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KS8995MA Datasheet(PDF) 28 Page - Micrel Semiconductor |
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KS8995MA Datasheet(HTML) 28 Page - Micrel Semiconductor |
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28 / 79 page  Micrel, Inc. KS8995MA/FQ Semptember 2008 28 M9999-091508 Half-Duplex Back Pressure The KS8995MA/FQ also provides a half-duplex back pressure option (note: this is not in IEEE 802.3 standards). The activation and deactivation conditions are the same as the ones given for full-duplex mode. If back pressure is required, the KS8995MA/FQ sends preambles to defer the other station's transmission (carrier sense deference). To avoid jabber and excessive deference as defined in IEEE 802.3 standard, after a certain period of time, the KS8995MA/FQ discontinues carrier sense but raises it quickly after it drops packets to inhibit other transmissions. This short silent time (no carrier sense) is to prevent other stations from sending out packets and keeps other stations in a carrier sense deferred state. If the port has packets to send during a back pressure situation, the carrier- sense-type back pressure is interrupted and those packets are transmitted instead. If there areno more packets to send, carrier-sense-type back pressure becomes active again until switch resources are free. If a collisionoccurs, the binary exponential backoff algorithm is skipped and carrier sense is generated immediately, reducing the chanceof further colliding and maintaining carrier sense to prevent reception of packets.To ensure no packet loss in 10BASE-T or 100BASE-TX half-duplex modes, the user must enable the following: • Aggressive backoff (Register 3, bit 0) • No excessive collision drop (Register 4, bit 3) • Back pressure (Register 4, bit 5) These bits are not set as the default because this is not the IEEE standard. Broadcast Storm Protection The KS8995MA/FQ has an intelligent option to protect the switch system from receiving too many broadcast packets. Broadcastpackets are normally forwarded to all ports except the source port and thus use too many switch resources (bandwidth andavailable space in transmit queues). The KS8995MA/FQ has the option to include “multicast packets” for storm control. Thebroadcast storm rate parameters are programmed globally and can be enabled or disabled on a per port basis. The rate is basedon a 50ms interval for 100BT and a 500ms interval for 10BT. At the beginning of each interval, the counter is cleared to zeroand the rate limit mechanism starts to count the number of bytes during the interval. The rate definition is described in Registers6 and 7. The default setting for Registers 6 and 7 is 0x4A (74 decimal). This is equal to a rate of 1%, calculated as follows: 148,800 frames/sec ¥ 50ms/interval ¥ 1% = 74 frames/interval (approx.) = 0x4A |
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