Installation manual

Glossary
ADSL
Strictly speaking, Asymmetric Digital Subscriber Line defines only a way of transmitting
broadband data (at speeds between 64kbps and 8Mbps) between a user’s premises
(home or office) and the local telephone exchange. In order to increase the number of
customers that it is possible to serve, the upstream speed (from the user) is lower than the
downstream speed (to the user), so the service is ‘asymmetric’. The scope of the definition is
similar to that of a modem, such as V.90; the format or meaning of the data is not defined
(except in the case of G.992.2 (G.lite) where ATM is defined as the low-level protocol).
However, ATM is de facto the standard low level protocol used with ADSL.
Since getting data to and from the local telephone exchange is not in itself of much use, and the
raison d’être of ADSL services is to enable high-speed access to the Internet, discussions of
ADSL (including this one) generally include how the data connection is extended to an Internet
Service Provider, and so, to the Internet. So, we assume that ADSL is used to carry TCP/IP data
but how this is done is not part of the ADSL specification.
ATM
This protocol is always used as the low level protocol (above the ADSL transmission layer).
This is because it is a flexible and convenient way for the telco to extend the user’s data
connection from the local exchange (where the ADSL connection ends) to the ISP.
Telcos have substantial experience of using ATM to carry broadband data within their networks.
Strictly speaking, ‘Full rate’ ADSL (G.dmt, G.992.1) does not specify that ATM is the low level
protocol, so another protocol could, in theory, be used, but ATM is the de facto standard.
G.lite (G.992.2) does, however, specify ATM as the low level protocol.
ATM connections are usually PVCs (permanent virtual circuits); the route is pre-configured
through the network. SVCs (switched virtual circuits) exist but are not widely used.
The owner of the data on an ATM link is described by two parameters: the VPI
(Virtual Path Identifier) and VCI (Virtual Channel Identifier). Each ATM switch defines in its
configuration the mapping between a VPI/VCI combination on one port and a different VPI/VCI
combination on another port. This configuration defines the route through the ATM switch.
Note that this means that a particular data connection may be given many different VPI/VCI
‘addresses’ as it passes through the network.
Authentication
Many ADSL connections employ PPP encapsulation in order that authentication can be
performed in the same way for ADSL users as it is for analogue modem and ISDN users.
Since the connection from the user to the ISP is, in effect, a point-to-point ATM connection
using a PVC with a fixed route defined by the telco, one could debate the need for
authentication. The ISP could, in theory, work out who you are from the ATM VPI/VCI that your
connection appears on. However, ISPs are not geared up to manage ATM VPI/VCIs but they are
geared up to handle PPP authentication, so this is currently the norm.
Allied Data Technologies46
CopperJet 81x
CJT 81x 02-10-2001 11:45 Pagina 46