Abbreviation ADSL(Asymmetric Digital Subscriber Line) stands for "Asymmetric Digital Subscriber Line", which emphasizes the difference in exchange rates initially inherent in this technology in the directions to the subscriber and back.

Asymmetry ADSL, at its core, involves the transfer of large volumes of information to the subscriber (video, data arrays, programs) and small volumes from the subscriber (mainly commands and requests).

Equipment ADSL, located on the PBX, and subscriber ADSL modem, connected to both ends of the telephone line, form three channels:

High-speed data transmission channel from the network to a computer (speed - from 32Kbit/s to 8Mb/s); high-speed data transmission channel from a computer to the network (speed - from 32Kbit/s to 1.5Mb/s); simple channel telephone communication, which carries regular telephone conversations.

The data transfer speed depends on the length and quality of the telephone line. The asymmetric nature of the data transfer rate is introduced specifically, since a remote Internet user usually downloads data from the network to his computer, and in the opposite direction there are either commands or a data stream of significantly lower speed. To obtain speed asymmetry, the bandwidth of the subscriber end is also divided between channels asymmetrically.

On the PBX side, a so-called access multiplexer should be located on the user line ADSL - DSLAM. This multiplexer selects subchannels from common channel and sends the voice subchannel to the PBX, and sends high-speed data channels to the router connected to DSLAM.

One of the main advantages of technology ADSL compared to analog modems and protocols ISDN And HDSL- the fact that voice support does not in any way affect the parallel transmission of data over two fast channels. The reason for this effect is that ADSL is based on the principles of frequency division, due to which the voice channel is reliably separated from the other two data channels.

The influence of cable parameters on the operation of ADSL equipment

Primary line parameters:(real)

Loop (pair) resistance, R [Ohm] from 10 to 1200 Insulation resistance, R [MOhm] more than 40 Loop inductance, L [mH] is usually not measured. Loop capacitance, C [nF] from 10 to 300 Capacitive asymmetry, C [nF] from 0 to 10, relative to ground.

note: measure on the damaged cable digital multimeter insulation resistance and capacitance is impossible! This is the first sign of a wet cable, “brokenness”, asymmetry...

Secondary line parameters:(basic)

Signal attenuation. from 5dB to 20dB - the line is excellent.
from 20dB to 30dB - the line is good.
from 30dB to 40dB - the line is bad.
from 50dB and above the line sucks.
(Upstream and Downstream have their own attenuation)

Noise level: RMS Noise Energy from -65dBm to -50dBm - excellent line.
from -50dBm to -35dBm - the line is good.
from -35dBm to -20dBm - the line is bad. (high probability of line damage)
from -20dBm and above, equipment operation is impossible.

Line frequency response.(examples below)

Note: With line noise levels ranging from -65dBm to -55dBm, normal equipment can operate over extreme distances. (up to 6 km or more with a core diameter of 0.5 mm) despite high signal attenuation (up to 50 dB) even at minimum parameters.

Measuring equipment: CableSHARK reflectometer from Consultronics. Reflectometer “990DSL CopperPro” from FLUKE Networks. Multimeters APPA 101 and UNI-T UT70D

First, let's look at what it looks like from the point of view ADSL modem ideal line.

Twisted pair. 5Cat. 720m. (assembled on twists from pieces) Loop resistance 160 Ohms. (24AWG)
Average noise level in the range 4kHz-2000kHz:
RMS noise -65 dBm (or less)
Loop capacitance 0.040 µF

Fig.1. Checking distance

Figure 2 shows the results of testing the resulting line.
Blue indicates the frequency response.
Green - noise level in the line.
DMT is indicated in red.

note: DMT (Discrete Multi-Tone), information flow is divided into several channels, each of which is transmitted on its own carrier frequency using QAM. Typically, DMT splits the 4 kHz to 1.1 MHz band into 256 channels, each 4 kHz wide. This method by definition, it solves the problem of sharing bandwidth between voice and data (it simply does not use the voice part), but is more complex to implement than CAP. DMT is approved in ANSI T1.413 and is also recommended as the basis of the specification Universal ADSL.

Fig.2. Line test results

Note: The longer the distance, the greater the line resistance, the worse the frequency response and the higher the signal attenuation. This mainly affects the Downstream (middle and end of the chart), i.e. connection speed ADSL modem towards the subscriber.

Real line:
Loop resistance 420 Ohm
The distance is approximately 2.5 km.
The operating capacity of the line is 0.12 µF.
Average noise level in the range 4kHz-2000kHz: RMS Noise -38dBm

DSLAM and modem from SIEMENS.
Theoretical speed:
7Mbps Downstream
800kbps Upstream

Real connection speed:
1Mbit/s Downstream
512kbps Upstream

The connection is stable.

There is slight damage to the line:
The cable is short-circuited, one of the conductors has a short to ground. As a result - low-frequency noise in the line when turned off ADSL equipment. plus when turned on ADSL equipment, due to the asymmetry of the line parameters, audible HF noise appears. Replacing the splitter is useless.

Using a reflectometer, you can “see” the damage. (presumably at a distance of 42.9 m it gets wet.) A little closer, the upward ejection is most likely an oxidized twist.

Fig.3. Damaged line

Fig.4. Noise on the line, mainly from the Mayak radio station (549KHz), etc.

Fig.5. Noise in the line, (Fig. 4 for more details)

Straight wire:
(copper pair without telephony, they like to call it a dedicated line. :)
Loop resistance 1067 Ohm
The operating capacity of the line is 0.18 µF.
Average noise level in the range 4kHz-2000kHz: RMS Noise -55.71dBm

DSLAM and modem from SIEMENS.

Real connection speed:
64Kbps Downstream
32kbps Upstream
(sometimes loss of synchronization)

Factory cross, noodles, twists... a very long distance to the automatic telephone exchange.
Stable operation of ADSL equipment on such a line is impossible.

External factors affecting the operation of ADSL equipment

All sorts of AVU lines, RF compaction, VDU signaling, and other DSL lines running in the same cable, in adjacent pairs, greatly interfere with the work. Especially if there are all sorts of cable defects, looseness/brokenness, cable getting wet, bends. All these devices create strong noise in the frequency range from 0 Hz to 100-200 KHz. (mostly) This causes a decrease in the outgoing signal ADSL (Upstream) up to its complete absence and, as a consequence, loss ADSL modem synchronization

At working together DSL and RF seals in the same cable on different pairs can cause crosstalk that interferes with analog telephony operation. (noise in the range of 1KHz and above)

Factory and industrial areas are greatly affected by all kinds of power equipment. Immediate proximity to the railway.

Fig.7. Interference from AVU lines, Peterstar HF seals, VDU alarms

As you can see in the graph, almost all the main noise is in the Upstream range. (beginning of the graph) The noise from the AVU lines and HF seals is constant, i.e. does not depend on the time of day. The alarm is usually activated from 19:00 to 09:00 and on weekends around the clock. Accordingly at this time ADSL works intermittently or doesn't work at all.

Fig.8. Operation of power electrical equipment

Very bad frequency response cable. High noise level, blocking almost the entire signal. Station part. DSLAM

Damage to the connecting multi-pair cable from DSLAM to cross-plinths:
Damage to cables, plinths, poor-quality cable sealing. On old cross-connects: cold soldering or non-soldered wrapping. The result is contact bounce. The result is an unsystematic loss of synchronization by the modem.
“Broken pairs” - can only be monitored with a tone generator + test tube with a high-impedance input. Incorrect cable cutting/installation. Poor/incorrect wiring of connectors. (The most difficult glitches to track. They are usually resolved at the installation stage)

Violation of installation technology crossover cable.

For example: when another pair of wires is passed through a cross-over eye, which already has many other cross-overs. And they do this with such force that the dragged pair rips off/burns the insulation on adjacent cross-connections. As a result: short circuit of conductors of different pairs to each other or to the ground.

Incorrect connection splitter/modem card in DSLAM. Incorrect connection of the splitter port to the line/station. Connecting a subscriber line to another DSLAM port. Sometimes they simply forget to make cross-connections. :) Equipment overheating.
Buggy software/firmware, failure of DSLAM to work with some types of subscriber equipment under certain conditions line parameters.

conclusions

Line resistance directly depends on the distance. Therefore, knowing the resistance, you can quite accurately calculate the distance between the subscriber and the telephone exchange. Knowing reference data ADSL modem, you can estimate at what speed the modem will connect. Unfortunately that's all. To find out the secondary parameters of a line, complex, expensive equipment is required. There is also an opportunity to see the average signal attenuation on Upstream and Downstream streams in some ADSL modems : ZyXEL 650, Cisco 800 series, in USB ADSL modems and others.

For example: at cable cross-section 0.5mm.sq. (0.085 Ohm/m) and loop resistance 1000 Ohm line length L = (1000/0.085)/2 = 5882 m. It should also be taken into account that in some areas the cable cross-section can be 0.4 mm.sq (0.133 Ohm/m) Thus. for modem ZyXEL 645R theoretical speed - 64 kbit/s

another example: Distance 5.5 km
Diameter of the core cable from the telephone exchange: 0.7mm
[to the nearest ten-pair branch from the main cable going to the subscriber's building] That is. Most of the cable from the telephone exchange to the subscriber has a copper core diameter of 0.7 mm
Loop resistance: 570 Ohm!!!
Loop capacitance: 0.3 µF
Maximum possible speed: 5M/640Kbit
Actual operating speed: 640Kbit/360Kbit (if you set it higher, the synchronization will fail)
Equipment: Cisco 800 series. There are two VoIP lines and Internet access.

At loop resistance 800 - 1000 Ohms the probability of failures/instabilities is very high. (in any case, 100% reliability cannot be guaranteed) It depends on your luck with the main cable. There are cases when the ZyXEL 645R works with minor glitches on a line with a resistance of 1200 - 1400 Ohms.

You can easily ruin a link even with a resistance much less than 800 Ohms. As a rule, this is everyone’s favorite “clove noodles” on the subscriber’s side. The maximum operating frequency is 180 kHz and, if desired, you can stir up 10BaseT through bleach (two pairs) ... but at what distance?

Old soviet telephone sockets. A kind of plug with a 1uF x 160V capacitor inside. The new ones, by the way, also do not shine with quality. The RJ11 plug made in China simply falls out of the sockets “Zrobleno in Belarus”. I haven’t seen RJ11 plugs made in Belarus, so such sockets are immediately thrown into the trash.

In apartments and offices with high humidity (old stock), the resistance of oxidized contacts can reach several hundred Ohms.

Sometimes narrow-minded “telephone operators” can make a telephone connection to an office/apartment through a forgotten radio input. The distribution box left over from the radio point. (a resistance of 300 Ohms is soldered onto each wire)

You can also look for diode blockers on the landing in the panel (if the line was paired a long time ago). We get a funny effect: the ADSL modem only works when the phone is off-hook. Or a forgotten HF filter from a private security alarm system.

If the line passes through the crossroads of an old plant/enterprise, then you get additional bonuses in the form: Four thermals per line. each has a resistance of 25-50 Ohms + inductance. Parallel line branches to other workshops, intermediate cross-connections, couplings, etc. System "Granite", against eavesdropping. Through it, the operation of Dial-UP equipment is difficult, and you can completely forget about ADSL.

Particular clinical cases:
Damage to the main cable insulation:(
Soaked couplings, “broken” couplings, etc.
A split pair is when the wires for a line are taken from different cable pairs.

Well, the simplest thing:
Incorrect connection of the splitter or microfilters.
In summer... Modem overheating.
Or after another thunderstorm - a burned out modem. :)

At loop resistance more than 1000 Ohms, the operation of an ADSL modem is almost impossible.