The challenges of implementing fax over cable networks are similar to those of voice. The two most significant issues are timing and lost packets. The delay of fax packets through a packet network causes the precise timing that is required for the fax protocol to be skewed and can result in the loss of the call. The fax-over-packet protocol compensates for the skewed timing of messages so calls are not dropped and the accuracy of faxed images is not compromised.

Lost packets can be an even more serious problem for IP–fax systems than for IP–voice systems. In a VoP application, the loss of packets can be addressed by replaying last packets and other methods of interpolation. Fax-over-packet applications, however, have more severe constraints on the loss of data because the fax protocol can fail if information is lost. The severity of the problem varies depending on the type of fax machine and whether error correction mode is enabled.

CableLabs is working on specific fax services that will be added to NCS to standardize the implementation of fax over cable networks. There is currently an optional fax relay service in the NCS protocol.