Yealink SIP-T19 E2 T4 Series, Руководство администратора

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Introduction
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SIP provides capabilities to:

Determine the location of the target endpoint -- SIP supports address resolution, name
mapping, and call redirection.

Determine media capabilities of the target endpoint -- Via Session Description Protocol
(SDP), SIP determines the “lowest level” of common services between endpoints.
Conferences are established using only media capabilities that can be supported by all
endpoints.

Determine the availability of the target endpoint -- A call cannot be completed because
the target endpoint is unavailable, SIP determines whether the called party is already on
the IP phone or does not answer in the allotted number of rings. It then returns a message
indicating why the target endpoint is unavailable.

Establish a session between the origin and target endpoint -- The call can be completed,
SIP establishes a session between endpoints. SIP also supports mid-call changes, such as
the addition of another endpoint to the conference or the change of a media characteristic
or codec.

Handle the transfer and termination of calls -- SIP supports the transfer of calls from one
endpoint to another. During a call transfer, SIP simply establishes a session between the
transferee and a new endpoint (specified by the transferring party) and terminates the
session between the transferee and the transferring party. At the end of a call, SIP
terminates the sessions between all parties.
SIP Components
SIP is a peer-to-peer protocol. The peers in a session are called User Agents (UAs). A user agent
can function as one of following roles:

User Agent Client (UAC) -- A client application that initiates the SIP request.

User Agent Server (UAS) -- A server application that contacts the user when a SIP request is
received and that returns a response on behalf of the user.
User Agent Client (UAC)
The UAC is an application that initiates up to six feasible SIP requests to the UAS. The six
requests issued by the UAC are: INVITE, ACK, OPTIONS, BYE, CANCEL and REGISTER. When the
SIP session is being initiated by the UAC SIP component, the UAC determines the information
essential for the request, which is the protocol, the port and the IP address of the UAS to which
the request is being sent. This information can be dynamic and will make it challenging to put
through a firewall. For this reason, it may be recommended to open the specific application type
on the firewall. The UAC is also capable of using the information in the request URI to establish
the course of the SIP request to its destination, as the request URI always specifies the host
which is essential. The port and protocol are not always specified by the request URI. Thus if the
request does not specify a port or protocol, a default port or protocol is contacted. It may be
preferential to use this method when not using an application layer firewall. Application layer