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Chapter 2

Protocol Attributes

Error-checking information follows the header and data fields. It is used

in CRC calculation.

Now you are at the end of the frame and it is again the same as a token

frame. Both data/command and token frames have end delimiters. The only
difference is that end delimiters end a token frame, but a data/command
frame has one more field.

The End delimiter field is only one byte long. It is used to indicate the

end of a data/command frame. The last field, the Frame status, or FS, has
two values that it can relay: frame-copied or address-recognized.

Fiber Distributed Data Interface (FDDI)

Fiber Distributed Data Interface (FDDI) is a LAN technology that has band-
width capabilities up to 100Mbps per ring. This along with other changes
makes it an improvement over Token Ring and Ethernet. FDDI uses some of
the same ideas as Token Ring. It uses fiber optics as its physical media, but
the topology is a dual ring instead of just a single ring as used by Token Ring/
IEEE 802.5.

Several advantages were achieved by the improvements made by FDDI.

The ring has 100Mbps bandwidth, which is much more than 10Mbps
10BaseT, or 16Mbps Token Ring. Another advantage was the fact that fiber
is a much better media than copper wire. It allows for higher bandwidth
capacities and throughput.

As we mentioned, FDDI uses some of the same ideas as Token Ring. It is

similar because FDDI uses tokens to control media access, uses a ring topol-
ogy, and has some of the same fault-tolerant features as Token Ring. Look
at Figure 2.27. You can see the back of three routers, each of which has a
FDDI processor board in it. Notice how the routers are connected. Port A on
Router A is connected to port B on Router B. Port A on Router B is con-
nected to port B on Router C. Port A on Router C is connected to port B on
Router A.

Each of the ports has two fiber optic connections. One fiber is considered

the primary ring and the other is considered to be the secondary ring. Data
flows in opposite directions on these two rings. Figure 2.28 shows that the
data on the primary ring flows clockwise, whereas the data on the secondary
ring flows counterclockwise.

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