Aloha protocol, Slotted aloha, Midas control channels – Comtech EF Data MIDAS 4 System and Design User Manual
Page 44: Outbound control channel, Inbound control channel
System and Design Manual, Revision 2
3–2
Theory of Operation
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Aloha is a random access protocol where the users send data at will, because
collisions of data from different users are possible, each user must listen for an
acknowledgement from the destination. The user either receives a negative
acknowledgement from the destination, or no acknowledgement for the pre-
determined timeout period. In both cases, the user must backoff a random
amount of time before retransmitting.
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Pure Aloha scheme can be improved with a small coordination between the
sending stations. Each user must transmit between synchronized pulses. These
slot-producing pulses are broadcasted to all stations.
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Dedicated satellite channels are used to transfer all network management and
call control information between the MIDAS Controller and the traffic nodes.
Two types of control channels are used in a MIDAS network:
• Outbound control channel
• Inbound control channel
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All network management and call control messages from the MIDAS
Controller to the traffic nodes are sent on a dedicated outbound control
channel whose characteristics can be summarized as follows:
• Pre-assigned digital SCPC channel using TDM.
• Transmits in continuous mode at 19.2 kbps, QPSK, FEC = 1/2.
• Slot size is 55 ms.
• Messages are contained in an HDLC frame.
• Each message contains the address of the destination traffic node(s),
or is broadcast to all the traffic nodes.
• Provides the synchronization pulses for the Slotted Aloha inbound
control channel.
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All network management and call control messages from the traffic nodes to
the MIDAS Controller are sent on a dedicated inbound control channel. The
inbound control channel characteristics can be summarized as follows: