Clocking

This actually applies to all T-carrier systems - T1, T2, and T3  -  the T-carrier systems all use the data bits for synchronization.  The framing bits are spread far apart, and therefore are unsuitable for synchronization - instead, they are use to mark the beginning of blocks.  Data is transmitted in both directions, but to explain this concept we will look only at the customer transmitting data to the carrier (the POP).  The timing is supplied by the T1 a clock signal which is sent to the customer - actually there is no isolated "clock signal"  -  there is just data that is clocked by the POP, being sent from the POP to the customer at 1.544 Mbps.  This data stream is used both for reception of information, and as the "clock signal" which the customer CSU/DSU uses to time it's own signals to send to the POP.  The CSU/DSU is configurable to use either it's own internal clock, or the carrier's clock  -  in virtually all cases, the technicians set it up to use the carriers clocking.

At all times, regardless of whether data is being transmitted.  therefore there is a clock signal available at the transmitting end.  The receiving end, even though it also has access to the same clock, must be able to determine the timing of the incoming signal without using that clock, since the timing may vary.  Therefore, the receiving end (the POP) will only use it's own clock for sending, and will use the data to determine the timing of the signal coming from the customer.