Cisco BTS 10200 Softswitch Manuel de l'utilisateur - Page 3
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The specifications used for page transmission (V.27ter, V.29, V.17) and fax negotiation (V.21) define
Note
how digital data is sent over analog telephone lines in the PSTN. Data modems are also able to use these
specifications even though most data modems have migrated to much faster speeds.
Fax Relay Basics
Fax relay is an important toll-bypass capability that can produce significant cost savings to end users of
packet telephony networks. As more services transition from the PSTN to packet networks, standards
are emerging to ensure interoperability between different vendors' equipment. Support of ITU-T.38 Fax
Relay in the Cisco BTS 10200 Softswitch is a move toward true voice-data convergence where the end
user can use the same lines for voice and data (fax) in heterogeneous (multi-protocol) networks.
The ITU-T.38 Fax Relay recommendation specifies the messages and data exchanged for facsimile
transmission in real-time between two Group 3 fax terminals communicating over Internet Protocol (IP)
networks based on the H.323 protocol. The fax transmission method prescribed in the ITU T.38
recommendation can best be described as a demodulation/remodulation procedure (demod/remod).
Fax relay is a technique used to overcome the deficiency in high compression voice codecs, such as
G729 or G723, when these codecs try to pass fax traffic. Since a fax call is treated as a regular speech
call, the DSP in each gateway is put into voice mode after which human speech is expected to be received
and processed. If a fax answer tone (CED) or calling tone (CNG) is heard during the call, the DSP does
not interfere with speech processing. It simply allows the tone to continue across the packet network call
leg as if it were a voice transmission.
A fax machine, after generating a CED or hearing a CNG, transmits a T.30 Digital Information Signal
(DIS) as part of fax handshaking. DIS is the initial message stating the capabilities of the terminating
fax machine. The terminating gateway's DSP detects the HDLC flag sequence at the start of the DIS
message and initiates a fax relay switchover. It unloads the high compression voice codec and loads a
lower compression fax codec to run the fax call.
Notification is also sent to the DSP on the other side of the packet network so that both DSPs on the fax
call are using the same fax codec. Notification mechanisms differ depending on the fax relay protocol
used. With the lower compression fax codecs loaded, the DSPs demodulate the T.30 HDLC frames,
extract the fax information, and pass it between the gateways using the standard T.38 fax relay protocol.
It is important to note that unlike inband faxing or fax passthrough, fax relay breaks down (demodulates)
the T.30 fax tones into specific HDLC frames, transmits the information across the packet network using
the T.38 fax relay protocol, then converts (re-modulates) the bits back into tones at the far side. The fax
machines on either end simply send and receive tones; neither terminal is aware that the fax relay process
is occurring or that part of the transmission is over the PSTN and part is over a packet network.
The information that follows can be difficult to understand if you aren't familiar with the messaging that
occurs during a typical fax transmission.
When page transmission occurs, a bit rate is used that is substantially higher than the initial 300 bps
used in the call setup negotiation. The bit rate used for the page transmission is confirmed during
training. Following are some of the common rates used in fax page transmission:
V.27ter – 2400/4800 bps
–
V.29 – 7200/9600 bps
–
V.17 – 14400 bps
–
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
Introduction
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