6 1/221 02-FGB 101 327 Uen A - December 2006
3 Ericsson W25
The Ericsson W25 will work as a fax gateway between the POTS interface
and the IP network using T.38 protocol. The fax terminal shall be connected
to one of the two POTS interface on the Ericsson W25, which is dedicated
for a fax machine. Connected to this fax port is a SIP user agent, which
manages and controls the media streams by using the SIP protocol (RFC
3261) towards the SIP server and the media Gateway.
3.1 Signaling
The Ericsson W25 conforms to the SIP protocol according to RFC3261.
Re-registration of the user agent is requested by the SIP server, but it’s
recommended to have the Ericsson W25 to re-register each hour.
3.1.1 Details over the SIP Implementation
The Ericsson W25 supports:
• Locating SIP server, RFC 3263.
• SDP according to RFC 2327.
• SDP usage according to RFC 3264.
• Media “hold” by using destination address 0.0.0.0
• Configuration of SIP message timers T1, T2 and B as defined in
RFC 3261.
• URI for fax calls, RFC 3966.
• PRACK method according to RFC 3262.
• Session timer, RFC 4028.
3.2 Security
The Ericsson W25 authenticates towards the VoIP system using the HTTP
Digest Authentication mechanism.
3.3 Media Stream
Due to the procedure of setting up of a fax call both G.711 and T.38 is
supported by the Ericsson W25. The PSTN gateway requires that the call is
first setup as a normal voice connection (e.g. with G.711) before a
switchover from voice to T.38 fax is done with SIP Re-INVITE practices.
3.3.1 Jitter Buffer
One important function in the Ericsson W25 is the jitter buffer, which is
necessary to have due to the latency and jitter in the IP network. The
access network contributes with most of the latency and jitter, so to reduce
this a HSDPA connection is requested.
