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Introduction

Most current fax devices are Group 3 compliant. Fax Group 3 is a standards-based technology that

consists primarily of International Telecommunications Union (ITU) T.4 and T.30 recommendations.

ITU T.4 pertains to how the fax image is encoded by a fax device, and ITU T.30 details the facsimile

negotiations and communication protocol. T.30 is the standard procedure for fax transmission in the

public switched telephone network (PSTN).

Group 3 fax devices were originally designed for use in the PSTN; however, the PSTN itself was

designed for human speech (35 Hz to 3,500 Hz), so Group 3 fax devices use analog encoding or

modulated signals. Fax machines are digital devices that use a modulated analog signal (audio tones)

to pass digital information through the PSTN.

Gateways in packet networks initially treat voice and fax calls the same. Both types of calls cause the

gateway to load a pre-configured voice compression codec in the digital signal processor (DSP). Voice

compression codecs are usually high compression codecs, such as G729 and G723, so less bandwidth is

used for voice calls. High compression codecs are optimized for voice and do a good job of conserving

bandwidth while maintaining voice quality. However, G.729 and other high compression codecs are not

optimized for fax transmissions. In fact, modulated fax signals usually do not pass correctly when high

compression codecs are used, and fax calls fail as a result.

Faxes can be transmitted successfully when codecs with lower compression ratios or no compression are

used, such as G.726 and G.711 with no echo cancellation or voice activity detection. This method of

sending faxes through voice codecs is referred to as inband faxing or fax passthrough. The initial analog

modulated signal is encoded and compressed by the codec on the originating gateway and passed across

the packet network as if it were a voice sample. The terminating gateway uncompresses and decodes the

sample and plays it out to the terminating fax machine.

Another technique, known as upspeeding, allows the originating gateway to initially load the configured

voice compression codec into the DSP for voice calls, and changes to a low compression codec if fax

tones are detected.

Fax Basics

Fax calls can be divided into two parts: fax negotiation and page transmission:

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Cisco BTS 10200 Softswitch T.38 Fax Relay Support

Half-duplex fax negotiation occurs at the beginning of a fax call. V.21 modulated high-level data

link control (HDLC) data frames are passed at a speed of 300 bps. These data frames are sent in a

standard sequence between the originating and terminating fax devices. During this exchange, the

two fax devices exchange capabilities and both agree on the fax session characteristics before page

transmission takes place.

Some capabilities that are exchanged and negotiated are page transmission speed (called "training"),

error correction mode (ECM), resolution, page coding, and scan time. Page transmission speed is

the speed at which the fax is going to send its information. Fax devices always try to "train" at the

highest transmission speed possible based on the parameters initially exchanged. The fax devices

retrain to a lower speed if training fails at a higher speed.

Page transmission occurs after the training part of the fax negotiation phase is complete using the

previously agreed upon parameters. The page information is coded into scan lines with a standard

resolution of 203 horizontal (H) dots per inch by 98 vertical (V) dots per inch. Fax images are

typically compressed and encoded using Modified Huffman (MH) encoding or Modified Read (MR)

encoding. MH encoding usually compresses fax transmission at a 20:1 ratio. MR encoding typically

provides a 20 percent compression improvement over MH, but is slightly less resistant to error.

Cisco BTS 10200 Softswitch T.38 Fax Relay