ZL50408GDC ZARLINK [Zarlink Semiconductor Inc], ZL50408GDC Datasheet - Page 41

• Manufacturer Part Number: ZL50408GDC
• Description: Managed 8-Port 10/100M 1-Port 10/100/1000M Ethernet Switch
• Manufacturer: ZARLINK [Zarlink Semiconductor Inc]

7.3
To avoid congestion, the Weighted Random Early Detection (WRED) logic drops packets according to specified
parameters. The following table summarizes the behavior of the WRED logic.
Px is the total byte count, in the priority queue x, can be the strict priority queue of RMAC ports and higher 3 priority
queues for GMAC port. The WRED logic has two drop levels, depending on the value of Px. Each drop level has
defined high-drop and low-drop percentages, which indicate the minimum and maximum percentages of the data
that can be discarded. The X, Y Z percent can be programmed by the register RDRC0, RDRC1. All packets will be
dropped only if the system runs out of the specific buffer resource, per class buffer or per source port buffer. The
WRED thresholds of each queue can be programmed by the QOS control registers (refer to the register group 8).
See Programming QoS Registers application note, ZLAN-42, for more information.
7.4
Although traffic shaping is not a primary function of the ZL50408, the chip does implement a shaper for every queue
in the GMAC port. Our goal in shaping is to control the average rate of traffic exiting the ZL50408. If shaper is
enabled, strict priority will be applied to that queue. The priority between two shaped queue is the same as in strict
priority scheduling.
Traffic rate is set using a programmable whole number, no greater than 64. For example, if the setting is 32, then
the traffic rate transmit out of the shaped queue is 32/64 * 1000 Mbps = 500 Mbps. See Programming QoS Register
application note, ZLAN-42, for more information.
Also, when shaping is enabled, it is possible for a queue to explode in length if fed by a greedy source. The reason
is that a shaper is by definition not work-conserving; that is, it may hold back from sending a packet even if the line
is idle. Though we do have global resource management, we do nothing other than per port WRED to prevent this
situation locally. We assume the traffic is policed at a prior stage to the ZL50408 or WRED dropping is fine and shall
restrain this situation.
7.5
The ZL50408 provides a rate control function on its RMAC ports. This rate control function applies to both the
incoming and outgoing traffic aggregate on each RMAC port. It provides a way of reducing the average rate below
full wire speed. Note that the rate control function does not shape or manipulate any particular traffic class.
Furthermore, though the average rate of the port can be controlled with this function, the peak rate will still be full
line rate.
Two principal parameters are used to control the average rate for a RMAC port. A port’s rate is controlled by
allowing, on average, M bytes to be transmitted every N microseconds. Both of these values are programmable.
The user can program the number of bytes in 8-byte increments, and the time may be set in units of 10us or 1ms.
The value of M/N will, of course, equal the average data rate of the traffic aggregate on the given RMAC port.
Although there are many (M,N) pairs that will provide the same average data rate performance, the smaller the time
interval N, the “smoother” the output pattern will appear.
In addition to controlling the average data rate on a RMAC port, the rate control function also manages the
maximum burst size at wire speed. The maximum burst size can be considered the memory of the rate control
mechanism; if the line has been idle for a long time, to what extent can the port “make up for lost time” by
transmitting a large burst? This value is also programmable, measured in 8-byte increments.
WRED Drop Threshold Management Support
Shaper
Rate Control
High Drop
Low Drop
Px > WRED_L1
Table 9 - WRED Logic Behaviour
X%
Y%
Zarlink Semiconductor Inc.
ZL50408
41
Px > WRED_L2
100%
Z%
BM Reject
100%
100%
Data Sheet