# 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

### 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 ‘systems architectures’.

### 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
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

Hubs broadcast any packets received to all other connections.

### Gateways

A device used to connect networks using different protocols so that information can be passed from one system to another. Gateways are used to connect LANs through to WANS like the Internet; in order to do this, they must translate the LANs frame into equivalent WAN frame and vice versa, this is because LANs use a protocol that is different from the protocols used on WANs like the Internet.
Define these and consider where and why they are used. In particular, consider how routing is achieved across the Internet and how local area networks are connected to the Internet via gateways.

### Subnetting

Subnetting is used to divide a network into smaller logical networks, e.g a LAN within a WAN. By dividing the address the responsibility for maintaining the network can be split between the WAN operator and the LAN operator. A subnet mask is used to identify which bits in an IP address specify its subnet, and hence which addresses it can send packets directly to, not via a router. 255.255.255.0 is the network mask for the segment 192.168.1