KS8999 Micrel Inc, KS8999 Datasheet - Page 23

• Manufacturer Part Number: KS8999
• Description: IC SWITCH 10/100 9PORT 208PQFP
• Manufacturer: Micrel Inc

Specifications of KS8999

• Applications: *
• Mounting Type: Surface Mount
• Package / Case: 208-MQFP, 208-PQFP
• Fiber Support: Yes
• Integrated Led Drivers: Yes
• Phy/transceiver Interface: MII/SNI
• Power Supply Type: Analog/Digital
• Package Type: PQFP
• Data Rate (typ): 10/100Mbps
• Vlan Support: Yes
• Operating Temperature (max): 70C
• Operating Temperature (min): 0C
• Mounting: Surface Mount
• Jtag Support: No
• Operating Temperature Classification: Commercial
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• For Use With: 576-1023 - BOARD EVAL EXPERIMENT FOR KS8999
• Lead Free Status / RoHS Status: Not Compliant, Contains lead / RoHS non-compliant

KS8999
Micrel
Backoff Algorithm
The KS8999 implements the IEEE Std 802.3 binary exponential back-off algorithm, and optional “aggressive mode” back off.
After 16 collisions, the packet will be optionally dropped depending on the chip configuration.
Late Collision
If a transmit packet experiences collisions after 512 bit times of the transmission, the packet will be dropped.
Illegal Frames
The KS8999 discards frames less than 64 bytes and can be programmed to accept frames up to 1536 bytes. Since the KS8999
supports VLAN tags, the maximum sizing is adjusted when these tags are present.
Flow Control
The KS8999 supports standard 802.3x flow control frames on both transmit and receive sides.
On the receive side, if the KS8999 receives a pause control frame, the KS8999 will not transmit the next normal frame until
the timer, specified in the pause control frame, expires. If another pause frame is received before the current timer expires,
the timer will be updated with the new value in the second pause frame. During this period (being flow controlled), only flow
control packets from the KS8999 will be transmitted.
On the transmit side, the KS8999 has intelligent and efficient ways to determine when to invoke flow control. The flow control
is based on availability of the system resources, including available buffers, available transmit queues and available receive
queues.
The KS8999 will flow control a port, which just received a packet, if the destination port resource is being used up. The KS8999
will issue a flow control frame (XOFF), containing the maximum pause time defined in IEEE standard 802.3x. Once the resource
is freed up, the KS8999 will send out the other flow control frame (XON) with zero pause time to turn off the flow control (turn
on transmission to the port). A hysteresis feature is provided to prevent flow control mechanism from being activated and
deactivated too many times.
The KS8999 will flow control all ports if the receive queue becomes full.
Half Duplex Back Pressure
Half duplex back pressure option (Note: not in 802.3 standards) is also provided. The activation and deactivation conditions
are the same as the above in full duplex mode. If back pressure is required, the KS8999 will send preambles to defer other
stations’ transmission (carrier sense deference). To avoid jabber and excessive deference defined in 802.3 standard, after a
certain time it will discontinue the carrier sense but it will raise the carrier sense quickly. This short silent time (no carrier sense)
is to prevent other stations from sending out packets and keeps other stations in carrier sense deferred state. If the port has
packets to send during a back pressure situation, the carrier sense type back pressure will be interrupted and those packets
will be transmitted instead. If there are no more packets to send, carrier sense type back pressure will be active again until switch
resources free up. If a collision occurs, the binary exponential back-off algorithm is skipped and carrier sense is generated
immediately, reducing the chance of further colliding and maintaining carrier sense to prevent reception of packets.
Broadcast Storm Protection
The KS8999 has an intelligent option to protect the switch system from receiving too many broadcast packets. Broadcast
packets will be forwarded to all ports except the source port, and thus will use too many switch resources (bandwidth and
available space in transmit queues). The KS8999 will discard broadcast packets if the number of those packets exceeds the
threshold (configured by strapping during reset and EEPROM settings) in a preset period of time. If the preset period expires
it will then resume receiving broadcast packets until the threshold is reached. The options are 5% of network line rate for the
maximum broadcast receiving threshold or unlimited (feature off).
January 2005
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KS8999