The Pivotal Role of SIP Proxy Servers in VoIP Communications

SIP Proxy Server

It is doubtful if VoIP would be playing such a central role in communications were it not for SIP or Session Initial Protocol, a communication protocol to manage sessions between two SIP addresses in which the SIP Proxy plays the role of “facilitator” as will be explained below.


Session Initiation Protocol or SIP is a signaling protocol that handles the task of initiating and terminating real time sessions. Sessions may comprise of voice, video and message. If you use internet telephony through softphones or through your VoLTE phones you must know that SIP is at work in the background. SIP works along with other protocols such as Session Description Protocol and with User Datagram Protocol, Transmission Control Protocol and Stream Control Transmission Protocol plus security through Transport Layer Security. If voice or video is to be transmitted then Real-time Transport Protocol also kicks in. The function of SIP is to initiate a communication session, find out end point, negotiate parameters to establish the session, teardown of the session in progress and then finally complete the session on termination of the call. It works in the request-response mode. There are user agents in IP called end points and these can be mobiles, softphones or SIP phones. Between the caller and the called party you have a SIP Proxy or a SIP server.

Why opt for SIP when you can use VoIP (as in WhatsApp and other “free” chat apps) is a question that often arises. The answer is that when you have SIP you can be assigned a phone number just like you have a phone number in PSTN landlines and that is essential for business communications.

The SIP Proxy

The SIP Proxy plays a crucial role and works both as a SIP server and SIP client in order to make requests. The primary role of the SIP Proxy is to handle routing with other functions such as enforcing policy being the secondary ones. The proxy interprets and reformats parts of the SIP request message before it is sent out to user agent servers and clients. The request may go direct or through several proxies in the network with each one making routing decisions and modifying the request before transmitting it further.

In simple terms the SIP Proxy manages SIP calls and is included in IP PBX setups.

Breakdown of how SIP server works

In essence, the SIP server is like an agent or a middleman for SIP calls. It transmits and ends calls through the use of stateless and stateful servers.

Stateful server

The stateful server can be compared to a “responsible agent”. It records requests and responses for future use. This has its uses. For example, if the first call attempt is unsuccessful then the stateful proxy will keep trying. It also proves helpful to pick up the call request and re-route it through another network. For instance, you can set up your IP PBX with a stateful server to receive calls at one extension during the day and at another during the night or on holidays.

Stateless server

The stateless SIP server simply acts as an intermediary to forward messages without recording any information.

Do you need SIP Proxy?

When you have a vendor set up your IP PBX, they usually take care of setting up the SIP Proxy or SIP server in the background. You may not know it but it is absolutely essential for a number of reasons:

Facilitate calls & security: as stated above, SIP Proxy facilitates calls and takes care of authentication. It can thus identify attempts at misuse and prevent frauds and prevent calls when a user does not have adequate balance in the account.

Inbound call distribution: Enterprise IP PBX may have a number of extensions in which case the SIP Proxy takes care of forwarding inbound calls to specific SIP devices according to assigned IP. It can keep ringing one number or try one extension after another until call is picked up.

Call records: As in the above case, call records may be important in small to large organizations. The SIP Proxy, configured as a stateful SIP server, will maintain records of all conversations.

SIP servers may be of different types:

Registrar server

When the user agent and its request are identified by the server then it functions as a registrar server. This is useful to authenticate users in a SIP network. Registration can be static in which it sends traffic to one PBX to a single IP address, or dynamic in which a domain address is made use of with the IP address changing dynamically.

Redirect server

This type of SIP Proxy receives requests through the registrar server database, finds the target recipient and routes traffic to that destination.

Location server

This server is responsible for handling caller location information to be passed on to the redirect server.

The IP PBX system may be configured and set up to work with suitable type of SIP Proxy which then works as the middleman to ensure perfect setup and transmission of signals and, on completion of a session, it clears the line for the next call.

Unlike analogue phones, digital or VoIP telephony breaks up audio into digital packets transmitted through the internet. Each packet will have a header and is assembled at the destination in the right sequence and then converted to audio stream. The SIP server’s task is also to assist with packet transfer by first sending signaling packets, confirming acknowledgement by target and then initiating packet transfer.

The SIP Proxy authenticates the caller, necessary in a business environment in order to prevent unauthorized use, and it does this through the IP address assigned to the user. The SIP server may also require use of Message Digest authentication in which it checks identity of SIP user agent and converts message to random alphanumeric code or MD5 hashing. This ensures security of VoIP communication.

VoIP offers immense benefits in that you can make long distance and international calls at a low cost and also exchange data as well as text and multimedia. SIP is at the core for business phones and the SIP Proxy keeps everything running smoothly.

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