Ericsson W25 Bedienungsanleitung PDF herunterladen (Seite 5)

1/221 02-FGB 101 327 Uen A – December 2006 5

2.1 Call Setup

To setup a call the SIP protocol is used. The picture below shows the SIP

signaling during a fax call.

Fax

Gateway

SIP

Server

W25

Dialing

Invite

Ring signal

200 OK

Detects off-hook

ACK

Audio (G.711)

Fax tone

detected

Re-Invite (T.38)

200 OK

ACK

T.38

Detects

on-hook

Bye

200 OK

Fax answer

Fax call

completed

2.2 T.38 and T30 Protocol

There are two protocols used to transfer a fax over IP; T.30 and T38.

T.30 is the protocol that describes the communication process between two

fax machines on a circuit-switched network.

T.38 is the protocol that describes the process for sending and receiving

faxes in real-time over a packet network. The protocol makes adjustments

for delay, jitter and dropped packets, which comes with the characteristics

of IP networks. Since Fax devices are sensitive to timing it would not be

reliable to send and receive faxes over an IP network.

The gateway sends and receives T.30 data from and to the fax and towards

the IP network it sends and receives T.30 data, which are wrapped in T.38

packet.

Since T.30 data is encapsulated in T.38 packets, the same protocol used

for faxing over traditional circuit switched networks, remains for every IP fax

connection. Thus it’s important to have a high level of T.30 interoperability

to be able to communicate with legacy fax devices.

The average bandwidth for the T.38 protocol is < 30 kbps, but peaks can be

up to 80 kbps.

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