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SIP T.38 cisco and asterisk

Could any one tell me why cisco use T38FaxMaxDatagram: 72 in sip's sdp?  Why 72?

I've an issue with t38 cisco+asterisk, 72 is too small for asterisk! 72 is too small at all.

T38FaxMaxDatagram:

Send: The T38FaxMaxDatagram attribute MUST be included. The value indicated MUST be

sufficient to accommodate the T.38 Internet Fax Packet (IFP) at the allowed maximum data

rate and packetization time, the RFC 2198 redundancy header, and the redundant IFP

packets. For T.38 carried over RTP, the T38FaxMaxDatagram value is defined as the RTP

payload. To allow a remote T.38 device to transmit using a 40ms packetization period, the

value SHOULD be 173 bytes for a 14.4 kb/s Group 3 transmission or 367 bytes for a 33.6

kb/s Super Group 3 transmission using a redundancy level of 1. If redundancy level of 2 is

applied, the value SHOULD be 255 bytes for a 14.4 kb/s Group 3 transmission or 546 bytes

for a 33.6 kb/s Super Group 3 transmission.

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1 REPLY
Cisco Employee

Re: SIP T.38 cisco and asterisk

This looks to be a bug in IOS and is documented in the following link,

http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCsm24777

You'll need to upgrade IOS to get the fix.

-Felipe

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