Special

characters are embedded in the data

stream to begin synchronization and to

maintain synchronization during the transmission,

allowing both computers to check

for and correct any variations in timing.













Asynchronous (serial)




asynchronous transmission A method

of data transmission that uses start bits and

stop bits to coordinate the flow of data so

that the time intervals between individual

characters do not need to be equal. Parity

also may be used to check the accuracy of

the data received.

See also communications parameters;

data bits; synchronous transmission.



Asynchronous (serial) transmission is simple, inexpensive technology ideally suited for transmitting small frames at irregular intervals such as keyboards or mouses. Because start, stop and parity bits must be added to each character being transmitted, however overhead of asynchronous transmission is high, often in the neighborhood of nearly 20-30 percent. Every bit is following another until is end or start’s bit.







Figure 3.0











Figure 3.1



Synchronous (parallel) transmission has one piece of wire for each transmitted bit. For synchronizing receiver with sender is used a clock pulse. Data are moved at clock pulse order. Data are moving faster than in asynchronous transmission because we don’t need start and stop bits. Synchronous transmission is usually use for printers and scanners. Figure 3.2













B2. Explain the process of signal modulation in networking transmission.



What is signal?



Signal is: a physical bearer which transmit messages by the use of environment,

conventional combination of amplitudes, frequencies, or periods of light, sound or

electric current



Modems transmit digital computer signal over telephone lines by converting them to analog forms.



In networking transmission are usually used two basic types of signals:



Analog signal. Analog information changes continuously and can take on

many different values. An analog clock’s hands move constantly,

displaying time on a continuous scale.







Digital signal. Digital information is characterized by discrete states. A light

bulb, for example, is on or off. A digital clock represents the

time in one-minute intervals and doesn’t change its numbers

again until the next minute. A digital clock can represent

exact minutes but not the seconds that pass in between.





Analog waverforms: usually have shape of sine waves. Constantly vary in one or more values, and these

changes in values can be used to represent data.