Passthrough transparently transports fax, modem, and text modulated data directly over a G.711 codec across the IP network. NSE-based passthrough and protocol-based pass-through are the two primary forms of signaling a switchover to VBD mode. All Cisco voice gateways support NSE-based passthrough, whereas only IOS gateways configured for the H.323 or SIP call control protocols support protocol-based pass-through.

IOS gateways support the Cisco proprietary NSE-based passthrough signaling method for H.323, SIP, MGCP and SCCP protocols. NSE-based passthrough is configured identically for H.323, SIP, and SCCP using the modem passthrough nse command. The MGCP NSE-based passthrough configuration has its own command syntax, and its main configuration command is mgcp modem passthrough voip. Both fax and modem calls are supported by NSE-based passthrough because the switchover is triggered by the shared stimuli tone of 21GG Hz.

Protocol-based pass-through uses messages within the voice signaling protocol to signal a switchover to VBD mode. This method is used primarily to interoperate with non-Cisco voice gateways. Protocol-based pass-through is only supported for H.323 and SIP protocols and is configured with the fax protocol pass-through command. In addition, protocol-based pass-through can be used only for fax calls because it is triggered by detection of the V.21 fax preamble.

IOS gateways also support text over G.711. This is a basic configuration that configures the gateway for static G.711 voice calls. The reason for this basic configuration is that the gateway has no way of distinguishing a text call from a voice call, so no switchover signaling or codec upspeeding occurs.

The 66G8 is a digital T1/E1 non-IOS gateway that supports passthrough for both fax and modem calls. Its passthrough configuration is done entirely from Unified CM gateway configuration screen. Older versions of code on the 66G8 supported NTE signaled passthrough. More recent versions support NSE-based passthrough but maintain NTE support only for backward compatibility.

The VG248 is an analog non-IOS gateway that is configured through a menu CLI. The passthrough configuration is accomplished by setting both port-specific and global configuration parameters. The VG248 has the same NTE support as the 66G8 and for the same reasons. NSE-based passthrough for both modem and fax calls is supported.

The ATA gateway is a non-IOS gateway that supports two passthrough modes of operation. The first passthrough mode is fax mode, which essentially configures the ATA only for static G.711 calls. This mode does not support NSE signaling nor codec upspeeding from high-compression voice codecs. The second passthrough mode is fax passthrough mode, which is the standard NSE-based passthrough with codec upspeeding. This mode operates like the NSE-based passthrough seen on IOS gateways.

The passthrough configuration on the ATA is very different from other voice gateways because it requires you to set the values of two different parameters based on certain bit value definitions. The ATA only officially supports fax calls.


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