When we place authentication system in our demilitarized zone (DMZ)

1) What happens when we place the authentication system in our demilitarized zone (DMZ)—that is, in the layer closest to the Internet?

2) What do we have to do to protect the authentication system? Does this placement facilitate authentication in some way? How about if we move the

authentication system to a tier behind the DMZ, thus, a more trusted zone? What are the implications of doing so for authentication performance? For security?