Modem Passthrough with NSE

The basic difference between the fax passthrough feature and the modem passthrough feature is whether the 2100 Hz answer tone from the answering fax/modem contains phase reversals. If the answer tone contains phase reversals, the modem passthrough feature is engaged but an additional NSE message will be triggered. Devices with transmission rates faster than a typical fax machine, such as high-speed modems (using V.34 modulation speeds and higher) and Super G3 (SG3) faxes, send an ANSam tone that contains phase reversals.

Figure 4-8 shows the message exchange for NSE-signaled modem passthrough. Just like in the fax passthrough case, a normal VoIP call is first established, and then an NSE-192 is sent upon the TGW detecting a 2100 Hz answer tone. In the case of a modem, however, this 2100 Hz answer tone is typically an ANSam.

Figure 4-8 Modem Passthrough Call with NSE Signaling

Figure 4-8 Modem Passthrough Call with NSE Signaling

NOTE As with NSE-based fax passthrough, the NSE call flow above remains the same no matter if the voice signaling protocol is H.323, SIP, MGCP, or SCCP.

The in-band signal detection by the TGW's DSP of the answer tone from the terminating modem will trigger the exact same codec upspeed and switchover to VBD mode seen in the fax passthrough case. The only difference is that in this case there is the additional detection of phase reversals in the answer tone. This phase-reversal detection instructs the DSP in the TGW to disable the echo canceller.

Also, this detection of an answer tone with phase reversals triggers a notification by the DSP to IOS that an answer tone with phase reversals was received. IOS responds with a command to the DSP to send an in-band RTP message to the OGW's DSP to also disable its echo canceller. The message is an NSE packet with event ID 193 (NSE-193). As illustrated in Figure 4-8, the TGW sends an NSE-193 and that is replied to by the OGW with the transmission of another NSE-193 message.

If low-speed modems are used that do not implement an ANSam tone, the NSE call flow will resemble fax passthrough in Figure 4-7 rather than modem passthrough in Figure 4-8. Low-speed modems are typically defined as 14.4 Kbps, and below and like the modulations used for fax, any 2100 Hz tones that are used do not contain phase reversals. So, an NSE-193 would never be triggered in the absence of phase reversals, and the call would proceed with just an NSE-192 as in Figure 4-7.

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