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A Mini project training on
BROADBAND ADSL/VDSL TECHNOLOGY IN BSNL
Submitted in partial fulfillment of the requirements for the award of theDegree of
BACHELOR OF TECHNOLOGY
in
ELECTRONICS AND COMMUNICATION ENGINEERING
by
M.RAMYA 09P31A0469
UNDER THE ESTEEMED GUIDANCE OF
Mr. N.RAJESH BABUM.Tech , Associate Professor,ECE
DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING
SRI SAI ADITYA INSTITUTE OF SCIENCE AND TECHNOLOGY
(Affiliated to JNTUK, Kakinada& Approved by AICTE, New Delhi)
Surampalem, East Godavari District
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Department of
Electronics and Communication Engineering
CERTIFICATE
This is to certify that the mini project report entitled
BROADBAND IMPLEMENTATION OF MPLS TECHNOLOGY IN BSNL
being submitted by
M.RAMYA 09P31A0469
This is to certify that the mini project work titledBROADBAND ADSL/VDSL
TECHNOLOGYIN BSNL is a bonafide work ofM.RAMYA carried out in partialfulfillment of the requirements under our guidance.
Project guide Head of the Department
Mr. N.RAJESH BABU, M.Tech Mr. R.V.V.KRISHNA,
Associate Professor , ECE M.Tech(Ph.D.)
HOD, ECE
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ACKNOWLEDGEMENT
Performing Mini Project is an important role in shaping up an
Engineering student for practical knowledge and to be update with latest
Technologies. First of all, I would like to express my attitude towards Head of
the Department of Electronics and Communication Engineering
Mr.R.V.V.KRISHNA, M.Tech(Ph.D.), Associate Professor and HOD,ECE for his
guidance throughout our Mini Project work. With great pleasure we want to
take this opportunity to express our heartfelt gratitude to all the people who
helped in making this Mini Project work a grand success.
We would like to thank Mr.N.RAJESH BABU , M.Tech ,Associate Professor,
ECE as my mini project guide for giving valuable suggestions.
First of all we are highly indebted to Principal Dr. CH.SRINIVASA
RAO, M.Tech, Ph.D., FIETEfor giving us the permission to carry out this Mini
Project.We would like to thank the HOD & Other Teaching Staff of ECE
Department for sharing their knowledge with us.
We thank Sri K.SURESH KUMAR S.D.E and Co-Staff Members, BSNL,
Rajahmundry for extending their utmost support and cooperation in providing
successful completion of the Project.
We thank Sri V.RAMESH BABU, Assistant General Manager(admn.) of
BSNL, Rajahmundry for extending their utmost support and cooperation in
providing all the provisions for the successful completion of the Project.
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ABSTRACT
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. The term
Xdsl covers a number of similar yet competing forms of DSL , including
ADSL , SDSL , HDSL , RADSL , and VDSL . xDSL is drawing significant
attention from implementers and service providers because it promises to
deliver high-bandwidth data rates to dispersed locations with relatively
small changes to the existing telco infrastructure .
xDSL services are dedicated , point-to-point , public network access over
twisted-pair copper wire on the local loop (last mile) between a
network service provider (NSPs) central office and the customer site , or
on local loops created either intra-building or intra campus. Digital The
benefits Subscriber Line (DSL) technologies has revolutionized Internet
access of DSL technology, coupled with the deregulation of the tele-
communications industry, have caused in increase in the number ofservice providers (xSP) offering DSL services.
Everyone from ILECs to CLECs to ISPs are offering DSL services to
homes and Businesses-with Asymmetric Digital Subscriber Line (ADSL)
currently being the most common and cost effective choice. Research
into DSL technologies has produced variants of ADSL to help resolve
issues users are faced with today, as well as plan for future
implementations. One of these variants is Very High Bit-Rate Digital
Subscriber Line (VDSL). VDSL differs from the other DSL technologies
primarily in the areas of speed and distance. Lower costs, competition
with other technologies and forward thinking for future bandwidth
requirements are contributing to making VDSL a variable technology for
even wider implementation. Currently the primary focus in xDSL is the
. development and deployment of ADSL and VDSL technologies and
architectures.
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INDEX
TABLE OF CONTENTS PAGE NO:
1. Asymmetric Digital Subscriber Line (ADSL) 6-11
1.1ADSL capabilities
1.2ADSL technology
1.3ADSL standards and associations
1.4ADSL market status
2. Very High Data Rate Digital Subscriber Line (VDSL) 12-14
2.1VDSL projected capabilities
3. ADSL 15-22
3.1Overview
3.2Operation
3.3Interleaving and fast path
3.4Installation problems
3.5Transport protocols
3.6ADSL standards
4. Digital Subscriber Line Access Multiplexer 23-26
4.1Path taken by data to DSLAM
4.2 Role of DSLAM
5. Conclusion 27
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CHAPTER-1
Asymmetric Digital Subscriber Line (ADSL)
ADSL technology is asymmetric. It allows more bandwidth
downstream from an NSPs Central office to the customer sitethan
upstream from the subscriber to the central office. This asymmetry,
combined with always-on access (which eliminates call setup), 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. ADSL transmits more than 6 Mbps to a
subscriber, and as much as 640 kbps more in both directions (shown in
Figure ). Such rates expand existing access capacity by a factor of 50
or more without new cabling. ADSL can literally transform the existing
public information network from one limited to voice, text, and low-
resolution graphics to a powerful, ubiquitous system capable of bringing
multimedia, including full motion video, to every home this century.
FIGURE 1
1.1 ADSL Capabilities
An ADSL circuit connects an ADSL modem on each end of a twisted-
pair line, creating three information channels - a high speed downstream
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channel, a medium speed duplex channel, and a basic telephone service
channel. The basic telephone service channel is split off from the
digital modem by filters, thus guaranteeing uninterrupted basic telephone
service, even if ADSL fails. The high-speed channel ranges from 1.5 to
6.1 Mbps, and duplex rates range from 16 to 640 kbps. Each channel can
be submultiplexed to form multiple lower-rate channels.
ADSL modems provide data rates consistent with North American T1
1.544 Mbps and European E1 2.048 Mbps digital hierarchies (see Figure
1.2) and can be purchased with various speed ranges and capabilities.
The minimum configuration provides 1.5 or 2.0 Mbps downstream and a
16 kbps duplex channel; others provide rates of 6.1 Mbps and 64 kbps
duplex. Products with downstream rates upto 8 Mbps and duplex rates
up to 640 Kbps are available today ADSL modems accommodate
Asynchronous Transfer Mode (ATM) transport with variable rates and
compensation for ATM overhead, as well as IP protocols. Downstream
data rates depend on a number of factors, including the length of the
copper line, its wire gauge, presence of bridged taps, and cross-coupled
interference. Line attenuation increases with line length and frequency
and decreases as wire diameter increases. Ignoring bridged taps ADSL
performs as shown in Table 1.1.
Table 1: Claimed ADSL Physical-Media Performance
Data
Rate
Wire
gauge
Wire
size
Distance
1.5 or
2Mbps
24
AWG
0.5mm 5.5km
1.5 or
2Mbps
26
AWG
0.4mm 4.6km
6.1Mbps 24
AWG
0.5mm 3.7km
1.5 or 2
Mbps
26
AWG
0.4mm 2.7
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Table 2: This chart shows the speeds for downstream bearer and duplex
bearer channels
Although the measure varies from telco to telco , these capabilities can
cover up to 95% of a loop plant, depending on the desired data rate.
Customers beyond these distances can be reached with fiber-based
digital loop carrier (DLC) systems . As these DLC systems become
commercially available , telephone companies can offer virtually
ubiquitous access in a relatively short time.
Many applications envisioned for ADSL involve digital compressedvideo . As a real-time signal, digital video cannot use link- or network-
level error control procedures commonly found in data communications
systems . ADSL modems therefore incorporate forward error correction
that dramatically reduces errors caused by impulse noise . Error
correction on a symbol-by-symbol basis also reduces errors caused by
continuous noise coupled into a line.
Downstream Bearer Channels
n*1.536 Mbps 1.536 Mbps
3.072 Mbps
4.608 Mbps
6.144 Mbps
N*2.048 Mbps 2.048 Mbps
4.096 Mbps
Duplex Bearer Channels
C Channel 16 kbps
64 kbps
Optional Channel 160 kbps
384 kbps
544 kbps
576 kbps
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1.2 ADSL Technology
ADSL depends on advanced digital signal processing and creative
algorithms to squeeze so much information through twisted-pair telephone
lines. In addition, many advances have been required in transformers, analog
filters, and analog/digital (A/D) converters. Long telephone lines may
attenuate signals at 1 MHz (the outer edge of the band used by ADSL) by as
much as 90 dB, forcing analog sections of ADSL modems to work very hard
to realize large dynamic ranges, separate channels, and maintain low noise
figures. On the outside, ADSL looks simple transparent synchronous data
pipes at various data rates over ordinary telephone lines. The inside, where all
the transistors work, is a miracle of modern technology.
To create multiple channels, ADSL modems divide the available bandwidth of
a telephone line in one of two ways-frequency division multiplexing (FDM)
or echo cancellation-as shown in figure 1.3. FDM assigns one band for
upstream data and another band for downstream data .The downstream path is
then divided by time-division multiplexing into one or more high-speed
channels and one or more low-speed channels .The upstream path is alsomultiplexed into corresponding low-speed channels .Echo cancellation assigns
the upstream band to overlap the downstream, and separates the two by means
of local echo cancellation, a technique well known in V.32 and V.34 modems.
With either technique, ADSL splits off a 4khz region for basic telephone
service at the DC end of the band.
Figure 2: Graph of Upstream and Downstream
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An ADSL modem organizes the aggregate data stream created by multiplexing
downstream channels, duplex channels, and maintenance channels together into
blocks, and attaches an error correction code to each block. The receiver then
corrects errors that occur during transmission up to the limits implied by the
code and the block length. The unit may, at the users option, also create
superblocks by interleaving data within subblocks; this allows the receiver to
correct any combination of errors within a specific span of bits. This in turn
allows for effective transmission of both data and video signals.
1.3 ADSL Standards and Associations
The American National Standards Institute (ANSI) Working Group
T1E1.4 recently approved an ADSL standard at rates up to 6.1 Mbps (ANSI
Standard T1.413). The European Technical Standards Institute (ETSI)
contributed an annex to T1.413 to reflect European requirements. T1.413
currently embodies a single terminal interface at the premises end. Issue II,
now under study by T1E1.4, will expand the standard to include a multiplexed
interface at the premises end, protocols for configuration and network
management, and other improvements.
The ATM Forum and the Digital Audio-Visual Council (DAVIC) have both
recognized ADSL as a physical-layer transmission protocol for UTP media.
The ADSL Forum was formed in December 1994 to promote the ADSL
concept and facilitate development of ADSL system architectures, protocols,
and interfaces for major ADSL applications. The forum has more than 200
members, representing service providers, equipment manufacturers, and
semiconductor companies throughout the world. At present, the Forums
formal technical work is divided into the following six areas, each of which is
dealt with in a separate working group within the technical committee:
ATM over ADSL (including transport and end-to-end architecture aspects)
Packet over ADSL (this working group recently completed its work)
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CPE/CO (customer premises equipment/central office) configurations and
interfaces
Operations
Network management
Testing and interoperability
1.4 ADSL Market Status
ADSL modems have been tested successfully in more than 30 telephone
companies, and thousands of lines have been installed in various technology
trials in North America and Europe. Severaltelephone companies plan market
trials using ADSL, principally for data access, but also including video
applications for uses such as personal shopping, interactive games, and
educationalprogramming.
Semiconductor companies have introduced transceiver chipsets that are
already being used in market trials. These chipsets combine off-the-shelfcomponents, programmable digital signal processors, and custom ASICs
(application-specific integrated circuits). Continued investment by these
semiconductor companies has increased functionality and reduced chip count,
power consumption, and cost, enabling mass deployment of ADSL-based
services.
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CHAPTER-2
Very-High-Data-Rate DigitalSubscriber Line (VDSL)
It is becoming increasingly clear that telephone companies around theworld are making decisions to include existing twisted-pair loops in their
next-generation broadband access networks. Hybrid fiber coax (HFC), a
shared-access medium well suited to analog and digital broadcast, comes up
somewhat short when used to carry voice telephony, interactive video, and
high-speed data communications at the same time. Fiber all the way to the
home (FTTH) is still prohibitively expensive in a marketplace soon to be
driven by competition rather than cost. An attractive alternative, soon to be
commercially practical, is a combination of fiber cables feeding neighborhood
optical network units (ONUs) and last-leg-premises connections by existing or
new copper. This topology, which is often called fiber to the neighborhood
(FTTN), encompasses fiber to the curb (FTTC) with short drops and fiber to
the basement (FTTB), serving tall buildings with vertical drops.
One of the enabling technologies for FTTN is VDSL. In simple terms, VDSL
transmits high speed data over short reaches of twisted-pair copper telephone
lines, with a range of speeds depending on actual line length. The maximum
downstream rate under consideration is between 51 and 55 Mbps over lines up
to 1000 feet (300 m) in length. Downstream speeds as low as 13 Mbps over
lengths beyond 4000 feet (1500 m) are also common. Upstream rates in early
models will be asymmetric, just like ADSL, at speeds from 1.6 to 2.3 Mbps.
Both data channels will be separated in frequency from bands used for basic
telephone service and Integrated Services Digital Network (ISDN), enabling
service providers to overlay VDSL on existing services. At present the two
high-speed channels are also separated in frequency. As needs arise for higher-
speed upstream channels or symmetric rates, VDSL systems may need to use
echo cancellation.
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2.1 VDSL Projected Capabilities
Although VDSL has not achieved ADSLs degree of definition, it has
advanced far enough that we can discuss realizable goals, beginning with data
rate and range. Downstream rates derive from submultiples of the SONET
(Synchronous Optical Network) and SDH (Synchronous Digital Hierarchy)
canonical speed of 155.52 Mbps, namely 51.84 Mbps, 25.92 Mbps, and 12.96
Mbps. Each rate has a corresponding target range:
Table 3:Target Ranges
Target
range(Mbps)
Distance
(feet)
Distance(meter
s)
12.96-13.8 4500 1500
25.92-27.6 3000 1000
51.84-55.2 1000 300
Upstream rates under discussion fall into three general ranges:
1.62.3 Mbps.
19.2 Mbps
Equal to downstream
Early versions of VDSL will almost certainly incorporate the slower
asymmetric rate. Higher upstream and symmetric configurations may only be
possible for very short lines. Like ADSL, VDSL must transmit compressedvideo, a real-time signal unsuited to error retransmission schemes used in data
communications. To achieve error rates compatible with those of compressed
video, VDSL will have to incorporate forward error correction (FEC) with
sufficient interleaving to correct all errors created by impulsive noise events
of some specified duration.
Interleaving introduces delay, on the order of 40 times the maximum length
correctable impulse. Data in the downstream direction will be broadcast to
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every CPE on the premises or be transmitted to a logically separated hub that
distributes data to addressed CPE based on cell or time-division multiplexing
(TDM) within the data stream itself. Upstream multiplexing is more difficult.
Systems using a passive network termination (NT) must insert data onto a
shared medium, either by a form of TDM access (TDMA) or a form of
frequency-division multiplexing (FDM). TDMA may use a species of token
control called cell grants passed in the downstream direction from the ONU
modem, or contention, or both (contention for unrecognized devices, cell
grants for recognized devices). FDM gives each CPE its own channel,
obviating a Media Access Control (MAC) protocol, but either limiting data
rates available to any one CPE or requiring dynamic allocation of bandwidthand inverse multiplexing at each CPE. Systems using active NTs transfer the
upstream collection problem to a logically separated hub that would use
(typically) Ethernet or ATM protocols for upstream multiplexing.
Migration and inventory considerations dictate VDSL units that can operate at
various (preferably all) speeds with automatic recognition of a newly
connected device to a line or a change in speed. Passive network interfaces
need to have hot insertion, where a new VDSL premises unit can be put on the
line without interfering with the operation of other modems.
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CHAPTER-3
ADSL
3.1 Overview
Figure 3: MODEM
A gateway is commonly used to make an ADSL connection.
ADSL differs from the less common symmetric digital subscriber line
(SDSL) in that bandwidth (and bit rate) is greater toward the customer
premises (known as downstream) than the reverse (known as upstream).This
is why it is called asymmetric. Providers usually market ADSL as a service
for consumers to provide Internet access in a relatively passive mode: able to
use the higher speed direction for the download from the Internet but not
needing to run servers that would require high speed in the other direction.
There are both technical and marketing reasons why ADSL is in many places
the most common type offered to home users. On the technical side, there is
likely to be more crosstalk from other circuits at the DSLAM end (where the
wires from many local loops are close to each other) than at the customer
premises. Thus the upload signal is weakest at the noisiest part of the local
loop, while the download signal is strongest at the noisiest part of the local
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loop. It therefore makes technical sense to have the DSLAM transmit at a
higher bit rate than does the modem on the customer end. Since the typical
home user in fact does prefer a higher download speed, the telephone
companies chose to make a virtue out of necessity, hence ADSL. On the
marketing side, limiting upload speeds limits the attractiveness of this service
to business customers, often causing them to purchase higher cost leased line
services instead. In this fashion, it segments the digital communications
market between business and home users.
3.2 Operation
Currently, most ADSL communication is full-duplex. Full-duplex ADSL
communication is usually achieved on a wire pair by either frequency-
division duplex (FDD), echo-cancelling duplex (ECD), or time-division
duplex (TDD). FDD uses two separate frequency bands, referred to as the
upstream and downstream bands. The upstream band is used for
communication from the end user to the telephone central office. The
downstream band is used for communicating from the central office to the
end user.Figure 4:Graph of frequency ranges of Upstream and Downstream
Frequency plan for ADSL. Red area is the frequency range used by normal
voice telephony (PSTN), the green (upstream) and blue (downstream) areas
are used for ADSL.
With standard ADSL (annex A), the band from 26.000kHz t
137.825kHz is used for upstream communication, whilecommunication.
PPSS
DDoowwnnssttrreeaa
UUppssttrreeaa
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Under the usual DMT scheme, each of these is further divided into smaller
frequency channels of 4.3125kHz. These frequency channels are
sometimes termed ''bins''. During initial training to optimize transmission
quality and speed, the ADSL modem tests each of the bins to determine the
signal-to-noise ratio at each bin's frequency. Distance from the telephone
exchange, cable characteristics, interference from AM radio stations, and local
interference and electrical noise at the modem's location can adversely affect
the signal-to-noise ratio at particular frequencies. Bins for frequencies
exhibiting a reduced signal-to-noise ratio will be used at a lower throughput
rate or not at all; this reduces the maximum link capacity but allows the
modem to maintain an adequate connection. The DSL modem will make a
plan on how to exploit each of the bins, sometimes termed "bits per bin"
allocation. Those bins that have a good signal-to-noise ratio (SNR) will be
chosen to transmit signals chosen from a greater number of possible encoded
values (this range of possibilities equating to more bits of data sent) in each
main clock cycle. The number of possibilities must not be so large that the
receiver might incorrectlydecode which one was intended in the presence of
noise. Noisy bins may only be required to carry as few as two bits, achoice
from onlyone of four possible patterns, or only one bit per bin in the case of
ADSL2+, and very noisy bins are not used at all. If the pattern of noise versus
frequencies heard in the bins changes, the DSL modem can alter the bits-per-
bin allocations, in a process called "bitswap", where bins that have become
more noisy areonly required to carry fewer bits and other channels will be
chosen to be given a higher burden. The data transfer capacity the DSL
modem therefore reports is determined by the total of the bits-per- bin
allocations of all the bins combined. Higher signal-to-noise ratios and more
bins being in use gives a higher total link capacity, while lower signal-to
noise ratios or fewer bins being used gives a low link capacity.
The total maximum capacity derived from summing the bits-per-bins is
reported by DSL modems and is sometimes termed ''sync rate''. This will
always be rather misleading, as the true maximum link capacity for user data
transfer rate will be significantly lower; because extra data are transmitted
that are termed ''protocol overhead'', reduced figures for PPPoA connections of
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around 84-87 percent, at most, being common. In addition, some ISPs will
havetraffic policies that limit maximum transfer rates further in the networks
beyond the exchange, and traffic congestion on the Internet, heavy loading on
servers and slowness or inefficiency in customers' computers may all
contribute to reductions below the maximum attainable. When a wireless
access point is used, low or unstable wireless signal quality can also cause
reduction or fluctuation of actual speed.
The choices the DSL modem make can also be either conservative, where the
modem chooses to allocate fewer bits per bin than it possibly could, a choice
which makes for a slower connection, or less conservative in which more bits
per bin are chosen in which case there is a greater risk case of error should
future signal-to-noise ratios deteriorate to the point where the bits-per-bin
allocations chosen are too high to cope with the greater noise present. This
conservatism, involving a choice of using fewer bits per bin as a safeguard
against future noise increases, is reported as the signal-to-noise ratio ''margin''
or ''SNR margin''. The telephone exchange can indicate a suggested SNR
margin to the customer's DSL modem when it initially connects, and the
modem may make its bits-per-bin allocation plan accordingly. A high SNR
margin will mean a reduced maximum throughput, but greater reliability and
stability of the connection. A low SNR margin will mean high speeds,
provided the noise level does not increase too much; otherwise, the connection
will have to be dropped and renegotiated (resynced). ADSL2+ can better
accommodate such circumstances, offering a feature termed ''seamless rate
adaptation'' (SRA), which can accommodate changes in total link capacity
with less disruption to communications.
Vendors may support usage of higher frequencies as a proprietary extension to
the standard. However, this requires matching vendor-supplied equipment on
both ends of the line, and will likely result in crosstalk problems that affect
other lines in the same bundle. There is a direct relationship between the
number of channels available and the throughput capacity of the ADSL
connection. The exact data capacity per channel depends on the
modulation method used.
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ADSL initially existed in two versions (similar to VDSL), namely CAP and
DMT. CAP was the ''de facto'' standard for ADSL deployments up until 1996,
deployed in 90 percent of ADSL installs at the time. However, DMT was
chosenfor the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called
''G.dmt'' and ''G.lite'' respectively). Therefore all modern
installations of ADSL are based on the DMT modulation scheme.
3.3 Interleaving and fast path
ISPs (rarely, users) have the option to use interleaving of packets to counter
the effects of burst noise on the telephone line. An interleaved line has a
depth, usually 8 to 64, which describes how many Reed-Solomon codeword
are accumulated before they are sent. As they can all be sent together, their
forward error correction codes can be made more resilient. Interleaving adds
latency as all the packets have to first be gathered (or replaced by empty
packets) and they, of course, all take time to transmit. 8 frame interleaving
adds 5 ms round-trip-time, while 64 deep interleaving adds 25 ms. Other
possible depths are 16 and 32.
"Fastpath" connections have an interleaving depth of 1, that is one packet is
sent at a time. This has a low latency, usually around 10 ms (interleaving adds
to it, this is not greater than interleaved) but it extremely prone to errors, as
any burst of noise can take out the entire packet and so require it all to be
retransmitted. Such a burst on a large interleaved packet only blanks part of
the packet, it can be recovered from error correction information in the rest of
the packet. A "fastpath" connection will result in extremely high latency on a
poor line, as each packet will take many retries.
3.4 Installation Problems
ADSL deployment on an existing plain old telephone service (POTS)
telephone line presents some problems because the DSL is within a frequency
band that might interact unfavourably with existing equipment connected to
the line. Therefore, it is necessary to install appropriate frequency filters at the
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customer's premises to avoid interference between the DSL, voice services
and any other connections to the line, for example in support of intruder
alarms "Red Care" being an example in the UK. This is desirable for the voice
service and essential for a reliable ADSL connection.
In the early days of DSL, installation required a technician to visit the
premises. A splitter or ''micro filter'' was installed near thedemarcation point,
fromwhich adedicated data line was installed. This way, the DSL signal is
separatedas close as possibletothe central office and is not attenuated inside
the customer's premises. However, this procedure was costly, and also caused
problems with customers complaining about having to wait for the technician
to perform the installation. So, many DSL providers started offering a "self-
install" option, in which the provider provided equipment and instructions to
the customer. Instead of separating the DSL signal at the demarcation point,
the DSL signal is filtered at each telephone outlet by use of a low-pass filter
for voice and a high-pass filter for data, usually enclosed in what is known as a
micro filter. This microfilter can be plugged by an end user into any 'phone
jack: it does not require any rewiring at the customer's premises.
Commonly, microfilters are only low-pass filters, so beyond them only low
frequencies (voice signals) can pass. In the data section, a microfilter is not
used because digital devices that are intended to extract data from the DSL
signal will, themselves, filter out low frequencies. Voice telephone devices
will pick up the entire spectrum so high frequencies, including the ADSL
signal, will be "heard" as noise in telephone terminals, and will affect and
often degrade the service in fax, data phones and modems. From the point of
view of DSL devices, anyacceptance of their signal by POTS devices mean
that there is a degradation of the DSL signal to the devices, and this is the
central reason why these filters are required.
A side effect of the move to the self-install model is that the DSL signal can be
degraded, especially if more than 5 voiceband (that is, POTS telephone-like)
devices are connected to the line. Once a line has had DSL enabled, the DSL
signal is present on all telephone wiring in the building, causing attenuation
and echo. A way to circumvent this is to go back to the original model, and
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install one filter upstream from all telephone jacks in the building, except for
the jack to which the DSL modem will be connected. Since this requires
wiring changes by the customer, and may not work on some household
telephone wiring, it is rarely done. It is usually much easier to install filters at
each telephone jack that is in use.
DSL signals may be degraded by older telephone lines, surge protectors,
poorly-designed micro filters, radio-frequency interference, electrical noise,
and by long telephone extension cords. Telephone extension cords are
typically made with small-gauge, multi-strand copper conductors which do
not maintain a noise-reducing pair twist. Such cable is more susceptible to
electromagnetic interference and has more attenuation than solid twisted-pair
copper wires typically wired to telephone jacks. These effects are especially
significant where the customer's phone line is more than 4km from the
DSLAM in the telephone exchange, which causes the signal levels to be lower
relative to any local noise and attenuation. This will have the effect of
reducing speeds or causing connection failures.
3.5Transport Protocols
ADSL defines three "Transmission protocol-specific transmission convergence
(TPS-TC)" layers:
* Synchronous Transport Module (STM), which allows the transmission of
frames of the Synchronous Digital Hierarchy (SDH)
* Asynchronous Transfer Mode (ATM)
* Packet Transfer Mode (starting with ADSL2, see below)
In home installation, the prevalent transport protocol is ATM. On top of ATM,
there are multiple possibilities of additional layers of protocols(two of them
are abbreviated in a simplified manner as "PPPoA" or "PPPoE"), with the all-
important TCP/IP at layer 4 of the OSI model providing the connection to the
Internet.
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Table 4:ADSL Standards
Standard name Common
name
Downstream
rate
Upstream rate
ITU
G.992.1
ADSL(G.
DMT)
8Mbit
/s
1.0Mbi
t/s
ITU
G.992.2
ADSL
Lite(G.Lit
e)
1.5M
bit/s
0.5Mbi
t/s
ITU
G.992.3/4
ADSL2 12Mb
it/s
1.0Mbi
t/s
ITU
G.992.3/4
Annex J
ADSL2 12Mb
it/s
3.5Mbi
t/s
ITU
G.992.3/4
Annex L
RE-
ADSL2
5Mbit
/s
0.8Mbi
t/s
ITU
G.992.5
ADSL2+ 24Mb
it/s
1.0Mbi
t/s
ITU
G.992.5
Annex L
RE-
ADSL2+
24Mb
it/s
1.0Mbi
t/s
ITU
G.992.5
Annex M
ADSL2+ 28Mb
it/s
3.5Mbi
t/s
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CHAPTER-4
Digital Subscriber Line Access Multiplexer
Figure 5: Siemens DSLAM SURPASS hiX 5625
A digital subscriber line access multiplexer (DSLAM, often
pronounced dee-slam) is a network device, often located in the telephone
exchanges of the telecommunications operators. It connects multiple customer
digital subscriber line (DSL) interfaces to a high-speed digital
communications channel using multiplexing techniques. By placing additional
DSLAMs at locations remote from the telephone exchange, telephone
companies provide DSL service to locations previously beyond effective
range.
4.1 Path taken by data to DSLAM
1.Customer premises: DSL modem terminating the ADSL, SHDSL or VDSL
circuit and providing LAN interface to single computer or LAN segment.
2.Local loop: the telephone company wires from a customer to the telephone
exchange or to a serving area interface, often called the "last mile" (LM).
3.Telephone exchange:
Main distribution frame (MDF): a wiring rack that connects outside subscriber
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lines with internal lines. It is used to connect public or private lines coming
into the building to internal networks. At the telco, the MDF is generally in
proximity to the cable vault and not far from the telephone switch.
xDSL filters: DSL filters are used in the telephone exchange to split voice from
data signals. The voice signal can be routed to a POTS provider or left unused
whilst the data signal is routed to the ISP DSLAM via the HDF (see next entry).
Handover distribution frame (HDF): a distribution frame that connects the last
mile provider with the service provider's DSLAM
DSLAM: a device for DSL service. The DSLAM port where the subscriber
local loop is connected converts analog electrical signals to data traffic
(upstream traffic for data upload) and data traffic to analog electrical signals
(downstream for data download).
4.2 Role of the DSLAM
Figure 6: xDSL Connectivity diagram
The DSLAM equipment collects the data from its many modem ports and
aggregates their voice and data traffic into one complex composite "signal"
via multiplexing. Depending on its device architecture and setup, a DSLAM
aggregates the DSL lines over its Asynchronous Transfer Mode (ATM), frame
relay, and/or Internet Protocol network (i.e., an IP-DSLAM using PTM-TC
Packet Transfer Mode - Transmission Convergence) protocol(s) stack.
The aggregated traffic is then directed to a telco's backbone switch, via an
access network (AN) also called a Network Service Provider (NSP) at up to
10 Gbit/s data rates.
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The DSLAM acts like a network switch since its functionality is at Layer2 of
the OSI model. Therefore it cannot re-route traffic between multiple IP
networks, only between ISP devices and end-user connection points. The
DSLAM traffic is switched to a Broadband Remote Access Server where the
end user traffic is then routed across the ISP network to the Internet.
Customer-premises equipment that interfaces well with the DSLAM to which
it is connected may take advantage of enhanced telephone voice and data line
signaling features and the bandwidth monitoring and compensation
capabilities it supports.
A DSLAM may or may not be located in the telephone exchange, and may
also serve multiple data and voice customers within a neighborhood serving
area interface, sometimes in conjunction with a digital loop carrier. DSLAMs
are also used by hotels, lodges, residential neighborhoods, and other
businesses operating their own private telephone exchange.
In addition to being a data switch and multiplexer, a DSLAM is also a large
collection of modems. Each modem on the aggregation card communicates
with a single subscriber's DSL modem. This modem functionality is integrated
into the DSLAM itself instead of being done via an external device like a
traditional computer modem.Like traditional voice-band modems, a DSLAM's
integrated DSL modems usually have the ability to probe the line and to adjust
themselves to electronically or digitally compensate for forward echoes and
other bandwidth-limiting factors in order to move data at the maximum
connection rate capability of the subscriber's physical line.
This compensation capability also takes advantage of the better performance of
"balanced line" DSL connections, providing capabilities for LAN segments
longer than physically similar unshielded twisted pair (UTP) Ethernet
connections, since the balanced line type is generally required for its hardware
to function correctly. This is due to the nominal line impedance (measured in
Ohms but comprising both resistance and inductance) of balanced lines being
somewhat lower than that of UTP, thus supporting 'weaker' signals (however
the solid-state electronics required to construct such digital interfaces is more
costly)
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CHAPTER-5
CONCLUSION
Because of the extremely high speeds that VDSL can accommodate, it is
being looked at as a good prospective technology for accommodating high
bandwidth applications like VoIP telephony and even HDTV transmission,
which ADSL is not capable of. Another very useful feature of VDSL stems from
thefact that it uses 7 different frequency bands for the transmission of data.
The user then has the power to customize whether each frequency band
would be used for download or upload. This kind of flexibility is very nice in
case you need to host certain files that are to be downloaded by a lot of
people. The most major drawback for VDSL is the distance it needs to be
away from the telephone exchange. Because of this, ADSL is still preferable
unless you live extremely close to the telephone exchange of the company
that you are subscribed to. Due to the limitations of VDSL and its high price,
its expansion is not as prolific as that of ADSL. VDSL is only widespread in
countries like South Korea and Japan. While other countries also have VDSL
offerings, it is only handled from a few companies; mostly one or two in most
countries. In comparison, ADSL is very widely used and all countries that offer
high speed internet offer ADSL. Hence VDSL is faster than ADSL and is not
widespread as ADSL. But still ADSL is better for homes that are much farther
from the DSLAM.
ADSL was born of the need for speed coupled with the desire for low cost
dedicated remote network access. There is no doubt that ADSL will
revolutionize the way we see the World Wide Web, and quite possibly
witness the demise of home entertainment as we know it. As the phoenix
from the flames we will see ADSL emerge heralding the coming of a new age
of remote multimedia. There is little doubt that ADSL will be around for a long
time to come, albeit under another name.
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If we are to truly realise the potential of the cyberspace concept we will need
to access it with as much convenience as turning on the television. With the
internet influencing our lives more and more each day, it will be high speed
ADSL connections that power the revolution. In the future people will view
ADSL like they view cable TV. That such a small object as an ADSL card may
wield such an influence over our lives may seem a little unbalanced, or is that
asymmetric
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BIBILOGRAPHY
References:
1. . Thomas; E. Gray (January 2001), RFC 3037: LDP Applicability, IETF2. de Ghein, Luc, MPLS Fundamentals, pp. 249326
3. Raza et al., Online routing of bandwidth guaranteed paths with local restoration usingoptimized aggregate usage information, IEEE-ICC 2005, retrieved 2006-10-27.
4. "Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice" by JohnEvans, Clarence Filsfils (Morgan Kaufmann, 2007, ISBN 0-12-370549-5)
5. Rick Gallaher's MPLS Training Guide (ISBN 1932266003).
6. S. Bryant; P. Pate (March 2005), RFC 3985: Pseudo Wire Emulation Edge-to-Edge (PWE3)Architecture, IETF
7. de Ghein, Luc, MPLS Fundamentals, pp. 249326.8. "AT&TFrame Relay and IP-Enabled Frame Relay Service (Product Advisor)", Research
and Markets, June 2007.
Websites:
1. www.google.com
2. www.wikipedia.com
3. www.networkworld.com
4. www.protocols.com
5. www.alttc.bsnl.co.in
6. www.itu.int
Text book
Telecommunication and switching systems and networks by thiagarajan viswanathan
http://www.google.com/http://www.google.com/http://www.wikipedia.com/http://www.wikipedia.com/http://www.networkworld.com/http://www.networkworld.com/http://www.protocols.com/http://www.protocols.com/http://www.alttc.bsnl.co.in/http://www.alttc.bsnl.co.in/http://www.itu.int/http://www.itu.int/http://www.itu.int/http://www.itu.int/http://www.itu.int/http://www.alttc.bsnl.co.in/http://www.protocols.com/http://www.networkworld.com/http://www.wikipedia.com/http://www.google.com/