Installation manual

Authentication will normally be performed by the BAS, probably by reference to a RADIUS

server.

BAS

The Broadband Access Server could be described as a highly flexible and configurable

ATM/PPP/IP switch/router. The function of the BAS is to ‘unwrap’ any encapsulation used to

carry the TCP/IP traffic over the ATM connection and pass the data into the ISPs normal

network. In addition, if PPP is one of the encapsulating protocols, the BAS performs the user

authentication (perhaps using an authentication server such as RADIUS to verify the

username/password combination).

Splitting this functionality from the DSLAM increases the implementation flexibility by allowing

the connection to the ISP’s IP network (and the PPP authentication, if necessary) to take place

at the ISP’s premises, across an ATM network from the DSLAM.

In some configurations, the BAS will allow switching between ISPs based on the

userid/password that the user supplies in the PPP authentication.

This architecture allows the ISP to replace the Access Server that terminates and authenticates

the PPP connection from customers with analogue modems and ISDN lines with the BAS for

ADSL customers. If PPP is used on the ADSL connection, then the ADSL user connected

through the BAS appears in the ISP’s network in exactly the same way as an analogue modem

or ISDN user; the same authentication server could be handling both conventional and ADSL

users. This explains the popularity of PPP as an encapsulating protocol.

The ‘Redback SMS100’ is an example of a popular BAS.

Bridged/Routed connections

Bridge-based solutions are perceived by some telcos and ISPs as the simpler, cheaper, entry

level ADSL offering. These telcos/ISPs tend to regard routed connections as ‘complicated,

expensive and advanced’. It is difficult to see how this can be justified, since the thinnest of

passive devices can use a routed connection using Microsoft’s PPP over ATM stack in

Windows 98SE, Me and 2000.

Note that some telcos launching services more recently (for instance BT) have chosen to

provide only routed solutions even for their low-end services, skipping the bridge.

Bridged solutions are normally either:

* Ethernet with LLC/SNAP (this is one of the options of RFC 1483)

* PPP over Ethernet (PPPoE - there is no ‘real’ RFC for this, only an ‘informational’)

Routed solutions are normally either

* PPP over ATM (PPPoATM - RFC 2364)

* IP with LLC/SNAP (this is another option of RFC 1483)

Allied Data Technologies 47

CopperJet 81x

CJT 81x 02-10-2001 11:45 Pagina 47