A technical question from a non-technical person. Any suggestions appreciated.

Discussion in 'VOIP' started by Graham Cross, Jan 20, 2005.

  1. Graham Cross

    Graham Cross Guest


    I work for a charity in the UK.

    We currently have two buildings, each with a Toshiba CTX PBX . These
    two buildings are connected via an 11MB radio link and Q sig enables
    the two phone systems to integrate, allowing our receptionist at the
    main site to handle / view all the call activity.

    The staff at the second building make and receive external calls via
    the ISDN installed at the main building.

    The radio link is also used for computer data traffic.

    The system works well a lot of the time. However there is an
    intermittent problem with calls cutting off at the second site. The
    usual scenario is that the call will end shorlty after the connection
    has been made. The phone will ring but then cut to an engaged tone -
    'call abandoned' will appear on the handset display. If an attempt to
    call the same number is made within a few minutes the same problem
    usually occurs. However, on another occasion the number can be
    contacted without problem.

    This problem seems to be mostly restricted to outgoing external calls.
    Calls made between sites to internal extensions are OK.

    As these 'internal' calls are OK it would seem that the problem is not
    with the link. Voice prioritisation has been implemented and we have
    no problem with the quality of calls.

    If anyone can shed any light on this it would be much appreciated.

    Thank you
    Graham Cross, Jan 20, 2005
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  2. Graham Cross

    Jon Wang Guest

    That's a pretty good description for a non-technical person!

    It seems to me that you're gonna need someone to monitor the ISDN line
    and probably engage the local PTT (Telco) and the PBX service vendor,
    or a third party with the right test equipment. You didn't elaborate,
    but I'd guess that it's an E1 and not the smaller individual ones that
    we refer to as BRIs. Bear with me, b/c I'm in the USA.

    I'd suggest that, armed with the right E1 monitor (for D-chl
    call-setup/teardow) one would be able to see if it's some kind of
    timeout problem where the handshaking isn't happening smoothly. I'd
    actually call the folks who installed the radio link, or the PBX
    people, to see if there's any debug mode that the PBX can printout a
    log of call-setup.

    These can be difficult to get resolved because there's several
    entities that can all point the finger at each other. The key here
    seems to be that the external calls initiated from the main bldg that
    has direct access to the ISDN doesn't seem to have the problem. So
    I'd be looking somewhere between the PBX at main bldg, the link delays
    (latency) due to any interface hardware problems/setup and then of
    course the far-end "remote" PBX.

    How long has the system been working before you noticed this problem?
    Is it possible it existed from day one, but just now is coming to be
    known? That might indicate a setup problem.

    Jon Wang, Jan 20, 2005
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  3. There are lots of possible causes including protocol errors that would need
    an expert to spot. But one possibility is simple congestion. To a certain
    extent this is unavoidable - just as someone makes an outgoing call, the
    last available "channel" is taken by an incoming call or another outgoing
    call. This "collision" can occur almost anywhere in the system. If the
    problem only occurs at busy times, this is a likely cause.

    Some billing problems also have symptoms like this - the call is set up but
    the system discovers it can't charge for the call and terminates it. But
    these tend not to be intermittent.
    Phil McKerracher, Jan 27, 2005
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