What is a SIP server?

SIP stands for Session Initiation Protocol. It is used to initiate, maintain, and terminate real-time communication sessions that include voice, video, and instant messaging. Applications of the SIP protocol include signaling and controlling multimedia communication sessions such as VoIP calls, video calls, and mobile phone calls over LTE. A SIP server is the main component of an IP PBX and mainly deals with the management of all SIP calls in the network. A SIP server is also referred to as a SIP proxy or a registrar.

What does a SIP server do?

SIP Server Diagram

Although the SIP server can be considered the most important part of a SIP-based IP PBX phone system, it only handles or manages sessions; more specifically, a SIP server can:

  • Set up a session between two (or more) endpoints (an audio conference would have more than two endpoints). When an IP Phone makes a call, it sends a request to the SIP server. The server reads the request and routes it to the correct destination. This could be a request within the network, to a remote device, or to an “off-domain” network, like the PSTN.
  • Handles call rules: The SIP server controls the call rules for any connected devices. For example, you can choose to configure to allow multiple phones to ring simultaneously, or sequentially, and set how long they ring before going to another destination, such as an extension’s voicemail.
  • Negotiate the media parameters and specifications for the session for each endpoint using the SDP protocol.

  • Adjust the media parameters and specifications of a session DURING the session (putting a call on hold, for example).
  • Substituting one endpoint with a new endpoint (call transfer).
  • Terminate a “session” or call.

Does a SIP server control media / audio?

The SIP server does not actually transmit or receive any media – this is done by the media server using the RTP protocol. Within the context of an IP-PBX environment, it is almost always true that the SIP server and its media server companion reside on the same machine.

Some very high-volume SIP servers (such as a large VoIP provider, for example), may separate their media server to a different machine to better handle the workload, and could also possibly distribute the load to multiple media servers.

How do SIP servers enhance security?

When a call is made to the PSTN network, an international destination, or any destination that charges, the proxy server can check the caller’s identity and credit. This helps minimize the risk of user impersonation and can flag suspicious activity to service providers at an early stage.

Further reading