What is latency

“Latency” is a general term which means, especially in widely understood computer systems, all kinds of delay during performing a given operation. Coming to more details, namely to computer networks, “latency” is defined as all kinds of stoppages or delays which occur during transmission of data packets, and in processing the received ones or the data prepared to be sent. There is often a possibility to encounter a notion “lag”, particularly popular in circles of computer games enthusiasts (gaming online with the use of the Internet), and among people intensively using the IRC service. Equally popular in a computer jargon is a term “ping” used interchangeably with “lag”. However, it is not entirely correct because ping is one of the basic tools for estimating delays of network transmissions.

Values of delays depend on many factors. In efficiently functioning local network, in which there is relatively small movement being a fraction of a maximum transmission medium’s bandwidth (e.g. 100 Mb/s with typical cabling in a form of twisted-pair cable of the category 5.), delays are lower than one millisecond. Using the Internet lines the average values can comprise themselves in a range from between ten and twenty milliseconds up to even a few seconds. While in case of the typical Internet activity like e.g. browsing web pages, chatting, downloading files or sending e-mail few-second lags are not much nagging, in situations when a quick answer of a server is required the matter looks completely different. Examples of applications like that are among others as follows:

Computer games – practice shows that delays of a 100 milliseconds order are still acceptable. Above this limit, especially for games of quick action, these lags make a game hard.
Video – watching films online, e.g. from YouTube requires relatively high constant line bandwidth. Delays of transmission of the range of 300 – 500 milliseconds can result in stopping a film, a loss of synchronization on a sound-image line or in some cases cause deformations of image.
Telecommuting, especially with graphics pad – delays of one second order many times cause breaking the connection and necessity of reconnecting, values oscillating around 300 milliseconds noticeably affect comfort of working.

Apart from delays caused by incorrectly functioning transmission lines, in some situations it is possible to encounter lags caused by system software, e.g. underdeveloped devices’ drivers, modules of operating some functions, etc. As characteristic examples of such situations it is possible to list the following ones:

Not much efficient software of the e-mail server – in case of many e-mails transmitting a single message is understood as intervals of time between receiving an e-mail by server and placing it in a user’s inbox can last even up to a dozen or so minutes. The typical SMTP server needs for that only a few milliseconds.
Errors in network interface cards’ drivers – some cheap network interface cards have errors in their software drivers which cause significant delays in data transmission. It happens often because of disobeying standards concerning forming the packets, an access to transmission medium or cooperation with a kernel of operating system. From the point of view of a final user a network interface cards with an incorrectly written driver has a very low bandwidth, very long anticipation time for connection (up to a dozen or so seconds, regardless of line parameters, used communication protocol and other features), and generally negative influence on the operating system’s stability.
Software which is not on time with converting incoming data – it is a slightly different situation from a case of free e-mail’s driver because it is connected with a too low computing power. Cases of this type can be observed while attempting to watch a movie online on a computer with a strongly outdated configuration. In spite of having an access to line of bandwidth theoretically significantly exceeding requirements demanded from a film watching it can become more difficult due to the hardware not being on time with decoding particular frames so that the user could receive a smooth image.