Couple of quesions on Alwayson AG's

Hi,

Couple of questions on Alwayson AGs.

1. What are the different modes in AG's? Is it same like in db mirroring we have HA mode with witness, High protection mode and High performance mode or any difference?

2. Do we still need to have an Witness server similar to db mirroring which allows

the capability of automatic fail over? How automatic failover is detected and initiated in case of AG's?

Thank you.