In this article, we present a basic architectural diagram of the elements of SIP (Session Initiation Protocol) and also include a brief description of them. As you know, SIP is the open standard protocol for delivering voice/ video/ multimedia communications. SIP resides in the application layer of the network – It establishes, modifies and terminates multimedia sessions (primarily voice and video) between intelligent devices.
User Agents – There are two types of user agents in a SIP environment – User Agent Clients, that issue SIP requests and User Agent Servers that respond to SIP requests. Eg. SIP Phones.
Registrar – A registrar is like a directory that acts as a repository to store SIP URL’s and associated contacts. Eg. LDAP/ User Directories.
Proxy Server – A Proxy Server does application level routing of SIP requests and responses. Eg. Call forwarding to a Voice Mail.
Home Proxies/ Location Services – SIP messages are usually sent to a user’s primary AOR (Address of Record), for example his land-line number. A Home Proxy in conjunction with Location Services can forward this to the preferred contact address (or) multiple contact addresses of the user, for example a SIP Client in Cell phone, Soft Phone in his computer, etc.
Redirect Servers – A Redirect Server directs the user agent to contact another server without forwarding its requests. Eg. Time of the day routing, Boss-Secretary feature, etc.
Presence Server – Accepts Presence information (Availability information), stores it and distributes it – from and to, endpoints. Eg. Status Messages like Busy, Available – Instant Messenger style.
Gateways – A gateway provides protocol translation (conversion) and inter-interoperability between SIP and other protocols like H.323, ISDN, MGCP etc. Eg. Analog Telephony Adapters.
Back to Back User Agent (B2BUA) – B2BUA processes requests as a SIP User Agent server and also acts as a SIP User Agent client. B2BUA maintains complete session state and participates in session requests. Eg. Initiation of Outbound calls.
Feature/ Application Servers – SIP is an extensible protocol and hence various feature servers and application servers work in the SIP framework to deliver various specialized applications. Eg. Text to Speech Application Server.
Except user agents (which needs to be separate), all the other components can either be located in a single server (physically) or separate servers (physically) depending on the volume of concurrent calls, number of sites, redundancy options, etc.
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