xDSL FAQ's

Floyd Kling 

DSL stands for 'Digital Subscriber Line', does that mean they send digital signals over the wires?

DSL does not mean the actual signal on the wires between the two modems are digital.  (When I say digital, I mean 1's and 0's).

Any signal that must travel a significant distance over an air or copper medium must have an analogue component.  The analogue component is then modulated (or encoded) with the digital information.  This is very similar to a radio station.  The radio station transmits an AM (Amplitude Modulated) or FM (Frequency Modulated) carrier signal.  These carrier signals are analogue and can travel longer distances.  .  

 

Since digital signals (1's and 0's), can only travel over very short distances by themselves, we encode them into a carrier signal.  The carrier signal is selected based on its travel and capacity capabilities.  We modulate this analogue carrier because the digital signal cannot travel very far by itself.

 

Even though ISDN was implemented before xDSL, it really is a type of DSL also.  In fact, ISDN uses 2B1Q modulation, which is the same modulation used for the first 4 wire DSL.

 

So my friend, in that regard, DSL is no different than the popular dial up or even ISDN modems, they are analogue signals encoded with digital signals transported over copper wires.

What is a 'Subscriber Line'?

In Telco (TELephone COmpany) jargon, subscriber line is the wires that go from the Central Office to the subscriber (that's you).  The Telephone Company considers your home or office to be a subscriber, so those wires or lines are called "subscriber lines".

How did the name 'Digital Subscriber Line' result?

DSL was designed for the movement of Digital Signals from the Telco to the subscriber. aka, Digital Subscriber Line

 

How does DSL get so much data over the same copper wires that my 56kbps modem cannot?

The popular 56Kbps modems are designed for operation over standard voice, dial up lines.  The 56K modulated signals are designed to occupy the narrow 4Khz voice band of your telephone lines.

 

The concept of xDSL is that there is a wider bandwidth available on the existing copper wires (between the CO and the subscriber) than just the 4Khz that was originally designed for voice traffic.  xDSL modulation utilizes much more than just the 4Khz bandwidth.  For instance:

· 128Kbps ISDN (2B1Q)uses about 400Khz of bandwidth

· 1.5Mbps 4Wire HDSL (2B1Q) uses about 425Khz over each pair

· 1.5Mbps 2Wire HDSL (CAP) uses about 375Khz

· 1.5Mbps 2Wire HDSL2(OPTIS) uses about 450Khz

· 384/384Kbps 2Wire ADSL (G.LITE DMT) uses about 500Khz

· 7Mbps 2Wires ADSL(Full Rate DMT) uses about 1.5Mbps

 

Your voice (and analogue modem signals) also go through the Telco switch and other equipment that limits the bandwidth to the 4Khz area.  xDSL is like a point to point leased line.  An xDSL modem is at both points of the connection and does not go through any bandwidth limiting devices, so there is more bandwidth available.

How does ADSL operate simultaneously with my voice line?

xDSL's spectrum begins above the 4Khz voice band.  This separation keeps the two spectrums from overlapping, up to 1.5Mhz and even higher over that copper pair.  The higher frequency is desirable because you can put more digital data on the wires.  However, longer wires are less friendly to higher frequencies, so there is a fundamental trade off between length and quality of the wires, verses speed. 

What's a "POTS Splitter" and why is it necessary with ADSL?

POTS is the Telco term for "Plain Old Telephone Service", and simply means your regular telephone service.  POTS Splitter, 'splits' out the phone from the DSL Signals

 

Since the spectrums of xDSL and POTS are fairly close to each other, there is some leakage of one spectrum, into the other.  The POTS splitter helps keep these two spectrums away from each other. 

 

"POTS Splitter" is rather old fashioned now.  Originally the phone installer would put the POTS Splitter at the POE (Point of Entry) of your house.  At that point two lines would be needed from the splitter, one for your house phones and the other pair for the ADSL signals to the ADSL modem. 

 

Since then the phone companies now put the DSL right on your existing wires and gives you "FILTERS" to split the two bands.  You're told to put one of these filters on every phone, FAX or answering machine on your premises.  You do not put one of these filters on the line going to your DSL modem. 

 

They simply didn't want the expense to run the extra pair of wires into your house, it is easier to just give you a pocket full of these 'splitter' filters and not touch your existing house wiring.

 

xDSL Rate vs Reach

Floyd Kling 

(10/22/08)

  

Rate vs Reach simply means how far a modem can Reach (distance in feet) at a particular Rate (speed in bps).

 

The transmission of any signal has a distance limit.  Newer DSL signaling, (like analogue modems) can be optioned to automatically change its speed depending upon the noise on the wires that carry the signal.  In these variable speed technologies, rate and reach are inversely proportional.  i.e. lower data speeds are more tolerant to noise and other line impairments therefore can operate over longer distances on the wires than higher speeds.

 

The type of modulation used for any transmission is a fundamental characteristic that will affect the Rate and Reach of a modem. Each has its advantages and disadvantages

·         2B1Q

·         CAP

·         DMT

·         OPTIS

 

Each modulation maybe implemented in either or both of the following:

·         Echo Cancellation

·         FDM (Frequency division multiplexing i.e. Split Band)

 

There are two main factors that effect the Rate/Reach relationship for any given modulation.

 

1.       In band noise - the noise that resides in the same frequency band as the received signal.

·         As the received signal level gets lower and lower, the signal gets closer to the noise floor of the receiver, therefore the receiver has a harder time discerning the desired signal from the undesired noise.

·         In band noise may also come from adjacent wires in the binder that carries signals in the same frequency band.  Some of the adjacent copper pairs very likely will carry the same services therefore carry signals in the same frequency band.  The DSL will consider any unwanted signals in the desired band as noise. 

·         Another Phnom is known as "Self Next" Many types of modems, DSL is no different, use a technique as "Echo Canceling".  This means that Modem A is sending Signals to Modem B on the identical carrier frequency as Modem B is sending to Modem A.  Well, you can see the problem in the two modems' receivers. 

It's kinda like two people talking at the same time.  Your voice is coming into your ears at the same time another persons voice is coming into your ears. Your ears must determine which voice to listen to.... not an easy task for electronics.  This is known as "Self Next". (more on Self Next  in another document).

 

2.       Impedance of the copper wire - Impedance is really 'resistance' at on the copper at a particular frequency.  The more resistance in the copper means more attenuation of the received signal.  The resistance in the copper is caused by;

·         Length of the copper

·         If the modems are far apart, the long copper means more resistance.

·         Diameter of the copper

·         Thinner copper has more resistance, therefore adds more attenuation per foot.

·         Terminations in the copper loop

·         Anything hanging on the wires will also attenuate the signal.

 

Factors that effect particular modulations' Rate/Reach are related to Resistance and Noise.

·         Adjacent binder interference (Noise)

·         Self Next (Noise)                 

·         Copper diameter (Resistance)

·         Copper length (Resistance)

·         Bridge Taps (Resistance)

·         Loading coils  (Resistance)

 

NOTE: Resistance, Impedance, Load and Attenuation are synonymous in this context.

 

 

xDSL Rate Reach Chart 

 

Examples of Various xDSL Rate/Reach performances (in process)

 

Technology	No of Pairs	Rate (bps)	Reach - Feet(meters) w/o noise
			22ga(.06mm)	24ga(.05mm)	26ga(.04mm)
SDSL - 2Wire - 2B1Q	1	128kbps		28,000(8,533)	22,000(6,706)
Symmetric	1	384k		22,500(6,856)	16,000(4,877)
	1	768k		18,500(5,637)	13,500(4,115)
	2	1.5mbps		11,000(3,352)	9,100(2,774)
	2	2.048m
HDSL2 - 2Wire - Cap	1	128kbps		29,100(8,800)	21,400(6,500)
Symmetric	1	384k		26,300(8,000)	19,000(5,800)
	1	768k		22,500(6,900)	16,500(5,000)
	1	1.5mbps		19,200(5,800)	14,000(4,300)
	1	2.048m		16,000(4,900)	12,000(3,700)
HDSL2 - 2Wire - OPTIS	1	128kbps
Symmetric	1	384k
	1	768k
	1	1.5mbps
	1	2.048m
ADSL - 2Wire - DMT	1	128kbps
(Upstream >128kbps)	1	384k
	1	768k
	1	1.5mbps
	1	2.048m
	1	3.0m
	1	6.0m
	1	8.0m

 

 

END