Telsey telecommunications
SIP Protocol
11
MT500--SIEN12
•
Stateless proxy mode
•
Re-direct server
The difference between these modes is that a
server in a stateful mode
remembers the incoming requests it receives, along with the responses it sends
back and the outgoing requests it sends on.
A
server in a stateless mode
forgets all information once it has sent a request.
These stateless servers are likely to be the backbone of the SIP infrastructure
while stateful-mode servers are likely to be the local devices close to the user
agents, controlling domains of users.
A
re-direct server
receives the request, but instead of passing it onto the next
server, it sends a response to the caller party, indicating the address for the
called user. This method enables the provisioning of the address for the caller,
in order make it possible to contact the called party directly at the next server.
The
SIP Registration Service
provides a means for a particular device to
register to use a SIP address. As seen at the beginning of this chapter, SIP
addresses use ‘URLs’ based on the same addressing scheme used in the web
and similar in form to an email address. The SIP address provides a single
address of record for the user that delivers a one number service for all
communications applications. Users can dynamically register the devices
through which they may be contacted for all types of applications. As a result,
people will no longer have to hand out multiple contact addresses as the system
will automatically handle the distribution of all types of calls appropriately
through the proxy and redirect servers.
SIP Event and Presence Servers
allow the effective sharing of information about
and between users and/or applications.
SIP Signalling
Here is reproduced a simple procedure of a call set-up:
a)
The caller (User Agent Caller) sends a request with the SIP URL
of the called party.
b)
If the client knows the location of the other party, is sends the
request directly to its IP address. If the location is unknown, the
client can send it to a SIP network server, which has been
locally configured.
c) The server will try to resolve the called user's location and
sends the request to them. There are many ways it can do this:
searching the DNS or accessing databases. Alternatively, the
server may be a redirect server that may return the called user
location to the calling client for it to try directly. During the
course of locating a user, one SIP network server can proxy or
redirect the call to additional servers until it arrives at one that
definitely knows the IP address where the called user can be
found.
d) Once found, the request is sent to the user. In the simplest
case, the user's telephony client receives the request, that is,
the user's phone rings. If the user takes the call, the client
responds to the invitation with the designated capabilities of
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