VoIP Networks - Interoperability
Interoperability is a term meaning the capability of different network nodes or applications systems to work together over various VoIP networks and topologies. Interoperability comes into play in the fax arena when people look at whether T.38 systems can work together over SIP trunking, or whether various devices can work with other devices.
On one level, fax systems running over VoIP networks can only interoperate if they can read images in the same format. For example, fax machines can transfer images over FoIP and VoIP networks if they are described in the TIFF (Tagged Image File Format) format. This is straightforward, as every fax device supports TIFF. Then again, issues arise in terms of the compression methods used to reduce fax image size. Modified Huffman (MH), Modified modified Huffman (MMH) or JBIG (Joint Bi-level Image Experts Group) compression are used in various fax devices. Although every fax device uses MH, the other compression types are optional in fax devices. It is the handshaking method defined in the T.30 standard, defined by the International Telecommunications Union (ITU), which determines which of the compression methods are supported in a best-method-available sorting system. If one of the devices only supports MH, then MH is the compression method that the fax call uses. Other wise it supports MMR or JBIG. In similar ways, handshaking determines the resolution of the fax (e.g, 200 dpi x 200 dpi or 300 dpi x 300 dpi), and the speed of the fax transmission (e.g., 14.4 kbs or 33.6 kbs). So, based on support of the T.30 protocol, every fax device theoretically interoperates with every other fax device. But T.30 is a very complicated protocol and its complexity meant there were devices that failed to comply with the full breadth of the T.30 protocol – and exceptions to the theoretical interoperability.
The T.38 protocol is based on the T.30 protocol but adds support for FoIP and VoIP networks, which use packet-switching rather than the circuit-switching used in the PSTN (public switched telephone network). T.38 adds the ability to spoof during a fax call, which is to fool the other fax device about when packets arrive (in packet-switching, packets may arrive out of order) which is not a problem with conventional telephone calls. T.38 will interoperate with T.30, but only if a gateway is located between the two nodes that converts the IP stream to the pulse-code modulation (PCM) used in circuit-switched networks.
Today, in T.38 calls moving over SIP trunks, interoperability problems may exist. These may be due to T.38 implementations varying slightly related to features. For example, many T.38 networks and terminals fail to support ECM (error correction method) which can cause excessive errors and lead a fax session to fail. Likewise, the transport protocol usually used in SIP VoIP networks is User Datagram Protocol Transport Layer (UDPTL) which doesn’t use handshaking to ensure reliability or data integrity. So, UDPTL provides an unreliable service and call data may arrive out of sequence, be duplicated, or be lost without warning. This can cause session failures. UDPTL is used for VoIP networks and installations with good reason, so fax must learn to live with it. ECM will be recommended for all future T.38 implementations, which will reduce session failures significantly.