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Figure 4.25. FDDI MAC data frame and token formats.
Now let's take a look at these formats:
• Preamble (64 bits or more)—Contains idle non-data symbols; it functions as
an interframe gap between data frames and tokens.
• Starting Delimiter (8 bits)—Indicates the beginning of the data frame or
token and uses a unique data symbol, distinct from normal data symbols.
• Frame Control (8 bits)—Indicates the frame priority class, if it is a beacon or
claim frame. It is also used to partition the data contained in the incoming
frame and to indicate the type of data contained in the information field
(LLC-PDU or SMT).
• Destination Address (16 or 48 bits)—FDDI uses the same address format as
Token Ring. The first 2 bits indicate if the address is a station or group and if
local or universal addressing is used. The remaining bits are used for the
station or group address.
• Source Address (16 or 48 bits)—Indicates the station that sent the frame.
• Information (0 to 4,472 bytes)—802.2 LLC-PDU or SMT data.
• Frame Check Sequence (32 bits)—Contains the CRC value for the frame. The
destination host recalculates the CRC value for the frame and compares it to
the one contained in the frame. If the FCS sequences match, the frame is
processed further.
• End Delimiter (32 bits)—Indicates the end of the data frame or token. It uses
a unique data symbol, distinct from normal data symbols.
• Frame Status (4 or 8 bits)—Used by the destination host to indicate that the
frame has been received.
The Station Management (SMT) Standard
The SMT acts in unison with the MAC, PHY, and PMD layers. Its role is to ensure the
proper operation of the ring and to manage interpretability between all the stations
on the ring. In instances where multiple FDDI controllers exist on a given station,
only one SMT interface exists. The primary functions of the SMT are as follows: