The fractional T1 is actually a whole T1. If you buy, for example, a half T1, the phone company can't actually give you a half a line, they give you a full T1 line and let you transmit and receive over 12 of the DS0 channels; the remaining channels just send 1s. Sound sneaky? It's not, really. After your fractional T1 gets to the telco, it's bundled with your neighbor's fractional T1 and actually fills a whole pipe internally (see Figure 2.5).
Figure 5 When purchasing a fractional T1, you actually get a whole T1. Think of the vertical columns as DS0 data pipes stacked 9 bits high. Some channels are available for data; the others (shaded) transmit nothing but 1s.
T3 is equal to 28 T1 circuits. A T3 can carry 44.736Mbps. A DS3 is actually part of the Plesiochronous Digital Hierarchy (PDH) network (plesiochronous means "almost synchronous"). Individual calls cannot be teased out of a DS3 frame; rather, the entire frame must be decomposed to its original constituent DS1 frames before individual calls can be added or removed from the data stream.
What Is Plesiochronous?
Synchronous signals are sent in the lock-step of marching fascist troops. The signals are clocked and governed by a super-accurate primary reference clock (PRC). This is usually an atomic clock. There is no room for deviation.
Plesiochronous signals are almost synchronous. They might be governed by two different atomic clocks. The signals are still very accurate, but nothing like synchronous. The difference between the two clocks is called the plesiochronous difference.
Asynchronous signals don't march in lock step at all.
Although PDH is the traditional method of multiplexing on T3s, it's being replaced by SONET (Synchronous Optical Network).