What is BRAS ?

A broadband remote access server (BRAS) routes traffic to and from the digital subscriber line access multiplexers (DSLAM) on an Internet service providers (ISP) network.

The BRAS sits at the core of an ISPs 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

What is DSLAM ?

A Digital Subscriber Line Access Multiplexer (DSLAM) allows telephone lines to make faster connections to the Internet. It is a network device, usually located at a telephone company central office (CO), that connects multiple customer Digital Subscriber Line (DSL)s to a high-speed Internet backbone line using multiplexing techniques. The DSLAM creates a network similar to a LAN but not subject to Ethernet distance limits, thus providing an Internet connection for the subscribers.

Hardware details
Customers connect to the DSLAM through ADSL modems or DSL routers, which are connected to the PSTN network via typical unshielded twisted pair telephone lines. Each DSLAM has multiple aggregation cards, and each such card can have multiple ports to which the customers lines are connected. Typically a single DSLAM aggregation card has 24 ports, but this number can vary with each manufacturer. The most common DSLAMs are housed in a telco-grade chassis, which is supplied with (nominal) 48 Volts DC. Hence a typical DSLAM setup may contain power converters, DSLAM chassis, aggregation cards, cabling, and upstream links. The most common upstream links in these DSLAMs use gigabit ethernet or multi-gigabit fiber optic links. read more...

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