Passthrough or VBD is a transport method for passing modulated data such as fax, modem, and text telephone protocols over an IP network. Transitioning to passthrough mode involves additional changes beyond a normal voice call. In Cisco voice gateways, this passthrough switchover can be signaled in one of two ways: NSE-based passthrough or protocol-based pass-through.

NSE-based passthrough uses Cisco proprietary NSE messages between the originating and terminating voice gateways to transition to passthrough. This signaling method is compatible with the common call signaling protocols of H.323, SIP, MGCP, and SCCP, but interoperating with third-party gateways is not possible.

Protocol-based pass-through is available only for fax calls because it is triggered by a V.21 preamble that is not present for modems. Unlike NSEs that communicate over the RTP media stream, protocol-based pass-through uses the voice signaling protocol to handle the pass-through transition. Only the call signaling protocols of H.323 and SIP support this signaling method, but interoperability with third-party gateways is possible.

For handling text telephone protocols, a manually configured version of passthrough must be used that is referred to as text over G.711. Text over G.711 can be used with any call signaling protocol or third-party gateway that supports the G.711 codec.

The ITU-T V.152 specification is a standards-based form of passthrough or VBD. Although this is a relatively new specification that is not widely implemented, it offers the benefits of both NSE-based passthrough and protocol-based pass-through without the current drawbacks.



Relay is the other transport method for passing modulated data over IP networks. However, relay uses a much different process than passthrough for establishing communication between fax, modem, and text devices.

Although passthrough digitizes the modulated waveform using pulse code modulation (PCM), relay decodes the modulated waveform and then passes the decoded frame over IP using a relay protocol. Because the modulated waveform sent by a fax or modem is simply carrying digital information, relay strips off this modulation and passes just the original binary information. On the other side, this binary information is remodulated into an analog waveform. Figure 5-1 provides an overview of a relay call.

Figure 5-1 Relay Call Overview

Modulated Data


Relay Protocol

Modulated Data

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Relay Relay Relay

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To accommodate fax, modem, and text devices, a different relay protocol must be implemented for each. The following sections address in detail the different relay types supported by Cisco gateways in addition to the fundamental concepts behind the relay transport method.

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