KS8997 Micrel Inc, KS8997 Datasheet - Page 18

• Manufacturer Part Number: KS8997
• Description: IC SWITCH 10/100 8PORT 128PQFP
• Manufacturer: Micrel Inc

Specifications of KS8997

• Applications: *
• Mounting Type: Surface Mount
• Package / Case: 128-MQFP, 128-PQFP
• Fiber Support: No
• Integrated Led Drivers: Yes
• Phy/transceiver Interface: MII
• Power Supply Type: Analog/Digital
• Package Type: PQFP
• Data Rate (typ): 10/100Mbps
• Vlan Support: Yes
• Operating Temperature (max): 70C
• Operating Temperature (min): 0C
• Pin Count: 128
• Mounting: Surface Mount
• Jtag Support: No
• Operating Temperature Classification: Commercial
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

KS8997
Micrel
Late Collision
If a transmit packet experiences collisions after 512-bit times of the transmission, the packet will be dropped.
Illegal Frames
The KS8997 discards frames less than 64 bytes and can be programmed to accept frames up to 1536 bytes. Since the KS8997
supports VLAN tags, the maximum sizing is adjusted when these tags are present.
Flow Control
The KS8997 supports standard 802.3x flow control frames on both transmit and receive sides.
On the receive side, if the KS8997 receives a pause control frame, the KS8997 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 KS8997 will be transmitted.
On the transmit side, the KS8997 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 KS8997 will flow control a port, which just received a packet, if the destination port resource is being used up. The KS8997
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 KS8997 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 KS8997 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 KS8997 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 KS8997 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 KS8997 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).
Programmable Features
Priority Schemes
The KS8997 can determine priority through three different means at the ingress point. The first method is a simple per port
method, the second is via the 802.1p frame tag and the third is by viewing the DSCP (TOS) field in the IPv4 header. Of course
for the priority to be effective, the high and low priority queues must be enabled on the destination port or egress point.
Per Port Method
General priority can be specified on a per port basis. In this type of priority all traffic from the specified input port is considered
high priority in the destination queue. This can be useful in IP phone applications mixed with other data types of traffic where
the IP phone connects to a specific port. The IP phone traffic would be high priority (outbound) to the wide area network. The
inbound traffic to the IP phone is all of the same priority to the IP phone.
802.1p Method
This method works well when used with ports that have mixed data and media flows. The inbound port examines the priority
field in the tag and determines the high or low priority. Priority profiles are setup in the Priority Classification Control in the
“EEPROM Memory Map” section.
KS8997
18
August 2003