Hi Wassim ,

Thanks for your question.

From the perspective of Cisco's implementation of FOIP , all fax calls begin as voice calls , after which they can escalate/switchover  to fax. There are triggers that allow the call to escalate to fax. For example. an ANSAM /CED tone detected by the DSP ; or a V.21 tone detected by the DSP.

Once the call formally escalates to fax , the actual fax data ( T.4 / T.30 ) has to be transported over the IP network. There are principally two methods by which this occurs.

1. Passthough

2. Relay

Passthrough : All the modulated fax data is transported using G711packets over the IP network( the actual fax tones are simply encapsulated in voice packets ) . To handle transmission of fax a few other changes occur on the gateway as well such as :

  a) Echo cancellation  is disabled

  b ) The jitter transitions to a fixed size ( it is adaptive by default ) 

 c ) Vad is disabled.

  d ) There is codec upspeed to G711.

For passthrough ; Cisco uses the following variants.

1. Modem passthrough - Here the transition to fax occurs via NSE packets that are exchanged between the originating and terminating gateway on receipt of a CED  or an ANSAM tone. ( This method is Cisco proprietary and will not work with other vendors )

2. Protocol based passthrough- Here the switchover/transition to fax is done using the call control protocol ( either H.323 / SIP ; MGCP & Sccp do not support this method ).

Relay : Here the fax data ( t.30 /t.4 ) is demodulated by the DSP's into binary data  & transmitted over the IP network over UDP/ TCP / RTP ( Cisco uses UDP ) the receiving side re-modulates this data into fax signals that are played out.

Relay does not have any restrictions of the call being G711 as such since the Fax data is not transmitted over voice packets and is generally more reliable.

For Fax relay there are the following methods.

Protocol based T.38 relay - This is the most popular implementation and is an industry standard. Here the fax data can be encapsulated over UDP , TCP or RTP. Cisco Uses UDP.The switchover/ transition to fax occurs via the call control protocol ( SIP , H.323 , MGCP ; SCCP does not support Protocol based T.38 )

NSE based T.38 relay- This is similar to modem passthrough in terms of how the call switches over to fax ; via the transmission of NSE packets between originating and terminating gateways. Once switchover is complete , normal T.38 encapsulation/ procedures are used.( note : SCCP supports T.38 NSE based relay ) 

Cisco Fax relay - Proprietary method that is supported by all call control protocols.

HTH ,

Kaustubh

Thanks for making it such an easy to understand terms. Could you please post any documents describing the commands for the same please?

Regards

Hi Wilson ,

I am working on that.

//Kaustubh

Hi Wilson,

Attached the document that explains the fax over IP.

Please check and let me know if you have any further queries.

Thanks,

Pawan

Hi Wilson ,

Thanks for reaching out , here are my thoughts.

1. I assume your question is on preparedness for FOIP when integrating with an ITSP?Correct me i'm wrong...

Below are a few things you can inquire about...

   a) What is the method over which they offer Foip services ( most ITSP's will do T.38 Version 0 which supports speeds upto 14.4 Bps) Some may use protocol based passthrough as well...regardless of whether they use protocol based passthrough or T.38 the idea is to make the fax method consistent throughout the network.

For example

Fax server----SIP---CUBE-----SIP------ITSP ( Fax server , ITSP , CUBE all need to do either T.38 or protocol based passthrough )

In extreme cases a few service providers do not formally "escalate/switchover" to fax and choose to stream fax t.30/t.4 data in G.711 packets this can cause severe issues.

b) In case of T.38 , you need to ask them whether they use multiple m= lines to switchover to fax. There are known issues on CUBE with regards to handling of switchovers with multiple "m= " lines.

For example , if the ITSP switches over to T.38 over SIP using the following m lines in the SDP , the call will fail on CUBE due to defects :

m=image 53200 udptl t38

m=audio 53190 RTP/AVP 8 0 101

2. Modem over IP is already in implementation , have a look at this link.

http://www.cisco.com/c/en/us/td/docs/ios/voice/fax/configuration/guide/15-2MT/vf_15_2MT_book/vf_fxmdm_oIP_ov.html#wp460072

Let me know if you have any queries.

HTH ,

Kaustubh

Use the command "show call active vice brief" and once your call switchover to fax call then output will change as below:
For modem passthrough: codec "g711ulaw"" and ""MODEMPASS nse"
For protocol pass-through: codec "g711alaw"

With SIP provider our gateway will work as CUBE and it will not initiate fax switch-over. We can have below considerations:
++ Don't use interworking protocol and better to change from H323 to SIP between CUBE and CUCM.
++ ATA 186 don't support T38 so use ATA 187.
++ T.38 with redundancy is the best choice for fax calls when packet loss is occurring.
++ I will recommend to use T.38 as primary and pass-through as fallback.
++ Here are some more benefits of T.38