For various reasons, data streams are often considered to be composed of various elements:
Bits –0 or 1
Characters –eight bit sequences
Blocks or frames –potentially variable numbers of bits
It is necessary to determine the start and end of these elements. The technique for doing this is synchronization.
There are two transmission modes:
asynchronousand synchronous.
We will examine bit synchronization for both synchronous and asynchronous transmissions
Asynchronous Transmission
Used primarily when the data to be transmitted is generated at random intervals. E.g.: a user typing at a keyboard communicating with a computer.
Generally used in applications where the data to be transferred consists of characters, each character being encoded using 7 or 8 binary bits, common coding schemesbeing ASCII and EBCDIC.
As data is transferred randomly there may be long intervals during which no data signal is present on the line. The receiver must be able to resynchroniseat the start of each new character received.
Disadvantage of Asynchronous Transmission
The use of the start and stop bits for each byte transferred means the method is inefficient in its use of transmission capacity.
The bit synchronisation method becomes less reliable as the bit rate increases.
So we look at some of the synchronous transmission schemes
Synchronous Transmission
Used for large blocks of data at higher bit rates. A frame of data is transmitted as a contiguous bit stream with no delay between each 8-bit element.
The receiver clock operates in synchronism with the received signal.
There are two methods of achieving this:
Embeddingthe clock information into the transmitted signal and having the receiver extract it.
Requires either return to zero or transition orientated scheme.
The receiver keeps a localclock, which is kept synchronizedwith the received signal using a Digital Phase Lock Loop (DPLL)
May be used for NRZ schemes
Requires sufficient transitions to keep synchronization
•Uses bit stuffing (more later)
Media Types
There are a number of transmission media types used for Data Communication. The choice of medium depends on:
Distance to be covered
Desired Bite Rate (in bits per second, bps)
Cost Considerations
Media are often categorised as:
Guided
Wireless
Satellite
Bits –0 or 1
Characters –eight bit sequences
Blocks or frames –potentially variable numbers of bits
It is necessary to determine the start and end of these elements. The technique for doing this is synchronization.
There are two transmission modes:
asynchronousand synchronous.
We will examine bit synchronization for both synchronous and asynchronous transmissions
Asynchronous Transmission
Used primarily when the data to be transmitted is generated at random intervals. E.g.: a user typing at a keyboard communicating with a computer.
Generally used in applications where the data to be transferred consists of characters, each character being encoded using 7 or 8 binary bits, common coding schemesbeing ASCII and EBCDIC.
As data is transferred randomly there may be long intervals during which no data signal is present on the line. The receiver must be able to resynchroniseat the start of each new character received.
Disadvantage of Asynchronous Transmission
The use of the start and stop bits for each byte transferred means the method is inefficient in its use of transmission capacity.
The bit synchronisation method becomes less reliable as the bit rate increases.
So we look at some of the synchronous transmission schemes
Synchronous Transmission
Used for large blocks of data at higher bit rates. A frame of data is transmitted as a contiguous bit stream with no delay between each 8-bit element.
The receiver clock operates in synchronism with the received signal.
There are two methods of achieving this:
Embeddingthe clock information into the transmitted signal and having the receiver extract it.
Requires either return to zero or transition orientated scheme.
The receiver keeps a localclock, which is kept synchronizedwith the received signal using a Digital Phase Lock Loop (DPLL)
May be used for NRZ schemes
Requires sufficient transitions to keep synchronization
•Uses bit stuffing (more later)
Media Types
There are a number of transmission media types used for Data Communication. The choice of medium depends on:
Distance to be covered
Desired Bite Rate (in bits per second, bps)
Cost Considerations
Media are often categorised as:
Guided
Wireless
Satellite
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