UNIT name: 7.12 xDSL techniques

Aim of the unit: The student recognises different DSL techniques and their basic features. Student will also come familiar how DSL techniques use channels and in what frequency range DSL`s operate. Understanding DSL`s restrictions in distance/data bitrate is also in important role in this unit.

1. Introduction

DSL or xDSL, is a family of technologies that provide digital data transmission over the wires of a local telephone network. DSL is technology that has developed to offer alternative for optical fiber. DSL –family include various techniques like ADSL (Asymmetric Digital Subscriber Line), HDSL (High-bit rate Digital Subscriber Line), RADSL (Rate Adaptive Digital Subscriber Line), SDSL Symmetric Subscriber Line) and VDSL (Very high speed Digital Subscriber Line).

Depending on DSL technology, line conditions and service level implemented DSL service speeds ranges from 256 kbit/s to 24,000 kbit/s. In ADSL technologies the upload speed is lower than download speed and equal in SDSL.

DSL Upload speed Mbit/s

Download speed Mbit/s

Max. distance km

Symmetry

ADSL, ITU-T G.992.1 1,0 8,0 5 asymmetric

ADSL2, ITU-T G.992.3/4 1,0 12,0 5 asymmetric

ADSL2+, ITU-T G.992.5 1,0 24,0 5 asymmetric

HDSL 2,048 2,048 3 symmetric

SDSL (One pair) 2,304 2,304 3 symmetric

SDSL (Two pairs) 4,608 4,608 3 symmetric

SHDSL (G.SHDSL) 4,608 4,608 3 symmetric

VDSL, ITU-T G.993.1 12 52 0,3 asymmetric

26 26 0,3 symmetric

VDSL2, ITU-T G.993.2 100 100 0,3 symmetric

1.1 DSL techniques

2. Operation

The local loop of the Public Switched Telephone Network was initially designed to carry POTS(Plain old telephone system) voice communication and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as voiceband or commercial bandwidth.

At the local telephone exchange the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore – according to the Nyquist theorem – any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).

The local loop connecting the telephone exchange to most subscribers is capable 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. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor. The pool of usable channels is then split into two different frequency bands for upstream and downstream traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether or not they are usable.

One of Lechlider's greatest contributions to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of synchronous DSL. This allowed Internet Service Providers to offer efficient service to consumers, who benefitted greatly from the ability to download large amounts of data but rarely needed to upload comparable amounts. ADSL supports two modes of transport: fast channel and interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where transmission errors are impermissible, even though resending packets may increase latency.

Because DSL operates at above the 3.4 kHz voice limit, it cannot be passed 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, nor can voice service be maintained past a certain distance without such coils. Some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this phone companies are efforting to remove load coils on copper loops that can operate without them, and conditioning lines to not need them through the use of FTTN(Fiber to the node).

All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Not long ago, the cost of such signal

processing would have been prohibitive but because of VLSI(Very-large-scale integration) technology, the cost of installing DSL on an existing local loop, with a DSLAM(Digital Subscriber Line Access Multiplexer) at one end and a DSL "modem" at the other end is orders of magnitude less than would be the cost of installing a new, high-bandwidth fiber-optic cable over the same route and distance.

Most residential and small-office DSL implementations reserve low frequencies for POTS service, so that with suitable filters and/or splitters the existing voice service continues to operate independent of the DSL service. Thus POTS-based communications, including fax machines and analog modems, can share the wires with DSL. Only one DSL "modem" can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection is to use a router that establishes a connection between the DSL modem and a local Ethernet, Powerline, or Wi-Fi network on the customer's premises.

Once upstream and downstream channels are established, they are used to connect the subscriber to a service such as an Internet service provider.

3. ADSL

Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call. By using a splitter or micro filters this allows a single telephone connection to be used for both ADSL service and voice calls at the same time. As phone lines are so varied in quality and weren't initially provisioned with ADSL in mind it can generally only be used over short distances, typically less than 5 km.

At the telephone exchange the line generally terminates at a DSLAM where another frequency splitter separates the voice band signal for the conventional phone network. The ATM stream carried by the ADSL physical layer is typically routed over the telephone company's data network to service center where the encapsulated IP packets are eventually routed onto a conventional internet network.

A for asymmetric

The distinguishing characteristic of ADSL over other forms of DSL is that the volume of data flow is greater in one direction than the other, i.e. it is asymmetric. Providers usually market ADSL as a service for consumers to connect to the Internet 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 together) 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 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.

Channels

ADSL 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. 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.

Each of these is further divided into smaller frequency channels of 4.3125 kHz. During initial training, the ADSL modem tests which of the available channels have an acceptable signal-to-noise ratio. The distance from the telephone exchange, noise on the copper wire, or interference from AM radio stations may introduce errors on some frequencies. By keeping the channels small, a high error rate on one frequency thus need not render the line unusable: the channel will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connections.

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 issues 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.

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.

1.2 Frequency plan for ADSL

Modulation

ADSL initially existed in two flavors (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 chosen for 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.

4. ADSL2

ITU G.992.3/4 is an ITU (International Telecommunication Union) standard, also referred to as ADSL2 (and Seamless Rate Adaptation). It extends the capability of basic ADSL in data rates. The data rates can, in the best situation, be as high as 12 Mbit/s downstream and 3.5 Mbit/s upstream depending on line quality. The distance from the DSLAM to the customer's equipment is usually the most significant factor in line quality.

5. ADSL2+

ITU G.992.5 is an ITU standard, also referred to as ADSL2+ or ADSL2Plus.Commercially it is notable for its maximum theoretical speed of 24 Mbit/s downstream and 3,5 Mbit/s upstream. Frequency range is from 26 Khz to 22 Mhz. Channels and frequencys varies even in established connections. Line quality plays very big role what comes to line speeds in ADSL2+. Line speeds drop dramatically after 1,3 km and continues to lower steeply till the end.

Technical differences vs. ADSL

ADSL2+ extends the capability of basic ADSL by doubling the number of downstream bits. The data rates can be as high as 24 Mbit/s downstream and 1 Mbit/s upstream depending on the distance from the DSLAM to the customer's home.

ADSL2+ is capable of doubling the frequency band of typical ADSL connections from 1.1 MHz to 2.2 MHz. This doubles the downstream data rates of the previous ADSL2 standard of up to 12 Mbit/s, but like the previous standards will degrade from its peak bitrate after a certain distance.

Also ADSL2+ allows port bonding. This is where multiple ports are physically provisioned to the end user and the total bandwidth is equal to the sum of all provisioned ports. So if 2 lines capable of 24 Mbit/s were bonded the end result would be a connection capable of 48 Mbit/s.

ADSL & ADSL2+ Performance

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5000

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20000

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30000

100 900 1700 2500 3300 4100 4900 5700 6500

Distance from DSLAM (metres)

Spee

d/D

ata

bitr

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(Kbp

s)

ADSL2+ADSL

1.3 Chart which shows how ADSL and ADSL2+ behaves in different distances

6. HDSL

High bit rate Digital Subscriber Line (HDSL) was the first DSL technology that uses a higher frequency spectrum of copper, twisted pair cables. HDSL was developed in the USA, as a better technology for high-speed, synchronous circuits typically used to interconnect local exchange carrier systems, and also to carry high-speed corporate data links and voice channels, using T1/E1 lines.

T1 circuits operate at 1.544 Mbit/s. These circuits were originally carried using a line code called Alternate Mark Inversion (AMI). Later the line code used was B8ZS. AMI did not have sufficient range, and required the application of repeaters over long circuits but also were subject to lightning and cable trouble such as inferior splices and backhoe fade. In troubleshooting these type of services, the *felt* frequency on each conductor is 772 Hz. and the repeaters are usually spaced every mile to 1.2 miles depending on cable size and the whim of the engineers. There is a positive and negative polarity to the side of the repeater. This is also true in HDSL carrier. in splicing this type of service the telcos placed the low voltage side of the repeater cable together and then the High voltage side together in the splice.. The telcos have a powering end to the circuit path and this gives the polarity and the repeaters are typically powered up to 130 volts dc. Usually if you see 130 volts there is trouble because the repeaters are running FULL power to try to compensate for the trouble. They require 60 milliamps and if they cannot get it they try to achieve it by raising the power.

The first attempts to use DSL technology to solve the problem were done in the USA, using the line code 2B1Q. This modulation allowed for a 784 kbit/s data rate over a single twisted pair cable.

With two twisted pair cables, the full 1.544 Mbit/s was achieved. The new technology attracted the attention of the industry, but could not be directly used worldwide, due to the differences between the T1 and E1 standards. A new standard was then developed by the ITU for HDSL, using the CAP (Carrierless Amplitude Phase Modulation) line code, that reached the maximum bandwidth of 2.0 Mbit/s using two pairs of copper.

HDSL can be used either at the T1 rate (1.544 Mbit/s) or the E1 rate (2 Mbit/s). Slower speeds are obtained by using multiples of 64 kbit/s channels, inside the T1/E1 frame. This is usually known as channelized T1/E1, and it's used to provide slow-speed data links to customers. In this case, the line rate is still the full T1/E1 rate, but the customer only gets the limited (64 multiple) data rate over the local serial interface.

HDSL gave way to two new technologies, called HDSL2 and SDSL. HDSL2 offers the same data rate over a single pair of copper; it also offers longer reach, and can work over copper of lower gauge or quality. SDSL is a multi-rate technology, offering speeds ranging from 192 kbit/s to 2.3 Mbit/s, using a single pair of copper. SDSL is used as a replacement (and in some cases, as a generic designation) for the entire HDSL family of protocols.

7. SDSL

Symmetric Digital Subscriber Line (SDSL) is a Digital Subscriber Line (DSL) variant with E1-like data rates (72 to 2320 kbit/s). It runs over one pair of copper wires, with a maximum range of about 3 kilometers. The main difference between ADSL and SDSL is that SDSL has the same upstream data transfer rate as downstream (symmetrical), whereas ADSL always has smaller upstream bandwidth (asymmetrical). However, unlike ADSL, it can't co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth. It is quite expensive and is mainly targeted at small and medium businesses who may host a server on site, (eg a Terminal Server or Virtual Private Network) who do want to use ADSL, but don't need the higher performance of a leased line.

SDSL was never properly standardised until Recommendation G.991.2 (ex-G.shdsl) was approved by ITU-T. SDSL is often confused with G.SHDSL and unfortunately, in Europe G.SHDSL was standardized by ETSI using the name 'SDSL'. This ETSI variant is compatible with the ITU-T G.SHDSL standardized regional variant for Europe.

Equipment routing SDSL support is usually proprietary equipment which only speaks to SDSL equipment from the same vendor, or to SDSL equipment from other vendors that use the same DSL chipset. Most new installations use G.SHDSL equipment instead of SDSL.

8. SHDSL

Symmetric high-speed digital subscriber line (SHDSL) is a telecommunications technology for Digital Subscriber Line (DSL) subscriber lines. It describes a transmission method for signals on copper pair lines, being mostly used in access networks to connect subscribers to Telephone exchanges or POP(Post Office Protocol) Access Points.

G.SHDSL was standardized in February 2001 internationally by ITU-T with recommendation G.991.2.

G.SHDSL features symmetrical data rates from 192 kbit/s to 2,304 kbit/s of payload in 64 kbit/s increments for one pair and 384 kbit/s to 4,608 kbit/s in 128 kbit/s increments for two pair applications. The reach varies according to the loop rate and noise conditions (more noise or higher rate means decreased reach) and may be up to 3,000 meters. The two pair feature may alternatively be used for increased reach applications by keeping the data rate low (halving the data rate per pair will provide similar speeds to single pair lines while increasing the error/noise tolerance).

The payload may be either 'clear channel' (unstructured), T1 or E1 (full rate or fractional), n x ISDN Basic Rate Access (BRA), Asynchronous Transfer Mode (ATM) or 'dual bearer' mode (i.e. a mixture of two separate streams (e.g. T1 and 'packet based') sharing the payload bandwidth of the G.shdsl loop).

The latest standardization efforts (G.SHDSL.bis) tend to allow for flexibly changing the amount of bandwidth dedicated to each transport unit to provide 'dynamic rate repartitioning' of bandwidth demands during the uptime of the interface and optionally provides for 'extended data rates' by using a different modulation method (32-TCPAM instead of 16-TCPAM, where TCPAM is Trellis-Coded Pulse Amplitude Modulation). Also, a new payload type is introduced: packet based, e.g. to allow for Ethernet-frames to be transported natively. (Currently, they may only be framed in ATM or T1/E1/...).

9. VDSL

VDSL or VHDSL (Very High Speed DSL) is an xDSL technology providing faster data transmission over a single twisted pair of wires. Compare HDSL (High data rate Digital Subscriber Line).

The maximum available bit rates are achieved at a range of about 300 meters (1000 ft), which allows for 26 Mbit/s symmetric access or up to 52 Mbit/s down – 12 Mbit/s up asymmetric access.

The Infineon 10Base-S™ (Ethernet over VDSL) technology delivers 10 Mbit/s, full duplex Ethernet over existing copper wire infrastructure up to 4000 ft/1200 m.

Currently, the standard VDSL uses up to 4 different frequency bands, two for upstream (from the client to the telco) and two for downstream. Recommendation allows two different band plans in area 25 Khz to 12 Mhz. They are called the band plan 997 and 998. The standard modulation technique is either QAM (Quadrature amplitude modulation) or DMT (Discrete multitone

modulation) which are not compatible, but have similar performance. The current mostly used technology is DMT.

These fast speeds mean that VDSL is capable of supporting new high bandwidth applications such as HDTV, as well as telephone services (Voice over IP) and general Internet access, over a single connection.

10. VDSL2

VDSL2 (Very High Speed Digital Subscriber Line 2) is an access technology that exploits the existing infrastructure of copper wires that were originally deployed for POTS services. It can be deployed from central offices, from fibre-fed cabinets located near the customer premises, or within buildings.

VDSL2 is the newest and most advanced standard of DSL broadband wireline communications. Designed to support the wide deployment of Triple Play services such as voice, video, data, high definition television (HDTV) and interactive gaming, VDSL2 enables operators and carriers to gradually, flexibly, and cost efficiently upgrade existing xDSL-infrastructure.

ITU-T G.993.2 (VDSL2) is an enhancement to G.993.1 (VDSL) that permits the transmission of asymmetric and symmetric (Full-Duplex) aggregate data rates up to 200 Mbit/s on twisted pairs using a bandwidth up to 30 MHz.

VDSL2 deteriorates quickly from a theoretical maximum of 250 Mbit/s at 'source' to 100 Mbit/s at 0.5 km (1640 ft) and 50 Mbit/s at 1 km, but degrades at a much slower rate from there, and still outperforms VDSL. Starting from 1.6 km its performance is equal to ADSL2+.

ADSL-like long reach performance is one of the key advantages of VDSL2. LR-VDSL2 enabled systems are capable of supporting speeds of around 1-4 Mbit/s (downstream) over distances of 4 to 5 km gradually increasing the bit rate up to symmetric 100 Mbit/s as loop-length shortens. This means that VDSL2-based systems, unlike VDSL1 systems, are not limited to short loops or MTU/MDUs only, but can also be used for medium range applications.

11. DSLAM

A Digital Subscriber Line Access Multiplexer (DSLAM) allows telephone lines to make faster connections to the Internet. It is a network device, located near the customer's location, that connects multiple customer Digital Subscriber Line (DSL)s to a high-speed Internet backbone line using multiplexing techniques.

What DSLAM do?

The DSLAM at the CO collects the digital signals from its many modem ports and combines them into one signal, via multiplexing.

Depending on the product, DSLAMs connect DSL lines with some combination of Asynchronous Transfer Mode (ATM), frame relay or Internet Protocol networks.

In terms of the OSI 7 Layer Model, the DSLAM acts like a massive network switch, since its functionality is purely Layer 2.

The aggregated signal then loads onto backbone switching equipment, traveling through an access network (AN) — also known as a Network Service Provider (NSP) — at speeds of up to 10 Gbit/s and connecting to the Internet-backbone.

The DSLAM, functioning as a switch, collects the ADSL modem data (connected to it via twisted or non-twisted pair copper wire) and multiplexes this data via the gigabit link that physically plugs into the DSLAM itself, into the Telco's backbone.

DSLAM is not always located in the telephone companys central office, but may also serve customers within a neighborhood Serving Area Interface (SAI), sometimes in association with a digital loop carrier. DSLAMs are also used by hotels, lodges, golfing estates, residential neighbourhoods and other corporations setting up their own private telephone exchange.

Besides being a data switch and multiplexer, DSLAM is also a large number of modems, each modem on the aggregation card communicating with a subscriber's DSL modem. This modem function being inside the DSLAM rather than separate hardware, and being wideband rather than voiceband, it isn't often called a modem. Like voiceband modems of standard v.32 and later, it has the ability to probe the line and train itself to compensate for echoes and other impairments, in order to move data at the maximum rate the line allows. This is also why twisted pair DSL services have a longer range than twisted pair (UTP) Ethernet.

Hardware details

Customers connect to the DSLAM through ADSL modems or DSL routers, which are connected to the PSTN network via typical unshielded twisted pair telephone lines. Each DSLAM has multiple aggregation cards, and each such card can have multiple ports to which the customers lines are connected. Typically a single DSLAM aggregation card has 24 ports, but this number can vary with each manufacturer. The most common DSLAMs are housed in a telco-grade chassis, which are supplied with (nominal) 48 volts DC. Hence a typical DSLAM setup may contain power converters, DSLAM chassis, aggregation cards, cabling, and upstream links. The most common upstream links in these DSLAMs use gigabit ethernet or multi-gigabit fiber optic uplinks.

1.4 Modules of Lucent Stinger DSLAM

IP-DSLAM

IP-DSLAM stands for Internet Protocol Digital Subscriber Line Access Multiplexer. User traffic is mostly IP based.

Traditional 20th century DSLAM used ATM (Asynchronous Transfer Mode) technology to connect to upstream ATM routers/switches. These devices then extract the IP traffic and pass it on to an IP network. IP-DSLAMs extract the IP traffic at the DSLAM itself. Thus it is all IP from there. Advantage of IP-DSLAM over a traditional ATM DSLAM is in terms of lower CAPEX/OPEX and a richer set of features and functionality. IP has a rich existing and ever-growing protocol set that one gets access to as soon as one moves to IP.

1.5 Example picture of different types of DSL networks

Picture explanations:

-ADSL2+ connection with POTS - basic home installation with two PCs and an analog phone.

-SHDSL connection – PBX connected with E1/T1 to SHDLS.

-VDSL2 connection – fast connection over twisted pair is installed as company´s backbone.

-DSLAM connections – DSLAM routes normal phone calls to PSTN via ATM and IP networking to internet via ATM/Ethernet depending on DSLAM and how it is installed.