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T.37 (Store-and-Forward) is an ITU standard which deals with sending  and receiving fax messages using an email. It defines the basic  communication procedures between the voice gateway and both the PSTN fax  and mail server used in a T.37 call flow. Additionally they cover the  formatting rules and encoding methods (MIME and TIFF) used in the  fax/email conversion process for T.37. The T.30 specification defines  the communication protocol between the T.37 voice gateway and the  PSTN fax machine. Communication between the T.37 voice gateway and the  mail server occurs via SMTP. MIME supplements SMTP by removing many of  the formatting restrictions imposed by RFC 822 and provides attributes that describe how the fax images are encoded  within the email so that the destination device can easily decode them.

Cisco's implementation of T.37 supports a very small subset of the  possible content types and encodings supported by MIME. For example, a  Cisco T.37 gateway can only process emails with plain text or enriched  text in the body of the email. Additionally, it only handles fax pages  that are graphically encoded as TIFF-F as an attachment to the email.

T.37 has two modes of operation:

  • OnRamp - A T.37 OnRamp gateway (must run on the  originating gateway) handles incoming PSTN fax calls, converts them to  an email, and forwards the email to an SMTP mail server
  • OffRamp - The OffRamp functionality (must run on a terminating gateway) turns incoming emails into a fax call

T.37 Store-and-Forward Call Flow


With the On-ramp process, a voice gateway handles incoming calls from  the standard fax machine or the PSTN and converts a traditional Group 3  fax to an e-mail message with a Tagged Image File Format (TIFF)  attachment. The fax e-mail message and attachment are handled by an  e-mail server while traversing the packet network. When acting as the  On-ramp gateway, the Cisco gateway receives faxes from end users,  converts them into TIFF files, creates standard MIME e-mail messages,  attaches the TIFF files to the e-mail messages, and forwards the  fax-mail messages to the designated SMTP server for storage. The gateway  uses the sending MTA and dial peers to complete these tasks. The  sending MTA, which is the Cisco gateway, defines delivery parameters  associated with the e-mail message to which the fax TIFF file is  attached. The delivery parameters include defining a return e-mail path  or designating a destination mail server.

OnRamp Configuration Guide


hostname OnRampGW

! Enable T.37 function on the gateway fax interface-type fax-mail

controller T1 x/y/z

framing esf
clock source line primary
linecode b8zs
pri-group timeslots 1-24 voice-port x/y/z:23

! Define the onramp script to be utilized by the POTS dial peer application
service onramp flash:app_faxmail_onramp.

! Link the onramp POTS dial peer to the script defined in the application submenu.

! Ensure that this OnRamp POTS dial-peer matches the correct incoming number dial-peer voice <tag_1> pots
service onramp
incoming called-number 5556667777
port x/y/z:23
dial-peer voice <tag_2> mmoip

! Define the IOS-bundled onramp script for the MMoIP dial peer. service fax_on_vfc_onramp_app out-bound
destination-pattern 5556667777
! Specify that this MMoIP dial peer handles fax information-type fax

! Indicate the email address of where the fax email should be sent (“To:” field) session target

! Request that an MDN message be sent back when the destination mail client has

! opened the fax message

! Request that DSN messages be sent dsn delayed
dsn success
dsn failure

! Define the value to be used in the CSI message sent to the originating fax machine. ! Wildcard that specifies that the information displayed is captured from the configured

! destination pattern. fax receive called-subscriber $d$

! Specify the destination email server that will receive the OnRamp fax email mta send server port 25

! Specify text that will appear in the “Subject:” field of the email header mta send subject Incoming PSTN Fax for 5556667777

! Indicate the filename that will be used for the fax image attached to the email mta send filename pstn_fax

! Insert a user comment to the email prefix header for additional identifying information mta send origin-prefix This is an incoming fax message from the PSTN

! Define the postmaster mail account for the onramp gateway mta send postmaster

! Specifies the hostname and username for the originator (“From:” field of email header. mta send mail-from hostname
mta send mail-from username FAXES

! Specify the hostname and username for where MDNs should be sent. mta send return-receipt-to hostname
mta send return-receipt-to username myaccount

In this the originating PSTN fax machine dials (555) 666-7777  to reach the onramp gateway.

The call arrives on voice port x/y/z:23 and matches the inbound POTS  dial peer <tag_1>, which is configured with the appropriate onramp  TCL script.

On the outbound side the MMoIP dial peer <tag_2> is matched  which directs the gateway to send the fax transmission as an email.

This working onramp configuration contains all of the required T.37  onramp commands as well as some of the optional ones that are commonly  seen.

Note that this onramp configuration can be combined with an offramp  configuration in order to create a single gateway that supports both  onramp and offramp functionality.


With the Off-ramp process, a voice gateway handles calls going out  from the network to fax machine or the PSTN and converts a fax e-mail  with TIFF attachment into a traditional fax format that can be delivered  to a standard fax machine or the PSTN. Off-ramp faxing requires that  the Cisco gateway act as an Off-ramp gateway to dial the POTS and  communicate with a remote fax machine (Group 3 fax device), using  standard fax protocols.

The Off-ramp gateway provides the following functionality:

  • Converts a fax-mail to TIFF file (or plain text file) into a  standard format and delivers it to the recipient. The Store-and-Forward  Fax application does not alter the TIFF or plain text file in any way  from its original format when converting it into a standard fax format.  The Off-ramp gateway uses the receiving MTA and dial peers to perform  the conversion.
  • Delivers an e-mail message as a standard fax transmission. The Cisco  gateway generates information that is appended to the top of each faxed  page (text-to-fax pages) and creates a fax cover sheet. The Off-ramp  gateway uses the receiving MTA, dial peers, and commands specific to  formatting the appended information and generating a fax cover sheet to  deliver e-mail messages as fax transmissions.
  • Uses only POTS dial peers to define the line characteristics between  the forwarding Off-ramp gateway and the fax device. Optionally  configure the MMoIP dial peers can be configured to define fax  compression schemes and resolution. This option is useful only if those  parameters are to be altered for the received fax-mails.
  • Defines the parameters associated with the gateway SMTP server,  using the receiving MTAs. This can be its SMTP host aliases, which can  be different than its normal DNS hostnames, or internal Cisco IOS  hostname.

OffRamp Configuration Guide


hostname OffRamp

! Define the TSI value to be sent by the offramp gateway to the termainting fax machine.
fax send transmitting-subscriber $s$

! Specify the left fax header to display the sender’s address, the center header to
! display the transmission time and the right header to display the page number.
fax send left-header $s$
fax send center-header $t$
fax send right-header $p$

! Specify that a fax cover page be included with offramp fax transmissions.
fax send coverpage enable

! Include email header information on the fax cover sheet.
fax send coverpage show-detail

! Specify a comment to be included on fax cover pages.
fax send coverpage comment OffRamp Fax From XYZ Corporation

! Enable T.37 faxing on the voice gateway.
fax interface-type fax-mail

! Specify the mail server where DSN messages are sent.
mta send server port 25

! Define additional hostnames to be used as an alias for this offramp gateway.
mta receive aliases
mta receive aliases

! Specify the number of simultaneous SMTP connections for the offramp gateway.
mta receive maximum-recipients 100

! Enable MDN response on the offramp gateway.
mta receive generate mdn

! Specify the location of the offramp TCL script.
! This script is referenced by the offramp MMoIP dial peer.
service offramp flash:app_faxmail_offramp.

voice-port 1/0:23

! Configure the offramp outbound POTS dial peer.
dial-peer voice <tag_3> pots
destination-pattern 4695550000
port x/y/z:23
forward-digits all

! Configure the inbound offramp MMoIP dial peer.
dial-peer voice <tag_4> mmoip
! Link this MMoIP dial peer to the TCL offramp script defined under the application submenu
service offramp
! Specify that this MMoIP dial peer handles fax
information-type fax
incoming called-number 4695550000
! Specify the optional use of MH image encoding.
image encoding MH
! Specify the optional image resolution of super fine.
image resolution super-fine
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