Communication Protocols

The Internet, the World Wide Web, and the Client–Server Model

A common mistake in exams is treating the Internet and the World Wide Web as the same thing. They operate at completely different levels of abstraction.

• The Internet is the physical global infrastructure of interconnected computer networks. It consists of copper cables, fibre-optic lines, wireless links, routers, switches, and servers connecting billions of devices worldwide.
• The World Wide Web (WWW) is an information service running on top of the Internet. It is a collection of multimedia web pages and digital documents interlinked by hyperlinks and addressed using Uniform Resource Locators (URLs).

In simple terms, the Internet is the roadway network, while the World Wide Web is one specific type of traffic travelling along those roads. Other traffic types, such as email and voice calls, use the same physical highway without being part of the Web.

The Client–Server Model

Web architecture relies on the client–server model:

• Client: A device or application (such as a web browser on a phone or laptop) that initiates communication by sending a request for data or a service.
• Server: A computer (or the program running on it) that listens for requests on the network, processes them, and sends back a response containing the requested resource or an error message.

For example, if you open a browser and request your school timetable: your device sends an HTTP GET request for /timetable (input); the school web server receives the request and fetches the timetable file (steps); and the server sends the HTML page back for your browser to render on screen (output).

Servers are generally named after their specific role: a web server delivers web pages, a mail server routes email, a file server stores shared documents, and a database server runs structured data queries. A single server can handle requests from thousands of clients simultaneously.

In contrast, a peer-to-peer (P2P) network has no central server. Every computer acts as both a client and a server, sharing files directly. Client–server networks are much easier to secure and back up centrally, whereas peer-to-peer networks have no single point of failure but are harder to manage.

Finding the Server: DNS

Routers navigate using numeric IP addresses, not human-readable domain names. When you enter https://www.curriculumonline.ie/index.html into a browser, the browser cannot connect immediately. It first asks a Domain Name System (DNS) server to look up www.curriculumonline.ie and return its numerical IP address. DNS acts like the phone book of the Internet. Once the IP address is returned, the browser opens its connection to that specific server.

Layered Architecture and the TCP/IP Suite

Managing network communication in a single step would be unmanageable. Networking solves this through abstraction, dividing communication into distinct layers. Each layer has a specific job and communicates only with the layers directly above and below it, hiding its internal complexity.

The 4-layer TCP/IP model powers the modern Internet:

Note on layer numbers: The layer numbers you sometimes see in networking — Layer 2 for switches, Layer 3 for routers — come from the theoretical seven-layer OSI model. Layer 2 corresponds to the link layer and Layer 3 corresponds to the network layer.

When you send data, it moves down the stack on your device. Each layer wraps the payload with its own control headers, a process called encapsulation. At the receiving end, the data travels back up the stack, and each layer strips away its corresponding header until only the application payload is delivered.

Application Protocols: HTTP and HTTPS

Hypertext Transfer Protocol (HTTP) is an application-layer request-response protocol used to transfer web resources like HTML pages, images, and stylesheets.

A typical HTTP exchange follows four steps:

1. The client browser issues an HTTP request, such as GET /index.html.
2. The web server receives and parses the request headers and retrieves the file.
3. The server returns an HTTP response with a status code (such as 200 OK or 404 Not Found) and the payload.
4. The browser parses the returned data and renders the page.

HTTP vs HTTPS

• HTTP sends data as clear, unencrypted text across port 80 by default. Anyone with access to intermediate routers or packet-sniffing software can read usernames, passwords, and form submissions in transit.
• HTTPS (Hypertext Transfer Protocol Secure) encrypts the entire HTTP exchange using TLS (Transport Layer Security), the successor to the older SSL (Secure Sockets Layer). It operates across port 443 by default. TLS uses cryptographic keys to ensure privacy, authenticate the server using digital certificates, and confirm message integrity so data cannot be altered during transit.

Transport and Network Layers: TCP, IP, and Packet Switching

Packet switching means a message is broken into small, individual pieces called packets. Each packet travels independently across the network and may take a different path before being reassembled at the destination. This makes network usage highly efficient because thousands of conversations share the same lines. It also provides resilience: if one route fails, remaining packets are routed around the breakdown. If a packet is corrupted, only that single packet needs to be re-sent, rather than the entire file.

Internet Protocol (IP)

Operating at the network layer, IP handles logical addressing and packet routing.

• An IPv4 address is a 32-bit number written as four decimal values separated by full stops, each between 0 and 255 (for example, 192.168.1.14). Because 32 bits only support roughly 4.3 billion addresses, IPv6 was introduced: a 128-bit address written as eight groups of hexadecimal digits (such as 2001:0db8:85a3:0000:0000:8a2e:0370:7334), ensuring ample addresses for all connected devices.
• IP attaches a header to each packet containing the source IP, destination IP, and packet length.
• IP is a connectionless, best-effort protocol. It forwards packets across routers towards their destination, but makes no guarantee that packets will arrive, arrive in order, or arrive without duplicates.

Transmission Control Protocol (TCP)

Operating directly above IP at the transport layer, TCP provides reliable, ordered, and error-checked delivery:

1. A three-way handshake (SYN, SYN-ACK, ACK) establishes an active connection between client and server before data flows.
2. TCP splits data streams into segments and tags each with a sequence number.
3. Each segment includes a checksum, a value calculated from the segment's bits. The receiver recalculates this checksum; if the numbers do not match, the damaged segment is discarded.
4. The receiving device sends an acknowledgement (ACK) back for every intact segment it receives.
5. If the sender's timer expires before receiving an ACK, TCP automatically retransmits the missing segment.
6. The receiver uses sequence numbers to reconstruct the data in original order, even if packets arrived out of sequence.

Voice over Internet Protocol (VoIP)

Voice over Internet Protocol (VoIP) is an application-level technology that digitises analogue voice audio into data packets so phone and video calls can travel over IP networks, replacing traditional analogue telephone lines.

The VoIP Transmission Cycle

1. A microphone captures voice waves as continuous analogue electrical signals.
2. An analogue-to-digital converter (ADC) samples and converts these signals into binary numbers.
3. Audio codecs compress the binary data to save bandwidth.
4. The digitised audio is packaged into IP packets and transmitted over the network.
5. At the destination, packets are unbundled and decompressed, a digital-to-analogue converter (DAC) turns the binary data back into a continuous analogue electrical signal, and the speaker converts that electrical signal into audible sound waves.

Why VoIP Uses UDP Instead of TCP

TCP's reliability mechanisms create delay. If an audio packet drops during a live conversation, waiting for an acknowledgement timeout and a retransmission means that packet arrives hundreds of milliseconds late. Hearing a voice sound half a second late is disruptive, whereas dropping that micro-fragment is barely noticeable. VoIP therefore runs over UDP (User Datagram Protocol) rather than TCP. UDP sends packets without handshakes or retransmissions, keeping latency minimal.

Latency, Jitter, and Buffering

• Latency is the time delay for a packet to travel from speaker to listener. High latency causes people to talk over one another.
• Jitter is the variation in packet arrival times caused by fluctuating network congestion. When packets arrive in bursts or with irregular gaps, the speech sounds robotic or clipped.
• To fix jitter, receiving devices use a playback buffer, briefly storing incoming packets so they can be played back at a smooth, constant pace. Increasing buffer size eliminates jitter distortion, but adds to overall latency.

Advantages and Disadvantages of VoIP

• Advantages: Substantially cheaper call costs (especially international calls using existing broadband); voice, video, and screen sharing can travel over a single network; users can keep their number on any Internet-connected device anywhere.
• Disadvantages: Call quality degrades if bandwidth is low or congestion is high; calls fail entirely during power cuts or Internet outages unless battery backups are present; emergency call geolocation is less precise than a physical landline.

Network Hardware Supporting Protocols

Protocols define software rules, but hardware components are required to transmit, forward, and convert physical signals:

• Network Interface Card (NIC): The hardware component inside every networked device (wired or wireless) that connects it to the physical network. Every NIC has a permanent, globally unique MAC address (Media Access Control address) burned into it during manufacturing.
• Modem: Short for modulator-demodulator. It converts digital data from a computer into an analogue signal for transmission over telephone, cable, or fibre lines from an Internet Service Provider (ISP), and converts incoming analogue signals back into digital data.
• Network Switch: Connects multiple wired devices within a Local Area Network (LAN). A switch inspects the destination MAC address of each incoming data frame and directs it straight to the specific port of the receiving device, rather than broadcasting it to all devices.
• Router: Connects different networks together (such as connecting your home LAN to your ISP's network). Routers inspect destination IP addresses on incoming packets and use internal routing tables to forward them along the best path.
• Wireless Access Point (WAP): Broadcasts radio signals allowing wireless devices to join a wired local network.
• Gateway: Connects two networks that use completely different communication protocols or data architectures, translating packet formats between them.
• Bridge: Connects two separate segments of the same local network, filtering traffic by MAC address to reduce network congestion.
• Repeater: Amplifies and regenerates electrical or optical signals that have weakened due to cable length (attenuation), extending the network's maximum physical reach.
• Transmission Media: The physical channels carrying data, including twisted-pair copper cables (Ethernet), fibre-optic cables (using pulses of light for high bandwidth, long distances, and immunity to electrical noise), and radio waves (Wi-Fi and mobile data).

Protocol Summary Reference

Use this core summary table to recall the syllabus protocols:

Key terms

Communication Protocol

An agreed set of rules that specifies how devices format, address, transmit, receive, and verify data across a network.

Internet

The global physical infrastructure of interconnected computer networks, cables, wireless links, switches, and routers.

World Wide Web (WWW)

An application-layer service of interlinked hypertext documents and media accessible via the Internet using web browsers.

Client–Server Model

A network architecture where client devices send requests for resources and centralised servers process and return responses.

DNS (Domain Name System)

An Internet service that translates human-readable domain names (such as examinations.ie) into numerical IP addresses.

Packet Switching

A transmission method where data is split into small packets that travel independently across network paths and are reassembled at the destination.

TCP

Transmission Control Protocol; a transport-layer protocol providing reliable, connection-oriented, sequenced, and error-checked packet delivery.

IP

Internet Protocol; a network-layer protocol responsible for logical addressing and packet routing across networks on a best-effort basis.

HTTP

Hypertext Transfer Protocol; an application-layer request-response protocol for fetching web resources in unencrypted plain text over port 80.

HTTPS

Hypertext Transfer Protocol Secure; an encrypted version of HTTP running over TLS/SSL on port 443 to ensure privacy and data integrity.

VoIP

Voice over Internet Protocol; an application technology that converts analogue voice into digital packets for real-time transmission over IP networks.

Sequence Number

A numerical tag added to each TCP segment so the receiver can detect missing packets and reorder segments correctly.

Checksum

A calculated numeric value included in a data packet header used by the receiver to detect bit errors and data corruption.

Latency

The time delay taken for a data packet to travel from its source across the network to its destination.

Jitter

The variation in packet arrival times caused by network congestion, resulting in stuttering in real-time streaming or VoIP.

MAC Address

Media Access Control address; a unique physical hardware identifier permanently assigned to a network interface card (NIC).

Router

A network device that forwards data packets between different networks by inspecting destination IP addresses.

Network Switch

A local network device that inspects destination MAC addresses to forward frames directly to their target device.