Synchronous Serial InterfacesData circuits acquired from the telphone company transmit data one bit at a time, or "serially". For this reason, routers typically use serial ports for use with telephone company circuits. Furthermore, digital circuits send a constant stream of bits at a constant rate, or "synchronously". Therefore, digital data circuits, such as 56k DDS and T1 circuits, are normally connected to a synchronous serial port on the router.
Normally a serial port is connected to a CSU/DSU of some sort. This is configured to handle all the specifics of the circuit it is connected to. The end result is a stream of data going to the router through a serial cable. CSU/DSU's normally have V.35 ports on them.
The speed, or clock, of the serial connection is determined by the available bits per second on the CSU/DSU. This can depend on what kind of circuit is being used and (in the case of a T1) how many channels are being used. A full ESF T1 will transmit 1.536 Mb/s or 1.536 MHz. Note that this is not the same as the clock rate of the actual circuit which also has framing bits.