North American standardization of SONET started in the mid 1980s with international standardization of SDH starting soon thereafter. The following are the essential features of SONET [ANSI95a] and SDH [ITU-T00a]:
A base TDM frame structure along with a similarly structured set of progressively larger frames (for faster signals).
A limited variety of “payload envelopes” (SONET) or “virtual con-tainers” (SDH) of fixed capacity (bandwidth) that are synchronously multiplexed into the base signal (or multiples of the base signal).
A flexible byte interleaving synchronous multiplexing method for placing the payload envelopes (or virtual containers) into the base signal such that short-term rate variations ...