MDS108
Data Sheet
used to select the transmission priority (as per the USE_TOS bit in the FCR register). The mapping of the priority
field values into either the high or low priority queue can be configured using the MDS108 configuration registers
AVPM and TOSPM.
If the system uses the TOS/DS field to prioritize packets, there are two choices regarding which bits of the TOS/DS
field are used. Bits [0:2] of the TOS byte (known as the IP precedence field) or bits [3:5] of the TOS byte (known as
the Delay/Throughput Reliability or (DTR) field) can be used to resolve the transmission queue priority. Either bit
group, [0:2] or [3:5], can also be used to resolve packet drop precedence, as per bits 6 and 7 of the register FCBST.
The MDS108 utilizes Weighted Round Robin (WRR) to schedule packets for transmission. To enable MDS108’s
intelligent QoS scheduling capabilities, the use of an external EEPROM to change the default register
configurations is required.
Weighted Round Robin is an efficient method to ensure that each of the transmission queues receives at least a
minimum service level. With two output transmission queues, the MDS108 will transmit X packets from the high
priority queue before transmitting a single packet from the low priority queue. The MDS108 allows the designer to
set the high priority weight X to a value between 1 and 15. If both queues contain packets, and the high priority
weight is set to the value 4, then the MDS108 will transmit 4 high priority packets before transmitting each low
priority packet.
The MDS108 also employs a proprietary mechanism to ensure the timely delivery of high priority packets. When the
latency of high priority packets reaches a threshold, the MDS108 will override the WRR weights and transmit only
high priority packets until the high priority packet delays are below the threshold. This threshold limit is 1 ms (last-in-
first-out). The MDS108’s proprietary scheduling algorithm is also designed to push low priority traffic through the
device faster, if necessary, to unclog congested queues.
Loading the appropriate values into the configuration registers enables the QoS scheduling capabilities of the
MDS108. QoS for packet transmission is enabled by performing the following four steps:
1. Select the TOS/DS or VLAN Priority Tag field as the decision-maker for IP packet scheduling. The selection is
made using bit 7 of the Flooding Control Register (FCR).
-
-
FCR[7] = 0: Use VLAN Priority Tag field to determine the transmission priority, if this Tag field exists
FCR[7] = 1: Use TOS/DS field for IP packet priority resolution
2. Select which TOS/DS subfield to use as the decision-maker for packet transmission priority if the TOS/DS field
was selected in step 1. The selection is made using bit 6 of the FCB Buffer Low Threshold Register (FCBST).
-
-
FCBST[6] = 0: Use DTR subfield to resolve the transmission priority
FCBST[6] = 1: Use IP precedence subfield
1
to resolve the transmission priority
3. Enable QoS using bit 5 of the Transmission Scheduling Control Register (AXSC). Set the transmission queue
weight for the high priority queue using bits 0 to 3.
4. Create the mapping from the value in the TOS/DS or VLAN Priority Tag field to the corresponding high or low
priority output queue. The mapping is created using the VLAN Priority Map (AVPM) and TOS Priority Map
(TOSPM) registers.
Note that for half duplex operation, the priority queues
2
must be enabled using bit 7 in the Transmission Scheduling
Control (AXSC) register to use QoS scheduling.
When QoS and flow control are enabled, the MDS108 will utilize enhanced WRR to schedule packet transmission,
and will use either back pressure or 802.3X flow control to handle buffer congestion. When QoS is enabled and flow
control is disabled, the MDS108 will utilize enhanced WRR to schedule packet transmission, and will use Weighted
Random Early Detection/Drop (WRED) to drop random packets in order to handle buffer congestion. Because of
1. IP precedence and DTR subfields are referred to as TOS/DS[0:2] and TOS/DS[3:5] in the IP TOS/DS byte.
2. In Half Duplex mode, QoS scheduling functions are disabled by default.
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