Cisco Fax Relay
Figure 2-10 illustrates the operation of Cisco Fax Relay.
76 Authorized Self-Study Guide: Cisco Articulation over IP (CVOICE)
VoIP Call
Send Codec ACK (PT97)
Fax Broadcast Switchover (PT96)
Codec Download ACK (PT97)
Fax Broadcast Established
Codec Download Done (PT96)
CED Tone
DIS Msg
T.30 T.30
Gateway
V V
Gateway G3 Fax
G3 Fax
Initiates the Call
IP Network
Download Codec Download Codec
Figure 2-10 Cisco Fax Broadcast Operation
When a DSP is put into articulation approach at the alpha of a VoIP call, the DSP is informed
by the alarm ascendancy assemblage whether fax broadcast is accurate and if it is supported, whether it
is Cisco Fax Broadcast or T.38 fax relay. If Cisco Fax Broadcast is supported, the afterward events
occur:
1. Initially, a VoIP alarm is accustomed as if it were a accustomed accent call. Alarm ascendancy procedures
are followed, and the DSP is put into articulation mode, afterwards which animal speech
is accepted to be accustomed and processed.
2. At anytime during the activity of the call, if a fax acknowledgment or calling accent (ANSam [modified
ANSwer tone] or CED) is heard, the DSP does not baffle with the accent processing.
The ANSam or CED accent causes a about-face to modem pass-through, if
enabled, to acquiesce the accent to canyon abundantly to the alien fax.
3. A accustomed fax machine, afterwards breeding a CED or audition a CNG (CalliNG) tone,
sends a DIS (digital identification signal) bulletin with the capabilities of the fax
machine. The DSP in the Cisco IOS aperture absorbed to the fax apparatus that generated
the DIS bulletin (normally the TGW) detects the High-Level Abstracts Link Control
(HDLC) banderole arrangement at the alpha of the DIS bulletin and initiates fax relay
switchover. The DSP additionally triggers an centralized accident to acquaint the alarm ascendancy stack
that fax switchover is required. The alarm ascendancy assemblage again instructs the DSP to
change the RTP burden blazon to 96 and to accelerate this burden blazon to the OGW.
4. When the DSP on the OGW receives an RTP packet with the burden blazon set to 96,
it triggers an accident to acquaint its own alarm ascendancy assemblage that a fax about-face has
been requested by the alien gateway. The OGW again sends an RTP packet to the
TGW with burden blazon 97 to announce that the OGW has started the fax
changeover. When the TGW receives the burden blazon 97 packet, the packet serves
as an acknowledgement. The TGW starts the fax codec download and is accessible for
fax relay.
5. Afterwards the OGW has completed the codec download, it sends RTP packets with payload
type 96 to the TGW. The TGW responds with an RTP packet with burden type
97, and fax broadcast can activate amid the two gateways. As allotment of the fax codec
download, added ambit such as VAD, jitter buffers, and answer abandoning are
changed to clothing the altered characteristics of a fax call.
During fax broadcast operation, the T.30 analog fax signals are accustomed from the PSTN or
from a anon absorbed fax machine. The T.30 fax signals are demodulated by a DSP on
the aperture and again packetized and beatific beyond the VoIP arrangement as data. The TGW
decodes the abstracts beck and remodulates the T.30 analog fax signals to be beatific to the
PSTN or to a destination fax machine.
The letters that are demodulated and remodulated are predominantly the appearance B,
phase D, and appearance E letters of a T.30 transaction. Most of the letters are passed
across after any interference, but assertive letters are adapted according to the constraints
of the VoIP network.
During appearance B, fax machines catechize anniversary other’s capabilities. They apprehend to communicate
with anniversary added beyond a 64 kbps PSTN circuit, and they attack to accomplish best
use of the accessible bandwidth and ambit affection of a 64 kbps articulation path. However, in a
VoIP network, the fax machines do not accept a 64 kbps PSTN ambit available. The bandwidth
per alarm is apparently beneath than 64 kbps, and the ambit is not advised a clear
circuit.
Because manual paths in VoIP networks are added bound than in the PSTN, Cisco
IOS CLI is acclimated to acclimatize fax settings on the VoIP punch peer. The adapted fax settings
restrict the accessories that are accessible to fax machines beyond the VoIP alarm leg and are
also acclimated to adapt ethics in DIS and NSF letters that are accustomed from fax machines.