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Introduction to Digital Subscriber Lines (DSL)

      by: Lawrence Harte and Robert Flood

Digital subscriber line is the transmission of digital information, usually on a copper wire pair. Although the transmitted information is in digital form, the transmission medium is usually an analog carrier signal (or the combination of many analog carrier signals) that is modulated by the digital information signal. The reason that this technology is important is that it allows the telephone company to use existing investment in copper wires to each home and business to deliver higher speed data services. So for the customers that are in a service area for DSL services, the two copper wires that provided their basic tele

This article is Part 1 of a 5 Part Series

 phone service can be used to deliver higher speed data services as well. DSL service requires special electronic equipment (Digital Subscriber Line Access Module [DSLAM]) to be connected to the two copper wires both at their origination point at the telephone company offices and at the customer's location (DSL Modem).
A DSL network is composed of several key parts; this includes a local access line provider, DSL access provider, backbone network aggregator, ISP provider, and other media providers. DSL services can be provided by a single service provider or may result from the combination of processes from different service providers. The communication network can be divided into several parts; local access lines 

Figure 1.1, Digital Subscriber Line (DSL) System 

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(copper), voice communications network (PSTN), high-speed digital subscriber line (DSL), aggregator (interconnection), Internet service provider (ISP) and content provider (media source). These network parts and the service providers who operate them, must interact to provide most DSL services. 

DSL systems are primarily installed and operated by post, telephone, and telegraph (PTT’s) and local exchange carriers (LEC's) are the established telephone network operators or companies that provide local telecommunications services. For some countries, PTTs are government operated telephone systems. In the United States, LEC's are granted franchises to provide telephone services to certain geographical areas as mandated by the Federal Communication Commission (FCC). Recently, deregulation and privatization of telecommunication systems worldwide have allowed the creation of new competing local exchange carriers (CLECs). CLEC's provide similar services as LEC's and PTTs. In some cases, CLECs provide services by leasing existing lines from incumbent local exchange carriers (ILECs) and reselling services on these lines. In other cases, CLECs install new communication lines or provide connection by wireless service. 

Figure 1.1 shows a simplified ADSL communication system that

consists of a digital subscriber line access mulitplexer (DSLAM), local distribution lines that start from a main distribution frame (MDF) wire cabinet that brings the connection to the digital subscriber line (DSL) modem at the customer's location. Modems in the DSLAM convert the digital signals from the internet to high frequency signals that travel down the telephone line to the DSL modem. The DSL modem converts the RF signals back to its original digital form so it can be provided to the customer's computer. Most DSL technologies (such as ADSL shown in this example) transmit the data information on frequencies about the audio channel. This allows for the simultaneous transmission of analog and data signals on the same telephone line. The highest frequencies are used transmission from the DSLAM to the DSL modem and frequencies just above the audio band are used to transmit from the data from the customer to the DSLAM. Typical DSL technology allows up to 6 Mbps to be transmitted to the customer and up to 640 kbps can be received from the customer.

Figure 1.2., IPTV Broadband Access Options

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Broadband Data

Broadband data is a term that is commonly associated with high-speed data transfer connections. When applied to consumer access networks, broadband often refers to data transmission rates of 1 Mbps or higher. When referred to LANs, MANs, or WANs, broadband data transmission rates are 45 Mbps or higher. 
Ultra broadband is a term that is commonly associated with very high-speed data transfer connections. When applied to consumer access networks, ultra broadband often refers to data transmission rates of 10 Mbps or higher.
Figure 1.2 shows the key types of broadband access providers that can be used to provide Internet television service. This diagram shows that some of the common types of broadband access systems that are 

available include powerline data distribution, cable modems, digital subscriber lines (DSL) and wireless local area network systems (3G wireless, WLAN, MMDS, and LMDS).

History of DSL

DSL service dramatically evolved in the mid 1990s due to the availability of new modulation technology and low cost electronic circuits that can do advanced signal processing (e.g., echo canceling and multiple channel demodulation). This has increased the data transmission capability of twisted pair copper wire to over 50 Mbps.
The data transmission capability of a DSL system varies based on the distance of the cable, type of cable used, and modulation technology. There are several different DSL technologies. Each of the DSL 

Figure 1.3., DSL Evolution

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technologies mixes different types of transmission technologies to satisfy a specific business need. Some DSL systems allow simultaneous digital and analog transmission and are compatible with analog POTS systems. 
   The first digital subscriber lines (DSLs) were developed due to the need for cost effective quality communication over copper wire. The first digital transmission system was the T1 line. This system had a maximum distance of approximately 6,000 feet and it required the use of repeaters for connections beyond 6,000 feet. 
   The T1 digital transmission system used a very complex form of digital transmission. A new high-speed digital subscriber line technology was developed to replace T1 transmission technology. HDSL systems increased the distance that high-speed digital signals could be transmitted without the user of a repeater/amplifier. The HDSL system did require 2 (or 3) pairs of wires to allow simultaneous (send and receive) up to 2 Mbps of data transmission. 
   To conserve the number of copper pairs for data transmission, symmetrical digital subscriber line (SDSL) technology was developed. Although SDSL systems offered lower data rates than HDSL, only 2 wire pairs were required. Since SDSL was developed, the HDSL system has evolved to a 2nd generation (HDSL2) that allows the use of 2 wire pairs for duplex transmission with reduced emissions (lower egress). 
   New efficient modulation technology used by ADSL systems dramatically increased the data transmission rates from the central office to the customer to over 6 Mbps (some ADSL systems to more than 8 Mbps). To take advantage of integrated services digital network (ISDN) equipment and efficiency, an offshoot of ISDN technology that was adapted for the local loop called ISDN digital subscriber line (IDSL). 
   Asymmetric digital subscriber line (ADSL) systems evolved to rate adaptive digital subscriber line (RADSL) allow the data rate to be automatically or manually changed by the service provider. To simplify the installation of consumer based DSL equipment, and low data transmission offshoot of ADSL developed that is called ADSL-Lite. 
Using similar technology as the ADSL system, very high-speed digital subscriber line (VDSL) was created to provide up to 52 Mbps data transfer rates over very short distances. The use of VDSL technologies is commonly used to interconnect systems or switches that are physically adjacent to each other (within a few hundred feet).
ADSL systems continued to evolve with the development of ADSL2. ADSL2 systems use a more advanced modulation technology over the 1.1 MHz of frequency bandwidth to increase the data transmission rate up to 12 Mbps (downstream).
   Asymmetric digital subscriber line 2+ (ADSL2+) is a double frequency band version of ADSL2. The ADSL2+ system increases the frequency ranges on the transmission line from 1.1 MHz to 2.2 MHz. This has the potential to increase the data transmission rate up to 24 


Mbps (downstream).
   Figure 1.3 shows the evolution of DSL systems. This diagram shows that high-speed digital subscriber line technology has been readily available since the 1970s. In the late 1990's, the addition of advanced signal processing technology allowed DSL technology to rapidly increase transmission speed to over 50 Mbps in short distances. 

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Introduction to DSL
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