AQA Alevel UNIT 4.9 Networking

AQA Alevel UNIT 4.9 Networking

1. Communication

   • TRANSMISSIONS
     • Parallel Transmission
       • Advantages
         • Faster, as more bits are transferred per cycle
       • Disadvantages
         • Less reliable over long distances
         • More expensive – more wiring needed
       • Description
         • Sending one bit at a time through different channels
     • Serial Transmissions
       • Advantages
         • Cheaper to produce hardware
         • Easy set up
         • Works well over longer distances
         • Wires less bulky
       • Disadvantages
         • Only one bit per cycle
       • Description
         • Sending one bit one after another through the same channel.
     • Synchronous Transmission
       • data is transmitted where the pulse of the clock of the sending and receiving device are in time with each other. The devices may share a common clock.
       • Synchronous means that the two devices which are communicating will synchronise their transmission signals. Using the system clock, the computer sending the data will control the transmission rate to be in time with the device or computer receiving the signal. If the two devices are not synchronised then data could be lost during transmission. Once they are synchronised the two devices can send and receive data without need for any further information.
     • Asynchronous Transmission
       • data is transmitted between two devices that do not share a common clock signal.
       • Asynchronous transmission does not require the permanent synchronisation of the sender's and receiver's system clocks. Instead, it synchronises only for the duration of the transmission by sending additional bits of information called start and stop bits.
     • Latency
       • Latency is the delay between the start of the transmission, and the start of the effect of transmission.
     • Protocols
     • Rates
       • Bit
         • Bit rate is one of the main measures of digital transmission. It is simply the number of bits transferred in a specified time.
       • Baud
         • Baud rate refers to the mechanical properties of the analogue wires used for digital transmission.
         • It is the number of signal changes per second that the wire can handle without the signal being garbled.
         • If only one bit is transmitted per signal change, then bitrate and baud rate are the same
       • Bandwidth
         • Bandwidth is the potential capacity of a transmission channel – it is the maximum theoretical bitrate

2. Networks

   • Wireless
     • A wireless network is still a collection of connected computers which can communicate, but instead of using electrical signals in wires, it uses pulses of electromagnetic energy to communicate.
     • WIFI
       • Each device that wishes to connect needs a Wireless Adaptor to transmit and receive signals. The signals sent follow international standards, so you know that your wireless adaptor will be able to communicate with any wireless access point worldwide.
       • 2.4 GHz band
         • The 2.4GHz band is the most widely used. There is enough space to carry 3 non-overlapping channels
       • 5GHz band
         • The 5GHz band can fit 23 non-overlapping channels reducing collisions. It has a shorter range than 2.4GHz, and is more costly to implement. Some home routers and equipment can use 5Ghz, some can’t
     • From a physical setup point of view, a wireless network is like a star network
     • Collisions
       • Only one device can be broadcasting at any one time or the signal is corrupted
       • A network’s signal is modulated at a specific frequency, which then doesn’t interfere with other frequencies.
       • CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance) is the technique used to allow devices on the same band to communicate without collisions. It is a very simple protocol, but you DO need to be able to describe it in an exam.
       • CSMA/CD
         • The device checks the channel to see if it is currently in use. If it is, it waits until it is free Once the channel is free, it broadcasts a packet of data and waits for a received signal
       • RTS/CTS (Request To Send/Clear To Send) is an extension to CSMA/CD When the channel is detected as clear, a request to send signal is sent. If a clear to send signal is received, then the transmission starts. This reduces the collisions within the network, although collisions from outside networks will still happen
       • Remaining Issues
         • We can now avoid interference between wireless networks, and void collisions within the network and with other devices.
       • SSID
         • SSID (Service Set Identifier)
         • The SSID is the network name, and it is attached to each packet of data sent
         • If the SSID on the transmitted packet matches the SSID of the Wireless Access Point, then it responds, otherwise it disregards it.
       • Hidden SSID
         • The Wireless Access Point can be configured not to broadcast its SSID, so devices will not know that it exists.
         • This is a VERY weak security feature and can be circumvented with little effort.
         • As packets sent to and from the network by authorised devices contain the SSID, they can be easily read by devices and the SSID discovered.
       • MAC filtering
         • The MAC address is the physical id of the Wireless Receiver, and is included in packets as they are transmitted.
         • Wireless Access Points can be configured to only accept packets from an approved list of MAC addresses, blocking unauthorised devices.
         • This is also a weak security feature as even though MAC address is unique to hardware, the software that adds it to the packet can be altered and custom MACs can be sent instead.
       • WPA
         • WPA (Wireless Protected Access) is a an encryption standard used by wireless networks
         • Each packet is encrypted using a key (a sequence of at least 6 alphanumerical characters).
         • The same sequence is required to decrypt the packet, so unless the network device has the key, it can’t send or receive any data.
         • WPA is the only reliable security measure for Wireless networks.
     • Wireless networks operate on two bands of unlicensed frequencies: 2400MHZ – 2500MHz (referred to as the 2.4GHz band) 5725MHz – 5875MHz (referred to as the 5Ghz band)

3. The Internet

   • Inter connected network
   • Structure of the Internet
     • The Internet can be visualised as millions of star networks connected together. There is not just one connection between each network
     • If you view a website, your computer is requesting an HTML page from a server. This server could be physically located in, for example, California.
   • Packet Switching
     • In order to keep data moving and maintain acceptable transfer speeds, a system called packet switching is used to transfer data.
     • Data is split up into small chunks called packets. Each packet is sent independently – each packet could take a different route, depending on what is fastest at the time. Various algorithms are used to attempt to find the quickest route. The packets are re-assembled at the other end.
     • Packets (3 parts)
   • Routers
     • Routers direct the packets to the appropriate destination. When a router receives a packet, it looks at the header and reads the recipient address. If the address is for a computer located on its network, it delivers it. If not, it passes it on to another router.
   • Gateways
     • They are used to connect 2 different types of network together. Gateways strip the header and footer information and re-build it in the format expected by the new network before passing it on.
   • IP Address
     • Pv4 uses 32 bit addressing, so there are only around 4 billion unique addresses. This is not enough for all devices on the internet to have their own IP address. Gateways are used to translate internal IP addresses to external IP addresses.
     • Internal IP addresses
       • A gateway sends packets from internal computers to the Internet, and changes the address to an external IP address (a unique one), and changes it back again when a reply arrives.
     • External IP addresses
       • Computers on the internet such as webservers have their own external IP address
   • Domain Names
     • A domain name (BBC.co.uk, Google.com) is simply a memorable identifier for a computer on the Internet. Domain Name System (DNS) servers are used to convert a domain name to its IP address.
     • Internet Registries
       • Since IP addresses are limited, Internet registries exist to allocate them Countries are allocated IP addresses in specific ranges by the global registry Registries within each country then allocate the IP addresses to businesses and ISPs.

4. Internet Security

   • Vulnerability
     • Malware (Malicious software) is software designed to harm systems or steal information.
       • Virus
         • malware that attempts to copy itself to other systems
       • Trojan
         • malware that appears to be something else
       • Worms
         • a type of virus that exploits vulnerabilities in networks to spread
     • In a perfect world, there would be no way for viruses to spread, but in reality there are systems they can exploit:
       • Human error
         • Sending an email, enticing the recipient to open an attachment. Copying to memory sticks and waiting for the user to run it on another machine. Creating a website that pretends to offer legitimate software.
       • Mistakes in code
         • Modern software in incredibly complex. All software has mistakes or design faults that can be manipulated. Hackers analyse software to find these mistakes, then write software that can manipulate them. Example: SQL Injection Attack
       • Poor practices
         • By keeping your operating system updated, and having current anti-virus and firewall software installed, the vast majority of threats are eliminated.
   • AntiVirus
     • Contains a database of known viruses and “signatures” to look for in code Scans files and running processes for these signatures Can then delete or quarantine any suspicious files Useless if not updated.
   • Firewall
     • Monitors network traffic (at the link layer) Blocks traffic to unrecognised ports, or banned IP addresses. Does not identify or delete viruses, but can prevent them from communicating.
   • Encryption
     • Encryption is making data unreadable to all but the intended recipient. There are two types: Private Key (symmetric) Public Key (asymmetric)
     • Private Key
       • Private Key encryption uses the same key to encrypt and decrypt. Example: Caesar Cypher The key must be shared in advance of communication.
     • Public Key
       • A different key is used for encryption and decryption. The decryption key can not be derived from the encryption key. The encryption key can not decrypt, and the decryption key can not encrypt. Only the encryption key is shared.
       • Computer A and Computer B want to communicate securely. A sends its public key to B B sends its public Key to A A uses B’s public key to encrypt messages and sends them B then decrypts them using its private key.
         • If the public keys are intercepted it doesn't matter as neither can be used to decrypt
   • Certificates & digital signatures
     • Certificates and signatures are used to prove that a file is what it says it is. A signature is a hash of the file being downloaded, encrypted with a private key. A certificate is a message attached to a website which has been encrypted with a private key. To check a website or file is valid, the certificate is downloaded and decrypted with the public key If the certificate contains the correct data, it is valid. If the file has been tampered with, the new file can’t be accurately re-encrypted without the private key.

Images and files in the map