Comtech EF Data MIDAS Version 4.X Basic Configuration - Manual (Page 67)

MIDAS 4.2 Basic Configuration, Rev. 1
Setup and Operation 2–33
Aloha Parameters
M M i i n n B B a a c c k k o o f f f f / /
M M a a x x B B a a c c k k o o f f f f
If a node does not receive an acknowledgement from the controller server, it
will pick a random number of frames between the selected Minimum and
Maximum Backoff values to wait before re-transmitting the last message sent.
This is done to minimize data collisions on the control channel. System default
settings are 1 slot minimum, and 10 slots maximum.
The access method by which remote nodes are accessing the inbound channel
is modeled after the Slotted Aloha random access scheme. This is due to the
fact that in most cases, the two causes of relative timing errors (namely local
oscillator drifts and node location uncertainty) are insignificant to the inbound
channel slot size. Slot timing drifts caused by local oscillator drifts are
insignificant, since the beginning of the slot is triggered based on the timing
message received (all remote nodes) on the outbound channel from the
controller server. Node location relative errors is typically in the order of few
tens of microseconds, for satellite with small inclination angles (e.g.,
~
± 15 µ
s
for satellites with 0.05
°
inclination angles, and 10
°
relative node latitude
differences). [For systems with large inclined satellites orbits e.g. 3
°
inclination, and networks where nodes have maximum large latitude
differences (say in the order of 10
°
), a guard time or an open loop burst
synchronization strategy should be employed.]
These parameters, permanently stored in the controller server database and sent
to the network elements (nodes) when they are enabled, are stored in static
RAM of the DAC. When there has been a response (or a request) that goes
unacknowledged by the NMS, these parameters control the minimum and
maximum amount of random backoff (delay in slots, referenced to a system
timing signal received from the outbound control channel) that the DAC will
wait before attempting to re-transit the unacknowledged message. The
minimum and maximum amounts of backoff are used to tune the inbound
control channel efficiency, regulating the re-transmission of messages to a time
which is not too soon, nor too long, after transmission of a message would
normally occur. This prevents further collisions without excessive delays for
the successful re-transmission of any one message.
The node receives the following configuration information from the controller:
No Ack Count = 12 + Min Backoff
Min Backoff = 0
Max Backoff = Max Backoff – Min Backoff