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May 27, 2008

The Efficiency of Moving Bits

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I started my engineering career in the early 1980’s as a designer for a modem company.  It was an amazing time since the Internet was growing in leaps and bounds and personal computers were making their big debut.  In those days the price of modems was similar to the price of gold – roughly $1 US per bit per second.  That’s meant if you wanted a 9600 bps modem (a museum piece today), you’d pay roughly $10,000 US for it.  Those were profitable times. 

The modems of those days were in rack chassis (no custom modem ICs existed yet) with hundreds of 74 series logic devices along with discrete analog filters, modulators, demodulators and line drivers made from op-amps and transistors.  The box weighed about 20 pounds and had a gigantic 50W power supply.  Not only were those early modems expensive to buy, they were very inefficient in moving the bits in terms of the power consumed. 

Things have improved over the last 25 years to where a household network has many high performance personal computers or game stations all connected together with 100 Mbps unshielded twisted pair (UTP) Ethernet drops or even wireless 802.11 access points.  These connections are managed by an Ethernet packet switch and typically a router / firewall connecting to a cable modem to provide upwards of a 10 Mbps connection to the Internet. 

But how would we compare those early (or even more recent) communication technologies to what’s available today.  How should we compare how efficient one technology or component is over another for moving information? Equation 1 provides a simple formula for creating a metric that does just that.    Equation1_3 Power is in watts and the transfer rate (fb) is in bits per second.  The variable “ch” is the number of channels in a system or device to normalize the result to a single channel.  The result is the data transfer efficiency (eb) in joules per bit (J / bit). 

This equation normalizes all coding and signal processing which allows you to compare how good a technology (system or device) is at using the least amount of energy to move a bit across a medium error free (i.e. a bit error rate or BER of less than 10-12).  If you want to normalize the length as well,  Equation2simply divide by the length (in meters) of the connection and the result is Joules per bit-meter (J / bit-m) as shown in Equation 2.  This allows technologies that drive various distances to be compared.

Let’s use equation 2 to calculate the efficiency of that old modem I worked on in the 1980’s.  It was capable of moving 9600 bits per second over 15,000 feet (4572 meters) using roughly 50 watts.  That yields a data transfer efficiency of 1.14 microjoules per bit-meter.  So every bit used roughly 1.14 uJ of energy to move it from my office to the telecom central office 3 miles away (worse case).   If the telephone switch was in the office building next door (1000 feet away), the number goes up to 17.1 uJ / bit-meter.  Compare this with a modern Data Over Cable Interface Specification (DOCSIS) cable modem which uses about 5 watts of power, goes the same distance (using coaxial cable) and moves up to 43 Mbps downstream (using 256 QAM) of error free data.  This equates to a data transfer efficiency of 25.4 picojoules per bit-meter.  That is an multiple improvement in transfer efficiency of more than 40,000 over the old modem technology – an amazing accomplishment.

Next time I’ll cover more on data transfer efficiency and we’ll look at bus architectures and interface technology.  If you have any thoughts (agree / disagree / don’t care), please drop me a comment here on the blog!  Thanks for reading and I hope to hear from you soon!


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