A broadband remote access server (BRAS) routes traffic to and from the digital subscriber line access multiplexers (DSLAM) on an Internet service provider's (ISP) network.
The BRAS sits at the core of an ISP's network, and aggregates user sessions from the access network. It is at the BRAS that an ISP can inject policy management and IP Quality of Service (QoS).
The specific tasks include:
Aggregates the output from DSLAMs
Provides user PPP sessions or IP over ATM sessions
Enforces quality of service (QoS) policies
Routes traffic into an Internet service provider’s backbone network
A DSLAM collects data traffic from multiple subscribers into a centralized point so that it can be uploaded to the router over a Frame Relay, ATM, or Ethernet connection.
The router provides the logical termination for PPP sessions. These may be PPP over Ethernet (PPPoE) or PPP over ATM (PPPoA) encapsulated sessions. By acting as the PPP termination point, the BRAS is responsible for assigning session parameters such as IP addresses to the clients. The BRAS is also the first IP hop from the client to the Internet.
The BRAS is also the interface to authentication, authorization and accounting systems (see RADIUS)
Reference:
http://en.wikipedia.org/wiki/Bras
