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TCP-group 1994

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Re: AX.25 "reserved" bits in SSID bytes

: > On page 7 of the AX.25 Level 2 Specification from ARRL, there is
: > explanation of two "reserved" bits in the SSID byte of each address,
: > that can be used "in an agreed-upon manner in individual networks".
: > It also says "When not implemented, they should be set to one".


: This is suprisingly important, the older Linux one used 0 and got caught
: out badly in some German networks where 0 meant something quire different.

Some -- most? -- nodes here in Germany run a derivate of AX.25 L2V2
called "DAMA" (demand assigned multiple access) which allows the
node (TNN or Flexnet) to controll the access to the channel. This
solves some problems like "hidden station" or "wrong parameters with
750 W and a 21 element Yagi". [Every user station ("slave") transmits
only when polled from the node ("master")]

The DAMA spec says that the "master" sets one of the reserved bits
(don't know exactly which) to zero to mark it as a "master". This
allows compatibility with the old AX.25 L2V2 scheme ("slaves" not
capable of the slave protocol should ignore the bit).

Joerg Reuter        Internet: dl1bke@melaten.ihf.rwth-aachen.de
                    ampr-net: dl1bke@db0pra
                    ax25    : DL1BKE @ DK0MWX.DEU.EU

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