How does the fax service work?

The Callnet fax service uses the T.38 standard to send and receive real time fax over IP, enabling fax machines from different vendors to interoperate. In the T.38 architecture, fax signals are demodulated at the voice/ fax gateway and sent over the IP network as IP fax packets using either TCP or UDP, depending on the service environment.

TCP provides 100% error-free transmission, but the downside is reduced real-time performance. Routers discard TCP packets first when congestion occurs. UDP has good real-time performance, since routers do not discard datagrams as readily as TCP packets. However, the downside of UDP is a reduced quality of service. It is possible for datagrams to get lost, arrive out of order, or to be duplicates.

At the other end of the IP network, the remote gateway decodes IP fax packets and regenerates fax signals.

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