Comtech EF Data DMD1050 - Manual (Page 36)

DMD1050 Satellite Modem Board Theory of Operation
MN-DMD1050 3–10
Revision 9
3.7.1.1 Configuring the Modem to use the Ethernet Data Interface
When the optional Ethernet Data Interface Card is selected, all of the Ethernet related menus can
be used to control the interface as follows:
Setup the TX Interface Menu to: (refer to Section 7.3.4.3 Figure 7-16 for Interface/TX Setup)
Set the Terrestrial Interface to Ethernet.
Set the Ethernet Flow Control as desired
Set the Ethernet Daisy Chain as desired
Set the Ethernet QOS Type as desired
Set the Ethernet QOS Queue as desired
Tx Clock is set to SCTE.
Set the Tx Clock Polarity to Normal.
Setup the RX Interface Setup Menu to: (refer to Section 7.3.4.3 Figure 7-17 for Interface/RX
Setup)
Set the Terrestrial Interface to Ethernet.
Set the Buffer Size to Zero.
Buffer Clock is set to Rx Sat.
Set the Buffer Clock Polarity to Normal.
3.7.1.2 Ethernet Flow Control
When disabled, if a packet is received for transmission and no packet buffer space is available,
the incoming packet is discarded.
When enabled, flow control is used to throttle the transmission station in order to avoid
overrunning the transmit buffers, which would in turn cause packets to be dropped. The
throttling mechanism used depends upon the interface and whether it is half-duplex or full
duplex.
3.7.1.3 Half-Duplex Flow Control
In half-duplex mode, the unit uses industry standard backpressure to support flow control as
follows:
When available buffer space is almost gone, the modem will force a collision on the input port
when it senses an incoming packet. This collision will cause the transmitting station to back off
and retry the transmission.
The interface will stop forcing 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 as follows:
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.