Data transmission

📌 Key points

1. Data is split into packets. Each packet has a header (destination IP address, packet number, originator's IP address), a payload (the actual data) and a trailer (an error-detection value such as a checksum, and a marker to show the end of the packet).
2. In packet switching, each packet can take a different route; routers decide the best route for each packet, so packets may arrive out of order and are put back in order using their packet numbers.
3. Serial transmission sends one bit at a time down a single wire, so it is reliable over long distances; parallel sends several bits at once down several wires, which is faster but only over short distances because the bits can become skewed (out of sync).
4. Simplex sends data in one direction only, half-duplex in both directions but not at the same time, and full-duplex in both directions at the same time.
5. USB is a serial connection: devices are detected automatically and the right drivers loaded, it is an industry standard, it can supply power, but the cable length is limited.
6. Interference can cause data to be lost, gained or changed. A parity check adds a parity bit so the number of 1s is even (even parity) or odd (odd parity); a parity byte and parity block can find the exact bit in error, but a parity check cannot detect an even number of changed bits.
7. In a checksum, the sender calculates a value from the data and sends it with the data; the receiver recalculates it and, if the values do not match, the data is sent again. In an echo check the receiver sends a copy of the data back to the sender to compare.
8. Automatic Repeat Query (ARQ) uses acknowledgements and a timeout: the receiver sends a positive acknowledgement if the data is correct (or a negative one if not); if the sender gets no positive acknowledgement within the timeout, it resends the data.
9. A check digit is an extra digit calculated from the other digits of a number (e.g. ISBN, barcodes) and is used to detect data entry errors such as an incorrect or transposed digit.
10. Encryption turns plaintext into ciphertext so intercepted data cannot be understood. Symmetric encryption uses the same key to encrypt and decrypt; asymmetric encryption uses a public key to encrypt and a matching private key to decrypt.

📖 Key terms

Packet

A small unit of data sent across a network, made of a header, payload and trailer.

Payload

The part of a packet that contains the actual data being sent.

Packet switching

Data is sent in packets that may take different routes and are reordered at the destination.

Half-duplex

Data can travel in both directions, but only one direction at a time.

Parity bit

An extra bit added to a byte to make the number of 1s even or odd, used to detect errors.

Checksum

A value calculated from a block of data and sent with it so the receiver can check for errors.

Automatic Repeat Query (ARQ)

Error-control method using acknowledgements and timeouts so data is resent if not received correctly.

Asymmetric encryption

Encryption that uses a public key to encrypt and a different, private key to decrypt.