1. Frame Format
   
   1. Original format (left) is very simple.
   2. IEEE expanded format (right) has not been widely adopted; most installations use the classic.
      1. When the type is >= 1500, it really is a type and the frame is classic.
      2. Otherwise, the LLC/SNAP is present
      3. The LLC holds source and destination type codes.
      4. If it contains a special value, the OUI and type are present, presumably to allow organizations to define their own types.
2. History
   1. Original Ethernet.
      1. “Thicknet,”, formally 10-Base-5. 10Mbps, 500m segments.
         
      2. Bus architecture. Broadcast, CSMA/CD.
      3. Thick coax, usually in the ceiling.
      4. Stations connected by a drop cable called a “Attachment Unit Interface” (AUI) cable.
      5. AUI cable connects to the main one using a clamp that pierces the insulation to connect the cable.
      6. Terminator is a resister that absorbs signals which reach it. Keeps them from bouncing back through.
   2. Thinnnet.
      1. “Thicknet,” formally 10-Base-2. 10Mbps, 200m (actually 185) segments.
         
      2. Similar bus architecture, broadcast, CSMA/CD.
      3. Thin coax, screw-in connection at each computer.
      4. Similar terminator required.
   3. Twisted pair (current).
      1. 10-Base-T. 10Mbps, later 100-Base-T and now higher. segments.
         
      2. Each computer connected to a hub.
      3. Wiring is twisted pair instead of coax.
      4. The hub is electrically a bus; still CSMA/CD. Hubs are later replaced with switches (next topic).
      5. The cables have something like eight wires in them, so I'm not quite sure they're “pairs,” but indeed twisted.
      6. Originally, different cables were wired differently to connect hubs to computers, and hubs to hubs.
      7. Most equipment now will automatically figure out what's up, and either cable connection can be used in either circumstance.
3. Switching.
   1. Building a larger network
      
   2. Simple Bridge.
      1. Hubs are broadcast media, and a simple Bridge is a repeater.
      2. If computer B sends a message, it is broadcast to A, C and the Bridge.
      3. The bridge sends it to Hub 2, which broadcasts it to X, Y and Z.
      4. But this not very efficient if the message is meant for C; all the forwarding an broadcast on Hub 2 is wasted.
   3. Leaning Bridges.
      1. When a packet is forwarded, record which side it came from.
      2. When a packet is received, see if we know where its destination is.
      3. Don't forward it if we know the destination is the same side it came from.
      4. After the learning bridge has been running a while, it will no longer forward packets unnecessarily
   4. Switches
      1. A switch looks like a hub, and is connected in the same way.
         
      2. Conceptually, each computer is connected through a learning bridge.
      3. Permits higher bandwidth than a hub: multiple independent conversations.
      4. Available in many sizes.