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CHAPTER 4: OPERATION OPERATION
MULTILINK ML1200 MANAGED FIELD SWITCH – INSTRUCTION MANUAL 4–3
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 ML1200’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.
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
10Mb 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.
Some NIC cards only auto-negotiate when the computer system that they are in is
powered. 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 Multilink ML1200 Fast Ethernet
switched ports operating at 100Mb half-duplex, the bit time delay is 50BT.
4.1.4 Flow-control, IEEE 802.3x standard
Multilink ML1200 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.
Multilink ML1200 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 ML1200 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.
When in Half-Duplex mode, the ML1200 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.