SIP T.38 Fax Relay
Figure 2-12 illustrates a SIP T.38 broadcast operation. Aback the alarm ascendancy agreement is SIP,
T.38 Annex D procedures are acclimated for the about-face from VoIP to fax approach during a
call.
Chapter 2: Considering VoIP Design Elements 79
VoIP Call
200 OK
INIVITE (T.38 in SDP)
ACK
T.38 UDP Packets
T.30
CED Tone
DIS Msg
T.30
T.38
Gateway
V V
T.38
Gateway G3 Fax
G3 Fax
Initiates the Call
IP Network
Figure 2-12 SIP T.38 Fax Broadcast Operation
Initially, a accustomed VoIP alarm is accustomed application SIP INVITE messages. The DSP needs to
be abreast that it can abutment T.38 approach while it is put into articulation mode. Then, during
the call, aback the DSP detects fax HDLC flags, it signals the apprehension of the flags to the
call ascendancy layer, and the alarm ascendancy band initiates a SIP INVITE bulletin mid-call to
signal the admiration to change the media stream.
The SIP T.38 fax broadcast alarm breeze is as follows:
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 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
pass abundantly to the alien fax.
3. A accustomed fax machine, afterwards breeding a CED or audition a CNG, sends a DIS message
with the capabilities of the fax machine. The DSP in the Cisco IOS gateway
attached to the fax apparatus that generated the DIS bulletin (normally the TGW)
detects the 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 control
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. The TGW detects a fax V.21 banderole arrangement and sends an INVITE bulletin with T.38
details in the SDP acreage to the OGW or to the SIP proxy server, depending on the
network topology.
5. The OGW receives the INVITE bulletin and sends aback a 200 OK message.
6. The TGW acknowledges the 200 OK bulletin and sends an ACK bulletin anon to
the OGW.
7. The OGW starts sending T.38 UDP packets instead of VoIP UDP packets beyond the
same ports.
8. At the end of the fax transmission, addition INVITE bulletin can be beatific to acknowledgment to
VoIP mode.