Difference between PDH and SDH

1. Plesiochronous Digital Hierarchy (PDH) :
PDH stands for Plesiochronous Digital Hierarchy. It is a telecommunications network transmission technology designed for the transport of large data volumes across large-scale digital networks. In PDH, multiplexing of 2 Mbit/s signals into higher order multiplexed signals. The laying cable between switch sites is very expensive. It increases the traffic capacity of a cable by increasing the bit rate. 

2. Synchronous Digital Hierarchy (SDH) :
SDH stands for Synchronous Digital Hierarchy and it refers to as a multiplex technology used in telecommunication. Synchronous Optical Network is internationally used. It is said to be a variation of SONET and is taken equal to SDH. It characteristics are founded on high-order multiplexing. These are the technologies that provide quite fast low-predicted network interconnection than PDH which stands for Plesiochronous Digital Hierarchy. 

Difference between PDH and SDH :

S. No.	PDH	SDH
1.	Reference clock is not synchronized throughout the network.	Reference clock is synchronized throughout the network.
2.	There is no synchronization between payload and frame.	There is synchronization between payload and frame.
3.	Its system has different frame structures at different hierarchy levels.	Its system has consistent frame structures throughout the hierarchy.
4.	In this, Physical cross-connections are provided on the same level.	In this, Digital cross-connections are provided at different signal levels.
5.	In this, rates are derived from basic rate of 1.544 Mbps. The maximum capacity is about 566 Mbps.	In this, rates are derived from basic rate of 155.52 Mbps. The maximum up to 40 Gbps rates can be derived from basic rate mentioned.
6.	The implementation cost of PDH is lower.	The implementation cost of SDH is higher.
7.	The multiplying method used in PDH is complex.	The multiplying method used in SDH is simple.
8.	It is incompatible with other signals such as ATM, FDDI, DQDB etc.	It is compatible with other signals such as ATM, FDDI, DQDB etc.
9.	There is no universal standard for PDH.	The universal standard exists for SDH.
10.	Poor PDH administration.	Better SDH administration.
11.	PDH is complex system in comparison to SDH.	SDH is relatively a simple system.
12.	In PDH systems, the network’s reference clock is not synchronized.	However, this is not the case with SDH systems, here reference clock is synchronized.
13.	Advantages- Enhancement in network management The core purpose of its creation was to support the transfer of massive amounts of data over digital devices using a variety of transmission mediums, including microwave, radio or fiber optic systems.	Advantages- Ability to carry broadband signals Enables speedy failure recovery It supports various operators and is multivendor. Permits for an unlimited increase in optical fiber bandwidth Rings provide switching protection for traffic. It offers LAN network transport services for interactive multimedia and video conferencing. Simple expansion to higher bit rates, improving the administration and maintenance procedure.
14.	Disadvantages- Difficultly in multiplexing and demultiplexing because different frames are used for transmission and the data layer in PDH.  Constrained maximum PDH bandwidth is 566 Mbps. There is a bit rate tolerance. Integration is quite challenging in connecting networks because each manufacturer has their own standards and PDH uses various multiplexing hierarchies so specialized interface equipment is needed to interwork the two hierarchies. De-multiplexing of entire system is required in order to access the lower tributary. Enables configuration for Point-to-Point networks.	Disadvantages- Complex system because of pointers that enables direct adding and dropping of lower-rate signals.  Requirement of complex system because of different ways for the management of traffic. It can be affected by computer viruses as software plays a big part in the SDH system.  Due to the high number of OH bytes used for OAM, it offers a lower bandwidth utilization ratio.