Purpose of a Gatekeeper ?

Can anyone help me understand the reason(s) why a Gatekeeper is necessary in
a VoIP network ?

And how are these reasons applied to ISP Services Provviders that offer
Prepaid VoIP calling card services, for example.

Thanks (novice VoIP....obvisoulsy)

 

It isn't. Think of a gatekeeper as an entity where you implement
access & dialing policy for a given set of machines speaking H.323,
called a zone. The gatekeeper registration, admission and status
(RAS) operations control who can register and call into/out of
the zone.

H.323 endpoints, like a Call Manager, can make use of a gatekeeper
but it is not required. Also there is no gatekeeper in environments
using other signaling protocols, like SIP.

-Steve