Data Transmission over Telephone Network
Facts About DSL
DSL is fast - DSL modems are much faster than analog modems. Each type of DSL provides different maximum speeds, from twice as fast to approximately 125 times faster than a 56.6K analog modem. The only speed limit with DSL is the speed of the Internet and all the different computers attached to it.
DSL doesn't tie up your phone line - DSL doesn't interfere with phone calls, even though it uses your regular phone line. What this means is that you can be on the Internet and you can pick up the phone and make a phone call on the same line. With DSL, you won't have to worry about missing calls, or logging off the Internet to order a pizza, and then logging back on when you're done with the call.
DSL is always available - Your DSL connection is always there. There's no need to dial up and listen to your modem squawk every time you want to do something online. And there's no frustration about the line dropping when you're in the middle of browsing or downloading. Want to check your e-mail? Set up your computer to check for new e-mail and notify you when you receive something instead of logging in and checking it yourself. Want to look at just one Web page? Just open your browser and look.
DSL is reliable - Phone company networks are among the most reliable in the world, experiencing only minutes of downtime each year.
Types of DSL
Asymmetric DSL (ADSL) reserves more bandwidth going downstream to the user and less going to the Internet. Imagine you're caught in a traffic jam on a highway during rush hour. What if you could take some of the lanes that are not as congested and switch their direction to ease the traffic jam on your side? This is exactly how Asymmetric DSL works—using more bandwidth for downloads to the user and less bandwidth for uploads from the user to the Internet. It is most attractive to Internet surfers and users of remote LANs, because they typically download much more data than they send.
G.dmt ADSL (also known as Full-rate ADSL) is the other standard for home DSL service. The G.dmt variety can download data at up to 8 Megabits per second, and send data upstream at up to 1.5 Megabits per second, if the modem is located within 10,000-12,000 feet of the phone company's CO (central office). Up to 18,000 feet away from the CO, G.dmt ADSL can reach up to 1.5 Megabits per second downstream. This type of DSL may require the telephone company to install a device called a "splitter" on the phone line, requiring an installation visit to your home. The big difference between G.dmt and G.lite (discussed below) is speed.
G.lite ADSL (also known as universal ADSL) is a new standard for DSL service that became available in mid-to-late 1999. The cost for equipment and service will be less than other varieties. It will also be easier to install than other varieties—you will be able to do it yourself. It is based on ADSL, and offers downstream speeds up to 1.5 Megabits per second and a maximum upstream data rate of 384 Kilobits per second. The major downside to G.lite ADSL is that if it is unacceptable for voice and entertainment applications.
RADSL (Rate Adaptive Asymmetric Digital Subscriber Line) - RADSL operates at the same bandwidths as ADSL—up to 7 Megabits per second downstream and up to 1.5 Megabits per second upstream, with the additional capability of adjusting bandwidth to the quality of the phone line during the data transmission, instead of just once at the start of the connection.
Symmetric DSL provides the same rate both ways, and is suited more to Web servers, corporate networks, and those who send out large quantities of data.
HDSL (High Bit-Rate Digital Subscriber Line) - HDSL is the most established of the DSL technologies. It is symmetric, with a maximum 1.5 Megabits per second traveling both ways over two copper phone lines, or 2 Megabits per second over three phone lines. It is often utilized as an alternative to T1 connections. (A T1 connection is a high speed, dedicated telephone line offering 1.54 Megabits per second of data transfer.) HDSL is limited to a distance of 12,000-15,000 feet. This range can be extended with the use of signal repeaters.
HDSL II (High Bit-Rate Digital Subscriber Line II) - HDSL II offers the same performance as HDSL, but over a single phone line.
SDSL (Symmetric Digital Subscriber Line) - SDSL offers a symmetric transmission of data at the same speed as HDSL, with two important differences: it can be done using only one phone line and the user must be no more than 10,000 feet from the phone company's central office. SDSL is the forerunner to HDSL II.
IDSL (ISDN Digital Subscriber Line) - IDSL is a hybrid of DSL and ISDN technologies. It uses the same data encoding technique of ISDN devices and delivers up to 144 Kilobits per second bandwidth. The difference between the two is that IDSL bypasses the congested phone network and uses the data network instead. Also, there is no call setup delay like you experience with ISDN connections.
VDSL (Very High Bit-Rate Digital Subscriber Line) - Currently in its experimental phase, VDSL is the fastest DSL technology, with rates from 13 to 52 Megabits per second downstream and 1.5 to 2.3 Megabits per second upstream. The tradeoff for this speed is that the maximum distance from the central office to the user must be between 1,000 and 4,500 feet.
DSL Service Providers
Because DSL uses phone lines, many of the same companies that provide Internet access with an analog modem will offer DSL service. There are three types of providers: 1) traditional telephone companies, 2) new, competitive telephone companies and, 3) ISPs (Internet service providers).
Traditional telephone companies include all the companies that used to provide telephone service in one area on a monopoly basis (also known as Incubent Local Exchange Carriers or ILECs). These include huge companies such as Southwestern Bell, Pacific Bell, Bell Atlantic, BellSouth, USWest, Ameritech and GTE, as well as very small companies that serve a single service area. The larger phone companies, as well as many of the smaller ones, offer DSL in at least part of their service area, with coverage areas increasing every month.
The new, competitive telephone companies (created after The Telecommunications Act of 1996 was passed) that compete to offer local phone services will also offer DSL service (also known as Competitive Local Exchange Carriers or CLECs).
Internet service providers (ISPs) provide access to the Internet. The ISPs that offer DSL usually don't own the equipment that makes the service possible. Instead, they buy the service from a traditional phone company or one of the newer competitive ones. The distinctions between telephone companies and Internet service providers are already blurred because ISPs can also be telephone companies. Also, many telephone companies sell Internet access. The terms NSP (network service provider) and USP (universal service provider) are coming into use to describe these companies that sell many different communication services.