Understand that data is broken down into packets to be transmitted

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Data Transmission – Breaking Data into Packets

Learning Objective

Explain why data is divided into packets for transmission, describe the structure of a packet, and identify the methods, hardware and technologies specified in the Cambridge IGCSE 0478 syllabus.

Why Break Data into Packets?

  • Efficient use of the link – packets from many users can be interleaved on the same physical medium.
  • Reliability – only the lost or corrupted packet needs to be resent.
  • Routing flexibility – each packet may follow a different path, avoiding congestion.
  • Error detection & flow control – control information is carried in every packet, allowing checks at each hop.

Packet Structure

A data packet consists of three parts.

Section Key fields / purpose Typical size (bytes)
Header
  • Source address (MAC or IP)
  • Destination address
  • Sequence number (order of packets)
  • Protocol identifier (TCP, UDP, etc.)
  • Control flags – e.g. SYN, ACK, FIN
 20 – 60
Payload (Data) The user information being transferred. Variable (up to 1500 bytes for standard Ethernet)
Trailer
  • Error‑checking code (CRC or checksum)
 4 – 8

Transmission Methods (Syllabus 2.1)

  • Serial transmission – bits are sent one after another on a single channel (e.g., fibre‑optic, Ethernet, USB).
  • Parallel transmission – several bits travel simultaneously on separate wires (typical inside a computer’s data bus).
  • Synchronous transmission – a continuous stream synchronised by a shared clock signal.
  • Asynchronous transmission – data is sent in discrete characters/packets framed by start‑ and stop‑bits.

Duplex Modes

Mode Definition Typical example
Simplex Data flows in only one direction. Television broadcast, keyboard → PC.
Half‑duplex Data can travel both ways, but not at the same time. Walkie‑talkie, Ethernet hub (old).
Full‑duplex Data can travel simultaneously in both directions. Modern Ethernet switch, USB.

USB – a modern serial, full‑duplex interface

USB sends data in packets, includes error checking, supplies power, and supports hot‑plugging. It therefore illustrates many of the concepts above (packet structure, serial transmission, full‑duplex).

Data Rate & Bandwidth (Syllabus 2.5)

The data rate (often called bandwidth) of a link is:

\( R = \dfrac{N}{t} \)

where R = bits per second (bps), N = number of bits transmitted, t = transmission time (seconds).

Worked example: A 2 Mb file is sent in 4 s.

\( R = \dfrac{2\,000\,000\ \text{bits}}{4\ \text{s}} = 500\,000\ \text{bps} = 500\ \text{kbit/s} \)

Packet Switching vs. Circuit Switching (Syllabus 2.6)

  • Packet switching – Data is divided into packets that are routed independently. Used on the Internet and most LAN/WAN networks.
  • Circuit switching – A dedicated communication path is established for the whole session before any data is sent (e.g., traditional telephone networks). The path remains reserved for the call’s duration, even if no data is being transmitted.

Error‑Detection Methods (Syllabus 2.2)

Method How it works Numeric example
Parity (even/odd) Add a single parity bit so that the total number of 1‑bits is even (or odd). Data = 1011001 (four 1’s). Even parity → parity bit = 0 (total still even).
Checksum Sum all data bytes (usually using 1’s‑complement) and send the result; receiver recomputes and compares. Bytes: 0x14, 0x3A, 0x07 → sum = 0x55. Send 0x55 as checksum.
Echo check (loopback) Sender transmits a test pattern; receiver immediately echoes it back for verification. Send 0xAA; if the returned pattern is 0xAA the link is OK.
Check digit (e.g., ISBN, barcode) Weighted sum of digits; the remainder determines a final digit used for validation. ISBN‑13 978030640615? → weighted sum = 112; 112 mod 10 = 2 → check digit = 10‑2 = 8.
ARQ (Automatic Repeat reQuest) Receiver detects an error (via CRC, parity, etc.) and asks the sender to resend the corrupted packet. Stop‑and‑wait ARQ – send packet, wait for ACK; if NACK or timeout, resend the packet.

Encryption (Syllabus 2.3)

  • Why encrypt? To protect the confidentiality and integrity of data while it travels across insecure networks.
  • Symmetric encryption – Same secret key is used by sender and receiver (e.g., AES). Fast, but key distribution can be difficult.
  • Asymmetric encryption – Uses a public key (open to everyone) and a private key (kept secret). Enables secure key exchange; common in HTTPS (RSA, ECDSA).

Network Hardware – Mini‑Box (Syllabus 2.4)

Component Function / key details
Network Interface Card (NIC) Provides the physical connection to a network; contains a unique MAC address (48‑bit hexadecimal, e.g., 00:1A:2B:3C:4D:5E).
MAC address Layer‑2 (Data‑link) identifier, written as six pairs of hexadecimal digits separated by colons.
IP address Layer‑3 (Network) identifier.
IPv4: 32‑bit, dotted‑decimal (e.g., 192.168.1.10).
IPv6: 128‑bit, hexadecimal groups (e.g., 2001:0db8::1).
Router Forwards packets between different networks using the destination IP address to decide the next hop.

Suggested Diagram

Flow diagram: data → packetisation → individual packets travel through one or more routers (different paths possible) → reassembly at the destination.

Key Points to Remember

  • Dividing data into packets makes transmission efficient, reliable and flexible.
  • The header carries source/destination addresses, sequence numbers and protocol flags; the payload carries the user data; the trailer provides error‑checking.
  • Serial vs. parallel, synchronous vs. asynchronous, and simplex/half‑duplex/full‑duplex are chosen according to distance, speed and application requirements.
  • Packet switching routes each packet independently; circuit switching reserves a dedicated path for the whole session.
  • Common error‑detection techniques (parity, checksum, echo check, check digit, ARQ) help maintain data integrity.
  • Encryption (symmetric and asymmetric) protects data from eavesdropping and tampering.
  • NICs, MAC addresses, IP addresses (IPv4 & IPv6) and routers are the essential hardware for packet‑based networks.

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