Hi There from going through the thrteadds of the last week I see that a lot of people are complaining of ADSL line speeds not getting the connectivity they pay for. So I went on a mission to come up with the best description of how ADSL works expressed in layman terms.
I found it [url=http://www.whichvoip.co.za/index.php?option=com_kunena&func=view&catid=70&id=696&Itemid=62]here[/url]
It is written by Mitchell Barker
This is a simple tutorial on the basics of ADSL testing and analysis with under-performing ADSL connections, very basic and as simple as possible so not going to get into any debates on all the possible variables.
Firstly let look at what collectively constitutes an ADSL circuit. ADSL uses two pieces of equipment, one on the customer end and one at the exchange.
Transceiver – This is situated at the customer\'s location. This is what is also known as an ADSL modem or router. For simplicity I will stick to modem for the duration of this document.
DSL access multiplexer (DSLAM) - The DSLAM is situated at the exchange and is used to receive/transmit customer connections.
Way back when 512k down was a fast as we could get, ADSL worked pretty well but as the higher speed offerings became available, things became a lot more complicated as Telkom started providing higher speed connections on copper backbone infrastructure that more often than not could not reliably cope with the higher bandwidth demand.
Key factors are, distance from the exchange and line condition. Line condition becoming more critical as distance increases. Let’s look at that one more closely.
ADSL’s performance is due to it running a series of channels, starting at around 100 kHz up till about 1 MHz on the copper lines between the exchange and the customer\'s premises.
This series of channels are split into two different frequency bands for “upstream” and “downstream” traffic, based on a predefined ratio. The split help to reduces interference. The DSL transceivers will continually monitor the quality of the channels and will add or remove them from service depending on whether they are usable or not.
The maximum bandwidth is about 1 MHz, and gives speeds of up to 8 Mbit/sec. (24Mbit/sec for ADSL2+). However, this bandwidth and speed is only available until around 2 - 2.5km from the exchange. At distances > 2.5km, things start to drop off considerably. At around 3.5km from the exchange you can expect 2Mbit/sec and at 6km about 1Mbit/sec.
The reason for this drop in performance is as the line gets longer, the signal is attenuated (weakens). The higher frequencies are attenuated more severely than lower frequencies and thus the reason you will find the uplink segment (e.g. the 512k from home to exchange) in this portion.
As distance increases more channels are taken out of service and the resultant reduced bandwidth gives a lower maximum ADSL connection speed.
Now where modems are “forced” to synchronize at higher speeds that line conditions permit, CRC errors will occur in abundance. (CRC or cyclic redundancy check is a non-secure hash function designed to detect accidental changes to raw computer data.)
Thus to sum up the above, the maximum ADSL speed is a function of distance, the diameter/radius of the phone wire and the actual ADSL technology employed by the Telco company.
The two places where things most often go wrong for ADSL users are the line and/or misconfiguration of the ADSL modem.
The best way to fault find is by studying you ADSL line stats and then applying a logical process of elimination to find the problem/problems
Let’s take a closer look at what or ADSL modems line stats are telling us.
Line stats are a set of figures that collectively can give a pretty good indication about the condition of your ADSL connection.
You need to look at all the figures collectively and not just rely on single ones like CRC error or Uplink/Downlink speeds. Each value is a chapter of a book that collectively will give you the big picture.
The most important stats are the ones related to the downlink (downstream) signal as this is the one that consumes the most bandwidth spectrum and will therefore be the most susceptible to line conditions.
The method used to get to the stats depends on the make and model of the modem, but the general rule would be to try and get into the Admin Advanced portions of the modems Web Interface to get to te most detailed statistics. (The User level access is normally limited to the most rudimentary info and usually not of much use)
If your modem does not provide line stat functionality, then it may be a good time to consider upgrading to a better modem. More often than not, you get what you pay for.
Which figures do I need to look for?
Naming does differ between the various manufacturers, but the important important stuff should go something like this:
•Upstream/Downstream Connection or Sync Speed or Rate
•SNR Margin or Noise Margin
•Line Attenuation or Atten or Loop Loss
•Details of any CRC/HEC/RS/FEC errors and Errored Seconds
Upstream/Downstream Connection or Sync Speed or Rate
This is the speed at which your modem synchronises with the exchange DSLAM equipment. Depending upon the provider, your sync speed can either be a Fixed Sync Rate or Rate Adaptive.
In SA our Fixed Sync Rate lines are 384kbps 512kbps and 4096kbps (“Fastest” package)
As per Telkom website - The Fastest DSL service has 4 sub-profiles, which are used to stabilize customers\' service that experience re-synchs (service interruptions) on their ADSL lines due to last mile distance, noise and attenuation. Therefore customers that experience problems on the service may be accommodated on any of the secondary profiles below:
min sync rate: 640kbps; max sync rate: 4096kbps
min sync rate: 640kbps; max sync rate: 3072kbps
min sync rate: 640kbps; max sync rate: 2048kbps
min sync rate: 640kbps; max sync rate: 1024kbps
To the best of my knowledge TelkomSA does not use Rate Adaptive ADLS in SA but Rate Adaptive lines will sync at the best speed they can for the length and condition of your line, therefore you may get slightly different sync speeds when you reboot your modem.
Keep in mind that the sync speed at which you connect is not going to be your actual throughput speed that you will download at, as there will be TCP/IP overheads etc. that will consume it’s fair portion. (around 12%)
SNR is Signal to Noise Ratio It is the measurement, in decibels, of the Signal strength in relation to the level of Noise on the line. The rule is, the higher your SNR, the better, as there is less background Noise.
Thus a Noise Ratio of 20db is better than as ratio of say 16db.
SNR will fluctuate throughout the day. Signal strength is usually best during the early part of the day. During the evenings additional noise often occurs due to more people being at home and turning on electrical equipment etc. which induces into the network, thus raising the noise floor.
Electromagnetic Noise can be introduced on your line from various sources around your home and neighbourhood, in as much as ADSL can also induce noise that can affect other user devices such as shortwave radios. Any man made or natural device can be a potential source for noise, but common culprits are motors, TVs, PCs, fluorescent lighting, air conditioners, pool pumps etc.
Then there is also \"cross talk\" which is interference that is induced into your line from other adsl connections running in the same cable between you and the exchange. Noise induction is reduced significantly when copper pairs are twisted and here is one of the main reasons we have so many problems with ADSL line quality in South Africa as when there is a shortage of good spare pairs, cable jointers often split pairs between joints when trying to repair an ADSL bearing line. (Taking one good wire from one pair and another from the next) – the result is two straight wires – NO LONGER TWISTED – becoming a long wire antenna for the reception of all sorts of electromagnetic signals. Result = dramatically raised noise floor = dramatically diminished reliable bandwidth.
We all know how difficult it is to hear someone talk in a crowded nightclub, music blaring and everyone else trying to communicate above the noise level. Well that’s your poor ADSL connections problem as well. When it gets real noisy it does not “hear” the full message and gets it wrong now and again, resulting in plenty of “what did you say” retries and CRC errors.
SNR Margin or Noise Margin
Whilst SNR Margin and SNR are related, they are not the same thing. SNR Margin is the difference between the actual SNR and the SNR required to run at a specific speed. For example, if your line needs min. 25dB of SNR to run at 4Mbps, and the actual line SNR is 30dB, then the SNR Margin would be 5dB.
ADSL modems usually show the SNR Margin and not the SNR value regardless of the display name.Depending on the manufacturer this may be \"Receive Margin\", \"SNR\",”SNRM, “Noise Margin”. \"Noise\" or any variation of these.
As with the SNR, the SNR Margin may fluctuate during the course of the day, probably lowest during the evenings. If your SNR Margin is too low this may lead to slower throughput or frequent disconnections. Thus as with SNR, The higher the SNR margin figure the better the line condition.
Looking at your SNR Margin figure will greatly assist in pinpointing ADSL faults. Line Attenuation and SNR Margin are independent figures and it is possible to have good attenuation, but poor SNRM - or vice versa.
Line Attenuation or Atten or Loop Loss
Line attenuation is relative to the line resistance (loop loss) on your line. The lower this figure the better, and the better chance you have of getting the faster speeds.
Attenuation in this context represents the reduction of the ADSL signal strength on the copper pair over distance and is measured in dB - decibels. The further you are away from the exchange the higher the loop resistance thus the higher your attenuation figure will be as the signal loss increases.
Attenuation is logarithmic thus each 3dB of attenuation halves the strength of the signal power received, therefore a line with 21dB of attenuation only receives 1/128th of the power, whilst a 30dB line would receives only 1/1024th. My own Billions screenshot example above shows Downlink attenuation of 48db thus only 1/65536th of the original power is reaching it’s destination. Not a good situation at all.
The ideal place to measure true attenuation is at the DSLAM in the exchange as our ADSL modem only gives us an average indication of how much the signal is attenuated over all the frequencies that it uses. Because the router measures against the frequencies available, thus when moving from a 512Kb to a 1Mb connection you might only see very slight increase in attenuation. However as you start moving to the higher data throughput options like 4Mb more frequency spectrum (bandwidth) is needed thus the higher frequencies also get used. Now the higher one goes in frequency the higher the attenuation for that given frequency over the same line.
Keep in mind that different modems may calculate attenuation average across the frequencies actually in use, whilst others may calculate the average across the entire frequency range available. This is also one reason why when swapping to a different router it may display a slightly different attenuation figure to the original one.
A rough attenuation guideline on the maximum reliable downlink sync speed one can expect.
>60 db 512Kb
Yes you probably could sync 4Mb to a 47dB Attenuated line, I have done so, but the connection drops as soon as you start putting some load on it, and massive CRC error counts are the order of the day. Currently I sync at 2Mb down and although I no longer have connection drops I still get a fairly constant CRC error count. The sad reality is that based on my distance from the exchange, I should drop to 1Mb. BTW you could probably still get a 384Kb ADSL to sync reliably on a line with 70-75db attenuation.
“Ok so now I know all this stuff, is there\'s anything that I can do to improve my speed?”
No, there isn\'t much you can do yourself as these figures are predominantly dependent on the cable between your home/office and the exchange. But now you know a little more than you did before (a good chance even more than the tech they send to fix your line) and you now understand the impact split pairs (remember them?) can have on line performance. You also understand that poor joints leads to higher loop resistance which leads to higher attenuation. And most importantly, that distance from the exchange plays a major role in the maximum ADLS speed available to you.
The bottom line is, monitor your stats, help your tech to help you, even if you give him a copy of this doc to read, and be prepared to downgrade to the lowest speed you find that you can reliably use. unquote