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This configuration note describes how to implement fax over IP on Cisco voice gateways.
Fax over IP enables interoperability of traditional analog fax machines with IP telephony networks. The fax image is converted from an analog signal and is carried as digital data over the packet network.
In its original form, fax data is digital. However, to transmit across a traditional PSTN, it is modulated and converted to analog. Fax over IP reverses this analog conversion, transmitting digital data over the packet network, and then reconverting the digital data to analog for the receiving fax machine.
The following sections provide an overview of fax over IP technologies supported on Cisco voice gateways:
Most Cisco voice gateways currently support two methods to transmit fax traffic across the IP network:
Fax relay mode is the preferred method to transmit fax traffic. However, if a specific gateway does not support Cisco fax relay, the gateway supports fax pass-through.
In fax pass-through mode, the gateways do not distinguish a fax call from a voice call. The fax communication between the two fax machines is carried in-band over a "voice" call in its entirety. All Cisco voice gateways support fax pass-through (see Table 1).
On IOS voice gateways, you can achieve more reliable fax transmissions using fax pass-through mode if you disable VAD on the gateway's dial peers. For more details, see Table 8.
Figure 1 illustrates how fax pass-through works. The fax traffic is transparently carried across the Quality of Service (QoS)-enabled IP infrastructure, and the data is not demodulated within the IP network.
Cisco fax relay does not involve Cisco CallManager; it is a gateway-controlled fax mode. In fact, most of the fax processing occurs in the digital signal processors (DSP), requiring only packet switching from the main processor (CPU) and some limited signaling to switch to fax mode. Therefore, the CPU overhead is very similar to a normal voice call.
Initially, a voice call is established. Once the V.21 preamble is detected at the terminating gateway, the originating and the terminating gateway negotiate the codec type. If the two sides cannot negotiate on a common codec and speed, the fax fails. If negotiation is successful, the fax transmission begins.
Cisco fax relay is supported using MGCP, H.323, or SCCP depending on the specific gateway type. See Table 1 for an overview of the supported protocols on specific platforms. The Cisco voice gateways support as many concurrent fax calls as G.711 voice calls.
As illustrated in Figure 2, the voice gateway demodulates the fax data before crossing the IP network. The voice gateway at the other end of the IP network demodulates it for transfer across the PSTN.
In addition to Cisco fax relay and fax pass-through, some of the Cisco voice gateway platforms (see Table 1) also support T.38 fax relay. The T.38 fax relay for VoIP H.323 feature provides standards-based fax relay protocol support these gateways. Because the T.38 fax relay protocol is standards-based, these gateways can inter-operate with third-party T.38-enabled gateways and gatekeepers in a mixed vendor network.
Although some Cisco voice gateways do not currently support T.38, Cisco is investigating adding this support to additional gateways in the future. Additionally, Cisco CallManager does not support T.38 at this time.
Because of the restricted availability of T.38 support on the voice gateways, detailed information about it is beyond the scope of this document. Briefly, you need to configure T.38 fax relay in both the originating and terminating H.323 gateways for the T.38 fax relay for VoIP to operate.
For additional information about implementing T.38 on these Cisco voice gateways, see the T.38 Fax Relay for Voice over IP H.323 feature description available at this location:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121newft/121t/121t3/dt_t38fx.htm
Table 1 provides an overview of the Cisco voice gateways, supported fax protocols and modes, and required software version. You can obtain the latest versions of Cisco software from the Cisco software downloads site:
http://www.cisco.com/kobayashi/sw-center/index.shtml
Table 1 Fax Support on Cisco Voice Gateways
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1 Cisco CallManager does not support T.38. See the "Understanding T.38 Support" section for details. 2 If using with the Cisco VG248, use 12.1.5YF1 or 12.1.5YF3 or later. See Table 8 for details. |
These sections provide details about configuring and verifying Cisco fax relay and tips for using fax pass-through:
If you are using a gateway that does not support Cisco fax relay (see Table 1) or if you choose to use fax pass-through instead of Cisco fax relay, follow these guidelines:
Refer to these tables for assistance in configuring Cisco fax pass-through on Cisco voice gateways:
For a detailed description of these and other VoIP commands available in Cisco IOS, refer to the following publications:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fvvfax_r/index.htm
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fvvfax_c/index.htm
You should also refer to the software configuration guides or command reference publications included with the specific hardware platform.
Table 2 provides an overview of common commands used on MGCP Cisco IOS voice gateways.
Table 2 Configuring Fax Pass-through on MGCP IOS Voice Gateways
Table 3 provides an overview of common commands used on H.323 Cisco IOS voice gateways.
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Note Dial peers can be configured for fax pass-through one at a time in dial-peer configuration mode or all at one time in voice-service mode. If a dial peer has been configured by both methods, the dial-peer configuration takes precedence over the voice-service configuration. |
Table 3 Configuring Fax Pass-through on H.323 IOS Voice Gateways
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Refer to these tables for assistance configuring Cisco fax relay on Cisco voice gateways:
For a detailed description of these and other VoIP commands available in Cisco IOS, refer to the following publications:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fvvfax_r/index.htm
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fvvfax_c/index.htm
You should also refer to the software configuration guides or command reference publications included with the specific hardware platform.
Table 4 provides an overview of common commands used on MGCP Cisco IOS voice gateways.
Table 4 Configuring Cisco Fax Relay on MGCP IOS Voice Gateways
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Table 5 provides an overview of common commands used on H.323 Cisco IOS voice gateways.
Table 5 Configuring Cisco Fax Relay on H.323. IOS Voice Gateways
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Table 6 provides information about disabling Cisco fax relay and ECM on non-IOS voice gateways.
Table 6 Configuring Cisco Fax Relay on Non-IOS Gateways
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Table 7 provides an overview of common commands used on H.323 and MGCP Cisco IOS voice gateways to verify fax status.
Table 7 Verifying Fax Status on Cisco Voice IOS Gateways
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Additionally, these tips can help you verify that Cisco fax relay is in use:
These recommendations and guidelines can assist you in best implementing fax support on Cisco voice gateways:
Quality of Service (QoS) indicates the level of service you can achieve on your network. Using QoS tools, you can set parameters to prioritize different types of traffic on the IP network. For example, you can adjust QoS parameters to grant priority to voice calls, which might be more susceptible to network degradation than data traffic.
QoS tools can assist you in lessening packet drops and delays to improve fax transmission. On Cisco voice gateways the QoS parameters do not distinguish between fax and voice calls. However, by using QoS tools, you can still improve the quality of service for IP telephony transitions, including voice and fax traffic.
When using QoS, make every effort to minimize the following:
For detailed information about implementing QoS in a Cisco IP telephony network, refer to the Cisco IP Telephony QoS Design Guide, which is available at this location:
http://www.cisco.com/univercd/cc/td/doc/product/voice/ip_tele/avvidqos/index.htm
These tips can help ensure the integrity of the fax calls:
The default fax mode on all Cisco IOS gateways is Cisco fax relay. For best performance, verify that you have Cisco fax relay on both the originating and terminating gateways. If two IOS gateways have differing transports, they will negotiate to use Cisco Fax relay.
The only non-IOS gateway that does not support Cisco fax relay is the Cisco Digital Access DT-24/DE-30+. If you connect this gateway to a Cisco IOS gateway, you should configure both gateways to use fax pass-through mode.
Cisco fax relay has been tested in multiple network scenarios with several brands of Group 3-compatible fax machines, including:
These fax machines, configured for T.30 signalling, were found to be interoperable with Cisco Fax Relay
For fax rates greater than 14.4kbps, Cisco voice gateways switch to fax pass-through mode even if they are configured for Cisco fax relay. However, fax machines with ECM enabled are highly sensitive to network condition such as packet delay and packet loss.
Refer to the following guidelines and analyze your specific network conditions before permanently enabling ECM on your fax machines. These guidelines were developed based on testing results from the Cisco IOS H.323 voice gateways:
Packet delay is the time it takes for a packet to reach the receiving endpoint after being transmitted from the sending endpoint. Delay consists of two components: constant delay and variable delay.
Constant delay in the network within permissible limits (up to about 1 second) does not have any effect on the fax performance.
Variable delay (jitter) can be caused by packets being routed through different paths to the same destination. Testing indicated that the Cisco IOS H.323 voice gateways can handle delays caused by jitter up to 240 milliseconds. Excessive jitter can cause packets to arrive out of order. See the"Effect of Packet Reordering" section for details.
Out-of-order packet arrival due to delay variation is much more critical for fax call than voice calls. Currently, Cisco fax relay does not reorder the packets at the receiving end. Instead, the gateway drops out-of-order packets, causing fax machines (in ECM mode) to initiate re-transmission.
To improve performance in networks with a high frequency of out-of-order packet arrival, disable ECM (see the "Disabling Error Correction Mode (ECM)" section).
T.30 fax machines with ECM enabled are highly sensitive to packet drop. Testing results indicate that the behavior of fax transmission in reaction to packet drop varies with different manufacturers. Some fax machines drop the call after a number of retransmissions, while others disable error correction.
However, most of the fax machines appear to accept packet drop in the range of 0.4 to 0.6% without slowing down to next speed. For example, a fax call originating with a 14.4 kbps transmission rate maintains the speed (with very few retransmissions) up to a 0.4 to 0.6% packet drop for a 30 page document.
If packet drops are closer to 1%, the fax call might be dropped due to an excessive number of retransmissions. Even if the call succeeds, the call durations will increase by four to five times because of the large number of retransmissions. Therefore, in a network with packet drop in the range of 0.8 to 1%, you should disable ECM (see the "Disabling Error Correction Mode (ECM)" section).
Pitney Bowes fax machines use a proprietary protocol to send faxes to and receive faxes from another Pitney Bowes fax machine. This proprietary protocol is not compatible with Cisco fax relay.
To use Pitney Bowes machines without disabling Cisco fax relay, you must make the following changes to the Pitney Bowes configuration.
Step 2 Enter Function Code 99.
Step 3 Enter Passcode ***1 and press Yes.
Step 4 Refer to the following settings to modify the options:
Step 2 Enter Function Code 99.
Step 3 Enter Passcode ***1 and press Yes.
Step 4 Refer to the following settings to modify the options:
Error Correction Mode (ECM) is intended to eliminate errors in the fax transmission. By default, ECM is enabled on Cisco IOS voice gateways. With ECM enabled, if packets drop, the receiving fax machine generates retransmission requests. If the packet drop is excessive, the call duration increases, and the call might be dropped.
You can disable ECM on the gateway itself rather than disabling it on multiple fax machines. However, if packet drops occur, the fax image quality might deteriorate. Therefore, you should disable ECM only after considering whether you want to risk compromising image quality rather than experiencing longer call durations or dropped calls. You should also monitor and evaluate the network to identify and resolve the cause of the dropped packets. See the "Effect of Packet Drop" section for additional information.
These topics can help you resolve difficulties supporting fax calls on Cisco voice gateways:
Known caveats are unexpected behaviors or defects in the software releases for a product. Table 8 contains information on known problems for fax support on Cisco voice gateways.
If you have a CCO account, you can search for known problems on the Cisco bug tracking system tool, called Bug Navigator II. To access Bug Navigator II, do one of the following tasks:
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These sections provide suggestions about resolving problems with fax transmissions when using particular configurations on specific Cisco voice gateways:
You might experience errors sending and receiving faxes with the Cisco Catalyst 4224 Voice Gateway or Cisco Catalyst 4000 Access Gateway module configured with T1/PRI interfaces.
The TDM clock reference on these systems can be derived from one of seven sources. These are the free running on-board clock and two recovered clock sources from each VIC slot that contains an E1/T1 card. Framing and CRC errors due to clock slips might be observed on E1/T1 interfaces if the on-board clock is used as the reference.
To resolve these errors, use the frame-clock-select 1 T1 1/0 command to select one of the E1/T1 ports as the primary reference. The default reference is the on-board clock.
If you experience intermittent errors using Cisco fax relay, verify that you are not experience excessive errors on the T1/E1 or PRI controller. Cisco fax relay has a low tolerance for excessive errors on the controller.
Use the show controller command to verify that the controller is up and that no alarms have been reported, and to display information about clock sources and other controller settings
If fax or voice transmissions passing through Cisco voice gateways disconnect, you might need to disable call-progress messages on the gateway.
By default, Cisco voice gateways have progress messages enabled. However, the connecting PBX systems might not recognize these messages, causing the calls or fax transmissions to drop.
You can enable alert messages rather than call-progress messages using the voice call send-alert command.
If you experience fast busy signals with voice and fax calls when using a Cisco Catalyst 4224 Access Gateway Switch configured with Fractional PRI, you should busy out the timeslots for which DSP resources cannot be allocated.
To do this, use the command isdn service dsl 0 b_channel #-# state 2, where #-# indicates the range of channels to be busied out. Use the show isdn command to verify the channels are out-of-service.
For additional information, see the Release Notes for Cisco Catalyst 4224 Access Gateway Switch for Cisco IOS Release 12.1(5)YE2 available at the following URL:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121relnt/acc_4224/1215ye2.htm
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