Technical papers | What is voice over IP?
What is Voice over IP?
Voice over IP, or Voice over Internet Protocol to use the full title, is simply a means of making telephone calls over a data network instead of over the traditional analogue public switched telephone network (PSTN).
The term VoIP describes the use of the Internet Protocol (IP) to transfer speech between two or more sites. Inherent in the term is the management of the protocol. In general, this means that the voice information is encoded into discrete digital packets and then transferred across an IP-based network.
There are many advantages to this method of telephony, primarily the cost savings that can be made by avoiding the use of the traditional PSTN. In addition to cost savings, the digital nature of VoIP allows easy administration, the implementation of additional services such as voicemail, and a reduction in the physical cabling required for new installations. However, due to the distributed nature of the internet the Quality of Service (QoS) can often suffer, and there are still a number of technical issues affecting the widespread adoption of VoIP.
The term VoIP was first used by the VoIP Forum (a group of major companies including Cisco, Vocalec and 3Com) to promote and develop the use of the International Telecommunications Union’s (ITU) H.323 protocol. The forum also worked on the standardisation of directory services and the use of touch-tone standards for accessing voicemail.
The ability to transfer voice over the internet, rather than the PSTN was first made possible in February 1995 when Vocaltec released its Internet Phone software. This software was designed to run on a standard personal computer (PC) equipped with a sound card, speakers, microphone, and modem. The software encoded and compressed the voice signal, converting it into IP packets that were then transmitted over the internet. However this PC-to-PC internet telephony only worked if both parties were using the Internet Phone software. Inherent in this technology were a number of severe limitations. For example, no formalised ringing protocols meant that it was necessary for both parties to pre-arrange the time of the call in order to be available to make the final connections themselves. Users also had to contend with the lack of any kind of directory service, poor quality, and frequent delays.
Internet telephony has made a number of important advances since 1995. Many software developers now offer PC telephony software but, more importantly, gateway servers are emerging to act as an interface between the internet and the PSTN. Equipped with voice-processing cards, these gateway servers enable users to communicate via standard telephones over great distances without using the ‘long distance’ telephone network.
How does VoIP work?
When a traditional call is made using the PSTN, the analogue lines are kept open between the two callers for the entire duration of the call; this is called a circuit. This small segment of the network is exclusively used for this call and will be unavailable to any other users until the call has finished. VoIP does not require such dedicated circuits as it translates voice signals into packets of digital data. These packets are then transmitted over Ethernet or wireless networks, with no part of the network used exclusively by callers.
? Becta 2004 Valid at September 2004 page 1 of 7
Review at December 2004
Becta | Technical paper | What is voice over IP?
VoIP requires the use of Codecs (COder-DECoders). These can be software, or hardware such as microphones, IP telephones or other similar devices. They are required to convert analogue signals (what we say and hear) into digital (for transmission over the network) and back into analogue.
Figure 1 (below) shows how schools can combine their existing voice and data networks to carry both voice and data traffic. In the example shown, the LEA provides schools A and B with a broadband connection that is used to carry both data and VoIP traffic. The schools incur no call costs when calling the LEA or each other as all calls are passed along existing connections. School C has not yet converted to VoIP and therefore runs two separate networks, one for voice and one for data. The LEA also has a VoIP gateway server that allows it to send and receive calls on the PSTN. Should school A or B wish to call outside the VoIP service area the calls pass through the gateway and out over the PSTN. Similarly, if an external site wishes to call school A or B the calls are routed into the VoIP gateway at the LEA and over the network to the school.
Figure 1 – a typical H.323 implementation of VoIP with other networks
? Becta 2004 Valid at September 2004 page 2 of 7