Digital Subscriber Line (DSL)

DSL (digital subscriber line), cable modem, and other high-speed technologies should be considered in areas where they are available.

DSL services are growing rapidly as local access carriers continue to install DSL access modules ("DSLAMs"). Most service providers break down their DSL services into three main categories: residential, SOHO (small office/home office), and enterprise. ADSL (asymmetric DSL, where up- and downstream speeds differ) appears to be the offering of choice for residential customers, while SDSL (symmetric DSL) is usually marketed to businesses because it has T1 or more speeds both ways.

Depending on the geographic location, installation time for DSL circuits can range anywhere from one week to over ten weeks. Quality of service (QoS) is an issue. Most service providers, because of the multiple risk factors, do not guarantee QoS with DSL service. For example, DSL can potentially be unavailable for a few hours or longer. However, unlike cable, DSL provides consistent bandwidth to the user and does not depend on how many other (unrelated) customers are using the service at any one time.

DSL, more prevalent than cable modem, has a number of potential advantages:

• Internet access for smaller offices.

• Backup Internet connection. Many organizations implement DSL as a fail-over device because it is so inexpensive compared to a T1. Although costs vary by provider, a business typically pays at least twice as much and sometimes three or four times as much every month for a T1 line as it would for DSL. In other words, for the cost of a single T1 1.54 Mbps connection, three 1.1 Mbps DSL connections can be supported.

• Primary connection for the Internet and data services (note: many organizations do not consider DSL sufficiently robust for their primary link, but if cost is the primary consideration, DSL will provide the necessary functionality).

• Decreased installation charges. Typically, installation costs for a T1 line are three to four times as much as installation and setup of DSL services. DSL uses traditional telephone lines as opposed to T1, which requires installation of special (conditioned) lines.

• Multiple pricing categories. Depending on needs, more or less bandwidth (line speeds) can be purchased (if available in the area).

• Combined data and voice services over a single connection (for small offices). Although not yet widely deployed, Voice-over-DSL (VoDSL), shown in Exhibit 1, is starting to be implemented in selected locations in the United States, such as Santa Clara, California, and Boston, Massachusetts. By combining multiple voice channels and data on one copper wire, local telecom companies can offer a competitive package to small businesses that need only Internet access and a few voice lines. Jim Greenberg, chief architect at Rhythms NetConnections Inc., estimates small businesses could save about 30 to 40 percent on additional voice lines and get it all from one company.

  Exhibit 1: Voice-over-DSL (VoDSL)

  
  

  
  

• An "always-on" technology, unlike ISDN, which requires a sign-on.

• More secure than cable modem, because the bandwidth is not shared with other users (due to DSL's copper wire to the user legacy ar architecture — dedicated to a single user).

Disadvantages include:

• DSL installation, while generally faster and cheaper than T1 or T3 installation, may experience technical problems. DSL runs over lines designed prior to World War I. It was originally intended to carry only miniscule traffic. For such a scrawny system to shoulder mountains of Internet data is akin to one writer's quip about a dog walking on its hind legs — "It is not done well, but you are surprised to find it done at all." Exhibit 2 shows an analysis of the percent of DSL installations completed versus time required for the install.

  Exhibit 2: DSL Installation History: Percentage of Installs over Time of Order

  
  

  
  
  
  

• Multiple parties are involved. Typically, when a local telephone company, ISP, and possibly a DSL provisioning company are involved at some point in providing the service, the potential for billing errors and increased repair time is greater.

• DSL bandwidth varies considerably. The distance from the POP largely determines the bandwidth that is available to a DSL customer. The distance limitation is usually considered 18,000 feet but "loop extenders" from companies like Symmetricom can extend the distance considerably. Exhibit 3 shows the relationship between bandwidth and distance from the central office.

  Exhibit 3: Distance versus Bandwidth for DSL

  
  

  
  
  
  
  

DSL and cable modems: High speed and low cost

New data networking services
New technologies, such as the Internet and video-on-demand, have caused a strong hunger in the marketplace for more bandwidth. Everybody wants to send more data at faster speeds. Many large companies pay for a T-1 connection to the Internet, but smaller businesses do not spend their money as freely. Most residences and many small businesses are far away from the telephone company central office, so it is too costly for carriers to offer advanced data services at an affordable price. The little guys have been left out—until recently.

DSL and cable modems: High speed and low cost
During the last few years, the phone companies have looked for new ways to offer high bandwidth services to small businesses and consumers. The two most prominent technologies that have recently stormed the market are digital subscriber line (DSL) service and cable modems. Both services offer bandwidth up to 1.544 Mbps for less than $100 per month. That means a consumer can get T-1 bandwidth without paying $1,000 a month for it.

DSL service is being widely adopted by both small businesses and consumers. Cable modems have been most attractive to consumers, probably because most homes are already wired for cable TV service.

Both DSL and cable modem service are dedicated connections. The line is always available for Internet use. Cable modems do not have to “dial-up” the specific ISP; they are always connected to the ISP. In addition to unlimited Internet access, most DSL and cable modem service providers give their customers e-mail accounts and Web site hosting as part of the monthly service.

According to Computer Economics, cable modem subscribers in the United States will increase from 5.7 million in 2000 to 27.6 million in 2005. DSL subscribers are expected to increase from 2.4 million in 2000 to 13.8 million in 2005.

DSL
As previously explained, the “last mile” of copper wiring from the telephone company’s central office to a business or residence has limited bandwidth capacity. End users demand lots of bandwidth, and phone companies want to earn revenue from this opportunity. DSL technology is a recent development that should satisfy both consumers and phone companies.

DSL deployment began in 1998. Since then, both computer manufacturers and telephone companies have not yet ironed out a single standard for DSL service. Consequently, numerous flavors of DSL are being offered today. The whole family of DSL services is often referred to as xDSL, with the “x” representing any number of other letters such as ADSL, CDSL, UDSL, VDSL, G.Lite, DSLLite, and freeDSL.

DSL offers a lot of bandwidth to small companies and consumers at phenomenally low rates. A typical DSL customer can receive 1.544-Mbps bandwidth across an ordinary copper telephone line for around $50 per month. That amount of bandwidth has previously only been available to businesses that paid as much as $1,500 per month for T-1 service.

DSL technology sends a digital signal across a traditional twisted-pair copper telephone line. Because the signal is never converted to analog, greater bandwidth is available. Like ISDN, DSL service can carry both voice and data simultaneously. A person can surf the Internet and talk on the phone at the same time. DSL uses a dedicated connection to the Internet.

Upstream and downstream
DSL has separate transmission rates for “upstream” and “downstream” data. A person surfing the Internet will receive large amounts of data “downstream” from the Web site because of the numerous graphics and files. The amount of data sent “upstream” is minimal, because the Internet surfer is only sending mouse clicks or occasional keystrokes. DSL service typically offers upstream rates of 128 Kbps, and downstream rates of 1.544 Mbps. Figure 1 illustrates DSL’s differing rates for upstream and downstream traffic.


Figure 1: DSL service uses standard copper telephone wires but can deliver T-1 bandwidth downstream from the carrier.

A DSL customer that wants to use his telephone and computer on the same line must have the signal separated so that the bandwidth can accommodate the phone’s analog signal and the computer’s digital signal.

DSL is available across the United States in most large and mid-size cities. Flashcom, one of the largest DSL providers, offers service for $49 per month. This includes Internet access, and if the customer signs a 24-month term agreement, the equipment is free and installation fees are waived. In Missouri, SBC provides DSL service for $39 a month, or $49 a month with Internet access included. A 12-month term agreement is required with this pricing, and the customer must purchase a “DSL modem” for $198. DSL is a flat-rate service; there are no monthly charges for usage.

Save money with DSL
Besides providing a large amount of bandwidth for Internet usage, DSL can also be used to lower existing telecom costs. A small independent insurance agency in Baltimore, for example, recently signed up for DSL service with a national DSL provider. The service provided a 512-Kbps dedicated connection to the Internet, e-mail, and Web site hosting, and the business could still make analog phone calls over the same line. The business previously paid more than $300 per month for all of these services. See Table 19.1 for a cost comparison of this change.

Cable modems
Cable modems are designed for high-speed Internet connections. Like DSL service, cable modems provide Internet access at different speeds downstream and upstream. The actual bandwidth for Internet connections over a cable line is 27 Mbps downstream and 2.5 Mbps upstream, but the total rate experienced by the end user is normally 1.544 Mbps. That is not too bad, though, considering that today many businesses pay more than $1,000 a month for this amount of bandwidth.

A new cable modem customer needs a service provider and the cable modem itself. Most service providers allow the customer to rent a cable modem; otherwise, the customer must pay around $300 for the device. With Time Warner’s Road Runner service, the subscriber pays a one-time installation fee of $100 and a monthly fee of $40. The monthly fee includes rental of the cable modem and unlimited Internet access. Cable modem users do not pay hourly fees for Internet use.



SONET and DWDM
Three basic types of data networking in use today: dedicated private lines, circuit switching, and packet switching. Specific services that use these technologies include ISDN, frame relay, ATM, and DSL. These services are commonly used by businesses.

Two other data networking technologies bear mentioning here: synchronous optical network (SONET) and dense wave division multiplexing (DWDM). Both SONET and DWDM are technologies used for transmitting data across fiber-optic lines. SONET and DWDM are used within carrier networks and rarely in a customer’s network. A single SONET connection is capable of simultaneously carrying 129,000 conversations.