Hi easkkimuthu, the link is not working simply because the hyperlink is not on the full address, so you should copy and paste the following into the address bar of you browser:
https://www.utdallas.edu/~dlm/3350%20comm%20sys/T-REC-G.Sup39-200602-I!!PDF-E.pdf
Regarding your question: I suggest a good book like Agrawal – Fiber Optic Communication Systems to have a basic knowledge of this matter.
Amplifiers are used to increase the link distance, but there are many factors to take into account (where they are placed – booster or preamplifiers? Their Noise Figure, if they create nonlinear impairments….) that it is better to start having some techincal background on this.
I hope my answer helps you, otherwise feel free to ask more clarifications!
Best Regards,
Alessandro