The Fiber Distributed Data Interface (FDDI)
is
a high speed local area network which has been defined as a standard
by an American National Standards Institute committee, ANSI X3T9.5
and by ISO.
FDDI is a 100 Mbps, token-passing, single or dual ring interface
that can be implemented with Fiber Optic or Unshielded Twisted-Pair
(UTP) media. A Timed Token Protocol (TTP) is used to control when
a station can transmit data to the network. A station can transmit
a message on the network only after it has received a token. Upon
receiving the token, a station begins transmitting data. The station
can transmit until the message is transmitted or until the TTP timer
expires. This allows all stations fair access to the ring. Once
the message is sent or the timer expires, the station generates
a new token and releases it on the ring. Any downstream station
with data to send can capture the token and repeat the timed-transmission
cycle.
A dual ring configuration for the network media provides a
secondary backup ring in case of a fault on the primary ring. It
is typically implemented as a campus backbone or within buildings
where a failure in the primary ring would have serious consequences.
A break in the primary ring causes the two stations on each side
of the fault to automatically wrap the data to the secondary ring.
Stations in a single ring configuration can only attach to the primary
ring. There is no secondary backup path in the event of a failure.
A typical FDDI network layout is shown in Figure 1-1 “FDDI Architecture”.
FDDI Devices |
 |
To promote ease of installation and maintenance, FDDI allows
for several types of networking devices. These include dual ring
and single ring concentrators, and dual ring and single ring stations.
Concentrators are the building blocks of an FDDI network. These
stations and concentrators connect to the FDDI ring as follows:
Dual Attachment Concentrator (DAC):
Connects to the dual ring and serves as a hub for single or dual
attachment devices. Also allows stations to be added and removed
from the ring with minimal interruption of network traffic.
Single Attachment Concentrator (SAC): Connects to
the primary ring through a DAC and serves as a single ring hub.
SACs can be stacked to form a span of trees topology.
Dual Attachment Station (DAS): Connects to a dual
ring. Both primary and secondary paths are connected.
Single Attachment Station (SAS): Connects only to
the primary ring. Generally, a SAS connects to the FDDI ring through
a DAC.
Network designers and administrators are given the opportunity
to balance the costs of installation and operation with the quality
of service demanded by each segment. For example, a dual set of
fiber running between stations is much more expensive than connecting
the stations with a single fiber. Thus, a dual ring topology is
typically used for major backbones in the system. Single rings are
generally used to branch off the backbones to various workgroups
and peripheral installations
|
| |
|
 | NOTE: DAC and DAS connect directly to the FDDI dual ring while
SAC and SAS only connect to a single, primary ring.Dual attachment
does not provide greater performance than single attachment. It
only provides a secondary means of communication in case of failure
on the primary ring. |
|
| |
|
ANSI FDDI Standards |
|
Figure 1-2 “FDDI Standards and the
OSI Model” shows the four
ANSI FDDI standards and how they combine to form a completely functional
fiber optic network. This figure also shows how these standards
fit into the OSI model.
The physical layer includes two pieces, the Physical
Medium Dependent (PMD) layer that provides the point-to-point
communications between stations in the network, and the Physical Layer
Protocol (PHY) layer that handles synchronization between
higher layer data and control symbols, and the code bit representation
which is transmitted on the medium.
The data link layer includes
the Media Access Control (MAC) standard and
the Logical
Link Control (LLC) standard. The MAC's primary function
is the scheduling, routing and delivery of Frames, the vehicles
used to transmit information on and off the ring. In an FDDI network,
information is transmitted sequentially, within frames, as a stream
of encoded symbols from one station to the next. The order of the
symbols within the frames is predetermined by the MAC standard.
The LLC provides a common protocol between the MAC and the network
layer. In addition to FDDI, the LLC standard also applies to IEEE
802.3, 802.4, and 802.5 networks.
The Station Management (SMT) standard
is a layer management entity which interfaces with the other sublayers.
It manages connections with the ring as well as station configuration
and ring configuration. HP EISA FDDI/9000 and HSC FDDI/9000 support
SMT version 7.3.
Printable version
