Fax and Modem Traffic Considerations

MX-ONE supports Fax over IP (FoIP) based on either T.38 fax relay or “clear channel”/G.711 fax pass-through. There are however a number of limitations to consider when fax shall be transferred over an IP network. The overall performance is highly dependent on the quality of the IP network i.e. the network impairments; packet loss, latency and jitter.

The different fax transport methods put different requirements on the network for reliable operation and performance.

With the T.38 fax relay method, defined by ITU-T, fax data are transmitted directly over an IP network without first being converted to an audio stream, which reduces significantly the required bandwidth. In a mixed packet based and circuit switch network transcoding has to be done between the different carrier methods causing extra latency which will have a negative impact on the overall operation.

G.711, defined by ITU-T, is optimized for transmission of voice in an IP network and any packet loss will not have a major impact on the voice quality but when transmitting fax data any loss of packets will have a significant impact on the overall operation.

Packet loss in IP network

T.38 has good protection against single packet loss and burst packet loss, especially with redundancy enabled i.e. FoIP operation in an IP network with several percent of packet loss will still give good fax operation.

G.711 has poor protection against both single packet loss and burst packet loss. Already at packet loss above 0,25% the fax operation is reduced noticeable and it can take very long time to transfer a document between two fax machines over the IP network.

Latency in IP network

Latency can vary a lot depending on network configuration, number of routers and transcodings etc.

Both T.38 and G.711 give similar performance and generally FoIP operates well with round trip delays less than 2 sec.

Jitter in IP network

Jitter is the variation of latency the network adds to transmitted packets. If a packet is coming too late to receiving side, due to network latency, it will be discarded i.e. there is a packet loss.

To reduce the sensitivity to jitter latencies, jitter buffers have been introduced in the network elements and as well in the IP PBX'es. Multiple jitter buffers will though introduce extra latency to the network.

Conclusions

While faxes and modems operate in real time they require that data are transmitted and received in a continuous stream. The overall fax performance is heavily depending on the network topology it will be transmitted over i.e. IP, circuit switched or a mixture of both. In a poorly maintained IP network or in network with many transcodings FoIP may not work at all.

MX-ONE is an IP based system where GW nodes are interconnected over an IP network. In order to get maximum fax performance with e.g. an analog fax it might be required to have the fax machine connected to the same GW node where all incoming/outgoing ISDN lines are connected. The analog interface used for fax could also be configured to use bearer capability 3.1 kHz audio (which gives clear channel, that is, bit transparency), and/or to use bearer capability conversion for fax traffic on incoming trunks, for better fax performance.

When migrating a circuit switched system, like MD110/Mitel TSW, to an IP switched MX-ONE it is especially important to replan the fax and modem set up in the new system with respect to the impairments coming with an IP based system.

How to configure the MGU board to interwork with fax or modem traffic please consult the descriptions for MGU Media Gateway Unit and MGU2/MGU2-X Media Gateway Unit.