On 4/2/2014 5:24 PM, Nathan Anderson wrote:
> On Wednesday, April 02, 2014 6:55 AM, Fred Goldstein <> wrote:
>
>> But in addition to that, I STRONGLY recommend a separate VLAN for the
>> voice-grade channels. With priority, or reserved bandwidth. TCP/IP in
>> normal operation manages its flow rate by having packets thrown away;
>> that's why the 1G LAN port on your PC doesn't blast a whole file at 1G
>> into a 2M link. It uses packet loss as a signal. TCP applications
>> retransmit and actual human voice is intelligible with some gaps, but
>> modems, including fax, are very unhappy.
> Do note that RTP is implemented over UDP, not TCP, so in VoIP, a dropped
> audio packet is a lost audio packet, not a delayed or even out-of-order audio
> packet (although those other two things can happen...they just aren't a
> result of retransmits, or at least not a retransmit initiated by Layer 4).
>
I guess my grammar was a bit rough there! So you're of course right.
TCP applications retransmit. (period) Actual human voice (which doesn't
retransmit, as it can't wait) is intelligible some gaps. Modes,
however, including fax, are very unhappy with gaps.
And stressing Nathan's previous note ("*what* doesn't work?"), this may
be one of those *rare* occasions when a video (YouTube anyone?) might
actually help. Although the audio alone is more important. If we could
(see and) hear the call being dialed by the originating fax, hear what
the ring sequence sounded like, and heard the response, with the speaker
belching the CNG tone all along, it might help identify the problem.
But really, fax and VoIP don't get along very well unless you really
tune the VoIP network up to support it.
And I know how some faxes are picky. My office fax line sat here
virtually unused for years, but my wife needs to receive faxes
regularly. Her fax is on a Comcast PacketCable (they call it VoIP but
it's really managed VuIP) line that is shared with her office phone and
answering machine. My fax (both are Brothers) can send hers a fax. The
answering machine gives its spiel, starts to listen, then the fax hears
CNG and cuts off the answering machine and sends modem tones. Just like
it's supposed to work. But the fancy new fax server system at the
courthouse just won't send to it. (Nor will some sizeable fraction of
other machines.) It will send to mine, which isn't shared with an
answering machine, but not one that is. Picky picky. Fax is like that.
--
Fred R. Goldstein k1io fred "at" interisle.net
Interisle Consulting Group
+1 617 795 2701
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