
    riz
    Guest

    yah thats fine but even in totorials you have ignored to add the layer 2 header in overall header size.

    #31511 Reply
    Webmaster
    Guest

    We omit layer 2 to keep the calculator simple. It is, after all, a free resource.

    The effect that layer 2 has depends on what datalink protocol is used. Sometimes, it is neglibigle; sometimes it is significant.

    #31512 Reply
    riz
    Guest

    plz tell when datalink layer makes a significant contribution and when it is negligible and how much contribution does it makes.

    and 2nd question is that in calculator you have use Earlang B formula out side the IP netork(i.e. you find only call blocking prbablity in PSTN network and after this you have add the header bandwidth and is telling total bandwidth while ignoring the blocking probablity with in the IP network (PLP:paket loss probablity due to overflow in queues and others)).

    #31513 Reply
    Webmaster
    Guest

    Your first question: It depends on the minimum payload a lay 2 protocol can support. If the minimum payload is large, then changing the size of other headers and the RTP payload makes no difference. You’ll need to research your specific layer 2 header.

    Your second point: Yes, that’s true. But, in my opinion, not significant in the real world.

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