What are the primary components of a SIP-Enabled Network?

SIP is designed for peer-to-peer communications, and uses a client-server (request/reply) model similar to HTTP.  A SIP client is a party that initiates a SIP request (i.e. wants to make a “call”).  A SIP server is an application that services SIP requests and sends responses (i.e. receives a “call”). During a session, a SIP endpoint, such as a phone, can serve as either a client or a server, depending on if it is initiating or responding to a request.

User agents are applications in SIP endpoints (such as a SIP phone) that interface between the user and the SIP network.  An agent can act as either a client or a server.  When making a call it acts as an User Agent Client (UAC), when receiving a call it acts as an User Agent Server (UAS).  A Back-to-Back User Agent (B2BUA) is an application that acts as an intermediary between two parties, but appears as an endpoint to both parties.  It serves as both an UAS/UAC simultaneously to process session requests.  An example of this is Avaya Communication Manager, which acts as a B2BUA that represents SIP endpoints to the PSTN network and PSTN endpoints to the SIP network.

• Registrar Server:  The Registrar registers users when they come on-line and stores information on the users logical identity, and the associated device
  (identified by IP address or URL) or devices they will allow for communications.
  
  
• Location Service:  A database that keeps track of users and the URL bindings that are “closer” to them.
  The location service gets its input from the registrar server and provides key information for the proxy and redirect servers.
  
  
• Redirect Server:  Maps a SIP request destined for a user to the URL of the device “closest” to the user. 
  For example, if a call is destined for johnsmith@avaya.com and the user is on the road, the Redirect Server may reply to the caller’s user agent
  (or to the requesting proxy server) with the contact address of the user’s cell phone.
  
  
• Proxy Server:  Services SIP requests by processing them and passing them along to other SIP servers. 
  A proxy server may act as both a server and a client, and can modify a SIP request before passing it along.
  A proxy is involved only in the set-up and teardown of communications. 
  Once a session is established, communications occur directly between the parties.
  
  
• Presence Server:  Accepts, stores, and distributes presence information.  The presence server has two distinct sets of clients:
  • Presentities (producers of information) provide presence information to the server to be stored and distributed
  • Watchers (consumers of information) receive presence information from the server