Communication Networks/DSL

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Digital Subscriber Line

Introduction

Digital Subscriber Line (also known as Digital Subscriber Loop) is a technology that transports high-bandwidth data, such as multimedia, to service subscribers over ordinary twisted pair copper wire telephone lines. A DSL line can carry both data and voice signals and the data part of the line is continuously connected. Digital Subscriber Line technology assumes that digital data does not require change into analog form and back. Digital data is transmitted to the system directly in digital data form, allowing wider bandwidth for transmission. Though, the signal can also be separated so that some of the bandwidth is used to transmit an analog signal allowing users to use telephone and computer simultaneously on the same line.

How DSL came into Action:-

The Telephone companies got pressure from the cable TV and the satellite industry as they were offering speeds up to 10Mbit/s and 50 Mbit/s respectively. While the telephone industry was only offering 56 kbit/s. As Internet grew as a new business prospect then the telephone companies realize that they need more competitive product, so that they can offer both telephony and Internet over the same local loop. This is the beginning of DSL.

Goals of DSL Service

The main goals of the xDSL services are as following:

It must work on the existing twisted pair local loops.
It must not affect customers existing telephones and fax machines.
It must be faster than the 56 kbit/s
And finally it should always remain ON.

How DSL works

DSL connects the computer to the Internet at speeds as fast as 52 Mbit/s, using the twisted pair copper lines that are commonly used for phone service. Apart from better download and upload times than traditional modems, DSL offers the benefit of always being ON; we don't have to dial up our Internet service provider every time we want to get on the Net. Since DSL connections are dedicated, so we don't have to share our bandwidth with other users as we do in cable modems. All of the DSL achieve their high speeds in the same way by sending data over previously unused frequencies in phone lines. Voice signals travel over phone lines at frequencies ranging from 0 kHz to 4 kHz. Standard modems use the same frequencies, but DSL uses frequencies between 25 kHz and 1 MHz. This extra bandwidth ensures that more data can be sent over the same line. This broadband connection requires special hardware at both, the consumer and phone company’s ends. On consumer’s end, a DSL modem modulates digital information from its computer to send it along phone lines. These signals are then translated by a Digital Subscriber Line Access Multiplexer (DSLAM) located at the phone company's nearest central office. The DSLAM separates the voice from the data signals, sending the data signal to an Internet Service Provider (ISP) and from there to the Internet.

xDSL technologies

Mainly there are two types of technology for the xDSL standard; one is the Discrete Multimode (DMT) which is the most widely used technology and another is the Carrier less Amplitude/Phase (CAP) system, which was adopted on many original installations.

CAP

The CAP method works by taking the entire bandwidth of the copper wires and simply splitting those up into 3 distinct sections or bands separated to ease interference. Each signal band is then allocated a particular task. The first band is in the signal range of 0 to 4 kHz and is used for telephone conversations. The second band occupies the range of 25 to 160 kHz which is used as an upstream channel, while the third band covers from 240 kHz up to a maximum (depending on conditions) of 1.5 MHz and is used as a downstream channel. This method was simple and effective as poor quality wires or large amounts of interference wouldn't affect the xDSL from working, instead it would just limit the range of the third band and result in slightly reduced speeds.

DMT

The DMT system is much more complex. It works by splitting the entire frequency range (bandwidth) into 247 channels of 4 kHz each and allocating a range of the lower channels, staring at around 8 kHz, as bidirectional to provide upstream and downstream channels. By splitting the bandwidth up in this way it effectively allows one connection to operate as if there were 247 modems connected to it, each of which operating at 4 kHz. The technology used in the DMT system is vastly more complex than that required for the CAP method as each of the 247 channels requires constant monitoring and assessment. If the system detects that a specific channel or range of channels are suffering from interference or a degradation in quality then the data stream must be automatically transferred to different channels. For the DMT system one need to place low pass filters into any telephone socket for making voice calls, because voice calls take place below the 4 kHz frequency and the filters simply block anything above this to prevent data signals interfering with the telephone call.

Most popular DSL Service

All the different types of DSL are known generally as xDSL, where x denotes all various types. The term xDSL covers a number of similar yet competing forms of DSL technologies, including ADSL, SDSL, HDSL, IDSL, and VDSL.

ADSL

The initially offered ADSL service worked by dividing the spectrum available on the local loop into three frequencies bands first one is for POTS (Plain Old Telephone System) the other is for upstream and the third one is for downstream.

But the most likely approach is called the DMT (Discrete MultiTone). It divides the available 1.1 MHz spectrum on the local loop into 256 independent channels of 4312.5 Hz each. Channel 0 is used for POTS (Plain Old Telephone System). Channel 1-5 are not used so that voice and the data signal cannot interfere with each other. From the remaining 250 channels one is used for downstream control and one is used for upstream control. The rest of the channels are available for user data. It is up to the service provider to determine how many channels should be allocated for upstream and downstream. Though 50-50 is possible, most providers allocate 32 channels for upstream and the remainder of the channels for downstream, because most users will download data more than they upload.

The speed provided by the ADSL (ANSI T1.413 and ITU G.992.1) is 8 Mbit/s downstream and 1 Mbit/s upstream. Within each channel a modulation scheme similar to V.34 is used and the sampling rate is 4000 Baud. The actual data is send through QAM modulation with 15 bits per baud.

ADSL Arrangement

In a typical ADSL arrangement the telephone company installs a Network Interface Device (NID) in the customer’s premises. A splitter is combined with the NID. It is an analog filter that separates the 0-4000 Hz band used by the POTS from the data. The POTS signal is routed to the telephone, and the data signal is routed to the ADSL modem. The ADSL modem is connected to the computer through an Ethernet card or USB port. At the other end of the wire towards the central office, a corresponding splitter is installed, where the voice portion of the signal is filtered out and sent to the voice switch. The signal above 26 kHz is routed to a DSLAM (Digital Subscriber Line Access Multiplexer), which contains the same kind of digital signal processor as an ADSL modem. Once the digital signal is recovered in the bit stream, packets are formed and sent to the Internet Service Provider. The one disadvantage of this system is that a company technician is needed to install the NID, which is very expensive for the company. So another splitterless design was standardized which is normally known as G.lite. The only difference was that a microfilter has to be inserted into each phone jack between the telephone or ADSL modem and wire. The microfilter for the telephone is the low-pass filter eliminating frequencies above 3400 Hz; the microfilter for the ADSL modem is a high-pass filter eliminating frequencies below 26 kHz. Though this system is not as reliable as having a splitter, it still requires a splitter in the end office.

Other DSL Services

ADSL2

ADSL 2 is similar to ADSL and typically the modems can be interchangeable. The difference is that ADSL 2 offers a downstream rate of up to 25 Mbit/s, while the upstream rate remains the same as regular ADSL, at 1 Mbit/s. The range of 15,000 feet from the central office also remains the same.

ADSL 2+

ADSL2+ is the next generation of ADSL Broadband, ADSL2+ services are capable of download speeds of up to an incredible 24 Megabits per second (depending on your equipment and the length of your copper line). ADSL2+ services are capable of upload speeds of up to 2.5 Megabits per second (Annex M) or 1 Megabit per second. ADSL2+ Broadband runs much faster than standard ADSL. This allows you to get faster speeds at longer distances from your telephone exchange (as per the graph), or get ADSL when you previously have not been able to in the past

SDSL

Symmetric Digital Subscriber Line (SDSL), a technology that allows more data to be sent over existing copper telephone lines (POTS). SDSL supports data rates up to 3 Mbit/s. 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 requires a special SDSL modem. SDSL is called symmetric because it supports the same data rates for upstream and downstream traffic

SHDSL

SHDSL stands for Symmetric High-Bit rate Digital Subscriber Loop. SHDSL is designed to transport rate-adaptive symmetrical data across a single copper pair at data rates from 192 kbit/s to 2.3 Mbit/s or 384 kbit/s to 4.6 Mbit/s over two pairs. With single-pair operation, SHDSL offers 192 kbit/s to 2.3 Mbit/s. Data rates are defined in increments of 8 kbit/s. With dual-pair operation (4-wire mode), SHDSL offers 384 kbit/s to 4.6 Mbit/s. Data rates are defined in increments of 16kbit/s. The line rate on both pairs must be the same.

VDSL

VDSL (Very High-Data-Rate Digital Subscriber Line) VDSL is basically ADSL at much higher data rates. It is asymmetric and, thus, has a higher downstream rate than upstream rate. The upstream rates are from 1.5 Mbit/s to 2.3 Mbit/s. The downstream rates and distances are listed in the following table. VDSL is seen as a way to provide very high-speed access for streaming video, combined data and video, video-conferencing, data distribution in campus environments, and the support of multiple connections within apartment buildings.

VDSL 2

VDSL 2, stands short for Very High Bit Rate DSL 2, is a type of Internet connection that uses the phone line, much like DSL. However, VDSL 2 uses 30 MHz of spectrum, has speeds of 100 Mbit/s, and has a range of 12,000 feet. These high capabilities allow for data to be sent in larger volumes, at a much faster speed, and over longer distances. It is no surprise why people are gaining interest in VDSL 2 for their Internet service.

Limitation of xDSL:-

DSL has one significant downside: The farther you are from the central office, the slower your connection is. As you move away from the central office, more distortion enters the line and the signal deteriorates. To counter this, the phone company slows down transmission rates, from 1.5 mbps to 384 kilobits per second, for example. But slowing the speed only works up to a point--if you live more than two miles from the nearest central office, you can't get DSL at all. According to the industry trade group ADSL Forum, about 60 percent of United States telephone customers live within areas that could support DSL.