What are common examiner traps in Switching and Performance Metrics?

Failing to convert bytes to bits (1 byte = 8 bits) when bandwidth is specified in bps/Mbps. Forgetting to include per-packet header overhead when calculating packet transmission delay. Multiplying the entire transmission time by the number of hops (m N Tt) instead of recognizing link pipelining ([N + m - 1] Tt). Unit mismatches between seconds, milliseconds (ms), and microseconds ( ). Confusing the number of routers (R) with the number of links/hops (m = R + 1). Double-counting propagation delay for intermediate packets in pipelined transmission instead of applying it to the leading or trailing packet. Unit conversion errors (e.g., kilometers to meters, Mbps vs bps, bytes vs bits). Confusing Go-Back-N sequence number space (2k Ws + 1) with Selective Repeat (2k 2 Ws). Using 1 + a instead of 1 + 2a for round-trip channel utilization. Rounding down instead of applying ceiling when determining integer bit-widths (\ \ 2(N) \ ). Confusing symmetric key pairs N(N-1)2 with asymmetric/public-key requirements (2N total keys). Overcounting directed pairs as N(N-1) instead of undirected shared secret keys N(N-1)2. Off-by-one errors like (N-1)2 or failing to account for self-communication exclusion. Confusing TCP and UDP requirements for control/lookup protocols (e.g., DNS typically uses UDP) vs. bulk transfer protocols (e.g., FTP uses TCP). Swapping the sender's private key with the receiver's public key during message signing versus message encryption. Reversing the decryption and verification order at the receiver end (the outermost encryption layer must be decrypted first before verifying the inner digital signature).

Switching and Performance Metrics

11 Qs · since 2013 · 17 marks · 0.7 marks/paperStandard yield

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In this topic area within Computer Networks, GATE tests foundational conceptual knowledge through direct 1-mark multiple-choice questions. The questions focus on matching standard… Guide

Type

GATE CSE 2026 Set 2Q21MCQ1 markMediumConsider a file of size 4 million bytes being transferred between two hosts connected via a path consisting of three consecutive links of bandwidth 2 Mbps, 500 kbps, and 1 Mbps, respectively. All processing delays and propagation delays are negligible. Assume that there is no other background traffic over the path and…
GATE CSE 2024 Set 1Q36MCQ2 marksMediumConsider a network path $\text{P—Q—R}$ between nodes $\text{P}$ and $\text{R}$ via router $\text{Q}$ . Node $\text{P}$ sends a file of size $10^6$ bytes to $\text{R}$ via this path by splitting the file into chunks of $10^3$ bytes each. Node $\text{P}$ sends these chunks one after the other without…
GATE CSE 2022Q59NAT2 marksEasyConsider a 100 Mbps link between an earth station (sender) and a satellite (receiver) at an altitude of 2100 km. The signal propagates at a speed of $3 \times 10^8$ m/s. The time taken (in milliseconds, *rounded off to two decimal places*) for the receiver to completely receive a packet of 1000 bytes transmitted by…
GATE CSE 2017 Set 2Q35MCQ2 marksEasyConsider two hosts $X$ and $Y$ , connected by a single direct link of rate $10^6$ bits/sec. The distance between the two hosts is $10,000$ km and the propagation speed along the link is $2 \times 10^8$ m/sec. Host $X$ sends a file of $50,000$ bytes as one large message to host $Y$ continuously. Let the…
GATE CSE 2015 Set 1Q21MCQ1 markEasySuppose that everyone in a group of $N$ people wants to communicate secretly with the $N-1$ others using symmetric key cryptographic system. The communication between any two persons should not be decodable by the others in the group. The number of keys required in the system as a whole to satisfy the…
GATE CSE 2015 Set 2Q8NAT1 markMediumA link has a transmission speed of $10^6$ bits/sec. It uses data packets of size 1000 bytes each. Assume that the acknowledgment has negligible transmission delay, and that its propagation delay is the same as the data propagation delay. Also assume that the processing delays at nodes are negligible. The efficiency…
GATE CSE 2015 Set 3Q28NAT2 marksMediumConsider a network connecting two systems located 8000 kilometers apart. The bandwidth of the network is $500 \times 10^6$ bits per second. The propagation speed of the media is $4 \times 10^6$ meters per second. It is needed to design a Go-Back- $N$ sliding window protocol for this network. The average packet…
GATE CSE 2015 Set 3Q36NAT2 marksMediumTwo hosts are connected via a packet switch with $10^7$ bits per second links. Each link has a propagation delay of 20 microseconds. The switch begins forwarding a packet 35 microseconds after it receives the same. If 10000 bits of data are to be transmitted between the two hosts using a packet size of 5000 bits,…
GATE CSE 2014 Set 2Q26MCQ2 marksMediumDiagramConsider the store and forward packet switched network given below. Assume that the bandwidth of each link is $10^6$ bytes / sec. A user on host A sends a file of size $10^3$ bytes to host B through routers R1 and R2 in three different ways. In the first case a single packet containing the complete file is…
GATE CSE 2013Q12MCQ1 markEasyThe transport layer protocols used for real time multimedia, file transfer, DNS and email, respectively are
GATE CSE 2013Q13MCQ1 markMediumUsing public key cryptography, $X$ adds a digital signature $\sigma$ to message $M$ , encrypts $\langle M, \sigma \rangle$ , and sends it to $Y$ , where it is decrypted. Which one of the following sequences of keys is used for the operations?

Topic guide

What does GATE test in Switching and Performance Metrics?

In this topic area within Computer Networks, GATE tests foundational conceptual knowledge through direct 1-mark multiple-choice questions. The questions focus on matching standard network applications to their corresponding underlying transport layer protocols (TCP vs. UDP) and identifying key-usage sequences in secure transmission protocols involving digital signatures and asymmetric encryption. The emphasis is on testing functional understanding of protocol services and layered security mechanisms.

Core concepts

Store-and-forward packet switching and pipelining
Transmission delay vs Propagation delay distinction
Header overhead trade-off during packet fragmentation/segmentation
Router processing and store-and-forward scheduling delay
Bottleneck link throughput in multi-hop paths
Stop-and-Wait protocol channel efficiency
Propagation delay vs Transmission delay
Bandwidth-Delay Product (BDP)
Channel utilization and efficiency
Go-Back-N window sizing constraints
Sequence number field bit-length requirements
Symmetric key cryptography key management overhead
Full-mesh pairwise communication topology
Combinatorics in network security and connectivity (selecting 2 endpoints out of N)
Transport Layer protocol selection (TCP reliability vs. UDP low-overhead/low-delay)
Application requirements for transport protocols (Real-time streaming, FTP, DNS, SMTP)
Digital signatures for authentication and non-repudiation using sender's private key
Public-key encryption for confidentiality using receiver's public key
Decryption sequence and signature verification using receiver's private key and sender's public key

Recurring question patterns

Pipelined Multi-Hop Store-and-Forward Delay
common · mixed · 2 marks · 2024, 2015, 2014
A file is partitioned into $N$ equal packets (or chunks) and transmitted across $m$ hops (or intermediate routers). Questions require computing total delivery time or comparing how packetization and per-packet headers trade off serialization latency against header overhead.
Point-to-Point End-to-End Latency Calculation
common · mixed · 2 marks · 2022, 2017
Direct single-link or single-hop satellite/terrestrial links where candidates must compute individual transmission delay ( $L/B$ ) and propagation delay ( $D/v$ ) or their sum to find total reception time.
Bottleneck Bandwidth and Throughput
occasional · MCQ · 1 marks · 2026
A multi-hop path with heterogeneous link capacities where candidates compute total transfer time governed by the bottleneck (minimum bandwidth) link.
Protocol Efficiency and Propagation Relationship
occasional · NAT · 1 marks · 2015
Given protocol efficiency (e.g. Stop-and-Wait ARQ) and link transmission parameters, determine link propagation delay or maximum distance.
Optimal Window Size and Sequence Number Header Calculation
occasional · NAT · 2 marks · 2015
Given link distance, propagation velocity, bandwidth, and packet length, compute transmission and propagation delays to find the window size needed for 100% channel utilization ( $W_s = 1 + 2a$ ) and find the minimum number of sequence number bits required by the protocol (e.g., Go-Back-N).
Pairwise Communication and Key/Link Counting
common · MCQ · 1 marks · 2015
Given a network of $N$ entities where every pair requires a dedicated, private symmetric key (or dedicated point-to-point circuit/channel), compute the total number of unique keys/links needed across the entire system.
Application to Transport Protocol Mapping
common · MCQ · 1 marks · 2013
Given an ordered list of common network services (e.g., real-time multimedia, file transfer, DNS, email), identify the correct sequence of transport layer protocols (TCP or UDP) utilized by each.
Asymmetric Cryptography Key Sequence Tracking
common · MCQ · 1 marks · 2013
Given a combined cryptographic workflow involving digital signing and confidential encryption between sender and receiver, identify the exact sequence of public and private keys used for encryption/signing at the sender and decryption/verification at the receiver.

Formulas GATE uses

Transmission Delay ($T_t$)

T_t = \frac{L}{B}

Used whenever a packet of size $L$ bits is pushed onto a link with transmission rate / bandwidth $B$ bps.

Propagation Delay ($T_p$)

T_p = \frac{D}{v}

Used to compute physical travel time of a signal over distance $D$ at propagation speed $v$ .

Total Pipelined Packet Transmission Time (Homogeneous Links)

T_{\text{total}} = (N + m - 1) \times T_t + m \times T_p

Used when sending $N$ store-and-forward packets over $m$ links with identical bandwidth and propagation delay, assuming zero queueing/processing delay.

Stop-and-Wait Link Efficiency

\eta = \frac{T_t}{T_t + 2T_p} = \frac{1}{1 + 2a} \quad \text{where } a = \frac{T_p}{T_t}

Used when analyzing the channel utilization/efficiency of a Stop-and-Wait protocol with negligible ACK transmission delay.

Bottleneck File Transfer Time

T_{\text{transfer}} = \frac{\text{File Size}}{\min(B_1, B_2, \dots, B_m)}

Used for end-to-end file transfer over a path of $m$ consecutive links with negligible propagation and queueing overheads.

Propagation Delay

T_p = \frac{\text{Distance } d}{\text{Propagation Speed } v}

Calculating signal transit time across the physical medium.

Transmission Delay

T_t = \frac{\text{Packet Size } L}{\text{Bandwidth } B}

Calculating time required to push packet bits onto the wire.

Delay Ratio

a = \frac{T_p}{T_t}

Normalizing propagation delay relative to packet transmission time.

Full Utilization Window Size

W_s \ge 1 + 2a

Finding minimum sender window size to achieve 100% channel efficiency.

Go-Back-N Sequence Number Bit-Width

k = \lceil \log_2(W_s + 1) \rceil

Finding the number of bits in sequence number field for Go-Back-N where maximum window size is $2^k - 1$ .

Total Unique Pairs for $N$ Entities (Full Mesh / Symmetric Key Count)

\binom{N}{2} = \frac{N(N-1)}{2}

Used when determining the total number of unique secret keys, connections, or dedicated circuits needed so every pair among $N$ participants can communicate privately in a symmetric system.

Examiner traps in Switching and Performance Metrics

Failing to convert bytes to bits (1 byte = 8 bits) when bandwidth is specified in bps/Mbps.
Forgetting to include per-packet header overhead when calculating packet transmission delay.
Multiplying the entire transmission time by the number of hops (m N Tt) instead of recognizing link pipelining ([N + m - 1] Tt).
Unit mismatches between seconds, milliseconds (ms), and microseconds ( ).
Confusing the number of routers (R) with the number of links/hops (m = R + 1).
Double-counting propagation delay for intermediate packets in pipelined transmission instead of applying it to the leading or trailing packet.
Unit conversion errors (e.g., kilometers to meters, Mbps vs bps, bytes vs bits).
Confusing Go-Back-N sequence number space (2k Ws + 1) with Selective Repeat (2k 2 Ws).
Using 1 + a instead of 1 + 2a for round-trip channel utilization.
Rounding down instead of applying ceiling when determining integer bit-widths (\ \ 2(N) \ ).
Confusing symmetric key pairs N(N-1)2 with asymmetric/public-key requirements (2N total keys).
Overcounting directed pairs as N(N-1) instead of undirected shared secret keys N(N-1)2.
Off-by-one errors like (N-1)2 or failing to account for self-communication exclusion.
Confusing TCP and UDP requirements for control/lookup protocols (e.g., DNS typically uses UDP) vs. bulk transfer protocols (e.g., FTP uses TCP).
Swapping the sender's private key with the receiver's public key during message signing versus message encryption.
Reversing the decryption and verification order at the receiver end (the outermost encryption layer must be decrypted first before verifying the inner digital signature).

How questions are asked

Compute total elapsed time across a multi-hop path (NAT/MCQ)
Compare transmission completion times for varying packet sizes with fixed headers (MCQ)
Calculate independent transmission and propagation delays (MCQ)
Derive link propagation delay or distance from ARQ efficiency (NAT)
Determine end-to-end transfer time over heterogeneous link capacities (MCQ)
Numerical calculation of delay metrics and header field sizes (NAT)
Symbolic algebraic formula selection for N communicating nodes
Ordered sequence matching of protocols to application list
Key sequence selection for end-to-end secure transmission pipeline

Paper trends

Shift from purely algebraic/comparative multi-case problems to precise numerical computation with pipelined timelines and router processing delays.
2024, 2015, 2014
Increase in NAT questions requiring exact unit conversions (seconds, milliseconds, microseconds) and decimal rounding.
2022, 2015
Introduction of heterogeneous multi-hop bandwidths highlighting bottleneck link capacity over intermediate router buffering.
2026
Performance metric questions are framed as multi-step numerical calculation problems (NAT) combining physical layer delay parameters with transport/data-link framing constraints.
2015
Questions on basic network counting and fundamental system parameters are presented as direct, conceptual 1-mark multiple-choice questions.
2015
Questions are tested as conceptual 1-mark single-correct MCQs targeting fundamental protocol choices and asymmetric key management workflows.
2013

Difficulty notes

Easy items test standard single-link transmission/propagation delay arithmetic or direct formula application ( $T_t + T_p$ ). Medium items involve multi-hop store-and-forward pipelining, non-zero switch processing delays, packetization with header overhead trade-offs, or bottleneck bandwidth identification with unit conversions.