Computer Networks

Answer

(iv) Both Session layer and Presentation layer

Explanation

The OSI model has 7 layers:

1. Physical
2. Data Link
3. Network
4. Transport
5. Session
6. Presentation
7. Application

The traditional TCP/IP model has fewer layers. Its Application layer combines the responsibilities of the OSI Application, Presentation, and Session layers.

Therefore, when comparing the two models, the Session and Presentation layers are separate layers in OSI but are not separate layers in the traditional TCP/IP model.

Option analysis

• (i) Application: Exists in both models, although its scope differs.
• (ii) Presentation: Separate in OSI; its functions are incorporated into TCP/IP's Application layer.
• (iii) Session: Separate in OSI; its functions are incorporated into TCP/IP's Application layer.
• (iv) Both session layer and presentation layer: Correct.

Final answer: (iv) Both session layer and presentation layer.

Answer

(ii) Host-to-host layer

Explanation

The OSI model has seven layers, while the TCP/IP model is commonly represented using four layers: Application, Host-to-Host (Transport), Internet, and Network Access.

The Transport layer of the OSI model provides end-to-end communication between processes. It handles functions such as segmentation, flow control, error control, and reliable delivery when TCP is used.

In the TCP/IP model, these responsibilities belong to the Host-to-Host/Transport layer.

Option analysis

• (i) Application: Corresponds broadly to the OSI Application, Presentation, and Session layers.
• (ii) Host-to-host: Correct. It corresponds to the OSI Transport layer.
• (iii) Internet: Corresponds to the OSI Network layer.
• (iv) Network access: Covers functions associated broadly with the OSI Data Link and Physical layers.

Final answer: (ii) Host-to-host.

Answer

(ii) Data Link Layer and Bit synchronization is the INCORRECT pair.

Explanation

Let's check each pair:

• (i) Network layer — Routing: Correct. The Network layer determines paths through an internetwork and performs routing/forwarding-related functions.
• (ii) Data Link Layer — Bit synchronization: Incorrect. Bit synchronization is primarily a Physical Layer function. The Physical layer is responsible for transmitting raw bits and maintaining synchronization between sender and receiver.
• (iii) Transport layer — End-to-end process communication: Correct. The Transport layer provides end-to-end/process-to-process communication and may provide reliability, flow control, and error control.
• (iv) MAC sub-layer — Channel sharing: Correct. The MAC sub-layer controls access to a shared transmission medium and determines when a device may transmit.

Therefore, the incorrect pair is (ii).

Final answer

(ii) Data Link Layer and Bit synchronization.

Answer

(iv) Both Session layer and Presentation layer

Explanation

The OSI model contains seven layers, while the traditional TCP/IP model groups several OSI functions together.

The OSI Presentation layer handles functions such as data translation/format conversion, encryption/decryption, and compression. The OSI Session layer handles session establishment, management, synchronization, and termination.

In the TCP/IP model, these functions are generally incorporated into the broader Application layer rather than represented as separate Presentation and Session layers.

Therefore, both the Session and Presentation layers are separate additions in the OSI model when compared with the traditional TCP/IP model.

Final answer: (iv) Both session layer and presentation layer.

Answer

Any two important functions of the Session Layer are:

1. Dialog control: It manages the dialogue between two communicating applications, including controlling which side can transmit at a particular time. It can support half-duplex and full-duplex communication.

2. Synchronization/checkpointing: The Session layer can insert synchronization points (checkpoints) into a long data transfer. If communication is interrupted, the transfer can resume from a checkpoint instead of restarting from the beginning.

Other valid functions

The Session layer is also associated with session establishment, session management, and session termination between applications.

Exam-ready answer

The Session layer establishes, manages, and terminates communication sessions between applications. Two functions are dialog control and synchronization/checkpointing.

For 2 marks: Write any two functions with one-line explanations.

Answer

(i) Mesh topology

Explanation

A mesh topology connects nodes with multiple dedicated links. In a full mesh, every node has a direct connection to every other node.

This gives mesh topology strong reliability and robustness: if one link fails, traffic can often use another path. Dedicated links also reduce contention and unwanted traffic compared with a shared bus.

Option analysis

• (i) Mesh: Correct. It is highly reliable, provides dedicated communication paths, and avoids the single shared communication medium of a bus.
• (ii) Ring: Nodes share a logical ring; a link/node failure can disrupt communication unless redundancy is provided.
• (iii) Star: Easy to manage, but the central device is a potential single point of failure.
• (iv) Bus: Uses a shared medium, so collisions/traffic contention can occur.

Final answer: (i) Mesh.

Answer

(iii) Wide Area Network

Explanation

WAN stands for Wide Area Network. It is a network designed to cover a large geographical area, such as cities, states, countries, or continents. The Internet is the largest example of an interconnected wide-area network.

Final answer: (iii) Wide Area Network.

Answer

(iii) Session layer

Explanation

The OSI model has separate Session and Presentation layers, whereas the traditional TCP/IP model combines their functions into its broader Application layer.

The Network, Transport, and Application functions are represented in both models, although the names and grouping differ. The Session layer is therefore the best answer among the options.

Final answer: (iii) Session layer.

Answer

A repeater is a Physical-layer networking device that receives a weakened or distorted signal, regenerates it, and retransmits it.

Purpose

Signals become weaker as they travel over long distances due to attenuation. A repeater restores the signal so the network can cover a greater physical distance.

Key point

A repeater does not make routing decisions or inspect IP/MAC addresses. Its primary function is signal regeneration at the Physical layer.

Exam-ready: A repeater regenerates and retransmits signals to extend the transmission distance of a network.

Answer

(ii) Bridge

Explanation

A bridge operates at the Data Link layer and connects LAN segments. It forwards Ethernet frames between LANs using MAC addresses.

• Bridge: connects LAN segments and forwards frames based on MAC addresses.
• Router: operates at the Network layer and forwards packets between IP networks.
• Repeater: regenerates physical-layer signals but does not make forwarding decisions based on addresses.
• Modem: performs modulation/demodulation for transmission over certain communication media.

Because the question specifically asks about transmitting frames from one LAN to another LAN, the expected answer is Bridge.

Final answer: (ii) Bridge.

Answer

(i) Host-to-host

Explanation

In the traditional TCP/IP model, the Host-to-Host (Transport) layer corresponds to the OSI Transport layer.

It provides process-to-process communication and can provide functions such as segmentation, flow control, error control, and reliable delivery through protocols such as TCP.

• Application → broadly combines OSI Application, Presentation, and Session functions.
• Host-to-host → corresponds to OSI Transport.
• Internet → corresponds to OSI Network.
• Network access → broadly covers OSI Data Link and Physical functions.

Final answer: (i) Host-to-host.

Answer

Twisting the conductors in a twisted-pair cable helps reduce electromagnetic interference (EMI) and crosstalk.

Each pair is twisted so that external interference affects the conductors in a more balanced way. The twisting also reduces the amount of electromagnetic radiation coupled between adjacent pairs.

Exam-ready

The twisting of wires reduces noise, electromagnetic interference, and crosstalk, improving the reliability of data transmission.

For 2 marks: Mention reduction of EMI/crosstalk and improved signal quality.

Answer

Baud rate is the number of signal symbols transmitted per second over a communication channel. Its unit is baud (symbols/second).

Important distinction

Baud rate is not always the same as bit rate. If each symbol carries more than one bit, then:

$\text{Bit rate} = \text{Baud rate} \times \text{bits per symbol}$

For example, if one symbol represents 2 bits and the baud rate is 1000 baud, the bit rate is 2000 bits/s.

Exam-ready: Baud rate = number of signal symbols transmitted per second.

Answer

(i) Topology

Explanation

Network topology describes the physical or logical arrangement of devices and communication links in a network.

Examples include:

• Bus: devices share a common backbone.
• Star: devices connect to a central switch/hub.
• Ring: devices form a logical/physical ring.
• Mesh: devices have multiple interconnections.

Therefore, the correct answer is (i) Topology.

Answer

(iv) None of the above

Calculation

Transmission delay is:
$T_t = \frac{L}{R}$

where $L$ is packet length and $R$ is transmission rate.

Given:

• $L=32$ bits
• $R=10$ Mbps $=10\times10^6$ bits/s

Therefore:
$T_t=\frac{32}{10\times10^6}=3.2\times10^{-6}$ s.

Convert to milliseconds:
$3.2\times10^{-6}\times1000=0.0032$ ms.

None of the listed numerical options equals 0.0032 ms.

Final answer: (iv) None of the above.

Answer

(iii) Optical fiber

Explanation

Optical fiber transmits data using pulses of light through glass or plastic fiber. It supports very high bandwidth and transmission speeds compared with traditional copper media.

Advantages include:

• Very high bandwidth.
• Low attenuation over long distances.
• Immunity to electromagnetic interference.
• Suitable for high-speed backbone and long-distance communication.

Therefore, among the given options, optical fiber has the highest transmission speed.

Final answer: (iii) Optical fiber.

Answer

(ii) Distance between the routers

Explanation

Transmission delay is the time required to push all packet bits onto the link:
$T_t=\frac{L}{R}$.

It depends on:

• $L$ = packet length
• $R$ = transmission rate

It does not depend on physical distance between routers. Distance affects propagation delay, which is approximately:
$T_p=\frac{d}{v}$.

Therefore, the correct answer is (ii) Distance between the routers.

Remember

Transmission delay → packet size + link rate

Propagation delay → distance + propagation speed