Comtech EF Data MIDAS Version 4.X NMS Configuration User Manual
Page 50
MIDAS 4.X NMS Configuration, Rev.6
2–22
Setup and Operation
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MIDAS uses dedicated satellite channels, called Control Channels, to
communicate all network management, monitoring and control, and call
control information between the NMS and the nodes. Based on functionality,
there are two types of control channels: Outbound and Inbound.
Outbound Control
Channel
All the network management, monitoring and control, and call control
messages from the NMS to the nodes are sent on a dedicated outbound control
channel, whose characteristics are summarized as follows:
• Pre-assigned digital SCPC channel using Time Division Multiplex
(TDM)
• Transmission is continuous mode at 19.2 kbps, QPSK, FEC1/2
• The slot size is 55 ms
• The outbound messages are contained in an HDLC frame
• Each message contains the address of the destination node(s), or is
broadcast to all the nodes
• Only one outbound control channel is supported at this time
Inbound Control
Channel
All messages from the nodes to the NMS are sent on the inbound control
channel, which is a digital SCPC channel specifically reserved for this purpose.
The Modified Slotted-Aloha access method is used on the inbound control
channel. The nodes trigger their transmit off the trailing edge of the NMS
outbound messages. The nodes transmit in burst mode at the typical data rate
of 19.2 kbps on the inbound control channel.
In the Slotted-Aloha access method, the bursts can collide resulting in lost
data. To guarantee the reception of a message at the NMS, the protocol relies
on an HDLC level acknowledgment being sent by the NMS for every inbound
message. If the message is not received by the NMS, then the message is not
acknowledged. In this case, the node retransmits the message after a random
delay.