Ethernet is a technology that specifies how data is transferred from one computer or device to another. It was developed in the late ’70s by Xerox Park Laboratories, USA and has been specified in the Institute of Electrical and Electronic Engineers (IEEE) 802.3 LAN Standards Model.

Example Bus Topology

Ethernet cards or network interface cards are commonly supplied with all newly manufactured PC’s and are the interface between the computer and the transmission medium or cable.

The Ethernet card controls the data being transmitted and received from the computer, Some of its many functions include fragmenting the data into manageable chunks called frames, to prepare it for transmission and to reassemble the received frames upon arrival. The card holds a unique address so that computers can locate each other on the network. Data carried on an Ethernet network conforms to a protocol called Carrier Sense Multiple Access with Collision Detection (CSMA/CD).

Older Ethernet networks would conform to a standard known as 10 Base 5 - which consisted of a half inch 50 -ohm coaxial cable sometimes referred to as ‘thick’ Ethernet. The 10 indicated the 10Mbps data transfer rate, Base refers to the Baseband transmission mode and 2 indicates a 185 (rounded to 200) metre recommended bus segment length or cable run.

It was possible to add additional length to an Ethernet bus, simply by adding a device called a repeater, which amplifies the signal, allowing the it to reach greater distances and prevent attenuation or weakening. Also, separate networks could be linked together by connecting a device called a Bridge. A Bridge would hold a small database of computers and their corresponding addresses, which after examination of the frame would determine whether or not the data is transmitted onto the linked network or stay on the source network.

There were many problems with this older style network configuration, mainly because each computer on the network had to contend with every other computer to send its data, as a result many collisions would occur and the network would become extremely slow.

Today, most Ethernet installations conform to a standard known as 100/1000 Base T. The numbers refer to the maximum data transfer rate - 100 Mbps or 1000Mbps (1 Gbps). The Base is the Baseband signal and T refers to the Twisted pair medium or wire that carries the data.

Example Star Topology

When using twisted pair cabling, a star topology is commonly used. Instead of each computer having to contend with other devices to send its signal, each computer can have a direct connection with a device called a switch (smaller networks may use a hub) which creates a virtual circuit with the destination device.

In the above configuration each each computer can potentially achieve its maximum data rate of 100 or 1000 Mbps.

Current is the flow of electrons forced into motion by voltage (electrical pressure). Current can only flow in closed loops or circuits, which are usually made from materials that are good conductors such as copper (Cu).

As discussed in the electrons ‘tech terms’, the electromagnetic force dictates that all like charges repel and all opposite charges attract. Following this law, when voltage is applied to a conductor and there is a path for the current, electrons will move from the negative terminal (which repels them) to the positive terminal (which attracts them).

Each electron moves only a relatively short distance to its neighbouring atoms’ orbit. But the movement occurs at near on the speed of light (300,000,000 metres per second), the process of the flowing electrons acts like a shunting effect.

Current can flow in different ways, Alternating current (AC) and voltages vary with time. By changing the polarity of the negative and positive terminals, so the negative becomes the positive and the positive becomes the negative, the current will flow one way and then reverse its flow to the opposite direction. The process is repeated at timed intervals.

Direct current (DC) always flows in the same direction and DC voltages always have the same polarity.

The amount of current in a circuit is measured in amperes (amps). An ampere is the amount of electrons that move past a certain point in the circuit every second.

To understand how computers store, perform arithmetic, and display the data on this page, you must have a basic understanding of the principles of Electron theory and Atoms.

All matter is comprised of molecules, which in turn are comprised of atoms. Some ancient Greeks believed that atoms were indivisible, hence the name ‘atom’ (Greek for ‘indivisible’). We now know that atoms are not indivisible. They are made up of a number of electrons orbiting a central nucleus made up of protons and neutrons.

Protons and electrons are electrical particles, protons are said to be positively charged and electrons negatively charged. They are both surrounded by an electrical force called the electromagnetic force.

Electrons

The force between two positive charges is repulsive, as is the force between two negative charges, but the force is attractive between a positive and a negative charge. This force keeps an electron in its orbit around the nucleus.

What has all this got to do with computers? Well I will try and explain the importance of electrons in modern technology.

Of the one hundred or so various atoms or elements, each has its own quantity of protons, neutrons and electrons. Some atoms have large amounts of electrons that orbit the nucleus in different layers. The various layers can hold only a maximum amount of electrons, for example, the inner most layer will only ever have a maximum of two electrons, the next layer can accommodate eight and a third layer is complete when eighteen electrons are whizzing around.

The ten inner most electrons are known as core electrons and the outermost are called valence electrons. These valence electrons are loosely bound to their parent nucleus and can be easily dislodged, allowing them to become ‘free’ electrons.

Silicon Atom

When atoms are bound together in elements such as silver (Ag) or gold (Au), the ‘free’ electrons can move from one atoms orbit into another. This movement of electrons causes a flow of electrically charged particles known as current or electricity.

Some elements consist of atoms that do not contain many ‘loose’ electrons and are not considered good conductors of electricity. Other elements such as copper (Cu) have a large amount of ‘loose’ electrons and as such are used in electrical wiring to carry the electric current.

The discovery of electric current gave way to the birth of electronics and modern technology as we know it. Devices such as computers rely on electric current to flow through circuits from one place to another and store the charge in various combinations that represent data.

Electrons can flow only when a circuit is complete. The diagram below displays a simple circuit (similar to how an electric torch would work). The chemical process within the battery causes a charge to be separated which provides a voltage, or electrical pressure, enabling electrons to flow. The negatively charged electrons are attracted to the positive charge in the battery. This provokes the electrons into traversing the copper wire, through the switch, through the bulbs filament and towards the positive charge which will complete the circuit.

Example Circuit

The circuits within computer chips use the same concepts as this very simple circuit. Although one main difference is that computers use electrical switches as opposed to a manual or mechanical switches (as seen in the above diagram). The development of the electrical switch, or transistor, led to the mass development of integrated circuits found in today’s microprocessors.

RAID is a method of storing data by using fault tolerant devices. There are many different RAID techniques, but essentially the concept is to store the same data on multiple hard disks resulting in the redundancy of information. RAID uses a technique known as striping, which involves partitioning each drive’s storage space into units or sectors.

There are at least nine types of RAID plus a non-redundant array (RAID 0):

• RAID 0 - Stripes data across multiple disks, no parity, no redundancy.
• RAID 1 - Disk mirroring, writes data to two identical partitions on separate hard disks creating a full backup. Separate controllers are used.
• RAID 2 - Writes data across multiple disks with error checking.
• RAID 3 - Stripes data one byte at a time and has a dedicated parity drive (for error checking).
• RAID 4 - Stripes data one sector at a time and has a dedicated parity drive (for error checking).
• RAID 5 - Stripes data and parity across multiple disks (at least 3). By mixing the parity across multiple disks a separate parity disk is not required and yet full data redundancy is achieved.

*Note with RAID 5 on an NT box the BOOT and SYSTEM partitions cannot be located on a RAID 5 disk array.

Object Orientated programming or OOP is a special way of developing computer programs. When programming languages were first developed they were procedural, that is the lines of code would be executed one after another until the program was completed. Many languages such as C and BASIC are still procedural based.

Object orientation takes a different view of the world. Object orientated programs try to model the world and break it down into small components. An example would be a PC, the PC has components such as a CPU, Motherboard, Graphics card, Mouse etc, each of which can be considered as an object. By connecting all these things we can create a complete computer.

OOP takes a similar approach. It will develop individual components that can be put together in order to build something new. Once a component has been developed, it may be reused in another application. In the same way that you can unplug a graphics card from one PC and plug it into another. This concept of reuse is central to the idea of OOP.

Streaming is the technique of transmitting data across the Internet in a constant flow or stream, as opposed to normal Internet traffic, which is transmitted under a request and acknowledgment basis. An analogy would be a hosepipe containing a constant stream of water, people wishing to share the water could each have their own pin-hole within the pipe.

The technique is well suited to Multimedia data being sent over the Internet, such as video and sound. With streaming video and sound, a Web user does not have to wait to download a large file before seeing the audio/visual file. Instead, the media begins to play as soon as each piece of data arrives.

The user usually requires a player, which is a special program that can uncompress and play the Multimedia data. A player can be either an integral part of a browser or a standalone application. Popular players include ‘Quicktime’ and ‘RealPlayer’.

Today, network designers are moving away from using bridges and hubs and are primarily using switches and routers to build networks. A switch is a device that will create an electronic circuit between a source and destination host.

The best way I can think to describe the function of a switch is to ask you to conjure the image of an old fashioned switchboard operator at a telephone exchange (you’re showing your age if you can!). In the early days of the telephone, the two handsets (caller and receiver) were connected by a dedicated cable (called a tieline).

When telephone networks were established, different telephone cables were connected via an exchange. A switchboard operator would physically connect the caller to the receiving party by plugging one line into a socket that connected to another line thus forming, for the duration of the call, a single dedicated line between the calling party and the called party.

A network with and without a Switch

Today, the switchboard operator has been replaced by a device called a switch. In computer networks (LAN and WAN) switches are used in a similar principle, forming virtual circuits between sending and receiving computers.

A topology (from the Greek topos: place) is a description of a particular locality in terms of its physical layout. In the context of computer networks, a topology describes pictorially the configuration or map of its layout. A topology influences a network’s cost and performance and each topology will dictate its own range of transmission medium and network types.

Example Star Topology

Examples of network topologies are:
Star - In a star topology all computers must be connected to a central hub or switch. The connected computers or devices can normally be attached or detached without causing detriment to any other devices on the network. If there are any defects with a device or its connection to the network it will have no effect on the other devices. However, if the central hub or switch becomes faulty, the whole network can be temporarily disabled.

Example Bus Topology

Bus - (from the Latin meaning route) is generally used for large general purpose networks although many newer configurations use the star or variations of it. All devices in a bus topology are connected to one main backbone medium such as a coaxial cable. The signal that traverses the bus normally takes a direct route along the medium to its destination, but the data it carries will have to be analysed by every connected device on the network in an effort to check whether the data is intended for that particular device. This type of configuration can be liable to collisions of data, resulting in slower network speeds.

Technically, bandwidth is the range of frequencies that can be passed over a transmission channel. Frequency is measured in hertz (Hz) or cycles per second. The larger this range, the greater amount of information can be sent through the channel.

An analogy would be a motorway with several lanes, each lane is the equivalent of a frequency band, the more lanes, the more traffic can flow past a certain point in a given amount of time.

Some people refer to bandwidth to express the rate at which data is being transmitted through a medium. In digital systems the rates are measured in bits per second (bps).