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ADSL Technology
Overview, line qualification and service turn-up
Executive Summary
Asymmetric digital subscriber line (ADSL) uses existing twisted pair telephone lines to create access
paths for high-speed data communications and transmits at speeds up to 8.1 Mbps to a subscriber. This
exciting technology is in the process of overcoming the technology limits of the public telephone
network by enabling the delivery of high-speed Internet access to the vast majority of subscribers
homes at a very affordable cost. This JDSU white paper provides an overview of ADSL technology and
a description of testing procedures used to qualify lines for DSL and verify service. The standard loop
architecture is illustrated in figure 1.
ADSL overview
Delivery of ADSL services requires a single copper pair configuration of a standard voice circuit with
an ADSL modem at each end of the line, creating three information channels a high speed down-
stream channel, a medium speed upstream channel, and a plain old telephone service (POTS) channel
for voice. Data rates depend on several factors including the length of the copper wire, the wire gauge,
presence of bridged taps, and cross-coupled interference. The line performance increases as the line
length is reduced, wire gauge increases, bridge taps are eliminated and cross-coupled interference is
reduced.
The modem located at the subscribers premises is called an ADSL transceiver unit-remote (ATU-R),
and the modem at the central office is called an ADSL transceiver unit-central office (ATU-C). The
ATU-Cs take the form of circuit cards mounted in the digital subscriber line access multiplexer
(DSLAM). A residential or business subscriber connects their PC and modem to a RJ-11 telephone
outlet on the wall. The existing house wiring usually carries the ADSL signal to the NID located on the
customers premises (see figures 1 and 2).
WEBSITE
:
www.jdsu.com
White Paper
MDF
NID
Cross Box
Pedestal
DLC
Splice case
Cross Box
NID
DSLAM
POTS
ATM
ADSL
ATU-Cs
POTS
splitter
DATA network
POTS switch
POTS
Splitter
ATU-R
POTS
ADSL
NID
Figure 1 ADSL loop architecture
Physical connectivity
At the central office, a main distribution frame collects the cables from many subscribers and uses a
splitter to distribute the data traffic to a DSLAM and routes the regular telephone traffic over an E1/T1
connection to the public switched telephone network (PSTN). The DSLAM mixes DSL services from
different subscribers into ATM virtual circuits. Often, a DSLAM concentrator is used in cases where an
ILEC or CLEC has many DSLAMs distributed over a large geographic area. The DSLAM contains
ATU-Cs where ADSL signals are multiplexed onto a high-speed interface connected to an ATM
network. This ATM network provides access to the internet through internet service providers (ISPs).
The DSL provider bundles the traffic destined for a given ISP and sends it over an E3/T3 or an STM-
1/OC-3c connection. A broadband remote access server (BRAS) terminates the subscribers IP session
and directs it to the Internet backbone.
In most cases, POTS splitters at the network interface device (NID) and central office allow the copper
loop to be used simultaneously for high-speed ADSL and POTS service. The POTS channel is split from
the ADSL channel by a passive, low-pass/high-pass filter that separates the signals low frequency for
POTS and high frequency for ADSL routing each to a separate wire pair. The splitter also protects the
ADSL signal from POTS transients originating from handsets going on-hook and off-hook. ADSL
service may be installed without using a splitter at the NID. Instead, micro filters are placed in-line
with the phone jack at each telephone location. While this configuration sacrifices some level of
performance, it allows the customer to self-install the CPE. Typically, micro filters are packaged with
the ADSL modem in a self-install kit. Figure 3 illustrates various test points for ADSL service.
White Paper: ADSL Technology
2
POTS
Splitter
ADSL and
POTS loop
POTS
ADSL and POTS
NID
ATU-R
10BT/ATM.25
10BT/ATM.25
Figure 2 Signal at the customer premises
White Paper: ADSL Technology
ADSL signal encoding
Traditional POTS uses a narrow 4-kHz base band frequency to transmit analog voice signals. This
means that even with sophisticated modulation techniques, current modem technology can only
achieve throughput of up to 56 kbps (downstream 56 K; upstream 32 K). To attain much higher
throughput (up to 8 Mbps), ADSL uses a frequency range from approximately 20 kHz to 1.2 MHz.
Frequency division multiplexing (FDM) creates multiple frequency bands to carry upstream and
downstream data. The lower 0 to 4 kHz frequency range is reserved for POTS service. The frequency
band from 25 K to 138 K is used to transmit upstream data, and the larger, higher frequency band from
138 K to 1.1 MHz is used for downstream data (see figure 4).
3
DSLAM
Central
office
ISP gateway
Network
element
manager
Customer
premises
ADSL service TP2
IP section TP3
End to end section TP4
Copper TP1
RSTN network
ATU-R
modem
POTS
splitter
ATM Data
network
Service
provider
Figure 3 ADSL network layers
Frequency
Amplitude
POTS
Upstream
Data
Downstream
Data
Frequency Division Multiplexing
Figure 4 Frequency bands
The American National Standards Institute (ANSI) and the International Telecommunication Union
(ITU) chose discrete multitone (DMT) modulation as the standard line code for ADSL. DMT, as its
name implies, divides the data bandwidth into 256 sub-channels, or tones, ranging from 25 kHz to
1.1 MHz. Upstream data transfer frequencies range from 25 kHz to 138 kHz, whereas downstream data
transfer frequencies range from 139 kHz to 1.1 MHz (see figure 4 and 5). Guardbands divide the three
frequency bands. Each tone has a bandwidth of 4 kHz and a spacing of 4.3 kHz; each supporting a
maximum number of 15 bits, as limited by its signal-to-noise ratio. Since the tones in higher fre-
quencies are more susceptible to attenuation and noise, higher frequencies usually carry fewer bits per
tone than lower frequencies. In addition to the normal data bits, an embedded operations channel
(EOC) is provided as a part of the ADSL protocol for communications between the DSL modem and
DSLAM. This device is used to provide in-service and out-of-service maintenance, retrieve a limited
amount of status information, and monitor ADSL performance. The EOC may be used in the future to
extend maintenance and performance monitoring.
ADSL is a fixed quality (fixed BER of 10
-7
), variable rate service. During training, the ADSL system
(ATU-R and ATU-C) evaluates the quality of the line by measuring the SNR and attenuation/ gain per
tone. It can then decide on the maximum data rate sustainable on the copper loop and still maintain a
BER of less than 10
-7
. This differs from most other digital transmission technologies (ATM, ISDN, etc.)
which are fixed rate, variable quality services (variable BER). So, to evaluate the QoS for ADSL, examine
line capacity and noise margin. The lower the capacity and higher the noise margin, the better the
signal. In adaptive mode, BER will always be 10
-7
or slightly less, depending on the fixed minimum
noise margin. After the line is evaluated, the max bandwidth is further reduced by the minimum noise
margin (set at the DSLAM). This is usually 6 dB to allow for changes in SNR and becomes the upper
limit for the data rate. Remember, the limiting factor is the BER and as long as BER remains less than
10
-7
, ADSL service requirements are met and synchronization will occur. The data rate may be less than
desired but it still meets specification requirements.
4
White Paper: ADSL Technology
Figure 5 Data bandwidth tones
Frequency
# of Bits
POTS
Upstream
Downstream
14
0
4
20
160
240
1100
(kHz) 5
White Paper: ADSL Technology
Quadrature amplitude modulation
ADSL uses quadrature amplitude modulation (QAM) (see figure 6) to achieve the 15-bit maximum
that any single tone can carry. This technique employs a combination of amplitude modulation and
phase shift keying. For example, a signal that transmits at three bits per baud requires eight binary
combinations to represent the signal. This example assumes two possible measures of amplitude and
four possible phase shifts, which allow for eight possible waves. Table 1 shows the correspondence
between each binary combination and amplitude and phase shift. Using the above technique, a large bit
stream can be broken down into three-bit words, as shown in the following example:
001-010-100-011-101-000-011-110
Figure 6 illustrates QAM-encoded signals of the above bit stream with each wave shifted in relation to
the wave that immediately precedes it.
Protocol stack
Data