GarrettCom 6K32TRC - Manual (Page 54)

Magnum 6K32T & 6K32TRC Managed Switch Installation and User Guide 05/07)
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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.
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 Magnum 6K32T &
6K32TRC Fast Ethernet switched ports operating at 100Mb half-duplex, the bit time
delay is 50BT.
4.4 Flow-control, IEEE 802.3x standard
Magnum 6K32T & 6K32TRC 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.
Magnum 6K32T & 6K32TRC 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 6K32T
& 6K32TRC 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 6K32T & 6K32TRC 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.
5.0
Magnum 6K32T & 6K32TRC Managed Switch Port Modules
This chapter describes each Port Module (PM), including appearance,
functionality, and status displays.