System information

FDDI Technology Basics
Book Title
5-70
Figure 5-7 FDDI Frame Format
The fields of an FDDI frame are as follows:
• Preamble—Prepares each station for the upcoming frame.
• Start delimiter—Indicates the beginning of the frame. It consists of signaling patterns that
differentiate it from the rest of the frame.
• Frame control—Indicates the size of the address fields, whether the frame contains asynchronous
or synchronous data, and other control information.
• Destination address—Contains a unicast (singular), multicast (group), or broadcast (every
station) address. As with Ethernet and Token Ring, FDDI destination addresses are 6 bytes.
• Source address—Identifies the single station that sent the frame. As with Ethernet and Token
Ring, FDDI source addresses are 6 bytes.
• Data—Contains either information destined for an upper-layer protocol or control information.
• Frame check sequence (FCS)—Filled by the source station with a calculated cyclic redundancy
check (CRC) value dependent on the frame contents (as with Token Ring and Ethernet). The
destination station recalculates the value to determine whether the frame may have been damaged
in transit. If it has been damaged, the frame is discarded.
• End delimiter—Contains nondata symbols that indicate the end of the frame.
• Frame status—Allows the source station to determine whether an error occurred and whether the
frame was recognized and copied by a receiving station.
CDDI
The high cost of fiber-optic cable has been a major impediment to the widespread deployment of
FDDI to desktop computers. At the same time, shielded twisted-pair (STP) and unshielded
twisted-pair (UTP) copper wire is relatively inexpensive and has been widely deployed. The
implementation of FDDI over copper wire is known as Copper Distributed Data Interface (CDDI).
Before FDDI could be implemented over copper wire, a problem had to be solved. When signals
strong enough to be reliably interpreted as data are transmitted over twisted-pair wire, the wire
radiates electromagnetic interference (EMI). Any attempt to implement FDDI over twisted-pair wire
had to ensure that the resulting energy radiation did not exceed the specifications set in the United
States by the Federal Communications Commission (FCC) and in Europe by the European
Economic Council (EEC). Three technologies reduce energy radiation:
Preamble Data FCS
Start
delimiter
Frame
control
Frame
status
End
delimiter
Source
address
Data frame
Destination
address
Preamble
Token
Start
delimiter
Frame
control
End
delimiter