Special Report: Next Generation Internet Applications

THE EVOLUTION OF OPTICAL COMMUNICATIONS

Optical communications has evolved out of the necessity for a transmission medium with increased bandwidth capacity. The evolution of fiber optic data communications has stemmed from the telecommunications industry's need to multiplex voice channels for interconnecting call distribution centers. In the 1930s communications engineers became aware that the current infrastructure deployment strategy would not support the service demand of future telephone usage. At the time, each concurrent connection required an allocated physical connection.[1] Massive physical networks were implemented and could not possibly have supported future expansions. The infrastructure problem was not strictly a United States issue. Europeans were also dealing with demand-based expansion problems. Early multiplexing techniques included frequency division multiplexing, which attained little success due to cost issues. In the late 1960s, research into a digitization technique called "Pulse Code Modulation" (PCM) was performed. PCM utilizes 8000 8-bit samples per second, creating a 64 kbps digital channel for carrying voice. Digitizing the audio channel made multiplexing easier. Early multiplexing techniques involved simple bit interleaving which proved ineffective for dense multiplexing operations.[1] At this time the European and United States telecommunications industries diverged. The United States systems multiplexed 24 64 kbps channels, and added some control overhead, to yield a 1.544 Mbps channel, referred to as a "T1". Europe, on the other hand, multiplexed 30 64 kbps channels and 2 control channels to yield a 2.048 Mbps channel, referred to as an "E1". Table 1 and Table 2 depict the different multiplexed voice services standards. The multiplexing differences between America and Europe laid the framework for critical integration problems that would have to be addressed.

Interoperability between the two networks became expensive and complicated. As further multiplexing was performed on the trunk lines, distance and vendor differences led to extensive transmission problems. Clock signal skew and distance-related delays caused signal timings to be compromised. To resolve these problems, the Plesiochronous Digital Hierarchy (PDH) standard was adopted by the International Telecommunications Union (ITU) in the 1970s. PDH allowed successful interoperability of these communications networks, but problems with network scalability and the introduction of additional value-added services for fiber optic media made PDH obsolete. To add or remove a data channel at a point along the network, extensive network infrastructure was needed to provide for the de-multiplexing, channel addition or removal, and remultiplexing of the channels.[1] PDH does not provide for channel addition without demultiplexing down to the channel hierarchy level of the channel to be added. Table 2: European Voice Service Rates [2] The court-ordered breakup of the American Telephone and Telegraph Company (AT&T) at the end of 1983 created the need for a new American National Standards Institute (ANSI) standard for the interoperability of the Regional Bell Operating Companies (RBOCs). Synchronous Optical Network (SONET), which allows for the addition or modification of a channel without extensive demultiplexing, became the United States standard. SONET was designed to be compatible with the United States PDH systems. Having observed SONET's integration benefits, the ITU adopted a Synchronous Digital Hierarchy (SDH) standard. SDH was designed to be interoperable with both the new U.S.-based SONET equipment and the large investment in European PDH equipment.[1] Both SONET and SDH operate by means of Synchronous Transfer Mode (STM) digital transmission. STM is a time division multiplexed technology with consistent framing every 125 microseconds.[1] Table 3 outlines the optical and electrical multiplexing services and rates for SONET and SDH. As shown in the Description column, the SONET/SDH rates were developed to accommodate the existing voice multiplexing hierarchies.

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