This is the Frequently Asked Questions (FAQ) section for ADSL.
A: It depends. Early on, the major factor should be availability of the service in your area. Most areas will have only one or the other. This should change in early 1999 to most areas having both offerings. Image that ... Cable Companies vs. Phone Companies, kinda like Godzilla vs. Rodan, well, you get the picture.
If you want a dedicated, high-speed connection with predictable bandwidth, and a dedicated IP address, then ADSL is your service. However, if are concerned with cost and don't mind the shared network thing, then Cable Modems may turn out to be a bit cheaper.
My belief is that Cable Modems will make some early inroads, but that ADSL will ultimately blow them out of the water, as long as the phone companies don't overprice it.
A: Sort of. The U.S. Bell Operating Companies have standardized on Discrete Multi-Tone (DMT) ANTs (ANSI T1.413) in their current rollout. Most others should follow their lead in the states. There are other types of ANTs, most notably Carrier-less Amplitude Phase Modulation (CAP), which, of course, are incompatible with each other.
A biased comparison from an DMT-based vendor on this subject can be found at the Aware. Still, it provides the best detail on this issue I have seen so far.
A rather expensive copy of the ANSI standard can be ordered at:
American National Standards Institute
ANSI Home Page
Asymmetric Digital Subscriber Line (ADSL) Metallic Interface
ANSI TI.413-1995
Note: ANSI TI.413 Issue 2 was released September 26, 1997
A: Yes, you can! Some ADSL ANT (at least the Alcatel version) has a ATM Forum 25Mbps interface, which connects to a PCI NIC card. However, I have not yet heard of any Linux drivers for such cards.
A: The basic problem is the 100 year old design of the copper loop. It works great for analog phone, but it presents a real challenge for a digital signal. Remember that the distance of a loop is inversely proportional to the data rate that it can carry. Rate-Adaptive technologies are great for making a digital signal work in many situations, but it can't provide a consistent bandwidth for all applications, especially for very long (over 18 kilofeet) loops. The different bandwidth that you see advertised reflect various marketing wars of vendors equipment, and the Telco struggle to finalize on a ''standard'' set of data rates. I think that the 384k/1.5Mbps will become the standard for now. The high bit rates will only be available for special application and/or situations, since they can only be provided on a small percentage of the available loops.
Also, check out the next question on the loop imparments that cause this to happen.
Load coils: in-line inductances that improve voice-frequency transmission characteristics of a telephone circuit. Essentially, a "load" steals energy from high frequencies and gives it to lower frequencies. Typically only used in very long (>9,000 ft) phone lines.
By "bridges" I assume you mean "bridged taps". In older neighborhoods, the phone wiring will have been used by more than one customer. Perhaps these customers lived at different (though near-by) addresses. The unconnected "spur" of wiring is a "bridged tab" on the currently connected circuit.
Digital loop carriers: there's a bunch of systems for carrying more than one voice transmission on a single pair of wires. You can shift the frequencies up or down, or you can digitize the voice transmissions and divide the telephone circuit by time or code or something. The more general term is "pair gain".
These things cause different problems for high-frequency communication.
Loads will completely mess up things by filtering high frequencies and passing low frequencies. They probably also change the "delay envelope", allowing some frequencies to arrive before others. One byte's tones will interfere with the next byte's.
Bridged taps act as shunt capacitances if they're long in relation to the signals wavelength, and they'll actually act as band pass filters if they're about 1/4 wavelength of the signal. That is, they'll pass particular frequencies freely. Particular tones of a DMT modem might get shunted back, rather than passed along to the receiving modem, reducing bandwidth for that telephone line.
Pair gain, digital or analog, limit the bandwidth available to one transmission in order to multiplex several on one wire. High and low tones of a DMT transmission get filtered out by the apparatus.
The book "Subscriber Loop Signaling and Transmission Handbook", by Whitham D. Reeve, , IEEE Press 1992, ISBN 0-87942-274-2 covers the math of how to calculate the effect of line length, bridged tap, etc on the transmission characteristics of a telephone line. It's pretty expensive, however.
A: Short Answer: Yes. Real Answer: The evolution of this technology is moving too rapidly for anyone to keep up to date in a HOWTO. A good source of ADSL ANTs is the ADSL Forum Home Page . Go to the Vendors pages to see what's happening.
However, I will provide a list of some of the current technology as of June 1998.
Examples: Flowpoint 2000 DSL (CAP), Netspeed Speedrunner 202 (CAP), Speedrunner 204 (CAP), 3COM Viper-DSL (CAP), StarNet Ezlink 500/100 (DMT), Westell ATU-R-Flexcap (CAP), Aware x200
Examples: Alcatel A1000 (DMT), Westell ATU-R-Flexcap2 (CAP)
Examples: Alcatel A1000 (DMT), Netspeed Speedrunner 203 (CAP), Ariel Horizon II
Examples: Westell ATU-R
Rumored to being pushed by Intel.
Examples: Netspeed PCI Runner (CAP), Efficient Networks Speedstream 3020 (DMT)
These are NOT endorsements of the products listed, just provided for illustration.;-).
Thanks to all those that contributed information to this HOWTO. I have anti-spammed their email addresses for their safety (and mine!). Remove the X's from their names.
A dictionary of some of the jargon I use in this FAQ.
The two wire twisted pair from the Telco Central Office that terminates at a customer location.
Asymmetric Digital Subscriber Line
ADSL Network Termination (a.k.a. the ADSL modem)
Asynchronous Transfer Mode - provides high-speed packet switching from 155 Mbps to (currently) 2Gbps. Used to provide backbone switching for the Internet.
ATM Forum Interface - 25Mbps speed, provided by a PCI NIC card.. One of the interfaces used between the ANT and PC.
Usually refers to one of two meanings -1) The Telco Building that houses Telephone equipment 2) The Telco Voice Switch that provides dial tone.
Customer Premises Equipment - The Telco term for customer equipment (i.e. the stuff you are responsible for fixing). Examples are CSU/DSU, modems, ANTs, and your phone.
Dynamic Host Configuration Protocol - The IP protocol used to set up dynamically assigned IP addresses.
The basic digital circuit for Telcos - offered at 56 kbps or 64kbps. Can support one analog voice channel.
Digital Subscriber Line Access Multiplexer - The Telco equipment that concentrates and multiplexes the DSL lines.
Digital Subscriber Line - A term describing a family of DSL services, including ADSL, SDSL, VDSL, etc.
See Section 2
Innovations Subscribers Don't Need; I Still Don't kNow or maybe Integrated Services Digital Network, a digital phone service that uses a single copper pair to run 2B (64k) + 1D(16k) channels that can be used for switched voice or data.
Internet Service Provider
Network Interface Device - The housing used to protect the ADSL splitter from the elements.
Network Interface Card - A PC card (PCI/ISA) that supports the required network interface. Usually an Ethernet 10baseT or an ATMF-25Mbps Card..
Plain Old Telephone Service - The service that provides a single analog voice line. (i.e. your phone line)
See "Recursion"
Subscriber Network Interface - The Telco term for the phone wiring housing on the side of your house. It designates the point between the Telco side and the Inside Wire. This is also called the Demarcation Point.
The passive device (low-bandpass filter) at the SNI that splits the ADSL signal into separate voice and data channels.
An ADSL installation that does not require the Splitter. For higher speeds, a RJ11 filter is placed on every extension phone jack where an analog phone is used, thus providing the filtering at the jack, rather than at the NID. For lower speeds, no filter is required.
Small Office HOme
a.k.a DS1 - A digital dedicated line at 1.544 Mbps, used for both Voice (24 DS0s) or Data.
a.k.a DS3 - A digital dedicated line at 44.736 Mbps, provides for both Voice (672 DS0s or 28 DS1s) or Data