📘 PAPER 6 – DATA COMMUNICATION & NETWORKS (UNIT 2 – DATA LINK LAYER & MEDIUM ACCESS CONTROL) university of allahabad
🔴 UNIT 2 – DATA LINK LAYER & MEDIUM ACCESS CONTROL
1️⃣ Data Link Layer – Introduction
✅ Definition
The Data Link Layer is the second layer of the OSI model.
It is responsible for reliable data transfer between two directly connected nodes.
✅ Main Functions
✔ Framing
✔ Error control
✔ Flow control
✔ Access control
✔ Physical addressing
2️⃣ Framing
🔹 Definition
Framing divides the data stream into manageable units called frames.
🔹 Methods of Framing
1. Character Count
-
Frame size specified in header
❌ Error-prone
2. Byte Stuffing
-
Special character added to data
✔ Used in HDLC
3. Bit Stuffing
-
Extra bit inserted after 5 consecutive 1s
✔ Used in modern networks
3️⃣ Error Detection Techniques
🔹 1. Parity Check
-
Adds 1 parity bit
-
Detects single-bit error
🔹 2. Checksum
-
Sum of data bits
-
Used in TCP/IP
🔹 3. Cyclic Redundancy Check (CRC)
-
Most powerful method
-
Uses polynomial division
✔ Highly reliable
✔ Widely used
4️⃣ Error Control
Error control ensures reliable data delivery.
Methods:
✔ Error detection
✔ Error correction
✔ Retransmission
5️⃣ Flow Control
Definition:
Controls the amount of data sent to avoid receiver overflow.
Methods:
-
Stop-and-Wait
-
Sliding Window
6️⃣ Sliding Window Protocol
Types:
🔹 1. Stop-and-Wait ARQ
-
Send one frame
-
Wait for ACK
-
Simple but slow
🔹 2. Go-Back-N ARQ
-
Sends multiple frames
-
Retransmits all frames after error
🔹 3. Selective Repeat ARQ
-
Retransmits only erroneous frame
-
Efficient but complex
7️⃣ Medium Access Control (MAC)
Definition:
MAC controls who can transmit data on shared medium.
8️⃣ Channel Allocation Methods
🔹 1. Static Channel Allocation
-
Fixed bandwidth
-
Inefficient for bursty traffic
🔹 2. Dynamic Channel Allocation
-
Bandwidth allocated as needed
-
Used in LANs
9️⃣ Multiple Access Protocols
🔹 ALOHA Protocol
Types:
-
Pure ALOHA
-
Slotted ALOHA
Pure ALOHA
-
Send anytime
-
High collision
-
Low efficiency (~18%)
Slotted ALOHA
-
Time divided into slots
-
Higher efficiency (~36%)
🔹 CSMA (Carrier Sense Multiple Access)
Types:
1️⃣ CSMA/CD (Collision Detection)
-
Used in Ethernet
-
Detects collision and stops transmission
2️⃣ CSMA/CA (Collision Avoidance)
-
Used in Wi-Fi
-
Avoids collision before transmitting
1️⃣0️⃣ IEEE Standards
| Standard | Description |
|---|---|
| IEEE 802.3 | Ethernet |
| IEEE 802.11 | Wi-Fi |
| IEEE 802.5 | Token Ring |
| IEEE 802.15 | Bluetooth |
1️⃣1️⃣ Data Link Layer Protocols
✔ HDLC
✔ PPP
✔ Ethernet
📌 EXAM IMPORTANT QUESTIONS (UNIT 2)
✔ Explain Data Link Layer
✔ Framing techniques
✔ Sliding Window Protocol
✔ ALOHA vs CSMA
✔ Error detection methods
✔ CRC explanation
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