Async, Sync and Line Coding

Question 0

How do Asynchronous systems work?

Answer 0

Basically has start and stop bits for each word

These systems require extra elements added to a transmitted data stream to inform the receive terminal where a word begins and ends. These systems do not require any linked timing device to maintain synchronicity

Question 1

What are the two common systems in use to maintain synchronicity?

Answer 1

Asynchronous systems

Synchronous systems

Question 2

How do synchronous systems work?

Answer 2

Have start and stops bits but at the beginning and end of the entire message or large quantities of data

These systems require extra elements added to a transmitted data steam to inform be receive terminal where a block of data begins and ends. To prevent errors occurring clocking devices at both transmit and receive terminals must be synchronised

Question 3

In a 1T1 signal how long is the stop bit?

Answer 3

1.5 bits

Question 4

Give an example of a signal that uses start and stop bits

Answer 4

1T1

Question 5

What does a FSK tele-printer do when the link between the transmit and receive terminals is in an idle state

Answer 5

Idle on one arm, either upper or lower

Question 6

To enable the receive terminal to determine when a data transmission has restarted and dropped out of the idle state what must first be sent?

Answer 6

A start but of a different polarity, otherwise the receiving terminal would not be able to recognise the start of the data transmission

Question 7

How do start and stop bits differ from each other?

Answer 7

Opposite polarity

Stop bit often bigger in size

Question 8

Within an asynchronous transmission, what problems may you encounter between the transmit and receive terminal clocks?

Answer 8

Could encounter drift or slippage as the terminal clocks are independent of each other, this loss of synchronicity is likely to corrupt the transmitted data

Question 9

What is the mechanism to start the receive terminal clock?

Answer 9

The leading edge of the first bit (the start bit)

Question 10

What is the term given to additional bits to the data?

Answer 10

Overhead

Question 11

If a signal has a single start and single stop bit along with 3 data bits, what is its overhead?

Answer 11

2/5 = 0.4

Question 12

What possible solutions are there to prevent drift between transmitter and receive terminals in a synchronous system?

Answer 12

1. To provide a separate clock line between the transmit and receive terminals
2. Embed the clocking information in the transmitted data signal

Question 13

What codes are applied to the data in synchronous systems to maintain synchrony between transmit and receive terminals?

Answer 13

Line Codes

Question 14

What are the aim of line codes?

Answer 14

To avoid long runs of 1s and 0s

Question 15

For sizeable blocks of data, which transmissions are more efficient, asynchronous or synchronous?

Answer 15

Synchronous as asynchronous requires far more overhead

Question 16

In a synchronous system, what does each block begin and end with respectively?

Answer 16

A pre and post amble bit pattern