Q1: What is DSL?
A1: DSL stands for Digital Subscriber Line.  DSL is a family of technologies (and an even larger family of products).  The family designation is often xDSL, where x can take on any one of several letters.  DSL technology increases the information-carrying ability of traditional copper wires.  DSL enables high-speed multimedia services such as video-on-demand, very high-speed Internet access, distance learning, remote corporate LAN access, and videoconferencing for anyone with access to a clean copper line.  While wires carrying analog signals are generally limited to 56 kilobits per second, once the switch to digital is made and DSL technology is employed, these same wires can support data rates well over a megabit per second.

Q2: How does ADSL work?
A2: ADSL allows you to take advantage of the unused bandwidth that already exists on your phone line.  ADSL enables one to use the remaining 99 percent of a regular phone line to transport high speed data, television, dense graphics, and movies without interfering with regular phone services.  This means that one could enjoy fast, permanent connection to the Internet while talking on the phone.

Q3: How does an end user get DSL?
A3: Most small office/home office and remote office customers sign up for DSL services with either the Incumbent Local Exchange Carriers (ILEC) or a Competitive Local Exchange Carrier (CLEC).  All of the major ILECs (also known as Regional Bell Operating Companies or RBOCs in many places) such as Bell Atlantic, BellSouth, SBC, and US West have announced major DSL initiatives which should begin to roll out by mid-1999.  In some instances, the Regional Bell Operating Companies are offering DSL service directly to end users.  In many communities CLECs have teamed with Internet Service Providers (ISPs) and have begun to take an active role marketing DSL to the public as a high-speed Internet service.  CLECs are also marketing directly to corporations that are interested in providing high-speed access to the corporate LAN.  The availability of DSL services from either ILEC or CLEC providers is very dependent on geographic location.  In some cases, corporate customers may use DSL technology themselves to connect up a campus-wide LAN, essentially extending an Ethernet network across large distances.

Q4: How much does DSL cost?
A4: The price varies and one can find out from his/her your ISP.  Although DSL service availability is still in the early stages, pricing in some area has been very aggressive.

Q5: How fast is ADSL?
A5: ADSL, depending on an user's individual line condition, its design, and line length, can offer speeds up to 9Mbps downstream (to the user) and up to 800Kbps upstream.

Q6: How do analog modems, cable modems, ISDN, and DSL compare?
A6: Analog modems use a telephone network as is.  Analog modems will complete a digital to analog conversion for transmission onto the network and vice versa on the receiving end.  Analog modem connection is the most ever-present form of data communications available today since each of the calls has to have a compatible modem.  Currently, the fastest speed for analog modem is 56Kbps while the slowest DSL connection is 144Kbps.

Cable modems provide high-speed, high-bandwidth Internet access over cable TV (CATV) networks.  However, the bandwidth is shared by all users on a particular cable segment.  Because cable networks use a bus topology, every cable modem in a neighborhood shares access to a single coaxial cable backbone.  The more subscribers in  that area, the less bandwidth available to each.  Thus the slower the connection rate and the more difficult it may become to establish a connection.  Another concern is that if the CATV is damaged, this could bring down all of the users on that line.  Cable modems provide a dedicated service over a shared media.  Cable modem Internet access is shared by many homes by sharing the same "line" to and from the Internet.  It offers high-speed, high-bandwidth Internet access over cable TV (CATV) networks.

ISDN (Integrated Services Digital Network) is a telephone company technology for digital transmission over an ordinary telephone copper wire as well as to other media.  ISDN typically provides digital service  in increments of 64Kbps channels with a maximum of 128Kbps.  ISDN requires adapters at both ends of the transmission so your access provider also needs an ISDN adapter.  ISDN requires the phone company to install services within their phone switches to support this digitally switched connection service.

DSL uses the existing ordinary copper telephone wiring to provide next generation high bandwidth services to the home and businesses.  DSL technologies offer a wider range of frequencies over the cable than traditional telephone services which in turn allow for greater bandwidth with which to send and receive information.  DSL is able to download data up to 1.5Mbits/s (G.Lite).

Q7: What is G.Lite and why is it getting so much attention?
A7: G.Lite is an industry term referring to ADSL technology that provides for data rates up to 1.5Mbits/s downstream and 512 kbits/s upstream.  ADSL is oriented toward the consumer market.  G. Lite ADSL dramatically improves consumers' internet experiences with the ability to deliver "always-on" internet access at high speed without any additional wiring.  With G. Lite ADSL, the existing analog phone will work simultaneously without a splitter.  (See Q8)

G.Lite originated from the Universal ADSL Working Group (UAWG).  This group, led by Compaq, Intel, and Microsoft, includes Lucent as well as many local exchange carriers (LEC) and other ADSL technology leaders.  The UAWG is advocating a mass market technology that does not require a residential splitter box or new wiring in the home, thus simplifying installation and promoting a plug-and-play solution.  The goal of the group is to accelerate an open, interoperable standard through the ITU and to encourage worldwide deployment.  This goal is being realized since the ITU has ratified G.992.2, the official ITU designation for G.Lite.

Q8: What is a splitter?
A8: A splitter is used to separate the POTS (plain ordinary telephone service) from the ADSL data services.  Full-rate T1.413 ADSL requires a sophisticated system of high- and low-pass filters to separate the voice and data services.  G.Lite, on the other hand, does not require a splitter at the client end and only a simple implementation at the central site.

Q9: How do I determine how far I am from my CO?
A9: The telephone company should be able to calculate the distance in airline and/or wire feet.  The closer you are the CO, the higher the speeds you can achieve using DSL.

Q10: Does DSL provide regular phone service also?
A10: Some ISPs and ILECS provide POTS service on the DSL line.  It requires that one install a small filter in front of each piece of equipment to be attached to the DSL line, such as a fax machine, telephone, or analog modem into the DSL line in addition to the DSL modem.  However, some ISPs do not provide POTS service on their DSL lines and will use a new line for DSL service only.

Q11: What hardware do I need to use DSL?
A11: An ADSL or other type of DSL line and a modem installed in your computer.  One will also need a POTS splitter in order to use POTS if the telephone company chosen provides POTS service if you are using ADSL Heavy.  G.Lite ADSL does not require a splitter.

Q12: Are there any DSL standards?
A12: Yes.  The International Telecommunications Union (ITU) recently approved six separate standards for DSL.  The two most important are G.992.1 and G.992.2.  G.992.2, also known as ADSL.Lite or G.Lite, is a splitterless version of the full ADSL standard (G.992.1), and is a compatible subset.  Both are based on DMT line coding, which was approved several years ago by ANSI as a standard known as T1.413.  Some of the equipment coming to market will be able to support both ADSL and ADSL.Lite.  The purpose of standards is to eventually provide a market situation where equipment from different vendors is interoperable, much like the situation today for data and fax communications.

Early trials of ADSL used CAP line coding and were not based on any standards.  There are also no standards for HDSL or VDSL, so equipment from different vendors will not interoperate.  The ITU is currently working on a standard for HDSL-2.

Q13: What DSL technology is being marketed to small businesses?
A13: Generally, DSL services to small businesses are marketed as high-speed Internet access.  These services are sold by ISPs, by competitive local exchange carriers (CLECs), and by the well known RBOCs, depending on the exact location.  In some cases the DSL service is marketed to businesses to enable high-speed, dedicated LAN connections for remote workers.  The RBOCs have been trialing proprietary versions of ADSL, but are moving towards ADSL.lite (G.lite) in 1999.  From the small and medium user's perspective, DSL services can look very attractive, especially when compared to traditional T1 connections.

Q14: Are there other types of DSL available?
A14: The complete picture of DSL gets complicated.  But in essence there are four basic members of the DSL family.

Name	Description	Data Rate	Comments
IDSL	ISP Digital Subscriber Line	160 Kbps	Provides two 64 Kbps channels and two 16 Kbps channels, one dedicated to signaling.  IDSL systems can use standard ISDN terminal equipment.  Cannot coexist on the same line as POTS.
HDSL	High bit rate DSL	1.544 Mbps	HDSL is widely used as the technology to deploy T1 circuits faster, more reliably, and at lower cost.  HDSL uses two pair of wires to deliver 1.5 Mbps in both directions.  A new version, HDSL-2 is emerging and will be standardized by committee.  HDSL-2 uses a new line coding and provides T1 speeds on a single pair of wires.  SDSL (symmetrical, single line DSL) is a closely related cousin that supports multirate services, hence sometimes called MDSL.
ADSL	Asymmetrical DSL	1-8.1 Mbps downstream  32-640 Kbps upstream  1.5 Mbps downstream (lite)  512 Kbps upstream (lite)	Speeds are distant-dependent and line quality-dependent.  Can co-exist with POTS service on a single twisted pair line.  Full rate ADSL (G.992.1) requires a splitter on the client premises.  A splitterless version (G.992.2) provides lower data rates.  Both standards use DMT line coding.  While ADSL is inherently rate adaptive, some companies call their ADSL products RADSL or RDSL to designate this.
VDSL	Very high bit rate DSL	13-52 Mbps downstream  2-52 Mbps upstream	Can be symmetrical or asymmetrical.  Runs over very short copper lines, hence requires much deeper reach of fiber.

In addition, there are three variations of line coding techniques used to spread high bit rate information over available spectrum within the twisted-pair wiring.  The line code situation is largely behind the scenes, but knowing the difference helps to explain the product proliferation.  The three line codes in use today are carrierless amplitude/phase modulation (CAP), discrete multitone (DMT), and two binary-one quaternary (2B1Q).  CAP and DMT uses techniques similar to those used in telephone modems, while 2B1Q is an all digital pulse coding method used in ISDN.  HDSL-2 will use a more sophisticated coding technique similar to 2B1Q.

Q15: What's all the fuss about splitterless DSL?
A15: One of the founding principles behind ADSL was the ability to use the already-installed copper wiring.  For customers where ADSL is adding high-speed data delivery on top of voice services, full rate ADSL is adding high-speed data delivery on top of voice services, full rate ADSL requires a splitter be installed at the network demarcation line, typically the side of the home or the building wiring closet (Data-only services over a separate twisted pair do not need a splitter).  In addition, full rate ADSL typically requires new internal wiring from the POTS splitter to the data devices.  The result is an installation obstacle; the phone company has to roll a truck and service technician to the customer site to install the splitter, thereby adding to the installation costs and time delays.  In addition, the customer must arrange for new internal wiring to connect up the data equipment.  Splitterless DSL does away with all of this.  By using ADSL.lite (G.lite), the same wire can carry voice and data without the use of a splitter in the customer premises.  Any phone jack in the home or office can be used for either voice or data services.  This set-up eliminates the need for the phone company truck roll as well as any new internal wiring.  The trade-off is the speed is limited to 1.5 Megabits per second.

Q16: What are the reseller opportunities presented by DSL?
A16: The DSL world is quickly moving to standards-based products.  This is changing the name of the selling game quickly.  Standards-based products will move through the same channels as telephone modems.  In fact, many future DSL modems will support V.90 analog modem functions as well.  DSL modems will be available both through traditional retail channels and will be bundled by platform OEMs into their performance class PC systems.  And DSL modems will be carried by value added partners and integrators who can install networks, routers, and hubs and connect business-computing resources to DSL networks provisioned by service providers.  In many cases, attractive opportunities will open up because resellers will be able to offer low cost DSL-based alternatives to T1 services.

Q17: What does a DSL network look like?
A17: The diagram below depicts several important possible configurations for an ADSL installation.  Service to the home in this example is ADSL.lite and the twisted-pair wiring to/from the central office is carrying both voice and data.  No POTS splitter at the customer premises is needed to separate the low frequency voice signals from the high frequency data signals.  The telephone system in the home is not altered in any way and the computer connection is made to any existing phone jack.  Inside the central office, a POTS splitter after the main distribution frame (MDF) sends the voice signal off to the voice switch while the data signals are aggregated by a DSL Access Multiplexer (DSLAM) and then sent via a switch or router off to the Internet Backbone.

The office application shown in the diagram is a data-only service, although many businesses may find it attractive to have voice and data on the same line, similar to the home example.  In this example, the DSL modem is connected to an Ethernet switch or hub and several PCs are thereby connected to high-speed data services.  At the central office, the MDF jumpers the subscriber line directly to the DSLAM.  Note, in many cases the DSLAM need not be located in the central office.  Many competitive local exchange carriers (CLECs) have been unable to obtain co-location privileges and are providing high-speed data services by installing their DSLAMs close to the telephone company's central office.  CLECs still use common twisted-pair wiring to provide their services.