Data Link Layer Communication Protocols and Cabling

Posted on Feb 28, 2025 in Computers

Link Layer

Protocols

  • Contention-Based Protocols

    Strife (Pure and Slotted ALOHA): Stations compete for transmission time. Pure ALOHA is optimal for low traffic (18.4% efficiency), while Slotted ALOHA improves efficiency to 36.8% by synchronizing transmissions into time slots.

    CSMA (Carrier Sense Multiple Access): Transmission is delayed if the medium is busy. Variations include:

    • 1-persistent CSMA: Wait until the medium is free (optimal for low, continuous traffic).
    • Non-persistent CSMA: Wait a random time if the medium is busy (fewer collisions, good for high traffic).
    • P-persistent CSMA: A mix of 1-persistent and non-persistent, suitable for moderate traffic.
    • CSMA/CD (Collision Detection): Stops transmission if a collision is detected, minimizing wasted time.

  • Controlled Access Protocols

    Bit Map Protocol: Each station is assigned a bit in a map; a ‘1’ indicates a desire to transmit. Optimal for high traffic. Efficiency is the ratio of data bits to total bits (data + map).

    Binary Reservation Protocol (BRAP): Improves Bit Map by assigning transmission time slots when a station sets its bit to ‘1’.

    Multi-Level Multi-Access (MLMA): Reduces the number of bits needed for reservation by using ranges.

  • Limited Contention Protocols

    Combine contention and controlled access. Optimal for average traffic.

    Tree Protocol: Adapts to traffic levels by adjusting the number of stations per group.

Structured Cabling

Offers flexibility using twisted-pair cables (UTP/FTP). Categories and bandwidths:

  • Cat3: 16MHz
  • Cat4: 20MHz
  • Cat5: 100MHz
  • Cat5e: 100MHz (Gigabit Ethernet)
  • Cat6: 200MHz (recommended for horizontal cabling)
  • Cat7: 600MHz (STP cable)

Cable Characteristics:

  • Attenuation (dB): Signal loss during transmission.
  • Impedance (Ω): Resistance of the medium to frequency.
  • Resistance (Ω): DC resistance of the conductor.
  • NEXT (dB): Near-end crosstalk (isolation between pairs).
  • FEXT (dB): Far-end crosstalk.
  • ACR: Attenuation-to-crosstalk ratio.
  • Power-Sum NEXT & Power-Sum ACR: Combined crosstalk values.
  • Return Loss (dB): Signal reflection at cable ends.
  • Propagation Delay (ns): Time for a signal to travel the cable.
  • Delay Skew: Difference in propagation times between pairs.

Horizontal Cabling: Uses RJ45 connectors (8 contacts). Maximum distance: 90m + 10m patch cable.

Vertical Cabling: Connects multiple distribution points. Uses fiber optic cable (e.g., 50/125 multimode) with ST or SC connectors.

Frame Format

Frames are serialized (bitwise) from LSB to MSB.

  • Preamble (7 bytes): Synchronization sequence.
  • Start Frame Delimiter (1 byte): Marks the beginning of the frame.
  • Destination Address (2-6 bytes) and Source Address (2-6 bytes): Identify sender and receiver.
  • Length/Type (2 bytes): Indicates the length of the data field or the protocol type.
  • Data/Payload (variable): Contains the actual data being transmitted.
  • Frame Check Sequence (4 bytes): Error detection using CRC.