14 - Communication and Internet Technologies CS

• The need for protocols: It sets a standard for communication. Both parties need to agree the protocol being used to ensure successful communication takes place.
• TCIP Protocols: Application layer, Transport layer, Internet layer, Data link layer
• Application layer - It sends files to the transport layer
• Transport layer - data is broken up into packets which are then sent to the internet layer. Ensures the packets arrive in sequence without errors.
• Internet layer: Identifies the intended network and host
• Data link layer: identifies and moves traffic across local segments
• Messages are split up into smaller groups of bits called packets
• A router is used to transmit packets of data. When packets arrive at a router, it decided where next to send them.
• Hypertext transfer protocol: defines the format of the messages sent and received.
• File transfer protocol: used when transferring files from one device to another via the internet or other networks
• Anonymous ftp: allows a user to access files without the need to identify who they are to the ftp server.
• ftp commands: a user is able to carry out actions that can change files stored on the ftp server.
• ftp server: the files that are downloaded are stored here
• SMTP: text-based protocol used when sending emails. Sometimes referred to as a push protocol.
• POP3/4 and IMAP: Are protocols used when receiving emails from the email server. These are known as pull protocols.
• Ethernet protocols: Ethernet connects a number of devices together to form a LAN. It uses protocols to control the movement of frames between the devices and to avoid simultaneous transmission by two or more devices.
• Peer to peer file sharing/BitTorrent protocol: allows for very fast sharing of files between computers.
• Swarm - a group of peers connected together is known as a swarm
• seed - a peer that has downloaded a file and has then made it available to other peers in the swarm
• tracker - the central server that stores details about other computers such as their downloads and uploads
• leech - a peer that has a negative impact on the swarm by having a poor share ratio. They download more than they upload.
• Circuit switching: A method of data transfer in which the message is sent over a dedicated communication channel.
• Packet switching: A method of data transfer in which the intended message is broken down into parts and is sent over whichever route is optimum in order to reach its destination.
• Pros of circuit switching

  • Dedicated circuit used for single transmission only
  • Whole bandwidth available
  • Faster data transfer rate than packet switching
  • Packets arrive in same order as sent
• Cons of circuit switching

  • Not very flexible
  • Dedicated channels require greater bandwidth
  • Nobody else can use the circuit/channel even when idle
  • No alternative routing if there is a fault on the dedicated line
• Pros of packet switching

  • High data transmission possible
  • Can overcome faulty lines by re-routing packets
• Cons of packet switching

  • Protocols are more complex than circuit switching
  • If a packet is lost, the sender needs to resend it (time consuming)
  • Needs large amount of RAM to handle the large amount of data
  • Does not work well with real time data streams
• Routing tables - Contains the information necessary to forward a packet along the shortest route to allow it to reach its destination. As soon as the packet reaches a router, the packet header is examined and compared with the routing table
• Outline what is meant by the term collision:

  A collision is when 2 values in the key field result in the same hash value. So the location identified may already be in use.
• Explain how a collision can be dealt with when writing records to a random file:

  A process of collision resolution is used. Search the overflow area and go through the spaces in a linear fashion. Then store the data item in the first available slot.