Fax Call Setup, Session Border Controller SBC

2.2.2.2Media 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).

2.2.3Session 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.

W25

Dialing

IP	IP
SIP Server	Fax GW

Fax call completed

Detects on-hook

Invite

200 OK

ACK

Audio (G.711)

Re-Invite (T.38)

200 OK 200 OK

ACK

T.38

Bye

200 OK

Ring signal

Detects off-hook

Fax tones detected

Fax answer

6	2/221 02-FGB 101 327 Uen B – November 2007