Tektronix CTS 850, Руководство пользователя

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Functional Overview
2–28
CTS850 SDH/PDH Test Set User Manual
Plesiochronous Digital Hierarchy (PDH)
Traditionally, digital transmission systems and hierarchies have been
based on multiplexing signals which are plesiochronous (running at
almost the same speed). Also, various parts of the world use different
hierarchies which lead to problems of international interworking, for
example, between those countries using 1.544 Mbit/s systems
(U.S.A. and Japan) and those using the 2.048 Mbit/s system.
In the PDH, tributaries and higher–order bit streams are allowed to
deviate from a pre–defined bit rate by a specified amount. The
justification (bit stuffing) process is required which brings all the
tributaries up to the same bit rate before multiplexing takes place.
However, the justification method makes it impossible to identify the
location of specific tributary channels within a higher–order bit
stream, without demultiplexing back down to the 2 Mb/s tributaries.
To recover a 64 kbit/s channel from a 140 Mbit/s PDH signal, it is
necessary to demultiplex the signal all the way down to the 2 Mbit/s
level before the location of the 64 kbit/s channel can be identified.
PDH requires ”steps” (140–34, 34–8, 8–2 demultiplex; 2–8, 8–34,
34–140 multiplex) to drop out or add an individual speech or data
channel. This is due to the bit–stuffing used at each level. Extra bits
are added (stuffed) in to the digital tributaries which increases the
speed of the tributaries until they are all identical.
140–34 DEMUX
34–8 DEMUX
8–2 DEMUX
34–140 MUX
8–34 MUX
2–8 MUX
140 Mb/s
140 Mb/s
8 Mb/s
2 Mb/s
34 Mb/s
Drop & Add
8 Mb/s
34 Mb/s
Figure 2–20: PDH Multiplexing by steps showing add/drop function