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Re: ATM vs. DS3
From: Ehud Gavron <GAVRON () ACES COM>
Date: Wed, 09 Jul 1997 19:52:23 -0700 (MST)

Hello,
      I just thought of something. We are in the process of purchasing a
4 Mb CIR from another backbone. Now, we have the choice of ATM or standard
T3 delivery (over a DS3 either way). Now, if we get ATM, that 4 Mb CIR
turns into:

[ (53-5)/53 ] * 4 Mb/s = 48/53 * 4 Mb/s = 3.62 Mb/s

Because an ATM cell is 53 bytes, but 5 bytes of that is header. Thus we
pay for 4 Mb/s and get 3.62 Mb/s.

Now, the question is, what is the overhead of the serial (ppp?) protocol
running over the T3? Since it is point to point, and not addressed, it
should be less, or is this not the case?

        <24 bytes per MTU.  If your MTU is 4000, your large frames
        buy you 3976/4K, which conveniently multiplies by 4M to give you
        3.9+M.

        Also, don't forget that ATM switching introduces a latency that
        a p2p connection doesn't.

        If you're truly having a point-to-point application, and you have
        a choice of a non-switched/routed protocol vs a p2p protocol...
        um... go with b :)

        E

Any input would be appreciated!

Josh Beck                                           jbeck () connectnet com
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