Data Link Layer

Data Link Layer

Data Link Layer provides error-free transmission over physical link.

Functions

1. Framing

Framing groups bits into frames.

Methods:

• Character count
• Flag bytes
• Bit stuffing

2. Error Detection

Error Detection detects errors in received frames.

Parity Check

Single Parity:

• Even parity: Even number of 1s
• Odd parity: Odd number of 1s
• Detects single-bit errors

Checksum

Checksum:

• Sum all data
• Send complement
• Receiver verifies sum

CRC (Cyclic Redundancy Check)

CRC:

• Polynomial division
• Remainder appended
• Strong error detection

Example: CRC-32

3. Error Correction

Error Correction corrects errors.

Methods:

• Hamming code
• Reed-Solomon

4. Flow Control

Flow Control prevents receiver overflow.

Methods:

• Stop-and-wait
• Sliding window

MAC Protocols

MAC (Media Access Control) protocols control access to shared medium.

ALOHA

Pure ALOHA:

• Transmit anytime
• Collision possible
• Efficiency: ~18%

Slotted ALOHA:

• Transmit at slot boundaries
• Better efficiency: ~37%

CSMA

CSMA (Carrier Sense Multiple Access):

• Sense carrier before transmit
• If busy, wait

CSMA/CD (Collision Detection):

• Detect collisions
• Back off and retry
• Used in Ethernet

CSMA/CA (Collision Avoidance):

• Avoid collisions
• Used in wireless

Token Passing

Token Ring:

• Token circulates
• Station with token transmits
• No collisions

Ethernet

Ethernet is widely used LAN technology.

Ethernet Frame Format

Fields:

• Preamble (7 bytes)
• SFD (1 byte)
• Destination MAC (6 bytes)
• Source MAC (6 bytes)
• Type/Length (2 bytes)
• Data (46-1500 bytes)
• CRC (4 bytes)

MAC Address

MAC Address is 48-bit (6 bytes) hardware address.

Format: XX:XX:XX:XX:XX:XX (hexadecimal)

Example: 00:1A:2B:3C:4D:5E

Unicast: First bit = 0

Multicast: First bit = 1

Broadcast: FF:FF:FF:FF:FF:FF

Switching

Switch

Switch operates at data link layer.

Functions:

• Forward frames based on MAC address
• Learning: Build MAC table
• Forwarding: Forward to correct port
• Filtering: Drop if same segment

Bridge

Bridge connects LAN segments.

Functions:

• Filter frames
• Forward frames
• Learning

GATE CS Important Points

1. Error Detection: Parity, checksum, CRC
2. MAC Protocols: ALOHA, CSMA/CD, CSMA/CA
3. Ethernet: Frame format, MAC address
4. Switching: Switch functions, learning
5. Flow Control: Stop-and-wait, sliding window

Practice Tips

1. Error Detection: Practice CRC calculations
2. MAC Protocols: Understand ALOHA efficiency
3. Ethernet: Know frame format
4. Switching: Understand learning and forwarding
5. Previous Year Questions: Solve GATE data link layer questions