Local Area Networks (LAN)

A single building or campus of few kilometers in size, LAN acts as privately owned networks. LANs are limited to a single building or group of buildings. By using telephone lines and radio waves, one LAN can be attached to other LANs over any distance. 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 means 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-mail or engaging in chat session users can also make use of LAN to interact with each other.
In today's world, Local Area Networks become fundamental part of communication. LAN applications are used in hospitals, educational facilities businesses, stock exchanges and warehouses. Local area network provides reader-friendly, comprehensive coverage of LAN technologies to be used in real-world applications.

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 network by three characteristics:
1)Their size
2)Their transmission technology and
3)Their topology.

LANs 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 of a single cable to which all the machines are connected like the telephone company party lines used in rural areas. Traditionally LANs operates at speeds of 10 to 100 Mbps with low delay and make very few errors. Newer LANs are working at speed of up to 100 megabits/sec.

Various topologies of LANs are:
1) Bus Topology:
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 100Mbps. Computers on an Ethernet can deliver data whenever they want to if two or more packets collide with each other than each computer just wait a random amount of time and tries again later.

2) Ring Topology:
A second type of topology used in broadcast system is the ring topology. In a ring topology, each bit circulates on its own not waiting for the remaining packet to which it belongs. Each bit circulates the entire ring in the time. It uses to deliver a few bits usually before the complete packet has been delivered. Like all other broadcast system, some limitation is required for arbitrating simultaneously accesses to the ring. IEEE 802.5 known as token ring is a popular ring based LAN working at 4 and 16 Mbps.

3) Star topology:
In Star 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.

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

Design considerations :

1) Cabling medium :
The most costless cable is Category 5 twisted-pair cable. This cable operates with standard Ethernet at a speed of 10 Million bits per second or 10 Mbps and "fast" Ethernet works at a speed 100 Mbps and is able to speeds up to 150 Mbps with existing technology. This cable is used in most LAN designs. Fiber-optic cable delivers more data but its materials and installation are much more expensive. In building proper installation of fiber-optic cable is very difficult to achieve because the cable should not be bent during installation.

2) Wiring plan
In the building 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. In refrigerator-sized cabinet, cables come together. Network transmission equipment are collected can be enclosed with doors and stand in a large room or it can be located as an open rack in a small room or closet. Wiring cabinets/closets can be attached with twisted-pair cable but fiber-optic cable provides higher speed.

3) Number of network connections (plug-in points) :
Every computer requires a network connection and each connection is consists of
(1) A plug-in point in the wall of the room
(2) Wiring from this point to the wiring cabinet/closet.
While deciding for network connections short and long-term technology aims need to be considered

LAN Hardware
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.

Conclusion
LANs are capable of delivering data at very higher rate than data can be transmitted over a telephone line; but the distances are restricted and there is also a restriction on the number of computers that can be attached to a single LAN.