Synchronous and Asynchronous Data Transfer

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.

http://www.modem.com/glossary/glos15.html

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.

http://www.modem.com/glossary/glos1.html

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.

http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci213406,00.html

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 job. Simplex links are built so that the transmitter (the one talking) sends a signal and it's up to the receiving device (the listener) to figure out what was sent and to correctly do what it was told. No traffic is possible in the other direction across the same connection.

You must use connectionless protocols with simplex circuits as no acknowledgement or return traffic is possible over a simplex circuit. Satellite communication is also simplex communication. A radio signal is transmitted and it is up to the receiver to correctly determine what message has been sent and whether it arrived intact. Since televisions don't talk back to the satellites (yet), simplex communication works great in broadcast media such as radio, television and public announcement systems.

A half duplex link can communicate in only one direction, at a time. Two way communication is possible, but not simultaneously. Walkie-talkies and CB radios sort of mimic this behavior in that you cannot hear the other person if you are talking. Half-duplex connections are more common over electrical links. Since electricity won't flow unless you have a complete loop of wire, you need two pieces of wire between the two systems to form the loop. The first wire is used to transmit; the second wire is referred to as a common ground. Thus, the flow of electricity can be reversed over the transmitting wire, thereby reversing the path of communication. Electricity cannot flow in both directions simultaneously, so the link is half-duplex.

Full duplex communication is two-way communication achieved over a physical link that has the ability to communicate in both directions simultaneously. With most electrical, fiber optic, two-way radio and satellite links, this is usually achieved with more than one physical connection. Your telephone line contains two wires, one for transmit, the other for receive. This means you and your friend can both talk and listen at the same time.

Half or Full-Duplex is required for connection-oriented protocols such as TCP. A duplex circuit can be created by using two separate physical connections running in half-duplex mode or simplex mode. Two-way satellite communication is achieved using two simplex connections.

http://www.inetdaemon.com/tutorials/theory/concepts/duplex_vs_simplex.shtml

Serial and parallel transmission

The communications links across which computers--or parts of computers--talk to one another may be either serial or parallel. A parallel link transmits several streams of data (perhaps representing particular bits of a stream of bytes) along multiple channels (wires, printed circuit tracks, optical fibres, etc.); a serial link transmits a single stream of data. At first sight it would seem that a serial link must be inferior to a parallel one, because it can transmit less data on each clock tick. However, it is often the case that serial links can be clocked considerably faster than parallel links, and achieve a higher data rate. A number of factors allow