Flashcards · 0478 · 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 is a data packet made up of?: A header (destination address, packet number and originator's address), a payload (the actual data) and a trailer (marks the end of the packet and can hold an error check).
Describe packet switching: The message is split into packets. Routers choose the route for each packet, so packets can travel by different routes and arrive out of order. The receiving device uses the packet numbers to reorder them.
Serial vs parallel transmission: Serial: one bit at a time down a single wire; reliable over long distances and cheaper cabling. Parallel: several bits at once down several wires; faster over short distances, but bits can arrive out of sync (skew) over long ones.
Simplex, half-duplex and full-duplex: Simplex: one direction only (e.g. computer to printer). Half-duplex: both directions, but only one at a time (e.g. walkie-talkie). Full-duplex: both directions at the same time (e.g. phone call).
Give one benefit and one drawback of USB: Benefit: a standard connector supported by most devices, which also supplies power and is detected automatically. Drawback: a limited maximum cable length and transmission speeds lower than some dedicated interfaces.

Why is error detection needed when data is transmitted?: Data can be corrupted, lost or gained in transit by interference, congestion or equipment faults, so the receiver must be able to tell whether the data arrived exactly as sent.
How does a parity check work?: Sender and receiver agree odd or even parity. A parity bit is set so the number of 1s in each byte is odd (or even). The receiver counts the 1s; a wrong total shows an error. It cannot detect two bits changing.
What does a parity block (parity byte) add to a parity check?: A parity bit is calculated for each row and each column of a block of bytes. The row and column that fail cross at the incorrect bit, so the error can be located and corrected.
Checksum vs echo check: Checksum: a value calculated from the data is sent with it and recalculated on arrival; different values show an error. Echo check: the receiver sends the data back to the sender to compare with the original.
What is a check digit, and how does it differ from parity and checksums?: An extra digit calculated from the other digits of a number such as an ISBN or barcode. It checks that one item of data has been entered correctly, not that a whole transmission arrived intact.
What is an automatic repeat query (ARQ)?: The receiver sends a positive acknowledgement if the data is correct, or a negative one if not. If the sender gets no acknowledgement before a timeout, or a negative one, it resends the data.

What is the purpose of encryption?: To scramble data (plaintext into ciphertext) so that if it is intercepted it cannot be understood without the key. It does not stop the data being intercepted.
What is symmetric encryption?: One shared key is used both to encrypt and to decrypt the data. The key must be sent to the receiver, and if it is intercepted the data can be read.
What is asymmetric encryption?: A mathematically linked public key and private key are used. Data encrypted with the receiver's public key can only be decrypted with the matching private key, which is never shared.
Why is asymmetric encryption more secure than symmetric for sending data?: The private key never has to be transmitted, so there is no shared key that could be intercepted. Anyone can have the public key, but it cannot decrypt the data.