Fax, Modem and TTY support. (part 5)

 The PSTN reliably supports fax, modem and TTY calls. Calls connect on almost every attempt and rarely fail. A VoIP network must provide a similarly reliable service. However, fax, modem and TTY traffic imposes some additional constraints beyond voice traffic.  Compared to voice traffic, fax, modem and TTY traffic is much more sensitive to packet loss but less sensitive to overall delay. In addition, lower-bit-rate codecs are optimized for voice traffic and cannot transport non-voice traffic. 

ITU-T T.38 defines how fax can be sent in an IP network as pure data, independent of the voice traffic.  However, it is a relatively recent standard and requires the use of a T.38 capable fax machine or T.38 gateway. The ITU-T has recently published equivalent standards for modems (V.150.0 and V150.1) and is currently working on developing equivalent standards for TTY traffic.  Alternatively fax, modem and TTY traffic can be supported successfully over a managed IP network by switching to a full rate codec (G.711). The media gateways need to detect a fax, modem or TTY call and switch to G.711. Silence suppression and echo cancellation also need to be turned off.  

Note that the detection and switch to G.711 needs to be performed in a timely manner, to allow the fax / modem to train at the highest possible data rate.

Auto-configuration
One significant difference between a POTS (plain old telephone service) network and a Next-Generation VoIP network is that for some architectures intelligent subscriber gateways or IP phones now reside on the customer premises. These complex devices need more configuration than a POTS phone, so auto-configuration of subscriber gateways becomes important as the network scales up.  Some of these requirements can be addressed using DHCP, but others require some form of management interface using UPnP, SNMP or LDAP.  Considerable work has been done in the DSL Forum to address auto-configuration of DSL equipment, but to date the issue of auto-configuration in VoIP networks has not been addressed.