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 Henderson, Sep 21, 2008
    1. Advertisements

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. The Martian

    Help Help Help Pentax S5i Help needed (Please)

    The Martian, Jun 4, 2008, in forum: Digital Photography
    Jun 20, 2008
  2. Iain

    Re: VoIP help needed please

    Iain, Sep 21, 2008, in forum: UK VOIP
  3. Graham.

    Re: VoIP help needed please

    Graham., Sep 21, 2008, in forum: UK VOIP
    Sep 21, 2008
  4. Harry Stottle

    Re: VoIP help needed please

    Harry Stottle, Sep 21, 2008, in forum: UK VOIP
    Harry Stottle
    Sep 21, 2008
  5. Woody

    Re: VoIP help needed please

    Woody, Sep 21, 2008, in forum: UK VOIP

Share This Page