[cisco-voip] Call Manager and Gatekeeper

[Guru Chakravarthy]

hi all, 
I am a new to VoIP and have few questions on
functionalities of Gatekeeper and call manager

1. Is gatekeeper required only in a multi site
environment with distributed call processing 
and 
Not Required in single site or a  multi site
environment with centralized call processing ? (is it
because all the call handling is done by call manager
it self ? ) 

2. Can a call manager replace the gatekeeper or vice
versa ? i.e in what deployment is gatekeeper essential
? 

3. in a pratical deployment scenarios(large/ small)
does any one make use of the H323 Annnex G border
element congigurations available in Cisco IOS 

http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/vvfax_c/callc_c/h323_c/323confg/3gwconf.htm#wp1039767

Thanks in advance.
Guru

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