A clue to the meaning of “synchronous data transfer” lies in the word “synchronous.” It derives, in part from the name “Chronos” who was the Greek God of time. “Syn” means “with” so this word tells us that the two sides in a synchronous data transfer keep time with each other. They use a common clock signal. “Asynchronous” is the opposite of “synchronous.” Each side in the data transfer uses its own clock signal.
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Time is important in data communication because information travels as an electric pulse that represents a stream of binary data. The receiving computer needs to know where each bit cuts off, so it knows when to chop off a signal and convert it into a number. Imagine a stream that contained 11110000. If the receiver used a clock that worked at half the speed of the sender, it would think the message was 1100 because it would allocate a longer period of time to each digit as represented by the electrical current. Each clock tick gives the receiver a point to sample the wire for a number.
A synchronous connection usually happens when a computer is directly connected to another device by a cable. The majority of synchronous connections happen over a “parallel” connection. This is because the two computers connected together for a data transfer need a common time signal. The parallel cable is terminated at each end by a standard plug. The plug contains a number of pins, each of which connects to a wire in the parallel cable. One of those wires is designated to carry a pulse that marks time. It is labelled as “strobe.” The sender of the data sends a signal down that wire with a clock pulse while it sends the data as a varying voltage down another wire. The receiving computer chops off the incoming data signal with each pulse that arrives down the strobe wire.
The other type of socket that computers traditionally have is called a “serial port.” The serial cable does not have a separate wire for a clock signal. It packages data so that the receiving computer can process them in its own time. Modern computers have fewer and fewer “serial” and “parallel” ports. The dominant connector for data transfer is the “USB.” USB stands for “Universal Serial Bus.” It is asynchronous.
Data transfers over the Internet follow the TCP/IP protocols. TCP/IP is asynchronous and does not include a separate signal for a clock pace. Similarly, the HTTP protocols that dictate the transfer of data for the World Wide Web do not involve any synchronization procedures.
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