Spirent SmartBits System, Reference Manual

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Chapter 9: Ethernet Cards
Gigabit Ethernet Testing, Clock Tolerance
132 SmartBits System Reference
Gigabit Ethernet Testing, Clock Tolerance
If a device is transmitting at a higher clock rate than the receiving device, the receiving
device will eventually have to either lose data or buffer it until it can process it. The longer
the continuous stream of data, the more data that must be buffered. Considering the
amount of data that is being transmitted per second at gigabit speeds, considerable buffer
space may be required. The following discussion reviews common gigabit Ethernet clock
problems and shows how to interpret SmartApplications Back-to-Back tests as an example
of how to view test results.
All references to bytes are to 8 bit octets, and all packets will be 64 bytes and have a
preamble of 64 bits including the Start Frame Delimiter.
Clock Tolerance Problem
A receiving port cannot receive at a slower rate than the transmitting device because the
receiving port derives its receive clock from the asynchronous data stream that it is
receiving. Therefore, its receive clock will be the same as the device transmitting to it.
Where the problem comes is that once the switching device receives the data, it must
process it and then transmit this data back out to its destination. The switch processor
clock is usually not the same clock as the receive clock and the transmit clock is also
usually a separate clock.
Assuming that the processor can keep up with the data stream, it must still be able to
transmit this data without loss. If the output transmitter of this switch is slower than the
device transmitting into it, eventually it could lose data. Tests have been designed to test
the ability of a switch to process data and forward it through its switching fabric fast
enough to keep up with large bursts of data. These tests treat the switch as a complete
system, and determine the robustness of a switching device.
The clock tolerance problem in both the device under test and the testing equipment can
cause difficulties when trying to obtain consistent results. This applies when different
types of test analysis equipment or different DUT models are used.
Example of Worst Case Scenario
With 64 byte packets at wire speed, Gigabit Ethernet transmits 1,488,095.238095 packets
per second at perfect clock tolerance. The 802.3z spec calls out for a clock tolerance of
plus or minus 100ppm (Parts Per Million). At plus 100ppm wire rate would be
1,488,244.047619 packets per second and at a minus 100ppm tolerance, wire rate would
be 1,487,946.428571. If we transmit into a port at plus 100ppm and the switch is
transmitting back out at minus 100ppm, then the switch would have to buffer 297.619048
packets per second.
Since we do not know what each switch is doing with the packets, such as stripping off the
MAC layer before buffering or adding management information, such as time stamps, we
assume that it is buffering 64 bytes for each 64 byte packet. That would be 19,047 bytes
per second for this port pair. To keep this simple, we assume unidirectional traffic and all