7.2 Networks

• Application layer- Transmitting

  Sender's email client creates data from user input
• Presentation layer


  Email data is converted into the common email format MIME and, if necessary, encrypted and/or compressed
• Session layer- Transmitting

  A session is begun, between sender and email server
• Transport layer- Transmitting

  Data is split into packets – which are numbered to reassembled - and reliable communication is established using TCP
• Network layer- Transmitting

  Finds the best path (route) for data transfer to the email server using it's IP address
• Data link layer - LLC layer- Transmitting

  Packets are made into frames, with error and flow control also being done
• Data link layer - MAC layer- Transmitting

  Email servers MAC address added to the frame
• Physical layer- Transmitting

  Frames are converted into bits which are sent as signals through a physical medium, such as WI-FI or Ethernet
• Receiving - Physical layer

  Bits are converted into frames
• Receiving - Data link layer - MAC layer

  Receives frame and MAC address is stripped from frame
• Receiving - Data link layer - LLC layer

  Turns frame into packets, checks data integrity
• Receiving - Network layer

  Checks packets target IP address to check it has been sent to the correct device if not retransmission is forced
• Receiving - Transport layer

  Packets are converted into MIME file and TCP is used to check the data
• Receiving - Session layer

  Session is established between email server and receiver email client
• Receiving - Presentation layer

  MIME file is converted into receiver email clients locally used format and, if necessary, unencrypted and/or decompressed
• Receiving - Application layer


  Email is presented to user through email client
• If ethernet is used then LLC handles flow and error control, otherwise it is done by the transport layer
• MAC address is only used in local transmission from within the same LAN, otherwise TCP/IP is used, however MAC is always included and will link to the next node of the route usually a router, if the MAC address of the next node or target node is unknown then ARP (address resolution protocol) is used to obtain it