Yealink SIP-T54S, Administrator'S Manual

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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 firewalls like to know what applications are flowing through which ports and it is possible