ACD82224 Datasheet, PDF (11/77 Pages) List of Unclassifed Manufacturers – 24 Ports 10/100 Fast Ethernet Switch

residue after the polynomial division, is the value of the frame check sequence. This value
should be equal to the FCS field appended at the end of the frame. If the value does not match
the FCS field of the frame, the Frame Bit Error LED of the port will be turned on once and the
packet will be dropped.
Illegal Frame Length & Extra-long Frame
During the receiving process, the MAC will monitor the length of the received frame. Legal
Ethernet frames should have a length of not less than 64 bytes and no more than 1518 bytes. If
the carrier-sense signal (CRS_DV) of a frame is asserted for less than 84 BT, the frame is
flagged with short event error. If the length of a frame is less than 512 BT, the frame is flagged
with runt error.
In order to support an application where extra byte length is required, an Extra long frame option
is provided. When the Extra long frame option is enabled (bit-11 of Register 25), only frames
longer than 1530 bytes are marked with a long event error. Frame length is measured from the
first byte of DA to the last byte of FCS.
Frame Filtering
Frames with any kind of error will be filtered. Error types include CRC, alignment, false carrier
sense, short event, runt, long event and jabber. An error frame will still be displayed on the MIB
interface, along with the error status indication.
Any frame heading to its own source port will be filtered.
When external ARL is used, the filtering decision will be made by the ARL. The ACD82224 will
act in accordance with the ARL’s decision.
If the Spanning Tree Support option (Bit 1 of Register 16) is set, a frame with DA equal to 01-80-
C2-00-00-00 will be forwarded to port-23 (the default CPU port) as a BPDU frame. If Spanning
Tree Support is not enabled, a frame with reserved group address specified in Table-3.5 of the
IEEE802.1d will be treated as a broadcast frame and will be forwarded to all the ports in the
same VLAN of the source port.
Jabber Lockup Protection
If a receiving port is active continuously for more than 50,000 bit times, the port is considered to
be jabbering. A jabbering port will automatically be partitioned from the switch system in order to
prevent it from impairing the performance of the network. The partitioned port will be re-activated
as soon as the offending signal discontinues.
Excessive Collision
In the event that there are more than 16 consecutive collisions, the ACD82224 will reset the
counter to zero and re-transmit the packet. This implementation insures there is no packet loss
even under channel capture situation. However, the ACD82224 has an option to drop the packet
on excessive collisions. When this option is enabled (bit-15 of Register 25), the frame will be
dropped after 16 consecutive collisions.
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