Network - Communication, Synchronous
Synchronous communications are those that depend on timing. In particular, synchronous transmissions are those that proceed at a constant rate, although this rate may change during different parts of a communication (or when the line quality changes). In synchronous communications, transmission elements are identified by reference to either an external clock or self-clocking, signal-encoding scheme. This is in contrast to asynchronous communication, in which transmission elements are identified by special signal values (start and stop bits). Synchronous communications can achieve very large bandwidths, eventually allowing speeds of over 100 Mbps. Unfortunately, as transmission rate increases, signal quality decreases, because each bit interval becomes extremely short. When an external clock is used for synchronous communications, the duration of test bits are timed, and the resulting values are used as the bit-interval value. It is necessary to resynchronize the transmission occasionally to make sure that the parties involved do not drift apart in their timing. This is a real danger, because even tiny differences in timing can have a significant effect when millions of bits are transferred every second in a communication. To avoid such a problem, many synchronous transmission methods insist that a signal must change at least once within a predetermined amount of time or within a given block size. For example, the B8ZS (bipolar with 8 zero substitution) signalencoding scheme is based on a requirement that a transmission can never contain more than seven 0 bits in succession. Before that eighth consecutive 0, a 1 bit will be inserted. Self-clocking, signal-encoding schemes have a transition, such as a change in voltage or current, in the middle of each bit interval. A self-clocking encoding method changes the signal value within every bit interval to keep the two parties in synch during a transmission. This works because each party can recalibrate its timing if it notices a drift. Self-clocking methods avoid the need to insert extra bits (as in the B8ZS encoding scheme). On the other hand, a self-clocking machine needs a clock at least twice as fast as the transmission speed in order to accomplish the signal changes within each bit interval. Expressed differently, this means you will not be able to transmit any faster than at half the clock speed on a machine. (You can effectively increase the speed by compressing files before transmission, thereby sending more information than the bit rate would indicate.) COMPARE Communication, Asynchronous