Transcript Lec # 19 - PCT Research Group

Lec # 19
Data Communication
Muhammad Waseem Iqbal
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Lecture Objectives
•
•
•
Introduction to DSL
Devices used in DSL Technology
DSL Equipment
– DSL Modem
– DSLAM
•
•
How DSL works?
DSL Modulation
– CAP
– DMT
•
•
DSL Advantages and Disadvantages
DSL vs. T1/E1 lines
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Introduction to DSL
• Digital Subscriber Line provides high speed
Internet access using regular telephone lines.
• It has the ability to move data over the phone lines
typically at speeds from 256kbps to 1.5Mbps – up
to 25 times quicker than the fastest analog
modems available today (56kbps)
• Digital Subscriber Line (DSL) technology is a
modem technology that uses existing twisted-pair
telephone lines to transport high-bandwidth data,
such as multimedia and video, to service
subscribers
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DSL
• DSL is a technology that allows high speed data
transmission over normal telephone lines at low
cost.
• There is minimal load on the CO
• The ISPs can provide multimedia services using the
much higher speeds than offered by normal modem
connections.
• There are many flavors of DSL, the higher the
frequency the greater the effect of distance and
crosstalk.
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Low Data Rates using Dial up Modems
• Why ordinary modems supports low data rates?
– Telephones were invented for carrying human
voice not for data
– Human voices can be carried in a frequency
range of 300 to 3,400 Hertz
– At the end point of local loop in the end office,
the wire runs through a filter that attenuates all
frequencies below 300 Hz and above 3400Hz,
almost 4KHz band
– Data are thus also restricted to this narrow
bandwidth.
– Standard phone service limits the frequencies
that the switches, telephones and other
equipment can carry.
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DSL Design Considerations
• A few Design considerations were:
– Service must work over existing infrastructure
(local loops)
– It must not effect the customers telephone and
fax machines
– It must be much faster than 56 Kbps
– It should always be on, with just a monthly
charge but no per-minute charge.
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What DSL does?
• When a DSL customer subscribes, the incoming
line is connected to a different kind of switch at the
end office, one that does not have this filter, thus
making the entire capacity of local loop available.
• In most cases, the wires themselves have the
potential to handle frequencies of up to severalmillion Hertz.
• Modern equipment that sends digital (rather than
analog) data can safely use much more of the
telephone line's capacity, and DSL does just that.
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Introduction to DSL
• Digital Subscriber Line (DSL) gives you
broadband access over your existing copper
telephone wires
• A DSL connection to the Internet is a highspeed, ‘always on’ (you don’t need to dial up
your ISP each time you want to connect)
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DSL standardization
• The ANSI T1E1.4 working group are responsible
for overseeing the development of the various DSL
technologies.
• The European Technical Standards Institute
(ETSI) also contribute from the European point of
view.
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How does DSL Technology Works?
• ADSL works by splitting the phone line into
2 frequency ranges .
– Voice Traffic (below 4KHz)
– Data Traffic (above 4KHz)
• Upstream
• Downstream
• This makes it possible to use the line for
phone calls and data network access at the
same time.
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DSL Equipment
• ADSL uses two pieces of equipment:
– DSL Modem / Transceiver
• At customer end
• point where data from the user's computer or
network is connected to the DSL line.
– DSLAM (DSL Access Multiplexer)
• At the service provider end (end office)
• DSLAM takes connections from many
customers and aggregates them onto a single,
high-capacity connection to the Internet.
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DSL Modem
• DSL Modem
– At the customer's location, there is a DSL
transceiver, which may also provide other
services.
– Most residential customers call their DSL
transceiver, a DSL modem.
– The DSL transceiver or modem can connect to a
customer's equipment in several ways, though
most residential installation uses USB or
10BaseT Ethernet connections.
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DSL Modem
– At the customer's location, there is a DSL transceiver,
which may also provide other services.
– Most residential customers call their DSL transceiver a
DSL modem. The engineers at the telephone company
or ISP call it an ATU-R (ADSL Termination Unit Remote).
– The DSL transceiver or modem can connect to a
customer's equipment in several ways, though most
residential installation uses USB or 10BaseT Ethernet
connections.
– An DSL modem performs the same task as a
conventional modem in that, it converts the digital
computer signals into analog signals that can be sent
down a telephone line.
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DSL modem
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DSLAM
• A DSLAM is a network device, usually at a
telephone company central office, that receives
signals from multiple customer DSL connections
and puts the signals on a high-speed backbone line
using multiplexing techniques.
• Depending on the product, DSLAM multiplexers
connect DSL lines with some combination of ATM,
Frame Relay, or IP networks.
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DSLAM
• A DSLAM is a network device, usually at a telephone
company central office, that receives signals from multiple
customer DSL connections and puts the signals on a highspeed backbone line using multiplexing techniques.
• It also known as ATU-C (ADSL Termination Unit - Central
Office)
• Depending on the product, DSLAM multiplexers connect
DSL lines with some combination of ATM, Frame Relay, or
IP networks.
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DSLAM
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ADSL Equipment Configuration
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PPPoE
• Protocol used in DSL technology
• PPPoE stands for Point to Point Protocol over
Ethernet.
• PPP is usually used over serial communications like
dial-up modem connections.
• Many DSL Internet service providers now use PPP
over Ethernet because of its added login and security
features.
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xDSL
• DSL is also known as xDSL, with the ‘x’ standing
for various kinds of DSL technologies.
• These technologies differ in the connect speed and
connection (asymmetric or symmetric) they provide
• The term xDSL refers to a number of similar yet
competing forms of DSL technologies
– E.g. ADSL, SDSL, HDSL, HDSL-2, G.SHDL,
IDSL,
and VDSL.
• xDSL promises to deliver high-bandwidth data
rates to dispersed locations with relatively small
changes to the existing Telco infrastructure.
• DSL Connection lets you use your telephone lines
for making and receiving calls and for Internet
access simultaneously
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DSL Technologies
•
•
•
•
•
•
SDSL
HDSL
HDSL-2
G.SHDSL
ISDN Digital Subscriber Line (DSL)
VDSL
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ADSL vs. SDSL
• A DSL service can be symmetric, in which
the downstream and upstream speeds are
identical (SDSL), or asymmetric in which
the downstream speed is faster than the
upstream speed (ADSL).
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ADSL
• Asymmetric Digital Subscriber Line (ADSL)
technology is asymmetric.
• It allows more bandwidth downstream—from
central office to the customer site—than upstream
from the subscriber to the central office.
• This
asymmetry
makes
ADSL
ideal
for
Internet/intranet surfing, video-on-demand, and
remote LAN access.
• Users of these applications typically download
much more information than they send.
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ADSL
• ADSL service has a maximum distance of 18,000
feet (5,460 m) between the DSL modem and the
DSLAM
• For speed and quality of service reasons, many
ADSL providers place an even lower limit on the
distance.
• At the upper extreme of the distance limit, ADSL
customers may experience speeds far below the
promised maximums, whereas customers close the
central office or DSL termination point may
experience speeds approaching the maximum
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ADSL Frequency Bands
• The ADSL divides the 1.1 MHz spectrum
available on the local loop into 3 frequency
bands:
– POTS
– Upstream
– Downstream
• This is achieved using FDM
• ADSL is a physical layer standard
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SDSL
• Symmetric Digital Subscriber Line (SDSL) is a rateadaptive version of HDSL and, like HDSL, is
symmetric.
• It allows equal bandwidth downstream from an
NSP's central office to the customer site as
upstream from the subscriber to the central office.
• SDSL supports data only on a single line and does
not support analog calls.
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DSL Advantages
1.
2.
3.
4.
5.
6.
7.
8.
Static IP Address (this is an assigned number
that is the signaling method used for all
communication over the Internet)
Surf and talk on the phone at the same time
Up to 140 times faster than analog modems
“Always On” connection (No waiting to “dial in”)
Unlimited Internet Access
Your home has its own dedicated connection
Your connection is highly reliability
Your connection is highly secure
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DSL Disadvantages
1.
2.
3.
4.
5.
6.
7.
Expensive
Distance dependence
Access
Asymmetry
Limited availability
Very new technology
Low or no CIR (Committed Information
Rate).
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