Material from Chapters 13.

1. Network Classifications
   1. Local Area Network (LAN). Single room, building, or campus.
      
   2. Wide Area Network (WAN). State or region up to the whole world.
   3. Metropolitan Area Network (MAN). Span a city. Seem to exist mostly just in theory.
2. This section is about LANs.
3. Connecting each computer point-to-point would be too expensive.
   1. Connecting n computers requies (n2−n)/2 links.
   2. Use shared connections.
4. LAN Topologies.
   
   1. Bus describes the original Ethernet. Current versions are essentially stars. Messages sent on the bus are delivered to all stations.
   2. Wired ring nets are not longer used, but many optical LANs use a ring. Messages travel once around the circle and are removed by the sender.
   3. Messages go to the hub, which send them out to some or all of the nodes.
5. Packet switching.
   1. As in other digital networking, LANs send data in packets.
      
   2. A way of sharing a channel. Any sender may send a packet when the channel is available.
   3. Receivers can check each packet as it arrives.
6. Standards Body
   1. Application Layer: World-Wide Web Consortium (W3C), Web Hypertext Application Technology Working Group (WHATWG), sometimes Internet Engineering Taskforce (IETF).
   2. Transport, Internet: Internet Engineering Taskforce (IETF).
   3. Physical: Institute of Electrical and Electronics Engineers (IEEE).
7. This section is in IEEE territory. IEEE LAN standards. Among many:
   1. 802.3 CSMA/CD (Ethernet)
   2. 802.5 Token Ring
   3. 802.11 Wireless LAN (WiFi)
   4. 802.15 Wirless PAN.
   5. 802.16 Wireless Broadband
   6. IEEE does not seem to standardize the optical net standards.
8. Frames
   
   1. Prelude (optional) contains known patterns for synchronization and marking the start of the frame.
   2. Header contains addressing and possibly error check or correction data.
   3. Payload is the actual data.
   4. Check is optional. Often the check is at the end to allow the endpoint to compute it as the message is flowing in or out.
   5. Postlude (optional) for marking the frame end, or checking timing.
9. Frame Marking
   1. Frames boundaries often marked by special bytes.
   2. This requires escaping when these values are needed in the frame.
      
   3. Called byte stuffing.
10. Some optical nets use 4B/5B coding.
    1. Each five-bit unit of transmission codes either control or four bits of data.
    2. For instance:

       Code	Meaning
       01101	T Control (Part of end delimiter)
       01110	Data 0110
       01111	Data 0111
       10000	Unused
       10001	K Control (Part of start delimiter)
       10010	Data 1000

    3. So no escaping needed.
11. IEEE LAN standards cover two main aspects
    1. Logical Link Control (LLC)
       1. How to address individual hosts.
       2. How to direct the payload within the receiving host.
    2. Media Access Control (MAC): How to share the medium in an orderly way.