IOS Gateway Fax Relay Configuration for H323 SIP and SCCP

The IOS command set for fax relay with H.323, SIP, and SCCP is relatively the same. Identical CLI command syntaxes can be used with each of these voice signaling protocols to achieve the same configuration results.

The H.323 and SIP protocols take this same command set a step further, and the whole fax relay configuration itself, from the command placement in the configuration to supported relay features, is identical. In fact, it is even possible to switch between the H.323 and SIP signaling protocols without altering the fax relay configuration.

The SCCP signaling protocol, on the other hand, has a few caveats and feature limitations that prevent identical configurations to SIP and H.323, even though the same command syntax is supported. These limitations and caveats for SCCP are discussed later in this section.

As for H.323 and SIP, all the fax relay commands are configured per call under the voice dial-peers or globally under voice service voip. More information on understanding voice service voip and its hierarchical relationship with voice dial-peers can be found in the section "IOS Gateway Passthrough Configuration" in the preceding chapter.

In addition to H.323 and SIP sharing the same fax relay command set, the Cisco fax relay and T.38 fax relay protocols themselves share the same commands. Unfortunately, this can cause some confusion when configuring fax relay. In an effort to make fax relay configurations a bit clearer, Table 10-1 details the commands applicable for T.38 and Cisco fax relay when H.323 and SIP are the voice signaling protocol.

Table 10-1 Fax Relay Quick Reference Configuration Guide for H.323 and SIP

Configuration

Commands

Cisco Fax Relay

T.38 Fax Relay

Enable command

fax protocol cisco

fax protocol t38

(Enabled by default)

Disable command

fax rate disable

no fax protocol t38

fax protocol none (dial-peer)

fax rate disable

fax protocol none (voice service voip)

fax protocol none (dial-peer)

fax protocol none

(voice service voip)

Table 10-1 Fax Relay Quick Reference Configuration Guide for H.323 and SIP (Continued)

Configuration Commands

Cisco Fax Relay

T.38 Fax Relay

Additional commands

fax rate

fax rate

fax nsf

fax nsf

fax-relay ecm disable

fax-relay ecm disable

(dial-peer)

(voice service voip)

fax-relay sg3-to-g3

(voice service voip)

Table 10-1 shows columns for both Cisco fax relay and T.38 fax relay. If T.38 fax relay is the chosen configuration option, the commands for enabling, disabling, and configuring T.38 fax relay properties are shown in the T.38 fax relay column.

In the Cisco fax relay column, the appropriate configuration commands are also shown. As mentioned previously, notice that the additional commands are the same for both Cisco fax relay and T.38 fax relay. These commands pertain to whichever fax relay type is currently configured on the IOS voice gateway.

TIP On the Cisco 5350, 5400, and 5850 series of voice gateways using the original NextPort

DSP architecture, only T.38 fax relay is supported. However, with the newer AS5X-FC cards, both T.38 and Cisco fax relay are supported on these platforms.

The SCCP voice signaling protocol currently possesses a major fax relay feature limitation that H.323 and SIP does not. As shown earlier in Table 7-4 of Chapter 7, SCCP does not support protocol-based T.38 fax relay. Only Named Signaling Events (NSE)-based T.38, fax relay is supported for SCCP, and H.323 and SIP can support either switchover method for T.38. This changes things slightly from a configuration perspective for SCCP.

In addition, all the relay configuration for SCCP must occur under voice service voip. With H.323 and SIP, relay configurations can occur under both voice service voip and the dial-peer itself. Table 10-2 highlights the fax relay commands available for SCCP under voice service voip.

Table 10-2 Fax Relay Quick Reference Configuration Guide for SCCP

Configuration Commands

Cisco Fax Relay

T.38 Fax Relay

Enable command

fax protocol cisco

(Enabled by default)

fax protocol t38 nse force

Disable command

fax protocol none

no fax protocol t38 fax protocol none

Additional commands

fax-relay sg3-to-g3

fax-relay sg3-to-g3

TIP Cisco IOS gateway support for Cisco fax relay and NSE-based T.38 fax relay with the

SCCP voice signaling protocol is introduced in Cisco IOS Releases 12.4(6)XE and 12.4(11)T.

Notice that because of the SCCP restriction of configuring the fax relay settings under voice service voip, additional configuration commands such as fax rate and fax-relay ecm disable are lost. These commands are present only under a dial-peer.

Tables 10-1 and 10-2 provide just a listing of the fax relay commands for H.323, SIP, and SCCP. To provide more detailed information, each of the commands from these tables and their arguments are now discussed.

The fax rate and fax protocol commands are probably the two most important fax relay commands. These two commands control the enabling/disabling of fax relay, the relay type, and the maximum speed that a fax relay call can transfer page data. Table 10-3 details the fax rate command, and Table 10-4 defines the fax protocol command.

Table 10-3 fax rate IOS Configuration Command

Command

Argument

Function

fax rate {12000 1 1440012400148001 7200196001 disable 1 voice} [bytes rate]

12000

14400

2400

4800

7200

9600

Specifies the maximum allowable speed for the fax relay call. The Cisco gateway will alter the fax messaging to ensure that the selected speed is not exceeded. Faxes may negotiate to speeds lower than the configured fax rate.

disable

Turns off fax relay completely for all calls matching the dial-peer.

voice

Enforces a fax transmission speed that is less than the bandwidth used by the voice codec configured on the dial-peer. For example, the default dial-peer codec is G.729, an 8 Kbps codec. The command fax rate voice ensures that the fax will not negotiate above 8 Kbps or a fax transmission speed of 7200 bps.

Note: This is the default setting for the fax rate command.

bytes

Configures the number of bytes carried in each fax relay data packet for Cisco fax relay. For T.38 fax relay, this option is only supported on the NextPort DSP products, including the 5350, 5400, and 5850. For other voice gateways, the T.38 packetization rate is fixed at 40 ms and cannot be changed. Raising this value from the default of 20 ms will lower bandwidth consumption at the expense of increased delay.

Table 10-4 fax protocol IOS Configuration Command

Command

Argument

Function

fax protocol {cisco 1 none 1 pass-through 1 system 1 t38}

cisco

Specifies Cisco fax relay as the fax relay protocol. When the fax V.21 preamble is detected by the gateway's DSP, a transition to Cisco fax relay is initiated.

Note: This is the default setting for the global configuration of this command under voice service voip.

none

Turns off fax relay and pass-through.

pass-through

Enables pass-through. See Table 9-3 in the section "IOS Gateway Protocol-Based Pass-Through Configuration" of Chapter 9 for additional information on this option.

system

Tells the dial-peer to use the global fax protocol command setting configured under voice service voip.

Note: This argument is available only within the dial-peer configuration, and it is the default setting.

t38

Sets T.38 as the fax relay protocol.

Note: The following settings are the default options for this command: fallback set to Cisco fax relay, low-speed and high-speed redundancy set to 0, and NSE-based transition disabled. See Table 10-5 for more detailed information on these settings.

If the t38 option is chosen for the fax protocol command shown in Table 10-4, additional T.38 specific arguments are made available via the CLI. These arguments for the fax protocol t38 command are detailed in Table 10-5.

Table 10-5 fax protocol t38 IOS Configuration Command

Command

Argument

Function

fax protocol t38 [fallback {cisco 1 none 1 passthrough {g711alaw 1 g711ulaw}} 1 ls-redundancy value [hs-redundancy value] 1 nse [force] ]

fallback

The fallback option provides an alternative transmission method for completing the fax call if the transition to T.38 fax relay fails. The fallback choice can be set to cisco for Cisco fax relay, none to disable fallback, or pass-through for protocol-based pass-through using a G.711 codec. More information on the T.38 fax relay fallback option can be found in the section "Fallback" in Chapter 7.

ls-redundancy hs-redundancy

Configures the redundancy settings for the T.38 fax protocol. Low-speed redundancy (ls-redundancy) is for the T.30 fax messaging, and high-speed redundancy (hs_redundancy) is for the pagetransmission data. A value of 0 disables redundancy, a redundancy level of 5 is the maximum setting for low speed, and 2 is the maximum setting for high speed.

Note: The default setting for both ls_ redundancy and hs_redundancy is 0.

nse [force]

This T.38 specific argument directs the gateway to use NSE packets for the T.38 switchover in what is termed NSE-based T.38 fax relay. If the other T.38 device does not indicate support for NSEs during the call setup, messages within the voice signaling protocol are used instead to signal the switchover to T.38 fax relay. However, the force option "forces" the gateway to use NSE packets for the transition regardless of whether NSE support has been indicated in the call setup.

Ancillary fax relay configuration commands include fax nsf, fax-relay ecm disable, and fax-relay sg3-to-g3. Table 10-6 and Table 10-7 discuss these commands and their functions.

Table 10-6 fax nsf IOS Configuration Command

Command

Argument

Function

fax nsf word

Configures the gateway to override the nonstandard facilities (NSF) value sent by the terminating fax device. The NSF is composed of a two-digit country code followed by a four-digit vendor code. More information on the T.30 NSF message can be found in the "DIS, NSF, and CSI Messages" section of Chapter 2, "How Fax Works." Specific NSF values between certain pairs of fax machines can cause proprietary encodings. These encodings can break fax relay. This is an extremely rare occurrence, but setting the NSF to a value such as all 0s will fix and prevent this from happening. This command is not supported in gateways using NextPort-based DSPs (such as the AS5350, AS5400, and AS5850). These platforms will always overwrite the NSF with 0xff. You should be aware that before 12.4(6) and 12.4(6)T, this command did appear in the CLI for the NextPort-based DSP platforms, but it had no effect on the NSF values.

Table 10-7 fax-relay IOS Configuration Command

Command

Argument

Function

fax-relay {ecm {disable}! sg3-to-g3 [system]}

ecm disable

Enforces the ECM fax negotiation parameter to always be disabled. This should increase the success rate of completed faxes for IP networks with impairments such as packet loss and jitter, but the fax image quality may suffer. Configuring this command also enables the packet-loss concealment feature. This feature causes fax image scan lines that are missing or corrupted to be repeated or replaced with white space to keep the fax transaction from failing. The significance of the ECM feature in fax transactions is discussed in detail in the section "Error Correction Mode" in Chapter 7.

Note: By default, this command is not configured, and ECM negotiations pass through the IOS voice gateway unaltered.

sg3-to-g3

[system]

Enables the Super G3 (SG3) spoofing feature that convinces SG3 fax devices that only G3 fax transmissions are possible. SG3 is not currently supported over fax relay. Ideally, SG3 should fall back to G3 when fax relay is being used, but this is not always the case, and unreliable faxing becomes a problem. This command configures the gateway to force the SG3 device into a G3 fax transmission mode. More information on SG3 faxing and options for handling it in IP networks can be found in the section "Super G3" in Chapter 7.

Note: This command is enabled by default under voice service voip. The system option, available only under the dial-peer, is also the default setting at the dial-peer configuration level. This means this feature is on by default for any fax relay call and would need to be explicitly disabled if that is the desired result.

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