CS 1.3 Exchanging Data

What is the difference between lossy and lossless compression?

Lossy compression discards some data permanently to achieve greater size reduction; lossless compression keeps all original data with no loss of quality

Give one benefit and one drawback of lossy compression

Benefit: greatly reduced file sizes, suitable for media streaming. Drawback: irreversible loss of data quality, not suitable for text or archival storage

Give one benefit and one drawback of lossless compression

Benefit: maintains original data integrity, best for text/data requiring accuracy. Drawback: larger file sizes than lossy, requires more bandwidth to stream

*A graphic designer needs to compress a high-resolution logo for professional printing — which compression type should they use and why?

Lossless compression, because it preserves the original image quality exactly, which is essential for professional branding and high-definition print use

What is Run Length Encoding (RLE)?

A lossless compression method that condenses sequences of identical, consecutive elements into a single value with a count, e.g. "AAAABBB" becomes "4A3B"

When is RLE most effective?

When data has lots of repetition/consecutive identical values, e.g. bitmap images with large blocks of the same colour

What is dictionary coding?

A compression method that replaces recurring sequences with shorter, unique codes, using a compiled 'dictionary' that maps original sequences to their codes

Give one difference between RLE and dictionary coding in terms of suitability

RLE is more effective when data has lots of repetition of consecutive identical values; dictionary coding is more versatile (works on any recurring sequence, not just consecutive repeats) but may need more computational resources

What is the purpose of encryption?

Converting readable data into an unreadable format to secure it from unauthorised access

How does symmetric encryption work?

The sender uses a key to encrypt data before transmission, and the receiver uses the same key to decrypt it

Give one benefit and one drawback of symmetric encryption

Benefit: usually faster, ideal for encrypting large amounts of data. Drawback: securely sharing the single key between sender and receiver is difficult — if intercepted, all messages can be decrypted

How does asymmetric encryption work?

It uses two keys: a public key (openly shared) for encryption, and a private key (kept secret by the receiver) for decryption; only the private key can decrypt data encrypted with the matching public key

Give one benefit and one drawback of asymmetric encryption

Benefit: solves the key-sharing problem since the public key can be shared openly without compromising security. Drawback: typically slower than symmetric encryption

*Why is symmetric encryption more suitable than asymmetric for encrypting a large database backup?

Because symmetric encryption is faster and more efficient for bulk data, and here the same person/system both encrypts and decrypts, so the key-sharing weakness of symmetric encryption isn't a problem

What is hashing?

A method of converting any data into a fixed-size string of characters called a digest, where the same input always produces the same hash, but even a small change in input produces a radically different hash

Is hashing reversible? How does this differ from encryption?

No — hashing is a one-way, irreversible process, unlike encryption which is designed to be reversible (decrypted) using a key

Give two common hashing algorithms

Any two of: MD5, SHA-1, SHA-256, SHA-3

How is hashing used for password storage, and why is this more secure than storing plaintext passwords?

When a user signs up, their password is hashed and the hash is stored (not the plaintext); at login, the entered password is hashed again and compared to the stored hash — if a database is breached, attackers only get hashes, not usable passwords

Why is hashing efficient for data retrieval in a database?

Hash digests have a fixed length, making them faster to compare than variable-length strings like email addresses, and a good hash function distributes keys uniformly across a hash table for balanced, quick lookup

How can hashing be used to verify data integrity?

If data is hashed before and after transfer and the two hashes match, this confirms the data was not lost or altered in transit, since identical data always produces identical hashes

Give one key difference between encryption and hashing in terms of reversibility

Encryption is reversible (can be decrypted back to original data with the correct key); hashing is irreversible (a one-way function, cannot be reversed to the original data)

What is a database, and give two benefits of using an electronic one over paper records

An organised collection of data allowing easy storage, retrieval and management; benefits include easier to add/delete/modify/update data, and multiple users from multiple locations can access it simultaneously

What is a field, a record, and a table in database terminology?

A field is a single piece of data; a record is a group of related fields representing one entry; a table is a collection of records with a similar structure

What is a primary key?

A unique identifier for each record in a table, usually an ID number

What is a foreign key?

A field in a table that refers to the primary key in another table, used to link tables and create relationships

What is a secondary key?

A field or fields that are indexed for faster searching

What is indexing in a database, and why is it used?

A technique that speeds up data retrieval by allowing the DBMS to go directly to relevant records rather than scanning every record, similar to using a book's index instead of reading page by page

What is a flat file database, and what problem does it cause?

A database that stores all data in a single table; it causes data redundancy (repeated data) which is inefficient and can lead to inconsistencies if not all copies of repeated data are updated

How does a relational database solve the problems of a flat file database?

By organising data into multiple linked tables using keys (primary/foreign), reducing data redundancy, making more efficient use of storage, and making the database easier to maintain

What is normalisation?

The process of organising a database to reduce data duplication and improve data accuracy and consistency, by applying a set of rules (normal forms)

What are the three requirements for a table to be in First Normal Form (1NF)?

It must contain only atomic (indivisible) values, have no repeating groups, and have a primary key

What is a partial dependency, and how does it relate to Second Normal Form (2NF)?

A partial dependency is when a non-key attribute depends on only part of a composite (multi-field) primary key rather than the whole key; for 2NF, a table must be in 1NF and have no partial dependencies

What is a transitive dependency, and how does it relate to Third Normal Form (3NF)?

A transitive dependency is when a non-key attribute depends on another non-key attribute rather than directly on the primary key; for 3NF, a table must be in 2NF and have no transitive dependencies

*A table has FilmID as its primary key, and a Description field that depends on a Certificate field (not on FilmID directly) — is this table in 3NF? Why/why not?

No — this is a transitive dependency, since Description depends on Certificate rather than depending directly on the primary key FilmID, so the table violates 3NF and Certificate/Description should be moved to a separate table

Name the three types (degrees) of entity relationship

One-to-one, one-to-many, many-to-many

*One customer can place many orders, but each order belongs to one customer — what type of relationship is this?

One-to-many

*An order can contain many products, and a product can appear on many orders — what type of relationship is this?

Many-to-many

How is a many-to-many relationship implemented in an actual relational database?

It must be "resolved" by creating a new link table between the two entities, which holds foreign keys referencing both original tables

What is an Entity Relationship Diagram (ERD)?

A diagram representing the entities (tables) in a database and the relationships between them, using boxes for entities and crow's foot notation for relationships

Name three ways data can be captured for a computer system

Any three of: forms, OMR (Optical Mark Recognition), OCR (Optical Character Recognition), sensors, barcodes, data mining

What is the difference between OMR and OCR?

OMR (Optical Mark Recognition) detects marked areas on paper (e.g. exam answer sheets); OCR (Optical Character Recognition) converts printed or handwritten text into digital, editable format

What is the SQL SELECT command used for?

To retrieve data from a database table, e.g. SELECT name, age FROM users; retrieves the name and age columns from the users table

What does the SQL WHERE clause do?

Filters data based on a specified condition, e.g. WHERE age > 30 retrieves only records where age is greater than 30

What does the SQL LIKE clause do, and give an example with a wildcard

Filters data based on a pattern match, e.g. WHERE name LIKE 'J%' retrieves records where name starts with 'J' (the % is a wildcard for any characters)

What does the SQL DELETE command do?

Removes records from a table that match a given condition, e.g. DELETE FROM Song WHERE Artist = "RandomBits" removes all songs by that artist

What does the SQL DROP command do?

Deletes an entire table from a database, e.g. DROP TABLE users

What is the SQL JOIN command used for?

To combine related data from two or more tables based on a common field (equivalent to an inner join)

What is referential integrity in a database?

A rule ensuring consistency between related tables, so that every value in a foreign key field either matches an existing primary key value in the related table, or is null (if allowed) — preventing "orphaned" records

Give one benefit and one drawback of enforcing referential integrity

Benefit: ensures data consistency and accuracy, prevents orphaned records. Drawback: can impact performance due to additional checks, and requires additional planning/design

What is a transaction in the context of a database?

A sequence of database operations treated as a single unit of work — either all operations succeed together, or none do (e.g. a bank transfer requiring both a withdrawal and a deposit to happen together)

What does ACID stand for?

Atomicity, Consistency, Isolation, Durability

What does Atomicity mean in ACID?

All operations in a transaction succeed or fail as a whole — if any part fails, the entire transaction is rolled back, so partial transactions never occur

What does Consistency mean in ACID?

A transaction must take the database from one valid (consistent) state to another, following all database rules and constraints

What does Isolation mean in ACID?

Transactions are isolated from each other — intermediate states of one transaction are not visible to other transactions, preventing conflicts

What does Durability mean in ACID?

Once a transaction has been committed, it persists even if the system subsequently fails — the completed data is not lost

What is record locking, and why is it needed?

A technique that prevents conflicting access to the same data by multiple transactions/processes at once, ensuring data consistency when multiple users try to read, modify, or delete records simultaneously

What is the difference between a shared (read) lock and an exclusive (write) lock?

A shared lock allows multiple transactions to read a record simultaneously but blocks modification; an exclusive lock allows only one transaction to access and modify a record, blocking all others from reading or writing until it's released

What is redundancy in a database, and what problems can it cause?

When the same piece of data is stored in more than one place in a database, either by accident or design; it can cause inconsistencies in the data and wasted storage space

What is the purpose of network protocols?

They define the rules and formats devices must follow to communicate over a network, ensuring successful, secure, and orderly data transmission

What is protocol layering, and why is it used?

Dividing network protocols into layers that each perform specific functions; it allows modular design (layers can be updated independently), simplifies troubleshooting, and promotes interoperability between different technologies

Name the four layers of the TCP/IP model, in order from top to bottom

Application layer, transport layer, internet layer, link layer

What does the application layer of TCP/IP do?

Interacts directly with software applications (e.g. web browsers, email clients) and prepares data for transmission by converting it into a suitable format

What does the transport layer of TCP/IP do?

Breaks data from the application layer into smaller packets, assigns each a port number, and is responsible for end-to-end communication between source and destination

What does the internet layer of TCP/IP do?

Adds a header to each packet containing the sender's and receiver's IP addresses, and is responsible for routing packets across the network

What does the link layer of TCP/IP do?

Prepares packets for transmission over the physical network by translating digital packets into electrical, optical, or wireless signals

What does HTTP stand for and what is it used for?

HyperText Transfer Protocol — the primary protocol for transferring web content (text, images, video) in a request-response client-server model

What does HTTPS add compared to HTTP?

Encryption for security, used for secure transactions like online banking and shopping

What does FTP stand for and what is it used for?

File Transfer Protocol — used for transferring files between hosts over a network, providing authentication and directory management

What is the difference between TCP and UDP?

TCP (Transmission Control Protocol) provides reliable, ordered, error-checked delivery of packets; UDP (User Datagram Protocol) is a simpler, faster, connectionless protocol that does not guarantee delivery, order, or error-checking

What does DNS stand for and what is its purpose?

Domain Name System — it translates human-readable domain names into the numeric IP addresses that computers use to locate servers

*Describe the process that happens when a user types a URL into their browser (DNS lookup)

The browser checks its cache for the IP address; if not found, it queries a DNS resolver, which asks a root server for the relevant TLD server, which points to the domain's authoritative DNS server, which finally returns the correct IP address, allowing the browser to request the webpage

What is a LAN (Local Area Network)?

A network contained within a small geographical area, typically owned entirely by a single entity, making it generally more secure than a WAN

What is a WAN (Wide Area Network)?

A network that connects multiple LANs over a large geographical area, typically using third-party connections (leased lines, fibre, satellite) provided by ISPs or telecoms companies

Give one advantage and one disadvantage of a LAN

Advantage: centralised management of updates/backups/security. Disadvantage: if hardware fails, the whole network may stop functioning, and access can be slow depending on network traffic

Give one advantage and one disadvantage of a WAN

Advantage: enables communication and data/resource sharing between offices in different locations. Disadvantage: typically slower than a LAN and more vulnerable to security risks since data travels across public networks

What is packet switching?

A networking method that breaks data into smaller packets, sends them separately (potentially via different routes), and reassembles them at the destination

Give one benefit and one drawback of packet switching

Benefit: efficient use of network resources as packets share bandwidth and can take different paths. Drawback: not ideal for real-time services like video calls, since packets can arrive out of order and need reassembly

What is circuit switching?

A communication method where a dedicated path is established between two devices for the whole duration of the communication, and all data travels along that same fixed route

Give one benefit and one drawback of circuit switching

Benefit: ideal for real-time services with a constant, steady transmission rate and no delays. Drawback: less efficient, as the dedicated path remains allocated even when no data is being sent, and it is more costly

*Why is circuit switching more suitable than packet switching for a live phone call?

Because circuit switching provides a constant, steady transmission rate with no delays and delivers data in order along a single fixed path, which is essential for smooth, real-time voice communication

What are the three main components of a data packet?

Header (metadata like source/destination IP), payload (the actual data), and footer/trailer (marks the end of the packet)

What is a hacker, in the context of network security?

An individual or group who exploits system vulnerabilities to gain unauthorised access to data

What is the difference between a virus and a worm?

A virus attaches itself to a legitimate program/file and needs that host to spread; a worm is a standalone program that can spread and replicate itself across networks without needing to attach to another file

What is a Trojan horse?

Malware that disguises itself as a legitimate program or file, but performs malicious actions (e.g. deleting data, damaging hardware) once installed

What is ransomware?

Malware that encrypts a user's files and demands a ransom payment to decrypt them

What is the difference between a DoS and a DDoS attack?

A DoS (Denial of Service) attack floods a server with requests from one source, causing it to crash or become unavailable; a DDoS (Distributed Denial of Service) attack does the same thing but uses multiple computers (bots) to send the requests

What is SQL injection?

An attack that inserts malicious SQL statements into an entry field on a website to exploit security vulnerabilities, potentially exposing a company's database to attackers

What is phishing?

Sending fraudulent communications (e.g. emails) that appear to come from a trustworthy source, tricking users into revealing sensitive information via a fake website link

What is pharming?

A cyber attack that redirects a website's traffic to a fake/bogus site, typically via malware downloaded without the user's knowledge, to steal sensitive information

What is social engineering?

Manipulating individuals (rather than exploiting technical vulnerabilities) to gain access to confidential information or systems, e.g. posing as IT support or leaving a tempting USB drive in a public place

Give three methods used to improve network security

Any three of: firewalls, secure passwords, anti-virus software, anti-spyware software, two-factor authentication (2FA), regular software updates, employee training

What is a modem, and what does it do?

A device (modulator-demodulator) that converts digital signals to analogue for transmission over telephone/cable lines, and converts analogue signals back to digital when receiving

What is a router, and what is its main role?

A device that joins two networks together (e.g. a LAN to the internet); it analyses data packets' headers to determine the destination and forwards them along the best path, using a routing table

What is a Network Interface Card (NIC)?

A hardware component that enables a device to connect to a network, with a unique MAC address, converting the computer's data into a network-friendly format

What is the difference between a hub and a switch?

A hub broadcasts any received data to every device on the network ("dumb" device, less efficient and less secure); a switch uses a lookup table of MAC addresses to send data only to the specific intended device, improving efficiency and security

What is a Wireless Access Point (WAP)?

A device that connects to a wired network and projects a Wi-Fi signal, acting as a central transmitter/receiver for wireless devices in a designated area

What is a client-server network?

A network where powerful, reliable server computers control the network and provide services (files, email, web access) to client devices that connect to them

Give one benefit and one drawback of a client-server network

Benefit: higher reliability and easier central management, as resources are managed centrally. Drawback: single point of failure — if the server goes down, services can become unavailable, and it's expensive to set up/maintain

What is a peer-to-peer network?

A network where all computers share equal responsibility and status, with no central server — each machine's user is responsible for their own security, backups, and data

Give one benefit and one drawback of a peer-to-peer network

Benefit: easy and inexpensive to set up, with no dependency on a central server. Drawback: lacks central control, which can lead to security issues, and it isn't suitable for large networks due to performance issues