In general, transmitting and reception of modem and fax over internet is a challenge. Due to the nature of the fax and modem, they are very susceptible to noise, delay and loss of signal. Unfortunately, these are some of the very common impairments in internet communications. Furthermore, the use of various codec and compression schemes make the situation more complicated. Most of the codec used on VoIP are optimized for human voice transmission, whereas Fax and Modem transmission are anything but voice. So you shouldn’t be surprised to see failure in sending fax and modem data over IP if you are using conventional codecs. To alleviate the problem, there are new protocols specifically designed to cater for these. For fax, the protocols are Fax over IP and Fax by pass. Fax over IP is using T.38 and Fax by pass is using G711. These are supported by Abacus. There are similar protocols for Modem. However, they are not part of Abacus implementation yet. If user is unable to use the relevant Fax / Modem over IP protocol due to the limitation of SUT or Abacus, the next best is to use G711 codec. There is still no guarantee on the quality of transmission. But the use of other codecs will definitely be even worse off.