GE ML1200 instruction manual Flow-control, Ieee 802.3x standard

CHAPTER 4: OPERATION OPERATION

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.

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 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.

4.1.4Flow-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.

MULTILINK ML1200 MANAGED FIELD SWITCH – INSTRUCTION MANUAL 4–3