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Fax Gateway T38

Hi all,

I'm having the folowing problem regarding sip t38 with the folowing scnario:

FAX-FXS-2801-GW---->SIPv2-TRUNK---->CM7.1.2----->CUBE------>SIP-TRUNK-PROVIDER

When i try to send a fax to the sip trunk the cube send a invite with t38 in the sdp to the callmanager but the the callmanager adds a second mline with g711a that prevents the gateway to switch to t.38.

No. Time Source Destination Protocol Info

33 19:58:38.705933 10.210.40.240 10.200.0.68 SIP/SDP Status: 200 OK, with session description

Frame 33 (1046 bytes on wire, 1046 bytes captured)

Ethernet II, Src: Cisco_bb:5a:80 (00:25:45:bb:5a:80), Dst: HewlettP_62:83:3e (00:23:7d:62:83:3e)

Internet Protocol, Src: 10.210.40.240 (10.210.40.240), Dst: 10.200.0.68 (10.200.0.68)

User Datagram Protocol, Src Port: sip (5060), Dst Port: sip (5060)

Session Initiation Protocol

Status-Line: SIP/2.0 200 OK

Message Header

Message Body

Session Description Protocol

Session Description Protocol Version (v): 0

Owner/Creator, Session Id (o): CiscoSystemsSIP-GW-UserAgent 7851 2943 IN IP4 10.210.40.240

Session Name (s): SIP Call

Connection Information (c): IN IP4 10.210.40.240

Time Description, active time (t): 0 0

Media Description, name and address (m): audio 0 RTP/AVP 8

Connection Information (c): IN IP4 10.210.40.240

Media Attribute (a): rtpmap:8 PCMA/8000

Media Description, name and address (m): image 18648 udptl t38

Connection Information (c): IN IP4 10.210.40.240

If i put the the folowing scenario:

FAX-FXS-2801-GW---->SIPv2----->CUBE------>SIP-TRUNK-PROVIDER

The fax send with t.38 with no problem, the only diference is on the invite with t.38 that does not have the first mline for g711a

Frame 33 (1046 bytes on wire, 1046 bytes captured)

Ethernet II, Src: Cisco_bb:5a:80 (00:25:45:bb:5a:80), Dst: HewlettP_62:83:3e (00:23:7d:62:83:3e)

Internet Protocol, Src: 10.210.40.240 (10.210.40.240), Dst: 10.200.0.68 (10.200.0.68)

User Datagram Protocol, Src Port: sip (5060), Dst Port: sip (5060)

Session Initiation Protocol

Status-Line: SIP/2.0 200 OK

Message Header

Message Body

Session Description Protocol

Session Description Protocol Version (v): 0

Owner/Creator, Session Id (o): CiscoSystemsSIP-GW-UserAgent 7851 2943 IN IP4 10.210.40.240

Session Name (s): SIP Call

Connection Information (c): IN IP4 10.210.40.240

Media Description, name and address (m): image 18648 udptl t38

Connection Information (c): IN IP4 10.210.40.240

Can any one tell me what is the diference on the gateway on receiving the two diferent invite.

Thanks

5 REPLIES

Re: Fax Gateway T38

It sounds like CUCM is adding this into the INVITE.

You may have an MTP involved here. If so, it needs to be on IOS, and you need to have 'codec passthrough' as one of the capabilities enabled.

I would 'debug ccsip messages' on both CUBE and the fax gateway, and see what the difference is. I haven't seen an IOS gateway add G711ulaw/alaw into a fax invite personally, so my suspicion is with CUCM.

-nick

Community Member

Re: Fax Gateway T38

Hi nick,

From the traces that i have taken i can see that the callmanager is adding the m line to the invite. I dont have any mtp involve in the call. I have to fing if is there any way to remove that m line from the invite.

Thanks

patcbr600

Re: Fax Gateway T38

What version CUCM?

Community Member

Re: Fax Gateway T38

CM 7.1.2

Community Member

Re: Fax Gateway T38

Hi,

Just for note, i have found in sip profile that the folowing option was checked by mistake:

Outgoing T.38 INVITE include audio mline

After uncheck everithing works

Thanks

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