THOMSON SpeedTouch, Configuration Manual

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Chapter 1
Introduction
E-DOC-CTC-20051017-0167 v1.0
4
1.1 Definitions and terminologies
1.1.1 Local End / Remote End
Local ends
Remote end
Irrespective whether a conversation happens between people or machines, there
are at least two parties involved. In this document the SpeedTouch™ is referred to
as the
Local end , and the Broadband Remote Access Server (BRAS) of the ISP the
Remote end .
The configuration at the remote end is fully controlled by the ISP.
The local end must be a mirror image of the remote end, but a few items can be
selected autonomously by the subscriber.
1.1.2 Virtual Channels
ATM
The popular term “DSL Services” is actually a misnomer because it refers not only
to the raw “DSL Technology” but also to virtual channels, encapsulation and
formatting of packets.
Indeed, on the one hand, DSL refers to advanced modulation/demodulation
techniques, applied to the copper wire between a subscribers’ home and the
telephone service provider’s central office. The net result of this technology is a
boost in bandwidth from a few tens of kilobytes per second to multiple megabits
per second.
On the other hand, as the distance covered by DSL is limited to only a few
kilometers, a wide area communication infrastructure is required to transport data
to ISPs and corporate data centres. For this purpose, Asynchronous Transfer Mode
(ATM) has been selected.
Virtual channels
A key aspect of ATM is its concept of Virtual Channels . For DSL service, an ATM
virtual channel is established between the DSL modem and the central office Digital
Subscriber Line Access Multiplexer (DSLAM) of the telephone service provider.
ATM cells
Another key aspect is the concept of ATM cells . Prior to DSL modulation, the
SpeedTouch™ must encapsulate user data in ATM cells. Modulated ATM cells
flowing along the virtual channel, are recovered in the central office of the
telephone service provider and switched to the ISP or corporate data centre of the
subscriber.
VPI/VCI
Each ATM cell carries two labels as part of its header:
Virtual Path Identifier (VPI)
Virtual Channel Identifier (VCI)
As a consequence, multiple virtual channels can reside on a single DSL line.
The SpeedTouch™ products support multiple combinations of Virtual Path
Identifiers (VPI) and Virtual Channel Identifiers (VCI), thus supporting multiple
Virtual Channels.
Commonly VPI is called virtual path and VCI virtual channel.