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Introduction to DSL

By Thomas Starr,John M. Cioffi,Peter J. Silverman,Massimo Sorbara

Date: May 9, 2003

Sample Chapter is provided courtesy of Prentice Hall Professional.

Learn how digital subscriber line (DSL) technology transforms an ordinary telephone line into a broadband communications link, much like adding express lanes to an existing highway.

Digital subscriber line (DSL) technology transforms an ordinary telephone line into a broadband communications link, much like adding express lanes to an existing highway. DSL increases data transmission rates by a factor of twenty or more by sending signals in previously unused high frequencies. DSL technology has added a new twist to the utility of twisted-pair telephone lines.
1.1 The Telephone Loop Plant

The twisted-wire pair infrastructure (known as the loop plant) connects customers to the telephone company network. The loop plant was designed to provide economical and reliable plain old telephone service (POTS). The telephone loop plant presents many challenges to high-speed digital transmission: signal attenuation, crosstalk noise from the signals present on other wires in the same cable, signal reflections, radio-frequency noise, and impulse noise. A loop plant optimized for operation of DSLs would be designed quite differently. Local-loop design practices have changed relatively little over the past 20 years. The primary changes have been the use of longer-life cables and a reduction in loop lengths via the use of the digital loop carrier (DLC). In recent years, primarily in the United States, many thousands of DLC remote terminals (DLC-RTs) have been placed in neighborhoods distant from the central office. Telephone and DSL service is provided directly from the DLC-RT. DSL performance is improved because the DSL signals traverse only the relatively short distance (generally, less than 12,000 feet) from the DLC-RT to the customer site. However, DSLs must cope with the huge embedded base of loop plant, some of which is 75 years old.

The term loop refers to the twisted-pair telephone line from a central office (CO) to the customer. The term originates from current flow through a looped circuit from the CO on one wire and returning on another wire. There are approximately 800 million telephone lines in the world.

The loop plant consists of twisted-wire pairs, which are contained within a protective cable sheath. In some parts of Europe and Asia the wires are twisted in four-wire units called "quads." Quad wire has the disadvantage1 of high crosstalk coupling between the four wires within a quad. Within the CO, cables from switching and transmission equipment lead to a main distributing frame (MDF). The MDF is a large wire cross-connect frame where jumper wires connect the CO equipment cables (at the horizontal side of the MDF) to the outside cables (at the vertical side of the MDF). The MDF permits any subscriber line to be connected to any port of any CO equipment. Cables leaving the CO are normally contained in underground conduits with up to 10,000 wire pairs per cable and are called feeder cables, E-side, or F1 plant. The feeder cables extend from the CO to a wiring junction and interconnection point, which is known by many names: serving area interface (SAI), serving area concept box (SAC box), crossbox, flexibility point, primary cross-connection point (PCP). The SAI contains a small wire-jumper panel that permits the feeder cable pairs to be connected to any of several distribution cables. The SAI is at most 3,000 feet from the customer premises and typically serves 1,500 to 3,000 living units. The SAI is a wiring cross-connect field located in a small outside cabinet that permits the connection of any feeder wire pair to any distribution wire pair. The SAI predates DLC. The SAI contains no active electronics, and is located much closer to the customer than the carrier serving area (CSA) concept originally developed for DLC.

1.2 DSL Reference Model¶

As shown in the generic DSL reference model in Figure 1.1, a DSL consists of a local loop (telephone line) with a transceiver at each end of the wires. The transceiver is also known as a modem (modulator/demodulator). The transceiver at the network end of the line is called the line termination (LT) or the transmission unit at the central end (TU-C). The LT may reside within a digital subscriber line access multiplexer (DSLAM) or a DLC-RT for lines fed from a remote site. The transceiver at the customer end of the line is known as the network termination (NT) or the transmission unit at the remote end (TU-R).

Figure 1 Figure 1.1 DSL Reference Model

The majority of DSLs are served via copper lines extending all the way from the central office to the customer's premises as shown in Figure 1.1. To address DSL's limited line reach, a repeater may be placed near the midpoint of the copper line to boost the signal on the line. However, installation of midspan repeaters has a high material and labor cost. More often, DSL service for customers in areas distant from a CO have DSL service enabled by the placement of a DLC-RT in their neighborhood as shown in Figure 1.2. The DSL signals traverse the relatively short copper wire between the customer and the DLC-RT. The link between the CO and DLC-RT is usually optical fiber. Alternatively, distant areas may also be served via a DSLAM located at a remote site; sometimes the DSLAM may be located within a building having many customers.

Figure 1.2 Digital Loop Carrier Reference Model

Figure 1.3 shows the network architectures for a voice-band modem, private-line DSL, and switched DSL services. The voice-band modem transmission path extends from one customer modem, a local line, the public switched telephone network (PSTN), a second local line, and a second customer modem. In contrast, the extent of DSL transmission link is a customer-end modem (TU-R), a local line, and a network-end modem (TU-C), with the digital transport through the remainder of the network being outside the scope of the DSL transmission path. The private-line service is not switched within the network and thus provides a dedicated connection between two endpoints. The private-line architecture may contain a service-provisioned cross-connection within the central office. The switched DSL service permits the customer to connect to many end points simultaneously or sequentially. Often, the TU-C for the switched DSL service resides within a DSLAM. The switching or routing functions may reside within the DSLAM or in separate equipment.

Figure 1.3 Comparative Network Architectures

1.3 The Family of DSL Technologies¶

Several species of DSL have resulted from the evolution of technology and the market it serves. The earliest form of DSL, 144 kb/s basic rate ISDN, was first used for ISDN service in 1986, and then was later applied to packet mode ISDN DSL (IDSL), and local transport of multiple voice calls on a pair of wires (DAML: digital added main line). Basic rate ISDN borrowed from earlier voice band modem technology (V.34), and T1/E1 digital transmission technology (ITU Rec. G.951, G.952).

As shown in Figure 1.4, DSL transmission standards have evolved from 14.4 kb/s voice-band modems in the 1970s to 52 Mb/s VDSL in the year 2001. This has been an evolution, with each generation of technology borrowing from the prior generation.

Figure 1.4 Evolution of DSL Technology (Note: Dates indicate publication of relevant standards.)

High bit-rate DSL (HDSL) was introduced into service in 1992 for 1.5 Mb/s (using two pairs of wires) and 2 Mb/s (using two or three pairs of wires) symmetric transmission on local lines. HDSL greatly reduced the cost and installation time required to provide service by reducing the need for midspan repeaters and simplifying the line engineering effort. HDSL is widely used for private line services, and links to remote network nodes such as digital loop carrier remote terminals and wireless cell sites. In 2000, HDSL2 was introduced to accomplish the same bit-rate and line reach as HDSL but using one pair of wires instead of the two pairs required for HDSL. Both HDSL and HDSL2 operate over CSA (carrier serving area) length lines consisting of up to 12 kft2 of 24 AWG wire, 9 kft of 26 AWG wire, or a proportionate length of mixed wire gauges. HDSL2 is spectrally compatible with other services in the same cable within the CSA line lengths but may not be spectrally compatible if a midspan repeater is used to serve longer lines. HDSL4, using trellis-coded pulse amplitude modulation (TC-PAM) for two pairs of wires, achieves spectral compatibility for 1.5 Mb/s transport on longer loops. By reaching up to 11 kft on 26 AWG lines without repeaters, HDSL4 further reduces the need for repeaters. The complementary pair of technologies—HDSL2 (for CSA lines) and HDSL4 (for longer lines)—provide a lower cost and spectrally compatible means to provide symmetric 1.5 Mb/s for nearly all lines. Chapters 4 and 6 discuss HDSL2 and HDSL4, respectively. Chapter 6 also addresses the symmetric SHDSL technology.

• Higher downstream bit rates are achieved via transmission asymmetry, using a wider bandwidth for downstream transmission and a narrower bandwidth for upstream transmission.
• Near-end crosstalk is reduced by partial or full separation of the upstream and downstream frequency bands.
• Simultaneous transport of POTS and data is achieved by transmitting data in a frequency band above voice telephony.3
• Use of advanced transmission techniques (trellis coding, Reed-Solomon codes with interleaving, and DMT modulation).
• Rate-adaptive transmission that adjusts to the highest bit rate allowed by the unique conditions for each line.

ADSL is widely used for applications benefiting from the bit-rate asymmetry, for example, high-speed Internet access and workstation access for small business offices and home work offices (SOHO). ADSL supports downstream bit-rates up to 8 Mb/s and upstream bit-rates up to 900 kb/s on short lines (less than 6 kft) with moderate line noise. However, to assure service to more lines with more noise, ADSL service is most often provided at bit-rates of 2 Mb/s or less downstream and 128 kb/s or less upstream. At mid-year 2002, there were 26 million ADSLs in service worldwide, with approximately 80 percent of the lines serving residential customers and 20 percent of the lines serving business customers.

The early deployments of ADSL employed a splitter at both ends of the line to combine the 0–3.2 kHz analog voice signal with the ADSL signals in a higher frequency band. The development of the "G.lite" (ITU Rec. G.992.2) standard introduced the concept of enabling customer self-installation without a splitter at the customer end of the line. This reduced the labor cost to install the service.

Field trials of G.lite demonstrated that an in-line filter must be inserted in series with most types of telephone sets to prevent problems for both the voice transmission as well as the digital transmission. Subsequently, it was determined that the in-line filters permitted effective operation of the full-rate ADSL (T1.413 and ITU Rec G.992.1), whereas G.lite is restricted to about 1.5 Mb/s downstream. As a result, the large majority of current ADSL installations use full-rate ADSL self-installed by the customer, placing an in-line filter by every telephone in their premises. Because ITU Recs. G.922.1 and G.992.2 were derived from the earlier T1.413 standard, all these ADSL standards are very similar, and most ADSL equipment supports all three standards.

Chapter 3 discusses ADSL in more detail.

SHDSL - Single-pair high-bit-rate¶

Single-pair high-bit-rate DSL (SHDSL) products were available by the end of 2000 based on the ITU Rec. G.991.2 standard. Like the nonstandard 2B1Q SDSL systems, SHDSL supports symmetric transmission at bit-rates from 192 kb/s to 2.32 Mb/s while providing at least 2,000 feet greater line reach than SDSL. Furthermore, the SHDSL specifications provided for the use of multiple pairs of wires and midspan repeaters to achieve greater bit-rates and line lengths. SHDSL uses trellis coded pulse amplitude modulation (TC-PAM), which is also used for HDSL2 and HDSL4.

Chapter 6 discusses SHDSL in more detail.

VDSL - very high-bit-rate DSL¶

Prestandard very high-bit-rate DSL (VDSL) systems were used in field trials in 2000. VDSL supports asymmetric bit-rates as high as 52 Mb/s downstream or symmetric bit-rates as high as 26 Mb/s. The key distinction of VDSL is its limitation to very short loops, as short as 1,000 feet for the highest bit-rates or up to about 4,000 feet for moderate data rates. The very short line length operation depends on shortening the copper line by placing an optical network unit (ONU) close to the customer site and then connecting one or more ONUs to the network with a fiber. Like ADSL, VDSL is a rate adaptive system that provides for simultaneous transmission of data and an analog voice signal.

Chapter 7 discusses VDSL in more detail.

xDSL - most types of DSL¶

The term xDSL applies to most or all types of DSL technology. Chapter 5 discusses ITU G.994.1 (g.handshake), which DSL transceivers use to negotiate a common operating mode.

Figure 1.5 shows the upstream and downstream rates supported by the various DSL technologies with the symmetric technologies (ISDN, SHDSL, HDSL) residing along a line of symmetry, and the rate adaptive technologies (ADSL, VDSL) covering a broad range of bit-rates with the corresponding maximum line lengths indicated.

Figure 1.5 DSL Data Rates

1.4 DSL Protocol Reference Model¶

Figure 1.6 shows the open systems interconnection (OSI) protocol reference model with additional detail shown for the physical layer. The structure within the physical layer is largely due to the contributions of Les Humphrey. The protocol reference model provides a structure to organize complex communications systems. The layered approach hides details of information from the subsystem, invoking the service of a particular layer. Thus, an application on a host requesting communication with its peer application does not need to know the details of its physical connection to a data network, communications used between itself and the network, or even the details of the protocols exchanged between the hosts supporting the application. The layered approach also simplifies the analysis of complex communication systems by segmenting the systems into several well-defined portions, facilitates reuse of portions of communications systems, and allows upgrades of portions of a communications system independently of each other.

ADSL filters explained

Written by Tomi Engdahl 2004

Basics of ADSL and telephone

Telephone wires were originally designed to carry "Commercial Speech" between your home and the telephone exchange. This uses a band of frequencies from 300 to 3400 hertz. this system is called PSTN (public switched telephone network).

ADSL uses frequencies very much higher than this speech band to carry fast data traffic. ADSL systems use typically frequencies between 25 kHz and around 1.1 MHz.

Because PSTN and ADSL systems operate at different frequencies, they can be carried though the same wire pair at the same when the operating conditions are right. Voice calls operate between 300Hz and 3.4KHz, and include also DC power (0-72V DC at on-hook condition, typically 0-60 mA current and lower voltage at on-hook) and rign voltage (typically 40-80 V AC at 20-25 Hz freuquency). The voice telephone system is matched to 600 ohm (or close to it) impedance at voice frequencies. ADSL technology operates between 26KHz and 1.1 MHz and is designed for around 100 ohms impedance. Because the two frequency spectrums do not overlap, it follows that both data and voice can be present at the same time on a single pair of copper wire. The different impedances have hostorical and technical reasons. The impedance of telephoen wiring is typically around 100-120 ohms at the frequencies ADSL system uses. The cable impedance is somewhat higher at voice frequency range, considerably higher than 100 ohms, and where where historical 600 ohms impedance comes to picture (cable might not be exactly 600 ohms for voice, but that's what devices are designed for).

Why splitters / filters are needed

When ADSL and PSTN work at the same line at the same time, the electronics inside a normal telephone can be problem for high frequency ADSL signals: the ADSL signals can be attenuated (high capacitance on telephone input, possible resonances inside telephone, impedance mismatch) and ADSL signals can be heard as noise on some telephones (phone electronics demodulates high frequency signal outside it's operating range to voice frequency noise). In order to keep these systems apart and stop them interfering with each other it is necessary to separate the two components from the telephone line in your home.

This is where the Filter / Splitter comes in. The ADSL POTS Splitter / filter allows taking the full advantage of the 1.1MHz copper line frequency spectrum, by stopping the telephone and ADSL systems from interfering with each other.

An ADSL filter is normally a small plastic box with a short lead that plugs into your phone socket and two outputs, one for your ADSL Modem and another for a telephone. Some filters have only one telephone output in them. ADSL filter select the band of frequencies for each of the outputs, phone or ADSL, and send just the correct band to the appropriate socket. The phone output gets only telephone frequencies (from DC to 3.4 kHz) and the ADSL output gets the higher freuquencies well (above 25 kHz).

For good system performance it is very important that all your other telephony equipment is separated from the ADSL signals by the use of a splitter / filter -- this equipment includes telephones, answering machines, "normal" computer modems, etc, etc.

Tips:

• All phones or other equipment must pass through a filter.
• Make sure that the ADSL signal is only passing through one Filter / Splitter.
• It can be the same Filter / Splitter for all of the phones.

How real life ADSL filters at home work

The signal to telephone output is generally just low-pass filtered so that voice frequencies (frequencies up to 3.4 kHz) get nicely though, but higher frequencies gets filterted. This filtering generally consista of LC low-pass filter designed to some suitable operating frequency between 4 and 20 kHz (between voice and ADSL bands). This kind of filter causes that the high frequencies of the ADSL signal will be severely attenuated (usually by at least 30dB with a good filter) so the signal reaching your telephone equipment does not contain such amount of high frequency signals that could cause noise. The telephone LC filter is also designed in such way that the filter impedance towards the line that carries ADSL signals is high at the high frequencies, meaning that those telephone equipment and cables related to them look like they are look to high freuquency signals that they would ne "disconnected from the main line". line.

The ADSL POTS splitter is simply a series of coupled inductors and parallel capacitors forming a low pass filter that attenuates the higher frequency ADSL data and permits only the voice frequencies to reach the telephone. The series inductor shows high impedance to high freuqencies, so the ADSL signals on the line are not attenuated.

General design specifications for an ADLS filter should be somethign like this:

• Return loss at voice frequencies (against 600 ohms) would be should be good enough.
• Should not alter voice band freuqncy response too much
• Should not have too high series resistance (commercial filters seems to have between 50 and 100 ohms for whole loop resistance)
• Filter must pass the POTS tip-to-ring dc voltages (typically o-72V)
• Filter must pass ring voltages well (40V to 80V rms at any frequency from15.3Hz to 68Hz with a dc component in the range from 0V to 72V)
• Filter must
• All requirements must be met in the presence of POTS loop currents (usually around 0-40 mA, can be up to 120 mA in some cases)

The ADSL output from filter (if it has such thing) is generally unfiltered line signal (normal home ADSL devices are not to be bothered with line voltage and voice signals.

ADSL splitters at the central office

When the operator install ADSL system to the central office, they install ADSL splitter filters on the central office end of the telephone wire. The filters at the central office have basically the same functional needs as the home units, they need to be able to keep different signals separate, and separate those two signals to different outputs. Typical central office ADSL splitter filter is a device that ghas many filters built into one package. For each outgoing line there is one PSTN connection (goes to centeral office telephone central equipment) and one for ADSL connection (goes to DSLAM rack that terminates ADSL connections). Typical ADSL splitter in central office has series capacitors (blocks telephone line DC well, attenuates ring signal sonciderably, but passes ADSL signals well) between line and ADSL output going to DSLAM.

Because ADSL splitter filter connects directly to the subscriber's loop media, it must also provide some surge protection from externally induced voltage which could damage any attached equipment or endanger humans interacting with the installed equipment. The ADSL splitters in the central ofice typically include overvoltage protection components to protect the ADSL DSLAM agains overvoltages on the line. Some filtered ADSL outputs provide protection from the high frequency transient and impedance effect that occur during POTS operations (ringing transients, on-hook, off-hook transient and so on).

• Ref. 1 : ETS 300 001 Attachment to Public Switched Telephone Network
• Ref. 3 : ITU-T K21 Resistibility of subscribers terminal to over-voltage and over-currents

http://en.wikipedia.org/wiki/DSL_filter

http://en.wikipedia.org/wiki/Digital_Subscriber_Line

http://en.wikipedia.org/wiki/ADSL_modem

http://pudeev.livejournal.com/tag/bcm6348

http://www.intersil.com/applications/printdoc/DSLModem(CPE).asp

http://en.wikipedia.org/wiki/Plain_old_telephone_service

• bi-directional, or full duplex, voice path with limited frequency range of 300 to 3400 Hz: in other words, a signal to carry the sound of the human voice both ways at once;
• call-progress tones, such as dial tone and ringing signal;
• subscriber dialing;
• operator services, such as directory assistance, long distance, and conference calling assistance;
• a standards compliant analog telephone interface including BORSCHT functions

In the United States, the pair of wires from the central switch office to a subscriber's home is called a subscriber loop. It is typically powered by -48V direct current (DC) and backed up by a large bank of batteries (connected in series) in the central office, resulting in continuation of service during most commercial power outages. The subscriber loop typically carries a "load" of about 300 Ohms, and does not pose a threat of electrocution to human beings (although shorting the loop can be felt as an unpleasant sensation).

http://www.nti-audio.com/Home/Solutions/ProductionTestSolutions/POTSInterfaces/tabid/88/Default.aspx

POTS Interface Testing

The RT-2X audio analyzer is a telecom tester optimized for high-level voice-band quality end-of-line tests on a production line.

• Line cards & modems
• Wired analog or digital telephone sets
• Telephone handsets

RT-2X is based on the multitone approach that allows simultaneously executing up to seven voice-band measurements by transmitting only a single test burst. This principle makes the RT-2X system ideally suited for high-volume production testing of telecom devices.

Additionally the LCL/RL option has been specifically tailored to access POTS devices and to execute standard POTS measurements at a COT (Central Office Terminal) or RT (Remote Terminal).

Testing the POTS portion of ADSL - Technology Information

Power loss (or line loss) is one of the most common measurements made on VF lines. Loss is measured with a dBm meter and is referenced to the measured attenuation of a 1004 Hz test tone transmitted at a level of O dBm. At the time of installation of the VF transmission line, the installer adjusts the active line components to yield 16 dBm +/- 1 dBm attenuation for the O dBm test tone. According to AT&T, long-term readings for this loss measurement should be 16 dBm +/- 4 dBm. This last figure must be met during routine line-loss test measurements.

http://www.epanorama.net/circuits/teleinterface.html

Telephone central                              Telephone equipment

Ground -----+                               
            |                          Hookswitch  /
          COIL 5H                             +--o/ o-------+
            |                                 |             )|| ___
          Resistor 200 ohm                    |         ____)||(   |
      2uF   |                            TIP  |        |    )||( Speaker
    --||----+--o---------------------------o--+       Mic   )||(___|
Audio               Line wire                          |    )||
    --||----+--o---------------------------o-----------+----+
      2uF   |                            RING      
          Resistor 200 ohm                          
            |
          COIL 5H
            |  
-48V DC ----+

http://ourworld.compuserve.com/homepages/Bill_Bowden/

1. PIC Security System Dials Your Cell Phone
2. Use Old Telephones as Intercom
3. Telephone In-Use LED Indicator
4. Telephone In-Use Relay Circuit
5. Telephone Ring Generator Using 60Hz Power Transformer
6. Telephone Ring Generator Using Switching Power Supply
7. Telephone Audio Interface

gornje sheme pod 3 i 4 bi mogle pomoći za testiranje postojanja signala od centrale.

E sad ne znam šta šalje centrala kada šalje DSL signal po slobodnoj parici ?