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Synchronous Digital Hierarchy 85

It became apparent that very expensive equipment was needed
at every central office to demultiplex and multiplex high-speed
lines across the backbone networks.

Consequently, the standards committees (both ANSI and the ITU)
began working on solutions to these multiplexing problems.The ulti-
mate attempt was to develop a synchronous transmission system
that could replace the plesiochronous transmission systems.
Table 4-1 presents a summary and comparison of worldwide
speeds and hierarchical arrangements. This table brings the differ-
ences to light when you compare the speeds and multiplexed chan-
nels combined.
In 1986, the Consultative Committee of International Telegraphy
and Telephony (CCITT) published a standard set of transmission
2
rates for SDH. The SDH standards finally emerged in 1992. These
are filed under the following standards:

G.707
G.708
G.709

Table 4-1
Hierarchical North American European Japanese International
Comparing the Level DS-n CEPT-n Level Rules
Speed Rates for
Various Levels of 0 64 64 64 64
the Digital 1 1,544 2,048 1,544 2,048
Hierarchy [in
2 6,312 8,448 6,312 6,312
kilobits per
second, (kbps)] 3 44,736 34,368 32,064 44,736

4 274,176 139,264 97,728 139,264

5 564,992 397,200 564,992

2 CCITT is now called the ITU-TS