Flashcards · 2210 · Topic 2

Data transmission flashcards

15 cards on Topic 2, subtopics 2.1 to 2.3. Pick a subtopic or study them all.

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What does the header of a data packet contain?: The destination address, the packet number and the originator's (sender's) address. The payload carries the data and the trailer marks the end of the packet.
Why can packets from one message arrive out of order?: In packet switching, routers send each packet by the best route available at the time, so packets can take different routes. The receiver puts them back in order using the packet numbers.
Which transmission method suits a long-distance link, and which a short high-speed cable?: Long distance: serial (one bit at a time on one wire, so no skew and cheaper cabling). Short, high-speed connection: parallel (several bits at once on several wires).
Give an example of simplex, half-duplex and full-duplex transmission: Simplex (one way only): keyboard to computer. Half-duplex (both ways, one at a time): walkie-talkie. Full-duplex (both ways at once): telephone call.
Why is USB widely used to connect devices?: It is a standard connector supported by most devices, it supplies power, devices are detected automatically and the right driver is loaded. Drawbacks: limited cable length and speed.

How does odd or even parity detect an error?: A parity bit makes the number of 1s in each byte odd (or even), as agreed. The receiver counts the 1s; if the total no longer matches the agreed parity, an error has occurred.
What can a parity check not detect?: An even number of changed bits, such as two bits flipped in the same byte, because the count of 1s stays odd (or even).
How does a checksum work?: A value is calculated from the data before sending and sent with it. The receiver recalculates the value from the data received; if the two values differ, an error has occurred.
How does an echo check work?: The receiver sends a copy of the data it received back to the sender, which compares it with the original. If they differ, the data is sent again. It cannot show which way the error happened.
What is a check digit used for?: Validating one item of entered data, such as an ISBN or bar code number. A digit is calculated from the other digits and compared with the one entered.
Describe automatic repeat query (ARQ): The receiver sends a positive acknowledgement for correct data or a negative one for an error. If the sender gets a negative acknowledgement, or none before the timeout, it resends the data.

Plaintext vs ciphertext: Plaintext is the original, readable data. Ciphertext is the scrambled data produced by encryption, which is meaningless without the key.
Symmetric vs asymmetric encryption: Symmetric: the same key encrypts and decrypts, so it must be shared securely. Asymmetric: a linked public/private key pair; the public key encrypts and only the matching private key decrypts.
Does encryption stop data being intercepted?: No. It makes intercepted data meaningless to anyone without the key, so it cannot be understood or used.
What is the risk with symmetric encryption?: The key has to be sent to the receiver. If the key is intercepted, anyone holding it can decrypt the data.