Showing results for 
Search instead for 
Did you mean: 

Ask the Expert: Foip on CUBE and Gateway’s

Lisa Latour
Frequent Contributor
Frequent Contributor

This ask the Expert Session's Topic is FOIP on gateway’s and CUBE using T.38, protocol based passthrough and modem passthrough. Fax over IP discusses about how Fax data ( T.30/T.4 ) is encoded and transported over VoIP networks.The many pitfalls of successful fax transmission, best practices, known integration issues with third party vendors.

Ask questions from Monday, July 6th to Friday, July 17th, 2015

Featured Experts

Pawan Srivastava  is currently working at Cisco TAC (Technical Assistance Center) under the Contact Center Technology for the Cisco Unified Contact Center Express Team (UCCX). In the past, he has worked with Call Manager & Multi-Services TAC teams and supporting customers and partners on Ip2ip gateways, CUCM, FOIP, UCCX with issues for over 4 years.


Kaustubh Inamdar  is currently working at Cisco TAC (Technical Assistance Center) under the Multi-service technology. With previous experience within the Call Manager TAC team. He has worked on IP2IP gateway, FOIP, CUCM, CUSP, VXML for customer and partners with multiple published documents on Foip and SRST Manager on the Cisco Support Community. CCIE voice certification ( # 42277 )

Find other

** Ratings Encourage Participation! **
Please be sure to rate the Answers to Questions


Wassim Aouadi


Could you give me a simplified explanation on how fax works on IP and how gateway protocols handle it?

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.




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


Hi Wilson ,

I am working on that.



Hi Wilson,


Attached the document that explains the fax over IP.

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




Wilson Samuel
Rising star
Rising star

Hello Gentlemen,

So far almost all the ITSP I have dealt with almost simply have declined to do provide FoIP services. However I am sure in near future there will be many who will, hence my questions would be:

1. When ordering a new ITSP services from a carrier, what are the questions or queries one as a customer or system integrator we should ask the carrier in order for a complete preparedness.

2. Is there a provision that exists or is in near future about Modem over IP as well?




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.


Let me know if you have any queries.





Terry Cheema

Hi Guys,

Thanks for taking out the time for this session. +5 for your posts here.

I am about to migrate a customer from existing TDM to SIP. The customer has some analog faxes, which are plugged into ATA/VG to convert analog to IP and PSTN connectivity is through E1 PRIs. Currently the fax protocol used is the default pass-through, below is the config from one of the gateway:

voice service voip
 ip address trusted list
 fax protocol pass-through g711alaw
 modem passthrough nse codec g711ulaw  <<

Is there any simple show command we can use to check the protocol used here? From the config I am assuming NSE G711ulaw defined under H323 is taking priority? Is that correct?

Also if we were to move the PSTN connectivity from the PRI to SIP, if the telco supports both the pass-through and T.38 is there still any benefit of changing the existing configs to T.38?


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


Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: