Comtech EF Data DMD1050TS, Installation And Operation Manual

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DMD1050TS Satellite Modem Board
Revision 1
Theory of Operation 3–14 MN-DMD1050TS
3.7.1.3 Half-Duplex Flow Control
In half-duplex mode, the unit uses industry standard backpressure to support flow control:
•
When available buffer space is almost gone, the modem causes a collision on the input
port when it senses an incoming packet. This collision causes the transmitting station to
back off, and retry the transmission.
•
The interface will stop causing collisions, as soon as free buffer space becomes
available.
3.7.1.4 Full-Duplex Flow Control
In full-duplex mode, the interface implements IEEE 8802.3x flow control:
•
When available buffer space is almost gone, the unit sends out a pause frame with the
maximum pause time, to stop the remote nodes from transmitting.
•
The interface sends out another pause frame with the pause time set to zero, as soon as
free buffer space becomes available.
3.7.1.5 Ethernet Daisy Chain
When Ethernet Daisy Chain is disabled, Port 2 on the Ethernet Data Interface operates normally.
Data received on Port 2 that is not addressed to other equipment on the LAN side, is transmitted
over the satellite.
When Port 2 is selected for Daisy Chain, any data received on Port 2 is forwarded to one of the
other LAN side ports (Port 1) and is not transmitted over the satellite. This is useful in a point-to-
multipoint configuration, as shown in Figure 3-6.
3.7.1.6 Ethernet Quality of Service (QoS) Type
When Normal QoS is selected, the interface sets a packet’s priority based on these criteria:
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IEEE 803.3ac Tag when present
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IPv4 Type of Service / Differentiated Services Field
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Ipv6 Traffic Class
When Port Based QoS is selected, the interface sets the priority of a packet based on the port on
which it arrived:
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Port 1 (JS2) has the highest priority
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Port 2 (JS3) has the second highest priority
3.7.1.7 Ethernet QoS Queue
When Fair Weighted queuing is selected, the interface transmits packets at a rate of 8, 4, 2 and
1, from the highest priority queue to the lowest, respectively. With Fair Weighted queuing, all
queues with data in them are guaranteed to receive some bandwidth.
When Strict Priority is selected, the interface transmits packets from the highest priority queue
until it is empty. It then begins transmitting data from the next highest priority queue. If higher
priority data arrives, the interface finishes the current packet, and then goes back to transmitting
packets from the higher priority queue, until it is empty again. Be careful when selecting Strict
Priority, because it is possible for the lower priority queues to be stalled indefinitely.