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The term "passthrough" can be very confusing when it comes to discussing it as a fax transport method. In its most general sense the term "passthrough" or "fax passthrough" simply refers to passing fax calls over an IP network using the G.711 codec. "Passthrough" can also be specifically used to refer to transporting fax using G.711 with an NSE-based switchover mechanism. Configuring the modem passthrough command in IOS voice gateways invokes NSE-based passthrough. This command results in the transporting of a fax call over the G.711 codec with NSEs signaling the switchover upon the detection of a 2100 Hz CED tone. The only way to distinguish which "passthrough" term is being referred to is by the context of the sentence where it is being used.
The term "pass-through" or "fax pass-through" also means transporting fax using the G.711 codec but the switchover does not used NSEs. Instead, the switchover is protocol-based and either the H.323 or SIP call control protocol is used. Fax pass-through is configured using the fax protocol pass-through command on H.323 and SIP voice gateways. "Pass-through" will only ever refer to passthrough using a protocol-based switchover. To keep the confusion at a minimum, use the terms "NSE-based passthrough" or "protocol-based pass-through" if the need to be precise arises when discussing fax passthrough.