[Asterisk-Users] Clarification - FAX on local network

[Bill Seddon]

A potential reason for the difference could be if Asterisk uses UDP (which I
think that I've read somewhere it does).  TCP is a protocol that demands
that transmitted packets are numbered and that the receiver both request the
re-send of packets that appear to be missing and order any packets that are
received out of order (the frame header includes a frame number).  In this
way, there is a fairly reasonable chance that the data sent will be
received. After all, where would this message be (which will be sent and
received using TCP) if sme of th chractrs wer misig.

By comparison, UDP was designed for use in environments where the
transmission control is not so important.  By reducing or removing the need
for transmission control, more frames can be squeezed into the available
bandwidth potentially providing higher data rates.  However, as data can be
lost UDP is only suitable for use in applications where the loss of some
data is not likely to be materially important.  An example is voice
transmission.  Others include video or web conferencing.  This is why SIP
phones include features like missing packet interpolation.

So I'd guess that Asterisk uses UDP, that Hylafax uses TCP and that sending
a TIF image via Asterisk is asking for trouble unless Asterisk can, under
prescribed circumstances, use TCP.



-----Original Message-----
From: asterisk-users-bounces at lists.digium.com
[mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of Lee Howard
Sent: September 14, 2004 7:29 PM
To: Asterisk Users Mailing List - Non-Commercial Discussion
Subject: Re: [Asterisk-Users] Clarification - FAX on local network

On 2004.09.14 11:10 Marty Mastera wrote:
> Ok, ok, I know there has been plenty of discussion on asterisk and fax
> -
> from this I understand:
> 
> 1)    First and foremost, use g.711 ulaw

Yes, the codec must be lossless.

> 2)    Packet loss, etc...makes faxing over the internet unreliable

I'm not sold on this theory yet.  I don't think that it's so much a 
matter of packet loss (this shouldn't occur regularly), but rather of 
latency.  Transmitting packets over a network, and in particular the 
internet, can result in latency delays that could, in theory, pose a 
problem for FoIP, but I've heard of so many people successfully doing 
FoIP with equipment other than Asterisk (i.e. using Cisco VoIP 
equipment), that I tend to believe that the reliability factor is more 
a consequence of SIP or the equipment used (Asterisk and, in my case, a 
Sipura SP-2000).

I have used a HylaFAX system connected to an SP-2000 for both sending 
and receiving faxes.  The fax call comes in to an FXO on the Asterisk 
server, which directs it over a small LAN to the Sipura.  This 
arrangement works tolerably well, but it's quite noticeably 
less-reliable than when the HylaFAX server is connected directly to the 
PSTN.  I don't know if the Sipura is to blame or if it's Asterisk, or 
if it's SIP design.  Although I've used a number of IAXys for voice, I 
haven't tried using one instead of the Sipura for fax.  (Testing that 
would be able to eliminate or isolate SIP+Sipura.)

Lee.
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