Local area network

From Citizendium, the Citizens' Compendium
Jump to: navigation, search
This article is developing and not approved.
Main Article
Talk
Related Articles  [?]
Bibliography  [?]
External Links  [?]
Citable Version  [?]
 
This editable Main Article is under development and not meant to be cited; by editing it you can help to improve it towards a future approved, citable version. These unapproved articles are subject to a disclaimer.
For more information, see: Locality of networks.

While there have been a great variety of local area network (LAN) technology and implementations, they share the property of interconnecting personal computers, servers, routers for access outide the LAN, and other computer network devices, within a relatively close distance of one another. LANs may be on physical media (e.g., twisted pair wire, coaxial cable, optical fiber) or operate over wireless media (i.e., wireless local area network (WLAN)).

LAN technical specifications must, at a minimum, define the way(s) in which an individual device connects to the transmission medium, and the way in which multiple devices share that medium with minimum interference. The first is often has a subset of a Physical Layer Specific (PLS) specifications that deal with variants of the basic medium. The second includes Medium Access Control (MAC), or the means for minimizing interference to a shared medium.

When LAN components are connected by physical media, the typical maximum distance between a computer and a device connecting it to the LAN does not exceed 100 meters, although there are many ways to extend the distance of connection, including interconnecting LANs anywhere that the Internet can reach. By interconnecting the interconnection devices, a single LAN can service a large building or campus, although operational considerations usually make it wise to separate a large location into multiple interconnected LANs,

History and standards

While there were many ad hoc means of interconnecting computing devices in a small area, the first widely accepted de facto standard was Ethernet, especially version 2 defined by the combination of Digital Equipment Corporation, Intel and Xerox. This is sometimes called DIX after the sponsors. The most common basic implementation used coaxial cable, at distances up to 500 meters, and at a data rate of 10 megabits per second (Mbps). See the Ethernet for more of the physical details. The key co-developers of DIX Ethernet were Robert Metcalfe and David Boggs.

Unfortunately, "Ethernet" has become an extremely generic term, which may only mean that one of the computer-to-LAN connectors used by one of the LAN standards is in use. Beyond that connector, there may well be a wide area network running for thousands of kilometers, or even into deep space. In addition to the connector, the frames, or units of data put onto the medium, often follow several standards developed for Ethernet variants.

Calling an access system Ethernet does not automatically make it successful or cheaper — David Thorne, presentation to IEEE Ethernet in the First Mile working group

LAN specifications involve several protocol layers. In the Internet Protocol Suite architecture, the various layers are grouped as "interface protocols" over which Internet Protocol, which is "agnostic" to the transmission system, runs. While LANs do not completely fit the Open Systems Interconnection Reference Model, most of their functions are at layer 1 (physical) and layer 2 (data link).

The DIX specifications were given to the Institute for Electrical and Electronic Engineers, a recognized standardization body. IEEE created Project 802 on Local Area Networks; the number comes from the date of its creation in February 1980.

There were some competing, or perhaps complementary, alternatives to Ethernet.

Ethernet works in practice, but allegedly not in theory — D.R. Boggs, J.C. Mogul, C.A. Kent [1]

In response, IEEE initially created several committees some usable with all the alternatives, and some bringing standardization to alternatives designed by different companies:

  • IEEE 802.1: dealt with overall LAN architecture, particularly the interconnection of LANs using MAC/LLC bridging
  • IEEE 802.2: dealt with Logical Link Control, a means of providing a standard data format independent of the MAC mechanism running below it.
  • IEEE 802.3: dealt with the carrier sense multiple access with collision detection (CSMA/CD) MAC used by DIX Ethernet, and the various means of physical interconnection, initially several types of coaxial cable and a type of optical fiber. The 802.3 group has produced well over 30 specifications for running the basic technologies at different speeds, on different media (i.e., the Physical Layer Signaling sublayer), and with various techniques for management, fault tolerance, and other specialized requirements.
  • IEEE 802.4: a PHY and MAC system, proposed by General Motors, which used the token bus MAC technology, and was optimized for the high electrical noise environments in factories. 802.4 has been replaced by other techniques, primarily 802.3 but also some specialized controller area networks
  • IEEE 802.5: a PHY and MAC system, proposed by IBM, that used the token ring MAC technology. It offered more predictable performance than 802.3, but it did not offer enough advantages to prevent 802.3 from overtaking it in the marketplace.

Virtual local area network

Virtual local area network (VLAN) techniques, standardized by the IEEE 802.1q working group, allow independent LANs to share the same physical media. While they still physically coexist and need to follow common MAC rules to avoid mutual interference, in a typical installation, an end user will only be able to participate in a limited number of the LANs, such as one for data and one for Voice over Internet Protocol (VoIP). It is more common to see a large number of non-interfering VLANs on cables in the infrastructure of a building or campus network.

Wireless LAN standards

There are a long list of other Project 802 projects, many beyond the scope of this discussion. Two important ones include;

  • IEEE 802.11: "wireless Ethernet", also called "WiFi"; a set WLAN technologies, of range of meters up to a kilometer or so
  • IEEE 802.16: "wireless local loop", also called "WiMax", originally intended for several niche applications, but now attractive for a range of WLAN applications requiring ranges of tens of kilometers.

It must be understood that while cellular telephony may indeed provide wireless data communications, it uses a signficantly different set of technologies, intended not for multiple computer interaction but the basic two-party model of telephone communications. Cellular telephony is a wireless extension of telephone networking.

References

  1. Boggs, David R.; Jeffrey C. Mogul & Christopher A. Kent (September 1988), Measured Capacity of an Ethernet: Myths and Reality, WRL Research Report 88/4