Spirent SmartBits System, Reference Manual

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Chapter 9: Ethernet Cards
Standard Ethernet Features
120 SmartBits System Reference
Carry ChainingBetween VFD1 and VFD2
Carry chaining allows the carry in of a VFD counter to be tied to the carry out of another
VFD counter, effectively combining disparate VFD’s into a larger VFD. Carry out from a
VFD occurs whenever the cycle count decrements to zero or when the VFD counter
overflows. This can be useful in applications where it is desirable to completely “roll
through” one counter while keeping another counter static. Stutter VFD’s allow the same
but more limited function. Carry chaining is only available for incrementing and
decrementing VFDs (i.e., not random-mode VFDs).
Collisions Detected
Note: Only works in half duplex mode; not applicable to full duplex.
When in half-duplex mode, a port will defer transmission when it senses another station
actively transmitting. When a collision occurs, the port obeys the IEEE standard truncated
binary exponential back-off timing algorithm. The back-off delay is an integer multiple of
the slot time. The number of slot times to delay before the nth retransmission attempt is
chosen as a uniformly distributed random integer in the range:
0 < slotTimes < 2 k
where:
k = min (n, BackoffTruncationExponent)
The Backoff Truncation Exponent parameter is configurable on a per-port basis in
SmartWindow and SmartLib. The minimum exponent is 1, and the default/maximum is
10. By setting this parameter lower than 10, it is possible to make a port “more
aggressive” than normal as the value of n increases.
The Collisions Event Counter indicates the cumulative number of collisions that have
occurred on the transmitting port of the SmartCard/module. The Collisions Rate Counter
displays the number of collisions per second.
CRC Errors
All SmartBits Ethernet cards can generate CRC-errored frames.
Data Integrity Check
The receiving card checks the frame contents for data integrity errors (CRC-16 errors) and
counts the frames with them.
Duplex Mode
Available modes: full (unidirectional) and/or half (bidirectional).