Specifications

Engineering Guidelines
262
G.711 Fax pass through performance guidelines
Due to the many variables involved in sending Fax data over G.711 pass-through on IP trunks,
there is no guarantee of reliable transport. However, practical experience has shown that, with
some careful network considerations, such a link can be made to work. These considerations
include:
• The IP trunk link must use G.711 only.
• The rate of packet loss on the link must be less than 0.1%.
• The link delay must be below 200 ms.
• Jitter must be less than 30 ms (ideally less than 20 ms).
With these settings, G3 FAX at V.17 speeds has been found to work with good reliability as
compared to standard TDM connections, however without error correction mechanisms such
connections should only be considered as best effort. Use of T.38 for transporting Faxes over
IP networks is strongly recommended.
T.38 – reliable Fax over IP transport
Under normal circumstances, transmitting Fax over IP should not be considered without
appropriate interfaces to provide signal redundancy and error correction. The two most
prominent protocols are T.37 and T.38, which allow a standard T.30 fax to be connected over
an IP network, T.37 is a store and forward protocol and T.38 is a real time protocol. These are
generally point-to-point connections and provide a means of toll bypass. Fax within a pure IP
environment makes little financial sense, considering that e-mail is far less sensitive to timing
issues, and generally uses an error-correcting IP protocol to ensure delivery.
Since G.711 Fax cut through can only be used successfully on very high quality networks it is
recommended that T.38 be used to support the transmission of FoIP. If the IP network introduces
too much latency, jitter or almost any packet loss Fax transmissions using G.711 pass through
will be error prone.
The T.38 protocol provides mechanisms to deal with network latency, jitter and packet loss.
Information pertaining to the use of T.38 for fax transmission can be found in “T.38 FoIP
Guidelines” on page 263.
G.711 modem pass through
Sending tones between IP end devices can be problematic as the voice stream data rates will
not be synchronized. This may result in gaps in the voice channel. Normally, these gaps are
infrequent and have little effect on speech. However, they do affect tones and therefore they
affect DTMF and MODEM tones. DTMF and MODEM devices can handle some data loss but
IP networks can introduce more than expected, resulting in poor performance of these services.
Because there is no guarantee that it will work, connecting Modems over IP trunks is not
recommended, however If it is necessary to transport Modem data across an IP trunk then the
following guidelines should be adhered to:
• The IP trunk link must use G.711 only.