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DA1BT > KAM 22.02.95 13:53l 114 Lines 5445 Bytes #-11459 (0) @ WW
BID : 2225LX0PAC8X
Read: DF7NZ DL6NBB GUEST
Subj: DAMA: NEW Feature (4 of 5)
Path: DB0AAB<DB0KCP<DB0ULM<DB0LX<DB0AAA<DB0CZ<DB0GE<LX0PAC
Sent: 950222/1029z @:LX0PAC.LUX.EU [Bourscheid JN39AV] BCM1.36
From: DA1BT @ LX0PAC.LUX.EU (Eifel_ARC)
To : KAM @ WW
Compatibility of DAMA and CSMA :
--------------------------------
One advantage of the DAMA method is that it does not require everybody to
change everything all at once. However as additional users convert their
TNCs to work with DAMA the more pronounced will become the increase of
throughput. Even stations that are waiting to switch over could help to
increase the areas throughput by changing a few operational parameters. For
example the delay between the reception of a frame and the TNCs response
(sometimes called T2 or DWAIT) should be reduced to a value under 1 second.
In addition the time interval from when an I-frame is sent to when the TNC
sends an RR# to ask for a pending ACK, should be set to a value that is
clearly higher than the time between two polls of the master (usually more
than 30 seconds at 1200 Bd).
To fully benefit from DAMA both the node and the user must work together in
the master/slave relationship. Assuming that the users TNC is capable of
both the normal and the DAMA mode, there still remains the problem of how
to tell the user to "turn DAMA mode on". There are several ways that this
could be done:
1. Automatic detection of the protocol version by means of the protocol
identifier byte or reserved SSID-octet-bits of the node (preferred
version).
2. Implementation of a channel specific parameter which controls the
protocol version.
3. Implementation of a new UPLINK command besides the current CONNECT
command.
4. Implement a further protocol element such as a SABM-frame (similar to
X.25) so that at connect time the node could alert the user to the
increased features.
In case #1 of the above it would be sufficient to tell the user to switch
DAMA mode only once, at connect time. This state would then remain in
effect until disconnect. However since there is no PID field in
SABM-frames this information has to be carried in some other way, such as
utilizing the dormant bit 5 of the master's SSID address field. It is
proposed that DAMA test versions set this bit to 0 to convey the necessary
information to the users TNC.
Conclusion :
------------
The existing AX.25 version was established in 1982 when packet radio was
not widespread as it is today. Most stations in the beginning were pretty
much equal and there was no distinction made between DTE and DCE functions.
However with the implementation of wide area networks not all stations are
performing the same function. In fact today the network nodes are acting in
DCE function considering their control and information exchanging aspects.
These functions will be better served with the implementation of DAMA.
The methods discussed in this article could increase the throughput on an
AX.25 channel tremendously. One advantage is the avoidance of system
breakdown which occurs with channel overload. Using DAMA, the throughput
will increase continuously up to its maximum. There is no foldback effect
like that which occurs using CSMA where at a special limit (above ca 60%)
the throughput is actually reduced.
There is also a strong "social" aspect of DAMA wherein even the weak
stations can work through the node reliably without being overpowered by
stations close to the node.
It is possible to make direct connections with other HAMs on the uplink
frequency unlike that of a duplex system. In addition the users TNCs still
retain the digipeater capability inherent in our present simplex system.
All protocol elements keep their original meaning which allows both
versions to be utilized on the same frequency, yet throughput increases as
more and more users switch over to the new method.
Literatur
Fox,T. AX.25 Level 2 protocol specifications AMRAD
Kauffels,F.J. Lokale Netze R.M}ller Verlag
Mahle,C. Satellite Scenarios and Technology IEEE J. on selected
Hyde,G for the 1990's Areas in communication
Inukai,Th. May '87
Schmidt,D.J. DAMA, ein neues Verfahren f}r cq-DL, 4/89
Packed-Radio?
Schmidt,D.J. Synchrone DF]-Protokolle mit 6809-Micro- TU-Script BS '81
Computern in heterogenen Sternnetzen
Tanenbaum,A. Computer Networks Prentice Hall Verlag
-------------------------------------------------------- July 24, 1992 ---
Note by DB2OS: The full DAMA protocoll is implemented in the latest
version of TheNetNode (TheNet for PC) as a DAMA MASTER
and TheFirmware (WA8DED Hostmode for TNC2) as a DAMA
SLAVE by NORD><LINK. DAMA is also supported by TFPCX, a
resident AX.25 software TNC for the PC (only external
Modem required, no TNC) and by TFKISS (former TFPCR).
TFKISS emulates a TNC with TheFirmware and any TNC
can be used if it can be switched into KISS-Mode!!!
DIGICOM and BAYCOM Software also now supports DAMA.
DAMA is now used on several german TNN digipeaters and
the results are of great promise.
----------------------------------------------------------------------------
Marcus Busch (DL1EKC)
Volksgartenstrasse 194 * CW forever *
D-41065 Moenchengladbach
Universitaet Duesseldorf
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