RE: hosted PBX/VOIP thru VPN?

From: Frank Bulk (no email)
Date: Sat Nov 15 2008 - 17:49:16 EST

  • Next message: Kevin Loch: "Re: Catalyst 6500 High Switch Proc"

    Nathan hit the nail on the head in his first sentence. With a VPN, if the
    latency is low enough to allow retransmission of the UDP-based RTP packets
    before the ATA's jitter buffer is starved, there can definitely be an
    improvement in audio quality. This was documented in a VoIP testing review
    that Network Computing performed several years ago. Since most jitter
    buffers range from 20 to 80 msec (2 to 4 times the sampling size), it's
    unlikely a hosted PBX with service delivered over the internet would benefit
    from a VPN.

    Frank

    -----Original Message-----
    From: Nathan Ward [mailto:]
    Sent: Wednesday, November 12, 2008 6:01 AM
    To: nanog list
    Subject: Re: hosted PBX/VOIP thru VPN?

    On 13/11/2008, at 12:39 AM, Aaron Wolfe wrote:

    > Because the broadband connection was so fast, TCP was able to
    > repair the impairments without reducing voice quality. "

    That works fine if latency+window size is low, so that segments are
    retransmitted quickly.

    You really should also do the math and factor in the latency that
    comes from doing something like this, assuming you lose a packet. G.
    114 recommends an end to end latency of no more than 150ms for voice
    applications, where over 400ms is unacceptable (between 150 and 400
    you should indicate that performance is not ideal).

    Finally, some audio codecs work well with fairly high amounts of loss
    - I'd recommend doing something like that first. iLBC does this really
    well. G.729 etc. do not - they rely on context, so a single packet
    lost results in several packets of lost audio (and so, silence). iLBC
    doesn't rely on context, and quality degrades during packet loss
    before you get silence.

    The i stands for Internet - so no surprise it works great in typical
    Internet conditions.

    --
    Nathan Ward
    

  • Next message: Kevin Loch: "Re: Catalyst 6500 High Switch Proc"





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