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MAC Sub-layer (Ethernet (Classic ethernet (Each type has a max cable…

- - - - Number of senders is more than allocated segments
      - Number of senders is not static
      - Bursty traffic
    - - Time division multiplexing
        
        Divide transmission time into segments
        
        User can only transmit frames in allocated segment
        
        Diagram
      - Frequency division multiplexing
        
        Place users on different frequencies
        
        Diagram
        
        Require guard bands in between modulated bandpass signals to avoid interference when demultiplexing
    - - Only good for fixed number of users
      - Not good for bursty traffic
      - Still used by TV, Radio (FDM) and 2G (TDM)
  - - - Independent transmission stations
      - Single channel for all communication
      - Simultaneous transmission results in damaged frames
      - Time: can begin continuous/discrete intervals
      - Carrier sense: detect channel use or not
    - - ALOHA
        
        User transmit frames whenever they have data
        
        Users retry after a random time if there are collision or no Ack received
        
        Requires no central control mechanism
        
        Efficient under low load, not under high load
        
        Slotted ALOHA
        
        User start sending only at beginning of defined slots
        
        Increase efficiency by reducing collision possiblity
        
        Introduces half a slot time of delay -> Better at high loads
      - Carrier Sense Multiple Access (CSMA)
        
        Require transmission state detection to determine transmission rights dynamically
        
        CSMA with collision detection
        
        Channel must be continually monitored
        
        Collision detected -> abort -> wait random period -> try again
        
        Used in half-duplex system
        
        Transmission period followed by contention period (where stations compete for transmission rights)
        
        Collision free protocols
        
        Bit Map protocol
        
        Reservation-based
        
        1-bit per station overhead (scalability problem)
        
        Division of transmission right + transmission event
        
        Binary Countdown protocol
        
        Stations send their address in contention slot
        
        Better scalability: Log N bits
        
        Start from the left, OR all bits; if does not correspond to your station's address at the bit, drop out
        
        Station that sees its full address is next to send
        
        Minimum packet size for reliable detection: 2*tao where tao is the time it takes to propogate over the Ethernet
        
        Persistent/non-persistent CSMA
        
        First check channel to detect other active transmissions
        
        1-persistent: wait until channel is idle; transmit one frame and check collisions. If collision, wait for a random time and repeat
        
        Non-persistent: if channel busy, wait for a random period and check again; if not, start transmitting
        
        p-persistent: if channel is idle, transmit with prob. p, or wait with prob. (1-p) and check again
        
        Diagram
        
        Limited Contention Protocols
        
        Increase probability of stations acquiring transmission rights
        
        Arbitrarily divide stations and use a binary algorithm to determine rights allocatio
        
        Adaptive Tree Walk Protocol
        
        All stations compete for right to transmit
        
        If collision occurs, binary division is used to resolve contention
        
        Depth-first-search
      - Comparison
        
        Throughput vs traffic flow: Aloha < Slotted ALOHA < 1-persistent < 0-persistent
        
        High loads, contention is less attraction: higher overhead associated with channel arbitration
        
        Low loads, collision-free is less attractive: higher delay between tranmissions
- - - - Random delay computed with Binary Exponential Backoff (BEB): [random number and 2^i - 1]
    - - Preamble is used to indicate that a frame is arriving in the channel
      - Start of frame is the flagbyte
      - Type & Length is to specify which process to give the frame to
      - Pad is to ensure that it reaches the minimum size of the message is reached (64B)
  - - - Much greater throughout for multiple ports
      - No need to CSMA/CD with full-duplex lines