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need help about question

Viewing 8 posts - 16 through 23 (of 23 total)
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  • #29799 Reply
    vivek manchanda

    Voice-over-IP (VoIP) is based on the UDP protocol. This is because UDP has a lower overhead than TCP. What is the effective percentage gain in throughput (of any given payload) using UDP vs TCP?

    #29800 Reply
    Tom Howard

    You know, what worries me most about this question is the assumption in it that VoIP uses UDP because of lower overheads.

    If you students want to stand out, challenge that statement in your answers. It is an oversimplification made by someone who has little understanding of the technology.

    Over their heads I guess. What do you think guys 😉

    #29801 Reply

    We are no students

    #29802 Reply
    Tom Howard

    Really? Why are you all asking exactly the same question then? It seems to be question 7 on your list.

    I have no problem helping students. But, this is a place for open an constructive discussion.

    #29803 Reply

    all the people are asking the effective percentage gain and Mr tom is only ans. the first part that udp is used because of its lower overhead i request him to answer what excatly will be the % gain

    #29804 Reply

    as far i know…it is not possible to give exat percentage gain……’coz it will depend on RTT, delays, etc., What do u say Sir ,TOM

    #29805 Reply

    its write but there must be a way to calculate the Efeective % gain
    can anybody tell me thati will be thankful
    thanks tom for your co operation

    #29806 Reply
    Abhishek Singh

    1)UDP has an 8 byte header, and TCP usually has a 20 byte header.
    Let payload is x bytes

    Throughput of voip will be given by formula:
    (Payload+ udp/tcp header + rtp header + ip header) * 8 * (voip bitrate/payload in bits)

    Terms :
    Payload is constant as specified in problem
    BitRate = bitrate defined by codec (ex. 8Kbps in G.729 )
    Packets per second= (bitrate / payload in bits )
    Num of packets required(dependent on payload) to send per second to maintain bitrate .
    Header = RTP header( 12 bytes) + TCP/UDP header(20/8 bytes) + IP header(20 bytes).

    Effective BW required by Voip call = (pay load + headers) * 8 * packets per second
    = (payload + rtp header + IP header+ TCP/UDP header) *8 * (bitrate / (payload *8))
    = ( pay load + 12+20+ TCP/UDP header) * 8 *(bitrate / (payload *8))

    BW gain = 1-{( pay load + 32+ 8) * 8 *(bitrate / (payload *8))}/{ ( pay load + 32+20) * 8 *(bitrate / (payload *8))}
    Bw gain=1-{pay load + 40}/{payload+52}
    Lets take payload = 20 bytes as in case of G.729
    Bw gain= 1-60/72
    =16.66% is the gain by using udp over tcp
    (gain is calculated over tcp)
    2) for each packet send tcp receives a ACK packet .
    this features wastes BW in case of real time applications like VOIP and media streaming
    because packet will be discarded at receiver if it is received late , but even then ACK packet will be sent by TCP. There is no such problem in UDP.

    3) Udp never decreses its send rate . while TCP can decrease its send rate because of its congestion control algorithm, which is an overhead in case of VOIP.

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