Digital Subscriber Line Technology

Bellcore first introduced digital subscriber line (DSL) technology in 1989 as a way to transmit video and television signals from telephone companies' central offices to end users over standard copper cable used for voice service. At that time, video-on-demand was perceived as the broadband application that would drive digital subscriber line (DSL) implementation. Asymmetric digital subscriber line (ADSL) service was proposed because of its higher speeds for downloading large video files. The main application for DSL has changed from video-on-demand to Internet access.

DSL is a dedicated, high-speed way to access the Internet or corporate files via a virtual private network (VPN). (See Chapter 5 for VPNs.) It operates over the same copper cabling already installed by the incumbent regional and independent telephone companies. It is an “always on” service in which modems don't dial a telephone number to reach the Internet. There are no usage charges and customers are billed a flat monthly fee regardless of how much time they spend online. It is less costly than T-1 or fractional T-1.

Regional Bell Operating Companies introduced digital subscriber line (DSL) services in the late 1990s as a competitive response to cable modems. There is particular interest in DSL service by RBOCs because it leverages their investment in copper cable in the outside cabling plant. The Bell Operating Companies, network providers such as AT&T, and competitive local exchange carriers such as Covad, Inc. and Internet service providers such as Earthlink sell DSL.

DSL is offered in symmetric and asymmetric versions. Asymmetric DSL, such as ADSL, has different speeds downstream and upstream. Downstream transmissions are from the Internet to the consumer. Upstream transmissions are from the consumer to the Internet. Incumbent telephone companies generally sell asymmetric service to consumers who often download larger files such as music than they send. Versions of DSL services sold to businesses are symmetric. They provide the same speed to the user when they are sending and receiving files. It is felt that businesses send as well as receive large files. For the most part, DSL is sold to small and medium-sized businesses because it is lower in cost than T-1 and does not have the usage fees associated with BRI ISDN. Examples of symmetric DSL services are IDSL, SDSL, HDSL and HDSL2. See Table 6.4.

Table 6.4. DSL Speeds and Cable Requirements

Digital Subscriber Line Service	Upstream Data Rate	Downstream Data Rate	Top Distance from Central Office	Voice	Comments
ADSL Asymmetric DSL	176 Kbps[*] 640 Kbps[**]	1.54 Kbps 7.1 Mbps	18,000 feet 12,000 feet	Yes	Offered by telephone companies for customers who want speeds higher than DSL lite. Uses one pair of wires.
DSL Lite Splitterless DSL	384 Kbps	1 Mbps	18,000 feet	Yes	Easier installation by telcos. Splitter not required. Operates on one pair of wires.
G.shdsl	192 Kbps 2.3 Mbps 4.62 Mbps	192 Kbps 2.3 Mbps 4.62 Mbps	40,000 feet 6,500 feet 6,500 feet		Single pair of wires speeds up to 2.3 Mb. Two pair speeds up to 4.62 Mbps.
HDSL High-bit-rate DSL	1.54 Mbps	1.54 Mbps	12,000 feet	No	Requires four wires; other DSL services only need two wires.
HDSL2	1.54 Mbps	1.54 Mbps	12,000 feet	No	Same as HDSL except only one pair of wires.
VDSL Very high-bit-rate DSL	640 Kbps 3 Mb	13 Mb 52 Mb	4,500 feet 1,000 feet	Yes	VDSL requires fiber optics on distances higher than these.
SDSL Symmetric DSL	Up to 1.1 Mb	Up to 1.1 Mbps	24,000 feet	No	Requires only one pair of wires. Offered by carriers such as WorldCom and AT&T and Internet service providers.
RADSL Rate-adaptive DSL	128 Kbps 176 Kbps 1 Mb	640 Kbps 1.54 Kbps 2.56 Mbps	21,300 feet 18,000 feet 12,000 feet	Yes	This is a version of ADSL; the speed varies according to condition of the copper. Requires one pair of wires.
IDSL ISDN DSL	128 to 144 Kbps	128 to 144 Kbps	18,000 feet	No	Used when distance to the central office is too far for SDSL. Operates on a single pair of wires.

^[*] Kbps = kilobits per second

^[**] Mbps = million bits per second

A benefit of DSL technology is its potential to relieve congestion caused by modem traffic on the public network. When DSL traffic hits the central office, it is routed on a data network that is separate from the voice network. Digital subscriber line technology packetizes data traffic and sends it on a parallel data network. Modem traffic, on the other hand, is carried on the public network along with voice traffic. Modems convert digital data to analog so that local and long distance companies cannot distinguish it from voice calling.

Because there are multiple “flavors” of digital subscriber line technology, some people refer to DSL as xDSL. Most DSL services run at different speeds, require dissimilar types of customer interface equipment, are positioned for different types of customers and can run over variable lengths of copper cables. They are listed next; additional information was given in Table 6.4.

• Asymmetric digital subscriber line (ADSL) is asymmetric. Incumbent telephone companies sell it mainly to residential customers. Downstream speeds on data that is sent to the customer from the Internet and on upstream data sent away from the customer to the Internet are different. ADSL is also known as full-rate ADSL.

• Digital subscriber line lite (DSL Lite), also known as Universal DSL, is the ITU-based G.lite standard for residential consumers. It is lower in cost to provision because a splitter to separate the data transmission from the voice call is not needed in the central office and at the customer site.

• Single-Pair High-Speed digital subscriber line (G.shdsl) offers higher speeds and operates over longer distances than symmetric digital subscriber line service.

• High-bit-rate digital subscriber line (HDSL) is symmetric, with the same speed upstream and downstream—1.54 million bits per second. Although the same speed as T-1, it uses different signaling than T-1. It does not have the reliability guarantees associated with T-1.

• High-bit-rate digital subscriber line 2 (HDSL2) offers the same speed as HDSL; however, only one pair of wires is required. This is a savings in outside copper required. It is designed as a single-pair replacement for HDSL.

• Very high-bit-rate digital subscriber line (VDSL) requires a combination of fiber and copper cabling because of its high speed. It can be used in combination with fiber for fiber-to-the-curb applications where, for example, customers might want broadcast-quality video simultaneously with high-speed Internet access. To date, it is rarely used.

• Rate-adaptive digital subscriber line (RADSL) is a variation of ADSL that overcomes varying conditions and lengths of copper cable. RADSL speeds adjust downward to overcome variations in outside cabling. Qwest uses RADSL in the 14-state territory where it operates its local telephone company (formerly U S West) business.

• Symmetric digital subscriber line (SDSL) provides the same speed upstream and downstream to the customer. This is the service that is offered by CLECs, mainly to business customers. Speeds of 160 Kbps to 1.1 Mbps depend on distance from central office.

• Integrated services digital subscriber line (IDSL) works with the same customer equipment as ISDN. However, this is a dedicated service with a fixed monthly charge without usage fees. IDSL operates as a single 144 kilobit per second pipe. It has no channels for voice.

Competitive Local Exchange Carriers (CLECs) and DSL

DSL was the first product sold by CLECs that promised lower prices and innovative service for Internet access to small and medium-sized businesses as a result of the Telecommunications Act of 1996. They sold symmetric DSL service mainly to business rather than residential customers. Some CLECs known as Data CLECs did not sell directly to end users—they acted as wholesalers and sold through ISPs, other CLECs and long distance carriers. Other Data CLECs sold directly to end users. They leased copper local loops from incumbent telephone companies and installed digital subscriber line access multiplexers (DSLAMs) in their central office. (DSLAMs aggregate traffic from many DSL modems and send it to Internet service providers.) They also provided the DSL modem/routers to small and medium-sized businesses. Retailers that bought DSL wholesale from data CLECs billed end users, provided customer service and Internet services such as email and Web hosting.

However, CLECs were largely unsuccessful. A number of factors contributed to their decline. Many of the Data CLECs claimed they were driven out of the market by the high costs and delays associated with providing service over RBOC facilities. Some of them also cited the fact that capital dried up from sources such as venture capital companies and they were not able to meet expenses. In addition, many of the Internet service providers who resold their DSL service did not pay their bills. For example, by October 2000, of 274,000 DSL lines that Covad installed, ISPs had not paid for 92,000. Many of them had gone bankrupt. Because of the steep competition, DSL service was priced extremely low, making it difficult to achieve profits while building out the infrastructure.

Another problem facing CLECs is that, for the most part, RBOCs are not required to sell space in digital loop carriers (DLCs) to competitors. (Digital loop carriers are also referred to as remote terminals. See later section on digital loop carriers.) To date, only Illinois and New York require incumbents to provide space, where technically feasible, in remote terminals for competitors' DSL access multiplexer cards. The lack of access to DLCs is hampering competitors' ability to overcome long cabling distances between subscribers and central office located DSL multiplexers. The FCC is exploring mandates to require incumbents to allow access by competitors to DLCs. For now, access requirements in remote terminals are up to state regulators as long as they follow FCC guidelines for technical feasibility. Data CLECs were Covad, Rhythms and NorthPoint. NorthPoint and Rhythms are no longer in business and Covad is in bankruptcy. The FCC is studying the issue of CLEC access to incumbent-owned digital loop carriers. RBOCs are concerned that access by competitors will be costly to arrange because of possible space, venting and heat issues in the digital loop carriers.

Because of the poor market conditions and the difficulty of making a profit, many CLECs and other resellers such as ISP Verio have stopped selling DSL. There are small DSL providers still selling service and long distance providers such as AT&T, which purchased NorthPoint's assets, that are still in the market. However, the largest market share, about 80%, belongs to incumbent telephone companies.

Incumbent Telephone Company DSL Offerings

Bell telephone companies' products initially were sold only to residential customers. These were ADSL services that supported only one computer per DSL modem. These offerings only supported dynamic not static IP service. Static IP service is required for customers with their own domain name who want Web hosting from their DSL provider. The dynamic IP service does not support this service because IP addresses are assigned at random rather than to the same device consistently. Customers with their own Web pages and domain names have their Web hosting at a different provider than their DSL provider when their provider does not support static IP service.

RBOCs are now starting to sell symmetric DSL service for small and medium-sized businesses and small office home office (SOHO). The DSL product is part of their strategy of growing their data communications business. They all are investing billions of dollars into new infrastructure for data communications products. While CLEC competition for DSL service is greatly diminished, Frame Relay, VPN and Gigabit Ethernet services from other companies represent viable competition to them.

The RBOCs offer a wholesale DSL product to ISPs that wish to connect their customers via DSL. For example, Qwest (formerly U S West) has signed a contract with MSN. In this arrangement, MSN sells the DSL service to its customers and provides the email and Web hosting. However, the subscriber traffic is routed over Qwest DSL facilities. The RBOCs have formed separate subsidiaries to provide the email and connections to the Internet and customer support for DSL.

DSLAMs—Digital Subscriber Line Access Multiplexers

DSLAMs, or digital subscriber line access multiplexers, are located at network service providers' sites. DSLAMs aggregate traffic from multiple DSL modems and combine it into higher speeds before sending it to the Internet or data networks. (See Figure 6.7.) It sends it out at speeds of T-3, or 44 million bits per second, and ATM speeds of OC-3, or 155 million bits per second. DSLAMs are located at central offices, digital loop carriers in neighborhoods (see below) and in the wiring closets of large apartment and office buildings.

Some DSLAMs also perform concatenation. They send data from attached devices in one adjacent stream instead of as individual streams. For example, streams of video may be sent as linked streams rather than being separated by data from other DSL devices. If a DSLAM performs concatenation, its speed is called OC-3C, or concatenated optical carrier level 3.

Customers have dedicated capacity between their DSL modem and the digital subscriber line access multiplexer (DSLAM) but not between the DSLAM and the Internet or the ISP. The connection between the DSLAM and an Internet service provider or is a potential site for network congestion. (See Figure 6.7.) If not enough capacity is available, a customer might experience delays.

Obstacles to Digital Subscriber Line Availability—Cost, Ease of Implementation and Availability

In the article, “My Kingdom for a DSL Line,” published in SmartMoney.com, 27 March 2001, a Gartner Group study found that as of January 2001, DSL was only available in 35% of the United States. DSL service is available mostly in metropolitan areas of the United States. It also is sold in Western Europe and Asia. However, the largest number of users, over 2 million by May 2001, is in the United States.

Digital Loop Carriers with DSL Equipment to Overcome DSL Distance Limitations

The fact that DSL only works up to 18,000 feet from the central office is a key factor limiting its availability. This is particularly true in countries and areas with less dense populations. Moreover, copper, not a mix of copper and fiber, is required to the nearest DSL access equipment.

When subscribers are located far from the central office, telephone companies often install digital loop carriers (see Chapter 5) closer to customers. Digital loop carriers (DLC) located between the central office and customers, are connected to the central office with fiber cabling. The DLC converts the optical signal to electrical and transmits it to customers over copper cabling. However, DSL requires copper cabling between the DSL modem and the digital subscriber line access multiplexer (DSLAM). Therefore, in order for DSL to operate where digital loop carriers are installed, the DSLAM must be installed in the DLC. SBC, BellSouth and Verizon are investing large sums of money to upgrade DLCs and install DSL-compatible equipment in them. This will increase the availability of DSL in suburban areas where central offices are typically more than 18,000 from many customers. See Figure 6.8 for a DSL connected to central offices via digital loop carrier equipment. Digital loop carriers are also referred to as remote terminals.

A contributing factor to low and uneven availability of DSL services is the fact that loading coils and bridge taps must be removed from copper lines. (Loading coils boost the signal on analog copper telephone wires that are far from the central office. Bridge taps enable the same copper wire from the central office to feed multiple locations.) Telephone companies remove loading coils but not bridge taps because of the cost.

A New Standard for DSL that Works over Longer Distances—G.shdsl

The new (2001) G.shdsl, single-pair high-speed DSL standard works over longer copper local loops than traditional DSL service. It is a standard that supports many different speeds and services. It enables service providers to lower their costs of provisioning DSL by supporting longer distances and a variety of speeds. For example, central office G.shdsl equipment also supports the HDSL2 1.54 million–bit per second service. The service is rate-adaptive. It negotiates the highest speed that the local loop supports during the “handshake,” the service setup portion of a data transmission session. It also negotiates the type of service to provision during the handshake. For example, T-1, E-1, ISDN, ATM and IP services all are supported. Most current equipment is suitable for data and video. However, voice transmissions are supported in the standard.

End-User Installation Difficulties

In addition to affordability and availability, ease of installation for end users is a major issue. Telephone companies send out technicians to install the modem. They also have support lines with limited support. However, for the most part, it is up to end users to determine how to program their computer to work with DSL. Small and medium-sized businesses are likely to hire systems integrators if they don't have knowledgeable staff.

A Reputation for Poor Service

By January 2001, class-action suits accusing them of poor service had targeted Verizon, Southwestern Bell and BellSouth. In addition, various Web sites are devoted to stories about delays in installation and network failures. The telephone companies have been accused of overselling and not having resources to support customers. Part of the problem is that records of copper cabling are not always accurate. For example, a telephone company might tell a consumer that DSL is available to their home. However, when the technician goes out to install the service, he or she discovers that bridge taps are installed or that the customer is too far from the telephone company equipment to qualify for service.

Hopefully, as the service matures and as outside cabling plants are upgraded with DSL equipment closer to customers, these problems will be resolved.

DSL Lite—Lower Cost Service

DSL Lite was developed to lower the cost of provisioning DSL service. DSL Lite, also called Universal DSL, works consistently on longer telephone company loops, the distance between the telephone company equipment and the customer's premise. DSL Lite achieves speeds of 1.5 million bits per second downstream and 384 thousand bits per second upstream. These speeds can be reached up to three miles from telephone company equipment.

DSL Lite also is known as splitterless DSL. A splitter is not required at the central office or at the customer premise. With full-rate ADSL, a splitter sits between the customer demarcation and the ADSL line. A splitter also is located at the telephone company equipment. The splitter, which is the size of two cigarette packs, separates the low-frequency voice signals from the high-frequency data signals. The fact that a splitter is not required saves the telephone company from having to dispatch a technician for installation.

Another benefit of DSL Lite is that consumers do not have to change their internal wiring. However, filters might be required to be plugged into each telephone jack. Some telephones are not built to standards. In these cases, the telephones might add noise to the telephone cable. The noise interferes with the data transmission. In these cases, filters screen out signals above the 4-kilohertz (KHz) range so interference is eliminated.

DSL—A Technical Explanation

DSL technologies use unused frequencies available in standard telephone wire for data transmission. Voice communications are carried on only 4 kilohertz of the available frequency of copper wires. This leaves the rest of the frequencies on copper cabling available for data communications. The American National Standards Institute (ANSI) and the European Telecommunications Standards Institute (ETSI) standard for ADSL and ADSL Lite is called T1.413. The T1.143 standard endorses discrete multitone modulation (DMT), a line-coding technique, for DSL modems. Modulation varies the frequency of data to be sent over telephone lines so that it is compatible with the provider's network.

DSL modems send data over copper cabling at frequencies above 4000 cycles per second. Discrete Multi-tone modulation (DMT) encodes and compresses data signals into 256 subchannels in increments of 32 kilobits. DMT essentially sends data over 256 discrete frequencies at the same time. It uses different frequencies, or subchannels, for the upstream portion to the network and other subchannels (frequencies) for the downstream transmission away from the carrier's network to the customer.

In addition to line coding and compression, DSL modems have other functions. Three of these functions are:

• Error correction

• Performance monitoring

• Routing

Routing functionality enables multiple personal computers to share one DSL line. All users can be online simultaneously using one DSL connection and a DSL modem with a built-in router. The connection from the modem to the computer is a 10/100base-T Ethernet connection (standards for 10 and 100 megabit per second transmissions over copper wiring).