When to Use H.323

Dial Plan Considerations

In a voice network that has CallManagers and H.323 gateways, you need to understand the interaction between the two so that you can properly configure call flow. On the CallManager, you configure a dial plan to send calls to the H.323 gateways when needed. On the H.323 gateways, you configure dial peers to forward calls out of the gateway. You can forward calls to a CallManager using a VoIP dial peer, to the PSTN as a POTS dial peer, to another gateway using a VoIP dial peer, or to directly connected voice ports as POTS dial peers.

You can control the treatment of incoming and outgoing calls with dial peers. In your dial planning, consider the need for such things as number translations or other digit manipulations, or call restrictions. If you are using SRST, be sure the dial plan will work both with and without CallManager.

You need at least one dial peer with a destination pattern to route outgoing calls. Default incoming POTS and VoIP dial peers exist, but you should specifically configure dial peers for incoming calls if you need nondefault services. The default VoIP dial peer includes the following configuration:

• It uses G729r8 codec.
• Voice activity detection (VAD) is enabled.
• Dual tone multifrequency (DTMF) relay is disabled.
• Preference is 0.
• Voice media has differentiated services code point (DSCP) of expedited forwarding (EF); signaling is AF31.
• Huntstop is disabled.
• Both Req-qos and Acc-qos are best-effort.
• No Tcl applications are applied.
• Fax relay is disabled.
• Playout-delay is 40 ms.

The default POTS dial peer includes the following settings:

• Direct inward dial is disabled.
• Preference is 0.
• Digit strip is enabled.
• Register the E.164 phone number with a gatekeeper.
• Huntstop is disabled.
• No Interactive Voice Response (IVR) applications are applied.

Because you must configure an H.323 gateway with the information you need to route calls, the configuration can get complex in a large network. You can simplify the configuration by creating a hierarchical structure using gatekeepers and perhaps a directory gatekeeper.

When you use a gatekeeper, the gateway routes all inbound calls to its local network, but it communicates with the gatekeeper for outbound calls to unknown phone numbers. This simplifies the configuration because the gateway does not need to know how to reach every phone number in your network.