Session Border Controller (Sbc); Fax Call Setup - Compaq W25 Application Note

2.2.2.2 Media Gateway (MGW)
The Media Gateway (MGW) provides interworking between PSTN and IP
media streams, i.e. conversion between circuit-switched TDM (Time
Division Multiplexing) bearer circuits and packet-switched media streams
(RTP).
2.2.3

Session Border Controller (SBC)

The SBC can also be called outbound proxy, which normally is a SIP aware
proxy/stateful firewall. If an SBC is used in the network, the User Agent has
to be configured to use the SBC for all SIP sessions. Usually the SBC is
placed together with the firewall so the only way in to the SIP server is
through the SBC. An SBC can also overcome problems with NAT in the
network.
2.3

Fax Call Setup

The picture below shows the SIP signaling during a fax call.
When sending a fax from a fax-machine connected to the W25, the off-
hook status as well as the called number (B-subscriber)is detected by the
W25. This is included in a INVITE-message which is sent to the SIP server.
The SIP server forwards the INVITE to the appropriate fax gateway, which
then calls subscriber B on the PSTN. When the B subscriber answers, a
voice call is setup using the voice codec G.711. At the same time as the B
Fax answers, it starts sending out CED (called station identification) tones,
which is detected by the Fax gateway. Now the Fax gateway sends a re-
invite to use T.38 protocol instead of voice codec G.711. The normal fax
training, e.g. negotiating fax transfer speed, is done using the T.38 protocol.
After the training, the pages are transferred and then the faxes go on-hook,
which then results in a SIP BYE message.
6
W25
Dialing
Re-Invite (T.38)
Fax call
completed
Detects
on-hook
IP IP
SIP Server SIP Server
Invite
Invite
200 OK
200 OK
ACK
Audio (G.711)
Re-Invite (T.38)
200 OK
200 OK
ACK
T.38
Bye
Bye
200 OK
200 OK
2/221 02-FGB 101 327 Uen B – November 2007
Fax GW Fax GW
Ring signal
Fax answer
Detects off-hook
Fax tones
detected