BANSAL INSTITUTE OF ENGINEERING &
DIGITAL SUBSCRIBER LINE
MR.PAWAN KUMAR SAINI
ANANT PRATAP SINGH
EC- 3 yr.
ROLL NO. 1042231009
It is pleasure for me to present this seminar regarding the
“DIGITAL SUBSCRIBER LINE (DSL)” Today, high-speed Internet access has
become quite a necessity, not only in the advanced nations but also in many of the
third world countries.
I would like to thank everyone who helped to see this seminar to completion. In
particular, I would like to thank my seminar incharge Mr. Ankur Shukla for their
moral support and guidance to comlete my seminar on time.
I am sincerely thankful to Mr. Pawan Kr. Saini(HOD EC.) for giving me
suggestion to complete my seminar in a right way and at the time.
I express my gratitude to all my friends and classmates for their support and help in
ANANT PRATAP SINGH
E.C 3 YR.
NAME OF TOPICS
HISTORY OF DSL
WHAT IS DSL?
COMPARATIVE STUDY OF DSL FLAVORS
WHAT IS CABLE INTERNET CONNECTION?
TYPES OF DSL
ADSL(Asymmetric Digtal Subscriber line)
VDSL(Very high-bit rate Digital Subscriber line)
RADSL(Rate-Adaptive Digital Subscriber line)
HDSL(High bit/data rate Digital subscriber line)
ISDN DSL(Integrated Service Digital Network)
DSL VS CABLE
SUMMARY OF DSL
LISTS OF FIGURES
BASIC DSL NETWORK
COMPARATIVE STUDY OF DSL FLAVORS
BASIC CABLE NETWORK
WORKING OF A SIMPLIFIED ADSL
GENERAL MODEM FOR DSL DEVICE
HISTORY OF DSL
MODEM (Modulator-Demodulator) used for communication between any two computers
through a PSTN line uses Modulation and Demodulation techniques for Digital-Analog and
Analog-Digital conversion, respectively.
In 1969, AT&T revised the tariffs to allow customer-provided devices (modems) to beConnected
to PSTN lines for transferring data, with the following conditions:
Restriction inoutput power and energy levels
Connection to the PSTN (Public Switched Telephone Network) thru the telephone companyprovided protective devices (Data Access Arrangement).
All the network-control signaling ( Dialling , Busy signals, etc.) performed with the Telephone
company -provided equipment at the connection point.Under these conditions, modem
technology had to come up with the following new Techniques in order to achieve higher data
rates (from 19.2 Kbps thru 56 Kbps):
Modem data rates are calculated using Claude Shannon‟s formula:
C = B * log2 (1+ S/N) where ,C = Channel Capacity (2400 to 2800 Hz)
B = Bandwidth (24000 bps)
S/N = Signal-to-Noise Ratio (24 to 30 dBs)
The limitations in achieving higher data rates thru these voice band lines are mainly due to the
core network and not the bandwidth availability of the copper wire of the telephone network.
Filters at the core of the network limit the voice-grade bandwidth to approximately 3.3 KHz. In
the absence of these filters, the copper wires can up to a few frequenciesMHz.It is precisely this
aspect that is made use of in the DSL technology, which came into existence in the late 1980s.
WHAT IS DSL?
Digital Subscriber Line (DSL) is a technology that provides digital data transmission over the
existing wires of local telephone network. DSL service is delivered simultaneously with regular
telephone on the same telephone line. This is possible because DSL uses a higher frequency.
These frequency bands are subsequently separated by filtering.
Fig1: Basic DSL network
Digital Subscriber Line (DSL) is a broadband high-speed Internet technology that brings highbandwidth information to home and offices over ordinary copper telephone lines.
It assumes that digital data does not require changing into analog form and back to digital form.
Digital data is transmitted directly to the computer, as is, exploiting the maximum
bandwidth and the wide range of unused frequencies available in the existing copper wire of
telephone networks for high-speed broadband communication. Moreover, the signal can also be
separated, if one chooses, so that some of the bandwidth is used to transmit an analog signal for
simultaneously using the telephone line for voice.
Following are some of the key features of DSL:
Distance sensitive technology.
Internet connection is always ON
Simultaneous use of the phone line for voice as well as data traffic.
Internet Connection is highly reliable and secure
High Speed (Mbps) vis-à-vis a regular modem (56 Kbps max).
Telephones are connected to the telephone exchange via a local loop, which is a physical pair of
wires. Prior to the digital age, the use of the local loop for anything other than the transmission of
speech, encompassing an audio frequency range of 300 to 3400 Hertz (voiceband or commercial
bandwidth) was not considered. However, as long distance trunks were gradually converted from
analog to digital operation, the idea of being able to pass data through the local loop (by utilizing
frequencies above the voice band) took hold, ultimately leading to DSL. The local loop
connecting the telephone exchange to most subscribers has the capability of carrying frequencies
well beyond the 3.4 kHz upper limit of POTS.
Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL
takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels
starting between 10 and 100 kHz, depending on how the system is configured. Allocation of
channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new
channels are deemed unusable.
Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a load coil. Load
coils are, in essence, filters that block out any non-voice frequency. They are commonly set at
regular intervals in lines placed only for POTS service. A DSL signal cannot pass through a
properly installed and working load coil, while voice service cannot be maintained past a certain
distance without such coils. Therefore, some areas that are within range for DSL service are
disqualified from eligibility because of load coil placement.
Fig2: Basic Technology(Operation)
Comparative Study of DSL Flavors
Fig3: Comparative study of different DSL flavors
What Is a Cable Internet Connection?
Cable internet connection is an internet connection designed to operate over existing cable TV
lines. Cable Internet access requires a Cable Modem (CM) at the customer's premises and a
Cable Modem Termination System (CMTS) at a cable operator facility. The two are connected
via coaxial cable because the coaxial cable used by cable TV provides much greater bandwidth
than telephone lines. Cable Internet systems can typically operate where the distance between the
modem and the termination system is up to 100 miles (160 km).
Fig4:Basic Cable network
TYPES OF DSL
ADSL(Asymmetric Digital Subscriber Line)
VDSL(Very high-bit-rate Digital Subscriber Line)
SDSL(Symmetric Digital Subscriber Line )
RADSL(Rate Adaptive Digital Subscriber Line)
HDSL(High bit/data rate Digital Subscriber Line)
ISDN DSL(Integrated Service Digital Network)
Asymmetric Digital Subscriber Line (ADSL):
Most home and small business users typically use ADSL.
The transfer of data from the internet to PC is much faster than the transfer from PC
Downstream speeds for ADSL range from 1.59 Mbps, while upstream speeds are up
Mbps, for a distance of 18,000 feet from the service providers premises.
Fig5: Working of a simplified ADSL
Very high-bit-rate Digital Subscriber Line(VDSL):
Provides high speed internet connection but works only over a short distance.
Symmetric Digital Subscriber Line (SDSL):
Does not allows to use the phone at the same time. But internet speed of sending and
receiving data is same.
Rate Adaptive Digital Subscriber Line (RADSL):
This service is a variation of ADSL but the modem
can adjust the internet speed .
High bit/data rate Digital Subscriber Line(HDSL):
HDSL was the first DSL technology that used a higher frequency spectrum of copper,
twisted pair cables.
Integrated service digital network(ISDN DSL):
Symmetric data rates of up to 144 kbps using existing phone lines.
Capable of using the same modem or terminal adaptor used for ISDN and is always
DSL is actually not a physical line, but a modem pair. One DSL modem is located at the
customer premises (Customer Premise Equipment or CPE) and another DSL Access Multiplexer
(DSLAM) is at the Central Office (CO). These two modems create a Digital Subscriber Line or
DSL. DSL modems transmit data at the rate of up to 160 Kbps over copper lines, up to 18000
feet.DSL Modem or DSL Transceiver, as it is also referred to as, is connected to the customer‟s
computer via USB or a 10-Base T connection.
DSLAM at the Central Office or the Access Provider is the one that actually makes the DSL
happen . It accepts connections from various customers and aggregates them into a single high
capacity connection to the Internet. In addition, the DSLAM also does IP routing and Dynamic
IP address assignment.
Fig6: GENERAL MODEM FOR DSL DEVICE
DSL V/S CABLE
Both DSL and Cable Modem technologies offer high-speed Internet access. Even though they
are similar in many respects, the two technologies differ on some fundamental aspects such as
speed, availability , bandwidth, security, reliability and quality of service.
DSL was first designed in the 1970„s whereas Cable„s HFC was designed in the 80s and 90s.
Roughly 80 to 85 percent of America's 68 million cable TV subscribers (say, 55 million or so)
can get high-speed access, says Instat-MDR analyst Mike Paxton. DSL access is a little spottier,
though Instat‟s Ernie Bergstrom puts the number above 50 million. In terms of worldwide
broadband market share, currently DSL has higher market share compared to Cable but in
coming years picture may change.
The topic of "which is better and faster" has been a highly debated topic, and still there doesn't
appear to be a clear winner. DSL offers users a choice of speeds ranging from 144 Kbps to
1.5Mbps. Cable modem download speeds are typically up to 2 times faster than 1.5Mbps DSL,
but the reason there is no clear speed winner is because cable technology is based on shared
bandwidth, with many factors influencing a user„s download speed. With shared bandwidth the
speed fluctuates depending on the number of subscribers on the network. With DSL, the
connection is kind of dedicated and not shared, and user gets more or less an invariable speed.
This is one reason why cable broadband providers don't often publish speed information.
In more rural areas with fewer subscribers, users bound to have faster download speeds than a
user in a metropolitan center. Because cable modem speeds fluctuate, it is difficult to gauge an
exact download speed. On the upload stream, however, cable and DSL are closely matched for
speed. Both DSL and cable Internet speeds are largely dependent on the service provider and
either the distance away from the switching station a user is or how many users are in an
Cable modem technology delivers “shared” bandwidth within the neighbourhood, while DSL
delivers“dedicated” bandwidth. DSL typically is provided in the range from 128 kbps to 1.544
Mbps, varying as the equipment.
DSL can be considered to be more secure of the two, more due to a perceived weakness in the
cable modem. All cable modem customers in a region belong to the same LAN, thus opening
avenues for file sharing and other menaces of a typical network neighbourhood. This calls for
security firewalls, strict authentication and packet filtering mechanisms to be built into cable
modem standard specifications.
Both DSL and Cable provide an “always on connection” capability, thereby giving a “Static IP
address” to the session; and in turn, providing a fixed target to network attackers. As a result,
many DSL and cable customers have to purchase a DSL Router or a Cable Router, that enhances
the functionality of a basic modem with security features such as packet filtering and network
address translation. Customers can choose from a variety of precautions offered by the
broadband router, to suit their applications.
Despite tall claims by vendors about out-weighing the benefits of one another, both DSL and
Cable modem providers are working towards winning customers and expanding their customer
base. Drastically cutting costs, providing more value-added services, improvising on the security
features and constantly withstanding competitive pressures from vendors of the other forms of
technology, both forms of broadband technology are here to stay for some more time.
Interactive Video – movies on demand , video on demand , video conferencing.
High-speed Data communication – Internet access , Telecommuting , Remote LAN
access ,specialized network access.
DSL service is always ON and we can still use the phone for voice calls.
Broadband speed is much faster than dial up service.
Fixed monthly billing, regardless of time usage.
Digital data is directly transmitted to computer as digital data which uses much wider
bandwidth for transmitting.
DSL has one significant downside:
As you move away from the central office , the connection becomes slower.
This results is distortion of signal.
Summary Of DSL
The growth in demand for access to the Internet by business and residential end users has been
increasing for a number of years. The content available on the Internet and the applications
required to access it have also developed to meet end user demand and become ever more
complex and sophisticated and have, in turn, driven demand for greater delivered bandwidths to
maximize operational performance.
Forecasts for growth in bandwidth requirements are based upon a number of factors, future
demand for services such as “Triple Play” using standard and high definition IP Television is
often cited as a driver for the provision of ever increasing bandwidths although there is
considerable debate over the dynamics of this market. This paper assumes that the demand for
higher bandwidths will continue to increase, and examines the capabilities and limitations and
the current and future roles that Digital Subscriber Loop (DSL) technologies have in delivering
ever higher bandwidths to end users to meet demand for access to developing services.
The development of Digital Subscriber Loop technologies has enabled the delivery of high
bandwidth broadband services to end users connected to the copper access network.
However, the intrinsic limitations of the copper cables impose a finite limit on the maximum
bandwidth that can be delivered to end users. Given that forecasts for bandwidth demand
indicate a continuous increase, DSL technologies seem to offer - at best - an interim solution to
the problem of providing genuinely fast broadband services.
Although end user demand for standard and high density IP Television is often hailed as
a major driver for the provision of high-bandwidth networks, the continuing slow take-up of
IPTV offerings in the UK would seem to make it unlikely that this sector will be sufficient to
drive a significant increase in bandwidth in the UK. Instead, it is the ever-greater requirements
for bandwidth to access more sophisticated content and to efficiently run the applications needed
to exploit them which is likely to make the ubiquitous provision of high-bandwidth services a
necessity, for both business and domestic users across a range of urban, rural and suburban
As requirements for universal Internet access combined with greater bandwidth increase, and the
associated problems with providing high bandwidths over legacy copper access networks
threaten to expand the phenomenon of the digital divide to include not just remote rural areas but
populous suburbs as well, so increased attention will need to be devoted to the underlying issues
of broadband access.
As already discussed, one obvious solution lies in the construction of Next Generation Access
networks using both hybrid copper/fibre networks and all-fibre networks to deliver bandwidths
of 50 Mbps and above to large numbers of end users. Negative aspects, however, include the
substantial levels of investment by network operators or public/private partnerships required to
make this a reality, and the degree of disruption inherent in laying new infrastructure by
trenching in existing built environments. Solutions to the latter exist in the possibility of running
fibre circuits through sewers, or in carrying them on overhead rights-of-way, but both suffer
from drawbacks in terms of the vulnerability to damage of the circuits and the potential difficulty
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Internet Engineering Task Force (IETF). ADSL MIB working group.