Configuring H323 Gateways

Options for setting up an H.323 gateway include the following:

■ Enable H.323 VoIP call services (required)

■ Configure an interface as an H.323 gateway interface (required)

■ Configure codecs (optional)

■ Configure Dual Tone Multifrequency (DTMF) and fax relay (optional)

■ Adjust H.225 settings (optional)

Depending on the deployment scenario, you might be configuring only the required features or many of the options available for an H.323 voice gateway.

H.323 Configuration Example

Figure 5-12 offers a sample topology that needs to be configured for H.323.

Figure 5-12 H.323 Configuration Example Topology

As a network administrator, you have been asked to deploy an H.323-based VoIP network between two corporate sites. Both sites are using a Cisco IOS H.323 capable gateway for the remote connection. You should deploy the network to meet network requirements:

■ Use H.323 as a signaling protocol.

■ A loopback interface is to be used as the connection to the remote site.

■ Multiple codecs must be available for negotiation in this order:

■ E.164-Address Registration must be configured, because some phones are directly connected to the gateway.

A four-step procedure can then be followed to configure an H.323 gateway according to network requirements:

Step 1. Enable H.323 VoIP services globally. Step 2. Configure an interface as an H.323 gateway.

Step 3. Create a codec voice class and populate with a list of codecs in order of their preference.

Step 4. Adjust H.225 timers.

These four steps are detailed as follows:

Step 1. Enable H.323 VoIP services globally.

Enter voice-service configuration mode and specify a voice-encapsulation type.

Router(config)#voice service {pots | voatm | vofr | voip}

The following is a description of the syntax:

■ pots: Plain Old Telephone Service (POTS) voice service

■ voatm: Voice over ATM (VoATM) encapsulation

■ vofr: Voice over Frame Relay (VoFR) encapsulation

■ voip: Voice over IP (VoIP) encapsulation

Voice-service configuration mode is used for packet telephony service commands that affect the gateway globally.

Select H.323-call-processing submode and enable the H.323 voice service configuration commands.

Router(conf-voi-serv)#h323

Activate the service. Router(conf-voi-serv)#no shutdown

Step 2. Configure an interface as an H.323 gateway.

Enter interface configuration mode for the interface that is connected to the gatekeeper.

Router(config)#interface interface

Assign an IP address to the interface. Router(config-if)#ip address ip-address network-mask

Identify the interface as a VoIP gateway interface. Router(config-if)#h323-gateway voip interface

Define the H.323 name of the gateway. Router(config-if)#h323-gateway voip h323-id name

Note Usually this step is the name of the gateway, with the gatekeeper domain name appended, for example: [email protected]

Designate a source IP address for the voice gateway. Router(config-if)#h323-gateway voip bind srcaddr ip-address

This command sets the source IP address to be used for this gateway. The IP address is used for outgoing H.323 traffic, which includes H.225, H.245, and RAS messages. You do not have to issue this command on the interface that you defined as the voice gateway interface (although it might be more convenient to do so). Use this command on the interface that contains the IP address to which you want to bind (for example, a loopback interface).

Configuring Codecs on an H.323 Gateway

Normally you configure only one codec when you configure a dial peer on a gateway. However, you can configure a prioritized list of codecs to increase the probability of establishing a connection between endpoints during the H.245 exchange phase.

Codec-order preservation is enabled by default in Cisco gateways running Cisco IOS Release 12.3(1) and later. No further configuration is needed.

Step 3. To configure codec negotiation, you must first create a codec voice class and populate with a list of codecs in order of their preference.

Enter voice-class configuration mode and assign an identification tag number for this codec voice class. Router(config)#voice class codec tag

This command only creates the voice class for codec selection preference and assigns an identification tag. Use the codec preference command to specify the parameters of the voice class, and use the voice-class codec dial-peer command to apply the voice class to a VoIP dial peer.

Specify a list of preferred codecs to use on a dial peer.

Router(config-class)#codec preference value codec-type [bytes payload-size]

Table 5-1 details the syntax options for this command.

Table 5-1 codec preference Syntax Description

Value The order of preference, with 1 being the most preferred and 14 being the least preferred codec-type The codec preferred. Values are as follows:

clear-channel: Clear Channel 64,000 bps g711alaw: G.711 a-law 64,000 bps g711ulaw: G.711 mu-law 64,000 bps g723ar53: G.723.1 ANNEX-A 5300 bps g723ar63: G.723.1 ANNEX-A 6300 bps g723r53: G.723.1 5300 bps g723r63: G.723.1 6300 bps g726r16: G.726 16,000 bps g726r24: G.726 24,000 bps g726r32: G.726 32,000 bps g728: G.728 16,000 bps g729abr8: G.729 ANNEX-A and B 8000 bps g729br8: G.729 ANNEX-B 8000 bps g729r8: G.729 8000 bps gsmamr-nb: Enables GSMAMR codec capability gsmefr: Global System for Mobile Communications Enhanced Full Rate (GSMEFR) 12,200 bps gsmfr: Global System for Mobile Communications (GSM) Full Rate (GSMFR) 13,200 bps ilbc: internet Low Bit rate Codec (iLBC) at 13,330 bps or 15,200 bps transparent: Enables codec capabilities to be passed transparently between endpoints (Note: The transparent keyword is not supported when the call-start command is configured.)

bytes payload-size (Optional) Number of bytes you specify as the voice payload of each frame. Values depend on the codec type and the packet voice protocol.

Enter dial-peer configuration mode for the VoIP dial peer. Router(config)#dial-peer voice tag voip

Specify a single codec or assign the previously configured codec selection preference list to the VoIP dial peer.

To specify a single codec

Router(config-dial-peer)#codec {clear-channel | g711alaw | g711ulaw | g723ar53 | g723ar63 | g723r53 | g723r63 | g726r16 | g726r24 | g726r32 | g726r53 | g726r63 | g728 | g729abr8 | g729ar8 | g729br8 | g729r8 | gsmefr | gsmfr} [bytes payload_size]

Tuning H.323 Timers

The last part of the scenario requires you to adjust some of the H.323 timers to meet network specifications.

Step 4. Adjust H.225 timers.

Create an H.323 voice class and enter voice class configuration mode. Router(config)#voice class h323 tag

To set the H.225 TCP establish timeout value for VoIP dial peers, configure the H.323 TCP establish timeout value.

Router(config-class)#h225 timeout tcp establish seconds

The number of seconds for the timeout range is 0 to 30. The default is 15. If you specify 0, the H.225 TCP timer is disabled.

Configure the SETUP Response Timeout value. Router(config-class)#h225 timeout setup value

This command sets the timeout value, in seconds, for the response of the outgoing SETUP message. Range: 0 to 30. Default: 15.

Enter dial-peer configuration mode for the VoIP dial peer. Router(config)#dial-peer voice tag voip

Assign the previously configured H.323 voice class to this VoIP dial peer. Router(config-dial-peer)#voice-class h323 tag

Enter voice-service configuration mode and specify a voice-encapsulation type.

Router(config)#voice service {pots | voatm | vofr | voip}

Enable the H.323 voice service configuration commands. Router(conf-voi-serv)#h323

To set a timer for an idle call connection, change the Idle Timer for concurrent calls.

Router(conf-serv-h323)#h225 timeout tcp call-idle {value value | never} The following is a description of the syntax:

■ value value: The timeout value, in minutes. The range is 0 to 1440. The default is 10. If you specify 0, the timer is disabled, and the TCP connection is closed immediately after all the calls are cleared.

■ never: The connection is maintained permanently or until the other endpoint closes it.

This command specifies the time to maintain an established H.225 TCP connection when there are no calls on that connection. If the timer expires, the connection is closed. If the timer is running and any new call is made on that connection, the timer stops. When all the calls are cleared on that connection, the timer starts again.

H.323 Complete Configuration

Example 5-1 illustrates the complete H.323 configuration example previously described. Example 5-1 H.323 Configuration Example

Router(config)#voice service voip Router(conf-voi-serv)#h323 Router(conf-voi-serv)#no shutdown Router(config)#interface loopback 0 Router(config-if)#ip address 10.10.1.1 255.255.255.0 Router(config-if)#h323-gateway voip interface Router(config-if)#h323-gateway voip h323-id gw1 Router(config-if)#h323-gateway voip bind srcaddr 10.10.1.1 Router(config)#voice class codec 100 Router(config-class)#codec preference 1 g711alaw Router(config-class)#codec preference 2 g729br8 Router(config)#dial-peer voice 500 voip Router(config-dial-peer)#voice-class codec 100 Or

Router(config-dial-peer)#codec g711alaw Router(config)#voice class h323 600 Router(config-class)#h225 timeout tcp establish 10 Router(config-class)#h225 timeout setup 10 Router(config)#dial-peer voice 500 voip Router(config-dial-peer)#voice-class h323 600 Router(config)#voice service voip Router(conf-voi-serv)#h323

Router(conf-serv-h323)#h225 timeout tcp call-idle never

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 pass-through.

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.

H.323 H.323

H.323 H.323

Figure 5-13 H.323 Fax Pass-Through Configuration Example Topology

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 | pass-through {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.

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

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.

Voice

San J°se Gateway

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.

Austin

DNIS:15125551750

Austin

DNIS:15125551750

Voice

San J°se Gateway

Figure 5-14 H.323 Fax Relay Configuration Topology Example

DNIS:15125551760

DNIS:15125551760

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.

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.

Example 5-3 H.323 Fax-Relay Configuration Example

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

Configuring H.323 DTMF Relay

DTMF is the tone generated when you press a button on a touch-tone phone. This tone is compressed at one end of a call. When the tone is decompressed at the other end, it can become distorted, depending on the codec used. The DTMF relay feature transports DTMF tones generated after call establishment out-of-band using either a standard H.323 out-of-band method or a proprietary RTP-based mechanism. For SIP calls, the most appropriate method to transport DTMF tones is RTP-NTE or SIP-NOTIFY.

Although DTMF is usually transported accurately when using high-bit-rate voice codecs such as G.711, low-bit-rate codecs, such as G.729 and G.723.1 are highly optimized for voice patterns and tend to distort DTMF tones. As a result, interactive voice response (IVR) systems might not correctly recognize the tones.

To specify how an H.323 or a SIP gateway relays dual-tone multifrequency (DTMF) tones between telephony interfaces and an IP network, use the dtmf-relay command.

You can use the following procedure to configure DTMF relay on a Cisco IOS gateway.

Step 1. Enter dial-peer configuration mode for the appropriate dial peer.

Step 2. Enable DTMF tone forwarding.

Router(config-dial-peer)#dtmf-relay {[cisco-rtp] [h245-alphanumeric] [h245-signal] [rtp-nte [digit-drop]] [sip-notify]}

The principal advantage of the dtmf-relay command is that it sends DTMF tones with greater fidelity than is possible in-band for most low-bandwidth codecs, such as G.729 and G.723. Without the use of DTMF relay, calls established with low-bandwidth codecs might have trouble accessing automated DTMF-based systems, such as voice mail, menu-based Automatic Call Distributor (ACD) systems, and automated banking systems.

Figure 5-15 and Example 5-4 illustrate a complete H.323 DTMF relay configuration example.

Gateway

Figure 5-15 H.323 DTMF Relay Configuration Topology Example

Gateway

Figure 5-15 H.323 DTMF Relay Configuration Topology Example

Example 5-4 H.323 DTMF Configuration Example

Router(config)#dial-peer voice 500 voip Router(config-dial-peer)#dtmf-relay h245-alphanumeric

Verifying an H.323 Gateway

Use the show gateway command to verify that an H.323 gateway is operational and to display the current status of the gateway.

Example 5-5 shows a sample report that appears when a gateway is not registered with a gatekeeper.

Example 5-5 show gateway Command with an Unregistered Gateway Router#show gateway

H.323 ITU-T Version: 4.0 H323 Stack Version: 0.1 H.323 service is up

This gateway is not registered to any gatekeeper

Alias list (CLI configured) is empty Alias list (last RCF) is empty

Example 5-6 shows a sample report that indicates an E.164 address has been assigned to the gateway.

Chapter 5: Examining VoIP Gateways and Gateway Control Protocols 275 Example 5-6 show gateway Command with a Registered Gateway

Router#show gateway

Gateway gateway1 is registered to Gatekeeper gk1 Gateway alias list H323-ID gateway1

H323 resource thresholding is Enabled H323 resource threshold values: DSP: Low threshold 60, High threshold DS0: Low threshold 60, High threshold and Active

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    How to configure codec on cisci route?
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