Ethernet Flow Control; Half-Duplex Flow Control; Full-Duplex Flow Control; Ethernet Daisy Chain - Radyne DMD20 Installation And Operation Manual

User Interfaces DMD20/DMD20 LBST Universal Satellite Modem

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 and is
described in the following sub-paragraphs:

4.8.1.1 Half-Duplex Flow Control

In half-duplex mode, the DMD20 uses industry standard backpressure to support flow control as
follows:
When available buffer space is almost gone, the DMD20 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 DMD20 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 DMD20 implements IEEE 8802.3x flow control as follows:
When available buffer space is almost gone, the DMD20 sends out a pause frame with the
maximum pause time to stop the remote nodes from transmitting.
The DMD20 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 DMD20 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.
4-58 TM103 - Rev. 2.5