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has a minimum frame size of 22 bytes and maximum size of 4,521 bytes. The
16Mbps frame has a minimum size of 22 bytes and a maximum size of 18,021 bytes.
Figure 4.19 illustrates the three frame formats.
Figure 4.19. The IEEE and IBM Token Ring frame
formats.
Now let's look at the fields that make up the 802.5/IBM Token Ring frames:
• Starting Delimiter (8 bits)—The starting delimiter contains non-data
encoding values. This is used by the stations as an indicator that a data or
control frame has arrived.
• Access Control (8 bits)—This field identifies if the frame is a data frame or a
token. It specifies the token's priority, sets the reservation for the next
generated token, and contains the monitor bit.
• Frame Control (8 bits)—Indicates whether the frame contains control
information or data.
• Destination Address (48 bits)—Like Ethernet addresses, these can either be
locally assigned or use a combination of an Organizationally Unique
Identifier (OUI) and a manufactured assigned address. Ethernet MAC and
Token Ring MAC addresses are not compatible. The Token Ring address has
five subfields. The first bit identifies the destination address as belonging to
a single end- station or as a group address. The second bit indicates if the
address is locally administered or if it is derived from the NIC. The next
subfield contains six hex digits (a null address is used if the address is locally
administered), making up the manufacturer's OUI. The next subfield
indicates which function, if any, is associated with the destination (such as
gateway, active monitor, or bridge). The remaining bits are used for the
station's unique ID, group address, broadcast address, and so on. In
addition to the formatting incompatibilities, it should also be noted that