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Network Performance: Transmission Delay, Pipelining, and Queuing Delay

Prof. Neminath Hubballi explains performance measurement fundamentals in computer networks, focusing on transmission delay across single and multi-hop links. The lecture covers store-and-forward architecture, pipelined transmission calculations for multiple packets, and how multiplexing induces queuing delay and packet loss in router buffers.

Computer
English · 6 topics
Checked against the video on 24 Oct 2026

Made by AI from the YouTube video Towards Performance Measurement - Part 01 | Computer Networks | Prof. Neminath Hubbali, IITI (opens YouTube in a new tab) by NPTEL-NOC IITM, then checked against it. All credit for the teaching goes to the creator. Tap a topic’s time to confirm a fact in the video.

Network Performance: Transmission Delay, Pipelining, and Queuing Delay

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In short

Prof. Neminath Hubballi explains performance measurement fundamentals in computer networks, focusing on transmission delay across single and multi-hop links. The lecture covers store-and-forward architecture, pipelined transmission calculations for multiple packets, and how multiplexing induces queuing delay and packet loss in router buffers.

Introduction to Performance Assessment

Network performance measurement analyzes how long data takes to travel from a sender to a receiver across intermediate network elements.
Although bits physically travel via serial transmission over physical media, networks logically aggregate bits into discrete chunks called packets.
A packet is defined as a collection of binary bits in transit across network links.
When direct connectivity between end hosts does not exist, intermediate devices such as routers and switches forward data hop-by-hop.
Must knowPerformance evaluation in networks fundamentally relies on calculating distinct delay components, specifically transmission delay and queuing delay.

Mechanics of Transmission Delay

Transmission delay represents the time required to push all the bits of a packet from a memory buffer onto the communication link.
The transmission rate of a link is determined by its capacity (bandwidth), measured in bits per second (b/s or bps).
If a packet has a length of L bits and the link capacity is R bits per second, the transmission delay for that link is given by L / R.
Must knowTransmission delay depends solely on packet length (L) and link capacity (R), and is independent of the physical distance between sender and receiver.
After bits are placed onto a link, they traverse the link to enter the buffer of the next node or destination host.

Store-and-Forward and Single Packet Transmission

Computer networks operate on a store-and-forward model where an entire packet must be received and stored at an intermediate router before it can be transmitted onto the next link.
For a single packet traversing two identical links of capacity R via one intermediate router, transmission happens sequentially: client-to-router and then router-to-server.
The total transmission time for one packet over 2 hops with identical rate R is L/R + L/R = 2 × (L / R).
Must knowOver N links (hops) having uniform capacity R, a single packet of size L incurs a total transmission delay of N × (L / R).

Pipelined Transmission with Multiple Packets

When multiple packets are sent across intermediate routers, pipelined transmission enables parallel activity on different links.
As soon as the first packet (P1) is completely received and stored at a router, the router begins transmitting P1 to the next node while the sender simultaneously transmits the second packet (P2) onto the first link.
For 3 packets transmitted across 1 intermediate router (2 hops) with identical link rates R, the transmission finishes in 4 × (L / R) time units instead of 6 × (L / R) under non-pipelined sequential transfer.
Pipelining significantly improves network throughput and reduces the total transmission completion time.
Must knowGeneral formula for pipelined transmission of P packets over N links of uniform capacity R is (N + P - 1) × (L / R).

Worked Example and Disparity in Transmission Rates

For a network with N = 3 hops, P = 10 packets, and a per-hop transmission delay L/R = 1 ms, the total transmission delay is (3 + 10 - 1) × 1 ms = 12 ms.
In practical networks, links often have disparate capacities due to differing transmission media such as copper wires and optical fiber.
If link 1 has capacity R1 and link 2 has capacity R2, the transmission time for a single packet of length L is (L / R1) + (L / R2).
Transmission delay on each hop varies inversely with that specific link's data-carrying capacity.

Multiplexing, Queuing Delay, and Packet Loss

When multiple senders transmit packets toward the same output link without coordination, their packets arrive asynchronously at an intermediate router.
Arriving packets are held in a memory buffer known as a queue and are scheduled for outbound transmission in first-come, first-served order.
Queuing delay is the time a packet spends waiting inside router queues before it can be transmitted onto the outbound link.
If the arrival rate of packets exceeds the router's processing and forwarding capacity, the finite buffer space gets exhausted.
Must knowWhen a packet arrives at a full router buffer, it cannot be accommodated and is dropped, causing packet loss.
In practical routers, queuing occurs at both input links and output links; total queuing delay is the aggregate waiting time across both queues and intermediate routers.

Delay Type	Determining Factors	Definition / Primary Cause
Transmission Delay	Packet size (L), Link capacity (R)	Time required to push all packet bits onto the link
Queuing Delay	Traffic arrival rate, Buffer size, Multiplexing	Time a packet waits in router buffers before transmission

Key terms

Packet: A collection of binary bits grouped together for transmission over a network
Transmission Delay: The time required to push all bits of a packet onto the physical communication link (L / R)
Store-and-Forward: A mechanism where a node must receive and buffer the entire packet before forwarding it onto the next link
Pipelined Transmission: A technique where consecutive packets are transmitted concurrently across successive links
Queuing Delay: The waiting time spent by a packet inside the router's buffer before transmission begins
Packet Loss: The discarding of a packet that occurs when it arrives at a router whose buffer (queue) is completely full

Formulas

d_trans = L / R: Transmission delay of one packet of size L over a link of capacity R
d_total_trans = N × (L / R): Total transmission delay of 1 packet across N uniform hops of capacity R
d_total_trans = (N + P - 1) × (L / R): Total pipelined transmission delay for P packets over N uniform hops of capacity R

Quick revision

Read these the night before the exam.

A packet is a logical chunk of binary bits transmitted serially over physical media.
Transmission delay equals packet length divided by link capacity: L / R.
Store-and-forward requires an intermediate router to receive the full packet before retransmitting it.
For 1 packet over N uniform links of rate R, total transmission time is N × (L / R).
Pipelined transmission allows multiple links to transmit different packets simultaneously.
For P packets over N uniform hops, the total pipelined transmission time is (N + P - 1) × (L / R).
Queuing delay is the duration a packet waits in router queues prior to transmission.
Packet loss occurs when a packet arrives at an intermediate router whose queue buffer is full.

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