zl50415 Zarlink Semiconductor, zl50415 Datasheet - Page 36

• Manufacturer Part Number: zl50415
• Description: Managed 16-port 10/100 M + 2-port 1 G Ethernet Switch
• Manufacturer: Zarlink Semiconductor

no greater than 64, and no greater than the peak rate. For example, if the setting is 16, then the average rate for
shaped traffic is 16/64 * 1000 Mbps = 250 Mbps. As a consequence of the above settings in our example, shaped
traffic will exit the ZL50418 at a rate always less than 500 Mbps, and averaging no greater than 250 Mbps. See
Programming QoS Register Application Note for more information.
Also, when shaping is enabled, it is possible for a P6 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 to prevent this situation locally. We
assume SP traffic is policed at a prior stage to the ZL50418.
7.7
The ZL50418 provides a rate control function on its 10/100 ports. This rate control function applies to the outgoing
traffic aggregate on each 10/100 port. It provides a way of reducing the outgoing 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 10/100 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 10 ms.
The value of M/N will, of course, equal the average data rate of the outgoing traffic aggregate on the given 10/100
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 10/100 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.
Example: Suppose that the user wants to restrict Fast Ethernet port P’s average departure rate to 32 Mbps – 32%
of line rate – when the average is taken over a period of 10 ms. In an interval of 10 ms, exactly 40000 bytes can be
transmitted at an average rate of 32 Mbps.
So how do we set the parameters? The rate control parameters are contained in an internal RAM block accessible
through the CPU port (See Programming QoS Registers application note and Processor Interface Application
Note). The data format is shown below.
As we indicated earlier, the number of bytes is measured in 8-byte increments, so the 16-bit field “Number of bytes”
should be set to (40000/8) 500. In addition, the time interval has to be indicated in units of 10 ms. Though we want
the average data rate on port P to be 32 Mbps when measured over an interval of 10 ms, we can also adjust the
maximum number of bytes that can be transmitted at full line rate in any single burst. Suppose we wish this limit to
be 12 kilobytes. The number of bytes is measured in 8-byte increments, so the 16-bit field “Maximum burst size” is
set to (12000/8) 1500.
63:40
0
Rate Control
39:32
Time interval
Zarlink Semiconductor Inc.
ZL50418
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31:16
Maximum burst size
15:0
Number of bytes
Data Sheet