SONET

520 Mbps, which is exactly equal to STS-3.
             Synchronous versus Asynchronous.
             Traditionally, transmission systems have been asynchronous, with each terminal in the network running on its own clock. 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 DS-3 signal specified at 44.736 Mbps + 20 parts per million (ppm) can have a variation of up to 1,789 bps between one incoming DS-3 and another.
             Asynchronous multiplexing uses multiple stages. When multiplexing an asynchronous signal such as DS-1 extra bits are added (bit-stuffing) to account for variations of each individual stream and combined with other bits to form a DS-2 stream. DS-2 is multiplexed up to higher rates in the same manner. Ultimately at the higher asynchronous rate, they cannot be accessed without demultiplexing.
             In SONET, the average frequency of all clocks in the same system will be synchronous or plesiochronous. Every clock can be traced back to a highly stable and accurate reference supply. Thus, base signal STS-1 rate is at a nominal 51.84 Mbps, allowing many synchronous STS-1 signals to be multiplexed without any bit stuffing. Thus, the STS-1s are easily accessed at a higher STS-N rate.
             SONET is backward compatible with current hierarchy, thus low speed synchronous virtual tributary (VT) signal can be inserted into an STS-1 and combined with other partial payloads (VT signals) to fill out the frame. This multiplexing requires no bit stuffing, and VTs are easily accessed.
             Physical Configuration.
             SONET transmission relies on three basic devices: STS multiplexers, regenerators, and add/drop multiplexers. STS multiplexers provide the interface between a tributary network and the SONET. Regenerators extend the length of the links possible between generator and receiver. Add/drop multiplexers allow insertion and extraction of SONET paths.