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Michael Hanes
Cisco Employee
Cisco Employee






david.jpgDavid Hanes is an engineer for the Cisco Customer Advanced Engineering (CAE) group supporting various emerging technologies through product testing and field trials. He is a technical expert in the area of fax over IP technologies and assists with network design and troubleshooting for critical fax over IP deployments. Since joining Cisco in 1997, he has worked as a Technical Assistance Center (TAC) engineer for the WAN, WAN Switching, and Multiservice Voice teams, a team lead for the Multiservice Voice team, and an Escalation Engineer covering a variety of voice and fax technologies.

Hanes is the coauthor of "Fax, Modem, and Text for IP Telephony" from Cisco Press. He holds CCIE certification #3491.



This document contains the anwers provided for the questions asked during the live "Ask the Expert" Webcast session on the Topic - T.38 Fax over IP Design and Best Practises. 


The series of Ask The Expert session is available in the Ask The Expert section of Cisco Support Community.


The Complete Recording of this live Webcast can be accessed  here.



T.38 FAX Over IP

Q. What are some best practices for configuring T.38 Fax Relay on a VG224 analog gateway running SCCP and STCAPP?

A. For any SCCP controlled analog gateway, the only method available for T.38 is an NSE-based switchover. Protocol-based switchover or standards-based switchover to T.38 is not supported with SCCP controlled endpoints


Q. T.38 with SIP or H.323 - any functional advantage for one or the other?

A. No real functional advantages to one or the other for T.38. Both support NSE or protocol-based switchover. SCCP controlled devices have some restrictions.


Q. Are there any plans for protocol-based T.38 support for SCCP?

A. No, not at this time. It's not being considered but can be if there is a business case presented to the Business Unit. This is probably best addressed through your account team.


Q. Is there any additional config required to enable T.38 v3, or can I just upgrade my IOS and expect the system to use v3 automagically? Is v3 Supported on both MGCP and SCCP?

A. Once IOS is upgraded, it still supports the Version 0 and you to need configure T.38 v3. The system is not going to turn on v3 automatically. v3 is definetly not supported in MGCP and SCCP. It's supported on H323 and SIP.


Q. Enabling T.38 redundancy requires more bandwidth?

A. Yes. High-speed and/or low-speed redundancy requires more bandwidth. Configuring level 2 redundancy for the high-speed data essentially doubles the bandwidth for a FoIP call.


Q. Did you just say that neither MGCP or SCCP supports T.38 v3? Only H323 or SIP?

A. Only H323 and SIP supports T.38 v3.



Q. Fax over IP (FoIP) is generally more affected by packet loss than VoIP. True or False

A. True. Mainly because even moderate packet loss with fax can cause scan line errors and a call to disconnect. Many voice codecs can utilize prediction algorithms to conceal for missed samples (at the same bandwidth), but T.38 requires redundancy (implying higher bandwidth usage) to be enabled to cope.



Q. Is there a dream fax configuration which can be configured on H323 gateways which works consistantly with different third party fax mechines.. ?

A. Basically standards-based T.38 fax on H323 or SIP voice gateways should work with the different third party fax servers and machines. However there may be some changes required depending upon the type of third party fax server and different software versions running.


Q. Is T.37 still supported? Is it going to removed from Cisco products or not?

A. T.37 is still supported in the latest version of IOS. There are no plans to remove support at this time.


Q. On a clutser deployment what is the permitted round trip delay ? allowed ?

A. For T.38, as discussed by David, it is 1000ms. For voice, it's 150ms.


Q. What would the basic MGCP configuration for NSE-based T.38 be?

A. NSE based switchover is the default for MGCP. The cli "no mgcp fax t38 inhibit" is need, but again, should be the default. To configure protocol-based T.38, the following are needed: mgcp package-capability fxr-package and mgcp default-package fxr-package.


Q. Do the Gateways with an LSI-based DSP chip also now support V34 or still just the TI DSP chip.

A. Yes, support for SG3 for T.38 is supported with the new LSI-based DSPs. SG3 is possible with passthrough as well.


Q. What is the difference between T.37 and T.38 ?

A. T.37 is store and forward faxing. The gateway accepts a fax call on the TDM side, converts it to an email and forwards it to an email server. T.38 is real time faxing over IP. Typically, the gateway doesn't participate in the fax call, and only relays it to another gateway or fax server.


Q. Besides the CUCM, are there also implementations models for the UC500?

A. Yes, the fax implementation on the UC500 is similar to that of a voice gateway. It is typically much easier as the fax machine is connected to the same gateway as the PSTN connection.



Q. Can you use a SCCP gateway (VG248) with MGCP PSTN gateway and use both methods (Standards-based T38 and NSE-based T38) at same time?

A. No, both switch over methods are not possible at the same time for T.38. On the VG248, only one option is possible.


Q. What is the difference between real-time FoIP and store and forward faxing?

A. Store and Forward or T.37 uses a script on the voice gateway to convert an inbound fax, from the PSTN, to a email with a TIFF attachment which it then forwards to an SMTP server which then sends to the end user as an email. "Real-Time" FOIP communicates between two Fax machines/devices and doesn't perform the conversion to an email/TIFF.


Q. Let's assume the following topology: Telco - PRI - GW (T.38 and Passthrough as backup) - MGCP - CUCM CUCM has a VG224 registered as SCCP Passthrough and a fax server doing T38 What is the best way to set this up?

A. Basically you have MGCP configured GW and then you have SCCP Gateway doing Passthrough with Fax server and have mix of this configuration aswell. When calls comes in Mgcp Gateway and SCCP GW VG224 negotiates Passthrough and terminates the call on fax server. VG224 has analog fax machine connected to specific port, change the port from SCCP control to use H323 / SIP and you can have T.38 for fallback or Passthrough.

Best solution would be change the Skinny control port (SCCP) to H323/SIP control port.


Q. A Rightfax server is connected to CUCM via SIP trunk. How it should be configured so we have less packet loss?

A. Packet loss is controlled by the Network, so a proper QoS policy between the gateway and fax server is needed and properly prioritizing FoIP traffic.


Q. Are there alternative software solutions (T.38 clients) to the Cisco Fax Server?

A. Yes, there are different alternative solutions available doing T.38 support by Third party vendors.


Q. I have a third party fax server that talks standards-based T38, is it possible to get a fax machine attached to a SCCP VG224 to connect to this fax server?

A. No, it will not be possible with SCCP controlled VG224. You'll need to convert the VG224 to MGCP, SIP or H.323 to get protocol/standards based T.38 switchover to work.


Q. What about the ATA186 and T.38

A. ATA 186 only supports Passthrough. New models of ATA relased this year (ATA187) has T.38 support and Voice GW VG224 also offers support for T.38. ATA187 does suppport T.38 but ATA 186 doesn't support T.38.


Q. What is the difference for fax-passthrough and faxpassthough in the IOS?

A. It dictates the switchover method: “Fax Passthrough” can refers to the more specific passthrough transport method using an NSE switchover to pass faxes over G.711. “Fax pass-through” details a switchover to passthrough using the call control protocol.


Q. Will SCCP protocol ever support standards-based T38?

A. No plans at this time.


Q. Is this session recorded for playback? Also, can I email additional questions after this session?

A. An archive will be available for viewing by November 19, 2010. For additional detail or if you have questions after this session concludes, please visit for more information.


Q. Can a network use both T38 and Passthrough at the same time using the same set of devices?

A. Yes, T.38 and Passthrough are triggered off of different tones. Both can be enabled and depending on the source, will be triggered.


Q. Related to the fax server question - are there any interoperability docs available like those on the Cisco Interoperability Portal?

A. No, but this presentation can be used as a reference. There really is no other options needed for the configuration to work. For more information,


Q. Is MGCP support coming for SG3 with T.38? Any idea on a time frame if its currently being worked on?

A. No, there is no plan to support SG3 on MGCP.


Q. Does the fax implementation works over SIP trunks on the UC500 without third-party applications?

A. Yes, but it depends on the SIP trunk provider and what they support either T.38 or fax passthrough.



Q. In what ways does T.38 outperform G.711 FAX over VoIP?

A. Bandwidth consumption. Fax over VoIP would require a higher fidelity codec such as G.711. Typical VoIP calls use G.729 which doesn't handle tones well as it is optimized for speech. T.38 will only use the BW required for the fax (ie. 14.4 overhead, etc).



Q. If the SIP trunk provider doesn't support T.38, can I use a Cube to transform from T.38 to G.711 to the provider and if yes is their an example IOS config I can use?


A. No. Cube can't transcode from T.38 to G.711.


Q. What are the weaknesses of T.38?

A. Weakeness is lack of RTP header which makes the T.38pProtocol not support features such as SRTP for secure transaction of FAX and CRTP for bandwidth savings which some customers might look for.



Q. Can a fax server be setup to print to specific printers instead of doing emailing?


A. Probably. Contact the fax server vendor for implementation details.



Q. What are the benefits using UDP instead of TCP for T.38 with RightFax?

A. UDP is ideal for real-time applications such as T.38 due to lower overhead. Retransmissions within TCP will not benefit a real-time application like VoIP or Fax over IP.



Q. Does Cisco support Secure Sip?

A. Yes, SIP TLS and SRTP is supported in Cisco IOS.






Q. Will the presentation be available for download or later review?

A. Yes, it will be available so that you can review and download.

It will be on the Cisco Support Community





Related  Information

Jorge Armijo
Level 4
Level 4

Great doc!


Jorge Armijo

Terry Cheema
VIP Alumni
VIP Alumni

+5 - Nice Work.


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