ISRO CS Computer Networks PYQs: Solved OSPF, CSMA and TCP Questions

How to identify the Computer Networks concept first?

Match the wording to the layer before reading the options:

• An address modified while crossing a router points to network-layer translation.

• OSPF and shortest-path computation point to link-state routing.

• Collision handling and wireless access point to data-link MAC protocols.

• Pseudo-header and one’s-complement arithmetic point to the TCP checksum.

• A zero advertised receive window points to the TCP persist timer.

1. Network Address Translation — ISRO CS 2014

Question

The process of modifying IP address information in IP packet headers while in transit across a traffic routing device is called:

• (A) Port address translation (PAT)

• (B) Network address translation (NAT)

• (C) Address mapping

• (D) Port mapping

Correct answer: (B) Network address translation (NAT)

Explanation

NAT changes IP address information as a packet crosses a translating router or gateway. PAT is a specific form of translation that also uses transport-layer port numbers so several private hosts can share a public address.

Because the stem speaks generally about modifying IP information, NAT is the correct umbrella term. “Address mapping” and “port mapping” are descriptive phrases, not the protocol name requested by the question.

2. OSPF routing algorithm — ISRO CS 2014

Question

What is routing algorithm used by OSPF routing protocol?

• (A) Distance vector

• (B) Flooding

• (C) Path vector

• (D) Link state

Correct answer: (D) Link state

Explanation

OSPF is a link-state interior gateway protocol. Routers flood link-state advertisements so each router can construct a topology database, then run Dijkstra’s shortest-path-first algorithm locally.

Flooding is part of distributing the information, but it is not the routing-algorithm family being asked for. Distance vector describes protocols such as RIP, while path vector describes BGP. Attempt the original item on the ISRO OSPF practice page.

3. Ethernet collision handling — ISRO CS 2016

Question

In Ethernet CSMA/CD, the special bit sequence transmitted by media access management to handle collision is called:

• (A) Preamble

• (B) Postamble

• (C) Jam

• (D) None of the above

Correct answer: (C) Jam

Explanation

A station that detects a collision sends a jam signal so every station in the shared collision domain recognises that the current transmission failed. It then stops and waits for a randomly selected backoff interval before trying again.

The preamble belongs to the beginning of an ordinary Ethernet frame and helps synchronisation; it is not a collision notification. Open the ISRO CSMA/CD practice question for another attempt.

4. Wireless LAN access — ISRO CS 2017

Question

Which media access control protocol is used by IEEE 802.11 wireless LAN?

• (A) CDMA

• (B) CSMA/CA

• (C) ALOHA

• (D) None of the above

Correct answer: (B) CSMA/CA

Explanation

IEEE 802.11 wireless LANs use carrier-sense multiple access with collision avoidance (CSMA/CA). A wireless station cannot reliably transmit and listen for a collision at the same time, so Wi-Fi attempts to reduce collisions before sending through carrier sensing, interframe spacing and random backoff.

CSMA/CD belongs to traditional shared Ethernet. CDMA is a channelisation method, and ALOHA is an earlier random-access approach. Reattempt the item on the ISRO CSMA/CA practice page.

5. TCP checksum and the pseudo-header — ISRO CS 2020

Question

Checksum field in TCP header is:

• (A) ones complement of sum of header and data in bytes

• (B) ones complement of sum of header, data and pseudo header in 16 bit words

• (C) dropped from IPv6 header format

• (D) better than md5 or sh1 methods

Correct answer: (B) ones complement of sum of header, data and pseudo header in 16 bit words

Explanation

TCP computes its checksum over the TCP header, payload and a pseudo-header derived from the IP layer. The pseudo-header protects important addressing and protocol information without becoming part of the TCP segment transmitted as TCP data.

The addition uses 16-bit words with end-around carry, followed by a one’s complement. Option A omits the pseudo-header and uses the wrong unit. Option C confuses TCP with the removed IPv6 header checksum: TCP’s checksum remains required.

6. TCP flow control and the persist timer — ISRO CS 2020

Question

The persist timer is used in TCP to:

• (A) To detect crashes from the other end of the connection

• (B) To enable retransmission

• (C) To avoid deadlock condition

• (D) To timeout FIN_WAIT1 condition

Correct answer: (C) To avoid deadlock condition

Explanation

Suppose a receiver advertises a zero window because its buffer is full. Later it sends a window update, but that update is lost. The sender still believes the window is zero and waits; the receiver believes it announced new space and also waits.

TCP’s persist timer breaks this deadlock by causing the sender to transmit a small window probe. The response carries the receiver’s current window. Retransmission timeout, keepalive and connection-state timers have different jobs.

Common traps in ISRO Computer Networks PYQs

For related subject-wise articles, continue with the ISRO CS DBMS PYQ walkthrough and ISRO CS Operating Systems PYQ guide.

A short revision loop

Redo all six questions without looking at the answers. For every miss, write one deciding phrase:

• NAT modifies IP information.

• OSPF is link state.

• Collision means jam.

• IEEE 802.11 means CSMA/CA.

• The TCP checksum includes the pseudo-header.

• The persist timer prevents zero-window deadlock.

Then solve the questions again from the options rather than from memory. That habit turns each fact into an elimination rule you can use under exam pressure.