8
ACD Confidential. Do Not Reproduce. Use under Non-Disclosure agreement only. Data Sheet: ACD821 12
INTRODUCTORY
The
reserved pool
guarantees each port will have a
fair network access possibility, even under the worst
traffic congestion situation. It takes about 50% of the
total buffer and is evenly allocated to each port as its
dedicated buffer slot. The dedicated slot is not shared
with other ports.
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 con-
gested 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 remain-
ing 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.
Under full duplex operation, 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,
a
max-pause-frame
( a pause frame with a maximum
time interval of FFFFh) will be sent to the sending
party to stop it from sending new frames. If pause-
frame based flow control is not enabled at that port,
the frame will be dropped. 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 linking party to enable
new frame transmission.
Under half duplex operation, 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, the port will execute back-pressure based
flow control by sending a jam pattern on each incom-
ing 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-13 of
register-25 is set), flow control will be triggered when
the broadcast queue is longer than the upper thresh-
old specified by
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 shorten below the lower threshold specified
by
Register-11
, a mini-pause-frame will be sent to
release the hold on transmission.
VLAN Support
(Registers 23 & 24)
The ACD821 12 can support up to 4 port-based security
VLANs. Each port of the ACD821 12 can be assigned to
up to four VLAN. On power up, every port is assigned
to VLAN-I as the default VLAN. Frames from the source
port will only be forwarded to destination ports within
the same VLAN domain. A broadcast/multicast frame
will be forwarded to all ports within the VLAN(s) of the
source port. A unicast frame will be forwarded to the
destination port only if the destination port is in the same
VLAN as the source port. Otherwise, the frame will be
treated as a frame with unknown DA. Each VLAN can
be assigned with a dedicated dumping port. Multiple
VLANs can also share a dumping port. Unicast frames
with unknown destination addresses will be forwarded
to the dumping port of the source port VLAN.
Security VLAN can be disabled by setting the corre-
sponding bit in the system configuration register (bit 8
of
Register 16, see Table 7.15
). When security VLAN
is disabled, each VLAN becomes a Leaky VLAN and is
equivalent to a broadcast domain. Four dumping ports
of four different Leaky VLANs can be grouped together
to form a fat pipe uplink (for example, port 0, port 1,
port 2, and port 3 can be grouped to form an 800 Mbps
uplink port). When multiple dumping ports are grouped
as a single pipe, each port has to be assigned to one
and only one VLAN. A unicast frame with a matched
DA will be forwarded to any destination port, even if the
VLAN ID is different. All unmatched DA packets will be
forwarded to the designated dumping port of the source
port VLAN. The broadcast and multicast packets will
only be forwarded to the ports in the same VLAN of the
source port. Therefore, a 200 to 800 Mbit/s pipe can be
established by carefully grouping the dumping ports,
and directly connecting with any segmentation switches.
Dumping Port
Each VLAN can be assigned with a dedicated dumping
port. Multiple VLANs can share a dumping port. Each
dumping port can be used for an up-link connection or
for a DTE connection. That is, the dumping port can be
used to connect the switch with a computer repeater