Posted By: technopediasite
The primary reason for the creation of SDH was to
provide a long-term solution for an optical mid-span meet between operators;
that is, to allow equipment from different vendors to communicate with each
other. This ability is referred to as multi-vendor interworking and allows one SDH-compatible
network element to communicate with another, and to
replace several network elements, which may have
previously existed solely for interface purposes.
The second major advantage of SDH is the fact that it’s
synchronous. Currently, most fibre and multiplex systems are plesiochronous.
This means that the timing may vary from equipment to equipment because they
are synchronized from different network clocks. In order to multiplex this type
of signal, a process known as bit-stuffing is used. Bit-stuffing adds extra
bits to bring all input signals up to some common bit-rate, thereby requiring
multi-stage multiplexing and demultiplexing. Because SDH is synchronous, it
allows single-stage multiplexing and demultiplexing. This single-stage multiplexing eliminates hardware complexity, thus decreasing the cost of equipment while improving signal
quality.
In plesiochronous networks, an entire signal had to be
demultiplexed in order to access a particular channel; then the non-accessed
channels had to be re-multiplexed back together in order to be sent further
along the network to their proper destination. In SDH format, only those
channels that are required at a particular point are demultiplexed, thereby
eliminating the need for back-to-back multiplexing. In other words, SDH makes individual
channels “visible” and they can easily be added and dropped.
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.
To recover a 64 kbit/s channel from a 140 Mbit/s PDH
signal, it’s 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.
Limitations of PDH Network
The main limitations of PDH are:
➤Inability to identify individual channels in a
higher-order bit stream.
➤Insufficient capacity for network management;
➤Most PDH network management is proprietary.
➤There’s no standardised definition of PDH bit rates
greater than 140 Mbit/s.
➤There are different hierarchies in use around the
world. Specialized interface equipment is required to interwork the two
hierarchies.
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