Synchronous method utilizes PCM digital channel to transmit data, if the data signal and the digital terminal clock is synchronized, at this time, the data signal of output data terminals is controlled by the PCM channel clock, therefore only needs to the data signal carries on the multiplex processing to be possible. Here the data terminal equipment is in the controlled subordinate status, therefore the flexibility is poor.
If the data signal and the data port clock are asynchronous, then the data signal can be used to fill the 64ke Bit S of the collection signal, which is asynchronous.
For a digital device, the receiver must have some way to know the beginning and end of the byte in the data stream. In asynchronous communication, the byte boundary is indicated by the start and stop bits. In synchronous communication, the timer mechanism helps the transmitter and receiver to be in sync state. A synchronous signal can occupy a single channel, but more often it is directly integrated into the signal.
Unipolar single polarity code has a voltage of 1, no voltage indicates O. No special coding. The single polarity code accumulates the DC component.
The Bipolar Bipolar code has a positive voltage of 1 and a negative voltage representing 0. The scheme reduces the power requirement and reduces the high level attenuation. The DC component of the bipolar code is greatly reduced, which facilitates transmission. The voltage state of the RZ Zero is returned to zero after a signal state. The pulse of the zero-code is narrower, according to the relationship between the pulse width and the bandwidth of the transmission band, so the zero-code is wider in the channel.
Nrzi (by 1 Inverse-phase zero) Nrzi code, regardless of whether the level is high or low, does not represent binary 1 and 0. But the voltage variation represents the binary 1. If there is no voltage change, the next one is 0; if there is a voltage change, the next one is 1. Non-zero code in the transmission difficult to determine the end of the A-bit and another start, you need some way to make the transmitter and receiver between timing or synchronization. Nrzi is used for slower RS-232 serial communications and data storage on hard drives. In a synchronous link, a long string of contiguous bits (possibly thousands of or 0) can cause problems. The receiver may lose synchronization and cannot detect the correct number of 0 in consecutive strings. Another problem is that the long string of 0 behaves as a DC, which does not pass through certain electrical components. Manchester coding and other solutions solve these problems by increasing the clock signal