[<< wikibooks] A-level Computing 2009/AQA/Problem Solving, Programming, Operating Systems, Databases and Networking/Communication and Networking/Networks
```== Local Area Networks ==
A local area network is a group or network of computers linked physically by cables. you should not think about the physical distance between the computers, more about the location of other computers within the network.

== Wide Area Networks ==
A set of links that connect geographically remote computers and LANs.

== Network adapter or Network Interface Card ==
A computer communicates on the network through a network adapter. A network adapter plugs into the motherboard of a computer and into a network cable. They convert data from the form stored in the computer to the form transmitted or received on the cable. Data is passed through electronics that calculate a checksum value, its own address and destination address. It is now known as a frame. The frame is transmitted a bit at a time onto the network cable, with the address sent first, then data and lastly, the checksum. Each network adapter card has a unique 48 bit hexadecimal MAC address.

== Topology ==
The structure of the connections that connect devices to a network

=== Bus ===

=== Star ===

== Bus Networks ==

A bus network relies on each computer being connected to one wire, the bus.
When a computer on the network wishes to transmit data, it is possible to do so immediately if no other data is detected on the bus.
If there is already data being transmitted on the wire then the computer has to retry later.
If two or more computers transmit at exactly the same time a collision occurs.
If a collision occurs the computer that was trying to send the data detects that the sending has failed and proceeds to try again at a different time.
At times when the network is in heavy use, lots of computers will be transmitting data therefore leading to a number of collisions. This causes the overall performance of the network to deteriorate.

=== Carrier Sense Multiple Access with Collision Detection (CSMA/CD) ===
To prevent collisions on a bus network, a set of rules can be used. An example of one of these sets of rules is CSMA/CD:

If the bus is quiet then transmit
If the bus is busy, listen until it is quiet, then transmit immediately
While transmitting monitor for a collision
If there is a collision, send a jamming signal to let the other computers on the network know there has been a collision, and stop transmitting
Wait a random amount of time before starting from step 1 again

== Star networks ==

A star network will have a switch at its centre.
All data transmitted from the individual computers on the network will go through the switch then sent on to their destination, therefore removing any congestion.You should be able to compare local area networking with standalone operation.

== Network Segment ==
A segment refers to a string of computers.
Segmentation refers to non switched Ethernet bus networks that are split to improve performance.

== Peer-to-peer networking ==
This a network system where each workstation has equivalent capabilities.
Peer-to-Peer LANs
Good for less than 10 users where users are located in a close proximity and security isn't an issue. However provides a limited growth for future network extensions.
Peer-to-Peer WANs
used to share files among a large number of users connected temporarily. E.g.: BitTorrent

== Server-based networking ==
Where resources, security and administration on a network is carried out by dedicated servers.

== Thin client computing ==
Thin-client computing is when all of a network's procession comes from a central server.  The thin-clients are computers used to connect to one central server to be able to form a network.  Some star formatted systems use thin-client networks.  Thin-client networks do not require the client computers to have any hard disk storage and they often have very little processing power. They are often called dumb terminals because of this. A thin-client network would be used on a small scale e.g. in a small office because a thin-client network has very little bandwidth. The central server is the application server and can run applications like Word, multimedia and send these to different users terminals.  Also, the central server can control user's login to see if the terminals are connected to the server.

== Web services ==
Explain these terms and describe situations where these might be used.
Compare and contrast thin client computing (software as a service, AJAX, Web 2.0, etc.) vs rich client computing (client-server, peer-to-peer), web services as examples of â€˜systems architecturesâ€™.

=== Inter-networking ===
Internetworking is formed when computers from one network communicates with computers from other networks.
These networks communicate via gateways.
An example of internetworking is the Internet, which has now become important and widely used in everyday life.

=== Routers ===
Routers and Bridges allow communication between computers on different segments.
A Router is a device that receives packets from a router or a host and uses the destination IP address the packets contain to pass them to another router or host. Routers are commonly connected to a network of other routers.
An example of how routers are used:
If a host of a network (network A) in one country(country A) wants to communicate with a host on a network(network B) of another country (country B), it will have to send packets to the gateway router on network A; this will pass the packets to the

local router -> regional router -> national router -> international router

until the network of country B is reached.

=== Bridges ===
A bridge holds a table of MAC addresses for each connected machine. Data packets received from one segment are only forwarded if they have a valid destination MAC address on the other segment. This helps to reduce overall traffic. Note that bridges are similar to switches but have only two connections.

=== Switches ===
Like a Bridge a Switch has a table of MAC addresses for each connected machine. Data packets received from one connection are only forwarded if they have a valid destination MAC address on the other connection. Unlike Bridges, Switches can have many connections.

=== Hubs ===