Chapter 16: XDSL

2012-02-09T16:45:00.000-08:00

xDSL

Overview

One of the major problems facing the incumbent local exchange carriers (ILEC) is the ability to maintain and preserve their installed base. Ever since the Telecommunications Act of 1996, there

has been mounting pressure on the ILECs to provide faster and more correct Internet access. In

order to provide the higher−speed communications abilities, these carriers have continually looked

for new means of providing the service.

The DSL family includes several variations of what is known as digital subscriber line. The lower case x in front of the DSL stands for the many variations. These will include:

Ø Asymmetrical digital subscriber line (ADSL)

Ø ISDN (like) digital subscriber line (IDSL)

Ø High bit−rate Digital Subscriber Line (HDSL)

Ø Consumer Digital Subscriber Line (CDSL)

Ø Single High Speed DSL (SHDSL)

Ø Rate−adaptive digital subscriber line (RADSL)

Ø Very high−bit rate digital subscriber line (VDSL)

Ø Single or symmetric digital subscriber line (SDSL)

ADSL is a technology being provided primarily by the ILECs because the existing cable plant can be

supported, and the speed throughput can vary, depending on the quality of the copper. However,

the most important and critical factor in dealing with ADSL technology is the capability to support

speeds between 1.5 Mbps up to 8.192 Mbps. At the same time, the ILEC can also support Plain Old

Telephone Service (POTS) for voice or fax communications on the same line.

What is DSL?

ADSL is the new modem technology to converge the existing twisted pair telephone lines into the

high−speed communications access capability for various services. Most people consider ADSL as

a transmission system instead of a modification to the existing transmission facilities. In reality,

ADSL is a modem technology used to transmit speeds of between 1.5 Mbps and 6 Mbps under

current technology. It is stated that in the future ADSL will support speeds of about 8.192 Mbps.

This definition of the higher range of ADSL speeds is one that is yet to be proven; however, with

changes in today's technology one can only imagine that the speeds will be achievable.

Emphases Technologies has lots of plans for potential profits. Emphases, a development-stage company, designed broadband communications equipment that lets telephone companies provide television over DSL lines. It has since shifted its development to middleware that allows telephone companies to deliver voice, Internet, and television service over Internet protocol (IP). It plans to market its systems to phone companies in areas with relatively little multi-channel television access, such as international markets and the rural US. It is also developing power cells that utilize nanotechnology through its Always Ready subsidiary.

DSL family are the services for converging voice, data, multimedia, video, and Internet streaming protocols services. It is through these demands that the carriers see their future rollout of products and services to the general consuming public. shown In Table

Current Data Rate	Wire Gauge	Distance in K Feet	Distance in Kilometers
1.5 to 2.048 Mbps	24	18	5.5
1.5 to 2.048 Mbps	26	15	4.6
6.3 Mbps	24	12	3.7
6.3 Mbps	26	9	2.7

Modem Technologies

Modems, or modulator/demodulator, were designed to provide for data communications across the

voice dial−up communications network. Through the use of modem technology users were able to transmit data across the voice networks at speeds varying between 300 bps to 33,600 bps. Although this may seem like high−speed communication, our demands and needs for faster communications quickly outstripped the capabilities of our current modem services, making the demand for newer services more evident. Higher−speed modems could be produced, but the economics and variations on the wiring system prove this to be somewhat impractical.

Benefits

•High-speed data service–DSL typically >10x faster than 56-kbps analog modem

•Always on connection–No need to “dial-up”

•Uses existing copper wires–Co-exists w/ POTS service

•Reasonably priced today and getting cheaper

Applications

•High speed Internet access

•SOHO

•Multimedia, Long distance learning, gaming

•Video on Demand

•VPN

•VoDSL

Figure shown: Modems are installed at the customer's location and use the existing telephone wires to transmit data across the voice network.

The Analog Modem History

In the early days of modem communications, the Bell telephone companies (or the independent

telephone companies) provided all services across North America. A customer desiring to transmit

data needed only to call the local supplier who would then install the dial−up telephone line, the

modem, and all associated services to accommodate the desired data rates available. Leased lines

were used when specific speeds or volumes were anticipated, but not guaranteed by the dial−up

services. Regardless of the modem and lines used, the main provider was the key ingredient. The

local providers supported only what they knew they could meet, so speeds were often kept very low

from a guaranteed standpoint.

A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from light emitting diodes to radio.

IDSL

DSL refers to a pair of modems that are installed on the local loop (also called the last mile) to

facilitate higher speeds for data transmission. Network providers do not provide a line; they use the

existing lines in place and add the DSL modems to increase the throughput. DSL modems offer

duplex operations — transmission in both directions at the same time. The speed of a DSL modem

may be 160 Kbps on copper at distances up to 18K using the twisted pair wires.

IDSL uses the Integrated Services Digital Network (Integrated Services Digital Network) Basic Rate Interface in ISDN transmission code.

IDSL is a technology developed by Ascend Communications (now part of Lucent Technologies). IDSL is only one possible technology in the Digital Subscriber Line approach (of which Asymmetric Digital Subscriber Line or Asymmetric Digital Subscriber Line is best known) and an expedient approach that allows use of existing ISDN card technology for data-only use.

The differences between IDSL and ISDN are:

Ø ISDN passes through the phone company's central office voice network; IDSL bypasses it by plugging into a special router at the phone company end

Ø ISDN requires call setup; IDSL is a dedicated service

Ø ISDN may involve per-call fees; IDSL may be billed at a flat rate with no usage charges

Figure: The IDSL line connection enables 128 Kbps in total simultaneously.

IDSL is a system in which digital data is transmitted at 128 Kbps on a regular copper telephone line (twisted pair) from a user to a destination using digital (rather than analog or voice) transmission, bypassing the telephone company's central office equipment that handles analog signals.

HDSL

High-bit-rate digital subscriber line (HDSL) was the first DSL technology to use a higher frequency spectrum of copper, twisted pair cables. HDSL was developed in the US, 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 lines. HDSL service types include HDSL1, HDSL2 and HDSL4 and are typically transmitted over twisted pair cables or over fiber optics.

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. Unlike later ADSL, HDSL did not allow POTS at baseband.

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.

Figure: The typical layout of the T1

To circumvent these cabling problems, HDSL was developed as a more efficient way of transmitting

T1 (and E1) over the existing copper wires. HDSL does not require the repeaters on a local loop of

up to 12K. Bridge taps will not bother the service, and the splices are left in place. This means that

the provider can offer HDSL as a more efficient delivery of 1.544 Mbps. The modulation rate on the

HDSL service is more advanced. Sending 768 Kbps on one pair and another 768 Kbps on the

second pair of wires splits the T1. This is shown in Figure.

Figure : HDSL is impervious to the bridge and splices.

The T1 is split onto two pairs. As already mentioned, HDSL runs at 1.544 Mbps (T1 speeds) in North America and at 2.048 Mbps (E1 speeds) in other parts of the world. Both speeds are symmetric (simultaneous in both directions). Originally, HDSL used two wire pairs at distances of up to 15K. HDSL at 2.048 Mbps uses three pairs of wire for the same distances, but no longer. The most recent version of HDSL uses only one pair of wire and is expected to be more accepted by the providers. Nearly all the

providers today deliver T1 capabilities on some form of HDSL.

SDSL

The goal of the DSL family was to continue to support and use the local copper cable plant.

Therefore, the need to provide high−speed communications on a single cable pair emerged.

Short for symmetric digital subscriber line, a technology that allows more data to be sent over existing copper telephone lines (POTS). SDSL supports data rates up to 3 Mbps. SDSL works by sending digital pulses in the high-frequency area of telephone wires and can not operate simultaneously with voice connections over the same wires.

SDSL is a rate-adaptive digital subscriber line (DSL) variant with T1/E1-like data rates (T1: 1.544 Mbit/s, E1: 2.048 Mbit/s). It runs over one pair of copper wires, with a maximum range of 10,000 feet (3,000 m). It cannot co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth.

Table summarizes the speeds and characteristics of the DSL technologies discussed. These

are the typical installation and operational characteristics; others will certainly exist in variations of

installation and implementation.

Service	Explanation	Download	Upload	Mode of Operation
ADSL	Asymmetric DSL	1.5 to 8.192 Mbps	16 to 640 Kbps	Different up and down speeds, one pair wire.
RADSL	Rate Adaptive DSL	64 Kbps to 8.192 Mbps	16 to 768 Kbps speeds	Different up and down. Many common operations use 768 Kbps. One pair wire.
CDSL	Consumer DSL	1 Mbps	16 to 160 Kbps	Now ratified as DSL−lite (G.lite). No splitters. One pair wire.
HDSL	High−data rate DSL	1.544 Mbps in North America,	1.544 Mbps	Symmetrical services. Two pairs of wire.
	2.048 Mbps	2.048 Mbps in rest of world
IDSL	ISDN DSL	144 Kbps (64+64+16) as BRI	144 Kbps (64+64+16) as BRI	Symmetrical operation.One pair of wire. ISDN BRI.
SDSL	Single DSL	1.544 Mbps, 2.048 Mbps	1.544 Mbps, 2.048 Mbps	Uses only 1 pair but typically provisioned at 768 Kbps. One pair wire.
VDSL	Very High data rate DSL	13 to 52 ± Mbps	1.5 to 6.0 Mbps	Fiber needed and ATM probably used.
SHDSL (G.SHDSL)	Single High−speed DSL or 384 Kbps	192 Kbps to 2.360 Mbps or 384 Kbps to 4.720 Mbps	192 Kbps to 2.360 Mbps Using 2 pair. to 4.720 Mbps	Using 1 pair.

Summary of DSL speeds and operations using current methods

xDSL Coding Techniques

Many approaches were developed as a means of encoding the data onto the xDSL circuits. The

most common are Carrierless Amplitude Phase Modulation (CAP) and discreet multitone (DMT)

modulation. Quadrature with Phase Modulation (QAM) has also been used, but the important part is

the standardization. The industry, as a rule, selected DMT, but several developers and providers

have used CAP. It is, therefore, appropriate to summarize both of these techniques. The SHDSL

technology uses a trellis−coded pulse amplitude modulation (TCPAM) technique to gain the benefits

of the single−pair services or two−pair service.

Provisioning xDSL

In the following figures, the various architectures of the xDSL implementations are shown. The point

to remember here is the goal of xDSL is to use the existing copper infrastructure and improve the

speed and throughput on the installed base of wires. Consequently, the installation process

attempts to minimize the added equipment (particularly at the customer's premises) and the labor

required to get the equipment installed.

the design of an ADSL model and the model components are shown. The intent of

the model is to show the infrastructure of the network from the customer premises to the network

provider. This model also shows the splitters in place to facilitate the ADSL model.

Chapter 16: xDSL

Chapter 16: XDSL