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Worked solutions

IB Computer Science HL · Paper 3

Computer Science, Higher Level

Session: November 2019
Marks: 30
Duration: 1 hour
Calculator: Not allowed
Questions: 4

Keep your copy of the paper open alongside. We don't reproduce the questions: each part below is labelled to match the November 2019 Global paper, which your school or the IB can provide.

Practice mode

Hide our working until you've had a go.

Questions

Questions 1–4 · 30 marks

Question 1

4 marks

Mobile hardware constraints and API architecture in Computer Aided Dispatch

Resource management
System fundamentals

(a)

2 marks

Identify two mobile device hardware resources that can constrain user experience during emergency reporting:
1) Battery power capacity: Continuous execution of GPS radios, screen backlights, and high-frequency cellular transmissions rapidly drains mobile batteries, threatening complete device shutdown during ongoing field crises.
2) Primary RAM and processor speed: Limited device memory and modest CPU processing power restrict the client's ability to render complex interactive vector maps or process local image compression before upload.

Mentor tip

Identify physical client-side hardware resources (battery life, RAM, CPU processing power, screen real estate, or GPS chip receiver) rather than backend server resources.

(b)

2 marks

Outline why an Application Programming Interface (API) is essential when interfacing mobile apps with CAD backends:
1) Abstraction and interface standardization: The API establishes a formalized contract defining endpoints, data formats, and parameter types, allowing mobile app developers to request CAD services without needing visibility into internal backend database schemas or server code.
2) Security enforcement and backward compatibility: The API gateway authenticates incoming mobile requests via cryptographic tokens, enforces rate limiting, and shields internal server binaries from direct public exposure while maintaining backward compatibility across evolving app versions.

Mentor tip

Explain abstraction (separating frontend client requests from backend logic) and centralized authentication/security checks.

Question 2

8 marks

Satellite trilateration mechanics and multi-tier architectural standards

Networks
System fundamentals

(a)

4 marks

Outline how a mobile device calculates its physical location using GPS satellite telemetry:
1) Time-stamped orbital broadcasts: GPS satellites in medium Earth orbit continuously transmit radio signals encoding their exact orbital position (ephemeris data) and precise transmission timestamps governed by synchronized onboard atomic clocks.
2) Pseudorange calculation via signal latency: The mobile GPS receiver logs the arrival time of each signal, multiplying the transit time delay by the speed of light ( $d = c \cdot Δ t$ ) to compute the distance (pseudorange) to each satellite.
3) Multi-sphere geometric trilateration: Each calculated distance defines an imaginary sphere centered at that satellite; the intersection of spheres from three satellites pinpoints two coordinate points in space, one of which represents the Earth's surface.
4) Clock error resolution and final positioning: A fourth satellite signal is required to resolve internal receiver quartz clock bias, yielding precise 3D positioning (latitude, longitude, and altitude).

d = c \cdot Δ t

Mentor tip

Walk through time-stamped radio signals, latency multiplied by the speed of light, and geometric trilateration across four satellites to resolve 3D coordinates and clock bias.

(b)

4 marks

Explain the importance of adopting a multi-tier architecture utilizing standard communication protocols:
1) Layered separation of concerns and independent scaling: A multi-tier architecture separates presentation, business logic, and database management into discrete decoupled layers. Teams can independently update, test, or scale application servers across multiple cluster nodes without re-architecting the database or rewriting mobile clients.
2) Interoperability and future-proofing through open standards: Utilizing standard protocols (such as HTTP/HTTPS, REST, JSON, and TCP/IP) ensures seamless communication across heterogeneous mobile platforms (iOS, Android) and municipal dispatch systems. It eliminates proprietary vendor lock-in, facilitates integration with third-party security appliances, and accelerates software maintenance using standard developer libraries.

Mentor tip

Divide your answer into the developer/scaling benefits of multi-tier layering followed by the interoperability and maintainability of standard network protocols.

Question 3

6 marks

High availability, network card redundancy, and cluster failover mechanisms

Computer organisation

6 marks

Explain why a network card failure in an active cluster server will not disrupt ongoing user sessions:
1) Hardware NIC teaming and link aggregation: Enterprise servers utilize redundant Network Interface Cards (NICs) configured in an active-standby or active-active link aggregation pair. If the primary NIC hardware fails, the local operating system detects link loss instantly and switches traffic to the secondary physical card without interrupting host connectivity.
2) Load balancer heartbeat monitoring: An external load balancer continuously dispatches heartbeat health-check probes (e.g., HTTP ping or TCP keepalive) to each node. Upon detecting that a server's interface is unresponsive, the load balancer flags the node as unhealthy and halts traffic routing to that physical machine.
3) Distributed session replication: User session states (including active incident reporting drafts and authentication tokens) are continuously mirrored across cluster nodes via distributed in-memory caches (such as Redis) or shared storage arrays rather than resting exclusively in isolated local RAM.
4) Seamless session failover: The load balancer re-routes the affected user's subsequent request packets to a designated surviving cluster node containing the mirrored session data.
5) User transparency: Because the target node retrieves the existing session context seamlessly, the user completes their incident submission without experiencing dropped connections, error prompts, or data loss.

Mentor tip

Detail the layered failover defense: dual redundant NICs (teaming) on the host, load balancer health-check detection, and synchronized distributed session replication to surviving servers.

Question 4

12 marks

Technical feasibility and ethical implications of guaranteed user anonymity

Networks

12 marks

Evaluate the technical feasibility and ethical dimensions of attempting to guarantee complete anonymity for emergency incident reporters:
1) Comprehensive digital footprints and logging across the network stack:
- ISP and network layer logging: Every packet traversing the Internet carries source IP metadata. Internet Service Providers (ISPs), intermediate autonomous system routers, and cellular tower base stations maintain mandated connection logs associating public IP assignments with subscriber accounts.
- Local network hardware identifiers: Local Wi-Fi routers and intermediate Ethernet switches log device Media Access Control (MAC) addresses in ARP tables, establishing hardware-level breadcrumbs.
- Server-side weblogs: Even if municipal application servers disable access logging, proxy firewalls and intrusion detection appliances routinely record request headers, timestamps, and source endpoints.
2) Technical anonymity anonymizers and their limitations:
- Onion routing (Tor) and multi-hop proxies: Routing reporting traffic through distributed onion networks encrypts payloads across multiple nodes, concealing the origin IP from the destination server. However, onion routing introduces severe latency, making real-time voice streaming impractical, and remains vulnerable to correlation attacks and malicious exit nodes.
- Application-level stripping: The client application could strip all device metadata (IMEI, phone numbers, GPS metadata, and user account IDs) prior to transmission. However, client-side malware or spyware on a compromised operating system can intercept raw inputs before encryption.
3) Physical and evidentiary traceability:
- The content of citizen reports often contains identifying situational details (unique vantage points, specific timestamps, or voice characteristics) that enable forensic reconstruction of the author's identity regardless of digital obfuscation.
4) Ethical dilemmas and public safety tradeoffs:
- Societal benefits of anonymity: Protects vulnerable citizens and whistleblowers from violent retribution when reporting organized crime, domestic violence, or municipal corruption, substantially increasing reporting rates of vital intelligence.
- Threats to emergency resource allocation: Guaranteed anonymity eliminates accountability, enabling malicious actors to inundate emergency dispatchers with hoaxes. Dispatching emergency personnel to fraudulent incidents strains public resources and endangers citizens facing authentic life-threatening crises.
5) Synthesis and judgment:
- It is technically unfeasible to guarantee 100% cryptographic anonymity against state-level or ISP-collaborating forensic investigations. Instead of promising absolute anonymity, Bangbai should implement robust pseudonymous confidentiality: encrypting submissions, enforcing strict access controls on whistleblower records, and stripping casual metadata while maintaining traceable accountability for emergency triage.

Mentor tip

Structure into two major dimensions: technical logging hurdles (IP traces across ISPs, MAC logging, Tor latency, forensic correlation) and the ethical conflict between whistleblower protection and hoax-call resource depletion.

Our analysis

Where the marks are

Each part sorted by the topic it mostly tests. Networks carries the most marks on this paper.

Networks16 marks · 53%
Q2, Q4
System fundamentals6 marks · 20%
Q1, Q2
Computer organisation6 marks · 20%
Q3
Resource management2 marks · 7%
Q1

Mentor insights

Traps to watch for

01
Listing backend server resources instead of smartphone resources in Q1(a) · Q1
The question specifies smartphone resources that constrain the user experience; do not discuss server CPU, cloud database capacity, or backend network bandwidth.
02
Confusing GPS trilateration with cellular tower triangulation · Q2
Satellite GPS calculates distances from orbiting atomic-clock satellites to solve intersecting geometric spheres (trilateration); cellular tower triangulation uses angular bearings from terrestrial towers.
03
Asserting that commercial VPNs or Tor provide absolute unbreakable anonymity · Q3
Complete anonymity cannot be guaranteed due to multi-tier logging at ISPs, cellular towers, intermediate routers, device-level spyware, and traffic correlation analysis.

Solutions and tips are GeorgeTheTutor's own; the questions are not reproduced here. Not affiliated with or endorsed by the International Baccalaureate Organization.

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Worked solutions

IB Computer Science HL · Paper 3

Computer Science, Higher Level

Session: November 2019
Marks: 30
Duration: 1 hour
Calculator: Not allowed
Questions: 4

Q1 Q2 Q3 Q4

Keep your copy of the paper open alongside. We don't reproduce the questions: each part below is labelled to match the November 2019 Global paper, which your school or the IB can provide.

Practice mode

Hide our working until you've had a go.

Questions

Questions 1–4 · 30 marks

Question 1

4 marks

Mobile hardware constraints and API architecture in Computer Aided Dispatch

Resource management
System fundamentals

(a)

2 marks

Identify two mobile device hardware resources that can constrain user experience during emergency reporting:
1) Battery power capacity: Continuous execution of GPS radios, screen backlights, and high-frequency cellular transmissions rapidly drains mobile batteries, threatening complete device shutdown during ongoing field crises.
2) Primary RAM and processor speed: Limited device memory and modest CPU processing power restrict the client's ability to render complex interactive vector maps or process local image compression before upload.

Mentor tip

Identify physical client-side hardware resources (battery life, RAM, CPU processing power, screen real estate, or GPS chip receiver) rather than backend server resources.

(b)

2 marks

Outline why an Application Programming Interface (API) is essential when interfacing mobile apps with CAD backends:
1) Abstraction and interface standardization: The API establishes a formalized contract defining endpoints, data formats, and parameter types, allowing mobile app developers to request CAD services without needing visibility into internal backend database schemas or server code.
2) Security enforcement and backward compatibility: The API gateway authenticates incoming mobile requests via cryptographic tokens, enforces rate limiting, and shields internal server binaries from direct public exposure while maintaining backward compatibility across evolving app versions.

Mentor tip

Explain abstraction (separating frontend client requests from backend logic) and centralized authentication/security checks.

Question 2

8 marks

Satellite trilateration mechanics and multi-tier architectural standards

Networks
System fundamentals

(a)

4 marks

Outline how a mobile device calculates its physical location using GPS satellite telemetry:
1) Time-stamped orbital broadcasts: GPS satellites in medium Earth orbit continuously transmit radio signals encoding their exact orbital position (ephemeris data) and precise transmission timestamps governed by synchronized onboard atomic clocks.
2) Pseudorange calculation via signal latency: The mobile GPS receiver logs the arrival time of each signal, multiplying the transit time delay by the speed of light ( $d = c \cdot Δ t$ ) to compute the distance (pseudorange) to each satellite.
3) Multi-sphere geometric trilateration: Each calculated distance defines an imaginary sphere centered at that satellite; the intersection of spheres from three satellites pinpoints two coordinate points in space, one of which represents the Earth's surface.
4) Clock error resolution and final positioning: A fourth satellite signal is required to resolve internal receiver quartz clock bias, yielding precise 3D positioning (latitude, longitude, and altitude).

d = c \cdot Δ t

Mentor tip

Walk through time-stamped radio signals, latency multiplied by the speed of light, and geometric trilateration across four satellites to resolve 3D coordinates and clock bias.

(b)

4 marks

Explain the importance of adopting a multi-tier architecture utilizing standard communication protocols:
1) Layered separation of concerns and independent scaling: A multi-tier architecture separates presentation, business logic, and database management into discrete decoupled layers. Teams can independently update, test, or scale application servers across multiple cluster nodes without re-architecting the database or rewriting mobile clients.
2) Interoperability and future-proofing through open standards: Utilizing standard protocols (such as HTTP/HTTPS, REST, JSON, and TCP/IP) ensures seamless communication across heterogeneous mobile platforms (iOS, Android) and municipal dispatch systems. It eliminates proprietary vendor lock-in, facilitates integration with third-party security appliances, and accelerates software maintenance using standard developer libraries.

Mentor tip

Divide your answer into the developer/scaling benefits of multi-tier layering followed by the interoperability and maintainability of standard network protocols.

Question 3

6 marks

High availability, network card redundancy, and cluster failover mechanisms

Computer organisation

6 marks

Explain why a network card failure in an active cluster server will not disrupt ongoing user sessions:
1) Hardware NIC teaming and link aggregation: Enterprise servers utilize redundant Network Interface Cards (NICs) configured in an active-standby or active-active link aggregation pair. If the primary NIC hardware fails, the local operating system detects link loss instantly and switches traffic to the secondary physical card without interrupting host connectivity.
2) Load balancer heartbeat monitoring: An external load balancer continuously dispatches heartbeat health-check probes (e.g., HTTP ping or TCP keepalive) to each node. Upon detecting that a server's interface is unresponsive, the load balancer flags the node as unhealthy and halts traffic routing to that physical machine.
3) Distributed session replication: User session states (including active incident reporting drafts and authentication tokens) are continuously mirrored across cluster nodes via distributed in-memory caches (such as Redis) or shared storage arrays rather than resting exclusively in isolated local RAM.
4) Seamless session failover: The load balancer re-routes the affected user's subsequent request packets to a designated surviving cluster node containing the mirrored session data.
5) User transparency: Because the target node retrieves the existing session context seamlessly, the user completes their incident submission without experiencing dropped connections, error prompts, or data loss.

Mentor tip

Detail the layered failover defense: dual redundant NICs (teaming) on the host, load balancer health-check detection, and synchronized distributed session replication to surviving servers.

Question 4

12 marks

Technical feasibility and ethical implications of guaranteed user anonymity

Networks

12 marks

Evaluate the technical feasibility and ethical dimensions of attempting to guarantee complete anonymity for emergency incident reporters:
1) Comprehensive digital footprints and logging across the network stack:
- ISP and network layer logging: Every packet traversing the Internet carries source IP metadata. Internet Service Providers (ISPs), intermediate autonomous system routers, and cellular tower base stations maintain mandated connection logs associating public IP assignments with subscriber accounts.
- Local network hardware identifiers: Local Wi-Fi routers and intermediate Ethernet switches log device Media Access Control (MAC) addresses in ARP tables, establishing hardware-level breadcrumbs.
- Server-side weblogs: Even if municipal application servers disable access logging, proxy firewalls and intrusion detection appliances routinely record request headers, timestamps, and source endpoints.
2) Technical anonymity anonymizers and their limitations:
- Onion routing (Tor) and multi-hop proxies: Routing reporting traffic through distributed onion networks encrypts payloads across multiple nodes, concealing the origin IP from the destination server. However, onion routing introduces severe latency, making real-time voice streaming impractical, and remains vulnerable to correlation attacks and malicious exit nodes.
- Application-level stripping: The client application could strip all device metadata (IMEI, phone numbers, GPS metadata, and user account IDs) prior to transmission. However, client-side malware or spyware on a compromised operating system can intercept raw inputs before encryption.
3) Physical and evidentiary traceability:
- The content of citizen reports often contains identifying situational details (unique vantage points, specific timestamps, or voice characteristics) that enable forensic reconstruction of the author's identity regardless of digital obfuscation.
4) Ethical dilemmas and public safety tradeoffs:
- Societal benefits of anonymity: Protects vulnerable citizens and whistleblowers from violent retribution when reporting organized crime, domestic violence, or municipal corruption, substantially increasing reporting rates of vital intelligence.
- Threats to emergency resource allocation: Guaranteed anonymity eliminates accountability, enabling malicious actors to inundate emergency dispatchers with hoaxes. Dispatching emergency personnel to fraudulent incidents strains public resources and endangers citizens facing authentic life-threatening crises.
5) Synthesis and judgment:
- It is technically unfeasible to guarantee 100% cryptographic anonymity against state-level or ISP-collaborating forensic investigations. Instead of promising absolute anonymity, Bangbai should implement robust pseudonymous confidentiality: encrypting submissions, enforcing strict access controls on whistleblower records, and stripping casual metadata while maintaining traceable accountability for emergency triage.

Mentor tip

Our analysis

Where the marks are

Each part sorted by the topic it mostly tests. Networks carries the most marks on this paper.

Networks16 marks · 53%
Q2, Q4
System fundamentals6 marks · 20%
Q1, Q2
Computer organisation6 marks · 20%
Q3
Resource management2 marks · 7%
Q1

Mentor insights

Traps to watch for

01
Listing backend server resources instead of smartphone resources in Q1(a) · Q1
The question specifies smartphone resources that constrain the user experience; do not discuss server CPU, cloud database capacity, or backend network bandwidth.
02
Confusing GPS trilateration with cellular tower triangulation · Q2
Satellite GPS calculates distances from orbiting atomic-clock satellites to solve intersecting geometric spheres (trilateration); cellular tower triangulation uses angular bearings from terrestrial towers.
03
Asserting that commercial VPNs or Tor provide absolute unbreakable anonymity · Q3
Complete anonymity cannot be guaranteed due to multi-tier logging at ISPs, cellular towers, intermediate routers, device-level spyware, and traffic correlation analysis.

Solutions and tips are GeorgeTheTutor's own; the questions are not reproduced here. Not affiliated with or endorsed by the International Baccalaureate Organization.

Want someone to go through your paper with you?

Our maths mentors took these exams recently. One-to-one, they'll find where your marks are going and fix it before May.

Find your maths mentor More past papers