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Computer Networks · Question Set
Data Link Layer & Error Control interview questions for placements and exams.
Questions
13
Included in this set
Subject
Computer Networks
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Difficulty
Mixed
Level of this set
Go through each question and its explanation. Use this set as a focused practice pack for Computer Networks.
The Media Access Control (MAC) address is a 48-bit identifier hard-coded into a network card. It uniquely identifies a device on a LAN. MAC addresses enable the Data Link Layer to deliver frames between local devices.
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A parity bit is an extra bit added to make the total number of 1s even or odd. If the received data does not match the expected parity rule, an error is detected. Though it cannot correct errors, parity checking provides a quick and inexpensive error-detection method.
The Data Link Layer ensures reliable transfer of frames between two nodes connected by a physical link. It groups bits into frames, adds headers and trailers, detects errors, and sometimes handles retransmission. This layer bridges the gap between the raw transmission of bits and structured network communication.
Framing is the process of dividing a stream of bits into manageable units called frames. Each frame carries synchronization information, addressing fields, and error-control bits. Without framing, the receiver cannot determine where one message ends and the next begins. Framing also helps in detecting transmission errors and ensures proper sequencing of data. Common techniques include byte count, flag bytes, and bit stuffing.
Cyclic Redundancy Check (CRC) uses polynomial division to detect errors in data frames. It is powerful and widely used in Ethernet and storage systems. The sender appends a computed CRC value to the frame; the receiver recomputes it and checks for mismatches.
Flow control ensures that a fast sender does not overwhelm a slower receiver with too much data. It uses mechanisms like Stop-and-Wait and Sliding Window to coordinate data flow. Proper flow control avoids data loss and buffer overflow.
Stop-and-Wait Automatic Repeat Request ensures reliability by waiting for acknowledgment of each frame before sending the next. If the acknowledgment is not received within a timeout, the sender retransmits the frame. This simplicity makes it reliable but inefficient for high-latency links.
The Sliding Window Protocol allows multiple frames to be sent before needing an acknowledgment, increasing channel utilization. The sender maintains a window of unacknowledged frames, and the receiver acknowledges as frames arrive, sliding the window forward. This reduces idle time and maximizes throughput, especially in long-delay networks. Variants include Go-Back-N and Selective Repeat ARQ.
In Go-Back-N, if a frame or acknowledgment is lost, the sender goes back and retransmits that frame and all subsequent ones. It’s simpler to implement but less efficient than Selective Repeat, which retransmits only the missing frames.
Selective Repeat ARQ retransmits only the specific frames that are lost or corrupted, not the entire sequence. Both sender and receiver maintain sliding windows and store out-of-order frames. This reduces redundant retransmissions, improving bandwidth usage. However, it requires more memory and complex synchronization to manage multiple acknowledgments.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) lets devices sense the channel before transmitting and detect collisions if they occur. If a collision is detected, transmission stops, and devices wait a random backoff time before retrying. It was key in early Ethernet but is largely obsolete in switched networks today.
Hamming Code uses redundant parity bits placed at specific positions in a binary data block to both detect and correct single-bit errors. Each parity bit covers certain data bits based on binary positions. When the receiver recomputes parity, a mismatch indicates which bit is corrupted. The bit is flipped to recover the correct data. Hamming Code is widely used in memory systems and demonstrates a clever trade-off between redundancy and reliability.
The Data Link Layer has two sublayers: the Logical Link Control (LLC) and the Media Access Control (MAC) sublayer. The LLC handles flow control, error detection, and logical addressing to interface with higher layers. The MAC sublayer controls access to the physical transmission medium, handles frame delimiting, and uses MAC addresses for local delivery. This division allows flexible adaptation to different physical media and access protocols.