Fax Server T38 Integration with a Cisco Voice Gateway

The current trend with fax server deployments is to use the T.38 fax relay protocol for integrating the fax server directly into IP networks. Two primary integration models are possible when connecting a T.38-enabled fax server with Cisco equipment. The first model involves the fax server communicating directly with Cisco voice gateways, and the other model involves the addition of Unified CM to handle the call control signaling between the fax server and the voice gateways. In this section, integrating a T.38 fax server solution with just Cisco voice gateways is covered. The next section discusses how T.38 fax servers can be connected to Unified CM.

T.38 fax servers can communicate with multiple voice gateways directly. This capability allows a single centralized T.38 fax server to handle the faxing tasks for any location with a voice gateway providing PSTN access. A single fax server can now offer the benefit of economies of scale while providing least-cost routing via the different points of presence offered by each voice gateway. Figure 8-4 shows a fax server solution where T.38 is used for communication with voice gateways in different geographical locations.

Figure 8-4 Fax Server T.38 Integration with a Cisco Voice Gateway


Figure 8-4 Fax Server T.38 Integration with a Cisco Voice Gateway


Cisco Switch Chassis Grounding Pad

In Figure 8-4, a fax server in San Jose can, with T.38 fax relay, send and receive fax documents in the locations of Sydney, Research Triangle Park, and Brussels. As long as a voice gateway in a certain location has IP connectivity to a fax server, a T.38 fax server can handle the faxing requirements for that location.

In many cases, Unified CM is added to the topology in Figure 8-4. The voice gateway is then configured to send fax calls to the fax server and then VoIP calls to Unified CM. In this instance, a single PSTN connection can be shared between the fax server and Unified CM. Based on the incoming DID number, the voice gateway can discriminate between voice and fax calls and route the incoming calls accordingly. Figure 8-5 demonstrates the concept of a fax server and Unified CM both using the same voice gateway independently of one another.

Figure 8-5 Voice Gateway Integrated Jointly with a Fax Server and Unified CM

Fax Server

Figure 8-5 Voice Gateway Integrated Jointly with a Fax Server and Unified CM

Fax Server

Unified CM

Even though Unified CM is involved from a voice gateway perspective in Figure 8-5, this type of solution still has the fax server directly communicating to a voice gateway. Conceptually, the deployment model in Figure 8-5 is exactly the same as Figure 8-4 from the fax server perspective. Neither the fax server nor Unified CM is aware of one another. The fax server never interacts directly with Unified CM.

If a voice gateway is Media Gateway Control Protocol (MGCP) controlled, adding fax server support requires the addition of another PSTN interface. Unlike the H.323 and SIP voice signaling protocols, which can share a PSTN interface between the fax server and Unified CM as shown in Figure 8-5, ports allocated to Unified CM via MGCP are dedicated resources. Therefore, if a voice gateway has a PSTN T1 connection that is MGCP controlled by Unified CM, another T1 interface will have to be provided for the fax server to access via H.323 or SIP.

As depicted in Figure 8-5, communication between a fax server and a Cisco voice gateway via T.38 fax relay must use the H.323 or SIP call control protocol. This choice of whether to use H.323 or SIP is somewhat of an arbitrary selection because Cisco voice gateways fully support both and so do most fax servers. If an existing H.323 or SIP infrastructure is already present or you are more comfortable with one protocol versus the other, this might make the selection process a bit easier.

Regardless of whether the SIP or H.323 protocol is chosen, the Cisco voice gateway always requires that the T.38 fax relay call start out as a standard VoIP call using a voice codec. Typically, the G.711 codec is used during this stage of the fax call setup between the fax server and the Cisco voice gateway. Although the Cisco voice gateway can support a wide array of voice codecs, fax servers typically support only the G.711 voice codec.

The fax server or the Cisco voice gateway can each signal a switchover to T.38 fax relay from the negotiated voice codec at any point after the initial VoIP call has been established. For the best interoperability between Cisco voice gateways and the fax servers, it is recommended that the terminating T.38 device initiate the switchover. Obviously, the terminating fax device can be either the fax server or the Cisco voice gateway depending on the direction of the call. Cisco voice gateways by default exhibit this behavior.

TIP Cisco voice gateways can handle the switchover to T.38 fax relay in two different manners:

by using the call control protocol itself or through the use of Named Signaling Event (NSE) packets. Because of the proprietary nature of Cisco NSE packets, Cisco voice gateways should always be configured to use a protocol-based T.38 switchover when interoperating with fax servers. You can find more information about how both protocol-based and NSE-based T.38 switchovers work in the sections "Protocol-Based Switchover for T.38" and "NSE-Based Switchover for T.38" in Chapter 5, "Relay." For Cisco voice gateway configuration assistance with protocol-based T.38 fax relay, see the section "IOS Gateway Fax Relay Configuration for H.323, SIP, and SCCP" in Chapter 10, "Configuring Relay."

The requirements for integrating a T.38 fax server with a Cisco voice gateway include the following:

• A Cisco IOS gateway with a voice port that is connected to the PSTN. Typically, this voice port is a digital connection such as a T1 or E1.

• A recent Cisco IOS version (12.3 or later) loaded on the voice gateway that supports H.323 and SIP.

• A fax server that offers T.38 support over the H.323 or SIP call control protocol.

In some respects, the requirements listed are generic because of the numerous fax server vendors and Cisco IOS voice gateways available. This leads to a large combination of potential integration possibilities, but the adherence of both the fax server and the Cisco voice gateway to the T.38 specification should mitigate any interoperability problems.

The configuration necessary for a Cisco voice gateway to interoperate with a fax server using T.38 fax relay is pretty basic in most cases. Although some caveats might apply for certain fax server vendors, implementing the following dial-peer configurations for H.323 and SIP should ensure a successful T.38 communication between the voice gateway and the fax server. Only the VoIP dial-peers configurations are shown in the following examples because the rest of the gateway configuration is what you use for a standard VoIP call. The VoIP dial-peer is where the main commands affecting fax server interoperability are configured.

Cisco voice gateways support H.323 Version 4 (starting in IOS Release 12.3(11)T) and the 1998 version of the ITU-T Recommendation T.38 using UDP transport layer (UDPTL) encapsulation. The T.38 configuration for a Cisco voice gateway is usually applied directly to the VoIP dial-peer. Example 8-1 highlights a Cisco voice gateway configuration for interoperating with a T.38 fax server using the H.323 signaling protocol.

Example 8-1 H.323 IOS Dial-Peer Configuration for Communicating with a T.38 Fax Server

iial-peer voice 6 voip incoming called-number .

Inbound fax calls to the voice gateway should match a T.38 enabled dial-peer. This can be accomplished by specifying a digit pattern or a "." that will match all calls

destination-pattern 14.. codec g711ulaw

Most fax servers support only the G.711 voice codec

fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback none

Enable protocol-based T.38 for this dial-peer

In Example 8-1, the H.323 VoIP dial-peer is configured for a typical G.711 voice call with the addition of the fax protocol t38 command. This command in Example 8-1 enables protocol-based T.38 for the dial-peer. You can tell that protocol-based T.38 is being enabled because of the absence of the nse keyword in the command line.

The fax protocol t38 command in Example 8-1 also specifies low-speed and high-speed redundancy values of 0. Check with your fax server vendor to confirm support and interoperability at higher settings.

In the past, the fallback setting in the fax protocol t38 command line has not been applicable to fax server integrations and it was typically set to none, as shown in Example 8-1. Be aware, however, that recently some fax servers have started supporting a fallback to G.711 if the T.38 call fails. Therefore, it is recommended to check with your fax server vendor concerning the support of T.38 fallback to the G.711 codec. If your fax server supports this feature, a fallback setting of pass-through should be used on the Cisco voice gateway. In the event of a T.38 negotiation failure, an alternative transport method is now available to handle the fax call.

Configuring protocol-based T.38 for an SIP dial-peer is identical to the H.323 configuration in Example 8-1 with the exception of the addition of the command session protocol sipv2. By default, all VoIP dial-peers use the H.323 signaling protocol, and the session protocol sipv2 is necessary to change the dial-peer signaling protocol to SIP.

TIP Be aware that the fax protocol t38 command used in Example 8-1 can also be configured globally under voice service voip rather than under the VoIP dial-peer. For more information about voice service voip, see the section "IOS Gateway Passthrough Configuration" in Chapter 9, "Configuring Passthrough."

Although issues such as misconfigurations, packet loss, jitter, and slips on digital circuits are problems that can affect fax server and voice gateway solutions, another common problem is T.38 fax relay interoperability. Because different vendors can interpret the T.38 specification differently, interoperability problems may arise between T.38 fax relay devices.

Troubleshooting for T.38 fax relay is covered in Chapter 12 of this book. You can use the show and debug commands covered in that chapter on the Cisco voice gateway to help narrow down any issues. However, if T.38 interoperability problems are encountered, the most helpful troubleshooting aid is a packet capture. Using packet captures to troubleshoot T.38 fax relay is covered in the sections "IP Troubleshooting Using Packet Captures" and "Analyzing T.38 Fax Relay Packet Captures" in Chapter 12.

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