How does a video arrive in order if its pieces take different routes?

Splitting data into packets

• Before data is sent across a network, it is broken into small pieces called packets.
• Each packet is sent on its own and the receiving computer joins them back together.
• Small packets share the network's links fairly: no single huge file blocks a link. If one packet is lost or damaged, only that packet needs to be sent again.

Inside a packet

• The header holds the destination address, the source address (the originator's address) and the packet number, used to put the data back in order.
• The payload is the actual data being carried.
• The trailer marks the end of the packet and can hold data used to check for errors.

Packet switching

• In packet switching, each packet is sent across the network separately.
• Routers read each packet's destination address and send it on towards its destination, one hop at a time.
• Each packet may take a different route, depending on which links are free or working.
• So packets can arrive out of order. Once the last packet arrives, the receiver uses the packet numbers to reorder them.

Parallel sends 8 bits at once. So why is USB serial?

One wire or many?

• Serial transmission sends data one bit at a time down a single wire.
• Parallel transmission sends several bits at the same time down several wires, e.g. 8 bits on 8 wires.
• Over the same short distance, parallel can move more data per second.
• Parallel is used over short distances, e.g. inside a computer or within an integrated circuit.

Why distance changes things

• Over a long parallel cable, bits sent together arrive at slightly different times. This is skew, and the data can be corrupted.
• Wires lying side by side can also cause interference with each other.
• Serial has no skew, so it is more reliable over long distances.
• Serial cables need fewer wires, so they are cheaper. Serial is slower than parallel over a short distance.

Which way can data go?

• Simplex: data travels in one direction only, e.g. a computer sending data to a monitor.
• Half-duplex: both directions, but only one direction at a time, e.g. a walkie-talkie.
• Full-duplex: both directions at the same time, e.g. a phone call.
• Each one combines with serial or parallel, e.g. "serial half-duplex".

The universal serial bus

• USB is an industry-standard interface that sends data by serial transmission.
• Benefits: devices are detected automatically and the right driver is loaded; one standard fits many devices; it can power or charge a device; newer versions are backward compatible.
• Drawbacks: maximum cable length is limited (5 m for USB 2.0); older versions may stop being supported.

One bit flips on the way. How can the receiver even tell?

When bits go wrong

• Interference during transmission can cause data loss, data gain or data change.
• The receiver never sees the original, so the sender adds extra check data.
• Parity check: one bit of each byte is a parity bit. Even parity: each byte holds an even number of 1s. Odd parity: an odd number.
• The receiver counts the 1s. A wrong count means an error, so the data is resent.

Finding the faulty bit

• One parity bit misses errors where two bits change: the count stays even (or odd).
• A parity block check sends several bytes, each with a parity bit, plus a parity byte: the parity bit for each column.
• The receiver checks every row and column. The faulty bit is where the failing row meets the failing column, so it can be corrected.

Checksum and echo check

• Checksum: the sender calculates a value from a block of data with an agreed algorithm and sends it too.
• The receiver recalculates it from the data received. Different values mean an error, so the data is resent.
• Echo check: the receiver sends the data back; the sender compares it with the original and resends if different.
• Drawback: an echo error may have happened on the way back.

Automatic repeat query

• ARQ (automatic repeat query) uses acknowledgements and a timeout.
• The receiver checks the data, e.g. with a checksum. No error: it sends a positive acknowledgement. Error: it sends a negative acknowledgement and the sender resends.
• Timeout: if no acknowledgement arrives within a set time, the sender resends automatically.
• This repeats until the data arrives correctly, or a set number of attempts is reached.

Check digits

• A check digit is a final digit calculated from the other digits. It catches data entry errors.
• Used in ISBNs (book numbers) and barcodes.
• On entry the check digit is recalculated and compared. A mismatch shows a mistake, e.g. a wrong or swapped digit.
• In an exam, the question tells you which check-digit method to use (ISBN-13 is one example).
• ISBN-13: multiply the 12 digits alternately by 1 and 3, add, take the remainder ÷ 10, and subtract it from 10 (10 gives 0).

How can you lock a message for someone you have never met?

Why encrypt?

• Data sent over a network can be intercepted by anyone tapping into the connection.
• Encryption uses an algorithm and a key to turn plaintext into ciphertext.
• Encryption does not stop interception. It makes intercepted data meaningless to anyone without the right key.
• Decryption uses a key to turn the ciphertext back into plaintext.

Symmetric encryption

• Symmetric encryption uses the same key to encrypt and to decrypt.
• So the sender must get a copy of the key to the receiver before any message can be read.
• If the key is intercepted while it is being shared, the attacker can decrypt every message sent with it.
• Sharing the key safely is the big weakness of symmetric encryption.

Asymmetric encryption

• Asymmetric encryption uses two different keys: a matching public key and private key.
• The public key can be given to anyone. It is used to encrypt messages for its owner.
• The private key is kept secret by its owner and never sent. Only it can decrypt what the matching public key encrypted.
• So no secret key ever has to travel across the network.

Big ideas

• Data is split into packets; each has a header (destination address, source address, packet number) and a payload.
• In packet switching, routers send each packet separately, so packets may take different routes and arrive out of order; the receiver reorders them using the packet numbers.
• A packet also has a trailer that marks its end.
• Serial: one bit at a time, one wire; reliable over distance, cheaper. Parallel: several bits at once; fast but only over short distances because of skew.
• Simplex is one way, half-duplex is both ways in turn, full-duplex is both ways at once.
• USB is serial: automatic detection and drivers, power supply, one standard; limited cable length.
• Interference causes data loss, gain or change; parity checks, checksums and echo checks detect it after transmission.
• A parity block (with a parity byte) can locate the exact faulty bit; ARQ uses acknowledgements and a timeout to get data resent.
• A check digit, recalculated on entry, catches data-entry errors in ISBNs and barcodes.
• Encryption doesn't stop interception; it makes intercepted data unreadable without the key.
• Symmetric: one shared key, which must be passed on safely. Asymmetric: encrypt with the receiver's public key, decrypt with their private key.