Version 6.2 251 February 2011
SIP User's Manual 8. IP Telephony Capabilities
The network packets generated and received during the bypass period are regular voice
RTP packets (per the selected bypass coder), but with a different RTP payload type
(according to the parameters FaxBypassPayloadType and ModemBypassPayloadType).
During the bypass period, the coder uses the packing factor, which is defined by the
parameter FaxModemBypassM. The packing factor determines the number of coder
payloads (each the size of FaxModemBypassBasicRTPPacketInterval) that are used to
generate a single fax/modem bypass packet. When fax/modem transmission ends, the
reverse switching, from bypass coder to regular voice coder is performed.
To configure fax / modem bypass mode, perform the following configurations:
IsFaxUsed = 0
FaxTransportMode = 2
V21ModemTransportType = 2
V22ModemTransportType = 2
V23ModemTransportType = 2
V32ModemTransportType = 2
V34ModemTransportType = 2
BellModemTransportType = 2
Additional configuration parameters:
• FaxModemBypassCoderType
• FaxBypassPayloadType
• ModemBypassPayloadType
• FaxModemBypassBasicRTPPacketInterval
• FaxModemBypasDJBufMinDelay
Note: 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.
Tip: 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
8.2.6.2.5 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).
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