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Networks



A computer network is a system of computers and workstations which are linked. A Local Area Network (LAN) is a network which is all on one site, such as a school or block of office. The workstations in a local area network are usually connected directly by cables. A Wide Area Network (WAN) is a network which connects large numbers of computers and terminals over long distances. The workstations in a WAN are connected by telephone or similar data links.

Data can be transmitted by:

1. wire cable. This conducts data as electrical signals an may be:

2. fibre optic cable (also known as optical fibre cable). 3. transmitted signals without cables Communication to WAN is usually archieved using a terminal and a modem with a telephone line. A modem (MOdulator-DEModulator) is a device which allows a computer or terminal to use the telephone line for communications.

Local Area Network (LAN)

LAN Hardware

LAN concept

A Local Area Network (LAN) is a information transport system for information tranfer between devices located on the same area, such as, an office building, a manufacturing plant, a hospital area, a university campus, or any other geographically confined area.

Any devide that communicates over a tranmission medium for example: computers, terminals, sensor (temperatures, security alarm, etc.) telephones, video camera, computer systems may be incorporated to LAN.

Why increasing interest on LAN

LAN Characteristics

A print server is a computer on a network which controls a printer. A file server is a computer on a netwrok dedicated to controlling a hard disc and handling users' files. The file server delas with the security of the files it controls. This includes checking passwords and user indentities.

LAN Capabilities

Advantages of Local Area Networks

Disadvantages of Local Area Networks

Requirements for a Local Area Network

LAN Hardware

1. The stations - these can be ordinary PCs:

2. The file server. This needs to be a powerful computer with: 3. A printer or printers LAN Software

1. Network management software to:

2. Applications software People

LAN Hardware Classification

Topology

A network topology is the patern ofr interconnection used among the various nodes of the network. In other word, topology is simply the way a network looks. Many topologies have been developed to cope with communications over a limited geographical area, but three major ones have had a major influence in LAN design and implementation.

Star Topology

A central hub to which all other nodes are connected by a single path.

Advantages

Disadvantages

Bus Topology

In this topology, various nodes are attached to a single long cable of transmissions medium (normally coaxial cable).

Advantages

Disadvantages

Ring Topology

All nodes are connected together in a ring but no one has control over the network. Data travels in one direction only.

Advantages

Disadvantages

Topology Selection

Media

The transmission media are cables that carry the signals through the establishment from node to node. Many types are available. Most popular types are: Each type has advantages and disadvantages over other. Therefore the following factors should be considered in media selection:

LAN Hardware Product

Ethernet Network

Ethernet is a high-speed linear bus network. It can use coaxial, twisted pair or fiber optic cables.

Thin Ethernet Cabling (10BASE-2).

Thick Ethernet Cabling (10BASE-5) Unshielded Twisted Pair Cable Network - uses 10BASE-T cables.

Token-Ring Network

A Token-Ring Network is a token-passing, as the name indicates. Each node attaches to a device call Multistation Access Unit (MAU). That sends the token from one node to the next in line.

For Token-Ring cabling (with small movable cabling system), follow the rules below.

ARCNET Network

ARCNET is token-passing star bus network. ARCNET networks traditionally use 93-ohm, RG-62/U coaxial cable. From the file server, the cable attaches to either a passive or an active hub. These hubs act as signal splitters, and for an active hub, a signal amplifier. From a hub, the cable can connect to another hub or a network node.

The cable-length distance between the farthest two nodes in the network cannot exceed 20,000 feet. This is the maximum distance a signal can travel on the cable in the allotted time (31 microseconds).

For ARCNET cabling (coaxial cable), follow the rules below.

Maximum cable distance depend on what you are connecting.
From To Maximum Distance
Network Node Network Node 2,000 feet
Network Node Active Hub 2,000 feet
Network Node Passive Hub 100 feet
Active Hub Active Hub 2,000 feet
Active Hub Passive Hub 100 feet
Passive Hub Passive Hub Does Not Work

Comparison

ARCnet Ethernet Token Ring
Speed 2.5 Mbps 10 Mbps 4/16 Mbps
Maximum Packet Size 512 bytes 1.5 KB 4 KB
Media Types Coaxial, Unshield Twisted Pair, Fiber Optic Thin/Thick Coaxial, UTP, Fiber Optic Shield Twisted Pair (STP), UTP
Standard Proprietary IEEE IEEE
Attributes Inexpensive, Reliable, Broad Vendor Support Fast, Broad Vendor Support Extensive Feature 
There is no one best LAN implementation. Each LAN type has its own good and bad points.

Factor to be weighted in implementing your LAN:

If cost is a driving concern, ARCnet is a good choice. ARCnet is simple to install and features very acceptable performance, especially in a typical office environment than on average contains fewer than 20 nodes.

If performance is a major concern, Ethernet should be considered. Ethernet cost bit more than ARCnet, but the speed and connectivity features offset the higher price. Therefore Ethernet has gained wide p;opularity in Local Area Networking.

For full-featured networks, Token-Ring is the Technology of choice. Token-Ring provides good performance plus features such as simple connectivity to the IBM mainframe system, simplified bridging for large network, and more sophisticated protocol-tuning mehcahism.

ARCnet, Ethernet and Token-Ring thrive in today's marketplace because each satif\sfies the requirements of certain customers. As long you know your requirements, you will be able to select the connection option that is best for you.


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