Fiber Optic Essentials

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