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MVTX1100 Datasheet(PDF) 11 Page - Zarlink Semiconductor Inc |
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MVTX1100 Datasheet(HTML) 11 Page - Zarlink Semiconductor Inc |
11 / 40 page MVTX1100 Data Sheet 10 Zarlink Semiconductor Inc. WRED allows traffic to continue flowing into ports on a switch, and randomly drops packets with different probabilities based upon each packet’s priority markings. As the switch congestion increases, the probability of dropping an input packet increases, and as congestion decreases, the probability of dropping an input packet decreases. In this manner, only traffic flows that have had packets dropped will be affected by the congestion. Other traffic flows will see no effect. The following table summarizes the WRED operation of the MVTX1100. It lists the buffer thresholds at which each drop probability takes effect. Table 1 - WRED Operation of the MVTX1100 The WRED packet drop capabilities of MVTX1100 are enabled by performing the following three steps: 1. Select the TOS/DS or VLAN Tag field as the control for packet dropping. The selection is made using bit 7 of the Flooding Control (FCR[7]) register. - FCR[7]=0, use VLAN Priority Tag field to map the drop priority if this Tag field exists - FCR[7]=1, use ToS/DS field for IP packet transmission priority mappin. 2. Select which TOS/DS Tag field to use for packet dropping provided that the TOS/DS field was selected in step 1. The selection is made using bit 7 of the FCB Buffer Low Threshold (FCBST[7]) register. - FCBST[7]=0, use DTR subfield to map the drop priority - FCBST[7]=1, use IP precedence subfield to map the drop priority 3. Set the drop mappings from the TOS/DS or VLAN Tag field to the high or low drop priority output flag. The selec- tion is made using the VLAN Drop Map (AVDM) and TOS Discard Map (TOSDML) registers. Note that to utilize the QoS function of the MVTX1100, flow control has to be disabled. 4.0 Buffer Management MVTX1100 stores each input packet into the external frame buffer memory while determining the destination the packet is to be forwarded to. The total number of packets that can be stored in the frame buffer memory depends upon the size of the external SBRAM that is utilized. For a 256 k byte SBRAM MVTX1100 can buffer 170 packets. For a 512 K byte SBRAM MVTX1100 can buffer 340 packets. In order to provide good QoS characteristics, MVTX1100 must allocate the available buffer space to low and high priority unicast and multicast traffic. This can be accomplished using the external EEPROM to load the appropriate values into MVTX1100 configuration registers. To allow the designer to set the minimum number of buffers provided for low drop priority unicast traffic, use the Low Drop Priority Buffer Threshold (LPBT[7:0]) register. To set the maximum number of buffers allocated for all multicast packets, use the Multicast Buffer Control (MBCR[7:0]) register. During operation MVTX1100 will continuously monitor the amount of frame buffer memory that is available, and when the unused buffer space falls below a designer configurable threshold, MVTX1100 will begin to drop incoming packets (WRED). This threshold is set using the FCB Buffer Low Threshold (FCBST[5:0]) register. WRED Threshold Drop Percentage Condition for High Priority Queue Condition for Low Priority Queue Drop Percentage for High-Drop Packet Drop Percentage for Low-Drop Packet Level 0 Total buffer space available in device is ≤LPBT 50% 0% Level 1 24 buffers occupied 72 buffers occupied 75% 25% Level 2 84 buffers occupied 100% 50% |
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Descripción similar - MVTX1100 |
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