Table of Content
- Introduction
- What is DSL
- ADSL Technology
- Types of ADSL
- ADSL Standards
- Frequency Allocations
- Quadrature Amplitude Modulation (QAM)
- Discrete Multi- tone Technology
- Limitations
- Error Control Codes
5. Crosstalk in ADSL
6. Shielding Cable Lines Decreases Crosstalk
7. Summary
8. References
__________________________________
Introduction
1969 was the year when the first internet connection was made by Charley Kline at the University of California at Los Angeles when he sent the first packet to Standford University. The fast pace life required an innovation of a new way of communication that satisfies the needs of the consumers who wants faster connection to the internet. Until recently, the available methods for transmitting and receiving data at high capacity were provided at a high cost. The methods used were microwaves and satellite networks. Only large companies were able to afford this type of service. However, small businesses and house holds need an access to high speed connection at an affordable cost. In 1988, ADSL (Asymmetric Digital Subscriber Line) was developed by Joe Lechleider at Bellcore. This technology made it affordable for households and small businesses to get fast connection. The next figure Shows an over view of an ADSL configuration.
What is DSL
Digital subscriber line known as DSL is a family of technologies that provide digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, although in recent years, many have adopted digital subscriber line as a more marketing-friendly term for the most popular version of consumer-ready DSL, ADSL.
Some of the members of the DSL family include:
- High Data Rate Digital Subscriber Line (HDSL), also covered in this article
- Symmetric Digital Subscriber Line (SDSL), a standardised version of HDSL
- Asymmetric Digital Subscriber Line (ADSL), a version of DSL with a slower upload speed
- ISDN Digital Subscriber Line (IDSL)
- Rate-Adaptive Digital Subscriber Line (RADSL)
- Very High Speed Digital Subscriber Line (VDSL)
- Very High Speed Digital Subscriber Line 2 (VDSL2), an improved version of VDSL
- Symmetric High-speed Digital Subscriber Line (G.SHDSL), a standardised replacement for early proprietary SDSL by the International Telecommunication Union Telecommunication Standardization Sector
- Powerline Digital Subscriber Line (PDSL), a high speed powerline communications solution which modulates high speed data onto existing electricity distribution infrastructure
- Etherloop Ethernet Local Loop
Asymmetric digital subscriber line uses existing twisted pair telephone lines to create access paths for high-speed data communications and transmits at speeds up to 8.1Mbps to a subscriber. The "asymmetric" in ADSL refers to the fact that the downstream data rate, or the data coming to your computer from the Internet, is traveling faster than upstream data, or the data traveling from your computer to the Internet. Upstream data rates are slower because web page requests are fairly miniscule data strings that do not require much bandwidth to handle efficiently. A common error is to attribute the A in ADSL to the word asynchronous. ADSL technologies use a synchronous framed protocol for data transmission on the wire.
- Types of ADSL
- ADSL that requires a voice/data splitter, commonly called a POTS Splitter (Plain Old Telephone Service) to be installed at the consumer's home or business location. The splitter separates voice from data transmissions. For simultaneous use of the telephone and data access, additional phone wires may need to be installed at your location. Full rate ADSL provides service up to a maximum range of 12,000 feet (about 2.0 miles) from the telecommunication provider company's central office to the end-user.
- ADSL
Lite technology often called Splitterless, G.lite or Universal ADSL and
now also known as G.992.2 does not require a POTS splitter to be
installed at the consumer's home or business. ADSL Lite provides
bandwidth downstream up to 1.5 Mbps and upstream up to 512 kbps. ADSL
Lite provides service up to a maximum range of 12,000 feet (about 2.0
miles) from the central office.
- ADSL Standards Bodies
A number of standards bodies are involved with DSL. In the USA,
committee T1E1.4, DSL Access, is sponsored by Alliance for
Telecommunications Industry Solutions (ATIS), and accredited by the
American National Standards Institute (ANSI). T1E1.4 has written
standards for Spectrum Management, VDSL, splitters and in-line filters,
HDSL4, HDSL2, ADSL, HDSL, ISDN, T1 lines, etc.The International
Telecommunication Union, Telecommunication Standardization Sector
(ITU-T) Study Group 15 Question 4 (SG 15 Q 4) has written global
standards for VDSL, G.shdsl, G.lite, G.dmt.bis, HDSL2, etc. Many
regional standards bodies use ITU
standards for DSL by creating
“pointer” standards which mainly reference the pertinent ITU standard
and add a few regional parameters such as using the 2.048 Mbps E1 rate
in Europe or the 1.544 Mbps T1 rate in North America. The
European Telecommunication Standards Institute (ETSI) Transmission and
Multiplexing 6 (TM6) writes European DSL standards. The Japan
Telecommunication Technology Committee (TTC) formulates Japanese
Standards. The Full-Services Access Network (FSAN) group does not write
standards per se, but has been active in defining VDSL. The DSL Forum
addresses end-to-end system aspects of DSL typically at layers
above
the physical layer. A new committee is IEEE 802.3ah Ethernet in the
First Mile (EFM), which is looking at DSL for carrying Ethernet
traffic. The pulse shape that was chosen for ADSL technology is a sinc
like pulse that dies off very fast, which can be seen in the following
figure.
- Frequency Allocation
ADSL uses Frequency Division Multiplexing (FDM) to 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. With standard ADSL (annex A), the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication. The next two figures shows a block diagram of how FMD is done, then a plot to show the frequency bands for each stream.
Figure : Block Diagram of Frequency Division Multiplexing
Figure: Frequency plan for ADSL. The red area is the frequency range used by normal voice telephony (PSTN), the green (upstream) and blue (downstream) areas are used for ADSL.
A
"splitter" (which is a filter), one at the user end and one at the
exchange end, separates the telephony signal from the ADSL signal. This means that telephone calls can be made at the same time that data is being sent or received. A
block diagram of the splitter is shown in the following figure. One can
see that it consists of low pass filter, which will extract the
telephone signal, and a high pass filter that extract the DSL
signal. The DC component that is use for transmitting the signal,
have to be blocked to get only the high frequency signals.
Figure :splitter of ADSL
Modulation
ADSL initially existed in two flavors (similar to VDSL), namely Carrier Phase/Amplitude Modulation (CAP) and Discrete Multi-Tone Technology (DMT). CAP was the actual standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs at the time. However, DMT was chosen for the first International Telecommunication Union (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
To better understand the functionality of DMT we'll discuss Quadrature Amplitude Modulation (QAM).
- Quadrature Amplitude Modulation (QAM)
Quadrature Amplitude Modulation (QAM) is a way of fitting information onto a limited frequency line, in the case of ADSL it is copper wire. QAM can split a single signal into 16 by using both phase and amplitude modulation. QAM uses a combination of sine and cosine waves at different phases to each other to produce these signals (sine and cosine average function always 90° out of phase, i.e. in quadrature). QAM uses four different amplitudes for each of the waves. in this way 16 different signal types are generated using all possible pairs of the amplitudes, e.g.A1sin(Ft)+A1cos(Ft),A1sin(Ft)+A2cos(Ft), A1sin(Ft)+A3cos(Ft),... This creates what is known as the QAM "constellation" a collection of 16 signals all representing 4-bit nibbles. This constellation is differential meaning it is 15° out of phase with the previous 4 bits, it is not referenced to a fixed signal. CAP/QAM is a single carrier technique.
Figure: QAM Transmitter Block Diagram
From the above figure, one can see that QAM uses I/Q modulation. First the flow of bits to be transmitted is split into two equal parts: this process generates two independent signals to be transmitted. They are encoded separately, then one channel (the one "in phase") is multiplied by a cosine, while the other channel (in "quadrature") is multiplied by a sine. This way there is a phase of 90° between them. They are simply added one to the other and sent through the real channel.
Figure : QAM Receiver Block Diagram
The receiver simply performs the inverse process of the transmitter.
As one can see the recieved signal gets multiplied by a cosine and sine
to extract the parts the were orthogonal to each other. In each branch
the following processing takes place. First, a low pass filter is used
to separate the signal from noise and any intersymbol interference.
Next, the Signal gets converted to digital signal so that ASK
(Amplitude Shift keying) demodulator can be used to to recognize the
different amplitudes.
- Discrete Multi-Tone Technology (DMT)
Discrete Multi-Tone Modulation (DMT)
uses 16-QAM constellation. Like QAM is all about fitting information
onto a line. DMT divides the entire bandwidth range on the formerly
analog passband limited loop into a large number of equally spaced sub
channels called subcarriers. This bandwidth is normally 1.1 MHz and is
divided into 256 subcarriers, starting at 0Hz. Each subcarrier occupies
4.13125 kHz, this gives a total bandwidth of 1.104 MHz on the loop.
Most DMT systems only use 249 or 250 of these subcarrier for
information. Generally the lower subcarriers #1 through #6 are reserved
for the 4 kHz passband for analog voice, giving ADSL the distinctive
ability to transmit data and carry a voice conversation on the same
line. 6 times 4.3125 kHz is 25.875 kHz so it is common that ADSL
services start at 25 kHz, this also means that a wide "guardband"
exists between analog voice and DMT transmissions. Generally the upper
subcarriers exhibit signal loss so those above #250 are not used. There
are 32 upstream carriers starting at #7, and 218 downstream channels
starting form #32, putting the "Asymmetric" in ADSL. If the upstream
and downstream channels overlap echo cancellation techniques are used
to prevent errors from transmitted signals being mistaken for recieved
ones. Some channels are also reserved for special purposes like pilot
signals. At activation ADSL devices measures each of these subchannels
for signal attenuation and noise, in a complex "handshake" procedure.
DMT can also monitor the channels for changing quality allowing a DMT
granularity (possible speed drop) of only 32 kbps against CAP's
granularity of 340 kbps. Each subcarrier operates a coding technique
based on QAM. The total throughput of it is the sum of all the QAM bits
sent on all of the active channels at a given time. The throughput is
maintained on a DMT connection by turning off subcarriers that
experience outside interference rather than retransmitting the entire
signal. DMT devices can be said to be Rate Adaptive DSL (RADSL) as each
subcarrier may transmit at a different rate than the others, depending
on the qualit
y
of the signal in each. one of the advantages of using DMT is that it
eliminates ISI (Intersymbol Interference), thus less error will occur.
Figure : Discrete Multi-Tone Modulation Block Diagram.
- Limitation
ADSL access is distance-sensitive. The connection speed depends on several factors:
- distance between the subscriber and the central office
- copper line wire gauge
- copper line wire speed increases with wire diameter because of the wire will have lower resistance. With high frequency and small diameter the Skin Effect arises and limits the distance since the signal gets attenuated because of the increased resistance. The skin effect is the tendency of an alternating current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor.
- presence of bridge taps - bridge taps are extensions, between the subscriber and the central office, that extend service to other subscribers. Bridge taps may increase the distance limit but slow down the access speed
Distance limitation of some different technologies is shown in the following illustration.
Forward Error Control (FEC)
ADSL deploys Forward Error Correction (FEC) to assure optimal performance by adding redundant block to the data to be transmitted. The redundant block is usually relatively small compared to the actual data being transmitted. The FEC block helps detect bits that the demodulator incorrectly decodes. The FEC scheme that ADSL uses is based on Reed-Solomon (RS) coding.
Scrambling
A scrambler randomizes bits that are passed to the modulator unit, so that different patterns of bit sequence will be equally probable. A descrambler removes the randomness. An even distribution of ones and zeros makes transmitted power output more stable.
Interleaving
Interleaving is to systematically separate contiguous bits. Each interleaved bit will be separated by a number of bits from its originally adjacent bits. De-interleaving is the reverse process. Interleaving is operated on FEC blocks. The simplest method is called block interleaving. In the interleaving algorithm, the bits are written into a buffer in rows but read out from columns. Each bit is then separated from its adjacent bit by the number of bits that equals the number of rows. In a de-interleaving algorithm, the bits are written into a buffer in columns but read out from rows. This simply reverses the interleaving process.
Framing
Framing describes how data is packaged with other transmission overheads. In ADSL(DMT), the downstream and upstream data channels are synchronized to the 4kHz symbol rate and are separated into two data area in an ADSL data frame, interleaved data buffer and fast (i.e. non-interleaved) data buffer
Each data frame must be transmitted in 250 microseconds due to its 4000 symbol rate. 68 data frames appended with a synchronization frame forms an ADSL superframe. Thus, a superframe is transmitted in 0.00025 x 68 = 0.017 second. However, the synchronization frame is not actually sent. Each data frame is sent within 68/69 x 250 microseconds.
Crosstalk in ADSL
Crosstalk is a phenomena caused by the electromagnetic interference between the different wires. Cross talk occurs when the electrical energy transmitted across the copper wire as a modulated signal radiates energy onto an adjacent copper wire located in the same cable. Cross talk can cause errors in data being transmitted on the line experiencing the cross talk. These errors occur because the energy that radiates to a neighboring copper wire combines with the signal on that wire and can alter the carrier wave form that was originally transmitted.
Two types of cross talk pose potential problems for DSL systems:
-
Near End Cross Talk (NEXT)
-
Far End Cross Talk (FEXT)
NEXT occurs when a receiving station overhears a signal being sent by a transmitting station at the same end of a neighboring line. (See Figure) FEXT occurs when a receiving station overhears a signal sent by a transmitting station at the opposite end of a neighboring line.
Figure: NEXT occurs when a receiving station overhears a signal sent
by a transmitting station at the same end of a neighboring line. FEXT
occurs when a receiving station overhears a signal sent by a
transmitting station at the far end of a neighboring line.
Generally speaking, NEXT poses more of a problem for DSL systems than FEXT does, particularly for downstream transmissions: There are more copper pairs closer together at the CO end of a loop running DSL than at the user's end of a loop running DSL. Also, NEXT signals haven't attenuated. Because the NEXT signals are strong, they may significantly alter the signal on the neighboring line. In contrast, FEXT signals have run the length of the line and, thus, are attenuated and not likely to significantly modify the signal on the neighboring line.
Depending on the transmission technique a DSL system uses, cross talk will be more or less of an issue. For example, some DSL systems (including many ADSL systems) use different frequency ranges for transmitting and receiving signals. This type of transmission technique is called frequency division multiplexing (FDM). (See Figure 3.) FDM-based systems do not typically have problems with NEXT from other FDM-based systems because the neighboring FDM-based systems are not receiving in the same frequency range as they are transmitting.
Figure
: FDM is a transmission technique that DSL systems commonly use. FDM
systems use different frequency ranges for upstream and downstream data
to avoid cross talk from like systems. ECH systems, in contrast,
overlap the frequency bands used for upstream and downstream data,
which allows for higher-speed downstream transmissions but is more
vulnerable to cross talk from like systems.
FDM-based systems use different frequencies for transmitting and receiving, however, these systems do not use available bandwidth as efficiently as Echo-Cancelled Hybrid (ECH) systems. ECH systems overlap the frequency band used for upstream and downstream data. As a result, ECH systems allow for higher transmission rates downstream. (See Figure 3.) On the other hand, ECH-based systems are more prone to interference from NEXT signals transmitted by other ECH-based systems. This interference occurs because the neighboring ECH-based systems are transmitting and receiving in the same frequency range.
Shielding Cable Lines Decreases Crosstalk
Data cables are classified according to the speed of the signal transmission they are guaranteed to support. Category (CAT) 3 is rated up to 16 MHz, CAT 4 to 20 MHz, CAT 5/5e to 100 MHz, proposed CAT 6 to 200 MHz with some tests required at 250 MHz, and proposed CAT 7 to 600 MHz. Actually, because application speeds have increased so quickly, most cabling done recently and continuing today uses CAT 5 or enhanced versions of the standard, CAT 5e. Very little CAT 4 is used.
Speed is affected by attenuation and crosstalk, both of which increase with frequency. The higher number categories, CAT 5, CAT 5e, and the proposed CAT 6, extract more and more performance from the basic twisted-pair medium. For example, higher speed cables generally have more twists per inch than slower ones.
Attenuation could be reduced by using larger diameter wire. But balanced against cost, the common 100-meter limit on cable length in LAN applications, overall cable diameter, and ease of installation, No. 24 wire is the norm with No. 22 optional. A 100-W characteristic impedance is standard, the stability, consistency, and accuracy of this value affecting data-transmission reliability.
Shielding greatly changes the characteristics of cables. Mechanically, shielding complicates cable termination, adds to a cable’s size and weight, and increases the minimum bend radius—you can’t make such tight bends with shielded cable. Electrically, shielding considerably reduces crosstalk among pairs in a cable, a major performance limitation in UTP (unshielded twisted-pair). Conversely, crosstalk can be compromised by the wire terminations made to connectors in a shielded system.
Conclusion
Currently ADSL
provides households and small businesses with the best technology to connect to
the internet at an affordable cost. ADSL
implementations are low error prone and effective. The methods of transmitting and receiving
data are already present in the telephone line.
However, with the increased use of cell phones households are moving
away from having a telephone line at home.
Eventually, every household and business won’t have a telephone line and
a wireless connection will be required.
Until this occurs, ADSL is the best and most affordable way of
connecting to the internet in a fast reliable manner.
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“Asymmetric Digital Subscriber Line.” Wikipedia.
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<http://www.itu.int/rec/T-REC-G.992.1/en>.
<http://en.wikipedia.org/wiki/ITU_G.992.1>.
16 Jan. 2007. 3 Nov. 2007
<http://www.walthowe.com/navnet/history.html>.
Telcordia Technologies, Inc. 2001. 26 Oct. 2007 <http://net3.argreenhouse.com:8080/dsl-test/Help/SM_paper.pdf>.
<http://williams.comp.ncat.edu/Networks/multip10.gif>.
