H.323 Gatekeepers
An H.323 gatekeeper, as illustrated in Figure 5-7, is an H.323 article on the LAN that provides
address adaptation and that controls acceptance to the LAN for H.323 terminals, gateways,
and MCUs. Gatekeepers are alternative nodes that administer endpoints in an H.323 network.
The endpoints acquaint with the attendant application the RAS protocol.
Endpoints attack to annals with a attendant on startup. When they appetite to communicate
with addition endpoint, they appeal acceptance to admit a alarm application a symbolic
alias for the endpoint, such as an E.164 abode or an e-mail address. If the gatekeeper
decides the alarm can proceed, it allotment a destination IP abode to the basic endpoint.
This IP abode ability not be the absolute abode of the destination endpoint, but it
might be an average address, such as the abode of a proxy or a attendant that
routes alarm signaling.
The Cisco attendant provides H.323 alarm management, including acceptance control,
bandwidth management, and acquisition casework for calls in the network.
256 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Note Although the attendant is an alternative H.323 component, it charge be included in
the arrangement if proxies are used.
Figure 5-7 H.323 Gatekeepers
The scope of endpoints over which a gatekeeper exercises its authority is called a zone.
H.323 defines a one-to-one relationship between a zone and a gatekeeper.
When a gatekeeper is included, it must perform these functions:
■ Address translation: Converts an alias address to an IP address
■ Admission control: Limits access to network resources based on call bandwidth
restrictions
■ Bandwidth control: Responds to bandwidth requests and modifications
■ Zone management: Provides services to registered endpoints
The gatekeeper might also perform these functions:
■ Call control signaling: Performs call signaling on behalf of the endpoint (gatekeeperrouted
call signaling)
■ Call authorization: Rejects calls based on authorization failure
Chapter 5: Examining VoIP Gateways and Gateway Control Protocols 257
V
Intranet Internet
H.323
Terminal
Address
Translation
and Access
Control
H.323
Terminal
H.320
Terminal
(ISDN)
H.323
Terminal
H.324
Terminal
(POTS)
Gatekeeper
MCU
PSTN
■ Bandwidth management: Limits the number of concurrent accesses to IP internetwork
resources (CAC)
■ Call management: Maintains a record of ongoing calls