Comtech EF Data MIDAS Version 4.X Basic Configuration User Manual
Page 61
MIDAS 4.2 Basic Configuration, Rev. 1
Setup and Operation 2–27
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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 controller server 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 controller server 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 kbit/s, 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 controller server 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 controller
server outbound messages. The nodes transmit in burst mode at the typical data
rate of 19.2 kbit/s 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 controller server, the
protocol relies on an HDLC level acknowledgment being sent by the controller
server for every inbound message. If the message is not received by the
controller server, then the message is not acknowledged. In this case, the node
retransmits the message after a random delay.