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#### Chapter Five

**Data Communications: Market Order 1973-1979**

**LSI Modems, Statistical Multiplexers and Networks**

**5.4 Wesley Chu and the Statistical Multiplexer 1966-1975**

Multiplexing is the
third important innovation in the technology trajectory of multiplexing after
frequency-division and time-division multiplexing (TDM). Chu’s theoretical work
in statistical multiplexing began in 1966 after he earned his PhD. in electrical
engineering from Stanford University and joined a new computer communications
studies group at Bell Labs (AT&T). Created in response to complaints
concerning interconnecting terminals and computers over telephone lines, the
study group began investigating terminal to computer communications. From
traffic measurements, the group learned terminals typically used about 5% of
the bandwidth of communication lines, leaving 95% of the bandwidth unused. The
question was: How to multiplex the traffic of many terminals over a
communication line to most efficiently use the bandwidth of the circuit and
thereby reduce communication costs?

Chu’s first thoughts turned to
improving time-division multiplexing (TDM). In TDM, each terminal connected to
the multiplexer has full use of the communication line on a rotating basis for
a dedicated, fixed period of time. (See Diagram 3.1 Time Division Multiplexer)
While a significant improvement over frequency-division multiplexing, TDM still
suffered inefficiencies because terminals in rotation frequently had nothing to
transmit. How then to give terminals with data to send control of the
communication line? (See Exhibit 6.1 Statistical Multiplexer.) A solution
required intelligence -- the multiplexer had to determine which terminals had
data to send and how to queue the terminals so as to optimally serve all
terminals -- and storage buffers, and both microprocessors and memory were
expensive.

Exhibit 6.1 Statistical Multiplexer

In 1969, Chu left Bell Labs to
teach at UCLA. While he continued to investigate statistical multiplexing, the
prohibitive cost of hardware precluded any immediate practical application.
Then in 1971, Intel introduced the 4004 microprocessor. Only it lacked the
computational power to perform the logical operations required of statistical
multiplexing. When Intel introduced the 8008 in 1972, Chu converted his design
to tat eight-bit microprocessor and began experiments using sixty-four
terminals as input. Again he found, one chip did not provide the needed
computational power, so he redesigned his system using two 8008’s and by 1975
had a working statistical multiplexer supporting sixty-four terminals. Chu
filed successful patent applications in 1976 and 1977 covering his statistical
multiplexing innovations. As it turns out, neither he nor UCLA ever asserted
their rights.