Lee Howard wrote:
On 2005.02.27 09:30 Martijn van Oosterhout wrote:
On Sun, Feb 27, 2005 at 09:10:48AM -0800, Lee Howard wrote: > Fax cannot handle a one-second delay. As Steve mentions in the > article, per-spec fax has some timings (particularly silence in > direction "switching") set at 75 ms +/- 20 ms. So if the delay gets > much larger than 75 ms, then there's likely to be trouble. Now, > some fax machines may tolerate larger delays, but that tolerance is > beyond the spec, and thus should not be used as a gauge.
Something's not right here.
Quite right. I'm sorry to have misled.
What happens is this (as an example scenario):
The receiver will, for an example, receive the post-page message. The sender expects a response to this. The receiver, however, is required to wait between 55 and 95 ms before transmitting the response. The sender will likely be looking for the post-page response immediately after transmitting the post-page message. Per spec the sender will only wait about 3 seconds (per-spec between 2550 and 3450 ms) before giving up wating and retransmitting the post-page message (and then re-expecting the response).
This is also slightly wrong. The gaps in the audio stream are specified as 75+-20ms. The response is specified as occuring a *minimum* of 75ms after the received carrier has ceased.
So if there is a steady 1000 ms lag between the sender and the receiver (both ways, meaning we assume that both ends could have the 1-second jitter buffer), what will happen is this:
The sender will finish transmitting the post-page message. One second later the receiver finishes getting it. The receiver will introduce its own required pause, and add to that the overhead of any processing required, and then it will return the signal. The sender will not get that signal for another 1000 ms. That means that for the total processing of that to occur the 2550 ms danger-zone time is nearly reached. Add to that buffer-time the latency time, and I'd say that you'd be looking at a signal failure quite certainly.
In real-world action, however, the 2550-3450 ms danger-zone time is practically never reached. In normal use that time is often very close to 400 ms.
So yes, 75 ms latency is not accurate for a command-response interaction between two fax machines. And, per-spec the response could, in theory, sustain a 1000 ms lag. However, that would far-exceed normal behavior, and I'd be surprised if it would not prove fatal to most fax communications.
I think this still allows significant buffering - say 500ms - without causing trouble. Extreme buffer would, however, be troublesome. 500ms, less the rollover time needed for the FEC, should give pretty good jitter buffering.
Regards, Steve
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