The Comparison Of PDH And SDH – Fiber Optical Networking

1.Introduction
With the rapid development of internet industry, the information content transmitted, exchanged and handled by network has greatly risen. As the transmission system of internet, PDH fails to meet the application of internet now, so SDH comes into being as required. This article is written to give you a better understanding of SDH and PDH through the comparison of SDH and PDH.

2.What Is PDH and SDH?
PDH, abbreviation from plesiochronous digital hierarchy, is a popular technology applied in the networks of telecommunication in order to transport huge amounts of data over digital transport equipment like fibre optic or microwave radio systems. As shown in Figure 1, in PDH, digital multiplexer’s inputs are of same bit rate and are derived from different clocks from different oscillators. Each will differ within tolerance of few clock periods. Hence it is called plesiochronous. The term plesiochronous is derived from Greek plēsios, which means near and close time and refers to the fact that PDH networks operate in a state where different parts of the network are nearly synchronised, but not quite perfectly synchronised.

Figure 1: The working principle of PDH

SDH, short for synchronous optical networking, are standardized technology that is used for high-speed data transmission of telecommunication and digital signals. It can transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). As shown in Figure 2, In SDH, digital multiplexer’s inputs are of same bit rate and are derived from common clock. Hence it is called synchronous.At low transmission rates data can also be transferred via an electrical interface. SDH system is designed to replace PDH system for transporting large quantities of telephonecalls and data traffic over the same fiber without synchronization problems, providing a simple and flexible network infrastructure.

Figure 2: The working principle of SDH

3.SDH Advantages over PDH
SDH is actually derived from PDH, but makes lots of improvements on the basis of PDH. Compared with PDH, SDH has a large number of advantages. The advantages enjoyed by utilization of SDH includes:
(1)The standardized optical interfaces make it very convenient for interconnection in lines.
(2)The world-standard frame structure and rate of digital signals make it easy to interconnect in the world.
(3)SDH has excellent ability of DXC.
(4)SDH is equipped with the powerful capacity of networking and network protection.
(5)Synchronous structure is flexible.
(6)SDH adopts the synchronous mapping, encapsulation and pointer to facilitate the add and the drop branches.
(7)SDH is cost-effective and reduces networking cost due to the transversal compatibility.
(8)SDH possesses forward and backward compatibility.

4.SDH Disadvantages over PDH
Though SDH has plenty of advantages compared with PDH, it still possess some disadvantages. The weaknesses of SDH is as follows.
(1)The utilization rate of SDH bandwidth is relatively low. In SDH, about one twentieth of data frames are used as the signals of management, thus resulting in the reduction of data frames utilized to transmit signals. SDH improves its maintainability but lowers the utilization rate of bandwidth.
(2)SDH is lacking in network security because of adopting the OAM as the monitoring and maintenance instrument of its following performances. As the common software, OAM is likely to be attacked by viruses or Trojans owing some bugs of designing and developing it.

5.Conclusion
With the advent of the information society, PDH fails to satisfy the internet requirements for transmitting signals. Though PDH has proved to be a breakthrough in the domain of digital transmission , it is replaced by SDH which is a very useful technology used in the telecommunication sector owing to its own weaknesses now. As the transmission system, SDH is able to transport large quantities of digital signals over the same fiber without synchronization problems, providing a simple and flexible network infrastructure. SDH system has brought a considerable amount of changes in the telecommunication networks that are based on the optical fibers as far as performance and cost are concerned.

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