Tutorial 1 Solutions CS331 · Chapter 01 Introduction · Part 01

Tutorial 1, worked solutions

Every question from the tutorial handed out by Dr. Sahar Amir, answered with the reasoning written out. Try each section from the blank tutorial first, then read these answers to check your method rather than only your final letter.

Covers Chapter 01, Introduction, Part 01: network topologies, transmission modes, network models, and the criteria of an effective network.

Section A

Scenario-based questions

Question 1.1 — connecting New York, London, Tokyo and Sydney

A multinational corporation needs to connect its headquarters with branch offices across continents, with high-speed transfer, reliability, and secure links for financial data.

Answer: a mesh topology, carried over a WAN. For four sites a full mesh is practical; larger deployments normally use a partial mesh between the busiest sites.

Mesh is the right fit because each requirement maps onto one of its properties:

  • Dedicated links. Every pair of offices gets its own point-to-point link, so the full capacity of that link carries only their traffic. No office competes for a shared line, which is what keeps transfers fast.
  • Reliability. If the New York to Tokyo link fails, traffic still reaches Tokyo through London or Sydney. One broken link never isolates a site.
  • Security and privacy. A message travels on the dedicated link between the two offices concerned, so no other site sees it in transit.
  • Fault isolation. A failing link is easy to identify and route around while it is repaired.

The cost is cabling: a full mesh of n sites needs n(n − 1)/2 links, so four offices need 4 × 3 / 2 = 6 links, and each site needs n − 1 = 3 interfaces. That is why real deployments lease WAN circuits or build a partial mesh rather than cabling a full mesh worldwide.

Question 2.1 — a 20-person single-floor office

All computers connect to one central device that manages traffic and lets staff share files and printers. It should be easy to manage and to expand.

Answer: a star topology.

The scenario names the deciding clue: a central device every computer connects to. Advantages:

  • Cheap to cable. Each device needs one link and one port, so a star needs n links where a mesh of 20 devices would need 190.
  • Easy to expand. Adding an employee means running one cable to a free port, with no change to the other stations.
  • Easy to manage and troubleshoot. Traffic passes through the hub or switch, so monitoring and fault detection happen in one place.
  • Robust against link failure. A broken cable takes down only the station on the end of it; the rest of the network keeps running.

The trade-off to state in an exam: the central device is a single point of failure. If the hub or switch dies, the whole network stops.

Question 3.1 — five research computers working together

Five powerful computers should communicate directly with each other, without relying on a central server, for maximum reliability and performance.

Answer: a peer-to-peer model, not a client/server model. This is a question about the network model, not the physical topology.

Key characteristics:

  • Each machine acts as both client and server, requesting and providing services.
  • There is no central server, so there is no single point of failure. Losing one machine leaves the other four working.
  • Work and resources are distributed across the peers, which suits a shared computational task.
  • It is cheaper to build, since no dedicated server hardware is required.
  • The weakness is administration: security, backups and access control have to be handled on every machine, so the model does not scale to large organisations.
Section B

Multiple choice

Question 1 — the physical path a message travels

  • a) Protocol
  • b) Signal
  • c) Medium
  • d) Path

The transmission medium is the physical path between sender and receiver: twisted pair, coaxial cable, fibre, or radio waves. It is one of the five components of a data communications system.

Question 2 — more than two devices sharing a single link

  • a) Point-to-point
  • b) Multi-point
  • c) Primary
  • d) Secondary

A multipoint (multidrop) connection shares one link among more than two devices, so the capacity is shared either spatially or by taking turns. A point-to-point connection dedicates the whole link to exactly two devices.

Question 3 — topology requiring a central controller

  • a) Mesh
  • b) Bus
  • c) Star
  • d) Ring

In a star, every device has a dedicated link to a central hub, and devices are not linked directly to each other. Mesh links every pair directly, bus shares one backbone, and ring passes traffic neighbour to neighbour, so none of those needs a controller.

Question 4 — the protocol suite of the current Internet

  • a) UNIX
  • b) NCP
  • c) TCP/IP
  • d) ACM

TCP/IP is the suite in use today. NCP is the distractor worth knowing: it was the earlier ARPANET protocol that TCP/IP replaced. UNIX is an operating system and ACM is a professional society.

Question 5 — frequency of failure and recovery time

  • a) Performance
  • b) Reliability
  • c) Security
  • d) Feasibility

Reliability is measured by how often the network fails, how long recovery takes, and how well it survives a catastrophe. Performance covers throughput and delay; security covers protection from unauthorised access, damage and viruses.

Section C

True or false

# Statement Answer Why
1 In full-duplex transmission, both stations can transmit and receive simultaneously. True That is the definition of full duplex, as in a telephone call. The capacity is shared between the two directions.
2 A LAN typically covers an area larger than 2 miles in diameter. False A LAN is normally limited to a room, a building or a campus. Anything spanning a town or country is a MAN or a WAN.
3 A protocol is the structure or format of data, specifically the order in which they are presented. False That describes syntax, which is only one element. A protocol is the full set of rules governing communication: syntax, semantics and timing.
4 In a bus topology, one break in the cable can stop all transmissions on the segment. True The backbone is shared, so a break splits the segment and reflects signals, stopping traffic on it. This is the classic weakness of bus.
5 An internet (lowercase i) is the same as the Internet (uppercase I). False An internet is any set of networks joined by internetworking devices. The Internet is one specific, global example of such a network.
Section D

Fill in the blank

# Question Answer
1 The five components are Message, Sender, Receiver, Transmission Medium, and __________. Protocol
2 In a __________ connection, channel capacity is shared by both devices at all times. Full-duplex
3 A task divided among multiple computers is __________ processing. Distributed
4 For n devices in a mesh topology, the number of cable links required is __________. n(n − 1)/2

Question 4 is the one to memorise with its companion: a full mesh needs n(n − 1)/2 links, and each device needs n − 1 ports.

Section E

Solving and understanding

Question 1 — cable links for 8 devices

Answer: mesh 28, ring 8, bus 1 backbone with 8 drop lines, star 8.

Topology Formula Working for n = 8 Links
Mesh n(n − 1)/2 8 × 7 / 2 = 56 / 2 28
Ring n Each device links to the next, and the last closes the loop 8
Bus 1 backbone + n drop lines One shared cable, tapped 8 times 1 (plus 8 drops)
Star n One dedicated link per device to the hub 8

State the assumption for bus: the backbone is one cable, and the drop lines connecting each station are counted separately. Say which convention you used and the marker can follow it.

Question 2 — half duplex against full duplex

Answer: in half duplex each station can send and receive, but only one at a time. In full duplex both can send and receive at the same moment.

Half duplex Full duplex
Direction Both ways, but one at a time Both ways simultaneously
Capacity The whole link belongs to whichever station is sending Capacity is split between the two directions
Example A walkie-talkie: you hold the button to talk, release it to listen A telephone call: both people can speak and hear at once

Question 3 — the three criteria of an effective network

Answer: performance, reliability and security.

  • Performance. Measured by transit time and response time, and expressed as throughput and delay. It depends on the number of users, the transmission medium, the hardware and the software. Throughput and delay pull against each other: pushing more data through raises congestion, which raises delay.
  • Reliability. Measured by frequency of failure, the time taken to recover from a failure, and robustness in a catastrophe.
  • Security. Protecting data from unauthorised access and from damage, and implementing policies for recovery from breaches and data loss.

Question 4 — why standards matter

Answer: standards make equipment from different manufacturers interoperate, and create an open market for that equipment.

  • Interoperability. A laptop from one vendor connects to a router from another because both implement the same standard.
  • A competitive market. Buyers are not locked to one supplier, which lowers prices and speeds up adoption.
  • Guidance for manufacturers. Vendors build against a published specification instead of guessing what their competitors did.
  • Examples. TCP/IP for internetworking, IEEE 802.3 for Ethernet, IEEE 802.11 for wireless LANs. Note the distinction: a de jure standard is approved by a recognised body such as ISO, ITU-T, ANSI or IEEE, while a de facto standard becomes one through widespread use.

CS331 · Data Communications and Computer Networks · tutorial prepared by Dr. Sahar Amir · solutions written for revision, check them against your own working.