by Julio Johansen

Created on: February 03, 2007   Last Updated: April 19, 2007

In the most general terms, a network provides a method for computers to send and receive data. The network consists of hardware to handle data and direct traffic and a medium to carry the data from computer to computer. Historically, this medium has been copper-wire network cables but, more recently, fiber optic and radio links have become popular.

The networks that are most widely known are the public Internet and the wired ethernetworks that comprise the Local Area Networks (LANs) in many homes and businesses. For wired ethernet, each computer on the network has to have a Network Interface Card (NIC) installed. The term NIC has persisted long after actual ethernet cards vanished and ports were built directly into newer computers.

The NIC allows you to plug in a network cable and provides an interface to the physical part of the network. All of the computers on the local network will be attached, by cables, to at least one hub or switch which facilitate communication between computers.

Both hubs and switches transmit data from one computer to another. However, there is a subtle difference in how the two direct traffic. When a hub receives data on one port, it repeats the data on all its other ports. A switch only repeats data on the port of the destination computer, which conserves bandwidth. Switches have largely replaced hubs and they permit higher rates of data transfer and more network traffic.

Wireless ethernet has become almost ubiquitous in recent years. It operates similarly to wired ethernet but with wireless links replacing physical network cables.

This versatility, having the choice of wired or wireless ethernet, arises from the layered logical structure that computer networking is built on. There are seven layers for a network, defined in the OSI (Open Systems Interconnection) model. These are, from highest to lowest level:
7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Data Link
1. Physical

The first five of these (Physical through Session) are what people usually mean when they talk about a computer network. I'll work my way up, from the Physical layer to the Session layer and briefly describe what goes on at every level.

1. Physical Layer.

The physical layer defines what the network is made of and what kinds of electronics run it. For ethernetworks, this includes the type of cables that are used and the voltages and bitrates used by devices on the network. For wireless networks, it describes the radio channels and broadcast power used

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