Overview of T37 Storeand Forward

Approved by the ITU-T in 1998, the T.37 specification is a conglomerate of standards that address different facets of the conversion process from fax to e-mail and vice versa. The major standards referenced in T.37 are detailed in Table 6-1.

Table 6-1 T.37 Standards



ITU-T T.30 and T.4

G3 fax standards that T.37 gateways must implement to converse directly

with traditional fax machines

RFC 821 and 1869

Covers SMTP and Extended SMTP (ESMTP) procedures for relaying mail

between mail transfer agents

RFC 2045-2049

Defines MIME (Multipurpose Internet Mail Extensions) and how it works

RFC 2301-2305

Details Internet fax procedures based on Internet mail

Generally speaking, these standards define the basic communication procedures (T.30 and SMTP) between the voice gateway and both the public switched telephone network (PSTN) fax and mail server used in a T.37 call. In addition, they cover the formatting rules and encoding methods (MIME and TIFF) used in the fax/mail conversion process for T.37. You can find ITU-T Recommendations at http://www.itu.int/ITU-T/ and RFC specifications at http://www.ietf.org/rfc.html.

The T.30 specification defines the communication protocol between the T.37 voice gateway and the PSTN fax machine. The operation of this protocol and a review of its component messages were covered in detail in Chapter 2, "How Fax Works."

The communication between the T.37 voice gateway and the mail server occurs via SMTP. The SMTP protocol is the unanimous choice for Internet mail today and it uses TCP to enforce a reliable transaction. The SMTP protocol and its operation within T.37 is discussed in detail in the next section.

MIME supplements SMTP by removing many of the formatting restrictions imposed by RFC 822. This allows the encapsulation of more complex nontext file formats as part of a standard e-mail message. In the case of T.37, MIME provides attributes that describe how the fax images are encoded within the e-mail so that the destination device can easily decode them.

The Cisco implementation of store-and-forward fax supports only a small subset of the possible content types and encodings supported by MIME. For example, a Cisco T.37 gateway can only process e-mails with plain text or enriched text in the body of the e-mail. In addition, it only handles fax pages that are graphically encoded as TIFF (Tagged Image File Format) or even more specifically as TIFF Profile F (TIFF-F) as an attachment to the e-mail. TIFF-F is defined in RFC 2301 and includes support for Modified Huffman (MH), Modified READ (MR), and Modified Modified READ (MMR) fax encodings. These fax encoding types are discussed in detail in the section "Page Encoding" of Chapter 2.

T.37 operates in two different modes: onramp and offramp. The onramp function handles incoming faxes to the voice gateway and coverts them to an e-mail, whereas the offramp function turns an incoming e-mail into a fax call. Each of these T.37 modes of operation is configured independently on a Cisco voice gateway and is discussed in more detail later in this chapter.

To this point, only real-time fax transport methods such as fax passthrough and fax relay have been discussed. T.37 takes a drastically different approach to transporting fax traffic over an IP network. Figure 6-1 illustrates this by showing the vastly different network path taken by a T.37 onramp and offramp store-and-forward fax call compared to the network path taken by a standard fax passthrough or fax relay real-time fax call.

Figure 6-1 Call Path Comparison for T.37 and Passthrough/Relay

IP Network

Originating Fax Machine

Email Server

Email Server

T.37 Call Path

Originating Fax Machine

T.37 Call Path

Passthrough/Relay Call Path

Passthrough/Relay Call Path

Terminating Fax Machine

Both fax passthrough and fax relay assist with the transport of the fax while the fax is actually transmitting. On the other hand, T.37 has a data storage component to it so that it can handle the fax to e-mail or e-mail to fax conversion process. Consequently, depending on the mode of operation, the T.37 gateway either completely terminates or originates fax calls to handle the conversion to or from an e-mail. This is the reason for the "store" portion of the moniker often given to T.37: store-and-forward fax.

This storing mechanism used in T.37 clearly breaks the real-time aspect of fax communication that is seen for fax calls over the PSTN or through an IP network using fax relay or fax passthrough. Although this has the drawback that it decouples the real-time nature of fax communication, it does provide the user with the added convenience of sending and receiving faxes directly from a computer through e-mail.

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