Radyne DMD2050, Operation Manual

Page: 113 / 214

User Interfaces DMD2050 Universal Satellite Modem
4-56 TM110 - Rev. 1.3
4.8.1 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.
4.8.1.1 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.
4.8.1.2 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.
The interface sends out another pause frame with the pause time set to zero as soon as free
buffer space becomes available.
4.8.2 Ethernet Daisy Chain
When disabled, Port 4 (JS4) on the Ethernet Data Interface operates normally. Data received on
Port 4 that is not addressed to other equipment on the LAN side, is transmitted over the satellite.
When Port 4 is selected for Daisy Chain, any data received on Port 4 (JS4) is forwarded to of the
other LAN side ports (Ports 1 - 3) and is not transmitted over the satellite. This is extremely
useful in a point-to -multipoint configuration as illustrated in Figure 4-7.