Asynchronous and Synchronous Multiplexing

Transmission hierarchies in the past have been built using asynchronous multiplexing systems. In asynchronous systems, each terminal in the network runs on its own clock. In digital transmission, clocking is one of the most important considerations. Clocking means using a series of repetitive pulses to keep the bit rate of data constant and to indicate where the ones and zeroes are located in a data stream.

Because these clocks are totally free-running and not synchronized, large variations occur in the clock rate and thus the signal bit rate. For example, a signal specified at 44.736 Mbps +20 parts per million (ppm) can produce a variation of up to 1,789 bps between one incoming signal and another.

Asynchronous multiplexing uses multiple stages. Signals such as asynchronous DS1's are multiplexed, and extra bits are added (bit-stuffing) to account for the variations of each individual stream and combined to form a DS2 stream. Bit-stuffing is used again to multiplex up to DS3.

The main problem with this procedure is that the lower multiplexed levels can not be accessed without de-multiplexing the entire signal. To add or drop a signal, the optical DS3 needs to be converted to copper DS3 and then de-multiplexed into individual DS1 signals before the required signal can be dropped. To add a signal, the reverse process has to take place.

In the synchronous SONET system, adding and dropping signals are much easier. The frequency of all clocks in the system will be the same (synchronous) or nearly the same (plesiochronous). The STS1 rate remains at a nominal 51.84 Mbps, allowing many STS1's to be stacked together without any bit stuffing. The multiplexing is made by byte interleaving.

Byte interleaving makes adding and dropping signals easy as shown in Figure 6. One channel is dropped by just taking out the corresponding byte. Another signal can be added in the same way, occupying the byte freed by the dropped signal.

Fig. 6  SONET Add/drop multiplexer

DS1 is transported as VT1 on SONET. VT stands for Virtual Tributary. VT1 occupies 1.7 Mbps which includes the DS1 1.544 Mbps and SONET overhead bits. Single-step multiplexing up to STS1 requires no bit stuffing, and VT's are easily accessed.