Asynchronous Vs Synchronous Vs Isochronous
Now we need to deal with the issue of synchronization, or timing. This requires synchronizing the transmitting and receiving devices. When I think of synchronization, I think of dancing, because if you and your partner are out of step, then disaster ensues. Fred Astaire and Ginger Rogers were dance partners that were always in step, or synchronized. Communications devices work the same way. There is bit, character (byte), and message (block frame packet) synchronization that must occur before data can be sent and received across a communications channel. Bit serial transmission places special demands upon sending and receiving equipment to perform bit, byte (character), and message synchronization. Figure 5-7 illustrates the problem of synchronization.
The problem illustrated in Figure 5-7 is to determine which is the correct interpretation of the incoming signal? Is the top case with alternating "1's" and "0's" correct, or is the bottom case with "0's" and "l's" sometimes repeated correct? From our perspective, we would most likely guess the bottom interpretation. Why? It seems that some pulses, or square waves, are longer than others.
Now let us put ourselves in the place of the receiving communications equipment. How does it know that some pulses are longer than others? It cannot view them like we can. Consequently, it must time them with a stopwatch (or clock). When timing is done and the correct bits are interpreted from the data stream, bit synchronization is being performed.
There are three techniques that are used for bit synchronization: asynchronous transmission, synchronous transmission, and isochronous transmission.
Bit synchronization—This is required for the receiving machine to determine when to sample (determine the voltage level) for a bit. The sampling rate is determined by a clock provided by the Data Communications Equipment (DCE, or modem) or the Data Terminal Equipment (DTE, or PC).
Character synchronization—This is required for receiving equipment (the DTE, or PC) to determine which bits are in a character. The DTE counts the bits to determine the character length. Agreement on character length is established and set into the PC and the server before communication begins. Asynchronous transmission uses 10 bits per character; synchronous and isochronous transmission use 8 bits per character.
Message synchronization—This is required for the receiving device (DTE, or PC) to determine the start and end of records or messages. The DTE, using handshakes and message formats established by the Level-2 data link protocol, performs message synchronization.
There are two ways to perform the timing for bit synchronization: with an internal clock, or wristwatch, in each device or with an external clock, or wall clock. Asynchronous transmission uses the wristwatch (internal clock) in every device. Synchronous transmission uses the wall clock (external clock). A wall clock works as the analogy here because most all wall clocks plug into an electrical outlet so they get their timing from the same source, the 60-cycle alternating electric current.
Asynchronous Transmission • In asynchronous transmission, both the sending and receiving devices have wristwatches. The wristwatch is their internal timer. They use the wristwatch and specific agreements on transmission speed, character length, and line terminating characters between sending and receiving devices to perform bit, character, and message synchronization.
Asynchronous transmission is a transmission technique in which each character of information is individually synchronized by using a "start" bit (start bit = 0) at the beginning of the character and a "stop" bit (stop bit = 1) at the end of the character (refer to Figure 5-8). In asynchronous transmission, bit synchronization is performed using the "start" and "stop" bits and a wristwatch (crystal oscillator) in every terminal device. This allows characters to be sent at random or irregular time intervals (asynchronous intervals) after the preceding character has been transmitted.
In asynchronous transmission, when no data is being sent, the transmitting device sends idle signals down the channel. The idle signals are continuous ones—"1's." The start bit is always a zero. The receiving device, seeing a 1 to 0 change on the channel, senses that data is beginning to arrive.
Clocked Units of Time
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