Spirent SmartBits System, Reference Manual

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Chapter 9: Ethernet Cards
Standard Ethernet Features
SmartBits System Reference 115
Address Stutter
VFDs may optionally “stutter”—keeping their values constant—for a specified number of
frame transmissions. The stutter counter is 12 bits wide.
Address Skipping, Mask-based
Mask-based address skipping allows VFD1 and VFD2 to be configured with a subnet
mask (a number of bits from 1 to 31). During operation, the VFD will skip host address
values that correspond to the subnet and broadcast addresses. For example, with a 24-bit
subnet mask, a 4-byte VFD will skip the values X.X.X.0 and X.X.X.255. This allows a
single stream to be configured with a VFD that can cover multiple subnets, intelligently
skipping undesirable source and destination addresses. Address skipping is available in
all VFD modes (increment, decrement, random).
Adjustable Preamble Length
Each port allows the length of the Ethernet preamble to be specified between 4 and 16
bytes in 4 byte increments (i.e. 4, 8, 12, or 16 byte preamble). The default value is 8 bytes
(7 preamble bytes plus 1 start frame delimiter byte).
Alignment, Dribble, Under/oversize Errors
Errored frames that can be generated and counted by the card.
The Alignment Errors Event Counter indicates the cumulative number of alignment errors
that have occurred on the receiving port of the SmartCard/module. The Alignment Errors
Rate Counter displays the number of alignment errors detected per second. An alignment
error is a packet that does not end on the octet boundary (regardless of CRC).
Alignment errors are not checked in packets greater than 2033 bytes.
Alternate Streams
An alternate blueprint transmitted in background mode every “n” frames where “n” is a
24-bit counter.
Background Pattern
In SmartWindow and SmartLib and in applications that have a Frame Editor, the
Background Pattern field defines the default contents of transmitted packets. It is called
Background because it may be over written by VFD (Variable Field Data) resources on the
SmartCard/module. The default pattern is all zeros.
The patterns 8-Fs, 8-0s and 8-0s, 8-Fs refer to bytes of 0x00 or 0xFF. The resulting pattern
for 8-Fs, 8-0s is:
0xFF,FF,FF,FF,FF,FF,FF,FF,00,00,00,00,00,00,00,00…