Cisco utilizes proprietary NSEs or Named Signaling Events to handle switchovers for a variety of different transport methods, especially for FoIP. NSE messages are sent inband using an existing voice RTP stream. All NSE packets use an RTP payload type of 100 by default. The NSE event IDs listed here are contained within the NSE RTP payload itself.
NSE Event IDs Used by Cisco Voice Products
NSE Event ID
NSE Event Definition
Triggered by the detection of fax CED or modem ANSAM (basically any 2100 Hz tone). This NSE instructs the other gateway to get ready for a fax/modem call. This is used for fax/modem passthrough when the "modem passthrough nse" cli is configured. This forces such changes as codec upspeed to g711, disabling of VAD, etc.
Triggered by a modem ANSAM tone (or phase reversals in any 2100 Hz tone). This message instructs the other gateway to disable echo cancellers.
Triggered by a local detection of carrier loss. There may be some other triggers as well including a certain amount of silence detected. This message instructs the other gateway to return to voice mode. Basically, all of the changes made by NSE-192 and NSE-193 are undone.
Sent by voice gateways to announce gateway controlled modem relay support. This was introduced as part of the Andante project in IOS 12.4(4)T.
Triggered by the detection of a V.21 fax preamble when T.38 is configured. This message initiates the switchover of the call from voice to T.38. This message is sent by the terminating fax gateway to notify the originating fax gateway of the switch to T.38.
Triggered by the reception of a NSE 200 along with the gateway confirming that it can comply with the request to switch to T.38. This message (basically an ACK) lets the other gateway know that the move to T.38 has been accepted.
Triggered by the reception of a NSE 200 along with the gateway rejecting the switchover to T.38. This lets the other gateway know that the move to T.38 has not been accepted (NACK).
Initially sent by the originating gateway to signal the switchover to modem relay (Tremolo project). This same message is also used as an ACK for the modem relay switchover by the terminating gateway.