Deploying Cisco® Voice over IP Solutions

Design Methodology for Large-Scale Dial Plans 173

latter case, the gatekeeper would forward the location request (LRQ) to the remote

gatekeeper either directly or through a directory gatekeeper. The remote gatekeeper would

ultimately respond with the address of the terminating endpoint.

The communication between gateways and gatekeepers is based on standard H.323v2

registration, admission, and status (RAS) messages. Gateways query gatekeepers for routes

using RAS admission request (ARQ) and admission confirmation (ACF) messages. Cisco

gatekeepers and directory gatekeepers also communicate with each other using RAS

location request (LRQ) and location confirmation (LCF) messages. Real-Time Transport

Protocol (RTP) provides the end-to-end transport functions. Figure 5-2 shows an example

of RAS messages sent between gateways and gatekeepers.

Figure 5-2 Example of RAS messaging when Phone A calls Phone B.

Design Methodology for Large-Scale Dial Plans

It’s important to apply some basic design principles when designing a large-scale dial plan.

Design options in this chapter will consider the following principles:

• Dial plan distribution

• Hierarchical design

• Simplicity in provisioning

• Reduction in post-dial delay

• Availability, fault tolerance, and redundancy

DGK

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ARQ ACF ARQ ACF

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DGK

GK

Gateway

A

Phone A Phone B

Gateway

B

H.225 Fast start setup

LRQ

LCF

H.225 Fast start alerting / connect

RTP

GK

V

V

V

V

174 Chapter 5: Designing Static Dial Plans for Large VoIP Networks

Dial Plan Distribution

Good dial plan architecture relies on effectively distributing the dial plan logic among the

gateway and gatekeeper components. Isolating H.323 devices to a specific portion of the

dial plan reduces the complexity of the configuration. Each component can focus on

accomplishing specific tasks. Generally, local POP-specific details are handled at the local

gateway; higher-level routing decisions are passed along to the gatekeepers and directory

gatekeepers. A well-designed network places the majority of the dial plan logic at the

gatekeeper and directory gatekeeper devices.

Hierarchical Design

Strive to keep the majority of the dial plan logic (routing decision-making and failover) at

the highest component level. The directory gatekeeper is generally considered the highest

device. By maintaining a hierarchical design, the addition and deletion of zones becomes

more manageable. For example, scaling of the overall network is much easier when

configuration changes need to be made to a single directory gatekeeper and a single zone

gatekeeper instead of all the zone gatekeepers.

Simplicity in Provisioning

You should keep the dial plan on the gateways and gatekeepers as simple and symmetrical

as possible. On the gateways, try to keep consistent dial plans by using translation rules to

manipulate the local digit dialing patterns. These number patterns can be normalized into a

standard format or pattern before the digits enter the VoIP core. Putting digits into a

standard format simplifies gatekeeper zone prefix provisioning and gateway dial-peer

management.

This methodology helps reduce dial-peer configurations on the outgoing POTS interface. If

the gatekeeper can be provisioned to direct only calls of a certain area code to a particular

gateway, then it is unnecessary to provision all of the individual gateways with their

respective area codes. Instead, you might be able to generalize the gateway configurations.

By normalizing the number, you also reduce the zone prefix search length, reducing the

time it takes to search for a zone prefix match. For example, if you have the 0118943xxxx

digit pattern, you can send the number as 8943xxxx and have the gatekeeper search on 89

as opposed to 01189.

Reduction in Post-Dial Delay

You should consider the effects of post-dial delay in the network. Gateway and gatekeeper

zone design, translation rules, and sequential LRQs all affect post-dial delay. Strive to use

these tools most efficiently to reduce post-dial delay.