Although we are in the second decade of the twenty-first century, fax machines do exist. That sends shivers down the spines of many! In fact, some industries such as healthcare, finance, and legal use faxes as a key component of day-to-day activities. Many channel partners have given up and either say, “Keep your POTS lines!” or “store-and-forward” or just settle for a plain ATA/gateway, accepting its inherent FoIP problems.
Fax was designed to meet the various challenges of the PSTN, but Fax over IP (FoIP) is a different beast, and must adapt to the transmission protocols that IP utilizes. No other solution available is designed to meet the challenges presented by FoIP like Smart FoIP, which is available on all New Rock HX4G and MX8G gateways, as well as the New Rock OM20G and OM50G PBXs. By eliminating the top-two problems with FoIP; the late T.38 re-invite and the G.711 clock-synchronization problem, Smart FoIP provides near-PSTN success rates.
FoIP and T.38 Background
Not all T.38 (the ITU protocol for real-time faxes in IP networks since 1998) implementations exhibit the same performance in the field, even if they conform to the T.38 recommendation. You could easily have a widely interoperable T.38 with an intolerably low transaction-success rate. It is difficult to verify interoperability, and it’s even more difficult to determine performance. And to make matters worse, the parameters of T.38 performance aren’t even widely known.Beyond interoperability, much of what we call T.38 performance is inherent in its implementation, not the specification. It’s safe to say that the best T.38 designs were developed by an engineer that has successfully fielded T.30, the protocol of terminating fax. This is because T.38 does not give any guidance on how to improve delay tolerance, for example, but, as we know, it is improved through so-called spoofing techniques implemented by skilled T.38-relay developers who thoroughly understand T.30. Better relays can handle up to five seconds of round-trip delay in the IP path.Smart ATA attaches a V.21 modem (along with other analysis algorithms) to the media streams at the beginning of a fax call. Smart ATA analyzes the decoded V.21 data to track the T.30 states of the calling and called terminals. The called terminal will repeatedly send its initial message (DIS) until Smart ATA sends its response. Once Smart ATA receives a complete DIS, it sends its response (DCS) within 75 milliseconds. Therefore, once this calling-terminal response (DCS) is received by the called terminal, uninterruptable G.711-based modem operations have begun, and the gateways can no longer switch the session to T.38 without possible corruption of the T.30 states being maintained in the endpoint terminals.With Smart FoIP, once Smart ATA detects the preamble to the calling fax terminal’s response, it will no longer accept the T.38 re-invite, continuing the transaction in G.711 mode and avoiding the session failures caused by the transition occurring during a modem session.
G.711 Pass-Through Fax
Of course, refusing a T.38 re-invite means continuing the session in what is called “G.711 pass-through mode.” But, you may have wondered, doesn’t Smart FoIP mean more G.711 pass-through faxes? And don’t they inevitably fail? That’s a good question because the answer is yes, G.711 fax sessions do often fail unless the session includes a fax relay with Smart FoIP’s 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 (the endpoint terminals) 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. Smart ATA includes buffer-management technology that eliminates PCM-clock-synchronization problems in G.711 pass-through and T.38 fax sessions.
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 re-transmitted by the off-ramp gateway’s local modem. If the remote fax is generating bits faster than the off-ramp gateway’s local modem can send them out to the fax terminal, off-ramp overflow eventually occurs. In the reverse case (off-ramp faster than transmitting fax terminal), 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).