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September 19, 2006

How Broad Is Your Band?

All electrons and photons travel at roughly the speed of light, 186,262 miles/hoursecond.  (embarassing mistake for a nerd to make.  Friend Andrew Odlyzko caught it in and also pointed out that, in a wire or fiber, they travel at only 2/3 this speed).

So how come some connections are described as being “faster” than other ones?  What make a connection worthy of being called “broadband”?  Why is one kind of broadband connection much better than another?  I have broadband; can I use it for VoIP service? For watching live sports events?  How do I know how good my broadband is?  How do I find out how good broadband service is likely to be BEFORE I buy it?  What is “good” broadband?  Is WiFi broadband?

Good questions.  I’ll answer them in the next couple of posts.  Nerds may want to skip this post and tune back in when I talk about actually measuring connections.

Let’s start with bandwidth (sometime called “speed” just to be confusing).  The bandwidth of connections is measured by how many bits can be transferred in a second.  On dialup if you’re really lucky you can transfer 52 thousand (a thousand bits is a kilobit) bits per second.  That’s called a bandwidth of 52Kbps.  Many broadband connections can handle millions of bits (megabits) per second.  A connection which can deliver 2 million bits per second would be called a 2Mbps or 2 meg connection.

Remember bits aren’t bytes.  File sizes are usually measured in bytes.  So a two meg connection AT UNOBTAINABLE BEST will take eight seconds to deliver a two million BYTE file.  There are eight bits in each byte (told you this wasn’t for nerds).  But there are some overhead bits in every transmission as well which is why the best is unobtainable.

Broadband connections often are asymmetric.  Asymmetric means the speed of download (to you)  is different than the speed of upload (from you).  For example, the satellite broadband connection from WildBlue I described here has a download speed of one megabit (1Mbps) but an upload speed of only 200 kilobits (200Kbps or .2Mbps).  Providers devote more bandwidth to download than upload because most residential users upload very little – perhaps just the URL of a website they want to see – and download much more – tan entire page of a website or an entire song or video.

However, if you work at home or upload videos or lots and lots of pictures or run a website of your own, then you may not be happy with an asymmetric connection because the stuff you send’ll take a long time to go.  As the Internet becomes more interactive and as more of us become content providers as well as content producers, asymmetry may become much too limiting.

OK, up or down, why is this called “bandwidth” if it’s about speed?  And I thought you said all electronic data goes at the speed of light anyway.  How can one connection be faster than another if everything moves at the same speed?

The term bandwidth is a holdover from the days when sending bits in parallel was the only way to get more data from here to there faster.  Obviously, if you have ten parallel wires between  points A and B, you can deliver ten times as many bits from A to B than if you had only one wire.  That is a “broader” connection and can deliver any given amount of data ten times as fast as fast as a single circuit.  However, engineers quickly found ways to put many virtual wires on one physical strand. For example, on a circuit built to carry sound (analog telephone circuits, for example), a bunch of bits can be sent in parallel by reserving certain frequency ranges for each of the parallel bits.  Now you know why dialup modems growl at each other.

Radio signals also divide into frequencies.  The more frequencies a radio connection has, the broader the pipe as far as sending data.  Ditto the frequencies of light in a fiber.

One more point before the history lesson ends: speed can also be obtained by sending “shorter” bits.  Since the bits all travel at the same speed, more short bits can fit down a given pipe in a given period of time than long bits – whether that pipe is a radio wave, a copper wire, or a fiber. The only information a bit has to convey is ON or OFF so, as long as the receiver is sensitive enough to understand them, short bits carry any given amount of data faster than long ones.

The inexorable progress of Moore’s Law – the price of a given amount of computing power is halved every eighteen months – means that bits can be both “narrower” (each consumes less frequency bandwidth) and shorter.  That’s why higher and higher broadband speeds are available at lower and lower prices.  That’s why basic DSL today is cheaper than dialup service was eight years ago.

Unfortunately, up and download speed aren’t the only factors which are important in judging a broadband connection.  Latency, jitter, packet loss, and reliability can sometimes be even more important than raw bandwidth depending what you want to use a connection for.  More in the next post of this series.

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