3Com VCX V7111, User Manual

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3Com
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VCX V7111 VoIP Gateway User Guide
255
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.
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
In the example above, V.152 implementation is supported (using the dynamic payload type
96 and G.711 mu-law as the VBD codec) as well as the voice codecs G.711 mu-law and
G.729.
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.
FXO Operating Modes
This section provides a description of the FXO operating modes and gateway configurations
for Tel-to-IP and IP-to-Tel calls.
IP-to-Telephone Calls
The FXO gateway provides the following FXO operating modes for IP-to-Tel calls:
ƒ One-stage dialing
ƒ Waiting for dial tone
ƒ Time to wait before dialing
ƒ Answer supervision
ƒ
Two-stage dialing
ƒ Dialing time
ƒ Disconnect supervision