Two or more computers that are physically connected together form a network.  A computer network consists of both hardware and software.  The hardware includes interface cards and cables that link computers together and the software includes the drivers and the network operating system that manages the network.  In order to connect computers together you need:

• Network adaptors (a board or adaptor that enables it to communicate with other computers.)
• Network adaptor drivers
• Cables
• Data circuits
• Network software

A network can include a few computers within a single department or many computers in several departments. Using Windows for Workgroups, Windows 95 or Windows NT you can organize a peer-to-peer network into a series of workgroups.  Each workgroup can be organized into logical groups such as by department. Each computer on the network is identified by its computer name, this can be any name but usually has something to do with the person using it.

A peer-to-peer network is a system where all computers can access resources on all other computers when given permission to do so.  All computers in this type of network work in the same way.  It does not matter if one workstation is running Windows for Workgroups or Windows NT.

The purpose of a peer-to-peer network is to share network resources.  Network resources include:

• Files
• Directories
• Disk Drives
• CD-ROM Drives
• Printers

Data or devices can only be used by others if you share them.  Shared data or devices on other machines can only be used if you connect to them.  

When a computer is attached to a network, it becomes a network node and is referred to as a workstation.

The client/server environment is one where one or more computers act as a server and workstations connect to them running client software.  

A client is a software program that is used to contact and obtain data from a Server software program on another computer. Each Client program is designed to work with one or more specific kinds of Server programs, and each Server requires a specific kind of Client. A Web Browser is a specific kind of Client.

Cabling

There are three main types of cabling used in computer networks, they are:

• Twisted pair
• Coaxial
• Optical Fiber

There are two measures that are important in data communications cabling.

• Data rate - The number of bits per second a cable can transmit.  The higher the cable can carry the higher data rate it will have.
• Bandwidth - The difference between the highest and lowest frequencies a medium can transmit.

Now that we know the types of cable and how the cables capacity is measured, we can compare each type.

Twisted pair cabling is when two insulated wires are twisted together.  Straight wires tend to act as antennas, the twisting helps reduce the amount of outside interference.  Twisted pair come in either two pair or four pair.  It has a data rate of 1-100 Mbps and a bandwidth of 200KHz.  The ethernet cable we use is Category 5 unshielded twisted pair. The standard connector for unshielded twisted pair cabling is an RJ-45 or and RJ-11 connector.  RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry.

Coaxial cabling is a conductive wire surrounded by an insulating layer, wire mesh and a protective outer cover.  It can transmit.  It has a data rate of 500 Mbps and a bandwidth of 500 MHz. The most common type of connector used with coaxial cables is the Bayone-Neill-Concelman (BNC) connector.

Optical fiber consists of a thins strands of glass.  Light rather than electricity travels through the cables.  It is a very efficient method for data communications over long distances.  It is completely resistant to electrical interference and can transmit enormous amounts of data, up to 1Gbps.

Network Topologies 

The topology of a network is its shape or the layout of its cable.  The factors considered in choosing a cable system are: its cost, throughput and integrity.  The types of network topologies are: Bus, Star, Ring,  Tree and Mesh.  Some network types can use more than one topology. For example, Ethernet networks can be laid out in a Bus, Star, or Tree topology, or any combination of the three.  To understand each network type requires first understanding the basic topologies.

The common bus topology is one where computers are connected using one or more parallel lines. In this method each device listens to the bus and examines it data traffic.  One popular form of bus network is Ethernet.  Its common bus typically is the Ethernet Cable, which consists of copper, optical fiber or combinations of both.  The major advantage of a bus is the ability to add new devices to the network easily.

In a ring topology computers are connected circularly.  Each computer in the ring can only communicate directly with the computer on either side of it.  If it wants to communicate with a device further away, it sends a message that passes through each device in between.  IBMs Token Ring is a variation of the ring topology.   

Star topology uses a central computer that communicates with other devices on the network.  Control is centralized, if a computer wants to communicate is has to go through the central computer or concentrator.

A tree topology can be thought of as being a "Star of Stars" network. In a Tree network, each device is connected to its own port on a concentrator in the same manner as in a Star.

A Mesh topology consists of a network where every device on the network is physically connected to every other device on the network. This provides a great deal of performance and reliability, however the complexity and difficulty of creating one increases as the number of nodes on the network increases. For example, a three or four node mesh network is relatively easy to create, whereas it is impractical to set up a mesh network of 100 nodes.  Mesh networks are not used much in local area networks (LANs) but are used in Wide Area Networks (WANs) where reliability is important and the number of sites being connected together is fairly small.

At Yale we have an enterprise network, it ties several type of networks together regardless of their topology.