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E1 · Data Transmission
Spec reference: Section E1 - How Data is Transmitted Key idea: Understand how data is transmitted between systems including transmission types, protocols, encryption and compression.
Communication channel types
| Type | Description | Example |
|---|---|---|
| Simplex | Data flows in one direction only | TV broadcast, keyboard to computer |
| Half-duplex | Data flows both ways, but only one direction at a time | Walkie-talkie |
| Full-duplex | Data flows both ways simultaneously | Phone call, most internet connections |
| Point-to-point | Direct connection between exactly two devices | Ethernet cable between two computers |
| Multi-drop | Multiple devices share a single communication line | Older bus network topologies |
Asynchronous vs synchronous transmission
| Feature | Asynchronous | Synchronous |
|---|---|---|
| Timing | No shared clock; each byte is wrapped in start/stop bits | Shared clock signal synchronises sender and receiver |
| Overhead | Higher (extra bits per byte) | Lower (no start/stop bits) |
| Use | Serial ports, older modems, simple devices | High-speed data links, networks |
| Example | USB keyboards, serial communication | HDMI, RAM data bus |
Parallel vs serial transmission
| Feature | Parallel | Serial |
|---|---|---|
| Bits sent | Multiple bits sent simultaneously on separate wires | One bit at a time on a single wire |
| Speed | Fast over short distances | Slower per wire but more practical |
| Distance | Short distances only (signal skew at longer distances) | Long distances |
| Cost | More wires, more expensive connectors | Cheaper, fewer wires |
| Examples | Old printer ports, internal memory bus | USB, SATA, Ethernet |
Note
Despite parallel seeming faster, modern high-speed connections (USB, SATA) use serial because at high frequencies, the timing differences between parallel wires cause errors.
Packet switching
Data is broken into packets for transmission across a network.
Contents of a data packet
| Field | Purpose |
|---|---|
| Source IP address | Where the packet came from |
| Destination IP address | Where the packet is going |
| Sequence number | Allows packets to be reassembled in the right order |
| Payload (data) | The actual data being transmitted |
| Checksum/error check | Allows the receiver to detect corruption |
| TTL (Time To Live) | Prevents packets looping forever on the network |
How packet switching works
- Data is split into packets at the source.
- Each packet is routed independently across the network.
- Packets may take different routes and arrive out of order.
- The receiver reassembles packets in the correct order using sequence numbers.
- If a packet is lost or corrupt, only that packet is resent.
Benefits: Efficient use of network bandwidth; resilient to node failures.
Protocols
A protocol is a set of rules governing how data is transmitted between systems. Both sender and receiver must use the same protocol.
| Protocol | Purpose |
|---|---|
| HTTP/HTTPS | Transferring web pages and data |
| TCP | Reliable, ordered delivery of packets (connection-oriented) |
| UDP | Fast, unreliable delivery (no guarantee of order or arrival) |
| IP | Routing packets across networks using IP addresses |
| FTP | Transferring files between systems |
| SMTP/IMAP/POP3 | Sending and receiving email |
| DNS | Translating domain names to IP addresses |
Encryption
Encryption scrambles data so it can only be read by someone with the correct key.
Simple ciphers
| Cipher | How it works | Weakness |
|---|---|---|
| Caesar cipher | Each letter is shifted by a fixed number (e.g. A becomes D with shift 3) | Only 25 possible shifts, easily broken |
| Vigenere cipher | Uses a keyword to apply different shifts to each letter | More complex but still breakable by frequency analysis |
Modern encryption in computer systems
| Type | How it works | Use |
|---|---|---|
| Symmetric key encryption | Same key used to encrypt and decrypt. Fast. | Encrypting stored data, VPNs |
| Public key encryption (asymmetric) | Two keys: a public key (shared openly) for encryption, a private key (kept secret) for decryption | HTTPS, digital signatures, email |
How HTTPS works
- Server sends its public key to the browser.
- Browser encrypts a session key using the public key.
- Server decrypts it using its private key.
- Both now share a symmetric session key for fast encrypted communication.
Compression
Compression reduces file or data size to save storage space or speed up transmission.
| Type | Description | Can restore original? | Use |
|---|---|---|---|
| Lossless | Removes redundant data without losing any information | Yes | ZIP files, PNG images, text files |
| Lossy | Permanently removes some data for much greater compression | No | JPEG images, MP3 audio, MP4 video |
Applications of compression:
- Streaming video: lossy compression allows HD video over limited bandwidth.
- Email attachments: lossless ZIP reduces file size for sending.
- Web images: lossy JPEG keeps page load times low.
Summary
| Term | Meaning |
|---|---|
| Simplex | One-way data transmission |
| Full-duplex | Simultaneous two-way communication |
| Packet switching | Breaking data into packets sent independently across a network |
| TCP | Reliable, ordered protocol ensuring all packets arrive correctly |
| Symmetric encryption | Same key for encrypting and decrypting |
| Public key encryption | Public key encrypts, private key decrypts |
| Lossless compression | Reduces size with no quality loss |