al. It is likely that this incoming signal will need to be sent to the Class 3 center. If that's the case, the Class 4 center creates a very special signal for transmission up to Class 3. Specifically, it takes four DS-1 signals that are coming into it (from Class 5 centers) and puts these signals together onto a single wire using TDM. It adds a few extra bits to help the Class 3 center identify the beginning of what it gets and the end of it. When it's all said and done, the signal that moves from Class 4 to Class 3 is a stream of bits with a bit rate of 6.312 Mb/s. That signal is called a DS-2 signal. A signal enters into a Class 3 switching center. This signal might need to move up to a Class 2 switching center. In that case, the Class 3 switching center puts together a special signal just for Class 2. It takes seven DS-2 signals that are coming into it (from Class 4 centers), and puts them together on a single wire using TDM. It adds a few extra bits to help the Class 2 office identify places where bit streams begin and end. Ultimately, it sends a signal to Class 2 that is a stream of bits with a bit rate of 44.736 Mb/s. This signal is called a DS-3 signal. Finally, it is possible in POTS that a signal arriving at a Class 2 switching center might be sent up to a Class 1 switching center. If that's the case, Class 2 puts together special signal for Class 1. Specifically, a Class 2 center takes five of the DS-3 signals that come into it (from Class 3 centers), and packages them together on a single wire using TDM. Including a few extra bits to make the package look nice for Class 1, the stream of bits sent to Class 1 has a bit rate of 274.1746 Mb/s. This signal is called DS-4. What I've said SO far is true, but not the complete truth. In general, it is possible in POTS that DS-1, DS-2, DS-3, and DS-4 signals could be found between any two switching centers. For example, I presented DS-3 as the signal Class 3 sends to Class 2 switching centers. It is also possible in POTS that Class 3 sends a DS-2 signal to Clas