Department of Computer Science
Helsinki University of Technology
Increment in the essence of internetworking has led into evolvong of many Local area network (LAN) technologies. LANs can be divided into several different subgroups based on different criteria, such as physical implementation or standardized technologies. These differ e.g. on data transfer speed, functioning distance and cost.
In the past decades the essence of internetworking has vastly increased. As always, demand breeds supply, and several different LAN technologies have emerged. Local area networks (LANs) can be implemented in several ways. They can be divided into several subgroups based on e.g. physical implementation or standardized technologies. Choosing the right kind of LAN solution for a particular purpose always depends on several issues, e.g. size of the network, location of the terminals, and usually also on the cost issues. This essay offers a brief overview on some of the most common LAN implementation techniques.
LANs can be divided into two quite different types: wired LANs and wireless LANs. As their names imply, wired LANs require fixed wiring whereas wireless LANs utilize radio or light waves as the transmission media. Wired LANs are much more common and usually less costly when used to e.g. interconnect all office equipment to provide shared use of printers and other resources. But if the layout of the interconnected computers is due to change often, a wireless network is worth considering, as also in the case of interconnecting handheld terminals and portable computers. There is also other physical division criteria: 
Naturally wired and wireless LANs use different topologies. The four topologies in common use for wired LANs are bus, ring, hub and star. Bus is a linear connection between the data terminals. A single network cable is routed through those locations that have terminals, and a physical connection (tap) is made to the cable for each terminal. Bus networks are often extended into an interconnected set of buses with special bus extenders. Ring topology is similar to bus topology except that in ring topology the two ends of the bus are connected to form a ring. Hub topology is a variation of bus and ring. In this case wiring from terminal always goes through hub first before going to another terminal. Hub consists of a set of repeaters that retransmit all the received signals to all terminals in that direction. Star topology means that the terminals are connected through a server or a router, which takes care of routing the signals into the right direction. Wireless LANs have two common topologies, namely fixed-wire replacement and ad hoc networks. The former is simply a LAN using wireless connection in a place where normally would be a wired LAN, but due to a high cost of installing wiring or often changing network layout. The latter is a real wireless LAN with possibly nothing else than portable-to-portable connections. 
Wired LANs use mostly three types of transmission medium, namely twisted pair, coaxial cable and optical fiber. Twisted pair consists simply of two wires twisted tightly together. Twisted pair is the cheapest to manufacture of the three mentioned. As it is also the cheapest and easiest to install, no wonder it is very widely used in LANs. With new cable manufacturing and copper refining technology and also improvements in the data transfer technology the engineers have been able to reduce the level of distortion and attenuation and increase the transferred data rate greatly, so twisted pair is still very substantial transmission medium. There are mainly two types of coaxial cable in use, namely thin wire and thick wire. Thin wire comes with 0,25" diameter and thick wire with 0,5" diameter. The advantage gained by the thicker cable is more than doubled functioning range. Optical Fiber is made of glass or plastic and can operate at data rates well in excess of those with twisted pair or coaxial cable. Thus it is also the most expensive of the three. Optical fiber also has a low level of distortion owing to its immunity to electromagnetic interference. 
There are many more or less standardized local area network technologies in use today. Here are introduced some of the most relevant.
Of the several LAN technologies Ethernet is by far the most popular. This is mostly due to the good availability and competitive pricing of the Ethernet technology. Because Ethernet is an open standard, the specifications and rights to build Ethernet technology are easily available to anyone. This has resulted in a large Ethernet market with several manufacturers and vendors. Additional to the traditional Ethernet, which functions on the speed of 10 Mbps, Fast Ethernet (100 Mbps) and Gigabyte Ethernet (1000 Mbps) standards have been introduced. They are strongly based on the traditional Ethernet standard, but provide capacity, which is sufficient also for foreseeable future purposes. Typically the traditional Ethernet LAN uses bus topology, newer ones often use star topology. Ethernet uses CSMA/CD control access method, which means that all the terminals are listening to the bus all the time. If they wish to transmit data they just check if there's traffic in the bus and if not, they star transmitting. In case a collision happens they just wait a random time and try again. [7,8]
The Token Ring network was originally developed by IBM in the 1970s. It is still IBM's primary LAN technology and is thus used in many large organization networks. Token Ring networks usually operate at speed of 4 or 16 Mbps. Token Ring uses star topology and token passing control access method. In token passing connected workstations form a logical ring. A single unique token is then passed in the ring from workstation to workstation continuously, and only the workstation in possession of token is allowed to transmit data. Token Ring technology offers much better error avoiding and detection methods than Ethernet, but due to it's higher price and lower speed it is seconded by Ethernet in general LAN popularity. [1,2]
FDDI (Fiber Distributed Data Interface) is a LAN technology mainly used as backbone in large university and organization networks. Topology is always dual ring, which makes it rather fault tolerant. As the name implies, FDDI uses optic fiber as transmission media. As the control access method FDDI uses timed token method, which id pretty much similar to token passing method. FDDI operates at the speed of 100 Mbps. Owing to FDDI's rather costly structure (dual fiber ring) and slow development newer techniques are slowly taking its place. 
VG100-AnyLAN is a network technology that provides a data rate of 100
Mbps makes interconnecting Ethernet and Token Ring networks possible. It
differs from Ethernet mainly by it's control access method, which is DPA
(Demand Priority Access). DPA makes 'polls' round the network to see which
terminals need to transmit data. Collisions are prevented and DPA has a
possibility for primitive priorisation. VG100-AnyLAN has a star topology
with a central hub, which is called root. [4,6]