Routing Calls between Zones
Call routing is based on either e-mail addresses (H.323 IDs—a string) or E.164
telephone numbers. However, E.164 gives you more flexibility.You can assign
www.syngress.com
Voice and Video Gatekeeper Design • Chapter 5 149
zones by area code, country code, area code + local exchange, or basically anything
you want.
When gatekeepers are attempting to route calls (resolve the endpoint address
of a call) they determine the destination zone using their zone prefix configuration.
This will allow the gatekeeper to locate the correct zone and the associated
gatekeeper. If the endpoint is part of a remote zone the gatekeeper will send a
location request to the appropriate gatekeeper to determine how to resolve the
address. If the endpoint is part of a local zone (or if the gatekeeper is servicing an
incoming call to one of its local zones), the gatekeeper will either find the exact
endpoint (which has registered with them) or an appropriate gateway.
If the gatekeeper needs to route a call to a gateway, it will select a gateway
based on the technology prefix the gateway used when it registered with the
gatekeeper. For example, when gateways register with the gatekeeper they typically
will register with one or more technology prefixes that they support.
Perhaps the technology prefix 1# is used to designate voice gateways while 2# is
used for ISDN gateways.When the gatekeeper receives a call request beginning
with 1# to one of its local zones, it will select one gateway from the pool of
gateways that have registered with that technology prefix.This requires callers to
dial the appropriate technology prefix based on the type of call (voice, ISDN, and
so on) they are placing.
On the gatekeeper, you can use the gw-type-prefix command for several
purposes. First, if a gateway is incapable of registering a technology prefix (such as
1#) you can use this command with the gw ipaddr keywords to manually define
the technology prefix a gateway supports.
Second, you can use the gw-type-prefix command to define a default technology
prefix to be used if a call does not have a technology prefix. Since voice
calls are so common, many organizations define voice gateways to be the default
technology and thus do not require callers to use any technology prefix when
placing voice calls. In the previous example, you could define 1# (voice calls) to
be the default technology prefix.Thus callers would not need to dial a preceding
1# for voice calls.Those calls would automatically get sent to the default technology
gateways: the voice gateways. Callers would only need to use a technology
prefix (2#) for ISDN calls.
Finally, the gw-type-prefix command can be used with the hopoff keyword
to force calls with a certain type of technology prefix to route through a particular
gatekeeper regardless of any zone prefix commands. For example, you may
have many zone prefixes defined to enable routing of voice calls. However, you
may want to route ISDN calls (a different technology prefix than voice calls) to a
www.syngress.com
150 Chapter 5 • Voice and Video Gatekeeper Design
particular gatekeeper (i.e., where the ISDN gateways reside) regardless of whether
the dialed number matches a zone prefix command.
Figure 5.5 shows two locations of a large international corporation. Only two
of the many locations within the corporation are shown. One site specializes in
large, enterprise products while the other produces smaller, intermediate products.