Cambridge AS & A Level9608

Communication and Internet technologies

Computer Science 9608 Chapter Notes

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Communication and Internet technologies
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In this chapter

  1. 1Data Transmission Media
  2. 2Switching and Data Flow
  3. 3Internet vs World Wide Web
  4. 4IP Addressing and DNS
  5. 5Wireless Communication Technologies
  6. 6Client-Side vs. Server-Side Processing

1. Data Transmission Media

Data transmission media are the physical pathways through which data is sent from one device to another. These can be wired (using cables) or wireless (using waves). The choice of medium depends on factors like cost, speed (bandwidth), distance, security, and the environment. Key characteristics to consider are bandwidth (the maximum data transfer rate), attenuation (the weakening of a signal over distance), and interference (unwanted noise from external sources that can corrupt the signal).

Key term

Bandwidth: The maximum rate of data transfer across a given path, typically measured in bits per second (bps).

Examiner insight

Examiners look for comparative answers. When asked to compare two technologies, you must explicitly mention both and use comparative words like 'higher', 'faster', 'less susceptible to', rather than just describing each one in isolation.

Common pitfall

Stating that wireless is always 'faster' than wired. While convenient, high-end wired connections like fibre-optic typically offer much higher and more stable speeds than most common wireless technologies.

Worked example 16 marks

A school is upgrading its network. It needs to connect two buildings that are 500 metres apart. Compare the suitability of using UTP (Unshielded Twisted Pair) copper cable versus fibre-optic cable for this connection. [6 marks]

  1. 1
    1. Fibre-optic cable is more suitable for this distance. UTP cable suffers from significant signal degradation (attenuation) over distances greater than 100 metres, whereas fibre-optic can run for kilometres with minimal loss.
  2. 2
    1. Bandwidth: Fibre-optic offers significantly higher bandwidth than UTP. This would support future needs and handle high traffic between buildings, such as video streaming or large file transfers.
  3. 3
    1. Interference: Fibre-optic cable transmits data as pulses of light, making it immune to electromagnetic interference (EMI) from power lines or other electrical equipment. UTP is susceptible to EMI, which could cause data corruption.
  4. 4
    1. Security: It is much harder to 'tap into' a fibre-optic cable without being detected compared to a copper cable, offering better data security.
  5. 5
    1. Cost: While the initial installation cost of fibre-optic cable might be higher, its superior performance, longevity, and lower maintenance make it a better long-term investment for a critical backbone link.
  6. 6
    1. Conclusion: For connecting buildings 500m apart, fibre-optic is the clear choice due to its superior range, bandwidth, and immunity to interference.

Recap

  • Transmission media are the physical channels for data transfer.
  • Wired media include twisted pair (copper), coaxial, and fibre-optic cables.
  • Wireless media use radio waves, microwaves, or infrared.
  • Fibre-optic cables offer the highest bandwidth and are immune to EMI.
  • Attenuation is the weakening of a signal over distance.
  • Interference is external noise that can corrupt data signals.

Quick check

  1. State one advantage of fibre-optic cable over copper cable.1 mark
  2. What is attenuation in the context of data transmission?1 mark

2. Switching and Data Flow

For data to get from a source to a destination across a network, a path must be established. This is done through switching. There are two main methods: circuit switching and packet switching. Circuit switching creates a dedicated, fixed connection (a 'circuit') for the entire duration of the communication, like a traditional phone call. Packet switching breaks the data down into small blocks called packets. Each packet is sent independently through the network and reassembled at the destination. The internet is built on packet switching. Data flow on a link can also be classified: Simplex (one-way, like a radio broadcast), Half-duplex (two-way, but not simultaneously, like a walkie-talkie), and Full-duplex (two-way, simultaneously, like a telephone conversation).

Key term

Packet Switching: A method of grouping data into packets that are transmitted over a digital network, where packets are routed independently and reassembled at the destination.

Common pitfall

Confusing packets with frames. Packets are a Network Layer (Layer 3) concept containing IP addresses, while frames are a Data Link Layer (Layer 2) concept containing MAC addresses. Essentially, packets are put inside frames for transmission on a local network.

Fun fact

The original idea for packet switching was developed in the 1960s to create a communication network that could survive a nuclear attack by having no single point of failure.

Worked example 14 marks

Explain two advantages of using packet switching over circuit switching for browsing the World Wide Web. [4 marks]

  1. 1
    1. Efficient use of network resources: With packet switching, multiple users can share the same network links. Packets from different users are interleaved, meaning the line is not tied up if a user is just reading a webpage and not actively downloading data. In circuit switching, the dedicated circuit would be idle for much of this time, wasting bandwidth.
  2. 2
    1. Robustness and Reliability: If a particular link in the network fails, packets can be dynamically rerouted around the fault. In circuit switching, if any part of the dedicated circuit fails, the entire connection is lost and must be re-established.

Worked example 22 marks

A security system uses a motion sensor to send a signal to a central alarm unit. The alarm unit can sound a siren but cannot send a signal back to the sensor. Identify and describe the mode of data flow used. [2 marks]

  1. 1
    1. Identification: The mode of data flow is Simplex.
  2. 2
    1. Description: This is because data travels in only one direction, from the motion sensor to the alarm unit. There is no communication in the reverse direction.

Recap

  • Circuit switching establishes a dedicated path for the entire communication session.
  • Packet switching breaks data into packets that are routed independently.
  • The Internet uses packet switching, which is efficient and robust.
  • Simplex data flow is one-way only.
  • Half-duplex is two-way, but not at the same time.
  • Full-duplex is two-way, simultaneous communication.

Quick check

  1. Which switching method does the traditional telephone network (PSTN) use?1 mark
  2. Give an example of a device that uses half-duplex communication.1 mark

3. Internet vs World Wide Web

The terms 'Internet' and 'World Wide Web' are often used interchangeably, but they are not the same. The Internet is the global network of interconnected computers—the physical infrastructure of routers, cables, and servers. It allows devices to communicate with each other using protocols like TCP/IP. The World Wide Web (WWW or 'the Web') is one of the services that runs on the Internet. It's a vast collection of interlinked documents (web pages) and other resources, accessed via URLs and displayed by web browsers. Think of the Internet as the road network and the Web as the collection of shops and houses you can visit using those roads. Access to the Internet is typically provided by an Internet Service Provider (ISP).

Key term

Internet: A global system of interconnected computer networks that use the Internet protocol suite (TCP/IP) to communicate between networks and devices.

Examiner insight

Clarity is key. A good answer will define the Internet as the infrastructure and the Web as a service running on that infrastructure. Using a clear analogy, like roads and cars or a library and its books, can earn high marks.

Common pitfall

Thinking the Web is the only thing the Internet is used for. Services like email (SMTP), file transfer (FTP), and online gaming all use the Internet but are not strictly part of the World Wide Web.

Worked example 14 marks

Using an analogy, explain the difference between the Internet and the World Wide Web. [4 marks]

  1. 1
    1. Analogy: The Internet is like the global postal system infrastructure – the mail trucks, sorting offices, and delivery routes.
  2. 2
    1. Internet Explained: It is the physical and logical network that allows data to be sent and received between any two connected points.
  3. 3
    1. WWW Analogy: The World Wide Web is like the letters and parcels that are sent via the postal system.
  4. 4
    1. WWW Explained: It is the information (like web pages, images, and videos) that travels across the Internet. It is a service that uses the Internet's infrastructure.

Recap

  • The Internet is the global hardware and infrastructure.
  • The World Wide Web is a service that runs on the Internet.
  • The Internet is a network of networks; the Web is a collection of information.
  • You use the Internet to access the Web, send emails, and make VoIP calls.
  • ISPs (Internet Service Providers) provide access to the Internet.
  • Protocols like TCP/IP govern communication on the Internet.

Quick check

  1. Is email part of the Internet or the World Wide Web?1 mark
  2. What is the acronym for the company that provides you with Internet access?1 mark

4. IP Addressing and DNS

Every device connected to the Internet needs a unique identifier, known as an Internet Protocol (IP) address. This address allows data packets to be routed to the correct destination. The current standard, IPv4, uses a 32-bit address (e.g., 172.217.169.68), which allows for about 4.3 billion unique addresses. As this supply is exhausted, the world is transitioning to IPv6, which uses a 128-bit address (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334), providing a virtually limitless number of addresses. Since remembering long strings of numbers is difficult for humans, the Domain Name System (DNS) was created. DNS acts as the Internet's phonebook, translating human-friendly domain names (like www.google.com) into their corresponding IP addresses that computers use to connect.

Key term

Domain Name System (DNS): A hierarchical and distributed naming system that translates human-readable domain names into machine-readable IP addresses.

Examiner insight

Examiners appreciate detailed, step-by-step descriptions of processes like DNS lookup. Mentioning the different levels of servers (root, TLD, authoritative) demonstrates a thorough understanding and will score higher than a simple 'it asks a server for the IP address' answer.

Common pitfall

Confusing a public IP address with a private IP address. A public IP is unique on the entire internet, assigned by your ISP. A private IP (like 192.168.x.x) is used only within your local network (e.g., home WiFi) and is not reachable from the outside internet.

Worked example 15 marks

Describe the steps involved when a user types 'www.cambridge.org' into their web browser until the IP address is found. [5 marks]

  1. 1
    1. The user's browser first checks its own cache, and then the operating system's cache, to see if it has recently looked up this domain.
  2. 2
    1. If not found, the request is sent to a local DNS resolver, usually operated by the user's ISP.
  3. 3
    1. The resolver queries a DNS root server. The root server doesn't know the IP address but directs the resolver to the Top-Level Domain (TLD) server for '.org'.
  4. 4
    1. The resolver then queries the '.org' TLD server. This server doesn't have the final IP but knows the authoritative name server for the 'cambridge.org' domain and provides its address.
  5. 5
    1. Finally, the resolver queries the 'cambridge.org' authoritative name server, which holds the actual DNS records. This server returns the IP address for 'www.cambridge.org' to the resolver.
  6. 6
    1. The resolver passes this IP address back to the user's browser, which can now initiate a connection to the web server.

Recap

  • An IP address is a unique numerical label for a device on a network.
  • IPv4 is a 32-bit address, while the newer IPv6 standard uses 128 bits.
  • The main reason for IPv6 is to solve the shortage of IPv4 addresses.
  • DNS translates domain names (e.g., bbc.co.uk) into IP addresses.
  • The DNS system is a hierarchical network of servers (root, TLD, authoritative).
  • Your computer caches DNS lookups to speed up future requests.

Quick check

  1. How many bits are in an IPv4 address?1 mark
  2. What is the primary function of the Domain Name System?1 mark

5. Wireless Communication Technologies

Wireless communication transmits data through the air using electromagnetic waves, removing the need for physical cables. Several key technologies operate at different scales. WiFi (Wireless Fidelity), based on the IEEE 802.11 standards, is used for creating Wireless Local Area Networks (WLANs). It allows devices to connect to a nearby wireless access point, which is typically wired to the internet. WiMAX (Worldwide Interoperability for Microwave Access), based on IEEE 802.16, is a longer-range technology designed to provide wireless broadband access over a larger area, like a city (a Wireless Metropolitan Area Network or WMAN). Cellular networks (e.g., 3G, 4G, 5G) provide mobile internet access over vast geographical areas using a grid of cells, each with a base station (cell tower).

Key term

WiFi (IEEE 802.11): A set of wireless networking protocols used for short-range data transfer and to create Wireless Local Area Networks (WLANs).

Fun fact

The term 'cell phone' comes from the fact that cellular networks divide service areas into a grid of 'cells', each with its own base station. As you move from one cell to another, your phone call is seamlessly 'handed off' between towers.

Worked example 14 marks

A coffee shop wants to offer internet access to its customers. Explain why WiFi would be a more suitable technology than WiMAX or a 4G cellular connection for this purpose. [4 marks]

  1. 1
    1. Scale and Purpose: WiFi is designed for Local Area Networks (LANs), covering a small area like a single building, which perfectly matches the coffee shop's requirement. WiMAX and cellular are for much larger areas (MANs and WANs) and would be overly complex and expensive.
  2. 2
    1. Infrastructure: The coffee shop can install a single wireless access point connected to its existing broadband line. This is a simple and low-cost setup.
  3. 3
    1. User Devices: Nearly all modern laptops, tablets, and smartphones have built-in WiFi capabilities, making it easy for customers to connect without needing special hardware.
  4. 4
    1. Cost: Providing access via WiFi is significantly cheaper than setting up a private WiMAX base station or paying for individual cellular data plans for each customer.

Recap

  • Wireless technologies use electromagnetic waves to transmit data.
  • WiFi (IEEE 802.11) is used for Wireless Local Area Networks (WLANs).
  • WiMAX (IEEE 802.16) is a longer-range technology for Metropolitan Area Networks (MANs).
  • Cellular networks (3G, 4G, 5G) provide mobile data over wide areas using cell towers.
  • The 'G' in 3G/4G/5G stands for 'Generation', with each offering higher speeds and capabilities.
  • A wireless access point (WAP) is the central hub for a WiFi network.

Quick check

  1. Which IEEE standard number is associated with WiFi?1 mark
  2. Which wireless technology is designed to cover an entire city?1 mark

6. Client-Side vs. Server-Side Processing

When you interact with a modern website, processing happens in two places: on your device (client-side) and on the web server (server-side). Client-side processing uses scripts that run in your web browser. It's used for tasks that need a fast response and don't require access to private data, like validating a form entry (e.g., checking if an email address has an '@' symbol) or creating interactive menus. JavaScript is the primary language for client-side scripting. Server-side processing happens on the web server before a page is sent to the browser. It's used for tasks that require access to a database, processing payments, or personalising content. Languages like PHP, Python, and Ruby are used for server-side scripting. The server runs the script, generates a plain HTML page as output, and sends that to the client's browser.

Key term

Server-Side Scripting: The processing of scripts on the web server to generate dynamic web page content before it is sent to the user's browser.

Examiner insight

Good answers clearly link the type of processing (client or server) to the specific task and justify why it is appropriate. For example, 'Database access requires server-side processing because the database is stored securely on the server and should not be exposed to the client'.

Common pitfall

Assuming client-side validation is enough for security. An attacker can easily bypass client-side scripts. Therefore, all data received from a user must be validated again on the server before it is used or stored.

Worked example 16 marks

A user is filling out a registration form on a website. The form requires a username that is not already taken. Explain where client-side and server-side processing would be used in this scenario. [6 marks]

  1. 1
    1. Client-Side (Initial Checks): When the user enters their email, a client-side script (JavaScript) can immediately check if the format is valid (e.g., contains '@' and '.'). This provides instant feedback without needing to contact the server.
  2. 2
    1. Client-Side (Password Strength): As the user types a password, a client-side script can check its strength (length, use of numbers/symbols) and update a 'strength meter' in real-time.
  3. 3
    1. These client-side checks are fast and reduce the load on the server.
  4. 4
    1. Server-Side (Username Uniqueness): To check if the chosen username is already taken, a request must be sent to the server. This is server-side processing.
  5. 5
    1. The server-side script (e.g., PHP) will receive the username, query the user database to see if it exists, and send a response back ('available' or 'taken').
  6. 6
    1. Server-Side (Final Submission): When the form is submitted, all the data is sent to the server. A server-side script validates the data again (for security), and if all is well, inserts the new user's details into the database.

Recap

  • Client-side processing happens in the user's web browser.
  • JavaScript is the main language for client-side scripting.
  • Client-side is used for interactivity, input validation, and fast feedback.
  • Server-side processing happens on the web server.
  • PHP, Python, and Ruby are common server-side languages.
  • Server-side is essential for database access, user authentication, and secure operations.

Quick check

  1. Would checking a user's password against a database be a client-side or server-side task? Why?2 marks
  2. Name the most common client-side scripting language.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State two physical transmission media and one wireless transmission medium.3 marks
  2. Explain the difference between attenuation and interference in data communications.4 marks
  3. A video streaming service sends movies to its customers. Would this be an example of unicast, multicast, or broadcast communication? Justify your answer.3 marks
  4. Describe the client-server model of communication as it applies to a user accessing a web page.4 marks
  5. Explain why the Domain Name System (DNS) is a necessary component of the internet.4 marks
  6. Compare IPv4 and IPv6, stating the bit-length of each and the main reason for the transition to IPv6.5 marks
  7. A large university campus covers several square kilometres and wants to provide blanket wireless internet coverage. Discuss the suitability of WiFi and WiMAX for this task.6 marks
  8. When you log into your online bank account, describe one task that is handled by client-side processing and one task that must be handled by server-side processing. Justify your choices.6 marks
  9. Describe the journey of a data packet from your computer to a web server in another country using packet switching. Mention the role of routers in this process.7 marks
  10. The Public Switched Telephone Network (PSTN) was originally designed for analogue voice calls using circuit switching. Explain how this infrastructure has been adapted to support modern digital internet access, such as broadband.8 marks

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