Cisco Certified Expert

VG248 Fax Relay Configuration

The VG248 is an analog gateway consisting of 48 FXS ports controlled directly by Unified CM using the SCCP protocol. Since code version 1.3(1), the VG248 has supported both Cisco fax relay and NSE-based T.38 fax relay. Neither Cisco modem relay nor Cisco text relay is supported by the VG248.

There are only two main screens on the VG248 where the fax relay configuration parameters are set. One screen sets global attributes for fax relay, and the other screen is attached to each individual port on the gateway. This allows certain fax parameters to be configured on a port-by-port basis.

To access the screen where the global fax relay attributes are located, make the following selections from the main VG248 menu.

Configure > Telephony > Advanced Settings

Figure 10-2 shows the global fax relay configuration parameters from the VG248 Advanced Settings screen.

The last three settings on the screenshot in Figure 10-2 are the only ones relevant to configuring fax relay on the VG248. The first setting (highlighted) is Fax Relay Payload Size. With a default of 20 bytes, this setting permits a user to select specific payload byte sizes to be used with the fax relay protocol.

Figure 10-2 VG248 Global Fax Relay Parameters Configuration

Advanced settings

ASRST provider Call preservation Ignore call type Media receive timeout Busy out off hook ports DTMF tone duration Echo cancelling policy Hook flash timer Hook flash reject period Distinctive ringing Passthrough signaling Passthrough codec

Fox relay pay Load size

Fax relay maximum speed Fox reloy ployout delay

(enabled: no timeout) (disabled) (disabled) (disabled) (default: 100ms) (default: use SLIC) (<country defoult>) (none)

(default: no call distinction) (IOS mode) (G.711 u-law)

(default: 14400 bps) (default: 3W

The last two parameters are Fax Relay Maximum Speed and Fax Relay Playout Delay. The default setting for the Fax Relay Maximum Speed parameter is a modulation rate of 14400 bps, but with this parameter the VG248 can be configured to enforce lower modulation speeds to save bandwidth. The Fax Relay Playout Delay parameter allows you to tweak the playout buffer to handle varying amounts of jitter but overall end-to-end delay is directly affected. Only in rare circumstances should this value be changed from its default of 300 ms. These global fax relay parameters are summarized in Table 10-24.

Table 10-24 VG248 Global Fax Relay Configuration Parameters

Configuration

Parameter Options Function

Fax Relay Payload Size 20-48 Configures the number of payload bytes for each fax relay packet.

Note: The default value is 20 bytes. Changing this value will alter the delay and bandwidth characteristics of the fax relay call.

Sets the maximum speed at which faxes can negotiate. A specific speed can be set or the voice bandwidth setting can be used. Like the IOS command fax rate voice, the voice bandwidth setting will prevent faxes from negotiating at a speed greater than the amount of bandwidth used by the voice codec.

Note: The default setting is 14400 bps.

Fax Relay Playout 100-700 Specifies the playout buffer size for both Cisco and

Delay T.38 fax relay in milliseconds

Note: The default setting is 300 ms.

Fax Relay Maximum Speed voice bandwidth 2400 4800 7200 9600 12000 14400

The other fax relay configuration screen is associated with an individual voice port on the VG248 itself. This screen of port specific parameters can be accessed through the VG248 main menu as follows:

Configure > Telephony > Port Specific Parameters > port

Figure 10-3 shows a screenshot of the Port Specific Parameters for port 1.

Figure 10-3 VG248 Port Specific Fax Relay Parameters Configuration

11 Port	selection	1 Port 1 parameters
1 ■
H 1	Enabled	«Status	(enabled) 1
1 z	Enabled	1 Ca 11 contra I mode	(standard)
3	Disabled	1 Caller ID	(enabled)
1 4	Disabled	1 MWI method	(lamp)
5	Disabled	1 VMWI variant	(-^country defaults-)
6	Disabled	1 Call supervision method (none)
7	Disabled	1 Input gain	(a)
1 8	Disabled	1 Output gain	m
9	Disabled	1 Dialing digit detection (default: use D5P)
10	Disabled	I Fax relay	(T.3B peer to peer)
11	Disabled	1 Fax relay ECM	(enabled)
1 12	Disabled	1 Fax relay NSF	(preserve value)
13	Disabled	1 Passthrough mode	(default: automatic)
14	Disabled		_________________ |
15	Disabled	31 Disabled	47 Disabled I
1 16	Disabled	32 Disabled	48 Disabled I
	■* ■ _	port in use press	"H' to enter range I

Only three parameters from the VG248 screenshot in Figure 10-3 are important for the configuration of fax relay. The first parameter is Fax Relay. This parameter is where fax relay on the VG248 is set to disabled, Cisco fax relay, or T.38 peer to peer.

When the Fax Relay parameter is set to disabled, fax passthrough can be used to handle fax calls. The Cisco fax relay setting and the T.38 peer to peer settings are only compatible with other Cisco gateways. Unlike Cisco fax relay, T.38 is not proprietary, but the NSE "peer-to-peer" switchover used on the VG248 works only with other Cisco gateways supporting NSE-based T.38 fax relay.

The second fax relay parameter is Fax Relay ECM. The only options here are enabled and disabled. When this parameter is enabled (default), the VG248 allows fax transactions using the ECM protocol to occur. When this parameter is disabled, the ECM feature is not permitted for fax relay calls.

The last fax relay parameter is Fax Relay NSF. This parameter has two options. The first option, preserve value, is the default and it keeps the NSF message exactly as the fax machine encoded it. The other option is override with 000000, and this option overwrites NSF values with all 0s.

Most of the fax relay parameters on this screen are exactly the same as what has been previously discussed with the IOS gateways and the 6608. Table 10-25 summarizes these parameters.

Table 10-25 VG248 Port Specific Fax Relay Configuration Parameters

Configuration Parameter	Options	Function
Fax Relay	Disabled	Disables fax relay, and NSE-based passthrough is used for fax calls.
	Cisco fax relay	Enables Cisco fax relay on the port. Note: This is the default setting.
	T.38 peer to peer	Enables NSE-based T.38 fax relay on the port.
Fax Relay ECM	Enabled/disabled	With ECM enabled, the VG248 allows fax machines to successfully negotiate the ECM feature. When ECM is set to disabled, the fax endpoints will not use ECM. Note: ECM is enabled by default for fax relay.
Fax Relay NSF	Preserve value	Allows the NSF to pass through the VG248 unaltered by the fax relay protocol. Note: This is the default setting.
	Override with 000000	Forces the NSF value to all 0s to prevent proprietary messaging between fax machines.

In most circumstances, configuring fax relay on the VG248 basically comes down to selecting the appropriate fax relay protocol, Cisco fax relay or T.38. The default setting of the other fax relay parameters are optimal for most situations.

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