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All the server components (Proxy, Redirect, Location) are mostly on one physical
device called Proxy server, which is responsible for keeping a client database and
connection establishing, maintaining and terminating, as well as call routing.
The
gateway VoIP-GSM gateway acts as a UA in any case (has the same
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®
functions as a VoIP phone), i.e. receives call set-up requirements and, on the basis of
its inner LCR table, routes calls to GSM networks.
None of the SIP-defined server components are integrated in the
gateway
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gateway.
SIP Signalling Messages
Below is a list of messages sent via the SIP:
INVITE – connection set-up request;
ACK – INVITE confirmation by the final message addressee;
BYE – connection termination;
CANCEL – failed connection cancellation;
REGISTER – UA registration with the SIP Proxy;
OPTIONS – server capability query.
The answers to the SIP messages are numerically coded as the case is with the http
protocol. Below are the most important ones:
1XX – information messages (100 – trying, 180 – ringing, 183 – progress);
2XX – successful request completion (200 – OK);
3XX – request forwarding needed (302 – temporarily moved, 305 – use Proxy);
4XX – error (403 – forbidden, 486 – busy here);
5XX – server error (500 – Server Internal Error, 501 – not implemented);
6XX – global failure (606 – not acceptable).
