Exchanging Data — OCR A Level Computer Science

📝 Questions (60)📚 Notes

Practice Questions

60 questions

What is the difference between synchronous and asynchronous transmission?

medium

What is a repeater?

easy

What does CSMA/CD stand for and where is it used?

medium

In serial data transmission, bits are sent:

easy

Explain the difference between serial and parallel data transmission, and give one example of where each is used.

medium

USB (Universal Serial Bus) transmits data using which method?

easy

Explain why serial transmission is generally preferred over parallel transmission for sending data over longer distances.

hard

In synchronous data transmission, the sender and receiver are kept in time using:

medium

Explain how 'start' and 'stop' bits are used in asynchronous data transmission.

medium

'Baud rate' is a measure of:

medium

What is a checksum?

easy

Explain how a parity bit is used in error detection.

easy

What does Hamming code provide that simple parity does not?

hard

What is the purpose of the checksum included in many network protocols?

medium

Describe how a hash function could be used to verify that a downloaded file has not been corrupted.

hard

What does CRC stand for, and what is it used for?

medium

Explain, in outline, how a Cyclic Redundancy Check (CRC) is used to detect errors in transmitted data.

hard

A system uses even parity. Which of the following bytes contains a parity error (assume the rightmost bit is the parity bit)?

medium

Explain a key limitation of using a single parity bit for error detection.

medium

Compared to a single parity bit, a checksum calculated over a whole block of data:

medium

A database table stores StudentID, Forename, Surname and TutorGroup. Which field is most likely to be the primary key?

easy

Explain the difference between lossy and lossless compression and why a word-processed document would always use lossless compression.

medium

A database stores customer orders. Each order belongs to exactly one customer, but a customer can place many orders. What type of relationship is this?

easy

Explain one benefit and one drawback of storing data in a normalised relational database compared to a single large unnormalised table.

hard

Run-length encoding (RLE) compression works best on data that contains:

medium

Explain how run-length encoding (RLE) compresses data, using an example.

medium

In Huffman coding, characters that occur more frequently in the data are given:

medium

Briefly describe how a Huffman tree is constructed and used to compress data.

hard

Which of the following file formats typically uses lossy compression?

easy

Explain the trade-off involved when choosing to use lossy compression for a photograph that will be shared online.

medium

What is the function of an SSL/TLS certificate?

medium

What is the difference between symmetric and asymmetric encryption?

hard

Explain why data sent over a public network (e.g. the internet) is often encrypted, even if it is also being error-checked.

medium

In public-key (asymmetric) cryptography, the public key is used to:

medium

Describe, in outline, the main steps of the SSL/TLS handshake when a browser connects securely to a website.

hard

A digital signature is created by:

medium

Explain how public-key (asymmetric) cryptography solves the 'key distribution problem' associated with symmetric encryption.

hard

In asymmetric encryption, which key must be kept secret by its owner and never shared?

easy

Explain the role of a Certificate Authority (CA) in the SSL/TLS handshake process.

medium

When a website's address shows 'https://' and a padlock icon in the browser, this indicates that:

easy

Describe the client-server model.

easy

What is the difference between circuit switching and packet switching?

medium

Explain the difference between star and mesh topologies.

medium

What does NAT do?

hard

What is the role of DNS?

easy

Explain what is meant by "client-server" networking, including the role of the server.

medium

A network uses the star topology. What happens if the central switch fails?

easy

Which of the following best describes the purpose of the Domain Name System (DNS)?

easy

What is the purpose of an IP address?

easy

Explain how a router uses a routing table to forward a packet towards its destination.

medium

Explain what a VLAN is and give one reason why organisations use them.

hard

What is the difference between TCP and UDP?

medium

Which layer of the TCP/IP model handles IP addressing and routing?

medium

What is a firewall and how does it protect a network?

easy

The TCP/IP model is typically described as having how many layers?

easy

Describe the role of the Application layer in the TCP/IP model, giving two example protocols.

medium

Which two protocols operate at the Transport layer of the TCP/IP model?

medium

Describe the role of the Link (Network Access) layer in the TCP/IP model.

medium

Which Application layer protocol is used by web browsers to request and receive web pages?

easy

Explain what is meant by 'encapsulation' as data passes down through the layers of the TCP/IP model before being transmitted.

hard

Revision Notes

Compression, Encryption & Hashing

⭐ Exam tip: The classic 6-marker: "Explain how HTTPS uses both asymmetric and symmetric encryption." Cover the handshake (asymmetric → exchange a session key) then bulk data (symmetric → speed).

Compression	Description	Quality	Examples
Lossless	Original reconstructed bit-for-bit	100%	RLE, Huffman, ZIP, PNG, FLAC
Lossy	Data permanently discarded for smaller size	Reduced	JPEG, MP3, AAC, H.264

Run-length encoding (RLE):Lossless: replaces a run of repeated values with a (count, value) pair. Excellent for images with large blocks of one colour (logos, icons); can INCREASE size for data with few repeats, since each non-repeat becomes a pair.

RLE example

Pixel row: WWWWWWBBBWWWWWWWWWWWWB  (22 pixels)
RLE:       6W 3B 12W 1B  → 4 pairs (8 values)
→ much smaller than 22 values.

Worst case: WBWBWB → 1W 1B 1W 1B 1W 1B
→ 12 values vs 6 → BIGGER (no repeats to exploit).

Dictionary / Huffman coding:Lossless variable-length coding: frequent symbols get short bit-codes, rare symbols get long ones, derived from a frequency-ordered binary tree. The decoder needs the tree/dictionary. Used in ZIP/DEFLATE and inside JPEG and MP3.

Huffman coding

Text "AAAABBBCCD"  freq: A=4 B=3 C=2 D=1
Build tree by repeatedly merging the two lowest:
  (D1,C2)->3 ; (3,B3)->6 ; (6,A4)->10
Resulting codes (shorter for frequent):
  A=0  B=10  C=110  D=111
Fixed 8-bit:   10 chars x 8 = 80 bits
Huffman: 4(1)+3(2)+2(3)+1(3)=4+6+6+3 = 19 bits
Compression ratio ≈ 4:1

Symmetric encryption:The SAME secret key encrypts and decrypts. Fast, so it is used for bulk data (AES). The weakness is key distribution — securely getting the shared key to the other party over an untrusted network.

Asymmetric (public-key) encryption:Each party has a public key (shared openly) and a private key (kept secret). Anything encrypted with the public key can only be decrypted with the matching private key. Solves key exchange and enables digital signatures, but is ~100–1000× slower than symmetric, so it is not used for bulk data.

Hashing & digital signatures:A hash function maps any input to a fixed-size digest — deterministic, fast, one-way and collision-resistant. Used for password storage (store the hash, never the password), file-integrity checks, and digital signatures: the sender signs the hash of a message with their private key; anyone can verify it with the sender's public key, proving authenticity and that the message was not altered.

TLS handshake (HTTPS)

1. Client → server: "hello", supported cipher suites
2. Server → client: digital certificate (contains
   server's PUBLIC key, signed by a CA)
3. Client verifies the certificate via the CA
4. A symmetric SESSION key is agreed using the
   server's public key (asymmetric)
5. All further data is encrypted with the fast
   SYMMETRIC session key (AES)

💡 Model answer for using both: "Asymmetric encryption securely establishes a shared symmetric session key without any pre-shared secret. Asymmetric is too slow for all the data, so once the key is agreed, fast symmetric encryption protects the actual traffic — giving both secure key exchange and high-speed bulk encryption."

⚠️ Common mistake: Hashing is NOT encryption. Encryption is reversible (with the key); hashing is one-way and cannot be reversed to recover the input — which is exactly why it is used to store passwords.

Databases & SQL

⭐ Exam tip: Normalisation to 3NF and writing correct SQL (SELECT…JOIN, INSERT, UPDATE, DELETE) appear most years. Learn the precise definition of each normal form.

Flat file vs relational:A flat file stores everything in one table, causing data redundancy (repetition) and update/insertion/deletion anomalies. A relational database splits data into linked tables, each about one entity, removing redundancy and improving integrity and consistency.

Normal form	A table is in this form when…
1NF	all values are atomic (no repeating groups), and each record is unique
2NF	it is in 1NF AND every non-key attribute depends on the WHOLE primary key (no partial dependency)
3NF	it is in 2NF AND no non-key attribute depends on another non-key attribute (no transitive dependency)

Keys & referential integrity:A primary key uniquely identifies each record. A foreign key is a primary key of another table used to create a relationship. Referential integrity means every foreign key value must match an existing primary key — preventing "orphaned" records that reference something which does not exist.

SQL — query, join and modify

SELECT name, grade FROM Student
WHERE grade >= 60
ORDER BY grade DESC;

-- join two tables
SELECT s.name, c.title
FROM Student s
INNER JOIN Class c ON s.classID = c.classID
WHERE c.title = 'Computer Science';

INSERT INTO Student (id, name, grade)
VALUES (7, 'Sam', 72);
UPDATE Student SET grade = 80 WHERE id = 7;
DELETE FROM Student WHERE id = 7;

Transactions & ACID:A transaction is a group of operations treated as a single unit. ACID guarantees: Atomicity (all operations succeed or none do), Consistency (the database moves from one valid state to another), Isolation (concurrent transactions do not interfere — achieved by record locking), Durability (once committed, changes survive power loss/crash).

Why atomicity matters

Bank transfer = two operations:
  1. subtract £50 from account A
  2. add £50 to account B
If the system crashes after step 1 only, £50 vanishes.
Wrapping both in ONE transaction means either BOTH
happen or NEITHER does — money is never lost.

💡 Quote the rule when normalising: not "split the table" but "remove the partial dependency to reach 2NF" / "remove the transitive dependency to reach 3NF". Examiners award the precise terminology.

⚠️ Common mistake: A common SQL slip: forgetting the WHERE clause on UPDATE/DELETE. "DELETE FROM Student;" deletes EVERY record. Always include the condition that selects the intended rows.

Networks, Protocols & Layers

LAN vs WAN:A LAN (Local Area Network) covers a small geographic area using infrastructure the owner controls (an office, a school). A WAN (Wide Area Network) connects LANs over a large area using third-party communication links — the internet is the largest WAN.

Packet switching vs circuit switching:Packet switching splits data into packets, each routed independently and reassembled in order at the destination — efficient (links are shared) and resilient (packets reroute around failures). Circuit switching reserves a dedicated path for the whole communication (traditional telephone calls) — guaranteed bandwidth but wasteful when idle.

Protocol	Layer	Purpose
HTTP / HTTPS	Application	Transfer web pages; HTTPS adds TLS encryption
FTP	Application	Transfer files between hosts
SMTP	Application	Send email
IMAP / POP3	Application	Retrieve email (IMAP keeps mail on server; POP3 downloads)
DNS	Application	Resolve a domain name to an IP address
TCP	Transport	Reliable, ordered, connection-oriented delivery
UDP	Transport	Fast, connectionless, no delivery guarantee
IP	Internet	Address and route packets across networks

Why use a layered model?

The TCP/IP stack (Application, Transport, Internet, Link) divides networking into layers, each with one responsibility. Benefits: each layer can be developed/changed independently (e.g. swap WiFi for Ethernet at the Link layer without touching HTTP), it standardises interfaces so products from different vendors interoperate, and it simplifies troubleshooting.

DNS resolution:When you type a domain, your computer queries DNS servers in a hierarchy (resolver → root → top-level-domain → authoritative) until it gets the matching IP address, which is then cached. Without DNS we would have to remember numeric IP addresses.

TCP/IP & sockets:A socket is a communication endpoint defined by an IP address + port number. A connection is uniquely identified by the four-tuple (client IP, client port, server IP, server port). Servers listen on well-known ports — 80 for HTTP, 443 for HTTPS.

💡 "State TWO advantages of packet switching": (1) efficient use of the network because links are shared between many communications rather than reserved; (2) resilience — if a link fails, packets are rerouted along a different path so the communication continues.

Web Technologies

HTML, CSS & JavaScript:HTML defines the structure/content of a page; CSS defines its presentation/style; JavaScript adds client-side interactivity and dynamic behaviour. Separating them keeps a site maintainable — one CSS file restyles every page consistently.

Processing	Runs	Pros	Cons
Client-side (JS)	In the user's browser	Fast/responsive; reduces server load; instant validation feedback	Code is visible/editable; can be disabled; depends on the device
Server-side	On the web server	Secure (code hidden); can access databases; consistent for all users	Adds server load; needs a network round-trip

Search engine indexing & PageRank

Search engines run web crawlers (spiders) that follow hyperlinks to discover pages and build an index mapping keywords to pages. PageRank measures a page's importance by the number AND quality of pages linking to it: a link from a high-ranked page passes more "authority" than one from a low-ranked page, and importance is shared out across each page's outbound links.

PageRank formula (idea)

PR(A) = (1 - d) + d * Σ ( PR(Ti) / C(Ti) )
  d      = damping factor (≈ 0.85)
  Ti     = pages that link TO page A
  PR(Ti) = PageRank of linking page Ti
  C(Ti)  = number of outbound links on Ti
Start all pages equal, then iterate until values
stabilise. A page gains rank from being linked to
by important pages that do not link to everything.

💡 Validation should be done on BOTH sides: client-side for instant user feedback and reduced server load, server-side because client-side checks can be bypassed (the user can disable JavaScript or edit the request). State both for full marks.

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