/mcpanalog modem. Both analog modems and fax machines are digital devices that must use a modulated analog signal to pass the digital information over the PSTN. This modulated signal can usually be heard as different audio tones. Gateways in a VoX network initially treat voice and fax calls the same. Both types of calls cause the gateway to load the configured voice compression codec in the digital signal processor (DSP). For more information about DSPs, see Voice Hardware: C542 and C549 Digital Signal Processors (DSPs). The voice compression codecs are usually high compression codecs so that less bandwidth is used for each voice call. High compression codecs, such as G729 and G723, are optimized for voice and compress the voice to a low bandwidth (8 kbps, which excludes overhead for G.729) yet maintain good quality, but G.729 and other high compression codecs are not optimized for fax. In fact, the modulated signals of fax transmissions usually do not correctly pass through when these codecs are used, and fax calls fail as a result. For more information about compression codecs, see Voice over IP - Per Call Bandwidth Consumption. Faxes can be transmitted successfully when codecs with lower compression ratios or no compression at all (such as G.726 and G.711 with no echo cancellation or Voice Activity Detection) are used. This method of fax transmission through the voice codec is usually referred to as inband faxing or fax passthrough. A technique known as upspeeding allows the gateway to initially load the configured voice compression codec into the DSP for voice calls and change it to a low compression codec if fax tones are detected. With inband faxing, the initial modulated signal is encoded and compressed by the codec on the source router and passed across the VoX network, just as if it was a voice sample. The termination gateway then uncompresses and decodes the sample and plays it out to the termination fax machine. Fax relay functions differently. It is a protocol that terminates the modulated signal, extracts the digital information, and then relays the digital information through the data network with data packets. At the termination side, the digital information is extracted from the packet, modulated, and played out. Fax Basics A fax call can be divided into two parts: fax negotiation and page transmission. The half-duplex fax negotiation occurs at the start 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 origination and termination fax devices. In this exchange, each fax device exchanges its capabilities, and both fax devices agree on the fax session characteristics before the page transmission takes place. This illustration shows a traditional fax call over PSTN.