BROADBAND ACCESS TECHNOLOGIES
DSL (DIGITAL SUBSCRIBER LOOP) TECHNOLOGY
Introduction:
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DSL Technology provides a point-to-point connection between a customer
location and the public Network over the existing local loop twisted pair
copper wire.
It provides a reliable and secure high-speed access to the Public network for
high-speed data transfer, interactive video as well as regular telephone
service simultaneously over the existing local loop.
Consider fig 8.1 and 8.2 in your textbook.
It uses either DMT (Discrete Multitone) or CAP (Carrier less Amplitude
Modulation) technique to provide various DSL services such as ADSL,
VDSL, SDSL, and RADSL etc.
The term x.DSL stands for various versions of DSL technology with different
data rates
CAP: A proprietary digital Modulation Technique based on QAM
(Quadrature Amplitude Modulation). You will learn QAM in the PCS
course.
DMT: Discrete Multitone:
The existing local loop can handle upto 1.1 MHZ BW
The first 4KHz BW is used for regular telephone voice service
Rest of the BW is divided into 256 channels each occupying a BW of 4.312
KHz as shown on P 261 of your text book
Each subchannel can carry upto 60 Kbps data rate. (4KHz15bits/Signal
Change)= 60 Kbps
Current designs allow 1.5 Mbps to 9 Mbps data rate depending upon the S/N
ratio of the line and distance between the customer location and the CO.
Channel 0 is used for voice
Channel 1 to 5 are not used to allow a gap between voice and Data
Channels 6 to 30 ( 25 channels) are used for up stream transmission and
control. One for Control and 24 for data. Thus upstream data rate is:
244KHz15bits/ Signal change= 1.44Mbps
Channel 31 to 255 (225 Channels ) are used for downstream transmission,
one for control and 224 for data. Thus the downstream data rate is :
2244K15 = 13.4 Mbps as shown below:
Voice
Ch 0
4K
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Upstream
Ch 6 to 30
26 K
Downstream
Ch 31 to 255
1.1 M
13.4 M
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Notice that these are theoretical maximum BW. The actual practical data
rate is reduced because of S/N Ratio of the link.
The actual data rates are as follows
Upstream :
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64Kbps to 1.5 Mbps
Downstream:
500Kbps to 9 Mbps
The DSL modems, when turned ON, check the quality of line and
automatically adjust the data rate
Fig below shows a complete DSL system.
FILTER
Local Loop
VOICE TO
PSTN
FILTER
PC
Adsl
modem
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DSLAM
TO
ISP
DSLAM stands for Digital Subscriber Line Access Multiplexer. It
performs dual function.
a) Acts as an ADSL Modem
b) Creates data packets for ISP and converts data packets from ISP into
DSL format
 DSL uses 2B1Q line encoding over the Subscriber loop. We learnt this
scheme in ISDN. In fact DSL is very similar to ISDN BRI service
 Study fig 8-9 p262 Text Book.
 ADSL frame format uses HDLC frame as shown in the fig below. Draw
this picture in the space provided
DSL Versions:
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ADSL: Asymmetric DSL:
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Hyder Khoja
Used for Residential customers
Operates on a single twisted wire pair
Provides 64Kbps to 250 Kbps up stream and 1.5 to 2 Mbps
downstream over 18000 ft for remote LANS and Internet Access as
well as voice connection to PSTN
Uses 2B1Q line encoding over the twisted pair wiring
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SDSL: SYMMETRIC DSL:
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Operates FDX on a single pair with Echo Cancellers over a distance
of 3KM (10,000 ft)
Suitable for T1/E1 circuits
Used for business and Web Server Applications
Divides the BW half-half for upstream and downstream transmission
with a data rate of 768 Kbps in both directions
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HDSL: HIGH BIT RATE DSL:
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Used for high speed digital transmission between corporate sites or
between customer and Local Office
Uses 2 twisted wire pairs ( 4 Wire Circuit)
Provides a data rate of 1.544 Mbps or 2.048 Mbps (T1 or E1 rates)
symmetrically in both directions
Maximum distance is 3.7KM
Uses 2B1Q encoding
VDSL: VERY HIGH DATA RATE DSL:
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Asymmetric
Provides 12.5 Mbps to 52.8 Mbps downstream and 1.5 to 2.3 Mbps
upstream over a single pair
Distance up to 1.35 Km ( 4500 ft)
Requires fiber Optic feed and ATM support
Cable operators are in the process of launching new service for
aggregated HDTV, voice and data signals using VDSL technology in
combination with GPON (Gigabit Ethernet Passive Optical
Network). This configuration works in conjunction with FTTN and
FTTC technologies. FTTN and FTTC provide intermediate
concentration points.
This configuration is shown in the fig below. Draw this picture in the
space provided.
RADSL: Read p265 to 267 text book and also study tables 8-1 and 8-2
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Data rate
(Mbps
1.5 to 2
1.5 to 2
6.1
6.1
Table below shows claimed data rates and corresponding AWG and
distances for different flavors of DSL
AWG
Distance (ft)
24
26
24
26
18000
15000
12000
9000
Wire Size
(mm)
0.5
0.4
0.5
0.4
Distance
(Km)
5.5
4.6
3.7
2.7
CABLE MODEM:
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Structure is already in place
Less susceptible to EMI
Allows simultaneous transmission of TV and Internet Signals up to a
speed of 30 Mbps downstream ( typically only 10 Mbps) and 768
Kbps upstream
Uses a shared architecture over a COAX cable as shown below.
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Complete system is shown on p271 Text
Traditional Cable Network :
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Fig below shows traditional cable network.
HFC: HYBRIDE FIBER COAX:
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Fig below shows Hybrid Fiber Coax Architecture.
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A combination of fiber and Coax
Up to 40,000 customers can be serves
Cable BW is shown below.
Upstream
Data
5
Video Band
6MHz/Channel
80 channels
42 54
Downstream
Data
550
750
Upstream:
- Uses QPSK to reduce Noise and Interference
- Uses 2 bits/baud
- Theoretical Data rate is 26 =12 Mbps
Downstream Transmission:
- Uses 64 or 256 QAM
- With 64 QAM, it uses 6 bits/ Hz. One bit for error detection and 5 for
data
- With 5 bits, the theoretical Data rate is 56 = 30 Mbps
- The actual data rate is about 10 Mbps
- Both upstream and Downstream BW is shared by several subscribers
- Uses TDM and FDM to share the channels
Customer Premises Equipment for Cable Modem Implementation:
- Fig below shows the Cable Modem implementation(Read p272-278Text
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ADSL/ CABLE MODEM APPLICATIONS:
There are four major applications of ADSL and Cable Modem Technologies.
1. Data Access
- Personal Shopping
- Corporate LANS
- Computer Telephony Integration
- Library Research
- Distance Learning
- Remote LAN Access
- WEB access
2. Home Video
- Interactive games
- Movies
3. Internet Access
- Surfing
- Research
- E Commerce
4. Video on demand
- ADSL-1 Provides one MPEG-1 channel on twisted pair wiring over
18000 ft
- ADSL –3 provides 4, 1.5 Mbps MPEG –1 or one MPEG-2 HDTV on the
subscriber loop over 9000 ft
Note: Because of limited capacity, ADSL is an inexpensive interim
solution for Video on Demand service until fiber proliferation.
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