Communication
Client-server, peer-to-peer, thin and thick clients
What you need to be able to do
Syllabus 2.1 Networks including the internet
- 2.1Explain the client-server and peer-to-peer models: the role of each computer, the benefits and drawbacks of each.
- 2.1Justify using a model for a given situation.
- 2.1Explain thin-client and thick-client and the differences between them.
You need: Networks: LAN and WAN. Next: Network topologies.
The client-server model
- The server is a powerful computer that stores the data / resources centrally and provides services: it receives requests, processes them and sends back the results.
- A client is a user's computer (or app / web browser) that sends a request to the server, waits, then outputs the response.
- The server is in charge: it controls access (log-ins, permissions), backups and security for everyone.
Everyday examples: a web browser (client) and a web server; a banking app (client) and the bank's server; a school's file server; an email server; an online game whose server holds every player's data.
| Benefits | Drawbacks |
|---|---|
| Central management: security, backups and software updates done once, on the server | Expensive: a server and specialist staff are needed |
| Data is in one place, so it is consistent and up to date for everyone | Single point of failure: if the server fails, clients can't work |
| Access rights control who sees what | The server can become a bottleneck when many clients make requests |
| Scales to many users (add a more powerful server) |
The peer-to-peer model
- All computers have equal status: there is no central server.
- Each computer provides access to its own resources and data (the data is distributed) and uses the others'.
- Each computer is responsible for its own security (and its own backups).
| Benefits | Drawbacks (with the reason the scheme wants) |
|---|---|
| Cheap and easy to set up: no server, no specialist staff | Reduced security: no central management, so the network is only as secure as the weakest computer; each computer is at risk from viruses on the others |
| No single point of failure: if one peer goes down the others keep working | No central backup: if one computer's data isn't backed up it is lost to everyone |
| Good for sharing files among a few users (and for torrent-style file sharing) | No central management of files / software: hard to keep versions consistent |
| A computer may respond slowly because others are using its resources; files are only available while that computer is switched on |
A peer-to-peer network is a model (who provides services), not a topology (how the cables run): it can use Wi-Fi, a star or anything else. The June 2021 report saw it confused with a mesh topology.
Thin client and thick client
| Thin client | Thick (fat) client | |
|---|---|---|
| Where the processing happens | on the server: the server performs all (most) processing and data storage | on the client: the client does most processing itself; the server does some |
| What the client does | only sends requests and displays the returned results | runs applications locally; resources installed locally |
| Network | heavily reliant on a constant connection to the server: no connection, no work | can keep working (at least partly) without the server |
| Client hardware | few local resources: cheap, low-power, low memory | powerful processor, memory and storage |
| Security / management | data stays on the server (not on the device); updates in one place | data on many devices; each must be updated and secured |
| Examples | browser-based marking software; a library catalogue terminal; cloud apps on a Chromebook | a gaming PC; a video-editing workstation; an app installed on a laptop that syncs later |
The June 2023 reports: thin- and thick-client answers were “too vague and could apply to any general client-server model”.
Give one statement about the client's role AND one about the server's role, specific to thin or thick.
Choosing a model: case by case
1 A school network
A school with 600 students and 300 computers. Which model? Justify.
Askedmodel + reasonsKnowmany users, needs controlRoutecentral management → client-server
- Client-server
- central control of security and user access rights (students can't see staff files)
- central backups of everyone's work every night
- software installed and updated once on the server
2 Three friends sharing photos
Three friends at home want to share photos between their laptops. Which model?
Askedmodel + reasonsKnowfew users, no budgetRoutecheap and simple → P2P
- Peer-to-peer
- no server to buy and little setup
- only three users, so central management isn't needed
3 An online multiplayer game
Players around the world play in one virtual world. Why is it client-server?
AskedexplanationKnowone shared worldRouteserver holds the state
- The web server stores all the data for each player; each player's browser is a client
- clients send requests over the internet; the server performs the action, updates the game data and sends the results to the players
4 Exam marking
Examiners mark scanned scripts in a browser; the scripts must never be stored on their own computers. Thin or thick client?
Askedtype + reasonKnowdata must stay centralRoutenothing stored locally → thin
- Thin client: scripts stay on the server, not on the examiner's computer
- examiners can use low-resource devices; but they can't mark if the server or connection is down
Mark-scheme focus
Describe the roles of the computers in a thin-client model (2 marks)
![(c) The computers in one room are set up as thin-clients in a client-server model. Describe the role of the different computers in this model. [2]](/notes-content/9618/communication/img/9618_s23_13_q02_c.png)
Credited
- Server performs all processes required by the task and / or data storage B1
- Clients only send requests to the server and display the returned results B1
Not credited
- “The server serves the clients” (circular)
- Answers that fit any client-server model with no thin-client detail
Why
Thin means the client does almost nothing: say so for the client, and say the server does the processing.
From: 9618/13/M/J/23 Q2(c)
Describe a thick-client model (2 marks)
![(e) The network runs as a thick-client model. Describe what is meant by a thick-client model. [2]](/notes-content/9618/communication/img/9618_s23_12_q01_e.png)
Credited
- The server performs minimal / some processing for the client B1
- The clients do most of their own processing independently // most of the resources are installed locally B1
Not credited
- “The client is more powerful” with no role described
- Only one side (client or server) described
From: 9618/12/M/J/23 Q1(e)
- Client-server roles: 2-4 marks, usually in a context (bank app, online game, school).
- P2P: features and drawbacks; each drawback needs its reason (“weak security” alone was not enough).
- Thin / thick: describe, or give characteristics applied to a given piece of software (4 marks).
Worked examples
Example 1 · Roles in a banking app (Paper 1, 4 marks) ★ Easy
![A bank allows customers to access their accounts using an application on a smartphone. (a) The system is a client-server model. Describe the roles of the different devices in this model. [4]](/notes-content/9618/communication/img/9618_s24_11_q05_a.png)
1 Read it with me what is actually happening?
Two devices to identify IN THIS SCENARIO, and a role for each: 1 mark to name it, 1 to describe it.
- roles of the different devicesserver and client, each named in the bank context
2 What is it asking for? the target, part by part
- (a)identify + describe server and client
3 What do we know? everything the question gives us
- bank, app, smartphone
4 Plan the route work backwards from what we want
FindKey ideaAlready have itWork out firstTrap
- Serverthe bank's computer
- Rolereceives and processes requests
- Clientthe customer's phone app
- Rolesends requests, outputs response
Which device is the client here?
The customer's smartphone running the app.
5 Now solve it have a go on paper first, then open the worked solution
- Server: the bank's computer that stores the account data B1
- It receives requests and processes them (e.g. looks up the balance) and sends back the result B1
- Client: the customer's smartphone / app B1
- It sends a request to the server, waits and outputs the response B1
6 Check it does the answer make sense?
Each device is named in the bank's context AND given a role ✓.
Example 2 · Peer-to-peer: features and two drawbacks (Paper 1, 6 marks) ★★ Medium
![Melinda and her friends set up a peer-to-peer network between their computers to share data. (a) Describe the key features of a peer-to-peer network. [2] (b) Describe two drawbacks to Melinda and her friends of using a peer-to-peer network. [4]](/notes-content/9618/communication/img/9618_s21_11_q04_ab.png)
1 Read it with me what is actually happening?
(b) is 4 marks for 2 drawbacks: each drawback (1) plus why it happens or what it means for Melinda (1).
- key featureswhat makes it P2P
- describe two drawbacksdrawback + reason, specific to P2P
2 What is it asking for? the target, part by part
- (a)two features
- (b)two described drawbacks
3 What do we know? everything the question gives us
4 Plan the route work backwards from what we want
FindKey ideaAlready have itWork out firstTrap
- (b)
- Securityno central management → weakest computer
- Backupnot backed up → lost to all
- Trapdrawbacks of any network
Why is “weak security” not enough?
The report: there must be a reason why the security is weak.
5 Now solve it have a go on paper first, then open the worked solution
- (a) All computers are of equal status B1; each computer provides access to resources and data // each is responsible for its own security B1
- (b) Reduced security / no central management of security B1 … only as secure as the weakest computer // each computer is at risk from viruses on the others B1
- No central management of backup B1 … if one computer's data is not backed up it is lost to all of them B1
6 Check it does the answer make sense?
Both drawbacks are specific to P2P (no central server), each with its consequence ✓.
Example 3 · Thin-client characteristics applied to marking software (Paper 1, 4 marks) ★★ Medium
![An assessment board scans exam papers onto a server; markers download them to mark and upload the marks. (b) The exam markers use software that operates as a thin-client. Complete the table by identifying two characteristics of a thin-client and describe how each characteristic will be used in this software. [4]](/notes-content/9618/communication/img/9618_s24_12_q03_b.png)
1 Read it with me what is actually happening?
A table: a thin-client characteristic in one column, how it shows up in the marking software in the other. Every description must mention the markers / papers.
- characteristica general thin-client feature
- use in this softwarethe same feature applied to the exam marking
2 What is it asking for? the target, part by part
- (b)two characteristics + their use
3 What do we know? everything the question gives us
4 Plan the route work backwards from what we want
FindKey ideaAlready have itWork out firstTrap
- Row 1
- Characteristicdata not stored on the client
- Usepapers stay on the server
- Row 2
- Characteristicreliant on the server / connection
- Usecan't mark if it goes down
5 Now solve it have a go on paper first, then open the worked solution
- Data is not stored on the client computer B1 → exam papers stay on the server, not on the examiner's computer B1
- Client is reliant on access to the server / network B1 → examiners cannot mark if their device can't reach the server B1
- (also: client needs few local resources → examiners can use low-resource devices; client performs minimal processing → software sends requests, server responds)
6 Check it does the answer make sense?
Each use line talks about examiners or papers, not computers in general ✓.
Example 4 · Why a browser game is client-server (Paper 1, 4 marks) ★★ Medium
![(c) The computer program allows users to play a game in a virtual world. Users can play from any computer with internet access using a web browser. The game allows users to interact with other users. Explain the reasons why the statement 'This computer game uses a client-server model' is correct. [4]](/notes-content/9618/communication/img/9618_s25_13_q03_c.png)
1 Read it with me what is actually happening?
Map the game onto the model: where is the data, who asks, who does the work, who gets the results?
- explain the reasons why … is correctshow each part of the model in the game
2 What is it asking for? the target, part by part
- (c)4 reasons
3 What do we know? everything the question gives us
4 Plan the route work backwards from what we want
FindKey ideaAlready have itWork out firstTrap
- Model
- Serverstores every player's data
- Clientthe player's browser
- Flowrequest → action → update → results
5 Now solve it have a go on paper first, then open the worked solution
- A web server stores all the data for each player B1
- Each player's web browser is the client B1, sending requests over the internet to the web server B1
- The server performs the action, updates the game data and sends the results to the players B1
6 Check it does the answer make sense?
All four points describe the game, not client-server in general ✓.
Summary
- Client-server: server stores and processes, clients request and display. Central control; single point of failure; costly.
- Peer-to-peer: equal peers, each shares and secures itself. Cheap; weak central security and backup.
- Thin client: server does the processing, client only requests and displays (needs a constant connection). Thick: client does most processing locally.
| If you see… | do this |
|---|---|
| “describe the roles” in a context | name the server and client in that context + what each does |
| “drawback of P2P” | drawback + the reason / consequence |
| “thin” or “thick” | one point about the client's role, one about the server's |
| “justify a model” | number of users, need for central security / backup, cost |
Can I do this?
- I can describe the roles of the server and the clients, in a given context.
- I can give benefits and drawbacks of client-server.
- I can describe peer-to-peer and its drawbacks, each with a reason.
- I can explain thin and thick clients and their differences.
- I can justify a model for a given situation.
Anything unticked: revise that section first. All ticked: take the quiz.
Quiz
The multiple-choice and guided questions need JavaScript. The concept checks and exam practice below work without it.
Concept checks
Answer each one in your head first, then open it.
1Is a web browser a client or a server?Show answer
A client: it sends requests (for pages) to web servers and displays what they return.
2Can one computer be both a client and a server?Show answer
Yes: in peer-to-peer every computer is both; even in client-server, a server can be a client of another server (e.g. a web server asking a database server).
3Why does a thin client need a reliable network?Show answer
It does no processing or storage of its own; every action is a request to the server.
Exam-style practice
New questions, not used above. Do them on paper, then mark your work against the official mark scheme.
A library provides computers that run a catalogue search program as thin clients.
- (a)Describe what is meant by a thin client.[2]
- (b)Give two benefits to the library of using thin clients.[2]
Show the mark scheme
| Part | Answer / working | Marks | Guidance |
|---|---|---|---|
| (a) | The server performs all / most of the processing and stores the data | B1 | |
| The client only sends requests and displays the results | B1 | ||
| (b) | e.g. cheaper client computers (few local resources needed) | B1 | |
| e.g. software / catalogue updated once on the server // data not stored on public computers, so more secure | B1 | ||
| Total: 4 marks | |||
Four students set up a peer-to-peer network to work on a group project.
Describe two benefits of using a peer-to-peer network for this project.
- [4]
Show the mark scheme
| Part | Answer / working | Marks | Guidance |
|---|---|---|---|
| No server needed … | B1 | ||
| … so it is cheap / quick to set up for a short project | B1 | ||
| No single point of failure … | B1 | ||
| … if one student's computer is off, the others can still share their own files | B1 | ||
| Total: 4 marks | |||
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