Lawrence G. Roberts
Packet Switching or Optical
One of history's main benefits is
that it lets us detect trends that help predict the future. Over the past
30 years since the Internet started, many clear trends have emerged. This
article very briefly examines the most critical of these trends, the one
that led to the creation of the Internet--the packet switching cost
Packet Network Cost Components
the early seventies, as the Internet started to grow, I first examined the
trends of the two components of packet network cost, switching and
illustrated in Figure
1. The computing cost to switch packets was decreasing by a factor of
two every 20 months, a rate that has remained fairly constant ever since.
The communications cost trend, however, was declining much slower, halving
every 79 months.
Because of this difference in the rates of decline, the computer cost
of switching crossed over in 1969 and became less than the communications
cost for moving a terabit of data. This meant that the computing power
needed for packet switching was no longer an inhibiting factor. Since the
alternative to packet switching was circuit switching, and typical data
interactions had a peak rate 15 times their average rate, the
communications cost for sending just the packets of data was 15 times less
than setting up a circuit at the peak rate. So after 1969, the Internet's
first year, packet switching had a significant advantage (7:1, quickly
growing to 15:1) over circuit switching for data.
Reversal to Optical Circuit Switching?
The cost trends continued unchanged until the early nineties, when
dense wave division multiplexing (DWDM) started to alter the
communications cost trend. Since 1992, DWDM has reduced the cost of fiber
capacity by a factor of two every 12 months, so that the decrease in
communications cost is now faster than that of the computing cost. At this
new rate, by 2010 the cost of communications could drop to that of
switching, and the curves could cross again. If this happens, a reversal
could occur, so that it might become less expensive to use optical circuit
switching instead of packet switching. Rather than lumping packets
together to fill a pipe, opening a fat pipe for the duration of a call
could become less expensive, even if the pipe was only 7 percent
Maximum Fiber Capacity
some controversy over the Shannon limit for the capacity of a fiber, but
experts agree that there is a limit to the total bandwidth we will be able
to achieve in a fiber, no matter how DWDM divides the channel. Current
estimates of this limit range from 10 to 100 terabits per second (Tbps).
This limit is expected to grow slowly with time, but probably no faster
than the decline in computing cost. Thus, by 2008, when the DWDM density
reaches 10,000 OC-192s per fiber (100 Tbps), we can expect the fast
decline in communications cost to stop and revert to a trend like that for
computing cost. At this point, the cost of the fiber component will still
be 10 times the cost of packet switching, so there is no real possibility
of a reversal to optical circuit switching in the foreseeable future.
Packet Switching Remains
Today, the time division multiplexing/circuit
switching market is starting to decline, and packet switching is taking
over voice as well as data. This trend is clearly going to continue. A
change to optical circuit switching because of DWDM-driven reductions in
communications cost appears unlikely, since computing will remain less
expensive than fiber capacity. Thus, computing can optimize the fill of
each fiber at less total cost than adding circuit-switched fibers, even if
the circuit switching is free.
Figure 1. Trends in packet switching computer cost and
1. L.G. Roberts, "Data by the Packet," IEEE Spectrum,
Vol. 11, No. 2, Feb. 1974, pp. 46-51.
Lawrence G. Roberts is chairman and CTO of
Packetcom Inc., a company that designs advanced IP router/switches with
improved QoS for the Internet. He was responsible for the design,
initiation, planning, and development of Arpanet. Roberts has BS, MS,
and PhD degrees from the Massachusetts Institute of Technology. He has
received numerous awards for his work, including the IEEE Computer
Pioneer Award, the IEEE Computer Society W. Wallace McDowell Award, and
the ACM SIGCOMM communications award.
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