Section A — 1 Mark Questions
1. Define the physical layer of the OSI model. [1 Mark]
Answer: The physical layer is the first layer of the OSI model, responsible for the transmission and reception of raw bit streams over a physical communication medium.
2. What is the main function of the physical layer? [1 Mark]
Answer: Its primary function is to transmit a stream of bits over a physical medium.
3. Name a common type of guided transmission medium. [1 Mark]
Answer: Twisted-pair cable.
4. Identify a device that operates at the physical layer. [1 Mark]
Answer: A Hub.
5. What does an unguided transmission medium use for signal propagation? [1 Mark]
Answer: Electromagnetic waves in free space or air.
6. Define a repeater. [1 Mark]
Answer: A repeater is a physical layer device that regenerates and strengthens a signal to travel longer distances.
7. What is the physical connector used for unshielded twisted-pair (UTP) cables? [1 Mark]
Answer: RJ-45 connector.
8. State one advantage of fiber optic cables over copper cables. [1 Mark]
Answer: Fiber optic cables offer higher bandwidth and are immune to electromagnetic interference.
9. What is the core component of a fiber optic cable? [1 Mark]
Answer: Glass or plastic core.
10. What does a coaxial cable consist of? [1 Mark]
Answer: A central conductor surrounded by an insulating layer, a metallic shield, and an outer jacket.
11. Define bandwidth in the context of physical media. [1 Mark]
Answer: Bandwidth is the capacity of a transmission medium to carry data, measured in bits per second or Hertz.
12. What is the primary purpose of a modem? [1 Mark]
Answer: To modulate digital signals into analog signals and vice versa for transmission over telephone lines.
13. Name a form of wireless transmission. [1 Mark]
Answer: Radio waves.
14. What is physical layer interface? [1 Mark]
Answer: The mechanical and electrical specifications for connecting devices to the transmission medium.
15. What type of signal does fiber optic transmission use? [1 Mark]
Answer: Light signals.
16. What is crosstalk? [1 Mark]
Answer: The unwanted coupling of signals between adjacent communication lines.
17. How does a hub handle incoming data? [1 Mark]
Answer: It broadcasts the incoming frame to all connected ports.
18. What is the purpose of shielding in a cable? [1 Mark]
Answer: To protect the signal from external electromagnetic interference.
19. Name a common wireless network standard. [1 Mark]
Answer: IEEE 802.11 (Wi-Fi).
20. What is the role of a physical layer protocol? [1 Mark]
Answer: To define the rules for data transmission over the physical channel.
Section B — 2/3 Mark Questions
1. Differentiate between guided and unguided media. [2 Marks]
Answer: Guided media uses physical paths like cables (wire) to conduct signals, while unguided media uses electromagnetic waves through free space (air, vacuum) without physical conductors.
2. List two primary services provided by the physical layer. [2 Marks]
Answer: 1) Physical characteristics of interfaces and medium. 2) Representation of bits as electrical or optical signals.
3. Explain the role of a repeater in a network. [3 Marks]
Answer: A repeater receives a digital signal, regenerates it to its original strength and shape, and retransmits it. This overcomes signal attenuation over long distances, allowing the signal to travel further than it could on its own.
4. Compare coaxial cable and twisted-pair cable. [3 Marks]
Answer: Coaxial cable has a higher bandwidth and better noise immunity due to its shielding compared to twisted-pair. Twisted-pair is cheaper and easier to install, making it the standard for local area networks, whereas coaxial is often used for long-distance broadband.
5. What are the components of a fiber optic cable? [2 Marks]
Answer: It consists of a light-conducting core, a cladding layer with a lower refractive index to keep light inside, and a protective outer jacket.
6. Describe the working of a hub. [3 Marks]
Answer: A hub is a multi-port connector device. When a signal arrives at one port, the hub replicates the signal bit-by-bit and broadcasts it to all other active ports in the network segment. It lacks intelligence and does not perform filtering.
7. Why is fiber optic transmission immune to EMI? [2 Marks]
Answer: Because it transmits data as pulses of light rather than electrical currents, it is not affected by electromagnetic fields created by power lines or electronic devices.
8. What are the primary advantages of wireless media? [3 Marks]
Answer: 1) Mobility: devices can connect from anywhere within range. 2) Ease of deployment: no cables need to be run through walls. 3) Scalability: easy to add new users without hardware reconfiguration.
9. What is the function of the physical interface? [2 Marks]
Answer: It defines the mechanical aspects (plugs, pins) and electrical aspects (voltage levels, timing of signals) required for a physical connection between devices.
10. Explain attenuation in transmission media. [2 Marks]
Answer: Attenuation is the loss of signal strength as it travels through a medium, which restricts the distance over which data can be successfully transmitted without signal amplification.
Section C — 5 Mark Questions
1. Explain the concept of the physical layer and its main responsibilities in detail. [5 Marks]
Introduction
The physical layer is the lowest level of the OSI model, acting as the foundation for all network communications. It deals with the mechanical, electrical, and procedural interfaces of the physical transmission medium.
Main Responsibilities
- Physical Characteristics of Interfaces: It defines the physical properties of the interface, such as the shape of the connector and the number of pins required.
- Representation of Bits: It defines the coding scheme (how 0s and 1s are represented) using voltage levels or light pulses.
- Data Rate: It defines the transmission rate (number of bits sent per second).
- Synchronization: It ensures that the sender and receiver are synchronized by providing clocking information.
- Transmission Mode: It defines the direction of transmission, such as simplex, half-duplex, or full-duplex.
2. Compare and contrast wired (guided) transmission media. [5 Marks]
Overview
Wired media guide signals along a physical path. The three main types are twisted-pair, coaxial, and fiber optic.
Comparison Table
| Medium | Bandwidth | Interference Immunity | Installation |
|---|---|---|---|
| Twisted-pair | Moderate | Low | Easy |
| Coaxial | High | High | Moderate |
| Fiber Optic | Very High | Excellent | Difficult |
Details
- Twisted-pair: Consists of pairs of insulated copper wires twisted together to reduce crosstalk. Common in office LANs.
- Coaxial: Uses a central conductor and metallic shield, suitable for high-frequency transmissions like cable TV.
- Fiber Optic: Uses light to carry data; provides the fastest and most secure transmission over long distances.
3. Describe the features and limitations of wireless (unguided) media. [5 Marks]
Introduction
Wireless communication uses the atmosphere or vacuum to carry signals. Unlike wired media, it does not rely on a fixed cable path.
Key Features
- Flexibility: Allows for mobile communication and connectivity in environments where laying cables is physically impossible or costly.
- Broadcasting: Data can be sent to multiple receivers simultaneously within range (e.g., satellite broadcasting).
- Standards: Relies on standardized frequency bands (like 2.4 GHz or 5 GHz) to manage spectrum sharing.
Limitations
- Interference: Highly susceptible to environmental conditions like rain, atmospheric noise, and other radio frequency signals.
- Security: Being broadcasted, wireless signals are inherently easier to intercept compared to closed-circuit wired media.
- Path Loss: Signals degrade rapidly over distance, requiring repeaters or high-power antennas for long-distance coverage.
4. Discuss the networking devices that operate at the physical layer, specifically focusing on hubs and repeaters. [5 Marks]
Introduction
Physical layer devices are essential for signal management and segment interconnection. They perform simple operations on the raw electrical signal without inspecting the data content (like frames or packets).
Repeaters
A repeater receives a signal that has weakened due to attenuation and retransmits it at its original power level. This effectively extends the distance of a network segment.
Hubs
A hub is essentially a multi-port repeater. It connects multiple workstations in a network. When a frame enters from one port, the hub floods the signal to all other ports. Because the hub broadcasts data to every connected device, it does not manage traffic congestion, which is a key limitation.
Comparison Summary
- Repeater: Typically used for signal restoration between two network points.
- Hub: Used for creating a central point of connection for several devices, creating a single broadcast domain.
5. Elaborate on the process of signal representation and the transmission of bits within the physical layer. [5 Marks]
Introduction
The physical layer must translate digital data (sequences of 1s and 0s) into signals that can travel over the specific chosen medium. This conversion is crucial for effective data delivery.
Signal Conversion Methods
- Digital-to-Digital: Using voltage changes to represent bits, such as using +5V for '1' and 0V for '0'. This is standard for short-distance wired communication.
- Digital-to-Analog (Modulation): For telephone lines or wireless, the physical layer uses a modem to represent digital bits by modifying the amplitude, frequency, or phase of an analog carrier wave.
- Optical Transmission: Uses the presence of light to represent '1' and the absence of light to represent '0' within fiber optic strands.
Factors Affecting Transmission
- Bandwidth Constraints: The transmission medium has a physical limit on the frequency it can support.
- Noise Floor: The physical layer must ensure the signal power remains above the background electrical noise to avoid corruption.
- Clocking: The physical layer ensures that both ends of the line agree on when a bit starts and ends, maintaining temporal synchronization.