Data transmissions practice questions.

A large file is split up before it is sent across a wide area network. Which items does the header of each packet carry?

• The complete original file, repeated in every packet
• The password of the user who sent the data
• The destination address and a sequence number
• The transmission rate guaranteed for the whole route

Answer: The destination address and a sequence number

The header states where the packet is going and where it belongs in the file, so switching devices can forward it and the receiver can rebuild the data in the right order. Repeating the whole file in every packet would defeat the point of splitting it up.

During a video call the data crosses a packet-switched network. Which statement describes the way the data travels?

• Packets may take different paths and are put back in order on arrival
• A dedicated physical circuit is reserved for the whole duration of the call
• Every packet is forced to follow exactly the same path
• The data is sent as one unit that cannot be divided

Answer: Packets may take different paths and are put back in order on arrival

Each packet is forwarded independently, so the route and the arrival order can differ and the sequence numbers are used to reassemble the stream. A reserved end-to-end circuit describes circuit switching, not packet switching.

Players complain that a game reacts slowly although the link has high bandwidth that is never fully used. Which quantity best describes the problem?

• Bandwidth, the maximum amount of data the link can carry
• Attenuation, the loss of signal strength along a long cable run
• Parity, the extra bit added to detect a changed bit
• Latency, the delay before transferred data starts to arrive

Answer: Latency, the delay before transferred data starts to arrive

Bandwidth is capacity per second while latency is delay, so a link can have spare capacity and still feel slow because every round trip takes too long. Buying more capacity does not shorten that delay.

Why is serial transmission used instead of parallel transmission for long-distance data links?

• Serial links need one wire per bit and therefore cost less to install
• Bits sent along parallel wires arrive skewed over long distances
• Parallel transmission can only carry data in one direction
• Serial transmission removes the need for any error checking

Answer: Bits sent along parallel wires arrive skewed over long distances

Over distance the wires of a parallel link behave slightly differently, so bits of the same word arrive at different times and the word is corrupted. A serial line carries one bit after another, so no skew between wires can build up.

A card reader at a barrier sends a card number to a controller and never receives any data back. Which transmission mode is being used?

• Full-duplex, because data can flow in both directions at once
• Half-duplex, because the two directions take it in turns
• Simplex, because data flows in one direction only
• Broadcast, because every device on the link receives the data

Answer: Simplex, because data flows in one direction only

Simplex means the channel is only ever used one way. Half-duplex would still require the reader to be able to receive, just not at the same moment as it sends.

A department sends very large text log files across a slow link every night. Which change reduces the transmission time without upgrading the link?

• Increasing the size of the header added to every packet
• Sending each file twice so that the receiver can compare the copies
• Using lossy compression so that every log entry is recovered exactly
• Applying lossless compression to the files before sending them

Answer: Applying lossless compression to the files before sending them

Compression reduces the number of bits that have to cross the link, so the same content needs less time at an unchanged data rate. Lossy compression would be smaller still but would destroy log entries that have to stay exact.