SNAX/XF and SNAX/APN Configuration and Management Manual

Configuring for Token-Ring Support
SNAX/XF and SNAX/APN Configuration and Management Manual—425836-006
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Description of Frames
The LPDU is passed from the LLC layer down to the MAC layer. At this layer, the MAC
header and trailer is attached, as illustrated by the data frame in Figure 9-11.
Figure 9-11 shows three types of frames in circulation on the ring:

The free token is the 3-byte frame that circulates in the absence of other traffic.
Between the starting delimiter (SD) and ending delimiter (ED) is a single byte
indicator for access control (AC). The bits in this byte indicate priority settings,
reservations, and the actual token bit itself.

The data frame contains a bit in the AC, to indicate whether the token is free.
Following the AC is the frame control (FC) byte, typically used to indicate whether
this is a data frame or a MAC frame. A data frame contains the LPDU and is used
to transfer information over the ring. A MAC frame is used to provide control
information between stations on the ring.
Following the FC are the destination and source addresses (DA, SA). Each station
attached to the ring has a unique MAC address, normally burnt into the ROM chip
set of the adapter card. The next field, if present, contains the routing indicator
(RI). The RI can be from 0 to 18 bytes in length, and is used to indicate the rings
and bridges used in routes between ring segments on a network.
The physical trailer is composed of the frame check sequence (FCS), the ending
delimiter (ED), and frame status (FS).

The MAC frame is similar in structure to a data frame, but instead of transporting
data over the ring, the MAC frame is used to transmit control information.
Figure 9-11. The MAC Frame
SD AC FC DA SA RI LPDU FCS ED FS
SD AC FC DA SA RI MAC FCS ED FS
SD AC ED
The Free Token
The Data Frame
The MAC Frame
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