Re: VoIP help needed please

Discussion in 'UK VOIP' started by Gordon Henderson, Sep 21, 2008.

  1. In article <>,
    Spence <spencelayhathotmaildotcodotuk> wrote:

    >I've been looking to see if I can find a spec for the bandwidth required
    >for VoIP, but have failed so far. Having said that, if 256kBit upload is
    >OK, then the maximum rate AFAICT is about 120Mbyte per hour, which on
    >current usage allows about 8 hours of calls a month with my spare Gig.
    >That'll do.


    The "basic" CODEC used is G711 - it goes by many names, a-alw, u-law,
    mu-law, PCM, PCM-a, PCM-u, G711a, G711u and probably others... The key
    thing is that the data rate is 64,000 bits per second. 8-bit samples,
    8000 times a second = 64,000 bits/sec.

    However, when you add the "IP" part, you typcially add on an extra
    16Kb/sec per call (SIP), so expect to use 80Kb/sec per VoIP call.

    And remember this is both ways. 80Kb/sec in *and* 80Kb/sec out.

    It's also per call, so 2 calls is using double the numbers. It's not
    uncommon for a company to make 100's of hours of calls a day when they
    have many staff making many concurrent calls.

    Another consideration is packets per second - it's 50 each way per call.

    Ways to decrease bandwidth usage would be to use a compressed CODEC. The
    best (arguably) is G729. This is encumbered with software patents in
    countries and honour such things though, however the basic bit-rate is
    an eigth that of G711. 8Kb/sec plus the fixed 16Kb/sec overhead of SIP.

    If you have a local asterisk based PBX, and your provider supports it,
    you can use IAX to "trunk" the calls rather than SIP - same codecs, but
    the 16Kb/sec overhead is only applied once, so you can squeeze a little
    bit more in.

    However, do'nt think it will work by calculation alone. 256Kb/sec upload
    would suggest 3 concurrent G711 SIP calls, but that's really pushing it
    to the limit and I'd expect something to "give" at that level, so stick
    to no more than 2 unless you're using a compressed CODEC, but then the
    other limit might hit you - packets per second. VoIP packets are small
    - Typically 160 bytes for G711 (50x160=8000) so 2 calls need a router
    to handle 2 x 50 packets per second each way - 200pps. Keep going and
    you might find that some routers can't handle the packet-rate, even
    though they can handle the data-rate, if the packets were bigger...
    or the ISP might flag this as something "odd" (Which I'm not aware of
    any doing so so-far, but ... while it's easy to make a network transmit
    vast quantities of data using large packets, doing it with small packets
    is something else)

    Using IAX, I have had 10 concurrent G711 VoIP calls over a busines quality
    ADSL line though (Entanet, 830Kb/sec upstream), and 18 concurrent G729
    ones though it.

    Gordon
    Gordon Henderson, Sep 21, 2008
    #1
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