Fax completes at speed of 9600, but T.38 indicates speed of 14,400
Symptom:
T.38 Fax calls are not completing at the data rate offered in either SIP or H.323 signalling on the Dialogic® IMG 1010 Integrated Media Gateway or Dialogic® IMG 2020 Integrated Media Gateway (IMG 2020), formerly referred to as Dialogic® BorderNet™ 2020 Session Border Controller.
In the case of SIP, the T.38 attributes in the Session Description Protocol offered by both endpoints may indicate V.17 (14,400) data rates:
a=cdsc:1 image udptl t38\r\n
a=cpar: a=T38FaxVersion:0\r\n
a=cpar: a=T38MaxBitRate:14400\r\n
In the case of H.323, the H.245 PDU RequestMode will include a Terminal Capability Set containing a T.38 entry that supports V.17 (14,400) data rates:
Item 0
CapabilityTableEntry
capabilityTableEntryNumber: 31
capability: receiveAndTransmitDataApplicationCapability (9)
receiveAndTransmitDataApplicationCapability
application: t38fax (12)
t38fax
t38FaxProtocol: udp (13)
udp: NULL
t38FaxProfile
.0.. .... fillBitRemoval: False
..0. .... transcodingJBIG: False
...0 .... transcodingMMR: False
version: 0
t38FaxRateManagement: transferredTCF (1)
transferredTCF: NULL
t38FaxUdpOptions
t38FaxMaxBuffer: 200
t38FaxMaxDatagram: 72
t38FaxUdpEC: t38UDPRedundancy (1)
t38UDPRedundancy: NULL
maxBitRate: 144
Server logs or reports on the originating or terminating fax endpoints may indicate speeds other than what is supported in the T.38 offered. This can lead to the suspicion that the rate offered by the IMG is not in fact supported, but offered in error.
Reason for the issue:
As a gateway, the IMG, if so configured, can provide T.38 fax relay capabilities for one or both legs of a fax call. Although an analysis of the IMG IP signaling may indicate full V.17 rates, the actual encapsulated rate end-to-end can be lower. The end-to-end rate will be indicated in the T.30 signaling, which is transparent to the IMG, but can be captured in an Ethernet trace, since the T.30 signaling is encapsulated in the T.38 data stream.
To verify the rate negotiated end-to-end, obtain an Ethernet trace of the IMG IP signaling. Within the trace, find the T.30 Digital Identification Signal (DIS) and Digital Command Signal (DCS) sent for the call. These two messages indicate the data rate offered in each direction.
In Wireshark, the display filter to isolate these fax control signals is:
(t30.FacsimileControl == 1) || (t30.FacsimileControl == 65)
In the DIS and DCS you will see the rates offered by the T.30 endpoints. In this case, one end offered up to V.17, while the other offered only V.29 / 9600:
Please click on the following image to display it full size 
Please click on the following image to display it full size 
Fix/Workaround:
Ensure T.30 fax endpoints support the maximum desired rates. Additionally, circuits must be noise-free end-to-end and IP segments free of excess latency and jitter.
Product List
Dialogic® IMG 1010 Integrated Media Gateway
Dialogic® IMG 1004 Integrated Media Gateway
Dialogic® IMG 2020 Integrated Media Gateway (IMG 2020), formerly referred to as Dialogic® BorderNet™ 2020 Session Border Controller
Related Documentation:
SIP FAX/Modem Support:
IMG 1010, IMG 1004:
IMG 2020:
H.323 FAX/Modem Techniques:
IMG 1010, IMG 1004:
IMG 2020:
First published: 07-Apr-2010
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