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The OFC transmission system used by the Southern Railways extends upto Trichy from Madras. It is a multiplexed transmission system based on the Plesiochronous Digital and Synchronous Digital Hierarchies. Like the digital microwave transmission system, both the digital OFC systems work with a primary rate of 2 Mbps ( 30 channel PCM ).
Plesiochronous Digital Hierarchy was intended as a solution for voice telephony channel multiplexing and is not suited for the high-bandwidth connections that the applications of today need. Additionally, PDH is not capable of efficiently multiplexing streams of different bit-rates.
Some Limitations of PDH are.....
1. It is not possible to extract a 2Mbps stream from a 140Mbps stream directly without the intermediate de-multiplexing levels of 34Mbps and 8Mbps. This is because it is not possible to pinpoint the frames corresponding to the 2Mbps stream ( we are interested in ) in the midst of the frames corresponding to the other streams that make up the 140Mbps stream. This directly means that extracting one single 2Mbps stream in a location where there is no need for more than that is very expensive, since the 3 demultiplexers have to be used, the stream extracted and then the remaining streams multiplexed again.
2. PDH systems do not have any built-in provision for operational maintenance,
and the operator cannot remotely configure mux-demux equipment. This requires
technical staff to be present for any reconfiguration at the site of a
mux-demux.
3. The lack of built-in mechanisms for operational maintenance also means that the operator of the system has to maintain careful records of the various interconnections. This has a direct effect as the traffic and interconnections increase, which might lead to operational errors. Hence, control is a major problem in a large PDH network.
In SDH, the basic transmission rate is the ( STM - 1 ) Synchronous Transport Module - 1, which has a bit rate of 155.52 Mbps. This basic rate was chosen to provide direct interoperability with the American transmission hierarchy SONET ( Synchronous Optical Network ) which has a basic rate of 51.84 Mbps, which is exactly one-third the STM-1 rate. The long-standing interconnection problem between North American and European transmission systems, beginning with the T- and E-carriers respectively, can finally be brought to an end. Very importantly, an STM-1 signal can carry a number of lower rate signals as payload, thus allowing PDH signals to be sent in an SDH network. This ensures that PDH equipment is not made obsolete by the advent of SDH.
Corresponding to each existing PDH rate, SDH defines a number of containers. While the containers are associated with PDH rates defined in ITU-T Recommendation G.702, they can also accommodate other payloads like ATM. The plesiochronous signal is put in a container, along with 'dummy' bits, similar to actual plesiochronous multiplexing. A Path Overhead ( POH ) is associated with each container in an STM-1 frame. The POH contains end-to-end ( also called the path ) error correction and control bits. This information remains the same, even if it traverses many SDH links. The container and the POH constitute the Virtual Container.
A line is a portion of the path which is between a multiplexer and a demultiplexer, where the demultiplexer is not the final destination, but merely an intermediate demultiplexer from where some streams in the aggregate stream are extracted. There is control information in an STM-1 signal that is relevant only for a particular line, called the Line Overhead ( LOH ). One very important control data in the LOH is the Pointer, that points to the beginning of a virtual container inside the STM-1 signal. This allows the intermediate demultiplexer to directly extract the relevant stream from the total signal, something that is not possible in PDH.
There is also the third overhead, called the Section Overhead ( SOH ) , which is used for limited error-detection and control between repeater stages in the link. An important difference between PDH and SDH is that higher order streams are obtained in SDH by byte-interleaving the lower streams, instead of bit-interleaving as is done in PDH. Therefore, obtaining higher transmission rates in SDH is relatively straightforward. STM - 4, corresponding to 622Mbps and STM-16 corresponding to 2.4Gbps are existing rates that many operators are working at.
While PDH made it difficult to even contemplate rates above 565Mbps, there is no such problem with SDH, simply because the STM-n structure is the same, for varying values of n=1,4,16...
The Southern Railway SDH system is presently 155Mbps ( STM-1 ) out of Madras. The space occupied by the SDH equipment was a fraction of that occupied by PDH equipment, which in fact supported a lesser bit-rate. Another thing which is instantly obvious is the ease of maintenance. The SDH STM structure has provision for sending maintenance information, and this makes it possible to remotely configure equipment. Using OA&M software running on a general-purpose computer suitably interfaced with the transmission equipment, it is possible to view the status of all repeaters and sections in the link, with the option of changing 'drop-insert' configurations at various points in the link. And all this can be operated sitting in Madras, truly as if by remote control !
manisridhar at hotmail dot com
