what is sdh,pdh?how exactly the call get connected?
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Answer Posted / ashish khandelwal
SDH is very useful equipment that is used for the
telecommunication sector in easy transfer of data. Earlier
PDH was widely used but due to some of its weaknesses, SDH
has replaced the use of PDH. But not everywhere. Point to
Point applications are still used mainly by PDH, and also,
it's cheaper.
PDH or the plesiochronous digital hierarchy is a popular
technology that is widely used in the networks of
telecommunication in order to transport the huge amounts of
data over the digital equipment for transportation like
microwave radio or fiber optic systems. The term
plesiochronous has been derived from the Greek work ‘plesio’
that means ‘near’ and ‘chronos’ meaning time. This means
that the PDH works in a state when the various different
parts of a network are clearly synchronized. But with the
change in technology, the PDH is now being replaced by the
SDH or what is popularly called as synchronous digital
hierarchy. The SDH is useful equipment that is used in most
of the telecommunications networks.
This PDH helps in proper transmission of the data that
generally runs at the similar rate but allows some slight
variation in the speed than the nominal rate. The basic
transfer rate of the data is 2048 kilobits per second. For
instance, in each speech transmission, the normal rate
breaks into different thirty channels of 64 kilobits per
second along with two different 64 kilobits per second in
order to perform the tasks of synchronization and signaling.
The typical rate of transmitting the data over the fiber
optic systems is 565 Mbit/sec in order to transport the data
in the long distance. But as the technology has improved
with the passing of time, now the telecommunication
companies have replaced the PDH equipment with that of the
SDH equipment, which has the capability of transmitting the
data at much higher rates as compared to the PDH system.
The SDH is an international standard that is highly popular
and used for its high speed data transfer of the
telecommunication and digital signals. This synchronous
system has been specially designed in order to provide a
simple and flexible network infrastructure. This system has
brought a considerable amount of change in the
telecommunication networks that were based on the optical
fibers as far as performance and cost were concerned.
The weaknesses that PDH faced paved way for the introduction
and use of the SDH systems. Although the PDH proved to be a
breakthrough in the field of digital transmission, the
weaknesses that made it less demanded includes:
1. Asynchronous structure that is rigid.
2. Restricted management capacity.
3. Non availability of world standard on the digital formats.
4. No optical interfaces world standard and without an
optical level, networking is not possible.
But if we compare the PDH system with that of the SDH
system, the latter one has a large number of advantages.
Some of the most common advantages enjoyed by the usage of
SDH include:
1. optical interfaces
2. capability of powerful management
3. world standard digital format
4. synchronous structure is flexible
5. cost effective and easy traffic cross connection capacity
and add and drop facility
6. reduced networking cost due to the transversal compatibility
7. forward and backward compatibility
Apart from all the advantages mentioned above, the SDH also
has various management capabilities such as performance
management, security and access management, configuration
management and the event or the alarm management. So, we can
clearly make a distinction between the PDH and SDH systems
so that as per the needs of the telecommunication, the
appropriate transmission system can be used.
Please note also, that PDH, if we talk as n x E1 G.703 lines
to be transmitted, can be developed only up to 16 E1 lines -
34 Mbit/s. But SDH - can reach 155 Mbit/s, which corresponds
to STM-1, can reach 622 Mbit/s - STM-4.
However, it has to be noted. that AD-net has done a huge
work bridging the gap between SDH and PDH systems, offering
customers a SDH systems - particulary from miniSDH and
microSDH range, and - close to PDH price! For example, most
of the suppliers, if we talk about more than 16E1 - are not
able to offer units at reasonable price and still 1U or even
2U high. AD-net have breakthrough unit called AN-SDH63,
which is just 1U high, and can send 63E1 over the same fiber.
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