Information Technology Reference
In-Depth Information
Figure5-7
FDDI Frame Format
Data frame
Start
delimiter
Frame
control
Destination
address
Source
address
End
delimiter
Frame
status
Preamble
Data
FCS
Token
Start
delimiter
Frame
control
End
delimiter
Preamble
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:
