Material from Chapter 14, though the book mentions the NIC in Chapter 15.

1. Methods to control access to the media so all stations don't send at once.
2. Channelization Protocols.
   1. Divide the medium into channels. We've seen these before.
   2. Frequency-Division Multiple Access (FDMA).
   3. Time-Division Multiple Access (TDMA).
   4. Time-Division Multiple Access (CDMA).
3. Controlled Access Protocols.
   1. A form of time multiplexing.
   2. Centrally controlled. Generally requires an extra, low-bandwidth control channel.
      1. Polling.
         1. Central controller prompts each device to send what it has.
         2. Usually round-robin, though other orders or priorities can be used.
      2. Reservation. Similar to polling, but hosts must notify controller that they have data to send.
      3. These methods work well with a star topology, but can be used with others.
      4. Failure of the controller will disable entire network.
   3. Token Passing. A more distributed control.
      1. There is a special message called a “token” which represents permission to send.
      2. The token is passed in rotation.
      3. When a host receives the token, it sends any data, then forwards the token.
      4. Possible failures include loss or duplication of token.
      5. Some privileged host is allowed to create a token if it seems missing, or remove what appears to be a duplicate.
      6. Most often used with ring topology, also with bus.
      7. Wired LANs in the past optical LANs now.
4. Random Access Protocols.
   1. These protocols use random behavior to avoid a need for centralized control.
   2. Coordination is local, distributed, and often after-the-fact.
   3. The general rule is: go ahead and try. If it fails, try again later.
   4. Types.
      1. Aloha. Historic, no longer used.
      2. CSMA/CD. Ethernet.
      3. CSMA/CA. WiFi
5. Aloha Protocol
   1. University of Hawaii.
   2. State-wide radio net.
      
   3. Only the central tower sends on the outbound frequency.
   4. It repeats messages from outlying stations for all.
   5. Outlying stations send whenever they like.
   6. Suppose two outlying stations send at the same time.
      1. Combination message is garbled.
      2. Checksum fails.
      3. Message is discarded.
   7. Outlying sender checks for this
      1. Listens for repeat of the sent message.
      2. If not heard within a time limit, resend.
      3. Each waits a random amount before resend to avoid another collision.
6. Carrier Sense on Multiple Access with Collision Detection (CSMA/CD).
   1. Ethernet.
   2. Wait for silence before sending a frame (Carrier sense)
   3. While sending, see that the signal on the bus matches what is being sent. If not, stop.
   4. Try again a random time later.
   5. For each re-send collision, double the range from which the random delay is chosen.
   6. Binary Exponential Back-Off
      1. The random delay is chosen from a range.
      2. On each repeated collision, the range is doubled.
      3. Doubling the range each time halves the probability of a new collision.
      4. Slows and scatters senders when the net is busy.
7. Carrier Sense on Multiple Access with Collision Avoidance (CSMA/CA).
   1. Hidden node problem.
      
      1. Consider nodes separated by the maximum range, d, of the wireless radio.
      2. If Computer 1 and Computer 3 both send to Computer 2 at the same time, there will be a collision.
      3. But neither Computer 1 nor Computer 2 can see it because they cannot see the competing signal.
   2. CSMA/CA
      1. Send a control message to the intended recipient. Wait for a reply.
      2. When a reply is received, send data.
      3. In case of a collision, the recipient will not answer.
      4. Senders will then delay and re-send, as CSMA/CD.
      5. Collisions are detected by the recipient rather than the sender.
         
8. Network Interface Card (NIC).
   1. Computers are connected to the network via a Network Interface Card (NIC).
      
   2. Appears as a peripheral device.
   3. Originally always a separate circuit board, now usually integrated into the mainboard.
   4. Handles
      1. Frame capture from the medium.
      2. Runs the access protocol for sending.
      3. Computes and creates check bytes (usually CRC).
      4. Discards packets not addressed to this host.
      5. Contains the assigned MAC address.
   5. Otherwise the network would place considerable added load on the CPU.