LAN - Local Area Network

LAN stands for Local Area Network. In other words, LAN is a network of computers, usually limited to some geographical area, such as building, campus and so on. LANs can also differ themselves by their size. Each LAN could be small and be composed of few, for example three, computers, or it can be huge, linking hundreds or even thousands computers. The rapid advance of technology and the big number of advantages in using them, LANs easily got implemented in many business and educational organizations.

There are several LAN types:

• Ethernet
• Fast Ethernet
• Token Ring
• Fiber Distributed Data Interface (FDDI)
• Asynchronous Transfer Mode (ATM)
• LocalTalk

Ethernet is the most popular LAN technology used. But what makes Ethernet so popular? The Ethernet type of LAN keeps a good balance between the ease of installation, speed and cost. Also, this type of network has a great support for almost every kind of nowadays network protocols, which makes Ethernet the most used network technology today. Ethernet uses a access method called CSMA/CD (Carrier Sense Multiple Access/Collision Detection). That means that every device listens to the cable. If the network is clear, the device will transmit, otherwise it won’t. Actually, if the line is busy, it will wait and try to transmit later. Of course, like many other network topologies, IEEE (Institute for Electrical and Electronic Engineers) defines the Ethernet standard as IEEE standard 802.3. This IEEE standard defines the ways by which the Ethernet tools communicate and function.

LANs which are in need of greater transfer speeds very often use the Fast Ethernet technology (IEEE 802.3u). The Fast Ethernet technology is a lot faster than the standard Ethernet, usually increasing the speed from 10 Mbps up to 100 Mbps (Mbps = Mega bits per second). The advantage in using Fast Ethernet is that it does not require major rewiring, meaning no big modifications of the wires should be done. There are three types of Fast Ethernet:

• 100BASE-TX: for use with level 5 UTP cables
• 100BASE-FX: for use with fiber-optic cables
• 100BASE-T4: for use with leve 3 UTP cables, plus extra two wires

From all these, 100BASE-TX is the most popular type.

Token Ring network technology is a unidirectional network technology. Actually, in Token Ring types of network, the message is separated in tokens and it travels in the ring, it is viewed by every device. When the message arrives to the destination, the device copies the message and marks it as read. At the end, the same message comes back to the sender, where usually the token is removed.

FDDI is defined by OSI and ANSI standards. It is a 100 Mbps LAN and it works by using the Token Ring technology (dual ring). It is designed for use with fiber-optic cables, but nowadays it can also use cobber cables ( then it is called CDDI ). FDDI can successfully operate at big distances, up to 124 miles.

ATM is a highly effective and high-performance network technology. It has its history back in the 1970s and 1980s, with the development of the broadband ISDN services. ATM implements switching and multiplexing functions and it is ideal for connecting devices with different speeds. The traffic in ATM networks in organized by using a fixed-length cells. The cells usually consist of 48 bytes of body and 5 bytes of header. The small length of every length ensures that the important transfers, like audio and video transfers, which are time-critical, are not delayed for a big time period. The use of the header guarantees effective switching of the cells.

Local Talk technology is developed by Apple Computer, Inc. It uses very similar access method to CSMA/CD, but this one is called CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). The main difference is that every device signals the intention to transmit before doing so. To connect computers through the serial ports, a LocalTalk adapter and a twisted pair cables are needed. The greatest disadvantage in this type of LAN is the speed. The maximum speed is only 230 Kbps (Kilo Bytes per second).