4.1 Networks

4.1 Networks

1. 4.1.1 Network Devices

   • Modems
     • Modulator Demodulator contains an ADC and DAC
     • Converts computer digital signals (modulates) into analogue signals for transmission over existing telephone line.
     • Converts analogue signals from a telephone line into digital signals (demodulates) to enable computer to process data.
     • Allows computers to connect to networks (internet) over long distances using existing telephone networks.
   • Network Hubs
     • Hardware device that can have number of devices/computers connected to it to make a LAN.
     • Main task: take any data recieved via one of the ports and broadcast it to every computer in the network.
       • Each computer/device will receive the data, whether relevant or not - hub not very secure/efficient method of data distribution
   • Switches
     • Similar to hubs, connects a number of computers together to make a LAN, but is more efficient in distributing data
       • Unlike hub, switch checks data packet received and works out its destination address and sends the data to the appropriate computers only. Switch is more secure at distributing data.
         • Each device or computer on a network has a media access control (MAC) address that uniquely identifies it. Data packets sent to switches will have a MAC address identifying the source of data and additional addresses identifying each device that should receive the data.
   • Bridges
     • Network device that links together two different parts of a LAN so that they can function as a single LAN.
   • Network interface cards
     • Any computer that is to be connected to a network, needs to have a network interface card (NIC).
       • Usually part of the device hardware and frequently contains the MAC address generated at the manufacturing stage.
   • Routers
     • A router is a network device that connects together two or more networks, so that data packets can be routed between different networks. A common use of a router is to join a home or business network (LAN) to the Internet (WAN).
       • The router will typically have the Internet cable plugged into it, as well as several cables connecting to devices on LAN.
         • Alternatively, the LAN connection might be wireless (WiFi), making the device a wireless router. (A wireless router is actually a router and wireless switch combined)
       • How a router routes data packets:
         • Routers inspect the data packages sent to it from any computer on any of the networks connected to it. Since every computer on the same network has the same part of an internet protocol (IP) address, the router is able to send the data package to the appropriate switch. The data will then be delivered using the mac destination address in the data packet. If this mac address doesn’t match any device on the network, it passes on to another switch on the same network until the device is found.
       • How a router stores computer addresses:
         • When a router receives a packet of data, it checks the destination IP address against the stored routing table. The routing table stores the MAC addresses of the device, the assigned IP address and the lease time the IP address is assigned for. The bits forming the destination IP address in the data packet are used to point to the correct route. The packet is send to a number of routers until it reaches its final destination.

2. A Network is a group of two or more devices that can communicate (share information and resources).

   • LAN (local area network) : A network confined to one building or site. Often it is a private network belonging to a business or organisation. Because LANs are small, they use cables or wireless for connections.
   • WAN (wide area network): A network which extends over a large geographical area. Often created by joining several LANs together. Use a variety of connection technologies: optical fibre cables, satellite radio links
   • WLAN (Wireless local area network): is a LAN that uses radio signals (wi-fi) to connect to computers instead of cables. At the centre of a WLAN is a router or wireless switch. More convenient than using cables, however difficult to make secure (must have robust password)

3. 4.1.3 WiFi and Bluetooth

   • Offer wireless communication between devices. Both use radio frequencies as carrier of data transmission.
     • WiFI
       • Wireless transmitter (WAP) receives information from a network via its connection. This transmitter converts the received information into radio waves and then transmits them.
         • Device receives radio waves via an installed wireless adaptor, which allows it to download the information from the data source.
           • WiFi is best suited to operating full-scale networks since it offers much faster data transfer rates, better range and better security than bluetooth.
             • A WiFi enabled device can access internet wirelessly at any access point (AP) or hotspot up to 100m away.
     • Bluetooth
       • Bluetooth sends and receives radio waves in a band of 79 different frequencies (known as channels) : these are all centred on a 2.45 GHz frequency.
         • Devices using bluetooth automatically detect and connect to each other, but they don't interfere with other devices since each communicating pair uses a different channel.
           • When a device wants to communicate it picks one of the 79 channels at random. If a channel is already being used, it randomly picks another channel. Spread-spectrum frequency hoping
             • To further minimise risks of interference with other devices, the communication pairs constantly change the channels they are using.
               • Bluetooth creates a secure wireless personal area network (WPAN) based on key encryption. Bluetooth used to transfer data between two or more devices that are very close, when speed of data transmission not critical, low bandwidth applications (sending music files from mobile phone to headset)
     • Transmission frequency used: Bluetooth 2.45GHz, WiFi 2.4, 3.6, 5.0 GHz.
     • Data transfer rate: Bluetooth 25Mbits/second, WiFi 250Mbits/seconds
     • Maximum effective range: Bluetooth 30m, WiFi 100m
     • Maximum number of devices connected: Bluetooth up to 7, Wifi depends on router used (can be 1 or many devices)
     • Type of data transmission security: Bluetooth Key matching encryption, Wifi WEP (Wireless equivalent privacy) and WPA (WiFi protected access)

4. Intranets and Internet

   • An intranet is a computer network based on internet technology that is designed to meet the internal needs for sharing information within a single organisation/company.
   • The internet is a worldwide collection of networks which allows a subscriber to send and receive emails, chat or browse the world wide web. The world wide web is the part of the internet which the user can access using a web browser, which is a software that allows the user to display and interact with pages and files from the web.

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