Configuring H.323 Fax Pass-Through and Relay
Tones used by fax machines can be degraded by a codec (for example, G.729) to the
point that they are unintelligible by a far-end fax machine. In an H.323 environment, two
approaches for preserving fax tones across an IP WAN include fax relay and fax passthrough.
Fax Pass-Through Example
Figure 5-13 shows an example of a topology containing a voice gateway that is configured
for H.323 and fax pass-through. Fax pass-through is configured to forward faxes
from a T1 (mu-law), and fax relay is disabled.
Chapter 5: Examining VoIP Gateways and Gateway Control Protocols 269
10.1.1.50
San Jose Austin
H.323
Gateway
H.323
Gateway
Ext. 550
IP Network
V V
Figure 5-13 H.323 Fax Pass-Through Configuration Example Topology
This scenario requires a company’s headquarters in San Jose to be able to fax to its
Austin office using fax pass-through. As a network administrator, your responsibility is
to configure the gateway to meet the requirements of the network.
Requirements dictate that you
■ Create a dial peer to match a destination pattern for the Austin fax machine extension
number.
■ Specify the session target for the VoIP dial peer as the Austin fax.
■ Configure Cisco fax pass-through to be used for fax support between the networks.
■ Specify the codec that is to be used for fax pass-through operation.
■ Specify the fax rate to be used for fax transmission to the Austin fax machine.
Complete the following steps to configure fax pass-through and relay with dial-peers.
Step 1. Enter VoIP dial-peer configuration mode.
Router(config)#dial-peer voice id voip
Step 2. Specify the destination pattern.
Router(config-dial-peer)#destination-pattern pattern
Step 3. Specify the session target for VoIP dial peer.
Router(config-dial-peer)#session target ipv4:ip-address
Step 4. Specify the fax protocol to be used for a specific VoIP dial peer.
Router(config-dial-peer)#fax protocol {cisco | none | system | passthrough
{g711ulaw | g711alaw}}
Use the fax protocol command in dial-peer configuration mode to configure
the type of fax relay capability for a specific dial peer. Note the following
command behavior:
■ fax protocol none: Disables all fax handling.
■ no fax protocol: Sets the fax protocol for the dial peer to the default,
which is system.
If the fax protocol (voice-service) command is used to set fax relay options
for all dial peers and the fax protocol (dial peer) command is used on a specific
dial peer, the dial peer configuration takes precedence over the global
configuration for that dial peer.
Step 5. Specify the rate at which a fax is sent to a specified dial peer.
Router(config-dial-peer)#fax rate {2400 | 4800 | 7200 | 9600 | 12000 |
14400} {disable | voice} [bytes rate]
Use this command to specify the fax transmission rate to the specified dial
peer.
The higher transmission speed values (for example, 14,400 bps) provide a
faster transmission speed but monopolize a significantly large portion of the
available bandwidth. The lower transmission speed values (for example, 2400
bps) provide a slower transmission speed and use a relatively smaller portion
of the available bandwidth.
270 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Note The fax call is not compressed using the ip rtp header-compression command
because UDP is being used and not RTP. For example, a 9600 bps fax call requires approximately
24 kbps.
If the fax rate transmission speed is set higher than the codec rate in the same
dial peer, the data sent over the network for fax transmission is above the
bandwidth reserved for Resource Reservation Protocol (RSVP).
The voice keyword specifies the highest possible transmission speed allowed
by the voice rate. For example, if the voice codec is G.711, the fax transmission
might occur at a rate up to 14,400 bps because 14,400 bps is less than
the 64 kbps voice rate. If the voice codec is G.729 (8 kbps), the fax transmission
speed is 7200 bps.
Example 5-2 illustrates the complete H.323 fax pass-through configuration example previously
described.
Example 5-2 H.323 Fax Pass-Through Configuration Example
Chapter 5: Examining VoIP Gateways and Gateway Control Protocols 271
Router(config)#dial-peer voice 550 voip
Router(config-dial-peer)#destination-pattern 550
Router(config-dial-peer)#session target ipv4:10.1.1.50
Router(config-dial-peer)fax protocol pass-through g711ulaw
Router(config-dial-peer)#fax rate 14400
Fax Relay Example
Figure 5-14 shows a topology example for a voice service VoIP and dial-peer configuration
of a voice gateway that is configured for fax relay.
DNIS:15125551750
DNIS:15125551760
10.1.1.50
San Jose
Austin
Voice
Gateway
Voice
Gateway
WAN
V V
Figure 5-14 H.323 Fax Relay Configuration Topology Example
This scenario requires a company’s headquarters in San Jose to be able to fax to its
Austin office using T.38 and Cisco fax relay. As a network administrator, your responsibility
is to configure the gateway to meet the requirements of the network.
Requirements dictate that you
■ Create two separate dial peers.
■ Configure T.38 fax relay for VoIP dial peers globally.
■ Configure Cisco Fax Relay for a single VoIP dial peer to override the global value.
■ Disable error-correction-mode for a specific dial peer.
■ Specify the fax transmission rate for a specific dial peer.
The default fax protocol is set to T.38 using the fax protocol t38 command to the voice
service VoIP configuration and then is overridden in dial peer 14152 to cisco. In addition,
the fax relay is set to support G3 fax machines and allows a maximum transfer
speed of 14.4 kbps for calls to 14151xx numbers.
You can use the following procedure to configure H.323 fax relay.
Step 1. Enter voice-service configuration mode and specify a voice-encapsulation
type.
Router(config)#voice service {pots | voatm | vofr | voip}
Voice-service configuration mode is used for packet telephony service commands
that affect the gateway globally.
Step 2. Specify the global default ITU-T T.38 standard fax protocol to be used for all
VoIP dial peers.
Router(conf-voi-serv)#fax protocol t38 [nse [force]] [ls-redundancy
value [hs-redundancy value]] [fallback {none | pass-through {g711ulaw
| g711alaw}}]
Use the fax protocol t38 command and the voice service voip command to
configure T.38 fax relay capability for all VoIP dial peers. If the fax protocol
t38 (voice-service) command is used to set fax relay options for all dial peers,
and the fax protocol t38 (dial peer) command is used on a specific dial peer,
the dial-peer configuration takes precedence over the global configuration for
that dial peer.
Step 3. Exit voice-service configuration mode.
Router(conf-voi-serv)#exit
Step 4. Enter VoIP dial-peer configuration mode.
Router(config)#dial-peer voice id voip
Step 5. Disable fax-relay Error Correction Mode (ECM).
Router(config-dial-peer)#fax-relay ecm disable
When this command is entered, the digital signal processor (DSP) fax-relay
firmware disables ECM by modifying the Digital Information Signal (DIS)
T.30 message. This is performed on DIS signals in both directions so that the
ECM is disabled in both directions, even if only one gateway is configured
with ECM disabled.
272 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Note This setting is provisioned when the DSP channel starts fax relay and cannot be
changed during the fax relay session.
Step 6. Enable the fax stream between two Super Group 3 (SG3) fax machines to
negotiate down to G3 speeds.
Router(config-dial-peer)#fax-relay sg3-to-g3
Example 5-3 illustrates the complete H.323 fax-relay configuration example previously
described.
H.323 Fax-Relay Configuration Example
Chapter 5: Examining VoIP Gateways and Gateway Control Protocols 273
Router(config)#voice service voip
Router(conf-voi-serv)#fax protocol t38
Router(conf-voi-serv)#exit
Router(config)#dial-peer voice 14151 voip
Router(config-dial-peer)#destination-pattern 15125551750
Router(config-dial-peer)#session target ipv4:10.1.1.50
Router(config-dial-peer)#fax-relay ecm disable
Router(config-dial-peer)#fax-relay sg3-to-g3
Router(config-dial-peer)#fax rate 14400
Router(config-dial-peer)#exit
Router(config)#dial-peer voice 14152 voip
Router(config-dial-peer)#destination-pattern 15125551760
Router(config-dial-peer)#session target ipv4:10.1.1.50
Router(config-dial-peer)#fax protocol cisco