V.152 Support - AudioCodes Mediant 4000 SBC User Manual

CHAPTER 15 Media
c=IN IP4 10.8.6.55
t=0 0
m=audio 6010 RTP/AVP 18 97
a=rtpmap:18 G729/8000
a=fmtp:18 annexb=no
a=rtpmap:97 telephone-event/8000
a=fmtp:97 0-15
a=ptime:20
a=sendrecv
m=image 6012 udptl t38
a=T38FaxVersion:3
a=T38MaxBitRate:33600
a=T38FaxMaxBuffer:1024
a=T38FaxMaxDatagram:122
a=T38FaxRateManagement:transferredTCF
a=T38FaxUdpEC:t38UDPRedundancy

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
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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