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Basic questions about telecommunications

Discussion in 'Electronic Repair' started by David Nebenzahl, Jan 1, 2011.

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  1. OK, this question is totally out of idle curiosity. No customers' jobs
    depend on it. No actual electronic repair issues are involved.

    Like a surprising number of people, I still have dial-up Internet
    access. (Yeah, I know, I'm living in the Stone Age.) So I'm quite
    familiar with various connections speeds. I also can observe my network
    traffic on my firewall's control panel (I use Sygate, a freebie, which
    I'm quite happy with).

    What I don't understand is why network traffic, at least as reported by
    Sygate, is so choppy. On a good day, I get a "fast" connection, meaning
    48 kbps, or maybe even (gasp!) 49.2; that's the fastest speed I ever get.

    What I see, invariably, is something like a triangular waveform, with a
    period of about a second, where the transmission speed varies from
    (usually) 4.4 and 5.9 K (I assume this is bytes, not bits, per second,
    but whatever). The speeds never change, at least not much. With a faster
    connection, it just stays at the higher speed longer, which flattens out
    the peaks of the "waveform".

    Why is this? I remember hearing that sending packets down a telephone
    wire is a "bursty" business; is that part of it?

    Why doesn't the connection just stay at one speed? Why does it alternate
    between these two speeds?

    Of course, this behavior is only when a single download is being done.
    If I'm loading a bunch of web pages and reading Usenet, the "waveform"
    becomes very chaotic. But it always seems to be alternating between
    various speeds.

    Not being an expert on such arcane things as packets and such, I can
    only guess that this is kind of like quantum physics, were electrons are
    only "allowed" to orbit at certain distances from the nucleus. Is there
    a set of standard, agreed-upon telecomm speeds? But why not just pick
    one speed and stay there?

    Maybe next time I'll ask about how I can get a good, fast connection but
    not be able to do anything because DNS isn't working ...


    NOTE: Please don't ask me to "Google it"! I post this here intentionally
    because I know there must be smart people out there who might be able to
    shed some light on the subject. Like I said, idle curiosity, and I'm too
    lazy to try to sort through the thousands of pages returned by a search,
    weed out the commercial sites, the bogus domain-squatters and
    web-scrapers, to get to some good "content" ...


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  2. There's a million reasons why speed varies.

    It could be line noise, where the modem continually re-negotiates for
    the best speed it can.

    It could be an iffy line connection, where it works "perfectly" at
    lower speeds, so the modem re-negotiates for a higher speed where it
    (consistently) fails and falls back again.

    It's a bit hard to diagnose this type of problem, more so if you have
    modem that does not have good diagnostic feedback on the last call.

    If it *IS* noise or line limitations, it might help to force your modem
    to 14.4, and see how your connection speed goes. You're looking at
    consistency rather than raw speed in this case.

    If it IS stable, your line might be going through a pair gain, used by
    Telstra to double up on a single copper pair, and splitting up when it
    gets near your place. It saves copper wire when they have many
    subscribers, plenty of nodes at the exchange, but little copper.

    If this is the case, you'll see phone calls work well (albeit sometimes
    with crosstalk), Fax works well (9600/14.4) and plays merry hell with
    modems that go faster than that.

    Good luck with convincing them to fix it though. Even if they DO have
    copper to spare, it ends up being a significant resource waster for one
    guy who's whining about his modem.
    Doesn't hurt to whine though, you might get lucky.

    Another option is that it has nothing to do with you or your carrier,
    but with your ISP. I've seen bodgy speed throttling techniques that
    don't work as well as they should. Though that said, they shouldn't be
    going through that anyway, as the modems have inherent speed limitations
    to keep them happy anyway.
     
  3. Thanks for your reply. However, I'm not really reporting or trying to
    fix a problem here. I think my connection, for the most part, works
    fine. (Actually, there is an easy fix for my problem: $$$.)

    I'm just trying to understand *why* it works that way (speed constantly
    bouncing between two "notches"). It seems like it's *supposed* to work
    that way; why?

    I was also going to say something about how it might have something to
    do with the negotiation process between client and host, the "REQ/ACK"
    process or the equivalent. Not knowing the details of TCP and all that,
    I can only guess at what this mechanism is.

    Details. I want the gory details.


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  4. I didn't entertain the packetising of data, because that is faster than
    the graph logging anyway, so you shouldn't even see it.

    Besides, from what I remember of modem links, it never behaved that way
    anyway, by the time the speed graph showed anything of interest, it was
    reasonably steady.

    What's the timing of your graph anyway? How frequently does it take a
    sample?
     
  5. D Yuniskis

    D Yuniskis Guest

    Hi David,

    *What* is being reported -- the number of bytes (bits) per second
    leaving some "upper level" of network software? Or, the actual
    speed that the *modem* is operating at? (I suspect the former
    unless your tool knows how to query *your* modem, directly,
    for this information).

    So, you have some process in the PC that starts and stops
    sending bytes to the modem. The modem probably has a buffer
    (even if it is a software modem *in* your PC) that it fills
    up with bytes "from" your PC... then, empties into the phone
    line... lather, rinse, repeat.

    The modem typically doesn't signal the PC after *each*
    byte has been pushed out onto the phone line -- that
    would involve too much overhead on both sides. Instead,
    it imposes a certain amount of hysteresis on the "signaling".
    So, a 100 byte buffer says "Full" when it receives it's 100th
    byte from the PC.... but, doesn't say "not full" until it
    has pushed maybe 80 of those bytes out onto the phone line.

    I.e., on a very short time scale, it looks (from the PC's
    point of view) like the modem is operating VERY FAST
    (gee, it gobbled up those 100 bytes in less than a millisecond)
    then very slow (gee, it hasn't asked for ANY more data for
    dozens of milliseconds).

    (No, this isn't your "problem". Rather, just an example of how
    things aren't "continuous time systems" when it comes to this
    sort of thing).

    Modern modems autonegotiate connection speeds -- based on the
    capabilities of the two modems talking to each other *and*
    the line conditions in effect from moment to moment.

    The connection speed is sort of like the "carrier frequency"
    (I am grossly misstating things here just to give you an analogy)
    of your local radio station. Your receiver has to be "tuned"
    to the same frequency as the transmitter for reception to occur.
    The two modems do this continuously while communicating.

    ON TOP OF that "carrier", you have your actual data rate.
    To further push the radio broadcast analogy, that's like
    the rate at which the announcer is talking -- someone who
    talks slow vs. someone who talks fast.

    [again, this is REALLY a bogus analogy]

    The system at the other end of the line (and there can be
    several of them cascaded, in effect) can throttle the
    data flow. E.g., there is something like the output
    buffer of *your* modem on the other end acting as an
    "input buffer". It fills and empties in non-continuous
    ways based on what *it* is talking to.

    Etc.

    You can also have underlying "infrastructure" that further
    distorts these numbers. E.g., the presence of a SLIC96
    can limit available bandwidth (though usually to something
    a lot less than what you appear to be seeing) simply because
    *it* has a limited bandwidth capability that it imposes
    on the lines that it services.

    Speeds upwards of 33.6Kbps tend to be more sensitive to line
    noise, distance from CO, etc.

    You could also have a really noisey line and the noise might
    come and go "periodically" -- forcing the modem(s) to change
    speeds and/or request retransmissions "regularly".

    <shrug> It's really hard to say what you are experiencing without
    actually looking at your line. As a starting point, you can see
    if you can query your modem for statistics about the last call,
    etc. (doing so varies by model, etc.)
    Hint: make a note of the IP addresses of various "stable" sites.
    When your name resolver isn't working, you can use these numeric
    IP addresses to verify that the problem is, in fact, with the
    name service and not "The Network".

    IIRC, there are some public sites that will provide a *manual*
    name service that you could use in a pinch (assuming you
    keep a record of their NUMERIC IP addresses)
     
  6. As I said, period seems to be about a second. The only time I see a
    straight line is if I upload something. I think FTP transfers are a lot
    more steady as well. Otherwise, the transfer speed (incoming) is
    constantly changing.

    I could post a snapshot if you like.

    Doesn't seem to be anything wrong with the modem, the phone line or the
    system. It just likes to behave that way.


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  7. Sure, I'd like to see the timing.
     
  8. Here:
    http://s786.photobucket.com/albums/yy147/bonezphoto/?action=view&current=Linespeeds.gif

    Time divisions are ~3 seconds. This transfer (downloading a picture)
    bounced between 3.0 and 4.4 K (which I ass-ume means kilobytes?).

    On longer transfers the pattern (sawtooth "wave") becomes very regular.


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  9. OK, you're talking about 9 seconds or so between each drop and rise,
    that's not a protocol issue, it's way too long. It might be a
    re-negotiation timing, or something your ISP is doing.
     
  10. No. Maybe I wasn't clear: the time between the time divisions (vertical
    lines) is about 3 seconds, so it's about a second between each drop
    and rise. The display updates about once a second. (3 samples/time div.)


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  11. That makes even less sense. If you meant 0.3 Seconds per horizontal
    division, that would equate to one second (or thereabouts) per each rise
    or trough, allowing that I counted about three divisions for each.
    So it appears you've left out a decimal point.

    For one second, that might be a bit quick for a re-negotiation, but I
    still don't buy the protocol packetising that's doing it.

    Have you tried using another graphing logger? It *might* be that it
    takes a snapshot, rather than a real average over the timing period.
     
  12. No. You're just not getting what I'm trying to describe.

    It's really simple. The display updates once a second. Each vertical
    division--the space between two vertical green lines--is 3 seconds. So
    there are 3 1-second samples between each pair of vertical lines. The
    smallest feature in the display (i.e., the resolution) is 1 second. Does
    that make sense?
    I don't *have* another graphing logger. This is the one that comes with
    the firewall.

    Here's another snapshot:
    http://s786.photobucket.com/albums/yy147/bonezphoto/?action=view&current=Linespeed44-59.gif

    Notice how regular the pattern is here (in this case, it's bouncing
    between 4.4 and 4.9 K/sec). The same flat tops of the waves (minus a few
    bobbles here and there). I've seen such a display that was absolutely
    perfectly uniform across the entire display on a long download. (This
    was downloading a PDF; line connection speed was 49.2, the highest
    possible speed with my setup).


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  13. You're describing two different timings.

    If it's three seconds per horizontal division, and as per the image, I
    saw (about) two to three divisions per peak or trough, (not counting the
    steady bit on the right, that equates to six to nine seconds per peak or
    trough. That's how it's counted.
    You're talking about the little variations? I get wider variations
    that that myself. I just did a test here, and I'm getting quite
    significant variations over the download. But, I'm on cable, and I
    don't have an active landline modem connection to test otherwise.

    In any case, now I officially don't know. Sorry for wasting our time. :)

    As far as speculation goes, one could guess that the internet IP
    packets might have something to do with that. Everything gets to your
    place in packets. If a few packets get there late, they're still
    ordered correctly, but just a little late.

    There would be significant (depending on your needs) "jitter" in that
    on average, you may have quite constant speeds, but from second to
    second, not. You basically don't have any control over the intermediate
    data handlers from the site you visit to your destination.
    It's actually a bad thing when you're using it for live voice/video
    communication (Skype etc), because that type of communications *needs* a
    constant supply of data, but the buffering works well enough that you
    don't notice too much.


    In effect, you're insulated from the effects of Real Life (TM).
     
  14. Phew. Finally; yes, that's what I'm talking about, the "little"
    fluctuations (between ~4K and ~5K in that second snapshot). They
    intrigue me because they're ubiquitous (they happen all the time) and
    they're sometimes so regular; it seems like a pattern that must have
    something to do with the underlying transfer mechanism. If it's indeed
    jitter, it's damned regular jitter.

    Since at this point we both seem to be going on pure speculation, I'll
    just thank you for your participation so far.


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  15. But as I watch the display during a download, I can see the rate *very
    regularly* alternating between two definite speeds (like 4.4 and 5.9 K).
    As you can see, the pattern is visible, at least according to this
    display. Doesn't that tell us something about how the transfer is taking
    place? Or is this just a regularly repeating roundoff error?

    In fact, I can tell when the transfer rate is faster or slower, based on
    the display: it'll bounce between the same two speeds--that never
    changes--but the "flats" on the upper part of the line will be longer,
    meaning it's spending more time at the faster speed. The difference is
    actually noticeable, in the time it takes to render web pages and such.


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  16. Meat Plow

    Meat Plow Guest

    LOL
     
  17. OK: Win2K SP 4; computer is (reading from the Windoze "System
    Properties" dialog here as I can't remember the exact MB brand): "x86
    Family 6 model 8" (Pentium IV???), running at, I believe, 700-something
    MHz, 786 MB RAM. Yeah, not enough RAM, not very fast clock speed by
    modren standards, but sheesh, should be able to keep up with a lousy 56K
    modem even running full blast, dontcha think?

    Test application is Firefox, which is recent (not that it should matter,
    right?): v3.6.8. The latest line-speed display I uploaded was while
    downloading a PDF of a few megabytes.

    Anything else you want to know? Can't tell you the model mfgr., except
    that it's a cheapie I got at the local computer guy's store. Nothing
    else fancy; no VPNs, PC-Anywhere, proxies, etc., etc.
    Thanks, I'll look into those.


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  18. What Firefox add-on? Didn't you read through the thread? The speed
    reporting is from my firewall, Sygate Personal Firewall.

    I just went to find their website and was dismayed to find out they've
    been bought out by Norton (ugh): http://us.norton.com/sygate


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  19. Regarding that: I looked at the first site and am intrigued, but don't
    understand something. I see you need a host and client to make this
    work, which makes sense, and as you point out you might (or might not)
    be able to use someone else's web-based host; but if not, what then? Can
    you somehow set up both host and client on your computah and have the
    packets make a "round trip"? (Although that would be a problem since I
    notice my upload speeds are a lot slower than download.) How would I use
    this to test my dial-up connection?

    Also, do I want to test using TCP, UDP or both? Which are used when I
    talk to my ISP?


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
  20. I did try it. Interesting.

    The test ran fine, but I was surprised to see the Java speed indicator
    zoom up to ~700 kbps. I figured there was something perhaps bogus about
    the speed test, but then I noticed that my Sygate monitor window also
    showed speeds I'd never gotten before; the incoming (download) speed
    went up to something like 120 K. Remember, my speeds usually bounce
    between ~4 and ~6 K here.

    So now I'm really confused. Are these transfer rates real? Remember, I'm
    on a 56K modem.

    Here's what they reported:

    Your download capacity of 627 kbps is very low compared to most
    broadband connections.*

    Your upload capacity of 493 kbps is acceptable.

    Your Quality of Service was measured at 30%, which shows that your
    connection is unable to produce a constant stream of data.


    * Except, of course, that I don't *have* a broadband connection!

    Regarding your generous offer to perhaps set up a host for me to use to
    check speeds, thanks, but remember this is just idle curiosity on my part.

    If I ever have a real problem, maybe I'll take up your offer, and then
    I'll really owe you one.


    --
    Comment on quaint Usenet customs, from Usenet:

    To me, the *plonk...* reminds me of the old man at the public hearing
    who stands to make his point, then removes his hearing aid as a sign
    that he is not going to hear any rebuttals.
     
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