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178 Chapter 8: Using DSL to Access a Central Site

Figure 8-1 Typical End-to-End ADSL System

The ATU-R is typically connected to an external splitter device. In some cases, however, the

external splitter is eliminated in lieu of an internal ﬁlter in the ATU-R and microﬁlters attached

to plain old telephone service (POTS) devices in the home. From the splitter, the loop is wired

to a Network Interface Device (NID) that serves as the demarcation point into the customer

premises. From the NID, the loop is connected to a splitter device in the central ofﬁce that splits

off voice trafﬁc and routes it to the PSTN. Data is connected to the ADSL Terminating Unit-

Central Ofﬁce (ATU-C). The user’s data trafﬁc is then typically routed across the ATM network

to an aggregation gateway or router.

Modulation Methods

Three modulation methods for encoding data onto the local loop are Carrierless Amplitude and

Phase (CAP), Discreet MultiTone 2 - Issue 2 (DMT2), and G.lite. DMT was selected as the

preferred standard for ADSL modulation. CAP technology is cost-effective and readily available.

G.lite is a simpliﬁed DMT encoding scheme that provides limited features to facilitate

interoperability and minimize end-user interaction.

Table 8-1 shows the maximum data rates for downstream and upstream, line-coding

technologies, and maximum reach. Note that the maximum-reach number is best-case,

assuming “clean copper.”

Sources of Interference

Many sources of interference can degrade the quality of DSL. For instance, loading coils are

used as a low-frequency (300 to 3300 Hz) ﬁlter but cannot be present for ADSL operation.

Table 8-1 ADSL Data Rates

Maximum Data Rate

Downlink/Uplink Line Coding Technology Maximum Reach

8 Mbps/1 Mbps CAP, DMT 18,000 feet/5.5 km

1.5 Mbps/640 kbps G.lite 18,000 feet/5.5 km

Local Loop

DSLAM

Voice Switch

CPE Splitter CO Splitter

ATU-C

ATU-R

Data Network

PSTN

Phone

0732x01.book Page 178 Monday, November 17, 2003 2:49 PM

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