EoY (chapter 2: Data Packets, Data Transmission Methods, Error Checking Methods (Not ISBN or Modulo11)

Question 1

Basic info about data packets

Answer 1

• sometimes called datagrams
• usually 64KiB (quite small data) since it’s easier to control than large amounts

Question 2

packet structure?

Answer 2

• packet header
• payload
• trailer

Question 3

what is in the packet header?

Answer 3

• IP address of the sending device
• IP address of the receiving device
• sequence number of the packet (to make sure the packets can be reassembled)
• packet size (to check if all the packets have arrived intact)
• value indicating how many total packets there are in the transmission

Question 4

what is in the payload?

Answer 4

• the actual data being sent in the packet (usually around 64KiB)

Question 5

what is in the trailer?

Answer 5

• some way of identifying the end of the packet
• an error checking method (CRCs)

Question 6

what is packet switching?

Answer 6

a method of data transmission in which a message is broken up into a number of packets

Question 7

how does packet switching work?

Answer 7

• file is split into packets
• each packet follows it’s own route
• routers determine the route of each packet (it always chooses the shortest possible path available)
• packets reach the destination and are reassembled

Question 8

benefits of packet switching

Answer 8

• no need to tie up a single communication line
• possible to overcome failed or busy lines by re-routing packets
• relatively easy to expand packet usage
• high transmission rate

Question 9

drawbacks of packet switching

Answer 9

• packets can get lost and need to be re-sent
• prone to errors with real-time streaming
• there is a delay while packets are being re-ordered at the destination

Question 10

types of data transmission

Answer 10

1. simplex
2. (full) duplex
3. half duplex
4. serial
5. parallel
6. asynchronous
7. synchronous
8. USB

Question 11

simplex?

Answer 11

data is transmitted one way only (ex. computer to printer)

Question 12

duplex?

Answer 12

data transmitted both was at the same time (ex. online gaming or face chatting)

Question 13

half duplex?

Answer 13

data transmitted both ways, but NOT at the same time (ex. walkie-talkie)

Question 14

serial?

Answer 14

• one bit sent at a time over a single wire
• (ex. computer to printer)
• sending data externally between devices

Question 15

parallel?

Answer 15

• multiple bits sent at the same time using multiple wires
• (ex. computer to modem)
• sending data internally

Question 16

why is serial used instead of parallel to connect devices to PCs?

Answer 16

• single wire has less chance of corruption
• single wire is less expensive
• more reliable over distances
• bits are still synchronized after transmission

Question 17

asynchronous?

Answer 17

• data transmitted in an agreed bit pattern
• data bits (1s and 0s) are grouped together and sent with CONTROL BITS
• retriever of the data knows where the data starts and ends
• prevents data becoming mixed up
• control bits make it possible to separate groups of data as they arrive

Question 18

synchronous?

Answer 18

• continuous stream of data
• data is accompanied by timing signals (generated by an internal clock) to ensure the sender and receiver are synchronized
• the receiver counts how many bits were sent and then reassembles them into bytes of data
• it is faster than asynchronous

Question 19

USB?

Answer 19

• USB = universal serial bus
• it is a data transmission method
• bits are sent one at a time
• devices are automatically detected
• USB only fits one way so it can’t connect incorrectly
• it’s industry standard
• backwards compatible

Question 20

what are the error checking methods?

Answer 20

• parity checking
• automatic repeat request (ARQ)
• checksum
• check digits
• echo checking

Question 21

why do we need to error check?

Answer 21

• there is always a risk that the data has been corrupted or changed after transmission

Question 22

why do errors happen?

Answer 22

• interference
• problems during packet switching (leads to data loss)
• skewing of data (causes bits to be out of synchronization)

Question 23

parity check?

Answer 23

• checks a byte of data
• a parity bit is added
• counts the number of 1s
• can be odd or even
• if parity is incorrect error is detected

Question 24

when will parity check not work?

Answer 24

• if two bits are interchanged (swapped)
• if there are multiple errors in the same byte

Question 25

ARQ?

Answer 25

- ARQ = automatic repeat request - uses acknowledgment ( a message sent but he receiver saying that the data was received) - checks performed on receiving data - if an error is detected a request is sent to resent data - the resend request is repeated until data is sent correctly - the acknowledgment that data is received is sent - if the acknowledgment is not received in the set time the data is resent

Question 26

checksum?

Answer 26

- value calculated from the data just before transmission - it is added and transmitted with the data - the value of recalculated after the transmission - values are compared to check for any errors

Question 27

echo check?

Answer 27

- when data is sent to another device, the data is sent back until it receives the correct data - echo checks has A LOT of problems (it can't know where the error occurred and is very repetitive and time consuming)

Question 28

check digits?

Answer 28

- check digit = final digit included in a code - used to identify errors in data entry caused by mistyping or mis-scanning