Chapter 4: Passthrough

Cisco Press

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The ITU-T has a specification titled V.152 that details a standards-based form of passthrough or VBD for faxes and modems over an IP infrastructure. Although relatively new, V.152 presents a passthrough solution that possesses the relative strengths of NSE-based passthrough and protocol-based pass-through without any of the previously mentioned drawbacks. Furthermore, V.152 supports text telephone protocols, too.

V.152 specifies two methods for transitioning to VBD mode from normal voice mode: payload type switching and SSE (State Signaling Event) messages. Payload type switching is similar to protocol-based pass-through. VBD parameters are negotiated in the call signaling protocol stack and a predefined VBD RTP payload type is established. V.152 recommends that the VBD payload type be a dynamic one.

VBD payload type switching can be accomplished with a number of different call signaling protocols. V.152 defines specific VBD parameters for SDP (used by SIP, MGCP, and H.248 gateways) and H.323.

SSE messages can also signal a V.152 VBD switchover. SSEs are defined in Annexes C, E, and F of V.150.1. These messages are similar to NSEs in that they notify other gateways of signaling events that are usually modem and fax related.

V.152 declares SSEs as optional for transitioning to VBD mode. If one or both gateways do not support SSEs, the payload type switching method must be implemented.

Although Cisco voice gateways do not currently support the recently ratified ITU-T V.152 standard, this specification is the next logical step for handling fax, modem, and text telephony in passthrough scenarios.


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.

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