Principles of operation of VoIP telephony

Principles of operation of VoIP telephony

The operation of VoIP telephony using the SIP protocol is based on the exchange of messages between the user and the service server in the client-server model. Users are defined as agents (UA – User Agent) both on the end device side and on the server side, but in the nomenclature related to the SIP protocol architecture, we distinguish two roles:
The role of the end user, which is the UAC (User Agent Client), i.e. the agent who sends the requests,
user role on the server side, which is UAS (User Agent Server), i.e. the agent that responds to the requests.
It should be noted here that each device, and especially the service server, plays the role of both UAC and UAS, because in the process of meeting the request from the end user, it can forward the received requests to other service servers and receive responses from them in order to meet the request from the end user.
In the SIP element architecture, there are also different types of servers that can de facto operate on a single physical machine, but which, from a logic point of view, have different functions:
Registrar Server – user registration server,
Location Server – stores information about the last IP address of the user, creates a database from which it generates answers to questions from Proxy Server and Redirect Server,
Proxy Server – receives and responds to user requests, logically is divided into Outbound Proxy Server (sending requests) and Inbound Proxy Server (receiving responses), in addition, it can be a Stateful Proxy Server, which remembers the path of communication exchange between agents and a Stateless Proxy Server, which does not realize the process of remembering the path of communication exchange,
Redirect Server – redirects users to other servers.
DNS servers are an important element of the SIP protocol elements architecture, thanks to which it is possible to determine the IP address of the user to whom the telephone call is to be made, if the user is registered in a domain other than the one that initiated the call.
In order to establish a telephone connection, the communication between devices uses query messages, which in the SIP protocol nomenclature are called methods (SIP Request) and messages that respond to queries in the form of specific codes (SIP Response). The codes are divided into six groups denoting a given type of event, each group containing its own subgroup with detailed codes denoting a specific response to a previously sent inquiry. The list of methods and code groups for the response is given below.
SIP Request – requests sent in signaling communication between VoIP devices, used to initiate a specific action by the other side of the call:
INVITE: invitation to establish a connection (session),
ACK: confirmation of acceptance of the invitation to establish a connection, based on a previously sent INVITE request,
BYE: end of conversation (session),
CANCEL: cancel the current request,
OPTIONS: check the capability of the server or UA receiving the request to execute the request, but without fulfilling the request,
REGISTER: request to register a user on a VoIP server (Registrar Server) with simultaneous indication of the user’s location on the Network,
List of additional requests that extend the possibilities of SIP communication:
WORK: temporary confirmation before final ACK confirmation,
SUBSCRIBE: requests information about the session status and user status, works in conjunction with NOTIFY,
NOTIFY: information about an event, for example a new incoming call, if the user can accept a waiting call or information about the status of a particular user of the following type: available, busy, I’ll be right back,
PUBLISH: Provides information to the server about a new event or status change,
INFO: sending additional information, for example, sending information during a session about no change in the modification of the session (connection) or transferring additional digits (dialing an extension number) in the DTMF code,
REFER: Initiation of the transfer of the call to the third party,
MESSAGE: sending a text message using SIP signaling,
UPDATE: Modify the parameters of a session (connection) during its establishment or during its duration, without interrupting it.
SIP Response – responses sent in the signaling exchange between VoIP devices in the form of three-digit messages to which specific meanings are assigned.
The basic groups of messages include:
1xx messages – information messages,
2xx messages – positive response messages,
3xx messages – response redirection messages,
4xx messages – response error messages from the user side,
5xx messages – messages about response errors on the server side,
messages 6xx

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