ACD82224 Datasheet, PDF (14/77 Pages) List of Unclassifed Manufacturers – 24 Ports 10/100 Fast Ethernet Switch

The common pool provides a deep buffer for the busy ports (e.g. server port) to serve multiple
low speed ports (e.g. client port) simultaneously. It helps to avoid head-of-line blocking. It takes
about 30% of the total buffer and is shared by all ports. It stores the congested traffics before the
flow control mechanism is triggered.
The extra pool is reserved only for ports with pause frame based flow control capacity. It takes
the remaining 20% of the total buffer. It is used to minimize the chance of frame dropping by
buffering for the latency of the pause frame based flow control scheme. It is used only after a
flow control mechanism is triggered.
Flow Control Scheme
Flow control activity is triggered when the buffer utilization exceeds certain thresholds specified
by the dedicated registers. Register-10 is used to specify the Upper and the Lower Thresholds of
the reserved buffer slot for each port. Register-11 is used to specify the Upper and the Lower
thresholds of the broadcast queue.
For full duplex with pause frame capability operation:
(1) If the buffer utilization of the reserved buffer slot for the source port has exceeded the
“Upper Threshold”, and the common pool has been used up.
(2) Then a max-pause-frame (a pause frame with a maximum time interval of FFFFh) will be
sent to the sending port to stop it from sending any new frames. If pause-frame based flow
control is not enabled at that port, the frame will be dropped.
(3) Once a Max-Pause-Frame is sent, if the utilization of the reserved buffer slot of the port
drops below the lower threshold, a Mini-Pause-Frame (a pause frame with minimum time
interval of 0) will be sent to the sending port to enable new frame transmission. But if the
utilization of the reserved buffer slot of the port does not drop below the lower threshold for
the maximum time interval, ACD82224 will not send another Max-Pause-Frame to the
sending port to prevent the Buffer Capturing by other ports.
For half duplex operation:
(1) If the buffer utilization of a port has exceeded the upper threshold of the reserved buffer slot,
and the common pool has been used up.
(2) The port will execute backpressure based flow control by sending a jam pattern on each
incoming frame. If backpressure flow control of the port is not enabled, the frame will be
dropped.
If the broadcast flow control is enabled (when bit-17 of register-25 is cleared), flow control will be
triggered when the broadcast queue is larger than the “Upper Threshold” in Register-11. All full
duplex ports with pause-frame capability will send a Max-Pause-Frame to its linking party. All
half-duplex ports with backpressure capability will jam incoming frames. After a Max-Pause-
Frame is sent, and if the broadcast queue is below the “Lower Threshold” in Register-11, a Mini-
Pause-Frame will be sent to release the hold on transmission.
Port-based VLAN Support (Registers 23 & 24)
The ACD82224 can support up to 4 port-based security VLANs. Each port of the ACD82224 can
be assigned to up to four VLAN. On power up, every port is assigned to VLAN-I as the default
Page 13 of 77
Confidential
Page 13