Information Technology Reference
In-Depth Information
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Registrar server:
This allows users to register their current locations. (This information is added to a location service.)
Registrar servers use the information to provide a lookup service that allows SIP UAs to be located.
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Location service:
This is created by a registrar server and is populated with bindings of address-of-record ([AOR], a user's
“public” address) to contact addresses. The location service can be used by proxy or redirect servers to retrieve information
relating to a called party's possible locations.
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Back-to-back user agent (B2BUA):
This entity receives requests but, to process them, has itself to generate requests.
Because it both processes and generates requests, its functionality is a combination of that of a UAC and a UAS. Note that it
differs from a proxy server in that it must participate in all requests corresponding to dialogs that it has set up.
A
dialog
is a SIP peer-to-peer relationship between UAs that exists for some time.
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Presence server:
This physical device is aware of the willingness and capability of tracked parties (presentities) to
communicate across a set of devices, and distributes this information to interested parties (watchers). Information about a
tracked party's willingness and capability to communicate is known as
presence information
.
CUCM functions as a B2BUA and a registrar server.
There are two SIP message types:
■
Request:
This message is sent by a client to a server that is used to invoke certain operations or functions.
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Response:
This message is sent by a server to a client that indicates the status of the request received from the client.
As already mentioned, request messages can invoke certain functions on a server. These functions are known as
methods
. A method
is specified in a request message sent by a client to a server, and they include the following:
■
INVITE:
When a UAC wants to initiate a session, it sends an INVITE request to a server. When this arrives at a UAS (it may
be forwarded by proxies), the UAS processes it and sends an appropriate type of response message.
■
ACK:
This message is sent in reply to a final response message from a server.
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BYE:
Terminates a session.
