Chapter 9: Glossary
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9 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. 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
