by Stephen Roberts

Created on: January 09, 2007   Last Updated: March 29, 2008

Voice over IP: What is it, and how is it implemented

Voice over IP overview:

Historically, there has been a division between the area of data networks and telephony networks. However, the line between these areas is more and more becoming blurred, with telephony networks carrying data (dial up Internet access for example) and data networks carrying telephony signals (the subject of this paper).

When we refer to a "Network", we are referring to any interconnection of nodes, which may be transmitting some form of data. The way in which the data is transmitted will depend on the architecture of the network (the way it is set up) and the data, which is being transmitted. The rules, which govern how the data is transmitted, are referred to as the protocol.

We can define two main divisions in the way in which data is routed, or switched, through the network.

(i) Circuit Switching
This is where a path is set aside for the nodes on the network who wish to communicate. The resources are reserved for the duration of their session. This technique is inefficient in terms of network resources as the resources are allocated when the users may not necessarily be using them. However, it offers high quality of service.

(ii) Packet Switching
This is where the data is divided into units called packets and then transmitted through the network. This is more efficient in terms of network resources however it can lead to packet loss and delay.

A major factor in transmitting a voice signal is the delay the signal encounters in traveling through the network. This is also known as latency. If the delay is too long, the users will end up talking over one another.

The public-switched telephone network, PSTN, is based on circuit switching, due to both its efficiency and historical reasons (when telephony networks were initially being set up, packet switching didn't exist). However, as pointed out above, we pay for the reliability with efficiency. For this reason, coupled with the increasing prevalence of Internet Protocol, IP, based networks, it is desirable to be able to transmit voice over IP.

Unfortunately, packet switched networks, such as IP networks, are unsuitable for transmission of voice. When considering the performance of such systems, we must decide what metrics to use in order to measure this. The obvious and most important factor is voice quality. As discussed above, this is affected seriously by latency. However packet loss and jitter will provide unacceptable quality also. Jitter

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