Communication and Internet Technologies

What you'll learn

This overview covers how data travels between devices and how the internet works, drawing together the data transmission and internet units of CIE IGCSE Computer Science (0478). It explains packets and packet switching, serial and parallel, simplex, half-duplex and full-duplex transmission, USB, methods of error detection (parity checks, checksums, echo checks and ARQ, and check digits), encryption, the difference between the internet and the World Wide Web, URLs, HTTP and HTTPS, web browsers, how web pages are located and retrieved using DNS, and cookies. Each section summarises what you must know; the dedicated sub-topic guides go into greater depth.

Questions in this area often ask you to describe a process in order (how a packet travels, how a web page is retrieved, how ARQ works) or to compare options (serial versus parallel, simplex versus duplex). Sequencing and precise terms earn the marks.

Key terms and definitions

Packet — A small unit of data sent across a network, made up of a header, payload and trailer.

Packet switching — Sending data in packets that may take different routes and are reassembled at the destination.

Router — A device that directs packets towards their destination.

Serial transmission — Sending data one bit at a time down a single wire.

Parallel transmission — Sending several bits at the same time down several wires.

Simplex / half-duplex / full-duplex — Data sent in one direction only / both directions but not at the same time / both directions at the same time.

USB (Universal Serial Bus) — A standard for connecting devices to a computer using serial transmission.

Parity check — An error check that counts the 1s in a byte to confirm odd or even parity.

Checksum — A value calculated from a block of data, recalculated on arrival to check for errors.

Echo check — The receiver sends the data back so the sender can compare it with the original.

ARQ (automatic repeat request) — An error control method using acknowledgements and timeouts to request resending.

Encryption — Scrambling data so that it cannot be understood without the key.

URL — The address of a web page, made of protocol, domain name and file name.

DNS — The system that translates domain names into IP addresses.

Cookie — A small text file stored by a website on a user's device.

Core concepts

Packets and packet switching

Data sent over a network is split into packets. Each packet has:

• a header containing the destination and source IP addresses, the packet number and sometimes the total number of packets;
• a payload — the actual data;
• a trailer that marks the end of the packet and may contain an error check.

In packet switching, each packet is sent independently. Routers examine the destination address and forward each packet along the best available route at that moment, so packets from the same file may travel different paths and arrive out of order. At the destination, packets are reordered using their numbers. If packets are missing, a request is sent for them to be re-sent.

Benefits: networks are used efficiently, and packets can be rerouted around busy or broken links. Drawbacks: packets can arrive out of order or be lost, and reassembly takes time.

Methods of transmission

Serial transmission sends bits one after another down one wire. It is reliable over long distances because bits arrive in order, with no risk of skewing (bits on different wires arriving at slightly different times). It is cheaper, needing fewer wires.

Parallel transmission sends several bits at once on several wires. It can be faster over short distances, but bits may arrive out of sync over long distances, and the cabling is more expensive.

Simplex sends data one way only (a computer sending to a printer). Half-duplex sends both ways but only one direction at a time (walkie-talkies). Full-duplex sends both ways simultaneously (a phone call or broadband connection).

USB

USB is a standard serial interface. Benefits: devices are recognised automatically and drivers are often installed automatically; connectors fit only one way (or are reversible); it supplies power to devices; it is an industry standard supported by most devices. Drawbacks: cable length is limited and transfer speeds are lower than some other connections.

Error detection

Data can be corrupted during transmission by interference. Methods to detect errors:

Parity check. The sender and receiver agree on odd or even parity. A parity bit is added so that the total number of 1s in each byte is odd or even. The receiver checks; if the parity is wrong, an error has occurred. A parity block (parity byte) checks rows and columns, allowing the exact bit in error to be located. Parity cannot detect errors where two bits change.

Checksum. The sender calculates a value from the data using an agreed algorithm and sends it with the data. The receiver recalculates it; if the values differ, an error has occurred and the data is re-sent.

Echo check. The receiver sends the received data back to the sender, which compares it with the original. If they differ, the data is re-sent. It cannot tell whether the error happened on the way there or back.

ARQ. After receiving data, the receiver sends a positive acknowledgement, or a negative one if an error is found. If the sender does not receive an acknowledgement within a set timeout, it re-sends the data automatically.

Check digits are used to detect errors in data entry, such as ISBNs and barcodes: a final digit is calculated from the others and recalculated when the code is entered or scanned.

Encryption

Encryption protects data if it is intercepted: the plaintext is converted into ciphertext using an algorithm and a key. In symmetric encryption the same key encrypts and decrypts, so the key must be shared securely. In asymmetric encryption, data is encrypted with the recipient's public key and can only be decrypted with their private key, which is never shared.

The internet and the World Wide Web

The internet is the global infrastructure of interconnected networks — the hardware, cables and protocols. The World Wide Web is a collection of websites and web pages accessed using the internet and a browser. Email, streaming and online gaming also use the internet but are not the web.

URLs, HTTP and HTTPS

A URL has three main parts, for example https://www.example.com/news/index.html:

• the protocol — https;
• the domain name — www.example.com;
• the web page or file name — /news/index.html.

HTTP is the protocol for transferring web pages. HTTPS is its secure version, encrypting data using SSL/TLS, so information such as passwords and payment details cannot be read if intercepted. Browsers show a padlock for HTTPS sites.

Web browsers

A browser renders HTML to display web pages and provides features such as bookmarks and favourites, a history of visited pages, multiple tabs, navigation (back and forward), storing cookies, an address bar, and checking that sites have valid security certificates.

How a web page is retrieved

1. The user types a URL into the browser.
2. The browser sends the domain name to a DNS server.
3. The DNS server looks up the matching IP address. If it does not have it, it asks another DNS server.
4. The IP address is returned to the browser.
5. The browser sends a request to the web server at that IP address.
6. The web server sends the web page's files (HTML and related files) back.
7. The browser renders the HTML and displays the page.

Cookies

Cookies are small text files stored on the user's computer by a website.

• Session cookies are temporary and deleted when the browser closes; they remember items in a shopping basket during one visit.
• Persistent cookies remain until they expire or are deleted; they remember login details, preferences and language settings, and can be used to track browsing and target adverts.

Cookies make browsing more convenient, but raise privacy concerns about tracking.

Comparing error-detection methods

Worked examples

Example 1: Packet switching

Describe how a file is sent across the internet using packet switching. (4 marks)

Answer: The file is split into packets, each with a header containing the destination address and packet number (1). Each packet is sent independently and routers send it along the best available route (1). Packets may take different routes and arrive out of order (1). At the destination, the packets are reordered using their packet numbers to rebuild the file (1).

Example 2: Parity

Using even parity, what should the parity bit be for the 7 bits 1011001?

There are four 1s, which is already even, so the parity bit is 0.

Example 3: ARQ

Explain how ARQ ensures data arrives correctly. (3 marks)

Answer: The receiver checks the data and sends an acknowledgement back to the sender (1). If an error is found, a negative acknowledgement is sent and the data is re-sent (1). If no acknowledgement arrives within a set timeout, the sender automatically re-sends the data (1).

Example 4: Choosing transmission

Explain why USB connections use serial rather than parallel transmission.

Answer: Serial transmission sends bits one at a time on a single wire, so bits do not become skewed and data arrives reliably. Fewer wires make cables cheaper and more flexible.

Common mistakes and how to avoid them

• Saying the internet and the web are the same.
• Forgetting the packet number in the header — it is how packets are reordered.
• Mixing up half-duplex and full-duplex. Half: one direction at a time.
• Saying an echo check identifies where the error happened. It cannot.
• Leaving out DNS when describing how a web page is retrieved.

Exam technique for "Communication and Internet Technologies"

• For process questions, number your steps and keep them in order.
• For comparisons, give matching points for each option (speed, reliability, distance, cost).
• Name parts precisely: header, payload, trailer; protocol, domain name, file name.
• Explain error checks by describing what the sender does, what the receiver does, and what happens if an error is found.

Quick revision summary

• Packets: header (addresses, packet number), payload, trailer; routers forward; reordered on arrival.
• Serial (one wire, reliable, long distances) vs parallel (many wires, short distances).
• Simplex, half-duplex, full-duplex.
• USB: automatic detection, power, standard; length limits.
• Error detection: parity (bit and block), checksum, echo check, ARQ; check digits for data entry.
• Encryption: symmetric (one key) and asymmetric (public and private keys).
• Internet = infrastructure; web = pages. URL = protocol + domain + file.
• HTTPS uses SSL/TLS; DNS converts domain names to IP addresses; cookies are session or persistent.