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DA1BT > KAM 22.02.95 13:49l 74 Lines 3642 Bytes #-11459 (0) @ WW
BID : 2225LX0PAC8W
Read: DF7NZ DL6NBB GUEST
Subj: DAMA: NEW Feature (3 of 5)
Path: DB0AAB<DB0KCP<OE9XPI<DB0CZ<DB0GE<LX0PAC
Sent: 950222/1027z @:LX0PAC.LUX.EU [Bourscheid JN39AV] BCM1.36
From: DA1BT @ LX0PAC.LUX.EU (Eifel_ARC)
To : KAM @ WW
Data transfer : User --> Node :
-------------------------------
As mentioned before, the node will send polls to all users that are
uplinked to it and the user will not respond until it receives this poll or
an I-frame from the node. It may be wise to point out that when a user is
polled he must always come back with some kind of response, even if it is
an RNR#. If the node fails to hear any kind of response from the user then
it assumes something went wrong (such as a collision) and moves on to the
next user on its polling list.
This method of always waiting for a poll before transmitting is the central
aspect used to avoid collisions in a situation where hidden stations exist.
This is in contrast to the usual CSMA method where several stations can
actually transmit at the same time. Additionally the problem of deadtime
collisions is resolved. Deadtime refers to the period from when the TNC
realizes the channel is free and starts transmitting, to when he has been
on the air long enough for other TNCs to recognize his carrier. This is
really not a rare case, as exemplified by the case where two or more TNCs
are waiting for a digipeaters carrier to vanish so that they can leap on
the frequency. Using DAMA the node will not acknowledge received frames the
instant it hears them. Instead it will first service all other uplinked
stations and then come back with an RR# to the sending stations I-frames
along with a poll to that station. This poll basically says "Have you got
any else for me ?".
Disconnecting :
---------------
If the master intends to cut the connection, it will send the usual
DISC-frame to the user. The user will then promptly respond with the
UA-frame (final bit set). If the node fails to receive the UA and again
sends a DISC-frame, the user will respond with a DM-frame. This is
identical to the actual CSMA version.
When the user wants to disconnect from the node, he will wait to send his
DISC-frame until polled by the master. AT this point it makes no major
difference whether the node responds to the user right away with a UA or
goes through another polling cycle to do so, however an immediate UA is
preferred.
UI-frames :
-----------
In CSMA as well as in a DAMA environment, the UI-frames are treated in a
special way. I.E. These frames are used to carry some information besides
the regular protocol traffic. Normally UI-frames are never sent from a user
to a node, and it is not good headwork to make a habit of making UI-frame
direct QSOs on the input frequency of a node. However, in contrast to a
duplex system, it is possible to actually do this. So although the rare
UI-frames will reduce the throughput to the CSMA value, it will not drop
the much lower ALOHA value that would occur with a duplex digi having a QSO
on its input frequency. UI-frames originated by the node are no problem
since all stations receive these frames.
Other protocol elements :
-------------------------
So we have gone from the beginning to the end in describing a complete DAMA
session. We have not translated each and every AX.25 element into one that
has a special significance to DAMA. This is not required since many of them
will keep their initial meaning. DM, RNR, REJ, etc will all be used as they
were before. The only deviation from the pure CSMA version is in the fact
that the users will only be allowed to transmit these frames after
receiving permission from the master (node) in the form of a poll. The node
will only transmit these frames after all other users on its list are
served by completion of one polling cycle.
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