Data packets

Large data is broken into small packets before being sent, then reassembled at the other end. Each packet has three parts:

• Header — the sender’s and receiver’s IP addresses, the packet number, and how many packets there are in total.
• Payload — the actual data being carried.
• Trailer — a marker for the end of the packet, plus an error-check (e.g. a checksum).

Packet switching: packets can take different routes across the network and may arrive out of order — the packet numbers let the receiver reassemble them correctly.

Inside a packet

Serial, parallel & direction

Serial transmission sends bits one at a time down a single wire — reliable over long distances. Parallel sends several bits at once down many wires — faster but only reliable over short distances (bits can arrive skewed).

Direction of flow can be:

• Simplex — one direction only (e.g. computer → printer, or a sensor sending readings).
• Half-duplex — both directions, but only one at a time (e.g. a walkie-talkie).
• Full-duplex — both directions at the same time (e.g. a phone call, broadband).

Serial, parallel or simplex?

Tap a statement, then tap the group it belongs to.

Which direction mode?

USB

USB (Universal Serial Bus) is the standard wired connection for peripherals. It uses serial transmission. Benefits:

• Connectors only fit one way, so it is hard to plug in wrongly.
• It is a widely supported standard — devices are recognised automatically and drivers are often installed for you.
• It can carry power as well as data, and can charge devices.

Drawback: the maximum cable length is limited (a few metres) and older USB versions have lower maximum transfer speeds than some alternatives.

USB benefit

Error detection methods

Transmission can corrupt bits, so we add checks:

• Parity check — an extra parity bit makes the number of 1s even (even parity) or odd (odd parity). If the count is wrong on arrival, an error is detected.
• Checksum — a value calculated from the data is sent with it; the receiver recalculates and compares.
• Echo check — the received data is sent back to the sender and compared with the original.
• Check digit — an extra digit added to the end of a number (e.g. barcodes, ISBNs) calculated from the other digits.

Your turn — parity bit

Which check?

Parity limitation

Encryption

Encryption scrambles data (plaintext) into unreadable ciphertext using a key. It does not stop data being intercepted — it makes it meaningless to anyone without the key. Two types:

• Symmetric encryption — the same key is used to encrypt and decrypt. Fast, but the key must be shared securely, which is a risk.
• Asymmetric encryption — uses a key pair: a public key (shared with anyone) to encrypt, and a matching private key (kept secret) to decrypt. Solves the key-sharing problem.

Key idea: in asymmetric encryption, data encrypted with a person’s public key can only be decrypted with their private key.

Symmetric vs asymmetric

The big ideas to know

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