R2511-HP MSR Router Series Voice Configuration Guide(V5)

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Configuration prerequisites
VoIP configuration is completed, IP calls can be made successfully, and fax machines are connected
correctly.
Configuration procedure
To configure a fax interworking protocol:
Ste
p
Command Remarks
1. Enter system view.
system-view N/A
2. Enter voice view.
voice-setup N/A
3. Enter voice dial program
view.
dial-program
N/A
4. Enter voice entity view.
entity entity-number { pots | voip } N/A
5. Configure a fax
interworking protocol.
• Configure the T.38 fax protocol:
fax protocol t38
[ hb-redundancy number |
lb-redundancy number ]
• Configure the standard T.38
(UDP) fax protocol:
fax protocol standard-t38
[ hb-redundancy number |
lb-redundancy number ]
Optional.
By default, the number of two kinds of
redundant packets is 0.
If the call control protocol is SIP, the
two commands can be used only for
the originator of the fax request
(using private T.38, standard T.38,
or fax pass-through protocol). When
a fax request is originated using
private T.38, standard T.38, or fax
pass-through protocol, the fax type is
decided according to the
configurations. The receiver of the
fax request responds to the originator
based on the type of the fax request,
and then establishes a fax call.
Configuring the pass-through mode
The fax pass-through technology was developed primarily for the purpose of compressing and
transmitting T.30 fax packets that cannot be demodulated through packet switched networks. With this
technology, the devices on two sides can directly communicate over a transparent IP link, and the voice
gateways (routers) do not distinguish fax calls from voice calls. After detecting a fax tone in an
established VoIP call, the voice gateway checks whether the voice codec protocol is G.711. If not, the
voice gateway switches the codec to G.711. Then fax data is transmitted as voice data in the pass-through
mode.
In the pass-through mode, fax information is in the format of uncompressed G.711 codes, is encapsulated
in RTP packets between gateways, and occupies a fixed bandwidth of 64 Kbps. Although the packet
redundancy mechanism can reduce the packet loss ratio, the pass-through mode is subject to factors such
as packet loss ratio, jitter, and delay. Therefore, it is necessary to ensure synchronization of the clocks on
both sides. Fax pass-through is called VBD by ITU-T. That is, fax or modem signals are transmitted over
a voice channel using a proper coding method. So far, the codecs supported are only G.711 A-law and
G.711 -law. In addition, when the fax pass-through function is enabled, the VAD function must be
disabled to avoid fax failures.
You can implement the fax pass-through function on the voice gateway (router) in the following ways:
• Configure the fax to work in the pass-through mode on both sides.