Prepared by : Parackattu.Akhil.B
Dept : Electronic And Communication
NEED OF SYNCHRONIZATION.
• Whenever an electronic device transmits digital (and sometimes analogue)
data to another electronic device.
• There must be a certain rhythm established between the two devices, i.e., the
receiving device must have some way of knowing, within the context of the
fluctuating signal that it's receiving.
• where each unit of data begins and where it ends.
• So there two types synchronization.
• In asynchronous the transmission of data is generally without the use
of an external clock signal, where data can be transmitted
intermittently rather than in a steady stream.
• Any timing required to recover data from the communication symbols
is encoded within the symbols.
• The most significant aspect of asynchronous communications is that
data is not transmitted at regular intervals, thus making possible
variable bit rate.
• And that the transmitter and receiver clock generators do not have to
be exactly synchronized all the time.
• In asynchronous serial communication the physical protocol layer, the
data blocks are code words of a certain word length.
• For example octets (bytes) or ASCII characters, delimited by start bits
and stop bits.
• A variable length space can be inserted between the code words. No
bit synchronization signal is required.
• This is sometimes called character oriented communication.
TWO CATEGORIES OF ASYNCHRONOUS.
• Strobe Control.
• Handshaking Control.
• This is one way of transfer i.e. by mean of strobe pulse supplied by one of the units to
indicate to the unit when the transfer has to occur.
• This method is used to accompany each data item being transferred with a control
signal. That indicates presence of data into the bus.
STROBE CONTROL METHOD.
• Strobe control method of data transfer uses single control signal for each transfer.
• The strobe may be activated by either source unit or destination unit
HANDSHAKING CONTROL METHOD.
• In case source unit the data transfer under the strobe. The source unit has no way of
knowing whether destination unit has received the data or not
• Similarly destination has no way of knowing whether source unit has place data on the
data bus or not
• So handshaking mechanism solve this problem by introducing second control signal that
provides a reply to unit that initiates to transfer.
sender stop Data Start Receiver
Here the data is transmitted intermittently rather than in a steady stream.
• Asynchronous activities take place outside of real time system.
• As asynchronous does not require a constant bit rate.
• For examples file transfer, email and the World Wide Web.
• Overhead of start and stop bits.
• Asynchronous transmission is relatively slow due to the increased
number of bits and gaps.
• In Synchronous the transmission of data is sent in a continuous stream at a constant
• Synchronous communication requires that the clocks in the transmitting and
receiving devices are synchronized running at the same rate.
• so the receiver can sample the signal at the same time intervals used by the
transmitter. No start or stop bits are required.
• For this reason synchronous communication permits more information to be passed
over a circuit per unit time.
TWO CATEGORIES OF SYNCHRONOUS
• Byte oriented protocols
• Bit oriented protocols
Byte Oriented Protocol.
• Early synchronous protocols were byte-oriented protocols, where synchronization
was maintained by transmitting a sequence of synchronous idle characters.
• when the line was not actively transmitting data or transparently within a long
• This protocol was developed by IBM in late 1960.
BIT ORIENTED PROTOCOL.
• Bit-oriented protocols are synchronous protocols that view the transmitted data as a
stream of bits with no semantics, or meaning.
• Control codes are defined in terms of bit sequences instead of characters.
Synchronization is maintained on an idle line by transmitting a predefined sequence
• Synchronous Data Link Control (SDLC) specifies that a station continue transmitting
a sequence of '1' bits on an idle line.
Sender DATA Receiver
Here data is transmitted in a steady stream.
• Over time the transmitting and receiving clocks will tend to drift
apart, requiring resynchronization.
• Synchronous communication is direct communication that occurs in
• That take place face-to-face, and as technology has evolved, can take
place irrespective of distance
• (ex. telephone conversations and instant messaging)
• Most common use is in the ASCII terminals.
• High speeds communication links established using synchronous
• The clock frequency should be same at both the sending and receiving
• No tolerance in clock frequency is allowed.