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Unit III: Data Link Layer and MAC Sublayer

⭐DATA LINK LAYER

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1. Data Link Layer Design Issues

• Purpose: The Data Link Layer (DLL) provides reliable communication between directly connected nodes and is responsible for framing, addressing, error detection, and flow control.

Key Design Issues:

1. Framing:

   • Definition: Process of dividing the stream of bits into manageable units called frames.
   • Methods:
     • Character Count: Frame includes a count of the number of characters in it.
     • Delimiter: Special characters (like flags) mark the start and end of frames.
     • Bit Stuffing: Insertion of non-data bits into the data stream to distinguish between data and control information.

2. Error Control:

   • Purpose: Detect and correct errors that occur during transmission.
   • Types:
     • Error Detection: Identifies presence of errors (e.g., parity, checksums).
     • Error Correction: Mechanisms to fix detected errors (e.g., Hamming code).

3. Flow Control:

   • Definition: Manages data transmission rates between sender and receiver to prevent overwhelming the receiver.
   • Techniques:
     • Stop-and-Wait: Sender transmits a frame and waits for an acknowledgment (ACK) before sending the next frame.
     • Sliding Window: Allows multiple frames to be in transit before requiring an ACK, increasing efficiency.

4. Medium Access Control (MAC):

   • Definition: Determines how multiple devices share the same communication medium.
   • Purpose: Prevents collisions and ensures fair access to the medium.

2. Elementary Data Link Protocols

1. Stop-and-Wait Protocol:

   • The sender transmits one frame and waits for an ACK before sending the next.
   • Advantages: Simple to implement.
   • Disadvantages: Inefficient due to idle time waiting for ACKs.

2. Go-Back-N Protocol:

   • Allows the sender to transmit multiple frames before needing an acknowledgment.
   • If an error is detected, the sender must retransmit the erroneous frame and all subsequent frames.
   • Advantages: More efficient than Stop-and-Wait.
   • Disadvantages: Can lead to retransmission of many frames due to a single error.

3. Selective Repeat Protocol:

   • Similar to Go-Back-N, but only retransmits frames that were detected as erroneous.
   • Advantages: More efficient than Go-Back-N, reduces unnecessary retransmissions.
   • Disadvantages: More complex to implement due to the need for buffering.

3. Error Detection and Correction

1. Parity Bit:

   • Definition: A single bit added to a string of binary data.
   • Types:
     • Even Parity: Ensures the total number of 1s is even.
     • Odd Parity: Ensures the total number of 1s is odd.
   • Limitations: Can only detect single-bit errors and cannot correct them.

2. Checksum:

   • Definition: A value calculated from the data in the frame, used to detect errors.
   • Process: Data units are summed, and the result is included in the frame. The receiver recalculates the checksum to verify integrity.
   • Limitations: May not detect all types of errors (e.g., two errors canceling each other).

3. Cyclic Redundancy Check (CRC):

   • Definition: A more robust error detection mechanism.
   • Process: Data is treated as a polynomial and divided by a fixed polynomial (generator). The remainder becomes the CRC code, appended to the data.
   • Advantages: Can detect burst errors and is widely used in network protocols.

4. Hamming Code:

   • Definition: An error correction code that adds redundant bits to the data.
   • Process: Uses specific positions for parity bits to allow the detection and correction of single-bit errors.
   • Advantages: Can correct one-bit errors and detect two-bit errors.

4. Switch Working

• Function: A device that connects multiple devices within a network, forwarding frames based on MAC addresses.

Operation:

1. Learning:

   • Switch builds a MAC address table by recording the MAC addresses of devices connected to each port.

2. Forwarding:

   • When a frame is received, the switch checks the destination MAC address against its table and forwards the frame to the appropriate port.

3. Filtering:

   • The switch will drop frames sent to the source MAC address to prevent loops and reduce unnecessary traffic.

4. Collision Management:

• In a switched network, each device has a dedicated bandwidth, which reduces the chances of collisions compared to shared media networks.

⭐MAC SUBLAYER

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1. Multiple Access Protocols

• Definition: Protocols that manage how multiple devices share the same communication medium to prevent collisions and ensure efficient communication.

A. ALOHA Protocol

1. Pure ALOHA:

   • Mechanism: Devices transmit data whenever they have it. After transmission, they wait for an acknowledgment (ACK).
   • Collision Handling: If a collision occurs (i.e., two devices transmit simultaneously), the devices wait a random amount of time before retransmitting.
   • Efficiency: Maximum throughput is 18.4%, meaning only a small fraction of transmissions can be successful.

2. Slotted ALOHA:

   • Mechanism: Time is divided into discrete slots, and devices can only transmit at the beginning of a time slot.
   • Collision Handling: Similar to Pure ALOHA, but collisions only occur within the time slot.
   • Efficiency: Maximum throughput is 36.8%, effectively doubling the efficiency compared to Pure ALOHA.

B. Carrier Sense Multiple Access (CSMA)

• Definition: A protocol where devices listen to the medium before transmitting to avoid collisions.

1. CSMA:

   • Mechanism: Devices sense the channel; if it is idle, they transmit; if it is busy, they wait.
   • Limitations: Collisions can still occur if two devices sense the channel as idle and transmit simultaneously.

2. CSMA with Collision Detection (CSMA/CD):

   • Definition: An enhancement to CSMA that detects collisions during transmission.
   • Mechanism:
     • Devices listen while transmitting. If they detect a collision, they stop transmitting immediately.
     • After a collision, devices wait a random backoff time before attempting to retransmit.
   • Applications: Widely used in wired Ethernet networks.
   • Efficiency: Performance decreases as network load increases due to increased collisions.

C. Random Access Protocols

• Definition: Protocols that allow devices to transmit whenever they have data, with methods to manage collisions.
• Examples: ALOHA and CSMA are both examples of random access protocols, where devices transmit without prior coordination.

D. Controlled Access Protocols

• Definition: Protocols that regulate access to the communication medium to avoid collisions and ensure orderly communication.

1. Polling:

   • Mechanism: A central controller polls devices in a round-robin fashion, allowing each one to transmit in turn.
   • Advantages: Eliminates collisions and provides predictable access.
   • Disadvantages: Can introduce delays and overhead.

2. Token Passing:

   • Mechanism: A token circulates in the network; only the device holding the token can transmit.
   • Advantages: Guarantees that only one device transmits at a time, preventing collisions.
   • Applications: Used in Token Ring and other token-based networks.

2. Ethernet Protocol

• Definition: A widely used networking technology for local area networks (LANs) that uses a specific set of rules for accessing the physical medium.

Key Features:

1. Frame Structure:

   • Preamble: Synchronizes the receiver with the sender.
   • Destination MAC Address: Specifies the recipient of the frame.
   • Source MAC Address: Specifies the sender of the frame.
   • Type/Length Field: Indicates the type of payload or the length of the data field.
   • Data Field: Contains the actual data being transmitted.
   • CRC: Used for error detection.

2. Access Method:

   • CSMA/CD: Uses the CSMA/CD protocol for managing access to the medium.
   • Collision Handling: If a collision occurs, the devices involved stop transmitting and wait for a random backoff time.

3. Variants:

   • 10BASE-T: 10 Mbps over twisted-pair cables.
   • 100BASE-TX: 100 Mbps (Fast Ethernet) over twisted-pair cables.
   • 1000BASE-T: 1 Gbps (Gigabit Ethernet) over twisted-pair cables.
   • 10GBASE-T: 10 Gbps Ethernet over twisted-pair cables.

4. Physical Layer Technologies:

   • Utilizes twisted pair cables, coaxial cables, or fiber optics depending on the variant.

5. Switching:

• Ethernet networks use switches to forward frames based on MAC addresses, increasing efficiency and reducing collisions.

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