Spirent SmartBits System, Reference Manual

Page: 186 / 278

Chapter 13: Token Ring Cards
Token Ring Operational Detail
178 SmartBits System Reference
Note: Neither the AC nor the FC byte can be switched off. Be careful when changing the
AC and FC bytes from their default values of 0x00 and 0x40 respectively.
•
MAC addresses . The MAC address fields are set from applications controls. These
fields can be overwritten by any VFD pattern. Users should exercise care doing this
because the
Routing Information Present marker (bit 7 of the first byte of the Source
Address) is only set automatically for the base header pattern; users have to ensure
that they do not set this incorrectly when overlaying the source address with a VFD.
If the MAC addresses are disabled, then both the Routing Information and the LLC
fields are also disabled.
•
Source Routing field . The Source Routing RIF (Routing Information) and LLC fields
are optional. If the entire header is enabled, then the frames produced will be LLC
TEST or Ethernet SNAP frames:
– If TEST frames are selected, then the user can control the Source and Desti-
nation SAPs.
– If Ethernet SNAP frames are enabled, then the user should set the fill pattern
up so that the custom background generates believable Ethernet frames.
•
LLC field . The RIF field controls the marker in the Source MAC Address and is
decoded to the extent that the length is taken from the encoded value in the first two
bytes and the maximum frame size is truncated to that in the RIF field. (An error in
the RIF specification will cause the Source Routing part of the header to be turned
off.)
If the frame length is too small to accommodate the header then the entire header –
except for the AC & FC bytes – will be switched off.
Background Fill Pattern
The background fill pattern behaves the same as all other SmartCards except that the fill
pattern will start immediately after the automatic headers. For example, if an Ethernet
SNAP header is enabled then the first two bytes of the fill pattern will be Ethernet frame’s
"type" field.
Interframe Gap and Ring Data Rates
Frames are specified in terms of a length - that exclude the CRC, FS and delimiters – and
an interframe
gap . This gap figure represents a countdown that will be the minimum time
the card will wait before reaquiring the token and transmitting another frame.
Gap figures are expressed internally in bit clock time and are computed from the supplied
value in nanoseconds by the card firmware. The interframe gap has a minimum value of
one bit clock time.