7. Fiber Optic Communication Standards 157
data rate mismatch (we will discuss SONET as an ATM physical layer
in a later section). An enhancement called FDDI-II uses time division
multiplexing to divide the bandwidth between voice and data; it would
accommodate isochronous, circuit-switched traffic as well as existing
packet traffic. Recently, an option known as low cost (LC) FDDI has been
adopted. This specification uses the more common SC duplex connector
instead of the expensive MIC connectors, and a lower cost transceiver
with a 9-pin footprint similar to the single-mode ESCON parts.
7.4 Fibre Channel Standard
Development of the ANSI Fibre Channel Standard (FC) began in 1988
under the X3T9.3 Working Group, as an outgrowth of the Intelligent
Physical Protocol Enhanced Physical Project. The motivation for this
work was to develop a scaleable standard for the attachment of both
networking and I/O devices using the same drivers, ports, and adapters
over a single channel at the highest speeds currently achievable. The
standard applies to both copper and fiber optic media, and uses the English
spelling “fibre” to denote both types of physical layers. In an effort to
simplify equipment design, FC provides the means for a large number of
existing upper level protocols (ULPs)—such as IP, SCI, and HIPPI—to
operate over a variety of physical media. Different ULPs are mapped
to FC constructs, encapsulated in FC frames, and transported across a
network; this process remains transparent to the attached devices. The
standard consists of five hierarchical layers [10], namely a physical layer,
an encode/decode layer which has adopted the DC-balanced 8B/10B
code, a framing protocol layer, a common services layer (at this time,
no functions have been formally defined for this layer) and a protocol
mapping layer to encapsulate ULPs into FC. The second layer defines the
Fibre Channel data frame; frame size depends upon the implementation,
and is variable up to 2148 bytes long. Each frame consists of a 4 byte
start of frame delimiter, a 24 byte header, a 2112 byte payload containing
from 0 to 64 bytes of optional headers and 0 to 2048 bytes of data,
a 4 byte CRC and a 4 byte end-of-frame delimiter. In October 1994,
the Fibre Channel physical and signaling interface standard FC-PH was
approved as ANSI standard X3.230-1994.
Logically, Fibre Channel is a bi-directional point-to-point serial data
link. Physically, there are many different media options (including cop-
per, multimode fiber, and single-mode fiber) and three basic network

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