2. Data Transmission

Question 1

Contents of a packet

Answer 1

• header
• payload
• trailer

Question 2

Contents of the header

Answer 2

• destination IP address
• originator’s IP address
• sequence number of the packet

Question 3

Packet switching

Answer 3

• data is broken/split/divided into packets
• each packet (could) take a different route
• a router controls the route/path a packet takes
• … selecting the shortest/fastest available route/path
• packets may arrive out of order
• once the last packet has arrived, packets are reordered
• if a packet is missing/corrupted, it is requested again

Question 4

Advantages of packet-switching

Answer 4

• Minimal interference and corruption (packets can be resent individually)
• Saves time and internet bandwidth (packets can be resent individually)
• Faster (each packet finds the quickest way around the network)
• Harder to hack (each packet contains minimal data, travels through network separately)

Question 5

Benefits of serial transmission

Answer 5

• data won’t be skewed
• less chance of interference/crosstalk/corruption/error
• transmission speed is adequate

Question 6

Benefits of parallel transmission

Answer 6

• faster transmission speed across short distances

Question 7

Advantages of USB

Answer 7

• devices are automatically detected and drivers are automatically loaded for communication
• cable connectors fit in only one way, preventing incorrect connections and ensuring compatible data transmission
• lots of support available from online and from retailers, as USB usage is standardised
• several different data transmission rates are supported
• newer USB standards are backwards compatible with older USB standards

Question 8

Disadvantages of USB

Answer 8

• maximum cable length is roughly 5 meters, so it cannot be used over long distances
• older versions of USB have limited transmission rates
• very old USB standards may not be supported in the near future

Question 9

Parity check

Answer 9

• every byte transmitted has one of its bits allocated as a parity bit
• the sender and receiver must agree before transmission whether they are using odd or even parity
• if the parities do not match an error has occurred
• although it cannot identify where the error is, the error can be pinpointed if a block check is used (there are horizontal and vertical parity checks)

Question 10

Checksum

Answer 10

• calculated using an algorithm from the data before transmission
• value is transmitted along with the data
• receiving computer recalculates this sum with the same algorithm using the data received
• if the checksums received and recalculated do not match, an error has occurred
• this packet can be requested to be resent

Question 11

Echo check

Answer 11

• transmits the received data back to the sender who checks the data to see if any errors occurred during transmission
• if so the data is resent
• not reliable as an error could have occurred during the second leg

Question 12

Check digit

Answer 12

• used to detect errors in numeric data entry (e.g. incorrect digits entered, omitted or extra digits, phonetic errors)
• a calculation is performed on the inputted data
• the calculated digit is compared to a stored value
• if it matches, the data entered is correct
• e.g. ISBN numbers and barcodes

Question 13

ARQ

Answer 13

• notifies the sender that an error has occurred and that the data received is incorrect
• if an error is detected the receiver sends a negative acknowledgement transmission to indicate the data is corrupted
• if no error is detected the receiver sends a positive acknowledgement transmission meaning the data is correct
• if the receiver does not send any acknowledgement transmission then the sender waits for a certain time period (known as time-out) before automatically resending the data
• this process is repeated until all data has been received and acknowledged