[Foip] FoIP-TG: Issues with V.8/V.34/V.152 call flows.
steveu at coppice.org
Thu Jun 25 10:29:00 EDT 2009
Wang, Chris wrote:
> Hi Michael,
> Thanks for the clarifications.
> Regarding T38MaxBitRate attribute (first defined by T.38 Amd2), it was
> designed to be used for purpose of bandwidth allocation considerations
> only. Using it to infer the modulation type is probably undesirable as
> it might lead to guessing, since multiple modulations share same bit
> rates. Although Gerard’s suggestion of using T38FaxVersion is viable,
> but most of the implementations in the field still declare version 0
> to allow wider interoperability.
If T38MaxBitRate is to work out bandwidth, why do machines supporting
V.34 say 33600, and things supporting V.17 say 14400? They should say
something at least a little higher, to allow for packetization
overheads. If there is any redundancy in transmission the number they
quote should be much higher. In practice its yet another woolly aspect
of the T.38 spec that nobody really knows what to do with. Its now so
broken in the field its useless.
> There is also the question of which GW should initiate the
> negotiation. Mule and Li’s draft recommended the fax detection should
> be tasked to the receiving GW, hence be the initiator of the
> negotiation. This draft although never got ratified into a standard
> track RFC, but it is widely used and implemented. Thus, if we want to
> let the emitting GW initiate the negotiations upon intercepting valid
> CM messages, we need to clearly state that this is only for a
> particular type of call flow, but not applicable to others (eg, non
> V.8 cases). One danger here is the negotiation between a V.34 emitting
> GW and V.17 receiving GW would likely lead to failure (ie, SIP 488
> response) because of T38FaxVersion being set to 3 and rejected by
> receiving GW.
T.38 properly defines who starts T.38 negotiation, although the field a
lot of things disobey the rules, and some machines foul up any call
where both ends start negotiation at the same moment.
T.38 D.2.2.3 says a call starting in T.38 mode starts with a T.38 invite
from the source of the call.
T.38 D.2.2.4 says that if a call starts in voice mode, the receiving
gateway is the one which should initiate a transition to T.38 is if
detects a FAX machine.
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