UNIT 3 DCN

You said:

ANSWER THESE QUESTIONS SEPARATELY IN DETAIL"Q1. Explain Framing, Flow control, Error
control in data link layer?
Q2. Explain High Level Data Link Control (HDLC), it’s configurations, Frame formats?
Q3. Explain ARQ methods: transmission efficiency, Piggybacking?
Q4. Explain in detail Medium Access Control Protocols: ALOHA, Slotted ALOHA, CSMA,
CSMA/CD? [10 marks]
Q5. Compare Pure ALOHA, Slotted ALOHA, CSMA, and CSMA/CD in terms of efficiency
and use cases.
Q6. Explain High Level Data Link Control (HDLC), bit stuffing and de-stuffing, Typical frame
exchanges?"

ChatGPT said:

Q1. Framing, Flow Control, Error Control in Data Link Layer

1. Framing: Framing is a function of the Data Link Layer that packages the data received from the
Network Layer into manageable units called frames. It adds necessary headers and trailers
around the data to create a frame that includes the data payload, source, destination addresses,
error-checking information, and control data. The main purpose of framing is to differentiate
between individual data units, ensuring that the start and end of each frame are clearly
identifiable. Framing methods include byte-oriented protocols (e.g., PPP), bit-oriented protocols
(e.g., HDLC), and character-count framing.
2. Flow Control: Flow control is the technique used to manage the rate of data transmission
between two nodes to prevent a fast sender from overwhelming a slow receiver. This is crucial to
avoid data loss and ensure efficient communication. Flow control methods include:
Stop-and-Wait: The sender transmits one frame at a time and waits for an
acknowledgment from the receiver before sending the next frame.
Sliding Window: This technique allows multiple frames to be sent before requiring an
acknowledgment. It uses a window size that determines how many frames can be sent at a
time, increasing transmission efficiency by utilizing available bandwidth effectively.
3. Error Control: Error control ensures that data is transmitted accurately between two nodes. It
involves detecting and correcting errors that occur during transmission. The common error
control mechanisms are:
Error Detection: Techniques like Cyclic Redundancy Check (CRC) and Parity Check are used
to detect errors in frames.
Error Correction: Mechanisms like Automatic Repeat Request (ARQ) and Forward Error
Correction (FEC) are used to correct errors. ARQ, for example, involves retransmitting
corrupted frames.

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 Q2. High Level Data Link Control (HDLC), Configurations, Frame Formats

1. High-Level Data Link Control (HDLC): HDLC is a bit-oriented synchronous data link layer
protocol developed by the International Organization for Standardization (ISO). It provides both
connection-oriented and connectionless services. HDLC is widely used due to its robustness,
error detection capabilities, and efficiency in handling data frames.
2. Configurations: HDLC can operate in three modes of configuration:
Normal Response Mode (NRM): A primary station controls communication, and secondary
stations can only transmit data in response to requests from the primary station.
Asynchronous Balanced Mode (ABM): Both stations can send and receive data
simultaneously, allowing for a balanced and flexible communication mode.
Asynchronous Response Mode (ARM): Similar to NRM, but the secondary stations can
initiate communication without waiting for permission from the primary station.
3. Frame Formats: HDLC uses three types of frames:
Information (I-frames): Used to carry user data and control information related to user
data. I-frames contain sequence numbers for flow and error control.
Supervisory (S-frames): Used for error and flow control purposes, such as
acknowledgment, request for retransmission, and flow control commands.
Unnumbered (U-frames): Used for control purposes, such as establishing and terminating
connections, and performing other management functions.

Q3. ARQ Methods: Transmission Efficiency, Piggybacking

1. Automatic Repeat Request (ARQ) Methods: ARQ methods are error-control strategies used to
ensure reliable data transmission over a network. The primary ARQ methods include:
Stop-and-Wait ARQ: The sender transmits one frame and waits for an acknowledgment
before sending the next frame. This method is simple but has low transmission efficiency
due to waiting times.
Go-Back-N ARQ: The sender can transmit several frames (up to a specified window size)
before needing an acknowledgment. If an error is detected, the sender retransmits all
frames from the erroneous one onwards.
Selective Repeat ARQ: Similar to Go-Back-N but only the erroneous frames are
retransmitted, improving transmission efficiency.
2. Transmission Efficiency: Transmission efficiency in ARQ refers to the ratio of successfully
transmitted frames to the total frames sent (including retransmissions). Selective Repeat ARQ
generally offers higher transmission efficiency compared to Stop-and-Wait and Go-Back-N ARQ
due to its targeted retransmission of only corrupted frames.
3. Piggybacking: Piggybacking is a technique used to improve transmission efficiency in duplex
communication. Instead of sending an acknowledgment (ACK) frame separately, the
acknowledgment is included with the data frame traveling in the opposite direction. This reduces
the number of frames sent and makes communication more efficient.

Q4. Medium Access Control Protocols: ALOHA, Slotted ALOHA, CSMA, CSMA/CD

1. ALOHA: ALOHA is a simple, random-access protocol used for wireless communication where
stations transmit data whenever they have data to send. It operates in a decentralized manner
without a central coordinator. The main drawback is the high possibility of collisions, leading to a
low maximum throughput of about 18.4%.

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 2. Slotted ALOHA: An improvement over pure ALOHA, Slotted ALOHA divides time into discrete
slots. Stations are allowed to transmit only at the beginning of a time slot, reducing the
likelihood of collisions. This results in a maximum throughput of 36.8%.
3. Carrier Sense Multiple Access (CSMA): CSMA is a protocol that listens to the channel before
transmitting data. If the channel is idle, the station sends data; if it is busy, the station waits.
Variants of CSMA include:
1-persistent CSMA: Continuously senses the channel and transmits immediately if idle.
Non-persistent CSMA: Senses the channel and, if busy, waits for a random period before
rechecking.
p-persistent CSMA: Used in slotted channels; transmits with a probability p when the
channel is idle.
4. CSMA/CD (Carrier Sense Multiple Access with Collision Detection): CSMA/CD is an
enhancement of CSMA used in wired Ethernet networks. It detects collisions during transmission
and stops sending, reducing network congestion. The station waits for a random backoff period
before attempting to retransmit. CSMA/CD is more efficient in a wired environment where
collision detection is easier.

Q5. Comparison of Pure ALOHA, Slotted ALOHA, CSMA, and CSMA/CD

Protocol Efficiency Use Cases

Low efficiency (max
Pure ALOHA Used in low-traffic, low-power wireless networks.
throughput ~18.4%)
Slotted Moderate efficiency (max Used in systems with moderate traffic and synchronized time
ALOHA throughput ~36.8%) slots (e.g., satellite communication).
Higher efficiency than ALOHA Used in wireless networks, LANs where collisions need to be
CSMA
(~60-90%) minimized.
High efficiency in wired Used in wired Ethernet networks to detect and handle
CSMA/CD
networks (~90%) collisions.

Q6. High Level Data Link Control (HDLC), Bit Stuffing and De-stuffing, Typical
Frame Exchanges

1. HDLC Overview: HDLC is a bit-oriented protocol that uses bit stuffing to ensure data
transparency and prevent flag sequences from appearing within the frame payload.
2. Bit Stuffing and De-Stuffing:
Bit Stuffing: To avoid confusion with the frame delimiter (flag sequence '01111110'), HDLC
inserts an extra '0' after any sequence of five consecutive '1's in the data. This prevents
accidental detection of the flag sequence within the data payload.
Bit De-Stuffing: The receiver monitors the incoming bit stream and removes any '0' that
follows five consecutive '1's, restoring the original data stream.
3. Typical Frame Exchanges: HDLC frame exchange involves the following steps:
Connection Establishment: The primary station sends a Set Asynchronous Balanced Mode
(SABM) command (U-frame) to establish a link.
Data Transfer: Information frames (I-frames) are exchanged between stations, with
sequence numbers for tracking.
Acknowledgment and Flow Control: Supervisory frames (S-frames) are used for
acknowledgment and flow control.
Connection Termination: A Disconnect (DISC) frame (U-frame) is sent to terminate the
connection.
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 ChatGPT can make mistakes. Check important info.

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