DS33R41 Maxim Integrated Products, DS33R41 Datasheet - Page 52

• Manufacturer Part Number: DS33R41
• Description: Network Controller & Processor ICs Inverse-Multiplexing Ethernet Mapper wit
• Manufacturer: Maxim Integrated Products

DS33R41 Inverse-Multiplexing Ethernet Mapper with Quad Integrated T1/E1/J1 Transceivers
configurable by the user but it is recommended that the high threshold be set approximately 96 packets from the
maximum size of the queue and the low threshold 96 packets lower than the high threshold. The device will send a
pause frame as the queue has crossed the high threshold and a frame is received. Pause is sent every time a
frame is received in the “high threshold state.” Pause control will only take care of temporary congestion. Pause
control does not take care of systems where the traffic throughput is too high for the queue sizes selected. If the
flow control is not effective the receive queue will eventually overflow. This is indicated by SU.QCRLS.RQOVFL
latched bit. If the receive queue is overflowed any new frames will not be received.
The user has the option of not enabling automatic flow control. In this case the thresholds and corresponding
interrupt mechanism to send pause frame by writing to flow control busy bit in the MAC flow control registers
SU.MACFCR.FCB, SU.GCR.JAME, and SU.MACFCR. This allows the user to set not only the watermarks but also
to decide when to send a pause frame or not based on watermark crossings.
On the receive side the user has control over whether to respond to the pause frame sent by the distant end (PCF
bit). Note that if automatic flow control is enabled the user cannot modify the FCE bit in the MAC flow control
register. On the Transmit queue the user has the option of setting high and low thresholds and corresponding
interrupts. There is no automatic flow control mechanism for data received from the Serial side waiting for
transmission over the Ethernet interface during times of heavy Ethernet congestion.
Figure 9-6. Flow Control Using Pause Control Frame
8
Receive Queue Low
Water
Rx
Receive Queue High
Data
Water Mark
Initiate Flow control
Receive Queue
Growth
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