1Which of the following is the primary responsibility of the Data Link Layer?
A.End-to-end delivery of packets
B.Routing of packets
C.Process-to-process delivery
D.Node-to-node delivery of frames
Correct Answer: Node-to-node delivery of frames
Explanation:
The Data Link Layer is responsible for the reliable transfer of data frames between two nodes connected by a physical medium (Node-to-node delivery).
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2In the context of framing, what is the purpose of Bit Stuffing?
A.To pad the frame to a minimum length
B.To increase the data transfer rate
C.To detect errors in the frame
D.To distinguish frame delimiters from data by adding extra bits
Correct Answer: To distinguish frame delimiters from data by adding extra bits
Explanation:
Bit stuffing involves adding an extra 0 bit after a sequence of five consecutive 1 bits to prevent the data from matching the flag pattern (usually $01111110$).
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3Which sublayer of the Data Link Layer is responsible for controlling access to the shared transmission medium?
A.Logical Link Control (LLC)
B.Physical Layer Control (PLC)
C.Network Control Protocol (NCP)
D.Media Access Control (MAC)
Correct Answer: Media Access Control (MAC)
Explanation:
The Data Link Layer is divided into two sublayers: LLC (upper) and MAC (lower). The MAC sublayer handles access to shared media.
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4In a Stop-and-Wait ARQ system, if is the propagation delay and is the transmission time, what is the link utilization efficiency?
A.
B. where
C. where
D. where
Correct Answer: where
Explanation:
The efficiency of Stop-and-Wait ARQ is given by , which simplifies to .
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5What is the Hamming distance between the codewords 10101 and 11110?
A.5
B.4
C.2
D.3
Correct Answer: 3
Explanation:
Hamming distance is the number of bit positions in which two codewords differ. . There are three 1s in the result, so the distance is 3.
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6To correct errors in a data stream, the minimum Hamming distance required in the coding scheme is:
A.
B.
C.
D.
Correct Answer:
Explanation:
To detect errors, a distance of is needed. To correct errors, a distance of is required.
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7In Cyclic Redundancy Check (CRC), if the generator polynomial is , what is the divisor in binary form?
A.1011
B.1001
C.1110
D.1101
Correct Answer: 1011
Explanation:
The polynomial corresponds to the binary sequence 1011.
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8When using a checksum for error detection, the sender calculates the sum of data segments using:
A.Floating point arithmetic
B.1's complement arithmetic
C.Modulo-2 arithmetic
D.2's complement arithmetic
Correct Answer: 1's complement arithmetic
Explanation:
In standard checksums (like Internet Checksum), segments are added using 1's complement arithmetic, and the result is complemented to form the checksum.
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9Which of the following best describes Piggybacking?
A.Sending multiple frames at once
B.Attaching an acknowledgment to a data frame going in the reverse direction
C.Retransmitting a frame immediately after a timeout
D.Using a token to control access
Correct Answer: Attaching an acknowledgment to a data frame going in the reverse direction
Explanation:
Piggybacking is the technique of delaying an acknowledgment (ACK) so that it can be attached to the next outgoing data frame.
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10In the Go-Back-N ARQ protocol, if bits are used for the sequence number, the sender window size must satisfy:
A.
B.
C.
D.
Correct Answer:
Explanation:
In Go-Back-N, the window size must be at most to avoid ambiguity between new frames and retransmissions of old frames.
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11In Selective Repeat ARQ, if the sequence number space is size , what is the maximum size of the sender window?
A.
B.
C.
D.
Correct Answer:
Explanation:
In Selective Repeat, the sender and receiver window sizes are typically equal and must satisfy . Thus, max window size is .
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12What is the standard length of a MAC address?
A.32 bits
B.64 bits
C.128 bits
D.48 bits
Correct Answer: 48 bits
Explanation:
A standard Ethernet MAC address is 48 bits (6 bytes) long, typically represented in hexadecimal format.
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13A Layer 2 Switch makes forwarding decisions based on:
A.Source MAC address
B.Destination MAC address
C.Destination IP address
D.Source IP address
Correct Answer: Destination MAC address
Explanation:
Switches look at the destination MAC address of the incoming frame to determine which port to forward the frame to.
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14Which of the following is the Broadcast MAC address?
A.127.0.0.1
B.AA:BB:CC:DD:EE:FF
C.FF:FF:FF:FF:FF:FF
D.00:00:00:00:00:00
Correct Answer: FF:FF:FF:FF:FF:FF
Explanation:
The address consisting of all 1s (FF:FF:FF:FF:FF:FF in hex) is the broadcast address, meaning the frame is intended for all stations on the LAN.
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15In Pure ALOHA, the vulnerable time (the time interval during which a collision can occur) is:
A.Indeterminate
B.
C. (Frame transmission time)
D.
Correct Answer:
Explanation:
In Pure ALOHA, a collision occurs if another station transmits within the time range , making the vulnerable time .
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16What is the maximum theoretical efficiency (throughput) of Pure ALOHA?
A.18.4%
B.36.8%
C.50%
D.100%
Correct Answer: 18.4%
Explanation:
The maximum throughput of Pure ALOHA is or 18.4%.
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17What is the maximum theoretical efficiency of Slotted ALOHA?
A.18.4%
B.36.8%
C.50%
D.80%
Correct Answer: 36.8%
Explanation:
Slotted ALOHA reduces the vulnerable time by half compared to Pure ALOHA. or 36.8%.
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18In CSMA/CD, what does the station do immediately after detecting a collision?
A.It stops transmitting and waits for a random time
B.It switches to Slotted ALOHA mode
C.It continues transmitting to ensure all nodes detect the collision (Jamming Signal)
D.It drops the packet permanently
Correct Answer: It continues transmitting to ensure all nodes detect the collision (Jamming Signal)
Explanation:
Upon detecting a collision, a station sends a brief Jamming Signal to ensure that all other stations on the segment are aware of the collision before stopping.
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19Which CSMA persistence method transmits immediately if the channel is idle, but transmits with probability ?
A.0-persistent
B.Non-persistent
C.1-persistent
D.p-persistent
Correct Answer: 1-persistent
Explanation:
In 1-persistent CSMA, if the station senses the channel is idle, it transmits immediately (probability 1). If busy, it listens continuously until idle.
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20The Binary Exponential Backoff algorithm is used in Ethernet to:
A.Assign IP addresses
B.Determine the window size for flow control
C.Calculate the CRC
D.Determine the waiting time after a collision
Correct Answer: Determine the waiting time after a collision
Explanation:
After a collision, stations wait a random amount of time determined by , where is chosen from $0$ to (Binary Exponential Backoff).
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21For CSMA/CD to work correctly, the minimum frame size must be:
A.
B.Equal to the header size
C.
D.
Correct Answer:
Explanation:
To detect a collision before finishing transmission, the transmission time must be at least twice the propagation delay ().
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22Why is CSMA/CD not typically used in Wireless LANs (802.11)?
A.Wireless signals travel faster
B.Wireless has no bandwidth limit
C.CSMA/CD is too fast for wireless
D.The Hidden Station problem makes collision detection difficult
Correct Answer: The Hidden Station problem makes collision detection difficult
Explanation:
In wireless, a sender cannot always detect a collision at the receiver (Hidden Station problem) and the signal strength of transmission overwhelms incoming collision signals.
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23Which protocol is used in Wireless LANs instead of CSMA/CD?
A.Slotted ALOHA
B.Token Ring
C.CSMA/CA (Collision Avoidance)
D.Pure ALOHA
Correct Answer: CSMA/CA (Collision Avoidance)
Explanation:
Wireless networks use CSMA/CA, which uses RTS/CTS frames and inter-frame spacing to avoid collisions rather than detecting them.
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24Which of the following is a Controlled Access protocol?
A.CSMA/CD
B.Token Passing
C.Ethernet
D.ALOHA
Correct Answer: Token Passing
Explanation:
Token Passing is a controlled access method where a station needs a token to transmit, eliminating collisions. ALOHA and CSMA are random access protocols.
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25In the Token Ring topology, what happens when a station holds the token?
A.It must forward the token immediately
B.It enters sleep mode
C.It can transmit data frames
D.It listens for collisions
Correct Answer: It can transmit data frames
Explanation:
Holding the token grants the station the exclusive right to transmit data on the ring for a specific holding time.
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26The IEEE standard for Ethernet is:
A.802.5
B.802.15
C.802.11
D.802.3
Correct Answer: 802.3
Explanation:
IEEE 802.3 defines the standards for wired Ethernet. 802.11 is Wi-Fi, 802.5 is Token Ring.
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27What line coding scheme is used in Standard Ethernet (10BaseT)?
A.Manchester Encoding
B.AMI
C.Differential Manchester
D.NRZ-L
Correct Answer: Manchester Encoding
Explanation:
Standard Ethernet uses Manchester encoding, which ensures a transition in the middle of every bit period for synchronization.
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28An Ethernet frame begins with a Preamble. What is its pattern?
A.Sequence of flag bytes
B.Alternating 1s and 0s
C.All 0s
D.All 1s
Correct Answer: Alternating 1s and 0s
Explanation:
The preamble is 7 bytes of alternating 1s and 0s ($101010...$) used to synchronize the receiver's clock.
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29Which field in the Ethernet frame follows the Preamble and signals the beginning of the frame?
A.Source MAC
B.CRC
C.SFD (Start Frame Delimiter)
D.Length
Correct Answer: SFD (Start Frame Delimiter)
Explanation:
The SFD is 1 byte ($10101011$) that immediately follows the preamble and indicates the start of the frame content.
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30What is the minimum frame length of a standard Ethernet frame?
A.128 bytes
B.32 bytes
C.64 bytes
D.46 bytes
Correct Answer: 64 bytes
Explanation:
The minimum Ethernet frame size is 64 bytes (18 bytes header/trailer + 46 bytes minimum payload) to ensure collision detection works properly.
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31If the data payload in an Ethernet frame is less than 46 bytes, what happens?
A.The CRC is omitted
B.The frame is sent as a 'runt' frame
C.The frame is discarded
D.Padding is added to make it 46 bytes
Correct Answer: Padding is added to make it 46 bytes
Explanation:
The data field must be at least 46 bytes. If the actual data is less, padding (extra 0s) is added.
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32The CRC in an Ethernet frame is located in the:
A.Header
B.Type field
C.Frame Check Sequence (FCS) field at the end
D.Preamble
Correct Answer: Frame Check Sequence (FCS) field at the end
Explanation:
The 4-byte CRC is stored in the Frame Check Sequence (FCS) field at the tail of the frame.
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33In 100Base-TX Ethernet, the '100' stands for:
A.100 meters maximum length
B.100 Mbps transmission speed
C.100 nodes maximum
D.100 MHz frequency
Correct Answer: 100 Mbps transmission speed
Explanation:
The number indicates the data rate in Mbps. 100Base-TX is Fast Ethernet running at 100 Mbps.
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34Which component splits a collision domain into multiple smaller collision domains?
A.Hub
B.Switch
C.Repeater
D.Cable
Correct Answer: Switch
Explanation:
A Hub broadcasts data to all ports, creating one large collision domain. A Switch isolates ports, creating a separate collision domain for each port.
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35What is p-persistent CSMA?
A.Station transmits immediately if idle
B.Station transmits with probability if idle, waits with probability
C.Station never transmits
D.Station transmits with probability 1 if idle
Correct Answer: Station transmits with probability if idle, waits with probability
Explanation:
In p-persistent CSMA (used in slotted channels), if the channel is idle, the station transmits with probability and defers to the next slot with probability .
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36Which Hamming code parity configuration (Even/Odd) ensures that the total number of 1s in the code is even?
A.Odd Parity
B.Zero Parity
C.CRC Parity
D.Even Parity
Correct Answer: Even Parity
Explanation:
Even Parity adds a bit such that the total count of 1s in the data plus the parity bit is an even number.
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37Calculate the Hamming code redundancy bits () required for a 7-bit data word (). (Using )
A.2
B.3
C.4
D.5
Correct Answer: 4
Explanation:
Using : If , (False). If , (True). So, 4 bits are needed.
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38What is a Burst Error?
A.A single bit flip
B.Error caused by router congestion
C.Two or more bits in the data unit have changed
D.Error in the header only
Correct Answer: Two or more bits in the data unit have changed
Explanation:
A burst error means that 2 or more bits in the data unit have changed from 1 to 0 or from 0 to 1.
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39The divisor in a CRC generator is always:
A.One bit more than the degree of the generator polynomial
B.One bit less than the generator polynomial degree
C.The same size as the data
D.Always 16 bits
Correct Answer: One bit more than the degree of the generator polynomial
Explanation:
If the polynomial degree is (e.g., ), the divisor will have bits.
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40In a switch, the Address Learning process involves:
A.Manually typing IP addresses
B.Sending a broadcast to ask for names
C.Reading the source MAC of incoming frames and mapping it to the port
D.Reading the destination MAC and storing it in the table
Correct Answer: Reading the source MAC of incoming frames and mapping it to the port
Explanation:
Switches learn network topology by examining the Source MAC address of every frame received and recording which port it came from.
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41Which mechanism is used for Flow Control in the Data Link Layer?
A.Manchester Encoding
B.Hamming Code
C.CRC
D.Sliding Window Protocol
Correct Answer: Sliding Window Protocol
Explanation:
Flow control prevents the sender from overwhelming the receiver. Sliding Window (and Stop-and-Wait) are mechanisms to achieve this.
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42What is the result of in Modulo-2 arithmetic?
A.1111
B.0101
C.0011
D.0100
Correct Answer: 0100
Explanation:
Modulo-2 addition is equivalent to XOR. . Result: $0100$.
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43In Polling (Controlled Access), who controls the link access?
A.The station with the highest IP
B.The station with the token
C.The Primary Station
D.Any station
Correct Answer: The Primary Station
Explanation:
In Polling, a Primary device controls the link and polls Secondary devices to see if they have data to send.
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44Gigabit Ethernet (1000Base-T) supports a data rate of:
A.10 Gbps
B.100 Mbps
C.10 Mbps
D.1000 Mbps
Correct Answer: 1000 Mbps
Explanation:
Gigabit Ethernet transmits at 1 Gigabit per second, which is 1000 Mbps.
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45What is the purpose of the Inter-Frame Gap in Ethernet?
A.To increase the data rate
B.To stuff bits
C.To allow the receiver to process the previous frame and prepare for the next
D.To detect collisions
Correct Answer: To allow the receiver to process the previous frame and prepare for the next
Explanation:
Ethernet requires a minimum idle time (96 bit times) between frames to give network interfaces time to recover.
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46Which of the following is NOT a Random Access Protocol?
A.CSMA/CA
B.ALOHA
C.CSMA/CD
D.Reservation
Correct Answer: Reservation
Explanation:
Reservation is a Controlled Access protocol. The others are Random Access (contention-based).
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47In Byte Stuffing, if the flag byte appears in the data, what is inserted before it?
A.A parity bit
B.A zero bit
C.Another flag byte
D.An Escape (ESC) byte
Correct Answer: An Escape (ESC) byte
Explanation:
Byte stuffing uses a special Escape byte to indicate that the following character is data, not a frame delimiter.
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48The Maximum Transmission Unit (MTU) for standard Ethernet is typically:
A.1500 bytes
B.64 bytes
C.1518 bytes
D.4096 bytes
Correct Answer: 1500 bytes
Explanation:
The maximum payload (data field) for a standard Ethernet frame is 1500 bytes.
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49In 2D Parity check, errors can be detected:
A.Only if there is a single bit error
B.In both rows and columns
C.Only in rows
D.Only in columns
Correct Answer: In both rows and columns
Explanation:
2D Parity organizes data into a table and calculates parity for both rows and columns, improving detection capabilities for burst errors.
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50What happens if a switch receives a frame with a destination MAC address that is not in its forwarding table?
A.It sends an ARP request
B.It sends it back to the sender
C.It floods the frame out of all ports except the receiving one
D.It drops the frame
Correct Answer: It floods the frame out of all ports except the receiving one
Explanation:
This is called 'Unknown Unicast Flooding'. The switch broadcasts the frame to find the location of the destination device.