In this post, I am going to talk about the 10 computer networking concepts that every professional should master. I will also include links to the main articles of each concept.
This theoretical model explains how networks behave within an orderly, seven-layered What is Asynchronous Transmission? Asynchronous Transmission This is in contrast to synchronous transmission in which some timer or clocking mechanism is used to ensure a steady flow of data between the devices. How it works In asynchronous communication, only about 80 percent of the transmitted bits actually contain data, while the other 20 percent contain signaling information in the form of start and stop bits.
Each symbol can represent or convey one or several bits of data. See Symbol Rate. A user unfamiliar with asynchronous communications and UART operations should refer to Reference 3 in section 8 of this document. Asynchronous communications is a serial data protocol that has been in use for many years. Normally, eight bits of data are transmitted at a time.
There are other less commonly used modes that can send 5, 6, or 7 bits of data. Each byte of data is framed by a start bit and a stop bit. A symbol is defined as a start, data, parity, or stop bit. It is common to define communications speed as bits per second.
The bit rate is defined as the inverse of the period of a unit symbol. Although the common standard bit rates are 50, 75, , , , , , , , , , , , , , , and , communications is possible at any rate provided that the sender and receiver use the same rate. Figure 6. We see that the idle state of the transmit signal is a logic one, or a high voltage level. A logic zero is sent when the signal is a low voltage level for one symbol time. Similarly, a logic one is sent when the signal is a high voltage level for one symbol time.
After the transmitting of the data there may be an optional parity bit. The parity bit is used for error detection and can be set for even parity or odd parity. Conversely, for odd parity, the parity bit is set high if the number of ones in the 8 data is even. The data byte is terminated with one or more successive stop bits. Stop bits are always a logic 1 or high voltage level. If the signal remains high for longer than one symbol period, the stop bit can be thought of as the stop or idle period and the communications signal may remain in the idle condition an arbitrarily long period of time.
Example of kB asynchronous serial data stream with odd parity. The receiving device unit must use the same bit rate as the sending unit. In the process of receiving serial communications, a processor may generate up to three error flags. A parity error is generated if the parity bit is the incorrect voltage level. The second type of error is a framing error that is generated if a low voltage level is received in the stop bit position. Thirdly, an overrun error is generated if a software instruction does not read the data byte from the receive buffer before the next byte of data is completely received, resulting in the new data byte overwriting the previous data byte.
Each new start bit synchronizes the sampling of the receiving unit. Generally, the receiver recovers the transmission using a clock that is an even multiple of the bit rate. When the receiving unit detects the falling edge of the START bit, it waits one half the symbol period and samples the receive data line. This is illustrated by the processor sample points in Fig. If the logic state of the line is zero, then it is recognized as a valid start condition. The receiver then waits full periods to sample the receive data line until all data bits, plus any parity bit, plus the stop bit have been received.
The order of the data bits is important as it dictates how the transmission is organized when it is received. It is viewed as a reliable data transmission method because a data bit is only sent if the previous data bit has already been received. Example of Serial Data Transmission Serial transmission has two classifications: asynchronous and synchronous. Asynchronous Serial Transmission Data bits can be sent at any point in time. Stop bits and start bits are used between data bytes to synchronize the transmitter and receiver and to ensure that the data is transmitted correctly.
The time between sending and receiving data bits is not constant, so gaps are used to provide time between transmissions. The advantage of using the asynchronous method is that no synchronization is required between the transmitter and receiver devices. It is also a more cost effective method. A disadvantage is that data transmission can be slower, but this is not always the case. Synchronous Serial Transmission Data bits are transmitted as a continuous stream in time with a master clock. The data transmitter and receiver both operate using a synchronized clock frequency; therefore, start bits, stop bits, and gaps are not used.
This means that data moves faster and timing errors are less frequent because the transmitter and receiver time is synced. However, data accuracy is highly dependent on timing being synced correctly between devices. In comparison with asynchronous serial transmission, this method is usually more expensive.
Serial transmission is normally used for long-distance data transfer. It is also used in cases where the amount of data being sent is relatively small.
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