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DA1BT > KAM 22.02.95 13:48l 92 Lines 4732 Bytes #-11459 (0) @ WW
BID : 2225LX0PAC8V
Read: DF7NZ DL6NBB GUEST
Subj: DAMA: NEW Feature (2 of 5)
Path: DB0AAB<DB0KCP<OE9XPI<DB0CZ<DB0GE<LX0PAC
Sent: 950222/1021z @:LX0PAC.LUX.EU [Bourscheid JN39AV] BCM1.36
From: DA1BT @ LX0PAC.LUX.EU (Eifel_ARC)
To : KAM @ WW
On the other hand, if the user (slave) actually receives the poll and
replies with sent I frames, the master will not acknowledge them until the
next time around after serving all the other active stations. If when
polled by the master, the user responds with an empty frame (Receive
Ready/Final), then the master will reduce the user in polling priority and
will skip him on the next time around.
As the activity on the frequency increases, the polling priority of
inactive users might be further decreased, but when these stations respond
with an I-frame they will again regain their original priority.
If you understand the description just given, you might think that you are
reading about AX.25 level 2 protocol and this is why DAMA has a chance of
working over amateur packet radio. AX.25 L2 provides all the protocol
elements that are needed to implement DAMA and no new syntax is required.
Most of the new functions required could be obtained simply by patching
existing operational parameters while the rest could be achieved by making
some minor changes to the TNC's firmware.
So how do we actually go about incorporating DAMA using AX.25 protocol ?
Due to the fact that there are no new syntax elements required, the
following description will only use standard AX.25 terms. Since CSMA
(Carrier Sense Multiple Access) as well as DAMA is used, please interpret
all future references DAMA as CSMA-DAMA. The term "poll" used throughout
this text in no way refers to the poll bit in the control field of packet
frames and this bit remains unchanged to ensure compatibility. The
different phases of the protocol will be described separately below.
Connect Establish :
-------------------
When a node attempts to connect to a user, the node adds the users ID to
it's polling list and begins to send SABMs to that station. If after a
certain amount of tries no UA is received, the user is assumed to be
inoperable and is removed from the polling list.
When a new user starts a connect sequence to the node, he begins by sending
SABMs to the master in a simple CSMA manner duplicating the existing method
used today. Collisions are possible during this phase, so it might be
necessary to repeat the SABMs several times until the node replies with a
UA. Once the node recognizes the users connection attempt, the users ID is
added to the polling list in a fashion very similar to the one used by
TheNet nodes (TheNet userlist) and the node (master) is now in control of
the uplink users station. After the user sends SABMs and the node replies
with a UA, the user replies with an RR0 to signal to the node that it had a
successful reception of UA.
Idle state :
------------
As long as no information transfer occurs between user and node, (idles)
then the node sends its polls as an RR with the corresponding count. If the
response by the user is just an RR#, then the time until the next poll to
the user will be lengthened to avoid unnecessary channel load. The exact
amount of time added is determined by the total channel activity.
If information transfer by other users on the node is high (as determined
by the number of I-frames being sent) then the amount of time added before
the next poll occurs to an inactive station is longer than in cases where
there is only very little channel activity. Thus when the frequency is
basically clear, the waiting times are reduced to a minimum so that no
decrease in channel throughput takes place. This is the principle of the
self-alignment mechanism of DAMA, where a channel is always regulated to
insure its maximum possible throughput.
If the node ever fails to receive an RR from the user (due to a collision
of the nodes poll or the users RR response) then the node will proceed on
to the other stations on its polling list. The node will come back and try
this station again after all the other users on its list have been
serviced. If after a certain number of transmitted polls this station still
has not answered, then it is considered to be unavailable by the node and
is dropped completely from the list. This is analogous to those "keep-alive
polls" that we have today.
Data transfer : Node --> User :
-------------------------------
There is no difference between regular CSMA and DAMA in this case. Because
it is always up to the master (node) to act first, it could send one or
more I-frames or a poll to the user. The user will acknowledge I-frames
immediately with an RR#, but could also send its own I-frames with the
corresponding count (having to correct the count on the sent I-frame serves
the same purpose as an ACK with AX.25). The meaning of the Poll/Final bit
remains unchanged.
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