CHAPTER15 Media
Mediant 800 Gateway & E-SBC | User's Manual
Simultaneous Negotiation of Fax (T.38) and Modem (V.150.1)
Relay
The device can negotiate fax relay (T.38) and modem relay (V.150.1) sessions in the same, already
established call channel. Fax relay sessions require bypass answering tone (CED) while modem
relay requires RFC 2833 answering tone. As the device is not always aware at the start of the
session whether the answering tone is fax or modem, it uses both methods for CED tone transfer
and sends both answering tone types. Only when the answering tone is detected, does the device
send the fax or modem. To support this functionality, you need to configure a Coders Group (in the
Coder Groups table - see Configuring Coder Groups) that includes the T.38, V.150, and G.711/VBD
coders.
This section is applicable only to the Gateway application.
V.152 Support
The device supports the ITU-T recommendation V.152 (Procedures for Supporting Voice-Band
Data over IP Networks). Voice-band data (VBD) is the transport of modem, facsimile, and text
telephony signals over a voice channel of a packet network with a codec appropriate for such
signals.
For V.152 capability, the device supports T.38 as well as VBD codecs (i.e., G.711 A-law and G.711
μ-law). The selection of capabilities is performed using the Coder Groups table (see Configuring
Coder Groups).
When in VBD mode for V.152 implementation, support is negotiated between the device and the
remote endpoint at the establishment of the call. During this time, initial exchange of call
capabilities is exchanged in the outgoing SDP. These capabilities include whether VBD is
supported and associated RTP payload types ('gpmd' SDP attribute), supported codecs, and
packetization periods for all codec payload types ('ptime' SDP attribute). After this initial
negotiation, no Re-INVITE messages are necessary as both endpoints are synchronized in terms
of the other side's capabilities. If negotiation fails (i.e., no match was achieved for any of the
transport capabilities), fallback to existing logic occurs (according to the parameter IsFaxUsed).
Below is an example of media descriptions of an SDP indicating support for V.152. In the example,
V.152 implementation is supported (using the dynamic payload type 96 and G.711 u-law as the
VBD codec) as well as the voice codecs G.711 μ-law and G.729.
v=0
o=- 0 0 IN IPV4 <IPAdressA>
s=-
t=0 0
p=+1
c=IN IP4 <IPAddressA
m=audio <udpPort A> RTP/AVP 18 0
a=ptime:10
a=rtpmap:96 PCMU/8000
a=gpmd: 96 vbd=yes
Instead of using VBD transport mode, the V.152 implementation can use alternative relay fax
transport methods (e.g., fax relay over IP using T.38). The preferred V.152 transport method is
indicated by the SDP ‘pmft’ attribute. Omission of this attribute in the SDP content means that
VBD mode is the preferred transport mechanism for voice-band data.
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