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  1. #1
    Join Date
    Apr 2006
    Location
    New England
    Posts
    189

    bacnet/lon tech FACTS ONLY question

    bacnet runs on IP, MS/TP, ARC156 (via co-processor in ALC). Lon runs at a fixed speed via lontalk (please correct if wrong).

    Can someone please point out TECHNICAL differences between the two transport methods and specifically:

    1) has anyone been able to measure the effective throughput/speed of LON with varied "saturation" rates of data on the bus? In other words, not just controller count on the network, but even if there were only 12 controllers on the network with lots of points bound between them.
    2) arc156 is "deterministic". how does lon handle collisions on the bus, and would it help if lon were deterministic?

    I really don't want this to spiral into another lon vs. bacnet thread so PLEASE keep responses factual & impartial. I am trying to understand the differences and relative benefits of different transport mechanisms of lon & bacnet.

    thank you.

  2. #2
    Join Date
    May 2002
    Posts
    9,564
    Lon has a collision avoidance superior to Ethernet. It's theoretical that you can load up a LON comm to 94% of full capacity. Practically, that is not the case. Taking care of wiring and terminations help greatly. Additionally, the network reduces or increases message slots on demand maximizing bandwidth.

    If you can find an archived grahamcontrols site, I find his way of explanation very simple to understand.

    EDIT: Or, call the US rep for Loytec. More information in his head than you can handle. http://www.loytec.com/assets/content...hite_Paper.pdf
    Last edited by sysint; 01-26-2008 at 09:55 AM.

  3. #3
    Join Date
    Mar 2003
    Location
    NY
    Posts
    2,145
    I can help with some of your questions. First, in lontalk the speed is determined by the channel medium that is used. FTT-10 is the most commonly used medium in this country and the speed is 78kbps. There is also a backbone channel that runs at 1.25Mbps and LON over IP will run up to 100Mbps as others. I believe the BACnet MS/TP is rs-485 and runs at 39kbps? Many other media are used at different rates such as 'Power Line' and Fiber. In my opinion the lontalk 'free topology' network structure is more desirable than the 'bus' constraints of MS/TP.
    Also note that an FTT-10 channel segment can accomodate 64 devices on a max. of 500 meters of wire. MS/TP is 32 devices?
    Lontalk uses 'predictive p-persisent CSMA for to handle collisions. It uses an algorithm that dynamically predicts and adapts the 'arbitration window' in the traffic cycle. Lontalk uses 'priority slots' and several different 'message services' for crititcal messages. The 'Acknowledged' message service requests a packet to be returned from the recieving device to verify receipt. The sender will continue to send the message until a receipt is returned.
    I have no data on saturation rates to offer but I am sure it is out there somewhere.

  4. #4
    Join Date
    Mar 2003
    Location
    NY
    Posts
    2,145
    sys, that's an interesting comment about Al. It can be challenging to speak with him on the phone as he lets all of that information out in a relatively short period of time.

  5. #5
    Join Date
    May 2002
    Posts
    9,564
    I think Loytec is a source of fair information. Hans, Hans-Joerg, Dietmar and Al are all knowledgeable people.

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