User's Manual

AirStream 4000/4001 WiMAX CPE User Manual

Information G.723 and G.729 cannot be enabled at same time. PCMu or PCMa Law can be

chosen in Line Configurations. Only one codec can be used at a time.

Most legacy fax devices adopt ITU-T Group 3 fax protocol published in 1990. The protocol is composed of

server standards that specify different parts the fax call. Amongst them T.30 specification defined procedure to

set up a fax call, determine the image size, encoding, transfer speed, the demarcation between pages, and the

termination of the call. T.30 also references the various modem standards for handshaking (V.21 Channel 2

Model at 300mbps), and for image transfer (V.27 Ter, V.29, V.17, and subsequently V.34).

The simplest approach to transfer fax over an IP network is the bypass mode. CPE or other gateway devices

simply samples all transmissions from fax machine using PCM (Pulse-code modulation) at 64kbps, and

transports the bit string to the other gateway at edge of the network.

Figure 4. Transfer of T.30 Faxes in Transparent Mode

Transparent transfer provides no protection to packet loss, and requires big buffer in the in the gateway device to

combat network delay jitter. Tolerance of round trip delay for the IP network is a stringent 2 seconds. Delay

longer than that will cause failure of fax transmission.

More reliable and preferred method of delivery fax via an IP network is fax relay. The T.38 fax relay standard

was devised in 1998 as a way to permit faxes to be transported across IP networks between existing G3 fax

terminals.

Figure 5. Transfer of T.30 Faxes using T.38 Fax Relay