BRAS Basic functions & logical connectivity

Hello,

     can any one explain me below points about BRAS & access users

1-What is BRAS ?

2-If BRAS is used   often used for HSI(high speed Internet) services   its means  that when access users go to internet then through BRAS? 

3-What are the devices used b/w access users & BRAS & whats its function?

4-Can we connect access user direct with switch then why used BRAS?

5-BRAS used for AAA  can we used router for that function?

6-Which type of connectivity of BRAS with Uplink IP CORE network means through IGP protocol?

7-How BRAS get internet routes ? BRAS have BGP peering with CORE network RR?

Detailed answer will be appreciated

Thank You