Technology Prefixes
A technology prefix is an optional H.323 standards-based feature that is supported by
Cisco gateways and gatekeepers and enables more flexibility in call routing within an
H.323 VoIP network. Technology prefixes are used to group gateways by type (such as
voice or video) or class or define a pool of gateways.
Technology prefixes are used to separately identify gateways that support different types
of services, such as video calls versus voice calls, where the gatekeeper can use this information
to correspondingly route traffic to appropriate gateways.
The network administrator selects technology prefixes (tech-prefixes) to denote different
types or classes of gateways. The gateways are then configured to register with their
gatekeepers with these prefixes. For example, voice gateways can register with techprefix
1#, H.320 gateways with tech-prefix 2#, and voicemail gateways with tech-prefix
3#. More than one gateway can register with the same type prefix. When this happens,
the gatekeeper makes a random selection among gateways of the same type. If the callers
know the type of device they are trying to reach, they can include the technology prefix
in the destination address to indicate the type of gateway to use to get to the destination,
as illustrated in Figure 8-20.
470 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
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Figure 8-20 Technology Prefixes
For example, if a caller knows that address 2125551111 belongs to a regular telephone,
the destination address of 1#2125551111 can be used, where 1# indicates that the
address should be resolved by a voice gateway. When the voice gateway receives the call
for 1#2125551111, it strips off the technology prefix and bridges the next leg of the call
to the telephone at 2125551111.
Cisco gatekeepers use technology prefixes to route calls when no E.164 addresses registered
(by a gateway) match the called number. In fact, this is a common scenario because
most Cisco IOS gateways can either register their H.323 ID or destination patterns. Cisco
Unified Communications Manager Express and Cisco Unified Survivable Remote Site
Telephony (SRST) can register their Ethernet phone’s directory numbers (ephone-dns) at
the gatekeeper. Without E.164 addresses registered, the Cisco gatekeeper relies on these
two options to make the call-routing decision:
■ With the technology prefix matches option, the Cisco gatekeeper uses the technology
prefix appended in the called number to select the destination gateway or zone.
■ With the default technology prefixes option, the Cisco gatekeeper assigns a default
gateway or gateways for routing unresolved call addresses. This assignment is based
on the registered technology prefix of the gateways.
The gatekeeper uses a default technology prefix for routing all calls that do not have a
technology prefix or for gateways that do not have a technology prefix defined. That
remote gatekeeper then matches the technology prefix to decide which of its gateways to
hop off. The zone prefix determines the routing to a zone just as the technology prefix
determines the gateway in that zone.
If the majority of calls hop off on a particular type of gateway, the gatekeeper can be
configured to use that type of gateway as the default type so callers no longer have to
prepend a technology prefix on the address. For example, if you use mostly voice gateways
in your network, and you have configured all your voice gateways to register with
technology prefix of 1#, you can configure your gatekeeper to use 1#* (that is, a 1# followed
by zero or more characters) gateways as the default:
Router(config-gk)#gw-type-prefix 1#* default-technology