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ADSL WAN Connections
ADSL Overview
READSL: Supporting Greater Distances
To make ADSL available to more customers, reach extended ADSL2
(READSL) was developed to support greater distances between a customer’s
premises and the public carrier’s CO. (READSL is an ADSL2 or ADSL2+
technology, which is sometimes called READSL and sometimes called
READSL2.) According to CommsDesign.com, READSL extends the reach of
ADSL “up to 2500 ft., allowing ADSL systems to reach as far as 20,000 ft.”
(Marcus Tzannes, “RE-ADSL2: Helping Extend ADSL’s Reach,” May 13, 2003.)
Currently, READSL2+ is designed to share the local loop with POTS traffic,
just as ADSL Annex A does.
Elements of an ADSL Connection
All WAN connections, including ADSL connections, consist of three basic
elements:
■ the physical transmission media, such as the cabling, switches, routers,
and other infrastructure required to create and maintain the connection
■ electrical signaling specifications for generating, transmitting, and receiv-
ing signals through the various transmission media
■ Data Link Layer protocols, which provide logical flow control for moving
data between WAN peers (the devices at either end of a WAN connection)
Physical transmission media and electrical specifications are part of the
Physical Layer (or Layer 1) of the Open Systems Interconnection (OSI) model,
and Data Link Layer protocols are part of the Data Link Layer (or Layer 2).
(See Figure 7-2.)
Figure 7-2. Physical and Data Link Layers of the OSI Model
Physical Layer
Data link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
1
2
3
4
5
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7
ATM
ADSL
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