What is a Local Area Network (LAN)?

LANs are privately owned networks, that are limited to a single building or group of buildings. However, using telephone lines and radio waves, one LAN can be attached to other LANs over any distance. Such a system of multiple connections of LANs is called a wide-area network (WAN).

Most LANs are used to connect workstations and personal computers. In a LAN, each host (individual computer) has its own CPU, with which it runs programs, but it also able to access data and devices anywhere on the LAN. This means that many users can share data information and expensive devices like laser printers. By sending e-mails or engaging in chat sessions, users can also make use of LAN to interact with each other.

In today's world, Local Area Networks have become a fundamental part of communication. LAN applications are used in hospitals, educational facilities, businesses, stock exchanges, and warehouses. LANs are typically identified by the following properties:

1. Multiple systems connected to shared medium
2. High total bandwidth
3. Low delay
4. Low error rate
5. Broadcast/Multicast capability
6. Limited geography (1-2 km)
7. Limited number of stations
8. Peer relationship between stations
9. Confined to private property

LANs are distinguished from other networks by three characteristics: their size, their transmission technology, and their topology.They are limited in size which means that the worst case transmission time is restricted and known in advance. Knowing this limitation makes it possible to make use of certain kinds of designs that would not otherwise be possible. It also simplifies network management.

LANs often make use of a transmission technology including a single cable to which all the machines are connected. Traditionally they operate at speeds of 10 to 100 Mbps with low delay, and make very few errors.

In a bus or linear cable network, at any instant one machine is allowed to transmit data or information. All other machines are needed to desist from sending. When two or more machines want to transmit simultaneously, an arbitration mechanism is used to resolve conflicts. The arbitration mechanism used may be centralized or distributed.

IEEE 802.3, known as Ethernet, is a bus-dependent broadcast network with decentralized control working at 10 or 100 Mbps. Computers on an Ethernet can deliver data whenever they want to. If two or more packets collide with each other, then each computer just waits a random amount of time and tries again later.

A second type of topology used in broadcast system is the ring topology. In this, each bit circulates on its own, not waiting for the remaining packet to which it belongs. It delivers a few bits usually before the complete packet has been delivered. Like all other broadcast systems, some limitation is required for simultaneous access to the ring. IEEE 802.5, known as token ring, is a popular ring-based LAN working at 4 and 16 Mbps.

In this topology, all devices are attached to a central hub. To install and manage star networks are relatively easy, but bottlenecks can occur because all data must pass through the hub.

A tree topology is a combination of the linear bus and star topologies. It includes groups of star-configured workstations attached to a linear bus backbone cable.

The most inexpensive cable is the Category 5 twisted-pair cable. This cable operates with standard Ethernet at a speed of 10 Million bits per second, or 10 Mbps. 'Fast' Ethernet works at 100 Mbps, at times even reaching 150 Mbps with existing technology. This cable is used in most LAN designs. Fiber-optic cables deliver more data but are expensive. Also, as fiber optic cables cannot be bent, it become very difficult to achieve proper installation.

The location of wiring cabinets/closets is based on distances between computers. The maximum distance should be 10 meters between a computer and a network wiring cabinet/closet. Network transmission equipment can be in a large room or can be located as an open rack in a small room or closet. Wiring cabinets/closets can be connected with twisted-pair cables, though fiber optic cables provide higher speed.

Every computer requires a network connection, and each connection consists of a plug-in point in the wall of the room, and wiring from this point to the wiring cabinet/closet. While deciding network connections, short and long-term technology aims need to be considered.

Ethernet and Token Ring are common types of LAN hardware. Ethernet is easily designed and consists of data transfer devices which are less expensive than similar Token Ring devices. Ethernet and Token Ring's performance are same in most situations.

While it is possible to connect Ethernet and Token Ring networks in the same location, it adds to cost and complexity. It is best to maintain the data communication infrastructure's configuration as simple as possible. Asynchronous Transfer Mode is another standard for data communication.

It is more expensive than Ethernet or Token Ring, but offers higher speeds, and will eventually offer better integration of computer-data, digitized voice (telephone), and video.

LANs are capable of delivering data at a very high rate as compared to a telephone line; but the distances are restricted, and there is a restriction on the number of computers that can be attached to a single LAN.