2.WiredAndWirelessNewtorks

L5 Protocols and layers
1. Protocols
  1. HTTP
    1. HTTP (hypertext transfer protocol) is used for accessing and receiving web pages in the form of HTML files on the Internet
  2. HTTPS
    1. HTTPS (secure protocol) encrypts the information so that it cannot be understood if it is hacked
  3. FTP
    1. Used for sending or retrieving files to or from a remote server or computer Works just like Windows’ File Explorer
  4. POP
    2. POP will download entire messages to your local device and delete them from the server
  5. IMAP
    1. These email retrieval protocols fetch message data and attachments from your remote mail server
    2. IMAP will only download the message header until the message is opened and will leave the message on the server This means other devices checking the same mailbox will also see the messages
  6. SMTP
    1. Email protocol used for transferring email between remote email servers Client email software sends message to outgoing mail server Mail server contacts destination mail server to pass on email using SMTP
  7. TCP/IP
    1. TCP (Transmission Control Protocol)
      1. Resends lost messages
      2. Detects errors
      3. Reassembles the packets at the other end
      4. Breaks up messages sent over the Internet into small chunks called packets
    2. IP protocol
      1. Routes the individual packets
    3. The TCP/IP protocol stack sending
      1. Application layer
        The network applications you are using operate on the Application layer The application selects the correct protocol to use depending on which application it is and what you are trying to do
      2. Transport layer
        The Transport layer creates the connection between two computers, or ‘hosts’ The two computers agree the communication settings and the size of the packets they will send and receive
        Data is then divided up into packets and numbered e.g. 1 of 6 Packets are reassembled by the recipient’s Transport layer Lost packets are resent
      3. Network layer
        Routers operate on the Network layer Destination addresses are written on to the packets ready for transmission
      4. Link layer
        The Link layer is the physical hardware that connects the two hosts such as the NIC and the cabling
      5. Layers are self-contained
    4. The TCP/IP protocol stack receiving
      1. The link layer removes the MAC address and passes packets up to the Network layer The Network layer removes the IP addresses and passes packets up to the Transport layer The Transport layer reassembles the packets and passes the data to the Application layer The Application layer uses the correct protocol to correctly display the data, web page or email for the user
  8. Email
    1. How email works
      1. Mail servers pass on or store emails until they are collected You must log in to a mail server to collect mail Examples of mail providers are Gmail or Yahoo
L1 The Internet
1. Internet Vs World Wide Web
  1. World Wide Web
    1. The World Wide Web, or simply Web, is a way of accessing information over the medium of the Internet. It is an information-sharing model that is built on top of the Internet. The Web uses the HTTP protocol, only one of the languages spoken over the Internet, to transmit data
  2. The Internet
    1. The Internet is a massive network of networks, a networking infrastructure. It connects millions of computers together globally
    2. Information that travels over the Internet does so via a variety of languages known as protocols.
2. IP address and MAC address
  1. IP address
    1. belongs to the router
    2. can change
    3. geographical location can be determined by its IP address
  2. Media Access Control (MAC) address
    1. belongs to the Network Interface Card (NIC)
    2. may have more than one
    3. Format 3B:14:E6:39:0A:2C
    4. never changes
3. Data packets
  1. Each packet is given a header containing
    1. The IP (Internet Protocol) address it is going to
    2. The IP address it has come from
    3. The sequence number of the packet
    4. The number of packets in the whole communication
    5. Error checking data
  2. Each data packet will contain the same number of bytes
4. Domain Name System (DNS)
  1. DNS technology allows you to type a name such as google.co.uk into your web browser It translates the web address google.co.uk into an IP address e.g. www.google.co.uk = 74.125.131.94
  2. When a DNS server receives a request not in its database, it will pass the request on to another server until it reaches one with the matching name and IP address
L2 Local Area Networks
1. Network topologies
  1. Star
    1. Advantages
      1. Fast data transfer as there are fewer data collisions
      2. If one cable fails the other workstations are not affected
    2. Disadvantages
      1. Requires additional hardware such as the central switch
      2. If the central device fails the whole network goes down
  2. Mesh
    1. Partial Mesh network
    2. Full Mesh network
    3. Wireless mesh networks
      1. The more nodes that use the network, the more the signal can travel
      2. Only one node needs a wired Internet connection – no other cabling or infrastructure is required
    4. Nodes act as routers for data in order to relay and propagate data in the network
    5. Partial mesh network model usually used, often in conjunction with star topologies to create larger networks
    6. Advantages
      1. No single point of failure – It is a self-healing network
      2. Expansion and modification can be done without disrupting the network
      3. Data can be transmitted from different devices simultaneously
    7. Disadvantages
      1. Can involve redundant connections
      2. Expensive to install cabling if using wired connections
      3. Network maintenance and administration is difficult
  3. There are many
2. Ethernet protocols
  1. Ethernet’ refers to a family of standard local networking ‘protocols’ or rules
  2. Two nodes attempting to transmit simultaneously will stop and each wait a random period before reattempting
  3. Two nodes attempting to transmit simultaneously will stop and each wait a random period before reattempting
  4. Ethernet transmission
    1. Ethernet systems divide data into frames, similar to Internet packets Each frame contains source and destination addresses and error checking data Frames are broadcast to all nodes – only the intended recipient will open the frame. Others will be discarded Frames which are detected to contain transmission errors are dropped or resent
3. Routers, hubs and switches
  1. Routers
    1. Sends data packets on their way in the best direction
  2. hubs
    1. Central, multi-plug adaptor for computers and printers in a network
    2. When a packet of data is received, it broadcasts the packet to all devices on the network
  3. switches
    1. Smart multi-plug adaptor only sends packets to the intended recipient, using its MAC address
    2. Reduces network traffic and increases speed
L4 Client-server and peer-to-peer networks
1. client-server structure
  1. client
    1. Role of the client
      1. The client sends requests to the server
      2. Waits for a reply
      3. Receives the reply
  2. Server
    1. A print server
      1. may organise printing on different printers
    2. A file server
      1. holds all the data files and databases and manages backups
    3. An email server
      1. may receive emails, detect and block spam, distribute emails to users
    4. A web server
      1. may host an external website
    5. Role of the server
      1. The server waits for requests from a client
      2. Acknowledges the request
      3. The server may need to pass the request (e.g. for a web page) to another server – in which case, it becomes the “client”
      4. The requested data is sent back to the client
  3. Client-server networking
    1. A central server is used to manage security
    2. Some files are held on the central server
    3. Some processing tasks are performed by the server
    4. Clients issue requests to the server for services such as email, file storage, backup and printing
    5. Suitable for many different types of organisation, small and large
    6. Can require specialist IT staff to administer the network
  4. In a school network, there may be dozens or even hundreds of clients and several servers
2. Peer-to-peer (P2P) structure
  1. Peer-to-peer networking
    1. Suitable for a small companies with fewer computers
    2. No central server controls files or security
    3. All computers can easily see files on all other computers
    4. All computers can communicate with each other without going through a server
    5. If a computer is switched off, data cannot be retrieved
3. Peer-to-peer on a WAN
  1. The P2P configuration can also be used for file-sharing websites
  2. This means that with thousands of people downloading, data can be passed between computers rather than just from the server
L3 Wireless networking
1. Wireless access point
  1. A wireless Access Point (AP) is a device that allows wireless devices to connect to a wired network using Wi-Fi
  2. The AP usually connects to a router (via a wired network) as a standalone device, but it can also be an integral component of the router itself
2. Wi-Fi Frequencies and channels
  1. The 802.11 standard uses five frequencies: 2.4GHz, 3.6 GHz, 4.9 GHz, 5 GHz, and 5.9 GHz
  2. Each frequency is subdivided into various channels: Each channel has a central frequency and bandwidth
  3. 802.11 standards
    1. 802.11b/g/n uses the 2.4GHz frequency 802.11a/n/ac uses the 5GHz frequency
    2. 2.4GHz
      1. Advantages
        Greater range and coverage
      2. Disadvantages
        More interference from other devices, and only three non-overlapping channels
    3. 5GHz
      1. Advantages
        Less crowded space with 23 non-overlapping channels with higher data transmission rates
      2. Disadvantages
        Fewer devices can use the 5GHz frequency, less able to penetrate through walls
3. Encryption
  1. Encryption terminology
    1. Plaintext: the original message to be encrypted
    2. Ciphertext: the encrypted message
    3. Encryption: the process of converting plaintext into ciphertext
    4. Key: a sequence of numbers used to encrypt or decrypt, often data using a mathematical formula
    5. Encryption algorithm : the formula for encrypting the plaintext
  2. Encryption techniques
    1. Private key (Symmetric encryption)
      1. This is more secure as it means that you never have to send or reveal your decryption key
      2. Two keys are used - one to encrypt and the other to decrypt data
    2. Public key (Asymmetric encryption)
      1. A single key is used to encrypt and decrypt a message and must be given to the recipient of your message to decrypt the data
  3. Caesar shift cipher
    1. A to B to C
  4. Cryptanalysis
    1. Brute-force attack
      1. Every possible key is tried On average, half of all possible keys need to be tried, so the longer the key, the more time it takes to find it
    2. Non-brute-force attack (cryptanalytic attack)
      1. eg: One important clue was that some messages always started with the words “Heil Hitler”
      2. Frequency analysis
  5. Modern ciphers
    1. using two very large prime numbers multiplied together
  6. Algorithmic security
By Samuel Houghton

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