T37 Onramp

By integrating e-mail with standard G3 fax, T.37 store-and-forward fax allows an added dimension of flexibility and convenience that is not possible with fax relay or fax passthrough. Faxes can only be sent and received to and from standard fax devices in the case of fax relay and fax passthrough. In addition to that functionality, T.37 allows receiving faxes as e-mail and sending e-mail that can be delivered as faxes.

A T.37 call is naturally segmented into two completely independent parts: onramp and offramp. Cisco voice gateways can be configured as an onramp gateway, an offramp gateway, or a combination of both.

An onramp gateway is responsible for receiving a G3 PSTN fax call and then forwarding that fax call as an e-mail to a mail server. Figure 6-2 shows a high-level view of this process.

Figure 6-2 T.37 Onramp

Figure 6-2 T.37 Onramp

Originating Fax Machine

Onramp Gateway

Email Server

Originating Fax Machine

Onramp Gateway

Email Server

T.30

SMTP

The onramp gateway in Figure 6-2 first communicates with the originating G3 fax device via the T.30 protocol. After the T.30 communication has been established and the fax page transmission attributes successfully negotiated, the originating G3 fax device sends the page data to the Cisco onramp gateway using the T.4 protocol. The T.4 protocol was covered in detail in the section "Page Encoding" of Chapter 2.

The onramp gateway then converts the received fax page data from the originating fax device into a TIFF file attachment. This conversion process on the T.37 onramp gateway uses an internal TIFF writer that encodes the images of the fax pages using the TIFF-F graphical encoding scheme. TIFF-F is the data format for compressed fax images used by standards-based T.37 gateways.

Following the TIFF conversion process, the onramp gateway constructs a standard MIME e-mail message. This e-mail contains the compressed images of the original fax pages as a TIFF attachment and is commonly referred to as a fax mail.

Finally, the onramp gateway forwards this fax mail to the appropriate mail server that is designated in its configuration. The onramp gateway and the mail server establish an SMTP communication on TCP port 25, and this fax mail is forwarded using the SMTP protocol as described in the previous section.

If necessary, this fax mail is forwarded between mail servers or MTAs until it arrives and is stored at the destination mail server. This fax mail can either be accessed as an e-mail by the recipient's mail client or UA or it can be forwarded via T.37 offramp to a standard G3 fax machine. The T.37 offramp function is covered in detail in the next section.

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