Communications

• Stand alone computer


  A computer that is not connected to a network
• Network

  a collection of computers that are connected together
• Computer Network Advantages

  users can share files and resources with others

  
  

  users access their files and resources on other PC on same net

  
  

  servers created to centrally control a number of operations

  
  

  communications can be sent to any computer on net

  
  

  peripherals can be shared
• Computer network Disadvantages

  Networks can be subject to security issues

  
  

  malware and viruses can be spread from PC to PC

  
  

  hard to remove viruses & malware

  
  

  if an element fails it could cause failure or issues

  
  

  high amount of network traffic can cause slowness
• LAN


  local area network covers a small geographical area. they are normally contained within a building or small site. can be found in homes schools and small businesses
• characteristic of LAN

  connection method - cables , fibre optic cables . radio waves

  
  

  ownership - owned by the individual or organisation

  
  

  transfer rate - high

  
  

  transmission errors - fewer errors

  
  

  security - fairly high
• WAN


  wide area network covers a large geographical area. WAN could cover a city , several branches of an organisation , a country or even several countries. a WAN is often a collection of connected LANs
• Characteristics of WAN


  connection method - cables , fibre optic cables . radio waves

  ownership - multiple people

  transfer rate - normally restricted

  transmission errors - greater chance

  security - can be difficult to manage due to the use of publicly shared cables
• PAN


  personal area network is limited to a single person. can be used for file transferral between devices , sharing internet between devices and printing
• Characteristics of PAN


  connection - wireless or wired depends on hardware

  ownership - owner is user of the network

  transfer rate - depends

  transmission errors - less likely

  security - high
• Ring network topology


  each computer is connected to two others loops backs to form a circle / ring .

  if computer fails the data can be rerouted and the network can still function. however expansion is impossible without bringing the whole network down.

  
• Star topology


  central device that all other computers are directly connected to such as a hub or switch. each computer is also indirectly connected to every other computer via this device.

  
• Bus topology


  one central backbone of cable that each computer on the network connects to . at the end of the cable is a terminator which stops data from continually being moved around the netowork

  
• Mesh topology


  mesh is expensive however if one computer fails it has little effect as the computers are connected to every other computer and data can find an alternative route

  
• Advantage of Bus topology


  only one main channel required

  easy to extend

  cheap

  station faults have no effect on rest of network
• Disadvantages of bus topology


  Fault diagnosis is difficult

  A fault in one section of main channel causes a fault in the entire channel

  More sophisticated protocols and hardware required

  Monitoring and auditing becomes complicated

  High rate of data crashes
• Advantage of Ring topology


  Signal is boosted at each station

  Node failure will not cause network failure, just rerouting
• disadvantage of ring topology


  Difficult to diagnose faults

  Extension of the network is impossible without bringing the rest of the network down

  Control software and equipment expensive
• advantage of star topology


  Shorter paths

  Simpler to monitor and audit

  Easier fault finding

  Simple access protocols

  Easy expansion
• disadvantage of star topology


  Loss of network if central hub fails

  Many cables needed

  Costly to expand

  Greater initial costs

  Central node slows communication between nodes
• advantages of mesh topology


  Fault in one channel doesn’t affect whole network

  Many transmissions can take place at any one time

  Excellent performance
• disadvantages of mesh topology


  Lots of wiring

  Expensive
• Wired network cables


  Coaxial cable

  Twisted pair cable

  Fibre-optic cable
• Wired network advantages


  Connections are generally faster and more than on wireless networks

  Easier to manage the security of the network
• disadvantages of wired network


  The need for cables, which can be impractical and requires management to avoid becoming a potential hazard

  Limited by the number of physical connections available

  Can only connect to a network available at that specific location

  Transmission media can be expensive depending on the size of the building
• advantages of wireless networks


  Can connect to networks from different geographical locations No limit on the number of physical connections

  Files can be transferred between devices more conveniently using wireless networks; e.g. sending a photo from your phone to a computer.
• disadvantages of wireless networks


  Data transfer speeds are generally slower and less reliable than wired due to distance, interference and packet loss

  Wireless networks are less secure; data can be more easily intercepted than with wired
• packet switching


  method of transferring data where each packet finds its own optimal route to its destination
• Packets


  Contain:

  a header - the sender & receivers IP, packet number , length of the packet and the protocol

  the payload - the data itself

  the trailer - contains the end of packet marker and error catching
• advantages of circuit switching


  Data is transmitted without delay once a connection is made

  Data is guaranteed to arrive in order
• disadvantages of circuit switching


  Nodes in the path cannot be used to transmit data from other sources while another transmission is taking place

  Establishing a connection can take time
• advantages of packet switching 


  Multiple packets can be sent over the network at the same time Can still work if a computer on the network fails
• disadvantages of packet switching


  Packets can arrive out of order

  Packets may not arrive in the expected time frame
• TCP/IP Transmission Control Protocol / Internet Protocol


  This sets rules for how devices on a network are connected.

  The TCP part manages the separation of a file into data packets to be sent across a network.

  It then manages the process of reassembling the data packets. The IP part gives each packet and address to allow it to reach its destination.
• HTTP Hypertext Transfer Protocol


  This is used by web browsers to access websites. It allows the browser to communicate with a web server.
• HTTPS Hypertext Transfer Protocol Secure


  This is a more secure version of HTTP. It encrypts and decrypts web pages and data as it is sent using this protocol.

  It is used by banks and online shopping sites to keep personal data secure
• FTP File Transfer Protocol


  Used in conjunction with TCP/IP to transmit files across a network.
• POP Post Office Protocol


  The POP3 protocol is used to retrieve emails from a server. An email server receives a user’s emails and they are then held on the server. Periodically, the user or the user’s email client checks the user’s mailbox on the server and downloads any mail using the POP3 protocol. When the email is downloaded, it is then deleted from the server by the protocol.
• IMAP Internet Message Access Protocol


  IMAP works in a very similar way to POP3. One difference, though, is that the email is not deleted from the server when the email is downloaded, until the user chooses to delete it.
• SMTP Simple Mail Transfer Protocol


  POP3 and IMAP are concerned with the downloading and reading of emails, but SMTP governs the sending of emails using a network.