Data Link Layer
Introduction
During data transmission, errors may occur due to:
- Noise
- Signal distortion
- Interference
- Hardware problems
Error control techniques are used to detect and correct these errors so that accurate data reaches the receiver.
Types of Errors
1. Single Bit Error
- Only one bit in the data is changed
- Example:
101101 → 101001
2. Multiple Bit Error
- More than one bit is changed
- Example:
110011 → 100001
3. Burst Error
- A group of consecutive bits is affected
- Example:
111000101 → 111111101
| Error Type | Description | Common Example |
|---|---|---|
| Single Bit | One bit changed | RAM error |
| Multiple Bit | Several bits changed | Network noise |
| Burst Error | Continuous bits affected | Wireless transmission |
Error Detection Techniques
Error detection only identifies errors, it does not correct them.
Vertical Redundancy Check (VRC) / Parity Check
Concept
- One parity bit is added to data
- Parity can be Even or Odd
Example (Even Parity)
| Data Bits | 1s Count | Parity Bit |
|---|---|---|
| 1011001 | 4 (even) | 0 |
| 1101011 | 5 (odd) | 1 |
Advantages
-
Simple and fast
Limitations
- Detects only single-bit errors
- Cannot detect burst errors
Longitudinal Redundancy Check (LRC)
Concept
- Data is divided into blocks
- Parity is calculated column-wise
Example: Used in magnetic tapes and barcodes
Advantages
- Better than VRC
- Detects some burst errors
Disadvantages
-
Still not fully reliable
Checksum
Concept
- Data blocks are added
- Sum is complemented and sent
- Receiver recomputes sum
Example
Used in:
- TCP/IP
- Internet communication
Advantages
- Simple
- Detects multiple errors
Disadvantages
- Cannot correct errors
- Less reliable than CRC
Cyclic Redundancy Check (CRC)
- Data treated as binary polynomial
- Divided by a generator polynomial
- Remainder is appended
Used In
- Ethernet
- Wi-Fi
- HDLC
Advantages
- Excellent error detection
- Detects burst errors
Disadvantages
- Complex calculation
- Cannot correct errors
Error Correction Techniques
Error correction detects and corrects errors without retransmission.
Hamming Code
Concept
- Uses redundant parity bits
- Detects 2-bit errors
- Corrects 1-bit error
Parity Bit Positions
Placed at positions:
2⁰, 2¹, 2², 2³ → 1, 2, 4, 8
Hamming Distance
Minimum number of bit changes required to convert one codeword into another.
Advantages
- Automatic error correction
- No retransmission required
Disadvantages
-
Extra bits increase data size
Comparison Table (Very Important for Exams)
| Technique | Detects Error | Corrects Error | Used In |
|---|---|---|---|
| VRC | Yes | No | Simple systems |
| LRC | Yes | No | Magnetic tapes |
| Checksum | Yes | No | TCP/IP |
| CRC | Yes | No | Ethernet |
| Hamming Code | Yes | Yes | RAM, ECC memory |
Error Detection vs Error Correction
| Basis | Error Detection | Error Correction |
|---|---|---|
| Purpose | Find error | Find & fix error |
| Retransmission | Required | Not required |
| Example | CRC | Hamming Code |
Real-Life Examples
- ATM transactions → CRC
- Internet data packets → Checksum
- Computer RAM → Hamming Code
- Barcodes → LRC
Important Exam Questions
- Explain types of transmission errors
- Describe VRC and LRC with examples
- Explain checksum and CRC
- Explain Hamming code with diagram
- Differentiate error detection and correction
Conclusion
Error detection and correction techniques ensure reliable data communication.
- CRC is best for detection
- Hamming Code is best for correction
Flow Control and Error Control
Introduction
In data communication, two major problems can occur:
- Flow Control – Sender sends data faster than the receiver can handle
- Error Control – Data gets corrupted or lost during transmission
To solve these problems, protocols are used.
Flow Control
Flow control is a technique that ensures the sender does not overwhelm the receiver by sending data too fast.
Real-life example: A teacher speaks slowly so that students can understand and take notes.
Error Control
Error control ensures that data is delivered correctly, even if errors occur, by:
- Detecting errors
- Correcting errors or
- Retransmitting data
Real-life example: If a WhatsApp message fails, it is sent again.
Stop-and-Wait Protocol
Concept
- Sender sends one frame
- Waits for ACK (Acknowledgement)
- Sends next frame only after ACK
Working Steps
- Sender sends Frame-1
- Receiver receives it and sends ACK
- Sender sends Frame-2
Advantages
- Simple to implement
- Good for small data transfer
Disadvantages
- Very slow
- Poor channel utilization
Real-life Example: Courier service delivers one parcel at a time and waits for confirmation.
Sliding Window Protocol
Concept
- Sender can send multiple frames before receiving ACKs
- Window size defines how many frames can be sent at once
Advantages
- Better efficiency
- Faster transmission
Disadvantages
-
More complex than Stop-and-Wait
Real-life Example: Sending multiple WhatsApp messages without waiting for delivery confirmation.
Go-Back-N ARQ Protocol
Concept
-
Sender sends multiple frames
- Receiver discards all subsequent frames
- Sender retransmits from the error frame onwards
Advantages
- Simple receiver design
- Efficient than Stop-and-Wait
Disadvantages
-
Wastage of bandwidth due to retransmission
Real-life Example: If one exam answer sheet is lost, all following sheets are re-submitted.
Selective Repeat ARQ Protocol
Concept
- Only lost or damaged frames are retransmitted
- Receiver stores correctly received frames
Advantages
- Best bandwidth utilization
- High efficiency
Disadvantages
- Complex implementation
- Requires more memory
Real-life Example: Teacher asks student to rewrite only the wrong answers, not the full paper.
Comparison Table (Very Important for Exams)
| Protocol | Frames Sent | Retransmission | Efficiency |
|---|---|---|---|
| Stop-and-Wait | One at a time | One frame | Low |
| Sliding Window | Multiple | Depends | High |
| Go-Back-N ARQ | Multiple | From error frame | Medium |
| Selective Repeat ARQ | Multiple | Only error frames | Very High |
Flow Control vs Error Control
| Basis | Flow Control | Error Control |
|---|---|---|
| Purpose | Control data speed | Ensure correctness |
| Focus | Sender-receiver speed | Lost/corrupted frames |
| Example | Sliding Window | ARQ Protocols |
Important Exam Questions
- Explain Stop-and-Wait protocol with diagram
- Describe Sliding Window protocol
- Explain Go-Back-N ARQ with example
- Explain Selective Repeat ARQ with advantages
- Differentiate Go-Back-N and Selective Repeat ARQ
Key Exam Tips
- Always draw sequence diagrams
- Mention ACK and retransmission logic
- Use comparison tables in answers
- Write real-life example for full marks
Conclusion
Flow control and error control protocols ensure efficient and reliable data transmission.
- Stop-and-Wait → Simple but slow
- Sliding Window → Faster
- Go-Back-N → Moderate efficiency
- Selective Repeat → Best performance
Channel Allocation Protocols
Introduction to Channel Allocation
Channel allocation refers to how multiple devices share a single communication channel efficiently without causing confusion or data collision.
Real-life example: Many students want to speak in a classroom → rules are needed so everyone can speak properly.
Categories of Channel Allocation Protocols
Channel allocation techniques are broadly divided into three categories:
- Random Access Protocols
- Controlled Access Protocols
- Channelization Protocols
Random Access Protocols
In random access, any station can transmit data at any time. If collision occurs, it is handled using retransmission rules.
ALOHA Protocol
Concept
- Stations transmit data whenever they have data
- If collision occurs, data is retransmitted after random time
Types
- Pure ALOHA – Transmit anytime
- Slotted ALOHA – Transmit only at fixed time slots
Advantages
- Simple
- Easy to implement
Disadvantages
- High collision
- Low efficiency
Real-life example: People speaking randomly in a group discussion.
CSMA (Carrier Sense Multiple Access)
Concept
- Station listens to the channel before sending
- If channel is free → transmit
- If busy → wait
Types
- 1-Persistent CSMA
- Non-Persistent CSMA
- p-Persistent CSMA
Example: Checking if someone is talking before speaking.
CSMA/CD (Collision Detection)
Concept
- Used in wired Ethernet
- Detects collision while transmitting
- Stops transmission and retries
Advantages
-
Reduced collision time
Disadvantages
-
Not suitable for wireless networks
CSMA/CA (Collision Avoidance)
Concept
- Used in wireless networks
- Avoids collision instead of detecting it
- Uses ACK and waiting time
Example
Wi-Fi networks (IEEE 802.11)
Controlled Access Protocols
In controlled access, stations take turns to transmit data.
Token Passing Protocol
Concept
- A special packet called token circulates
- Only the station holding the token can transmit
Advantages
- No collision
- Fair access
Disadvantages
-
Token loss can stop communication
Example: Passing a microphone in a meeting.
Channelization Protocols
Channelization divides the channel into smaller parts and assigns each part to a station.
FDMA (Frequency Division Multiple Access)
Concept
- Channel divided into frequency bands
- Each user gets a separate frequency
Example: FM radio stations
TDMA (Time Division Multiple Access)
Concept
- Channel divided into time slots
- Each user transmits in its time slot
Example: Mobile communication
CDMA (Code Division Multiple Access)
Concept
- All users use same frequency & time
- Unique codes separate users
Example: 3G mobile networks
Comparison Tables (Very Important for Exams)
Protocol Category Comparison
| Category | Collision | Efficiency | Example |
|---|---|---|---|
| Random Access | Yes | Medium | ALOHA |
| Controlled Access | No | High | Token Passing |
| Channelization | No | Very High | TDMA |
Random Access Protocol Comparison
| Protocol | Collision Handling | Used In |
|---|---|---|
| ALOHA | Retransmission | Satellite |
| CSMA | Sense channel | LAN |
| CSMA/CD | Detect collision | Ethernet |
| CSMA/CA | Avoid collision | Wi-Fi |
Channelization Techniques Comparison
| Technique | Division | Used In |
|---|---|---|
| FDMA | Frequency | Radio |
| TDMA | Time | GSM |
| CDMA | Code | 3G |
Important Exam Questions
- Explain channel allocation protocols
- Describe ALOHA and CSMA
- Differentiate CSMA/CD and CSMA/CA
- Explain FDMA, TDMA, and CDMA
- Compare random access, controlled access, and channelization
Exam Tips
- Always draw simple diagrams
- Use comparison tables
- Mention real-life examples
- Write protocol advantages & disadvantages
Conclusion
Channel allocation protocols ensure fair, efficient, and collision-free communication.
- Random Access → Simple but collision-prone
- Controlled Access → Organized and fair
- Channelization → Highly efficient