4.5
Auto-negotiation, full-duplex mode
Full-duplex Ethernet provides separate Transmit and Receive data paths, enabling
simultaneous bi-directional collision-free data movements on a port. The network
topology must be a “star” type, not a “bus” type. With full-duplex mode, the cable
distance is only limited by the physical layer line driver and cable attenuation. There are
no collision-domain restrictions or limitations.
The RFL 3000 4K-Series Switches perform half- or full-duplex mode auto-
negotiation independently on all switched ports. If the device or node on the other end of
a port’s attached cable supports auto-negotiation, the RFL 3000 4K24 Switch will
negotiate to run full-duplex. If the attached device or node doesn’t support auto-
negotiation (for example, if it is a 10 Mb repeater or a standard 100Mb hub or older
Switch w/ Fixed 100 Full duplex), the 4K24-Switch’s RJ-45 ports will default to operate
at half-duplex after the negotiation.
4.6
Flow-control, IEEE 802.3x standard
RFL 3000 4K- Series 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 speed
difference makes the 10Mb port 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.
RFL 3000 4K-Series 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 switched 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 indicate 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 4K24-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.
