AudioCodes Mediant 800 MSBG User Manual page 447

SIP User's Manual
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8.3.7.2.5 Fax / Modem NSE Mode
In this mode, fax and modem signals are transferred using Cisco-compatible Pass-through
bypass mode. Upon detection of fax or modem answering tone signal, the terminating
device sends three to six special NSE RTP packets (using NSEpayloadType, usually 100).
These packets signal the remote device to switch to G.711 coder (according to the
parameter FaxModemBypassCoderType). After a few NSE packets are exchanged
between the devices, both devices start using G.711 packets with standard payload type (8
for G.711 A-Law and 0 for G.711 Mu-Law). In this mode, no Re-INVITE messages are
sent. The voice channel is optimized for fax/modem transmission (same as for usual
bypass mode).
The parameters defining payload type for the proprietary AudioCodes' Bypass mode
FaxBypassPayloadType and ModemBypassPayloadType are not used with NSE Bypass.
When configured for NSE mode, the device includes in its SDP the following line:
a=rtpmap:100 X-NSE/8000
(where 100 is the NSE payload type)
The Cisco gateway must include the following definition: "modem passthrough nse
payload-type 100 codec g711alaw".
To configure NSE mode, perform the following configurations:
IsFaxUsed = 0
FaxTransportMode = 2
NSEMode = 1
NSEPayloadType = 100
V21ModemTransportType = 2
V22ModemTransportType = 2
V23ModemTransportType = 2
V32ModemTransportType = 2
Version 6.2
When the device is configured for modem bypass and T.38 fax, V.21 low-
speed modems are not supported and fail as a result.
When the remote (non-AudioCodes') gateway uses G711 coder for voice and
doesn't change the coder payload type for fax or modem transmission, it is
recommended to use the Bypass mode with the following configuration:
•
EnableFaxModemInbandNetworkDetection = 1
•
FaxModemBypassCoderType = same coder used for voice
•
FaxModemBypassM = same interval as voice
•
ModemBypassPayloadType = 8 if voice coder is A-Law; 0 if voice coder
is Mu-Law
447
8. IP Telephony Capabilities
February 2011