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2 System Overview
Fax over IP is based on Voice over IP. The Voice over IP system utilizes IP
technology over broadband networks and uses SIP for control signaling
and RTP for media. The differences between Fax and Voice are the
codecs. For voice the normal voice codecs are G.711, G.729, AMR etc, but
T.38 is used for fax. The voice and fax media are both sent in RTP packets.
Fax over IP
Analog lineAnalog line
SIP ServerSIP Server
Fax GWFax GW
PSTN Networks
W25
Access Network
T.38
T.30T.30
Figure 1 - Typical network for VoIP running Fax (T.38)
The fax machines still use the analog interface and the T.30 fax protocol. In
the picture above one fax is connected to the dedicated fax port on the
Ericsson W25 and the other fax to the PSTN.
The Ericsson W25 has a SIP User Agent associated to the fax port. The
User Agent registers in the SIP server. The User Agent originates and
terminates the FoIP (Fax over IP) calls and use the SIP protocol to manage
and control the media streams (T.38). T.38 enables real-time facsimile
communication over an IP network.
W25 does NOT support fax in clear channel i.e. G.711. This is not feasible
due to, primarily, lack of phase-synchronization between the endpoints. It
may be possible to transmit shorter fax messages but the method would
not be proven robust enough to be useful.
2.1 The Fax over IP solution
To enable a fax call a SIP server and PSTN gateway is required. There is
actually no need for extra features more than a basic call, i.e. no
supplementary services are required.
The SIP server needs to support SIP according to RFC 3261 and the Fax
gateway needs to support SIP and T.38. In addition to this the operator
might have requirements on charging etc, which is not taken into account in
this document. There are several products on the market which fulfill the
basic requirements.
4 2/221 02-FGB 101 327 Uen B – November 2007