Router sequence of operations

Discussion in 'Broadband' started by Graham J, Nov 22, 2014.

  1. Graham J

    Graham J Guest

    Can anybody help me with an authoritative reference explaining exactly
    what happens as a router makes or loses an internet connection, please?

    Modem component sees the ADLS or VDSL signal, trains, and after about a
    minute achieves sync.

    Router component sees this, sends the PPP authentication, and in due
    course receives an IP address, DNS, and routing information. We know
    this sometimes takes several tries - so it could be a minute or two.

    This sequence is much the same whether the modem and router are separate
    components, or integrated into a single box. It's also similar for ADSL
    and FTTC using VDSL.

    If the modem loses sync, then the connection fails, and the process
    repeats - the modem re-acquires sync, then the router re-authenticates.


    While the connection is up, does the router ever repeat the
    authentication process?

    Does it ever happen that sync is maintained - but the PPP session is lost?

    If so, what prompts the router to restart the authentication process?

    Graham J, Nov 22, 2014
    1. Advertisements

  2. it gets a bad response or no resposnse at the PPP level

    Some detail:

    The training process is one of selecting narrow bands of frequencies
    that work, or don't work. These frequencies are in the LW/MW/SW radio
    band are all are subject to interference from a variety of sources, ADSL
    on adjacent lines being one, and MW transmitters another.

    Once the basic ADSL link is UP, you are connected to the DSLAM at the

    The next step is to send PPP over ATM frames to the ISP of choice. You
    do this by FIRST sending your name/password to BTs own Radius servers
    which proxy for your ISP, and accept your connection, and then BT sets
    up an ATM virtual circuit over its backhaul to the ISP itself where you
    log in again, and get a basic IP link set up and get given your DHCP
    info - what IP addrress to use and if you use it, what DNS servers to use.

    That's why there is a bit of ATM configuration in your router - VCI etc etc.

    NOW: its possible for sync to the DSLAM to fail, if noise on the line
    happens, and you will then re-sync to the exchange without upsetting tor
    VPC back to the ISP. So you can get retrains occasionally.

    It is also possible for the ISP to disconnect and require
    re-authentication (actually with CHAP you are constantly
    re-authenticating anyway) and the ISP may issue you with a new IP
    address and or DNS target as well.

    Quality routers will have diagnostics and logs to show this process in
    action and information about what frequencies are in use and whether or
    not the ATM based backhaul is working OK.

    The Natural Philosopher, Nov 22, 2014
    1. Advertisements

  3. Graham J

    Graham J Guest

    The Natural Philosopher wrote:

    Is there a specification that routers should adhere to saying how
    frequently the authentication process should be repeated?
    What you write is tautological. If it doesn't restart the
    authentication process then it will not send a PPP request.

    So what causes the router to send a PPP request, it the PPP session is
    already up?
    Yes, I've seen that happen
    How frequently?

    The case in point is an Openreach-supplied VDSL modem connected to the
    Ethernet port on a Vigor 2820 router; at a site which is unattended.

    From time to time the connection fails. (Actually twice in the first
    two weeks of service, both times mid-morning, first for about an hour,
    the second time for about 15 minutes.) The router only shows how long
    the WAN connection has been up, and the VDSL modem tells us nothing
    useful, given that there is nobody on site to look at its green lights.
    I have nothing at the site that will receive syslog output.

    The ISP (Zen Internet) can only confirm that the service disconnected.
    The information they get from Openreach tells them the number of
    disconnections in the previous 24 hours, but nothing useful like exactly
    when, or whether the cause was authentication failure, or sync failure.
    Neither can Openreach tell them the error counts on the VDSL service,
    or SNR margin and loop attenuation; and of course I can't find that out
    from the VDSL modem; so I can't tell whether the connection is marginal.

    I monitor using F8Lure so I see the exact disconnection and reconnection

    So I'm looking for information to understand all the possible failure
    Graham J, Nov 22, 2014
  4. it is sending PPP packets all the time, some are keepalives and some
    contain CHAP authentication packets. And some contain user data.
    timers...? at either end.
    I would say minutes not hours. It all depends on how the gear is set up.
    Zen should be able top see that and also tthe router may log what
    happened in its own logs.
    get a VDSL router that can do some logging.
    The Natural Philosopher, Nov 22, 2014
  5. Graham J

    grinch Guest

    The 2820 can syslog to a usb stick in its usb port and gives good
    info to the state of the PPP session. As it is not directly connected to
    the actual VDSL I don't think you will get any line stats from it.

    Set the time up using an NTP server then you know the time is correct

    It has the ability to send keepalives and also to ping a given IP
    address, if it cant ping said address I am not sure if it will try to
    re-authenticate again, try google for their website, but I think it is

    try google for PPP CHAP VDSL IP PPPoE XTALK REIN

    Have a rummage round in wikipedia you should find an explanation of how
    dsl works just ignore what connectivity method it uses adsl/vdsl

    There are routers which directly use the vdsl so you don't need the BT
    modem such as a Cisco 867 but that is against the BT t&Cs.
    grinch, Nov 22, 2014
  6. Graham J

    Graham J Guest

    The Natural Philosopher wrote:

    Thanks - and to quote:

    "At random intervals the authenticator sends a new challenge to the peer"

    I asked them, and specifically - no - they can't see the cause of the
    failure. They might be able to determine if the line is in sync when
    the connection has failed, but that relies on me seeing the fault and
    ringing them to ask.
    I have a Vigor 2860 ready to take to the site for just that purpose.
    Graham J, Nov 22, 2014
  7. Graham J

    Graham J Guest

    grinch wrote:

    Might be useful - but necessitates a trip to the site to plug in the USP
    stick, and a second trip to collect it.

    When the connection is up I have remote management of the router. Can I
    then use the router to interrogate the contents of the USB stick? OK I
    will have to read the manual and try it on a test router.
    It uses this mechanism to decide whether to bring up a backup WAN
    connection using 3G. Also it maintains a VPN. Not clear that it
    invokes a re-authentication cycle.
    Graham J, Nov 22, 2014
  8. Graham J

    grinch Guest

    No you can read it remotely once installed but put a reasonable sized
    USB stick as if memory serves it's about 10MB per day .

    Please don't take that as fact I have not used my 2820 since I got vdsl
    installed 2 years ago.You can of course delete days when nothing happens

    Why not suck it and see I don't think it will break anything
    grinch, Nov 22, 2014
  9. Graham J

    Martin Brown Guest

    Most of the failures I encounter on my rural line are where the sync is
    nominally fine but the transport mechanism and SNR margin has failed.
    Analysis of the SNR margins shows completely insane values but the modem
    fails to do anything useful about it and presses on regardless. In this
    state all packets get dropped on the floor by timeout/retries.

    User disconnect and reconnect notably fail to do anything useful at all!

    The modem can be roused from this state only by cycling the power or a
    full hardware reset from its hidden "advanced" menu. If you could detect
    this bad state at the far end and force a reboot that might help.

    My previous routers had similar behaviour but slightly worse SNR so I
    use the present one despite its rare but annoying need for hard reset. I
    don't know what it is about rural lines but they have interesting
    faults. I have had two days without dialtone where only one phone would
    ring and although CLID was displayed answering the phone didn't work!

    Another peculiarity I have seen with some routers is that to make them
    be stable for streaming video or audio you have to disable plug & play.
    I have no idea why this works but it does!
    You might want to consider putting the errant router on a clock to cycle
    the power once a day. I am just celebrating being back online after two
    days with no dialtone or ADSL due to OpenReach handiwork.

    I am hoping for no further disruption for a while.
    Martin Brown, Nov 26, 2014
  10. Graham J

    Graham J Guest

    Martin Brown wrote:

    Have exactly this fault in North Lopham (Norfolk) since yesterday. No
    dial tone, just a faint clicking sound. Incoming calls ring the phone,
    but can't be answered - the caller continues to hear ring tone.

    ADSL is just about connecting at dial-up speeds.

    ISP tells me that it is a widespread fault which will require "traffic
    management" to cure - not expected to be fixed until Friday.

    I suspect exchange equipment failure ...
    Graham J, Nov 26, 2014
  11. Graham J

    Graham J Guest

    Just discovered - traffic managment means road traffic management - so
    they have to dig a hole in the road to fix the problem ...
    Graham J, Nov 26, 2014
  12. Graham J

    Martin Brown Guest

    In my case so did the recipient. The brr brr was audible on the line.
    What surprised me was that the CLID still worked OK.

    Last time around it was the transpose. You could pick up a non ringing
    phone and talk to someone if you prearranged a time to do it!
    Mine did too but with loss of sync every five minutes resulting in speed
    spiral of death. I now have a 4.6Mbps downlink sync speed and a BRAS of
    0.5Mbps which my clueless ISP and OpenReach are too lazy to reset. I am
    hoping it will retrain upwards tomorrow or after four hours.
    They told me the root cause of my failure was a "protection unit" gone
    bad which was replaced and apparently broke a large number of sites. The
    local farm shop is seriously unhappy about this outage as no working
    phone means losing Christmas and other routine orders.

    It all started out as a very "noisy" line with crosstalk between POTS
    voice subscribers. I mean really noisy - enough for phantom phone bells.

    Escalating it did eventually work though my ADSL speed is an order of
    magnitude down on what it should be and Orange customer service sucks.
    (perhaps it should be renamed Lemon)
    Martin Brown, Nov 26, 2014
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.