From aa-asterisk.org.uk wiki
BRAS (supposedly pronounced Bee-Razz, not bras) stands for Broadband Remote Access Server. It is the equipment which authenticates your login and passes it to your ISP's RADIUS server. Much like a Microsoft RAS server or pppd, it handles the PPP connection which is made once the line has been synced.
The BRAS rate (sometimes known as IP rate or line rate) is the rate at which IP packets will be sent down your line. This is not necessarily as fast as your sync speed, because of ATM, PPP, LCP overheads etc.
The maximum BRAS rate available on an 8Mbit/s BT Wholesale ADSL Max line is around 7.15Mbit/s, and that is with a maximum sync rate of 8128kbit/s (which is the ATM rate). The BRAS rate is calculated in 0.5Mbit/s steps.