Special Report: Next Generation Internet Applications
Early successes like the American Airline's Sabre airline reservation system, the "Wizard of Avis" car rental system, and the Federal Express package tracking system fueled enterprise network development. Corporations and military organizations quickly outgrew 64 kbps channels and demanded higher capacity DS1 and DS3 channels in North America and the equivalent E1 and E3 channels in other settings. The PDH multiplex architecture became economically prohibitive in the face of widespread demand for higher than 64 kbps transmission channels.
5.1 Transitioning the Multiplex Layer from SONET to DWDM Changing market demand and the widespread introduction of fiber optic cables during the 1980s drove yet another transition within the multiplex layer of the global communications infrastructure. North America and the global communications community adopted nearly identical multiplexing standards referred to as SONET in North America and SDH within the ITU-T. SONET and its global counterpart SDH build upon the 51.84 Mbps, OC-1 multiplex format. Multiples of the OC-1 format create 155.52 Mbps OC-3, 622.08 Mbps OC-12, 1.24 Gbps OC-24, etc., as previously described in section 1.0.[41] The total bandwidth of the multiplexed optical channel depends upon the maximum speed of the electronics used in the SONET OEO subsystem.
Today, electronic technology limits the commercial SONET transmission speed to a 9.95 Gbps, OC- 192 multiplexed optical channel. High capacity, OC-192 SONET systems are expensive. Practically speaking, they fail to provide value when combining channel speeds greater than OC-24 onto an OC- 192 multiplexed channel.
SONET has its greatest application in combining virtual tributary (VT) channels at VT- 1.5 (1.728Mbps), VT-2 (3.456Mbps) and VT-6 (6.912Mbps) onto OC-48 or OC-192 fiber optic channels with efficient add and drop functions. To date, frame relay has been the most successful packet service provided by the carriers. Typical frame relay services fit comfortably within the SONET VTs at rates from 128 kbps to 1.544 Mbps. Rapid growth in Internet data traffic, high-speed ATM networks, and voice-over-packet networks are supplanting frame relay, the virtual tributary architecture, and the voicecentric networks that SONET virtual tributaries were designed to support.
In the early 1990s, the Internet changed network usage patterns, creating an explosion in backbone traffic and a six-fold increase in subscriber line usage. Business requirements grew beyond frame relay and 1.544 Mbps access network transmission speed. Telephone subscriber holding times, the time that a telephone line is in use, were typically within the range of four to five minutes before the Internet became popular. Average Internet subscriber line holding times now fall within a range of 15 to 30 minutes.
Carriers who built single-mode fiber networks during the late 1980s and early 1990s were caught by surprise, having not foreseen rapid growth of corporate LANs and the Internet. They had planned on using SONET to gradually increase the capacity of their established fiber. Rapid Internet traffic growth quickly upstaged plans to use SONET as the primary multiplier of installed fiber capacity. By the end of 1996, nearly 100 percent of the major carriers were fully lit on allimportant routes. On a broader scale, the carriers were 85 percent lit on their overall fiber capacity. Exploding demand put the carriers in a position of turning down customer requests for new service.
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