Chapter 4: Passthrough

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Table 4-3 Cisco NSE Event IDs Used for Passthrough

NSE Event Number

Description of Operation


Triggered by the detection of 2100 Hz tone, which is typically a fax CED or modem ANSam tone. This message instructs the remote gateway to switch over to passthrough (VBD) mode (upspeed codec to G.711, disable VAD, set the jitter buffer to a fixed value, and so on).


Triggered by a modem ANSam tone (or phase reversals in any 2100 Hz tone). This message instructs the other gateway to disable echo cancellers.


Triggered by a local detection of 4 seconds of silence or carrier loss detection. This message instructs the remote gateway to return to voice mode. Basically, all the changes made by NSE-192 and NSE-193 are undone.

When VoIP calls are transitioned to fax and modem passthrough calls using NSEs, the NSE signaling occurs within the RTP media stream. The signaling protocol is generally unaware that this out-of-band messaging is even occurring. So, NSE-based passthrough is supported in just about all Cisco voice gateways implementing the common signaling protocols of H.323, Session Initiation Protocol (SIP), Media Gateway Control Protocol (MGCP), and Skinny Client Control Protocol (SCCP).

The procedures that a fax call and a modem call use to set up NSE-based passthrough are slightly different. Therefore, the cases of fax passthrough and modem passthrough are analyzed independently and in more detail.

Fax Passthrough with NSE

In this section, the information exchange and negotiations between a TGW and an OGW for an NSE-signaled fax passthrough call are covered. Figure 4-7 illustrates this message flow for a typical fax call.

Figure 4-7

Fax Passthrough Call with NSE Signaling

Note - The NSE signaling flow shown in Figure 4-7 does not indicate a specific voice signaling protocol. NSE-based fax passthrough is compatible with H.323, SIP, MGCP, and SCCP, and the call flow from an NSE perspective is identical.

When the calling fax machine places a call to the answering fax machine, the call is initially set up as a simple voice call between the voice gateways. At this point, with both gateways having been configured for NSE-based passthrough, the DSP on the TGW is listening for a 2100 Hz answer tone.

When the terminating fax machine answers the call, it transmits its 2100 Hz CED tone. The DSP on the TGW detects and recognizes this tone. This triggers a codec upspeed and a switch to passthrough or VBD mode on the TGW.

In addition, detection of the fax CED by the TGW triggers a notification by the DSP to IOS that a fax call has been detected. In response, IOS sends an instruction to the DSP to send an in-band RTP signal with payload type 100 to the OGW instructing it to upspeed its codec and switch to VBD mode, too. This signal is an NSE with event ID 192 (for example, NSE-192) and is shown in Figure 4-7. The OGW replies by sending an NSE-192 back to the TGW.

Tip - In reality, more than one NSE message is passed between the gateways to protect against packet loss or other network problems. Typically, the NSE message is repeated three times. For the NSE-192 messages in Figure 4-7, three NSE messages are sent from the TGW to the OGW, and then three messages are sent in the reverse direction as a response. This is true of practically all NSE signaling messages, but for simplicity this is just shown as one NSE message in the NSE call-flow diagrams.

At this point, both gateways have completely transitioned from voice mode to VBD mode. The fax negotiation will proceed normally as PCM packets over the IP network. The modulated data is end to end between the two fax machines.

Although not shown in Figure 4-7 because it is rarely seen, an NSE-194 may occur at the completion of the fax call if the fax machines do not hang up. An NSE-194 will occur after 4 seconds of silence and will switch the passthrough call back to voice mode. However, because fax machines usually immediately disconnect upon call completion, it is not necessary for the voice gateways to transmit NSE-194 messages.

Modem Passthrough with NSE

The basic difference between the fax passthrough feature and the modem passthrough feature is whether the 2100 Hz answer tone from the answering fax/modem contains phase reversals. If the answer tone contains phase reversals, the modem passthrough feature is engaged but an additional NSE message will be triggered. Devices with transmission rates faster than a typical fax machine, such as high-speed modems (using V.34 modulation speeds and higher) and Super G3 (SG3) faxes, send an ANSam tone that contains phase reversals.

Figure 4-8 shows the message exchange for NSE-signaled modem passthrough. Just like in the fax passthrough case, a normal VoIP call is first established, and then an NSE-192 is sent upon the TGW detecting a 2100 Hz answer tone. In the case of a modem, however, this 2100 Hz answer tone is typically an ANSam.

Figure 4-8

Modem Passthrough Call with NSE Signaling

Note - As with NSE-based fax passthrough, the NSE call flow above remains the same no matter if the voice signaling protocol is H.323, SIP, MGCP, or SCCP.

The in-band signal detection by the TGW's DSP of the answer tone from the terminating modem will trigger the exact same codec upspeed and switchover to VBD mode seen in the fax passthrough case. The only difference is that in this case there is the additional detection of phase reversals in the answer tone. This phase-reversal detection instructs the DSP in the TGW to disable the echo canceller.

Also, this detection of an answer tone with phase reversals triggers a notification by the DSP to IOS that an answer tone with phase reversals was received. IOS responds with a command to the DSP to send an in-band RTP message to the OGW's DSP to also disable its echo canceller. The message is an NSE packet with event ID 193 (NSE-193). As illustrated in Figure 4-8, the TGW sends an NSE-193 and that is replied to by the OGW with the transmission of another NSE-193 message.

If low-speed modems are used that do not implement an ANSam tone, the NSE call flow will resemble fax passthrough in Figure 4-7 rather than modem passthrough in Figure 4-8. Low-speed modems are typically defined as 14.4 Kbps, and below and like the modulations used for fax, any 2100 Hz tones that are used do not contain phase reversals. So, an NSE-193 would never be triggered in the absence of phase reversals, and the call would proceed with just an NSE-192 as in Figure 4-7.

Protocol-Based Pass-Through for Fax

The other method used to trigger a transition from voice mode to passthrough mode is in the messaging of the VoIP call signaling protocol. Although the call signaling protocol's primary responsibility is to set up and tear down the VoIP call, it can also be used to transition to passthrough mode.

At this point, a distinction needs to be made between the term passthrough and pass-through. The term passthrough is what has been used until now when referring to the VBD feature in general and the mode where the passthrough switchover is signaled by NSE packets. This terminology will not change. However, to indicate a transition to VBD mode via messages in the voice call control protocol, the term pass-through is used going forward.

This terminology difference arises from a command-line interface (CLI) configuration convention used on Cisco IOS voice gateways. The modem passthrough nse command enables the VBD feature for fax and modem calls using NSE signaling. Thus when handling a fax call using NSE signaled VBD, this is commonly referred to as "fax passthrough" or "NSE-based passthrough."

The command fax protocol pass-through indicates that a fax call is being handled by the signaling protocol. Therefore, if the VBD mode is signaled via protocol signaling messages, this is commonly referred to as "fax pass-through" or "protocol-based pass-through."

Note - The two configuration methods for passthrough are covered in detail in Chapter 9, "Configuring Passthrough."

There is no protocol-based pass-through for modems. Only fax calls can take advantage of pass-through using the call signaling protocol. Unlike NSE-based passthrough, which is triggered by a 2100 Hz tone, protocol-based pass-through can be triggered only by a fax V.21 preamble.

The principal reason to use the protocol-based signaling rather than the Cisco proprietary NSE-based signaling for a fax passthrough call is that it allows for interoperability with third-party voice gateways. NSE-based signaling is proprietary, so protocol-based signaling is the only solution for Cisco voice gateways to interoperate with voice gateways from other vendors.

The two fax pass-through implementations that are discussed involve the H.323 and SIP protocol stacks. H.323 and SIP are the only supported call signaling protocols for pass-through signaling on Cisco voice gateways.

Tip - There is not a protocol-based pass-through solution for the MGCP protocol stack on Cisco voice gateways due to protocol conflicts and reliability concerns. NSE-based passthrough should be used instead.

Fax Pass-Through with H.323 Signaling

H.323 fax pass-through occurs when a VoIP call is set up using the H.323 protocol stack, and then H.323 messages are used to transition the call to pass-through mode. Figure 4-9 illustrates pass-through using the H.323 call signaling protocol.

Initially, the VoIP call is established using the H.323 signaling protocol. When the terminating fax machine answers the call it plays a 2100 Hz CED tone. However, unlike NSE-based passthrough, this tone does not trigger a pass-through switchover.

Following the CED tone, the terminating fax machine begins the transmission of the DIS and optional NSF and CSI fax messages. These messages are flagged with a V.21 preamble. The V.21 preamble is the signal that triggers the transition to pass-through.

Upon detection of the V.21 preamble by the TGW, a passthrough switchover is initiated with the H.323 protocol stack. H.245 request mode messages signal the switchover to the G.711 codec while H.245 logical channel messages are responsible for closing the previous voice mode logical channels (typically using a high-compression codec such as G.729), opening the new logical channels for the G.711 codec, and acknowledging the same.

Upon completion of the H.245 message exchange, the fax pass-through session is established. The fax call should now be able to complete successfully.

Figure 4-9

Fax Pass-Through Call with H.323 Signaling Protocol

Fax Pass-Through with SIP Signaling

With SIP as the call signaling protocol, fax pass-through is very similar to H.323. SIP sets up a normal VoIP call, and then when the V.21 preamble is detected, SIP handles the transition to pass-through mode. Figure 4-10 illustrates fax pass-through with SIP as the call signaling protocol.

Figure 4-10

Fax Pass-Through Call with SIP Signaling Protocol

After the initial VoIP call is established using the SIP signaling protocol, a V.21 preamble detected by the TGW triggers the switchover to pass through. However, unlike NSE-based passthrough, the SIP protocol will handle the pass-through transition rather than NSE messages.

A SIP re-INVITE is issued by the TGW to transition to pass-through mode. The most important occurrence here is the codec upspeed to G.711. The pass-through parameters enclosed within the SIP re-INVITE are confirmed in the SIP 200 OK message. A SIP ACK from the TGW completes the pass-through switchover.

Text over G.711

Text telephony poses a major challenge to both NSE-based and protocol-based passthrough mechanisms. There is not a common tone implemented by the different text telephony protocols that can be focused on to trigger a passthrough switchover. So, to pass text telephone protocols in a passthrough mode, a manual version of passthrough must be configured. This is referred to as text over G.711.

With text over G.711, dedicated voice connections (using dial-peers on IOS gateways) are configured specifically for the use of text telephone protocols. These text-specific configurations on the voice gateways force all calls across this connection to use the G.711 codec while disabling silence suppression or VAD.

From a call-flow perspective, there is no need for any switchovers using NSEs or the call signaling protocol because the call is set up from the beginning to handle text telephone traffic. Figure 4-11 illustrates text over G.711.

Figure 4-11

Text over G.711 Call

Text over G.711 is signaling protocol independent. So, voice signaling protocols such as H.323, SIP, MGCP, and SCCP are compatible with G.711 over text because these protocols can be configured on voice gateways to set up simple G.711 VoIP calls with silence suppression/VAD disabled.

A Future Look at ITU-T V.152

Both NSE-based passthrough and protocol-based pass-through have their positives and negatives. NSE-based passthrough works for modem and faxes, but this solution is Cisco proprietary and excludes Cisco voice gateways from interoperating with other vendors. On the other hand, protocol-based pass-through offers third-party interoperability, but this is only an option for fax calls. Modem calls do not work with protocol-based pass-through.

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