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Addresses Source and destination addresses.
Data Actual user data e.g., LLC PDU.
FCS Frame Check Sequence.
End Delimiter Marks the end of frame.
Frame Status Used by receiving station to record receipt status.
The Frame Control field specifies the frame type i.e., whether it is an LLC PDU or some kind of MAC control frame. The Frame Status field is used by the
receiving station to indicate that it has successfully received the frame and extracted its data, so that when the frame reaches the originator, it can be disposed of.
9.3. ANSI FDDI Standard
Fiber Distributed Data Interface FDDI is a high-speed LAN protocol designed by ANSI for use with optical fiber transmission media. It is capable of achieving data
rates in order of 100 mbps, and a network size in order of 1000 stations. Furthermore, with the high reliability of optical fiber, station distances in order of
kilometers and a geographic spread of the LAN in order of hundreds of kilometers are feasible.
9.3.1. Topology
FDDI utilizes a ring topology that involves two counter-rotating rings and two classes of stations see Figure 9.104. Class A stations are connected to both rings. Each
class A station also has a bypass switch i.e., short circuit which when activated causes the station to be excluded from the ring without affecting the ring continuity.
Class B
stations are only connected to one of the rings via a concentrator. The concentrator also provides a bypass switch. Furthermore, because each of the B
stations is independently connected to the concentrator, it can be switched off without affecting the other stations. Typically class A represents the more important
stations e.g., servers and class B represents the less significant stations e.g., infrequently-used PCs and terminals.
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Figure 9.104 FDDI ring topology.
Class A
Class B Class A
Class B Class A
Class A Concentrator
PIU
PIU
PIU PIU
PIU
PIU
9.3.2. Token Ring Protocol
The FDDI protocol is specified by the ANSI X3T9 standard. It is a token ring protocol similar to IEEE 802.5 but with an important difference: stations can transmit
even if the token they receive is busy. When a station receives a frame, it examines the address of the frame to see if it matches its own address, in which case it copies
the data. In either case, if the station has nothing to transmit, it passes the frame to the next station. If it does have a frame to transmit, it absorbs the token, appends its
frames to any existing frames and then appends a new token to the result. The whole thing is then sent to the next station in the ring. As with earlier token ring
protocols, only the originating station is responsible for removing a frame.
Figure 9.105 illustrates the FDDI frame structure, which is almost identical to the IEEE 802.5 frame structure, except for the different physical size of the fields, and
that it includes a Preamble and excludes the Access Control field.
Figure 9.105 FDDI frame structure. Field
Description
Preamble Special bit pattern for synchronization purposes.
Start Delimiter Marks the beginning of frame.
Frame Control Denotes the frame type.
Addresses Source and destination addresses.
Data Actual user data MAC or LLC.
FCS Frame Check Sequence.
End Delimiter Marks the end of frame.
Frame Status Used by receiving station to record receipt status.
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The FDDI protocol requires that each station maintains three timers for regulating the operation of the ring:
•
The token holding timer determines how long a transmitting station can keep the token. When this timer expires, the station must cease transmission and
release the token.
•
The token rotation timer facilitates the normal scheduling of token rotation between stations.
•
The valid transmission timer facilitates recovery from transmission errors.
9.4. Further Reading
Introductory texts on LANs and MANs include: Kessler and Train 1992, Stallings 1993a, Stamper 1993, and Martin et al 1994. Other useful texts include:
Chorafas 1989, Fortier 1992, and Hunter 1993. Zitsen 1990 and Layland 1994 discuss the internetworking aspects of LANs. Hegering and Lapple 1993
describe the Ethernet in detail. Gohring and Kauffels 1992 discuss token rings. Jain 1993 and Nemzow 1994 provide detailed descriptions of the FDDI technology.
