22 Mar The Long-Fax-Failure Problem Revealed Commetrex Solves Problem That Has Plagued the Industry
Posted at 21:02h
, pcm clock-sync
In my last blog post, I explained how slow signaling in carrier networks can result in the failure of a fax session and how Commetrex’ patent-applied-for technology solves the problem. But, you may have wondered, doesn’t Commetrex’ solution mean more G.711 pass-through faxes? And don’t they inevitably fail? That’s a good question because the answer is yes, they do inevitably fail unless the session includes a fax relay with Commetrex’ proprietary buffer-management technology.
Carriers have done a great job of virtually eliminating dropped packets, but PCM clock-synchronization problems remain. The problem results from jitter buffer under-run and over-run caused by the PCM clocks at opposite ends of the link not being equal, which is always the case. The question, of course, is how unequal are they and how long is the fax? The more unequal they are the quicker the session fails. Long-enough G.711 pass-through faxes and even long T.38 sessions can fail if the jitter buffers are not effectively handled. Commetrex’ relay technology and BladeWare media servers include proprietary buffer-management technology that eliminates PCM-clock-synchronization problems in G.711 pass-through and T.38 fax sessions.
In BladeWare, our HMP fax-server platform, terminating G.711 IP faxes use the incoming G.711 packet stream for timing purposes. For every 20ms of G.711 data it receives, for example, the system generates an equal amount of data for transmit, removing clock-synchronization errors. So BladeWare’s G.711 data are exactly in sync with the remote relay’s sample clock, and we never overflow or underflow our G.711 buffers, nor does the remote gateway. Problem solved.
However, in relay-to-relay T.38 operations, there are two analog PCM sample clocks: one at the remote transmitting fax and the other at the local re-modulating modem. These two clocks always have a different rate. Bits generated at the transmitting endpoint fax terminal must be retransmitted by the off-ramp gateway’s local modem. If the remote fax is generating bits faster than the gateway’s local modem can send them out to the fax terminal, off-ramp overflow eventually occurs. Goodbye, fax session! In the reverse case, the on-ramp modem will run dry since the off-ramp gateway is sending the bits out faster than it receives them, and T.38 relay will have to spoof some bits to keep the transmitter running (provided you have a well designed relay, of course). Underflow is not as much of a problem, since the relay can insert additional flags in V.21 data or padding bits at the end of a line of image data (does your relay do that?). But overflow is a problem as valid data must be tossed, and modems just hate that.
Enter Commetrex innovation. Our T.38 relay uses a variable-delay jitter buffer. What that means is that the jitter buffer starts at zero delay for each sequence of incoming packets. If packets are delayed, the buffer will increase in length. This gives us the maximum protection against clock drift, as the clocks are re-synched at the start of each new modem operation. Problem solved for T.38!
But what about G.711 pass-through, you ask? More innovation is our answer. The new technology explained in last week’s post will also be included in a new release of our gateway software, which many of the industry’s OEM use. It allows their gateways to make the same intelligent decision of whether to accept a T.38 re-invite. This same package includes patent-applied-for fax-aware jitter-buffer management that is specific to G.711 pass-through fax and eliminates PCM-clock sync problems in gateways.
Bottom line? Performance matters. Experience matters. Rock-solid FoIP is finally here!