Ch4 Short Answer Questions 1: What Does The Data Link Layer

Ch4short Answer Questions1 What Does The Data Link Layer Do What

Ch4 Short Answer Questions:

1. What does the data link layer do? What are its primary responsibilities? Where does the data link layer sit in terms of the simplified five layer network model?
2. What is media access control and why is it important? What are two examples of controlled access methods and contention based media access methods? When might one access method be preferred over another in a network, and why? Under what conditions do contention-based media access control techniques outperform controlled-access techniques (i.e., have lower response time)? Explain.
3. Compare and contrast roll call polling, hub polling (or token passing), and contention. Which is better, hub polling or contention? Explain.
4. Errors normally appear in ______________________________, which is when more than one data bit is changed by the error-causing condition. Is there any difference in the error rates of lower speed lines and of higher speed lines? What kinds of lines are more prone to errors?
5. Briefly define noise. Describe five types of noise and the underlying causes of this noise. Which type of noise is likely to pose the greatest problem to network managers? What does error look like in a data network?
6. How do amplifiers differ from repeaters?
7. What are three ways of reducing errors and the types of noise they affect?
8. Describe three approaches to detecting errors, including how they work, the probability of detecting an error, and any other benefits or limitations.
9. Briefly describe how even parity and odd parity work. Give an example of even parity with a 7-bit ASCII code, for the following: , using a 0 start bit and a 1 stop bit.
10. How does cyclical redundancy checking work?
11. How does forward error correction work? How is it different from other error correction methods? Under what circumstances is forward error correction desirable? What is one type of forward error correction by name? Given a diagram like that in the text of forward error correction, be able to explain how the system works.
12. Compare and contrast stop-and-wait ARQ and continuous ARQ.
13. Describe from a high level perspective the packet layout for SDLC, Ethernet, and PPP.
14. What is transparency?
15. What is transmission efficiency? How do information bits differ from overhead bits? What are three issues which might affect packet throughput rates? Which is better for file transfer, large packet sizes or small packet sizes?
16. What is asynchronous transmission v. synchronous transmission? Describe one protocol of each type.
17. Think about a recent world impact event and how useful the technologies of communications might have been related to helping people impacted by that event (Hurricane Katrina, Pakistan earthquakes, or some other large world event). How might telecom have a positive role in large scale human events? Use insight gained from the technological underpinnings provided in the course. Be creative.

Sample Paper For Above instruction

The data link layer plays a crucial role in computer networks by providing node-to-node data transfer, error detection and correction, and media access control. It operates immediately above the physical layer and is responsible for framing data packets, managing access to shared media, and ensuring error-free transmission between directly connected devices. Located in the second layer of the OSI model, the data link layer acts as a safeguard against transmission errors and facilitates reliable communication across physical links.

Media access control (MAC) is fundamental because it determines how devices on a shared communication medium coordinate access to prevent collisions and ensure efficient data transfer. Two common controlled access methods are Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA). Contention-based methods include Carrier Sense Multiple Access with Collision Detection (CSMA/CD) and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). In networks with predictable traffic patterns and critical latency requirements, controlled access methods are preferred. Conversely, contention-based methods outperform controlled ones in highly dynamic environments where traffic loads fluctuate, as they tend to have lower response times by allowing devices to transmit as soon as the medium becomes available.

Comparing roll call polling, hub polling, and contention reveals differing efficiencies and complexities. Roll call polling involves a central controller querying each device in turn, which can be efficient under certain conditions but may introduce delays. Hub polling or token passing involves passing a token among devices, granting permission to transmit, reducing collisions, and increasing efficiency. Contention allows devices to transmit whenever they detect the medium is free, risking collisions but simplifying access. Generally, token passing is considered more efficient than pure contention, especially in high-traffic networks, as it minimizes collisions and retransmissions.

Error commonly appears in burst errors, where multiple bits are corrupted simultaneously due to specific disturbance events. Research shows higher speed lines tend to experience fewer errors due to better shielding and shielding measures, but lines in noisy environments, such as wireless links, are more prone to errors. Noise refers to unwanted disturbances in signal transmission. Types of noise include thermal noise caused by electron movement, intermodulation noise from interference among different signals, crosstalk between parallel lines, impulse noise from electromagnetic interference, and background noise from natural sources. Impulse noise often poses the greatest challenge because it can cause significant data corruption in short bursts, complicating error detection and correction.

Amplifiers differ from repeaters primarily in their function: amplifiers increase the strength of a signal without regenerating it, leading to amplified noise, whereas repeaters regenerate the original signal, restoring it to its original form and reducing noise accumulation. Error reduction can be achieved through techniques like shielding, proper grounding, and using high-quality transmission media—all of which combat different noise types. Error detection strategies include parity checks, checksum, and cyclic redundancy check (CRC). Parity methods add a parity bit to detect odd or even number of bits in error. CRC uses polynomial division to encode data and detect errors with high probability, although it does not correct errors.

Even parity involves adding a parity bit to ensure the total number of 1-bits in a data word is even, while odd parity ensures an odd total. For example, in an 8-bit ASCII code, if the data bits have an odd number of 1s, an additional parity bit is set to 1 in even parity schemes, making the total count even. Cyclical redundancy checking (CRC) employs polynomial division of the message bits, appending a checksum at the end. It detects common transmission errors with high efficiency.

Forward error correction (FEC) involves adding redundant data to transmitted messages so that errors can be detected and corrected at the receiver without the need for retransmission. Unlike ARQ protocols which request retransmissions upon error detection, FEC anticipates errors and corrects them proactively. It is especially useful in situations with high latency or unreliable channels like satellite communications. Reed-Solomon codes are one common FEC technique, offering robust correction capabilities for burst errors.

Stop-and-wait ARQ involves sending a data frame and waiting for an acknowledgment before transmitting the next, which can be inefficient over long distances. Continuous ARQ allows multiple frames to be in transit simultaneously, improving throughput by pipelining data. The packet layout differs for SDLC, Ethernet, and PPP, with each protocol defining specific fields for addressing, control, and error checking. Transparency in data transmission refers to methods that allow special control characters to be transmitted without misinterpretation, often via stuffing techniques.

Transmission efficiency reflects the ratio of useful data bits to total bits sent, including overhead. Higher efficiency means less bandwidth wasted on protocol overhead. Smaller packet sizes increase the probability of error per packet but reduce retransmission costs, while larger packets improve throughput but are more susceptible to errors. Asynchronous transmission sends data in irregular intervals with start and stop bits, suitable for low data rates, while synchronous transmission employs continuous streams synchronized by a clock signal for high-speed data transfer. Protocols exemplify each method, such as RS-232 for asynchronous and Ethernet for synchronous.

In recent natural disasters like earthquakes or hurricanes, communication technologies have played vital roles. For example, satellite phones and wireless networks provided critical communication links when traditional infrastructure was damaged. Telecom networks help coordinate rescue efforts, deliver real-time updates, and facilitate aid distribution on a large scale, demonstrating the importance of advanced communication systems in disaster response and humanitarian aid.