Difference between SONET and SDH

1. Synchronous Optical Network (SONET):
SONET stands for Synchronous Optical Network. It is a communication protocol, developed by Bellcore which is used to transmit a large amount of data over relatively larger distances by using optical fiber. By SONET, multiple digital data streams are transferred at the same time over the optical fiber. 

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

Difference between SONET and SDH :

S. No.	SONET	SDH
1.	SONET stands for Synchronous Optical Network.	SDH stands for Synchronous Digital Hierarchy.
2.	It is a digital hierarchy interface for the optical transmission.	It is a network node interface, user-network interface and U reference-point interface to assist BISDN.
3.	It is developed by ANSI (American National Standards Institute).	It is developed by ITU-T (International Telecommunication Union – Telecommunication Standardization Sector).
4.	It has total number of transport overhead of 9×3 = 27 bytes.	It has total number of transport overhead of 9×9 = 81 bytes.
5.	It can transmit data in only synchronous mode.	It can transmit in both synchronous mode and asynchronous mode.
6.	It does not provides better transmission rates as compared to SDH.	It provides better transmission rates as compared to SONET.
7.	Optical Carrier level-1(OC-1) is the fundamental unit of SONET. This unit serves as the basis for other rates which are as follows-  OC-3 OC-12 OC-18 OC-24 OC-36 OC-48 OC-96 and OC-192	Synchronous Transmission Module level-1 (STM-1) is the fundamental unit of  SDH.
8.	Advantages- Extreme effectiveness Decreased cost Easy demultiplexing Permits transmission of all traffic types  It provides network resilience capabilities. Both legacy and future networks can use it.	Advantages- The ability to transmit PDH signals currently in use. Rings provide switching protection for traffic. Rapid failure recovery possible SDH allows for limitless sudden increases in optical fiber bandwidth. It is a dynamic, self-repairing network. It is simple to connect to various networks. Simple expansion to higher bit rates that improves the administration and maintenance procedure.
9.	Disadvantages- No interoperable standards exist. With few channels, low cost is efficient. Issue of bandwidth effectiveness at higher capacities.	Disadvantages- The SDH system largely relies on software. It is therefore susceptible to computer viruses. Complexity is associated with the system because of lower rate signals that are added or removed directly are archived using pointers.  Complex equipment is required because there are so many different management traffic types and options.