06-10-2016, 02:36 PM
A review of Plesiochronous Digital Hierarchy (PDH) and
Synchronous Digital Hierarchy (SDH)
1458046286-pdhpaper.pdf (Size: 57.73 KB / Downloads: 35)
ABSTRACT
With the advancement in telecommunications, packet
traffic is rapidly becoming the mainstream of data
traffic. The use and deployment of Synchronous Digital
Hierarchy (SDH) networks for interconnection has
gained traction worldwide due to its flexibility and
standard for interconnecting multiple vendors, low
operating cost and the high quality of service it provides.
Plesiochronous Digital Hierarchy (PDH) on the other
hand has been used before the introduction of the SDH
standard and it also provides a means to transport large
quantity of data via digital equipment such as radio wave
systems, optic fibre and microwaves. In this paper, we
shall discuss both PDH and SDH technologies and
identify some of the features of both and the issues in
PDH that brought about the introduction of the SDH
technology
INTRODUCTION
In Modern telecommunication systems, the increasing
demand for new services, like video and data, calls for
more complicated transmission methods, higher
communication speeds, and more complex network
topologies. These requests, in turn, impose high design
accuracy and perfect synchronization techniques of data
signals [5]. The term ‘Synchronization’ is nowadays
broadly used in telecom to encompass the methods that
enable oscillators at different locations to be set to the
same frequency within specified limits. With the
introduction of Pulse Code Modulation (PCM) for
telephony in the late 1960s which allows a single line to
be used by multiple signals; using a digital time-domain
multiplexing where the analog telephone signal is
sampled, quantized and transmitted, network
communications were being changed into digital
technology and the demand for a bigger bit rate also increased. Plesiochronous Digital Hierarchy (PDH) was
introduced by ITU-T G.702 [1] to cope with the
increasing demand for higher bit rates; it uses a basic
multiplex of 2Mbps with other stages of 8, 34 and
140Mbps. Due to the fact that PDH wasn’t quite
synchronous, multiplexers uses a little overhead on their
high speed trunks to help cater for the differences in the
data rates of streams in ports with low speed. Due to the varying developments adopted by different
networks, interconnecting gateways between networks
was expensive and difficult; also PDH was not flexible
which made monitoring and management more difficult
to realize. Synchronous Digital Hierarchy (SDH) was
developed to fix some of the limitations experienced in
PDH. As more people began to use SDH, management
capabilities increased because of the comprehensive
monitoring and the high capability management
throughout the network [4].
II. PLESIOCHRONOUS DIGITAL
HIERARCHY (PDH)
Plesiochronous Digital Hierarchy (PDH) was the
standard originally for telephone networks. PDH uses
time division multiplexing [3]. It was also designed to
support digital voice channels running at 64kbps, was
designed to use a No Store and Forward method which
puts a strict restriction between the Transmitter (TX) and
the Receiver (RX) and a Plesiochronous method was
adapted for use which implies (nearly synchronous) [3].
PDH networks evolved, as isolated links connecting
analog switching systems for Public Switched Telephone
Networks.
Different standards were used in PDH which made it
difficult to connect different networks. The figure below
shows the different hierarchy adopted in PDH for US,
Europe and Japan.
The T-1 carrier system is adopted as a United States
(US) standard; it uses 24-voice channels which are the
results of quantization, sampling and coding using TDM
framing and the PCM standard [3]. Also additional
signaling channel of 1 bit is used and the T-1 speed is
1.544Mbps.
A. Multiplexing techniques in PDH
To move a multiple of 2Mbps data from one point to
another, these data streams are multiplexed in groups of
four, which is done by taking one bit from the first
stream and one bit from the second stream and one bit
from every other stream. In multiplexing, the
transmitting multiplexer adds additional bits in order for
the receiving end to decode which bits belongs to a
particular 2Mbps stream of data in order to reconstruct
the original data streams. The additional bits added are
called “stuffing bits” or “justification bits”.
B. PDH Synchronization
In PDH, every device has its own clock making network
wide synchronization impossible. Also, errors occur
during synchronization because every clock is different.
The solution to preventing this error is by inserting and
removing surfing bits to the frame called bit stuffing [3].
The problem of synchronization is solved by Frame
Alignment Word (FAW).
If a multiplexer clock rate is higher than the tributary
rate, it is called positive stuffing and this can be used for
up to 140Mbps systems. On the other hand, if the
multiplexer clock is lower than the tributary rate, it is
called negative stuffing. When the MUX clock rate and
the tributary bit rate are the same, it is called positive- negative stuffing or justification. In positive stuffing, the
steps are performed: Data is written in a temporary buffer. When transmitting the data to a faster
transmission channel, data is read from the buffer
at a higher rate.
Whenever the buffer is meant to be empty, a
stuffing bit is normally transmitted rather than the
actual data itself. When a stuffing bit is to be sent, a signal is sent to
the receiver so that the stuffing bit can be
removed at the receiving end.
In PDH, because a different frame is used both on the
transmission and data layer, multiplexing and de- multiplexing operation is very complex.
C. Limitations of PDH
1) PDH is not flexible: The difficulty involved in
identifying individual channels in a higher bit stream
order means that multiplexing must be performed for the
high bit rate channel down through all multiplexing
levels until the ideal rate is located, this requires a lot of
multiplexing cost and its expense.
2) It is inefficient: In PDH, it is difficult to get slower
tributaries from high speed rates [3].
3) Lack of performance: Since the performance of
PDH systems cannot be monitored, it is difficult to
provide a good performance to the system. Also there are
no international agreed standards for monitoring the
performance of PDH and no management channels.
4) PDH lacks standards: Every manufacturer has its
own standards; PDH also has different multiplexing
hierarchies making it difficult to integrate interconnecting
networks together.
5) Inefficiency in high bandwidth connections: PDH is
not ideally suited for high capacity or high bandwidth
connections [3].
Other disadvantages and limitations of PDH include: Accessing lower tributary requires the whole
system to be de-multiplexed. The maximum capacity for PDH is 566Mbps
which is limited in bandwidth. Tolerance is allowed in bit rates. PDH allows only Point-to-Point configuration. PDH doesn’t support Hub.
D. Migrating to Synchronous Digital Hierarchy (SDH)
Since the emergence of standard bodies in the 1990’s,
Telecom providers sparked the standardization process
since the PDH system was no longer scalable to
accommodate high capacity bandwidth and was not
guaranteed to support traffic growth [3]. The optical
technologies were gradually becoming commonly used
and interoperability amongst different providers was very
difficult. Synchronous Optical Network (SONET) was the American standard and Synchronous Digital
Hierarchy (SDH) was proposed and the European
standard.
SDH provided a vendor independent and sophisticated
structure that resulted into the development of new
applications, new network equipments and management
flexibility than the PDH. Other services that resulted due
to the evolution of SDH include:
a) High / low speed data
b) LAN interconnection
c) Voice
d) Services such as HDTV
e) Broadband ISDN.