As we progress through this chapter, we must keep in mind that there are two quite
different PCM standards. On the one hand there is a North American standard, which
some call T1; we prefer the term DS1 PCM hierarchy. The other standard is the E1
hierarchy, which we sometimes refer to as the “European” system. Prior to about 1988,
E1 was called CEPT30+2, where CEPT stood for Conference European Post and Telegraph
(from the French). Japan has sort of a hybrid system. We do not have to travel
far in the United States to encounter the E1 hierarchy—just south of the Rio Grande
river (Mexico).