When two devices communicate with each other using a packet-switched network, it takes a certain amount of time for information to be generated by one device, travel to the other device and be received by the second device. The total time that it takes for this chunk of information, commonly called a packet, to travel end-to-end is called network delay. There is another type of telecommunications network called a circuit-switched network. In a circuit-switched network, the network delay is considered to be only the travel time from one device to another, excluding the output and intake times.
A typical computer network is a packet-switched network, thus when talking about computer networks, network delay takes into consideration all three of these delays. They are more formally called transmission or serialization delay, propagation delay and processing delay. Some network devices, such as routers and switches, also introduce additional types of delays into a network, for instance, queuing delays. These additional delays, if they occur, are included in the total time for network delay.
In the field of computer networking and network engineering, network delay is an important consideration. Varying and excessive delay can cause unwanted side effects such as packet jitter and lost packets. Packet delay variation, or jitter, creates problems such as garbled telephone calls and shaky video playback. Packet loss can result in dropped telephone calls, longer download times and slow web page retrieval.
Network delay can be caused by many things, including improper network design, the natural characteristics of different media, excessive network traffic and malfunctioning or poorly configured devices. Network designers strive to reduce the causes of delay. They use utilities and tools such as ping, traceroute, cable testers and network analysis software to troubleshoot where delay problems are occurring and what might be causing them. Most operating systems allow a person to test delay by issuing the ping or traceroute commands from the command prompt.
As networks continue to grow in speed and capacity, it becomes increasingly important to understand how long it takes for information to travel from one point to another and where delays occur. The information that travels through networks is mixed together from multiple sources and separated many times as it travels. Phone calls, web pages, downloads and many other types of traffic are mixed and separated repeatedly. If network delay is not kept to a minimum, increasingly more information could be lost in transit as network loads increase and congestion occurs.