SIP, VoIP, Trunks and Gateways: Acronyms and Jargon Demystified

SIP is an acronym for Session Initiation Protocol. SIP can be described as a signalling protocol that is used for controlling communication sessions over Internet Protocol (IP). Examples of the types of communications sessions controlled by SIP include voice and video calls, instant messaging, and online games. The SIP protocol can be used for creating, terminating, and modifying sessions that can consist of several media streams.

SIP uses a type of transaction (request / response) in order to control the exchanges between the the participants of the communication session. Examples of the type of SIP requests include register, invite, Ack, Cancel, and Bye. Examples of SIP responses include Provisional (1xx), Success (2xx), Redirection (3xx), and Client Error (4xx).


VoIP is an acronym for Voice Over Internet Protocol, which essentially refers to the collection of technologies, communication protocols, and transmission techniques required to deliver voice calls over Internet Protocal (IP). The main difference between traditional calls over the PSTN (Public Switched Telephone Network) and VoIP calls is that VoIP calls are delivered over the internet, whilst calls on the PSTN are delivered over physical analogue lines.

Calls are able to be transmitted over the internet by the process:

  1. Signalling is initiated in order to set up a channel for the call
  2. The voice signal is then digitised and encoded prior to transmission
  3. This signal is then broken up into packets and then transmitted over the internet
  4. This process is then reversed when the call reaches the recipient of the call

SIP Trunks

SIP trunking is a VoIP Protocol based upon SIP. SIP Trunking allows ITSPs (Internet Telephony Service Providers) to deliver telephony services and unified communications (voice, video and other media) to a SIP based PBX (Private Branch Exchange). It is also possible to provide SIP trunking on a PBX that isn’t SIP based by using a SIP Gateway to convert from ISDN (Q931) to SIP.

The benefits of using SIP trunks include:

  • Calls that are made over the internet are far cheaper than calls made from traditional landlines. Furthermore, it is very common for calls made to different sites within the same business to be free as calls are originated and terminated on the same network (on net)
  • SIP trunks make use of your existing data connection, and as a result can negate the requirement for line rental, saving your business money.
  • Adding SIP to compliment your existing ISNDs can give your business added resilience. If an issue were to manifest on your ISDN lines, then your business can continue to make and receive calls via the SIP trunk

Media Gateways

A media gateway is a device, that acts as an entry point to another network. In a typical setup, the gateway will be associated with a router and a switch. The router will know where to route a particular packet of data that arrives at the gateway, whilst the switch will setup the path in and out of the gateway for particular packets of data.

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