One frequently-used application for the Managed DFG48000 Switch copper ports is to connect one of
them using a fiber media converter to another Switch in the network backbone, or to some other
remote 100Mb device. In this case, it is desirable to operate the fiber link at 100Mb speed, and at
either half- or full duplex mode depending on the capabilities of the remote device. Standard
commercially available Fast Ethernet media converters mostly do not support auto-negotiation
properly, and require that the switched port to which they are connected be at 100Mb fixed speed.
Attachment to 10/100 auto-negotiation ports typically will not work properly. The DFG48000 Switch’s
RJ-45 ports handle this situation by configuring the ports as per desire through MNS software port
settings and can check the port status of each port after the change.
When DFG480XX RJ-45 copper ports are set for auto-negotiation and are connected to another auto-
negotiating device, there are 4 different speed and F/H modes possible depending on what the other
device supports. These are: (1) 100Mb full-duplex, (2) 100Mb half-duplex, (3) 10 Mb full-duplex and
(4) 10 Mb half-duplex. The auto-negotiation logic will attempt to operate in descending order and will
normally arrive at the highest order mode that both devices can support at that time. (Since auto
negotiation is potentially an externally controlled process, the original “highest order mode” result can
change at any time depending on network changes that may occur). If the device at the other end is
not an auto-negotiating device, the DFG48000’s RJ-45 ports will try to detect its idle signal to
determine 10 or 100 speed, and will default to half-duplex at that speed per the IEEE standard.
NOTE – Some NIC cards only auto-negotiate when the computer system that they are in is
powered up. These are exceptions to the “negotiate at LINK – enabled” rule above, but may
be occasionally encountered.
When operating in 100Mb half-duplex mode, cable distances and hop-counts may be limited within that
collision domain. The Path Delay Value (PDV) bit-times must account for all devices and cable lengths
within that domain. For DFG48000 Fast Ethernet switched ports operating at 100Mb half-duplex, the bit
time delay is 50BT.
4.5 Flow-control, IEEE 802.3x standard
DFG48000 Switches incorporate a flow-control mechanism for Full-Duplex mode. The purpose of flow-
control is to reduce the risk of data loss if a long burst of activity causes the switch to save frames until
its buffer memory is full. This is most likely to occur when data is moving from a 100Mb port to a 10 Mb
port and the 10Mb port is unable to keep up. It can also occur when multiple 100Mb ports are
attempting to transmit to one 100Mb port, and in other protracted heavy traffic situations. DFG48000
Switches implement the 802.3x flow control (non-blocking) on Full-Duplex ports, which provides for a
“PAUSE” packet to be transmitted to the sender when the packet buffer is nearly filled and there is
danger of lost packets. The transmitting device is commanded to stop transmitting into the DFG48000
Switch port for sufficient time to let the Switch reduce the buffer space used. When the available free
buffer queue increases, the Switch will send a “RESUME" packet to tell the transmitter to start sending
the packets. Of course, the transmitting device must also support the 802.3x flow control standard in
order to communicate properly during normal operation.
Note:
When in Half-Duplex mode, the DFG48000 Switch implements a back-pressure algorithm on
10/100 Mb ports for flow control. That is, the switch prevents frames from entering the device by
forcing a collision indication on the half-duplex ports that are receiving. This temporary “collision” delay
allows the available buffer space to improve as the switch catches up with the traffic flow.
General information -
Auto-negotiation per-port for 802.3u-compliant switches occurs when:
-- The devices at both ends of the cable are capable of operation at either 10 Mb or 100Mb speed
and/or in full- or half-duplex mode, and can send/receive auto-negotiation pulses,
and
-- The second of the two connected devices is powered up*, i.e., when LINK is established for a port,
or
-- The LINK is re-established on a port after being lost temporarily.