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Discussion in 'Networking & Security' started by andrew123, Jan 18, 2010.

  1. andrew123 New Member

    Jan 18, 2010
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    Hi Guys,

    I'm an Installation and Repair technician with Telus.

    I'd like to post a few quick things about some of the DSL issues some of you are having, as well as some myths and some technical solutions that may help you troubleshoot a problem with DSL.


    Let's talk about Speeeeeeed. DSL companies say that DSL is quicker, Cable companies say that Cable is quicker, well, they're both right. DSL is a dedicated connection, and while you can't pump as much bandwidth over it as a Coax, it is more consistent, because Coax connections are token ring, everyone is on the same network. (that's starting to change as Coax companies make changes to their networks too become more competitive). However in a situation where a Coax user is in a neighborhood that doesn't have many subscribers, they can easily achieve speeds that are unattainable by DSL. On a busy day, your cable connection might be pretty much unusable in during primetime hours, while DSL stays at the same rate (unless it's affected by line conditions).

    Now getting back to DSL, let's talk about how the signal originates. It all starts at the CO, with the DSL equipment. It's the source of the signal. Now, we want to get this signal out to our subscribers and there are a variety of different methods of getting this out to our users..

    We'll start with method number one which is the most common for people who are served directly from the CO, it goes over POTS (Plain Old Telephone System -- the copper). Well, in order to serve people dialtone AND DSL over the same line, we have something called a DSLAM (Digital Subscriber Line Access Multiplexer), which multiplexes (combines) the signal from the DSL equipment and the line cards that generate dial tone, onto the cable pair heading out towards the phone poles or underground from the CO to your house. Now, for brevity I will mention that there is lots of other stuff I didn't cover here, the IDF, MDF, Coils etc, but rest assured that stuff doesn't really apply unless you work 'On the rack'.

    (Incidentally, if you're served 'dark dsl' or 'dsl no local' which are two names for just a DSL signal with no dialtone, you're connection is still going through a DSLAM, but you're only getting talk battery from the line card with no dialtone.)

    For the 'record', the phone company relies entirely on records, there is no magic system for figuring this stuff out, we have assignments of every phone number, address etc, and we do run wire jumpers, use wire-wrap (not as common anymore). It really is a cool place to work.

    Well, ok, so yes the signal runs all the way down the phone poles. Now we should do a quick detoure and talk about DSL. Firstly, there are a few different generations of DSL, first DMT, then DMT2 and now ADSL2+. ADSL2+ being the coolest by far, altho DMT2 is not bad. Now, these sort of all work in a similar manner, so for brevity I won't talk about how exactly the signal is generated, but heres how the signal is generated in case you wanted to know...

    In order for us to be able to send you internet, we need to be able to send large amounts of data over copper pair. Regular phone lines have a voltage of -48V (new standard is -52V) DC. When your phone rings that's a 90V AC pulse on top of the DC voltage. What we do is use voltage to represent the data, and in the DSL world we do this with a little divvy called Quadrant Amplitude Modulation. QAM is simply the bit encoding scheme we use to convert binary data into ac data, yep, it's method of DAC. QAM uses symbols and constellations to represent data, the phase of the voltage represents the symbol in the constellation, and each symbol in the constellation is mapped to a particular bit value. So lets say we have 30v at a 30 degree phase, well that might be mapped to bit value 0000, but if we crank the voltage up to 33v at a 30 degree phase, our symbol now changes to a bit value of 0001, that's because the constellation is mapped for different voltage/phase combinations. This reason is why DSL is so sensitive to line conditions, as the impedance of a line increases with distance (attenuation), or mutual induction from other twisted pairs in the cable count affect the DSL signal (noise), all the sudden, we may not be able to map a voltage of 33v to a phase of 30 degrees, it may get 'skewered' due to line conditions, effectively cutting down the bandwidth that we have available. DSL also needs a good ground reference, as QAM is modulated with reference to ground, if the ground reference at the other end is more than a 6ohm difference, it causes all sorts of headaches.

    Ok, so that's DSL in a nutshell, im certain I've missed finer points, but it's mostly interesting to know, even in my job I usually am troubleshooting wiring problems and line conditions.. but hey, it's cool to know. (also know that climbing a pole isn't as hard as it looks).

    So lets say, for illustrative purposes, that you are lucky to have a direct cable connection to the CO thru underground, and we don't need to go through a SAC box. I just googled Serving Area Concept and here is a link http://connectedplanetonline.com/mag/telecom_rethinking_access_network/ to check out. It will probably explain it better than I can because I really want to get into talking about how to get the best possible DSL connection.

    (I just realized that I will be explaining a lot of stuff in order to explain the stuff I wanted to explain, but it's interesting to know).

    So, DSL works best if it has a dedicated line to your house/modem from it's point of origin (notice I said point of origin, sometimes we put DSLAMS and DSL Equipment in SAC boxes to really crank out the speed).

    Now a dedicated line is important, it used to be the phone company would run a 25 pair down a neighborhood of 30 houses, well, lets say for arguments sake that at one time there where never more than 24 houses with phone on that street. If Mr.X from house A decided to move down the street to house B, well, we wouldn't give him a new pair, we'd simply hook up a new drop to his pair going down the street, and leave the old house connected, if a lady moved into Mr X's old house from another house on that street, well, we'd splice the drop pair at her new house (Mr X's old house) onto line that was on her old house, what you end up with is something that looks like a tree, one trunk (the main pair) with a bunch of little limbs (the bridge taps).

    Bridge Taps Kill DSL. So, when you get DSL, we go down the line, and remove every one of those pesky 'limbs' that used to be on your line, and then when we get to your house, we cut your line, so anyone that may have been using it further down the road previously won't affect your service, that's called a clear-cap. After we condition the line, we check it for Tip to Ring, Tip to Ground and Ring to Ground shorts, opens, metallic faults etc. Then we check for attenuation and Signal to Noise (SNR, or as we like to call it 'Shnir'). If all is good, then the DSL is now at your phonebox / demarcation point.

    Here is where it becomes interesting for you guys. Remember, DSL doesn't like bridge taps, it want's a dedicated signal, well, 80% of the time, the DSL is going into a house that has all sort's of short little bridge taps, your inside jack wiring! (and the SHORT bridge-taps are the worst ones of all, you gotta think that the DSL signal, the VOLTAGE is now being shared across all your jacks, this is why you hear the DSL signal without a filter!).

    So, let's talk about filters. Those ones that you put on each phone? They work, but they're crap because you're not getting the best signal you could be getting, and hey, you're paying for it right?... So here's the solution, we put a special filter on your phone box, we call it a POTS SPLITTER (Plain Old Telephone System Splitter). Essentially, this is a bandpass filter, that filters out the PHONE to one output, and the DSL signal to the other output (there is no filtering on the modem output). This allows is to remove ALL the bridge-taps in your inside wiring, and dedicate a jack to the DSL modem, and presto, you now have a sweet bridge-tap free ADSL signal.. AND you don't need those crappy filters on your phone. POTS Splitters are cheap too, why they don't send one out with modems is beyond me.

    Anyways... 80% of the problems I encounter in my day to day work are Inside the Home, so trust me when I say putting a pots splitter in your phone box makes a world of difference! (with correct installation) :)

    Questions? Post 'em here. I'm certain I've made some errors on here so post them too.

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