Multizone Topology

To solve the single-zone problem of reducing the call volume of the network to the capabilities of the lowest-capacity WAN link, you can design multiple gatekeeper zones into the network. A good practice in designing multiple zones is to create one zone per site, as shown in Figure 8-24.

Figure 8-24 Simple Enterprise Multizone Topology

HQ-Gatekeeper Site 1-Gatekeeper Zone 1 to Any Zone Limited to

Internal Zone HQ Traffic Is Unlimited Zone HQ to Zone 1 Limited to 128 kbps Zone HQ to Zone 2 Limited to 256 kbps

128 kbps

Internal Zone HQ Traffic Is Unlimited Zone HQ to Zone 1 Limited to 128 kbps Zone HQ to Zone 2 Limited to 256 kbps

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HQ-Gatekeeper Site 1-Gatekeeper Zone 1 to Any Zone Limited to

128 kbps

Zone 2 to Zone 1 Limited to 128 kbps Zone 2 to Zone HQ Limited to 256 kbps

Site 2-Gatekeeper

Site 2-Gatekeeper

The Site 1 gatekeeper limits the number of calls active in Site 1, regardless of where those calls originate or terminate, to two simultaneous calls by limiting the available bandwidth to 128 kbps. Because there is only one gateway at Site 1, there is no need to configure a limit for the intrazone call traffic. All interzone traffic is limited to two calls to protect the WAN link connecting Site 1.

At Site 2 there is also a single gateway, and therefore no need to limit the intrazone call traffic. There are separate interzone limits for the following scenarios:

• Calls between Site 2 and the Headquarters site are limited to a maximum of four calls on the WAN link connecting Site 2.

• Calls between Site 2 and Site 1 are limited to a maximum of two calls on the WAN link connecting Site 1.

The Headquarters site has a similar configuration to Site 2 except that calls are unlimited within Headquarters; not because there is a single gateway, but because there is ample bandwidth between the gateways at this site.

In the network topology in Figure 8-24, gatekeeper CAC provides sufficient granularity needed to protect voice conversations across the low-speed WAN links. Consider another network topology in which there are multiple gateways per zone, however, with each gateway at the remote sites having a separate WAN connection to the aggregation point. Figure 8-25 shows such a network topology.

Figure 8-25 Complex Enterprise Multizone Topology

Zone 1 to Any Zone Limited to 128 kbps

Figure 8-25 Complex Enterprise Multizone Topology

Zone 1 to Any Zone Limited to 128 kbps

Remote Site 2 Zone 2

Zone 2 to Zone 1 Limited to 128 kbps Zone 2 to Zone HQ Limited to 256 kbps

Remote Site 2 Zone 2

Zone 2 to Zone 1 Limited to 128 kbps Zone 2 to Zone HQ Limited to 256 kbps

Of the three gateways in remote Site 1, the slowest WAN access link can carry a maximum of two simultaneous calls. Because the gatekeeper bandwidth limitation is configured per zone and not per gateway, there is no facility within gatekeeper CAC to limit the calls to specific gateways within the zone. To ensure protection of voice conversations, the zone bandwidth must be configured for the slowest link in the zone. For both remote Sites 1 and 2, the slowest link is 128 kbps bandwidth or two calls.

This configuration ensures proper voice quality at all times, but it is also wasteful of the gateways that could terminate more calls without oversubscribing their WAN bandwidth. In this network configuration, CAC is activated too soon and reroutes calls over to the PSTN when in fact they could have been carried by the WAN. In this type of topology, gatekeeper CAC is not sufficient to protect voice quality over the WAN link, and, at the same time, optimize the bandwidth use of all WAN links.

The last configuration to consider is an SP network where the gateways in the POPs are connected via Fast Ethernet to the WAN edge router, as shown in Figure 8-26.

Figure 8-26 Service Provider Topology with Multiple Gateways per Zone

Service Provider WAN Backbone

Figure 8-26 Service Provider Topology with Multiple Gateways per Zone

Service Provider WAN Backbone

In this network, gatekeeper CAC is sufficient, even though there are multiple gateways per zone, because the connections to specific gateways within the zone are not the links that need protection. The bandwidth that needs protection is the WAN access link going into the backbone that aggregates the call traffic from all gateways. A gatekeeper bandwidth limitation for the zone indeed limits the number of calls over that link. It is assumed that the OC-12 backbone link is overengineered and requires no protection.

In summary, a multizone gatekeeper network offers the following CAC attributes:

• The WAN bandwidth at each connecting site can be protected, provided each site is also a zone. For small remote sites in an enterprise network, this often translates into a zone per gateway, which might not be a practical design.

• The bandwidth within a site can be protected if necessary, but this is frequently of little value because there is only one gateway in the site (small remote offices, or a customer premises equipment [CPE] entry point to an SP-managed network service) or because a high-speed LAN is between the gateways (large sites and SP POPs).

• Gatekeeper CAC is a method well suited to limit the number of calls between sites.

Gatekeeper CAC cannot protect the bandwidth on WAN segments not directly associated with the zones. For example, gatekeeper CAC cannot protect the backbone link marked with 20 calls in the simple enterprise topology shown in Figure 8-24, unless the slowest link approach is followed.

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