I have some questions to ask, hope that you can help me..

1. How can we calculate a dead zone? I see that in http://en.wikipedia.org/wiki/Optical_time-domain_reflectometer, the dead zone for 10ns pulse width is about 1.5m. But in http://xa.yimg.com/kq/groups/16193174/1957821756/name/OTDR+Guidebook0605.pdf, it's said that 8/3m. Which one is true? And how can I get it? In there, it is said that it's a theoritically, if it's true, can someone give me a reference please?

2. I asked a professor via email and he said that I can calculate the length of the Dead Zone by using this formula : The length L of the dead zone is given by the relaxation time t of the receiver and the group velocity Vg of the fiber (L=Vg. t). Can someone tell me is it true? And if it's true, what is the value of the 'relaxation time t of the receiver' if I'm using an Avalanche Photodiode?

3. How can we calculate the length of the noise at the end of fiber when we are using an OTDR? I need a formula or some references to know the length because I'm making a simulation and not using the real thing.