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DA1BT > KAM 22.02.95 13:37l 81 Lines 4068 Bytes #-11459 (0) @ WW
BID : 2225LX0PAC8U
Read: DH1FBK DL5MFD DJ5ZH OK1XMP DF7NZ DL6NBB GUEST
Subj: DAMA: NEW Feature (1 of 5)
Path: DB0AAB<DB0KCP<DB0MWS<DB0RBS<DB0HOM<DB0GE<LX0PAC
Sent: 950222/1016z @:LX0PAC.LUX.EU [Bourscheid JN39AV] BCM1.36
From: DA1BT @ LX0PAC.LUX.EU (Eifel_ARC)
To : KAM @ WW
From: SM0ETV@SM0ETV.STHLM.AB.SWE.EU
To : NETSYS@EU
DAMA - A NEW METHOD OF HANDLING PACKETS ?
==========================================
by Detlef J. SCHMIDT, DK4EG Steinbrecherstr. 22 D-38106 BRAUNSCHWEIG
NORD><LINK e.V.
c/o Peter G}lzow, DB2OS
Allensteiner Str. 5
D-30880 LAATZEN
Germany
Translation : Mark Bitterlich, WA3JPY
Reprint : Pierre Cornelis, ON7PC
Lately it seems we are hearing more and more stories about hams who are
having trouble using their local node or digipeater. It seems that the user
has no trouble hearing the digi, but the digi doesn't seem to hear the user
at all. The symptoms almost match those where the receiver at the digi site
is either dead or close to it. While that kind of failure is always a
possibility, it is not the subject of this article.
The condition that this paper will talk about is one where the above
symptoms do actually occur, but not from any lack of receiver sensivity.
Instead it is due to the digi's receiver hearing too many signals all at
once and the remote user pretty much gets lost in the "noise".
The reason for this becomes obvious when we consider that while all the
users may hear the digi/node just fine, they in many cases don't hear each
other. Thus in some cases, more than one station will transmit at the same
time causing packet collisions. This situation is referred to as "a hidden
station" problem, and for remotely located users access to his or her
favorite digipeater become difficult to impossible during rush hour
periods.
This is not a new problem, and in fact there are other services
experiencing the same difficulties. A real world example is ships on the
open sea trying to gain access to a communication satellite.
Several different experiments have been made to overcome this dilemma on
amateur packet radio. One possible solution that is being pursued is
through the use of full duplex digipeaters (BTMA), however there are
several disadvantages to this approach. In a full duplex the hardware
expense will normally be much higher and the system will occupy two
frequencies but will only realize the maximum throughput of one. A better
approach might be to increase the throughput by reducing the collisions on
a single channel system rather than spreading the load onto two channels.
It would be ideal if we could incorporate a system that did this with
something so minor as software change (such as replacing the EPROM in a
TNC) or by changing some operational parameters.
One of the methods used that attempts to solve the hidden station problem
while still using a single frequency is called DAMA (Demand Assigned
Multiple Access). A description of this method follows.
In a connection oriented protocol environment, an end user will try to
connect to the master (satellite) by means of a slotted ALOHA method
(channel access without any coordination). Collisions might occur during
this phase but they are tolerable since they are relatively rare. Once a
connect request is recognized by the master, the connecting stations
identification is added to the polling list and from this point on the
master controls all connected stations. Permission to send data is granted
by means of polls which might be included in ACK packets or even in
transferred data frames. so in this case a user will only be allowed to
transmit after receiving "permission" in the form of a poll sent from the
master station. Once permission is granted several frames might be
transmitted in a block. However, if the user does not respond within a
given time frame (say around 1/2 second) then the master assumes that the
poll got clobbered or the user never received it for some reason. The
master then passes permission to to transmit to all other active stations
and when completed comes back to the first user and gives him another
chance.
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