In the world of internet communication, where devices and applications need to "speak" to each other to exchange information, SIP (Session Initiation Protocol) acts as a common language.  It provides a set of rules and procedures that allow different devices to initiate, modify, and terminate multimedia communication sessions like voice and video calls.

Within this SIP-enabled environment, different components play specific roles to ensure smooth and efficient communication.  At the forefront are the User Agents (UAs), which can be thought of as the communicators. They are the endpoints that initiate and receive communication requests. These UAs can act as both clients, initiating requests, and servers, responding to those requests.

Then we have the clients, which are typically the applications used by people, like softphones on computers or communication apps on smartphones. They generate requests to initiate communication sessions.

On the receiving end are the servers, which handle these requests according to predefined rules. There are various types of servers, each with a specific function. Proxy servers act as intermediaries, forwarding requests to the appropriate destination. Redirect servers guide clients towards alternative routes if the original destination is unavailable. Registrars keep track of user locations, and Location servers store this information for efficient routing.

Together, these components form a dynamic ecosystem that enables real-time communication over the internet. They ensure that requests are properly routed, users are located, and communication sessions are established and managed effectively.

NOTE : The components described here is more about generic SIP network components. In case of IMS, it tend to have more components. Regarding IMS network components and architecture, refer to this note.

User Agent

A User Agent (UA) is an application or device that acts as an endpoint in the SIP communication process. It can function in two roles based on the context of the communication:

• User Agent Client (UAC): The UAC is responsible for initiating requests and sending them to a server or another endpoint. For instance, when a user starts a voice or video call, the device or application acts as a UAC to generate the required SIP messages.
• User Agent Server (UAS): The UAS receives requests generated by a UAC, processes them, and sends back appropriate responses. For example, when an incoming call request is received, the receiving device or application acts as a UAS to process and respond to the request.

It is important to note that a single UA can switch between the roles of UAC and UAS depending on whether it is initiating or responding to a request.

Client

The term Client typically refers to the end-user device or application that interacts with the SIP network. This could include:

• A softphone running on a computer, smartphone, or other devices, which uses SIP to make and receive calls over the internet.
• Messaging applications that rely on SIP for real-time communication.

Clients are responsible for generating SIP requests when initiating a call, message, or other communication session. These requests are typically sent to a server, which manages the signaling and routing of the session.

For example:

• When a user makes a call, the client device generates a SIP INVITE request and sends it to a proxy server.
• The proxy server routes the request to the intended recipient.

Server

Server is an entity that handles the request coming from the clients in a predefined rule. There are several different types of servers described as below.

• Proxy Server:
• The proxy server is the most common type of SIP server.
• It acts as an intermediary, routing requests from the client to the appropriate destination (e.g., another server or endpoint).
• The proxy server does not process media but focuses on signaling.
• For example, if the recipient's location is unknown to the client, the proxy server forwards the request to the appropriate server or recipient.
• Redirect Server:
• A redirect server provides information to the client about an alternate route or address for the intended recipient.
• This is commonly used when the recipient has changed their location temporarily or permanently.
• Instead of forwarding the request, the redirect server informs the client where to send the request.
• Registrar:
• The registrar server is responsible for handling registration processes for clients.
• When a client registers with the network, the registrar server stores the client's location (e.g., IP address) and other details in a database.
• Location Server:
• The location server maintains the addresses and registration information of users.
• This information is updated by the registrar server and is used to determine the current location of a recipient for routing SIP requests.

How These Components Work Together

In a typical SIP communication session, various components collaborate seamlessly to establish, manage, and terminate sessions. The process begins with the client, acting as a User Agent Client (UAC), sending a request such as an INVITE to initiate a session. This request is routed through a proxy server, which may consult a location server to determine the recipient's current address. The recipient's device, acting as a User Agent Server (UAS), processes the request and sends an appropriate response back through the network. If the recipient's location has changed, a redirect server provides the client with updated address information to ensure successful communication. Throughout this process, the registrar ensures that all clients are properly registered and reachable within the network, maintaining a reliable and efficient communication framework.

In a typical SIP communication session:

• A client (acting as a UAC) sends a request, such as an INVITE, to establish a session.
• The request is routed via a proxy server, which may interact with a location server to find the recipient's address.
• The recipient's device (acting as a UAS) processes the request and sends a response back through the network.
• If the recipient has moved or changed locations, a redirect server informs the client of the new address.
• The registrar ensures that all clients are properly registered and reachable.