1. Link layer
   --> Getting closer to reality; this is typically the domain of EE
   --> The concept of frequency ranges and bandwidth
   --> The notion of capacity
   --> Shared vs. unshared communication media
2. TDMA and FDMA
   --> Centralization required
   --> Not good for bursty traffic
   --> Need distributed/contention-based protocols.
3. ALOHA
   --> Problem setting: terminals in islands and central computers
   --> No carrier sense.
   --> Using ACKs to detect collisions
   --> Optimal transmission probability
   --> Optimal utilization
4. Prelude to carrier sense
   --> Real networks are a little less challenging.
   --> Can use carrier sense to listen before you speak.