As we progress through this chapter, we must keep in mind that there are two quite different PCM standards. On one hand there is a North American standard which some call T1, and 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 from the United States to encounter the E1 hierarchy, just south of the Rio Grande (Mexico).
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