Fax passthrough is the transportation of fax communications over an IP network using a low compression codec like G.711. Fax passthrough occurs using one of two methods on Cisco voice gateways, NSE-based passthrough or protocol-based pass-through. This page discusses in detail NSE-based passthrough.
How NSE-based Passthrough Works
NSE-based passthrough is simply the transport of fax or modem communications using the G.711 codec in conjunction with Cisco proprietary Named Signaling Event (NSE) packets used for the switchover. The NSE packets and the switchover itself are critical elements of NSE-based passthrough because all fax calls start off as voice calls. Only upon determining that the call is a fax or modem call do the voice gateways switchover to a passthrough mode from voice mode. In the case of NSE-based passthrough, NSE messages are the critical component that handles the switchover. Figure 1 details how NSE messages perform the switchover for NSE-based passthrough.
Figure 1: Flow Diagram for Fax Passthrough Call Using NSE Switchover Signaling
In Figure 1, a G3 fax call is initiated from left to right. Upon detection of the fax CED tone by the terminating gateway (TGW), an NSE-192 is from the TGW to the originating gateway (OGW), requesting a switchover to passthrough. This switchover includes a codec upspeed to G.711, disabling of VAD, and jitter buffer changes. The OGW responds with an NSE-192 message as well confirming that the switchover has occurred on the originating voice gateway.
Foe high speed modem calls or Super G3 (SG3) fax calls, which use a V.34 high speed modem modulation, the NSE-based passthrough call flow looks like Figure 2. In addition to the NSE-192 that is triggered by the 2100 Hz ANSam tone, the phase reversals of the ANSam tone also trigger an NSE-193. The NSE-193 is used to signal the disabling of echo cancellers in the voice gateways. Unlike normal fax calls, high speed modem calls and SG3 fax calls need echo cancellation disabled for reliable communication.
Figure 2: Flow Diagram for Modem Passthrough Call Using NSE Switchover Signaling
Here are the important things to know about NSE-based passthrough for fax faxes and modems:
The 2100 Hz CED tone played from the terminating fax is the stimuli tone for initiating NSE-based passthrough for normal G3 fax devices. If the DSP on the TGW never detects this tone then the switchover to passthrough will never occur.
Upon detecting the 2100 Hz CED tone, the TGW initiates the passthrough switchover using an NSE-192 message. The OGW responds with an NSE-192 message.
The NSE-192 message upspeeds the codec to G.711, disables VAD, and sets the jitter buffer appropriately.
The NSE-193 is triggered by an ANSam tone that is only found with high speed modem and SG3 fax calls. The NSE-193 signals that the echo cancellers on the voice gateways need to be disabled.
NSE-based Passthrough Product Support and Configuration Tips
NSE-based passthrough is the most widely supported fax and modem transport method for Cisco voice products. From legacy products to the latest IOS voice gateways, this transport method is always available even though it may be referenced by a different name depending on the product. One major caveat with NSE-based passthrough is that it is Cisco proprietary because of the NSE messages that are used for the switchover. Therefore, NSE-based passthrough will not work with any third party products. It should only be configured between Cisco voice products.
Cisco Product Support for NSE-based Passthrough
Cisco Voice Product
IOS Voice Gateways
NSE-based passthrough for faxes uses the command, modem passthrough for the H.323, SIP, and SCCP protocols under the voice dial-peer or voice service voip or the global command mgcp modem passthrough voip for the MGCP call control protocol. While it is somewhat confusing to configure a command with the word" modem" in it for fax passthrough, these commands do transport modem calls as well. Make sure that both T.38 and Cisco fax relay are disabled as these transport methods take precedence over NSE-based passthrough if they are configured concurrently. For the official Cisco documentation on configuring NSE-based passthrough, please refer to the doc Configuring Modem Passthrough.
NSE-based passthrough on the VG248 is enabled by default but you must make sure that fax relay is set to disabled on the "Port Specific Parameters" configuration screen because fax relay take precedence. Also, it is recommended to set "Passthrough Mode" to "Default:automatic" on the "Port Specific Parameters" screen and to set the "Passthrough signaling" to "IOS mode" on the "Advanced settings" screen. The specific G.711 variant for passthrough codec is set on the "Advanced settings" screen as well. More information on configuring NSE-based passthrough on the VG248 can be found in the document Faxing over IP Using Catalyst 6608 and VG248 (CCO)
6608 and 6624 Catalyst Blades
NSE-based passthrough on the 6608 and 6624 blades are enabled by default but you should make sure that fax relay is disabled on the Unified CM gateway configuration screen because it takes precedence. The NSE type should be set to "IOS gateways". Passthrough redundancy is available on these blades but it should not be enabled unless you are sure the peer gateway also supports this feature and has it enabled. More information on configuring NSE-based passthrough on the VG248 can be found in the document Faxing over IP Using Catalyst 6608 and VG248 (CCO)
NSE-based passthrough only works between Cisco voice products because of the Cisco proprietary NSE signaling messages used in the switchover from voice mode to passthrogh mode.
NSE-based passthrough consumes a lot of bandwidth because of the required G.711 codec. Allocate about 83 Kbps per call and 170 Kbps per call if redundancy is enabled. These numbers assume an ethernet/frame relay layer 2 encapsulation. A feature such as Cisco's IP RTP header compression can significantly lower these numbers at the expense of router CPU cycles.
All forms of passthrough are very sensitive to packet loss, even with redundancy enabled. As little as .1 and.2% packet loss can cause failures.
Make sure that all digital links in the fax path are free from impairments such as slips.
For IOS voice gateways confirm that an inbound VoIP dial-peer exists on the TGW that is configured for modem passthrough.
If you suspect a switchover problem, you can confirm NSEs are being sent and received on IOS voice gateways using the command debug voip rtp session named-event.
Sample Output from debug voip rtp session named-event
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