Spirent SmartBits System, Reference Manual

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Chapter 13: Token Ring Cards
Token Ring Operational Detail
SmartBits System Reference 177
that the maximum byte count (product of the sequence length and number of frames
in a set) has to be less than 2048 bytes.
The VFD patterns are overlaid in sequence so that VFD2 will overlay VFD1 and VFD3
will overlay VFD1 and VFD2.
The offset of the VFDs is computed from the start of the Access Control byte.
VFDs will not appear outside their frame boundaries. If they are placed outside, then they
will be truncated or switched off as appropriate. VFDs 1 & 2 can be placed between the
first byte of the Destination MAC address and the last byte of the frame payload. VFD3
can be placed between the Frame Control byte and the last byte of the frame payload.
The fill pattern does not start at the first byte of the frame. The first byte, the Frame
Control , is set from the Token Ring specific dialog box or is overwritten by the first byte
of a VFD3 pattern overlaid at byte offset 1. The fill pattern will start after this byte if
preset headers (see below) are disabled or at the first byte following the header if these are
enabled.
VFDs are reset to their initial values on every start command or on a step command that
follows a parameter change without an intervening start command.
Random Length Frames, Random Gaps and Random Data
The user can specify whether the frames are to be random length, to have random gaps
between them and whether they have random data in them. VFDs 1 & 2 can also be
specified as random.
Random frame lengths, frame gaps and frame data is not truly random because frames are
precalculated before the card is started and then reused while the card is transmitting. The
frame lengths and gaps will repeat every few hundred frames or so as the basic frame
structures cycle. VFD random patterns are also cyclical but because these are calculated
for each frame from a 32 bit feedback shift register their cycle time is quite long.
Editable Token Ring Headers
Networks users have to be careful about what data is placed in the Token Ring specific
parts of generated frames. Incorrect data can cause other stations to malfunction or ring
faults. Users can avoid these problems by using an optional pre-formed frame header that
acts like a set of Token Ring specific templates. The header comes in five parts:
•
Access Control (AC) byte . The Access Control byte is the first byte following the
Start Delimiter. All offsets in the Token Ring Smartcard are referenced from this byte.
This byte can be altered by a user command, changing all fields except the
token
marker. (This byte cannot be changed.)
•
Frame Control (FC) byte . The Frame Control byte is set from either an applications
control or by overwriting it with the first byte of a VFD3 pattern. This byte is preset to
0x40 when the optional LLC field is enabled.