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EXTERNAL ADSL USB MODEM USERS’ MANUAL
2.2 Overview
2.2.1 A
BOUT
ADSL
Asymmetric Digital Subscriber Line (ADSL) technology provides high-speed data
access across regular phone lines (copper wires) by making use of previously
unused frequency bandwidth above the voice band. By placing the ADSL signal
above the frequency of the voice signal, ADSL service is able to coexist on the same
line with your telephone service. ADSL is asymmetric in the sense that it provides a
higher data rate in the downstream (receive) direction than in the upstream
(transmit) direction. Asymmetric operation is ideal for the home or small office
because files and information are downloaded more frequently than uploaded.
There are several standard types of ADSL modulation techniques including
Discrete Multitone (DMT) and Carrierless Amplitude and Phase (CAP). The
External ADSL USB Modem supports the following DSL standards: ANSI T1.413
Issue 2, ITU G.992.1 (G.DMT), and ITU G.992.2 (G.lite).
2.2.2 P
ROTOCOL
S
ELECTION
The External ADSL USB Modem can be easily connected to a USB port on the PC
via a standard USB cable. The Modem is fully software upgradable so that new
features and updates may be added by simply loading a new version of the device
driver onto your PC.
ADSL modems employ ATM (Asynchronous Transfer Mode) framing. ATM is a
protocol that divides packets into small fixed-sized cells for rapid transmission over
high-speed networks. The ATM protocol allows various types of traffic (for
example, data, voice, and video) to be securely and efficiently carried over the
same network. ATM is being widely deployed by telecommunications carriers in
their backbone networks. Two types of ATM connections are possible: PVC
(Permanent Virtual Circuit) and SVC (Switch Virtual Circuit).
Several different protocols are used on top of ATM. The protocol required in your
configuration depends on the equipment deployed by your DSL service provider.
There are several possibilities:
1. Point to Point Protocol (PPP) Over ATM (RFC 2364): PPP provides session
setup, user authentication (login), and encapsulation for upper-layer
protocols such as IP (Internet Protocol). Using PPP makes the modem
appear as a dial modem to the operating system. Dial-Up Networking is used
to establish a connection. PPP is supported by either the WAN (Wide Area
Network) driver or the ATM driver.
