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Introduction
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The term LAN originates from the Xerox Corporation Palo Alto Research Center where the Ethernet LAN was developed during the 1970s. This early specification was refined by a joint team from Digital Equipment Corporation (DEC), Intel and Xerox, and became the predominant method for networking personal computers in the early 1980s.
Meanwhile, research was in progress at IBM's Zurich labs on an alternative to Ethernet. The IBM method utilised a ring topology with token access control which became known as the Token Ring network. The IBM Token Ring LAN was formally announced in 1985 after further development into commercial hardware and software products at IBM's Research Triangle Park, and is still popular today.
Halsall [HAL92] states that "local area networks are used to interconnect distributed communities of computer-based DTEs located within a single building or a localized group of buildings." Among the DTE (data terminal equipment) devices may be office workstations, peripherals, minicomputers, or intelligent instrumentation equipment. As an example, a LAN may be used to interconnect workstations or computer-based equipment distributed around the buildings of a university campus, factory or hospital.
In general, however, a LAN is controlled by and located within a single establishment and is regarded as a private network. If the network is spread over an area of greater than, say, 10km², it is regarded as a metropolitan area network (MAN), especially if it links a number of LANs together or is distributed around a town or city. A network located over a greater geographical area than this is a wide area network (WAN).
The geographical constraints of a LAN, combined with the advanced transmission technologies available, enables data transfer rates of many millions of bits per second on the network. These transmission rates, coupled with a reliable access control method, permit a large number of digital devices to share the common physical medium with minimal interference, while allowing large blocks of data to be transferred using simple error-recovery procedures and data management protocols.
A LAN has been described by Devargas [DEV92] as a "layered system of interconnected workgroups". This emphasises the point that a LAN can be used in the workplace or learning environment to facilitate teamworking and information sharing between employees/students.
Popular network operating systems such as Novell Netware, IBM LAN Server and Microsoft Windows NT Advanced Server provide facilities for management of groups using LAN workstations in the form of 'directory services' or 'user services'. Therefore, the LAN in conjunction with appropriate software/hardware can provide an additional function as an information management tool, controlling the data flow and providing an overview of an organisations' systems.
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 Figure 1: Local Area Network ShowingMedium Attachment Unit (MAU)
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A number of LAN topologies are available including bus, ring, star and tree. Also, a plethora of different transmission media is used including shielded twisted pair and unshielded twisted pair (STP/UTP), coaxial, multi-mode and fibre-optic cable. Wireless LANs are now available which use radio and infra-red waves for data transmission.
There has been a substantial increase in the number of LAN installations over recent years. By 1996, nearly 1 million network server operating systems were forecast to be in use by the US Fortune 1000 companies alone. Inter-LAN communication via routers and bridges has been developed to cope with the increasing complexity of LAN installations.
LAN Glossary and Frequently Asked Questions (FAQ)
A complete glossary of commonly used LAN terms and acronyms has not been attempted. http://web.syr.edu/~jmwobus/comfaqs/lan-glossary.html is a fairly comprehensive LAN glossary. Other similar glossaries include the Internet 'Request For Comments' RFC 1208. http://web.syr.edu/~jmwobus/comfaqs/big-lan-faq.html is a Frequently Asked Questions list for LANs.
The IEEE 802 Project was formed by the American Institute for Electrical and Electronic Engineers to develop standards for Local Area Networks.
Ethernet-based Standards (IEEE 802.3, IEEE 802.3u)
The standard for Ethernet-type LANs in a bus topology is IEEE 802.3. Basic Ethernet (10Base-T) networks have a bit rate of 10 Mbps. All Ethernet-type networks utilise the CSMA/CD (Collision Sense Multiple Access with Collision Detection) access method. In CSMA/CD, all messages between devices are broadcast - sent to all stations in order to find the right destination. If a collision occurs, each station waits for a random interval then tries again. Ethernet is a general-purpose network, and as the most popular network technology it has a very widespread use, but it is most suitable for office-like environments.
The recent Fast Ethernet (100Base-T) standard, IEEE 802.3u, is very similar to conventional Ethernet but is faster (100 Mbps bit rate.)
Token Passing-based Standards (IEEE 802.4, IEEE 802.5, ANSI X3T9.5)
In addition to Ethernet, attention was focused on token-access protocols where a token passes from station to station, giving each station in turn permission to transmit on the network. Token-passing on a bus topology (Token Bus) is the subject of the IEEE 802.4 standard. Development of the Token Bus standard was led by General Motors, and it is mainly used in manufacturing environments for applications where reliability is absolutely critical (e.g. process control). As this material focuses on general-purpose LANs, Token Bus is not discussed further.
The Token Ring standard, IEEE 802.5, specifies a token-passing LAN on a ring topology. Development of the Token Ring standard was heavily influenced by IBM, and there are many similarities between the IBM Token Ring network and the IEEE 802.5 standard. Token Ring is intended for use in commercial and light industrial environments. The standard is not designed for use in heavy industrial environments, although the deterministic nature of the access method makes it more suitable than Ethernet for applications such as process control. Token Ring-based technologies with priority are particularly suitable for multimedia applications.
Fibre Distributed Data Interface (FDDI) is also based on token-passing and a ring topology, but extends and modifies Token Ring so that very high speed transmission on optical fibre networks is possible. The FDDI standard was proposed by the American National Standards Institute (ANSI) X3T9.5 Committee. Copper Distibuted Data Interface (CDDI) is a development of FDDI technology for copper wire.
Future LAN Standards (IEEE 802.12, ATM)
A new high-speed (100 Mbps) network technology has been proposed by Hewlett Packard. 100VG-AnyLAN uses an access method called Demand Priority, which is similar to Token Ring but without the expensive overheads incurred by token-passing. The new technology can be integrated into existing Ethernet and Token Ring networks fairly easily (at a price). A standard for 100VG-AnyLAN networks, IEEE 802.12, was agreed recently.
Asynchronous Transfer Mode (ATM) is a communications protocol based on small, fixed-length cells that can carry data, voice and video at very high speeds. ATM is not specifically a LAN technology, but it can emulate significant parts of existing IEEE LANs, including Ethernet and Token Ring.
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| Figure 2: Relation of the LAN Model to the OSI Reference Model |
| LANs Home | Token Ring | FDDI / CDDI | Ethernet | Future LANs | References |
Adrian Catchpole | A.G.Catchpole@comp.brad.ac.uk