AL103 Revision 1.0
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Collision based backpressure can cause a late collision.
After 16 consecutive collisions, the MAC could drop frames. The AL103 has an
option not to drop frame after 16 collisions. However, the end terminal may still
drop frames.
Therefore, we recommend the use of carrier based backpressure as the preferred method for half-
duplex flow control. In this mode of operation, we also recommend that the IPG of the JAM signal
should be set at less than 96BT. This is because if the IPG is at 96 BT, the far end terminal might
still be able to transmit the frame and cause a collision. The excessive collision could cause frames
to be dropped.
The AL103 also supports collision-based backpressure for customers that prefer collision-based
backpressure.
3.8.2 Full Duplex Flow Control (802.3x)
In the full-duplex mode, the AL103 will transmit and receive the frame in accordance to 802.3x.
Note that the transmission channel and the receiving channel operate independently.
In the incoming direction, whenever the receive frame buffer of a port is full, the MAC of the port
will send out a PAUSE frame with its delay value set to maximum. The PAUSE frame will deter any
incoming frame from flowing into the port. After the receive frame buffer is reduced below the
backpressure watermark level (register 04, bit 11:10), the MAC of the port will then send out a
PAUSE frame with the delay value set to zero to resume receiving the incoming frame flow.
In the outgoing direction, whenever a incoming PAUSE frame with a non-zero delay value is
received through a port, the MAC of the port will stop the next frame transmission after the ongoing
frame transmission is finished. It will start its pause timer and resume frame transmission either
after the pause timer expired or when a PAUSE frame with a zero delay value is received.
When the 802.3x flow control option is elected, the device will program the appropriate bit in the
auto-negotiation capability field. When the AL103 is used in the full-duplex mode, it is
recommended that flow control should be turned on which prevents the buffer from overflow and
loss of frames. If the connected device has no 802.3x capability, then the recommended link setting
is half-duplex.
3.9 Queue Management
The AL103 ports have an advanced queue management algorithm for optimal switching
performance. All frames received by AL103 are stored into the shared memory. If the frame is
unicast type, the location of the frame in the buffer is then passed to the destination output queue
manager. It is up to the Destination Output Queue Manger to extract the frame from the buffer and
transmit. If the output queue manager receives more frames than it can send out, it simply stores the
locations of the frames and transmits them after transmitting the current frame.
There are two ways to manage the output queues. One method is that all eight output queues will
share the frame buffer to the shared memory limit, without limit to each individual queue. When
this method is chosen by setting bit 00.15 to “0,” shared buffer memory is allocated to the incoming
frame from any port as long as free buffer is available. When extreme cases of congestion are
experienced, such as traffic merging into a single port or speed mismatch for a long period of time,
a single output queue may occupy the entire shared buffer causing other ports to drop frames. In
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