Description Page 61 of 378
ITG Trunk 2.0 ISDN Signaling Link (ISL) Description, Installation and Operation
Fax support
The ITG ISL Trunk card transfers T.30 protocol (G3 Fax) implementations
over the IP network. Near real-time operational mode is supported where two
T.30 facsimile terminals are able to engage in a document transmission in
which the T.30 protocol is preserved.
The ITG ISL Trunk uses the T.38 protocol on the connection between a pair
of ITG ISL Trunk nodes.
The call acts in the s ame way as a ga te way-to -g a tew ay H.32 3 c al l. Th e cal l is
setup using the normal voice cal l process (t hat is , th e n ormal voice call c odec
negotiation process occurs and the corresponding codec payload size and
jitter buffer valu es are used). Whe n the call setup i s complete, the two G3 Fax
terminals are linked . The DSP detects the f ax call setup tones and switches to
handle the fax call. For the re mainder of the call, the par ameters administere d
for the fax call are used (for example, payload size).
Some implications of the Fax call setup process are the following:
• a voice codec must be configured, even if only fax calls will be made
• both ends of the call must be able to negotiate to a common voice codec
for the calls to be successful.
All T.30 session establishment and capabilities ne gotiation are carried out
between the terminals through the ITG ISL Trunk cards over the IP network
using the T.38 protocol. In terms of the Inte rnet fax ser vice roles , the ITG ISL
Trunk card acts as both the fax on-ramp gateway and the fax off-ramp
gateway, depending on the call direction.
The on-ramp gateway demodulates the T.30 transmission received from the
originating G3 Fax terminal. The T.30 facsimile control and image data is
transferred in an octet stream structure, using a Real Time Protocol (RTP)
payload, over User Datagram Protocol (UDP) transport mechanism.
Signaling specified by H.323 V.2 protocol is used for ITG to ITG call setup.
Modules supporting facsimile transmission ar e respons ible for the followin g:
• fax speed detection and adjustment
• protocol conversion from G3 Fax to RTP payload for fax data transfer