No, it’s not usually due to dropped packets when your subscribers say, “I can send a single-page fax with good reliability, but multi-page faxes are unreliable.” Even if you and your carrier partners support T.38, not all faxes will end up switching to T.38, remaining in G.711 pass-through mode. And no matter what you do, you’re going to have FoIP failures if you get a burst of dropped packets in G.711 mode. But when is the last time you saw a Wireshark extraction of the PCM where packet loss was reported? In today’s networks, it’s rare, but failures of a G.711 pass-through FoIP call are, unfortunately, not rare.
It’s caused by PCM clock-sync problems.
Over the PSTN, the clocks in a fax-receive modem are synchronized to the PCM clock in the transmitter with a phase-locked loop, so clock differences are not a problem in TDM modems. But in FoIP systems, the clock in the receive modem for the on-ramp/calling gateway, which is bringing the modem-image bits into the system, is different from the gateway’s system clock, which is driving the T.38 image packets out to the IP network. So there’s the possibility of a gateway either running out of bits to send, possibly (depending on the gateway) causing the transmit modems on the receiving gateway (SIP peer/off-ramp gateway) to run dry, or the buffers in the calling gateway will overflow, causing the call to fail, depending on whether the sending-endpoint terminal’s clock is slower or faster.
If both clocks use a 50-PPM (News - Alert) oscillator, the worst case difference is 100-PPM. So, if the gateway starts with a 200-msec jitter buffer, it may get eaten up in 32 seconds, which is the image-transfer time of many one-page faxes. But if the buffer is not initialized between pages, there will be a problem on the second page.
But if the transaction is G.711 pass-through, how would the gateway know that the fax is between pages, allowing the buffers to be initialized? Commetrex (News - Alert), in a patent application filed in November 2010, disclosed a method of doing so. If you’re interested, the application has been published by US PTO as serial number US-2011-0109936-A1.
To find out more about Commetrex visit the company at ITEXPO West 2012 to be held Oct. 2-5 at the Austin Convention Center in Austin, TX. Visit Commetrex in booth 722. For more information on ITEXPO (News - Alert) West 2012, click here.
Edited by Juliana Kenny