Synchronous and Asynchronous

Synchronous and asynchronous

Synchronous and Asynchronous data transfer are two methods of sending data over a phone line. In synchronous data transmission, data is sent via a bit-stream, which sends a group of characters in a single stream In order to do this, modems gather groups of characters into a buffer, where they are prepared to be sent as such a stream. In order for the stream to be sent, synchronous modems must be in perfect synchronization with each other. They accomplish this by sending special characters, called synchronization, or syn, characters. When the clocks of each modem are in synchronization, the data stream is sent. In asynchronous transmission, data is coded into a series of pulses, including a start bit and a stop bit. A start bit is sent by the sending modem to inform the receiving modem that a character is to be sent. The character is then sent, followed by a stop bit designating that the transfer of that bit is complete.

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Analog and digital

"Analog" refers to information being presented continuously, while "digital" refers to data defined in individual steps. Analog information's advantage is its ability to fully represent a continuous stream of information. Digital data, on the other hand, is less affected by unwanted interference, or noise. In digital computers, data is stored in individual bits, which have a value of either 1 (on) or 0 (off). If graphed, analog signals are shaped as sine waves, while digital signals are square waves. Sound is analog, as it is always changing. Thus, in order to send information over a phone line, a modem must take the digital data given it by the computer and convert it into sound, an analog signal. The receiving modem must convert these analog signals back into the original digital data.

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XON and XOFF

Xon/Xoff (sometimes written "X-on/X-off" or "XON/XOFF" and pronounced eks-AWN eks-AWF) is a protocol for controlling the flow of data between computers and other devices on an asynchronous serial connection. For example, a computer typically sends data to a printer faster than the printer can print. The printer contains a buffer where data is stored until the printer catches up with the computer. If the buffer becomes full before the printer catches up, a small microprocessor in the printer sends back an X/off signal to stop sending data. When enough data is printed and buffer storage becomes free, the printer sends an X/on signal telling the computer to resume sending data. The "X" stands for "transmitter" so the X/on and X/off are signals to turn a transmitter on or off. The actual signal for X/on is the same bit configuration as the ASCII Ctrl-Q keyboard combination. The X/off signal is the Ctrl-S character. When you define your modem to your computer's operating system, you may need to specify the use of flow control with X/on/Xoff or with CTS/RTS (Clear to Send/Ready to Send). When sending binary data, Xon/Xoff may not be recognized because it is character-encoded.

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Simplex and duplex

Simplex communication is permanent unidirectional communication. Some of the very first serial connections between computers were simplex connections. For example, mainframes sent data to a printer and never checked to see if the printer was available or if the document printed properly since that was a human