Hi Sumati,
SONET/SDH is a huge topic which cant be explained over the forum.The basics i can share with you.SONET/SDH protocol is very useful in data transmission over communication medium.Transmission over a communication medium has various factors to be taken into consideration like frame number,error checking etc which are very common in all the protocols.Google the topic.For basics start with the matter in Wikipedia and then find the attachment for the specification and description of SONET/SDH in detail.I have some more docs with me.If interseted please get back to me after finishing this pdf.
Thanks