There are 4 basic topologies with variations of networksBus TopologyBus consists of a single linear cable called a trunk. Data is sent to all computers on the trunk. Each computer examines EVERY packet on the wire to determine who the packet is for and accepts only messages addressed to them. Bus is a passive topology. Performance degrades as more computers are added to the bus. Signal bounce is eliminated by a terminator at each end of the bus. Barrel connectors can be used to lengthen cable. Repeaters can be used to regenerate signals. Usually uses Thinnet or Thicknet.

both of these require 50 ohm terminator good for a temporary, small (fewer than 10 people) network But its difficult to isolate malfunctions and if the backbone goes down, the entire network goes down. Star TopologyComputers are connected by cable segments to a centralized hub. Signal travels through the hub to all other computers. Requires more cable. If hub goes down, entire network goes down. If a computer goes down, the network functions normally.most scalable and reconfigurable of all topologies.

Ring Topology

Computers are connected on a single circle of cable. usually seen in a Token Ring or FDDI (fiber optic) network Each computer acts as a repeater and keeps the signal strong => no need for repeaters on a ring topology No termination required => because its a ring Token passing is used in Token Ring networks. The token is passed from one computer to the next, only the computer with the token can transmit. The receiving computer strips the data from the token and sends the token back to the sending computer with an acknowledgment. After verification, the token is regenerated. relatively easy to install, requiring ;minimal hardware.

Mesh

The mesh topology connects each computer on the network to the others Meshes use a significantly larger amount of network cabling than do the other network topologies, which makes it more expensive. The mesh topology is highly fault tolerant.Every computer has multiple possible connection paths to the other com-puters on the network, so a single cable break will not stop network communications between any two computers.

Star Bus Topology

Several star topologies linked with a linear bus. No single computer can take the whole network down. If a single hub fails, only the computers and hubs connected to that hub are affected. Star Ring TopologyAlso known as star wired ring because the hub itself is wired as a ring. This means it's a physical star, but a logical ring. This topology is popular for Token Ring networks because it is easier to implement than a physical ring, but it still provides the token passing capabilities of a physical ring inside the hub. Just like in the ring topology, computers are given equal access to the network media through the passing of the token. A single computer failure cannot stop the entire network, but if the hub fails, the ring that the hub controls also fails.

Hybrid

Mesh most important aspect is that a mesh is fault tolerant a true mesh is expensive because of all the wire needed another option is to mesh only the servers that contain information that everyone has to get to. This way the servers (not all the workstations) have fault tolerance at the cabling level.

Does your business take green IT seriously? Well it should, because ignoring climate change could cost you money and harm your credibility.There are now over 1.1 billion computers in operation worldwide, collectively producing about one billion tonnes of CO2 through their electricity requirements. E-waste is serious headache too with computers, mobile phones and electronic gadgets now accounting for 5% of the world's garbage (www.unep.org).But surely small IT companies don't need to be worrying about that kind of thing? Actually, they do - there are solid reasons why all businesses should be going greener, and not just because of the do-gooder's warm fuzzy feelings to be had! Over 70% of PCs will not be recycled when disposed of over the next five years, and globally we will dispose of 512 million. We are now legally obliged to dispose of PC equipment properly, and that normally incurs charges. Thankfully there is a free and simple, if underused, alternative; lots of people in the world are in dire need of our "outdated" computers, so donate them to the likes of www.computeraid.org.

For safety, I would suggest you scrub your drives first (try dban.sourceforge.net). More regulation info at www.netregs.gov.uk.Perhaps more importantly, consumers and businesses are increasingly taking note of how the products and services they use impact the environment. Memset (www.memset.com) became the UK's first "carbon neutral" Web host last Summer and that has definitely helped us win more business, which is also probably why so many other dedicated server hosts are following suit.Carbon offsetting. Things like travel, electricity usage and product manufacture have a "carbon footprint"; the quantity of greenhouse gases directly or indirectly produced as a result of those activities. Most activities are impossible to make 100% green, but you can offset the effective carbon impact by investing in carbon sequestration projects (eg. planting trees) or in greener power generation facilities (eg. wind farms), thus becoming "carbon neutral".

Organisations like the CarbonNeutral Company (www.carbonneutral.com) and the Carbon Trust (www.carbontrust.co.uk) can guide you through offsetting, and it is neither expensive nor difficult. For example, an average small office with 10 staff might have equivalent emissions of 20 tonnes CO2/year, which would probably only cost around £200/year to offset.As well as giving you more credibility in today's increasingly enviro-friendly world, taking a few hours to review your carbon footprint can lead to some worthwhile cost savings as well. A simple example is turning off (or hibernating if, like me, you hate rebooting and getting back to where you were each morning) your PC at night.

A recent study by Fujitsu estimated that the UK alone wastes £123m on electricity powering PCs left on out-of-hours. See www.energysavingtrust.org.uk for general energy-efficiency tips.Virtually greenerCarbon offsetting is all good stuff, but when it comes to IT power consumption, prevention is better than cure. Demand for high-availability, centralised server resource is growing relentlessly, and high-density computing uses a lot of energy. Even a base-spec 1U rackmount server will burn 100-200 Watts continuously, and once you fill a few racks and add in cooling requirements you are looking at a whopping electricity bill! With energy prices doubling every few years datacentres' power consumption is fast becoming a major issue for IT business, and is now the main cost underlying server hosting.

The oddity from our perspective is that the vast bulk of servers in our datacentres idle most of the time, with perhaps 90% never getting close to full capacity. While many applications are best hosted on their own dedicated server (better security, for example), few need the full resource of a modern multi-core, gigahertz machine. That is where virtualisation comes in; the latest generation of virtual machines, using the hardware-assisted virtualisation in new AMD and Intel's chips (eg. www.vmware.com, www.miniserver.com), are operationally indistinguishable from a physical server but use 5-20% of the electricity.

The reduced power and hardware costs give you significant savings while not costing anything you actually needed in the first place, and at the same time you are tackling climate change.It can be argued that all such measures are a dribble in the ocean, and some have serious doubts about the efficacy of our whole approach to climate change. However, being more climate-friendly is not difficult for most IT businesses and almost certainly helps, so can you really afford to ignore the risks, or miss out on the benefits of going greener?

Article written by Kate Craig Wood