Introduction

Nowadays all around the world has increased demands on networks both from the point of view of ensuring high reliability communications, and expansion of services provided to subscribers. Satisfaction of needs in communication, development and modernization of telecommunication networks, can be implemented on the basis of new technologies, such as optical communication lines, digital switching systems and digital transmission systems.

Intensive progress of digital transmission system (DTS) due to their significant advantages compared to analog transmission systems. The main advantages of DTS before the ATS are:

• higher noise immunity;

• he independence of the transmission quality, the length of the line;

• the stability of the parameters of the DTS channels;

• the effectiveness of the use of the channel bandwidth for the transmission of digital signals;

• simple mathematical signal processing;

• the possibility of building a digital communication network;

• high technical and economic indicators.

Advantages of DTS can be most distracting in a digital communication network. Such a network contains only the digital paths that connect network nodes and end systems digital switching and digital subscriber units.

Let’s talk about fiber optic transmission systems (datalinks).

 Most systems operate by transmitting in one direction on one fiber and in the reverse direction on another fiber for full duplex operation. Most systems use a "transceiver" which includes both transmission and receiver in a single module. The transmitter takes an electrical input and converts it to an optical output from a laser diode or LED. The light from the transmitter is coupled into the fiber with a connector and is transmitted through the fiber optic cable plant. The light from the end of the fiber is coupled to a receiver where a detector converts the light into an electrical signal which is then conditioned properly for use by the receiving equipment. The sources used for fiber optic transmitters need to meet several criteria: it has to be at the correct wavelength, be able to be modulated fast enough to transmit data and be efficiently coupled into fiber.

Just as with copper wire or radio transmission, the performance of the fiber optic data link can be determined by how well the reconverted electrical signal out of the receiver matches the input to the transmitter. The discussion of performance on datalinks applies directly to transceivers which supply the optical to electrical conversion.

Every manufacturer of transceivers specifies their product for receiver sensitivity (perhaps a minimum power required) and minimum power coupled into the fiber from the source. Those specifications will end up being the datalink specifications on the final product used in the field. 

All datalinks are limited by the power budget of the link. The power budget is the difference between the output power of the transmitter and the input power requirements of the receiver. The receiver has an operating range determined by the signal-to-noise ratio (S/N) in the receiver. The S/N ratio is generally quoted for analog links while the bit-error-rate (BER) is used for digital links. BER is practically an inverse function of S/N.

The operating range of a data link will look like this figure of BER vs received optical power for a typical fiber optic transceiver. There must be a minimum power at the receiver to provide an acceptable S/N or BER. As the power increases, the BER or S/N improves until the signal becomes so high it overloads the receiver and receiver performance degrades rapidly. More on power budgets and the similar "loss budget" which is the estimate of fiber optic cable plant loss.

So, consider Plesiochronous Digital Hierarchy (PDH). What does Plesiochronous Digital Hierarchy (PDH) mean?

The plesiochronous digital hierarchy (PDH) is a telecommunications network transmission technology designed for the transport of large data volumes across large scale digital networks.

The PDH design allows the streaming of data without having isochronous (clocks running at identical times, perfectly synchronized) to synchronize the signal exchanges. PDH clocks are running very close, but not exactly in time with one another so that when multiplexing, signal arrival times may differ as the transmission rates are directly linked to the clock rate.

PDH allows each stream of a multiplexed signal to be bit stuffed to compensate for the timing differences so that the original data stream could be reconstituted exactly as it was sent.

PDH is now obsolete and has been replaced by synchronous optical networking and synchronous digital hierarchy schemes, which support much higher transmission rates.

The term plesiochronous means "nearly synchronous". PDH supports a data transmission rate of 2048 Kbps. The data rate is controlled by a clock in the device that generates the data. 

With multiplexing signals, the clock rate on each stream within the multiplex can vary very slightly. This can occur for a number of reasons, and is sometimes referred to as "jitter". When a multiplexed stream arrives, there has to be a mechanism for reconstituting the various streams into the original signal form. With signals arriving at various different end-times, there has to be a way to get them all to be available for inverse multiplexing in a simultaneous manner, so PDH bit-stuffs the signals until they are all the same length, at which point they can be successfully demultiplexed. The stuffed bits are then discarded.