COMPSCI110 · Introduction to Computer Systems
Computer Networks
Module 6 of University of Auckland COMPSCI110 covers how computers communicate over the Internet: protocols, the choice between packet and circuit switching, the tiered ISP hierarchy, the four sources of packet delay (with the examinable formula d_trans = L/R), throughput and bottleneck links, and the five-layer Internet model with encapsulation. It carries 5 marks on the final exam — one of the heavier modules — and the delay calculations are reliable marks if you keep the units straight.
What this chapter covers
- 01Protocols (rules for message format, order and actions) and guided vs unguided media
- 02Packet switching (dynamic sharing, no reservation, buffers can drop packets) vs circuit switching (a fixed, guaranteed reserved rate)
- 03Forwarding (move a packet input-port → output-port via a local table) vs routing (compute the network-wide path)
- 04The ISP hierarchy: access ISPs can't all interconnect directly, so a tiered structure with tier-1 transit and IXPs; large content providers run private networks
- 05The four sources of nodal delay: d_proc + d_queue + d_trans + d_prop, with d_trans = L/R and d_prop = d/s (s ≈ 2×10⁸ m/s)
- 06Throughput and the bottleneck link: end-to-end rate ≈ the minimum link rate on the path
- 07The five-layer Internet stack (application, transport, network, link, physical) and encapsulation — each layer adds its own header
Transmission delay, propagation delay and the bottleneck
- +1Transmission delay is d_trans = L / R = 3×10⁶ bits ÷ 1.5×10⁶ bits/s = 2 s. This is the time to push all the bits onto the link, so it depends on packet size and link rate.
- +1Propagation delay is d_prop = d / s = 3×10⁶ m ÷ (2×10⁸ m/s) = 0.015 s = 15 ms (3000 km = 3×10⁶ m). It depends on distance and signal speed — NOT on packet size.
- +1These are two of the four delay sources; the other two are processing (checking errors, choosing the output link — sub-millisecond) and queuing (congestion-dependent). Total nodal delay = d_proc + d_queue + d_trans + d_prop.
- +1End-to-end throughput is set by the slowest (bottleneck) link: min(1.5 Mbps, 1 Mbps) = 1 Mbps. Adding faster links wouldn't help — the 1 Mbps hop caps the rate.
Key terms
- Protocol
- A set of rules governing the format, order and actions of messages exchanged between communicating devices — what a message looks like, when it may be sent, and how each side responds. The Internet is a stack of protocols, one set per layer.
- Packet vs circuit switching
- Packet switching splits data into packets that share link capacity dynamically with no reservation — efficient for bursty traffic, but a packet meeting a busy link waits in the output buffer and may be dropped if no buffer is free. Circuit switching reserves a fixed, guaranteed rate for the whole call, wasting capacity when idle but never contending.
- Transmission delay (d_trans = L/R)
- The time to push all of a packet's bits onto the link: packet length L (bits) divided by link rate R (bits/s). It scales with packet size and inversely with link speed, and is independent of how far the link stretches.
- Propagation delay (d_prop = d/s)
- The time for a bit to travel the length of the link: distance d divided by signal speed s (about 2×10⁸ m/s in typical media). It depends on distance, not on packet size or link rate — the opposite dependencies to transmission delay.
- Bottleneck link & throughput
- Throughput is the actual bits-per-second delivered between sender and receiver. On a path of several links it is limited by the slowest one — the bottleneck link — so end-to-end throughput ≈ min(link rates). Upgrading any other link changes nothing until the bottleneck is fixed.
- Five-layer Internet model
- Top to bottom: application (HTTP/SMTP — messages), transport (TCP/UDP — segments), network (IP/routing — datagrams), link (Ethernet/Wi-Fi — frames) and physical (bits on the wire). End systems run all five; routers use network/link/physical; each layer adds its own header (encapsulation) on the way down.
Computer Networks FAQ
What are the four sources of packet delay?
Processing delay (the router checks the packet and picks an output link — usually sub-millisecond), queuing delay (waiting behind other packets, which grows with congestion), transmission delay d_trans = L/R (pushing the bits onto the link) and propagation delay d_prop = d/s (the bits travelling the link's length). Their sum is the nodal delay at one hop; end-to-end delay adds up the nodal delays along the path.
What's the difference between packet and circuit switching?
Circuit switching reserves a dedicated, fixed-rate path for the entire session — guaranteed bandwidth, but idle capacity is wasted. Packet switching sends independent packets that share links on demand — far more efficient for bursty computer traffic, at the cost of possible queuing and, if buffers fill, packet loss. The Internet is packet-switched; the classic reason is that reservation doesn't suit bursty data.
Can AI help me with computer networks in COMPSCI110?
Yes, as a study aid. Sia can work a delay calculation with you, check your unit conversions (bits vs Mbit, metres vs km), explain why the bottleneck link caps throughput, and quiz you on the five layers and their data units. Use it to understand the method and rehearse — it explains and checks your reasoning but does not do graded work or the exam for you, and the University of Auckland's academic-integrity rules apply to controlled assessments.
d_trans vs d_prop — how do I tell which is which?
Look at what the numbers describe. Transmission delay is about the packet and the pipe's width: L/R, bits divided by bits-per-second. Propagation delay is about the pipe's length: d/s, distance divided by signal speed. A quick tell: doubling the packet size doubles d_trans but leaves d_prop alone; doubling the link length doubles d_prop but leaves d_trans alone. Mixing them up is the module's most common mark-loser.
What are the five layers of the Internet model?
Application (HTTP, SMTP — the unit of data is a message), transport (TCP/UDP — a segment), network (IP and routing — a datagram), link (Ethernet, Wi-Fi — a frame) and physical (raw bits on the medium). End systems implement all five; a router works up to the network layer. As data goes down the stack each layer wraps it in its own header — that's encapsulation.
Exam move
Networks is 5 marks and splits into calculation and recall — prepare both. For calculation, drill the two delay formulas until you never confuse them: d_trans = L/R (size over rate) and d_prop = d/s (distance over speed), watching units carefully (convert Mbit ↔ bits and km ↔ m before dividing). Practise a full nodal-delay sum and a bottleneck-throughput question (throughput = the minimum link rate, never the total). For recall, make crisp flashcards for the facts that appear as MCQs: packet vs circuit switching (dynamic sharing vs guaranteed reserved rate), forwarding (local table) vs routing (network-wide path), why access ISPs can't all interconnect directly, and the five layers with their data units (message/segment/datagram/frame/bits) in order. Finish by being able to explain encapsulation in one sentence — each layer adds a header on the way down — because that ties the stack together.
Working through Computer Networks in COMPSCI 110? Sia is AskSia’s AI Computer Science tutor — ask any COMPSCI 110 Computer Networks question and get a clear, step-by-step explanation grounded in how COMPSCI 110 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
