Topic in Computer Networks & Cloud Computing
The data link layer is layer 2 of the OSI model, responsible for framing bits, physical (MAC) addressing, and error and flow control across a single link. Practice items test error detection with parity bits, checksums and CRC, and correction with Hamming codes. Flow-control questions cover stop-and-wait, Go-Back-N and Selective Repeat sliding windows, including window size limits. Medium access topics include ALOHA, CSMA/CD with binary exponential backoff, the 64-byte minimum Ethernet frame and why it depends on propagation delay, plus frame headers and trailers, ARP and RARP, and the split between the LLC and MAC sublayers.
Below are 30 practice questions from a pool of 210 Data Link Layer MCQs, one of 10 topics in Computer Networks & Cloud Computing. Each shows the correct answer with an explanation; when you are ready, take a timed quiz to test recall under exam conditions.
Each question below shows the correct answer with a full explanation. Use these to build conceptual understanding before attempting a timed quiz.
Q1. What is a MAC address?
A MAC (Media Access Control) address is a 48-bit physical address burned into the network interface card (NIC) by the manufacturer.
Q2. Which error detection method appends a calculated value at the end of the data?
All three methods (parity check, CRC, and Hamming code) append redundant bits at the end of data for error detection or correction.
Q3. How many bits are in a MAC address?
A MAC address is 48 bits (6 bytes) long, typically represented as six pairs of hexadecimal digits (e.g., AA:BB:CC:DD:EE:FF).
Q4. What is the main function of the data link layer?
The data link layer is responsible for node-to-node (hop-to-hop) delivery of frames between directly connected devices.
Q5. What does CSMA/CD stand for?
CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. It is the access method used in traditional Ethernet networks.
Q6. Which sublayer of the data link layer controls access to the shared medium?
The MAC sublayer controls how devices share access to the transmission medium. It defines protocols like CSMA/CD and token passing.
Q7. What is framing in the data link layer?
Framing is the process of dividing a stream of bits from the network layer into manageable data units called frames.
Q8. Simple parity check can detect which type of errors?
Simple (single-bit) parity check can only detect an odd number of bit errors. If an even number of bits are flipped, the error goes undetected.
Q9. Which protocol is used to resolve an IP address to a MAC address?
ARP (Address Resolution Protocol) is used to map a known IP address to a MAC address on a local network.
Q10. In Ethernet, what happens when a collision is detected?
When a collision is detected in CSMA/CD Ethernet, the devices stop transmitting, send a jam signal, and wait a random time (backoff) before retrying.
Q11. If the generator polynomial for CRC is x^3 + 1, what is the binary divisor?
The polynomial x^3 + 1 = x^3 + x^0 corresponds to binary 1001 (bit positions 3 and 0 are set).
Q12. In Selective Repeat ARQ, what is the maximum window size if the sequence number has m bits?
In Selective Repeat, the maximum window size for both sender and receiver is 2^(m-1) to avoid ambiguity between new and retransmitted frames.
Q13. Which access method does IEEE 802.11 (Wi-Fi) use?
IEEE 802.11 uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) because collision detection is impractical in wireless networks.
Q14. What is the minimum frame size in standard Ethernet (IEEE 802.3)?
The minimum Ethernet frame size is 64 bytes (including header and trailer). This ensures that collisions are detected before the entire frame is transmitted.
Q15. What is the purpose of bit stuffing in HDLC?
Bit stuffing inserts a 0 after five consecutive 1s in the data to prevent the flag pattern (01111110) from accidentally appearing within the frame content.
Q16. In the Go-Back-N ARQ protocol, what happens when an error is detected in frame n?
In Go-Back-N, when an error is detected in frame n, the receiver discards all subsequent frames, and the sender retransmits frame n and all following frames.
Q17. What is a VLAN (Virtual LAN)?
A VLAN is a logical grouping of network devices that functions as a separate LAN segment, regardless of the physical location of the devices.
Q18. What is the window size at the receiver in the Go-Back-N protocol?
In Go-Back-N, the receiver window size is always 1, meaning it can only accept frames in order. Out-of-order frames are discarded.
Q19. What is the purpose of the LLC sublayer?
The LLC (Logical Link Control) sublayer provides flow control and error control services. It also provides multiplexing for network layer protocols.
Q20. In the Hamming code, how many redundancy bits are needed for a 4-bit data word?
Using the formula 2^r >= m + r + 1 where m=4: 2^3 = 8 >= 4 + 3 + 1 = 8. So 3 redundancy bits are needed.
Q21. What is the exponential backoff algorithm used for in Ethernet?
The binary exponential backoff algorithm determines how long a station should wait before retransmitting after a collision. The wait time increases exponentially with each successive collision.
Q22. What is the purpose of the IEEE 802.1Q tag in Ethernet frames?
IEEE 802.1Q adds a 4-byte tag to Ethernet frames to identify which VLAN the frame belongs to. The tag includes a 12-bit VLAN ID field.
Q23. What is the difference between persistent and non-persistent CSMA?
In persistent CSMA, a station continuously senses the channel and transmits immediately when idle. In non-persistent CSMA, a station waits a random interval before sensing again if the channel is busy.
Q24. What is the Hamming distance needed to correct t errors?
To correct t errors, the minimum Hamming distance must be 2t + 1. This ensures that even with t bit errors, the received codeword is closer to the original than any other valid codeword.
Q25. In the PPP (Point-to-Point Protocol) frame, what is the role of the LCP (Link Control Protocol)?
LCP is responsible for establishing, configuring, maintaining, and terminating the point-to-point link. It handles authentication, compression negotiation, and link quality monitoring.
Q26. In Ethernet, why is there a minimum frame size requirement of 64 bytes?
The minimum frame size ensures that a frame is still being transmitted when the collision signal returns from the farthest point on the network (collision domain). This is based on the round-trip propagation delay.
Q27. In the HDLC protocol, what type of frame is used for flow and error control without carrying any data?
S-frames (Supervisory frames) in HDLC are used for flow and error control. They carry ACK, NAK, and flow control information but no user data.
Q28. What is the maximum number of VLANs supported by IEEE 802.1Q?
IEEE 802.1Q uses a 12-bit VLAN ID field, supporting 2^12 = 4096 VLANs. VLAN 0 and 4095 are reserved, leaving 4094 usable VLANs.
Q29. CRC-32 used in Ethernet can detect all burst errors of length up to how many bits?
CRC-32 can detect all burst errors of length 32 or fewer bits. It can also detect most burst errors longer than 32 bits with very high probability.
Q30. In a sliding window protocol with a window size of 7 and sequence numbers 0-7, what is the maximum sender window size in Go-Back-N?
In Go-Back-N with m-bit sequence numbers (2^m = 8 here), the maximum sender window size is 2^m - 1 = 7 to prevent ambiguity.
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