Not all T.38 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 and have 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 that thoroughly understand T.30. Better relays can handle up to five seconds of round-trip delay in the IP path.
Through rigorous testing, Commetrex has learned the effect on call set-up performance of how an ATA, a gateway, or a fax server handles late-arriving T.38 re-invites. Signaling delays within a carrier network can cause a gateway without Smart FoIP to effectively kill a fax session by blindly accepting a T.38 re-invite from its off-ramp peer in a non-V.34 session.
To solve this problem, we developed Smart FoIP, Commetrex’ licensed software that includes patent-applied-for technology that puts intelligence into whether to accept a T.38 re-invite, eliminating this as a cause of failed sessions, boosting transaction success rates, often by 10%.
On one large-footprint IP network we tested, fully 10-percent of the T.38 re-invites took over five seconds to arrive, with most of those resulting in transaction failures. These calls averaged over 8 seconds from the initial Invite’s 200OK to the subsequent re-Invite. Applying our new technology took care of the problem, boosting an IP-based fax-broadcast server’s transaction success rate to equal that of a multi-line intelligent ISDN fax board.
An ATA or gateway with Commetrex’ proprietary relay technology attaches a V.21 modem (along with other analysis algorithms) to the media streams at the beginning of the call. The on-ramp gateway 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 the calling terminal sends its response. Once the calling fax terminal 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 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 the on-ramp gateway 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.
In the case of an IP-based fax server there are no modems since the server is connected directly to the IP network, and the T.30 state machine and the T.38 protocol engine are both on the same system. Therefore, there is no need for a V.21 modem since the server maintains its own T.30 calling-terminal state. The server will accept T.38 re-invites up to the point where it has received the called terminal’s DIS, refusing all subsequent re-invites.
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”, making support for G.711 pass-through in server applications a Smart FoIP prerequisite. As Commetrex’ BladeWare MMTF, and T.38 Fax Relay all support both T.38 and G.711 pass through, Commetrex’ TerminatingT38 does not. Smart FoIP is now included in BladeWare beginning with Release 2.4. Of course, gateways and ATAs support both, and they are plagued by this problem unless they have Commetrex technology with Smart FoIP inside.
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. Commetrex’ Smart FoIP relay technology and BladeWare media servers include 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 since we are slaved to its clock.
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 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).
Enter Commetrex innovation. Smart FoIP 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.
Smart FoIP is an inherent feature in BladeWare. Support for both T.38 and G.711 is required for Smart FoIP to work in BladeWare. Smart FoIP is optional in Multi-Modal Terminating Fax (MMTF) and T.38 Fax Relay licensed media technologies.
Smart FoIP is a module that receives events from the modem package and sends events to the OEM’s call-control and buffer-management software. Without Smart FoIP, none of the licensed technologies listed above have any interface with the licensee’s call-control system. But Smart FoIP, since it detects modem events and sends messages to call control, effectively creates an intelligent bridge between the two. This package is fully documented in a developer’s manual.
Commetrex has led the industry effort for T.38 interoperability since January 2002, when it launched the T.38 Interoperability Test Lab. Recently, Commetrex has done extensive interoperability testing with the BladeWare HMP telephony platform, with international carriers, service providers, and enterprises.
- Smart FoIP for MMTF: PN 60324
- Smart FoIP for PowerRelay for T.38: PN 60325