THEORY EXAMINATION (SEM–VI) 2016-17 COMPUTER NETWORK

COMPUTER NETWORK – ECS601

B.Tech (SEM VI) | Section-wise Solved Answers

SECTION – A

(Explain the following – 2 marks each)

(a) User access in ISDN

ISDN provides digital end-to-end connectivity. User access in ISDN is achieved using Basic Rate Interface (BRI) and Primary Rate Interface (PRI), which allow users to transmit voice, data, and video over the same network.

(b) Star topology: advantages and disadvantages

In star topology, all nodes are connected to a central hub or switch. It is easy to install, manage, and troubleshoot. However, failure of the central device brings down the entire network, and cabling cost is higher.

(c) ALOHA vs Slotted ALOHA

ALOHA allows stations to transmit anytime, leading to high collision probability. Slotted ALOHA divides time into slots and allows transmission only at slot boundaries, reducing collisions and doubling efficiency.

(d) Requirements of CRC

CRC requires polynomial division, a generator polynomial agreed by sender and receiver, and redundant bits added to detect transmission errors accurately.

(e) Reasons for congestion in a network

Congestion occurs due to high traffic load, insufficient bandwidth, slow routers, buffer overflow, or sudden bursts of data transmission.

(f) Extracting net-id and host-id from IP address

The net-id and host-id are extracted using the subnet mask. The network part is identified by masked bits, and the remaining bits represent the host.

(g) Piggybacking

Piggybacking is a technique where acknowledgment information is attached to outgoing data frames to improve channel efficiency.

(h) Duplication control in transport layer

The transport layer uses sequence numbers to identify duplicate packets and discards repeated data to ensure reliable delivery.

(i) Use of HTTP

HTTP is used for communication between web browsers and servers to transfer web pages, images, and multimedia content.

(j) Email gateways

Email gateways include SMTP gateways, X.400 gateways, and POP/IMAP gateways used to transfer emails between different mail systems.

SECTION – B

(Attempt any five – explained properly)

(a) Issues in Data Link Layer and Protocols

The data link layer handles framing, error control, flow control, and access control. Protocols are designed using the layering principle to ensure modularity and reliable node-to-node delivery.

(b) Network Topologies

Common topologies include bus, star, ring, and mesh.
Each topology has its own advantages in terms of cost, reliability, and scalability, and disadvantages such as fault tolerance or maintenance complexity.

(c) Stop-and-Wait ARQ – Link Utilization

Given:
Frame size = 10 Kbits
Bandwidth = 10 Mbps
Propagation delay = 270 ms
Error probability = 10⁻³

Stop-and-Wait has low utilization on long-delay links like satellite channels because the sender waits for acknowledgment before sending the next frame.

(d) Line Coding in FDDI

FDDI uses 4B/5B encoding combined with NRZI. This ensures sufficient transitions for synchronization and reduces signal loss.

(e) TCP Header, Working & TCP vs UDP

TCP provides reliable, connection-oriented service using sequence numbers, acknowledgments, and flow control.
UDP is connectionless, faster, but unreliable.

(f) Three-Way Handshaking

TCP establishes a connection using SYN, SYN-ACK, and ACK packets. This ensures both sender and receiver are ready for data transmission.

(g) TELNET and its Working

TELNET allows remote login over a network. It uses a client-server model where commands entered at the client are executed on the remote machine.

(h) FTP and Active vs Passive FTP

FTP transfers files using separate control and data connections.
Active FTP requires the server to initiate data connection, while Passive FTP allows the client to initiate both connections.

SECTION – C

(Attempt any two – long answers)

(3)

(i) OSI Reference Model

The OSI model consists of seven layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. Each layer has specific responsibilities to ensure end-to-end communication.

(ii) GO-BACK-N Performance Issues

GO-BACK-N retransmits all frames after an error, causing inefficiency in high-error or long-delay networks.

(4)

(i) Count-to-Infinity Problem

In distance vector routing, incorrect routing updates can cause routers to continuously increase path cost, leading to slow convergence.

(ii) Reliable Delivery by Transport Layer

The transport layer ensures correct delivery using sequencing, acknowledgments, error detection, retransmission, and flow control.

(5) SNMP Protocol

SNMP is used for network management. It consists of managed devices, agents, network management systems, and MIBs. It supports monitoring, fault detection, and configuration management.