U5. Data Link Layer and Network Layer

Question 1

1. Data Link Layer – Design Issues

Answer 1

Framing: Divides data into frames for synchronization.
Error Control: Detects and corrects transmission errors.
Flow Control: Maintains pace between sender and receiver.
Acknowledgment & Retransmission used for reliable delivery.

Question 2

2. Services Provided by Data Link

Answer 2

Framing

Reliable Delivery

Error Detection & Correction

Flow Control

Question 3

3. Framing Techniques

Answer 3

Character Count

Flag bytes with byte stuffing

Flag bits with bit stuffing

Physical layer coding violations

Question 4

4. Error Control Mechanisms

Answer 4

Error detection: Parity bits, CRC (Cyclic Redundancy Check).

Error correction: Hamming Code.

Uses ACK/NAK and retransmission for reliability.

Question 5

5. Flow Control Methods

Answer 5

Prevents sender from overwhelming receiver.

Stop-and-Wait Protocol

Sliding Window Protocol (used in TCP/IP)

Question 6

6. Protocols

Answer 6

(a) Simplex Link Layer Protocols
One-way communication (sender → receiver).
No flow/error control – very basic.

(b) Sliding Window Protocol
Allows multiple frames in transit.
Maintains window of acceptable frames

Question 7

7. Medium Access Control (MAC) Sublayer

Answer 7

Controls access to shared media (e.g., Ethernet, Wi-Fi).
(a) Channel Allocation
+ Static
+ Dynamic
(b) Multiple Access Protocols
+ ALOHA
+ CSMA

Question 8

ALOHA

Answer 8

Simple random access protocol.

Two types: Pure ALOHA (low efficiency), Slotted ALOHA (better).

Question 9

CSMA (Carrier Sense Multiple Access):

Answer 9

Listens before transmitting.

CSMA/CD (Collision Detection) – used in Ethernet.

CSMA/CA (Collision Avoidance) – used in Wi-Fi.