IT Trenches

Mar 6 2009   4:45PM GMT

Network speed & capacity are NOT the same

Troy Tate Profile: Troy Tate

There is often a misunderstanding of what network speed is versus capacity. The speed of data transfer is not just a function of the capacity (bandwidth) of the link, but of the distance between the endpoints. To find distances: Go to http://www.infoplease.com/atlas/calculate-distance.html to get distances between locations.

Considerations about the impact of latency and network throughput:
Networks do not transfer data in a continuous stream, as many people think, but in small packets. The server sends a packet to your computer, which sends an acknowledgement back (TCP/IP protocol). Upon receipt of the acknowledgement the server sends the next packet. This is called handshaking, it’s a little game of ping-pong.

The speed of light is 299792 kilometers per second. The maximum number of ping-pongs per second is therefore 299792 divided by twice the distance between you and the server. If the server is 1000 kilometers away that’s 149 ping-pongs per second. Every ping-pong is 1 packet, so if the packet size is 1 bit the server can only send you 149 bits per second. The speed of the network is immaterial, even a gigabit network cannot break the speed of light. The server is not sending data while waiting for the acknowledgement, waiting means less throughput, so the speed is reduced because of the distance. The further away,
the lower the speed.

Here’s some information that I came across but do not know the original source that will also help explain this situation.

Would you say that a Boeing 747 is three times “faster” than a Boeing 737? Of course not. They both cruise at around 500 miles per hour. The difference is that the 747 carries 500 passengers where as the 737 only carries 150. The Boeing 747 is three times bigger than the Boeing 737, not faster.

Now, if you wanted to go from New York to London, the Boeing 747 is not going to get you there three times faster. It will take just as long as the 737.

In fact, if you were really in a hurry to get to London quickly, you’d take Concorde, which cruises around 1350 miles per hour. It only seats 100 passengers though, so it’s actually the smallest of the three. Size and speed are not the same thing.

NOTE: In the internet world, there is no such thing as a Concorde. Data speed is limited to the speed of light. The speed of light in a vacuum is ~300,000 km/sec. – p.s. The Concorde jet service ceased operations in 2003 (http://en.wikipedia.org/wiki/Concorde)

On the other hand, If you had to transport 1500 people and you only had one plane to do it, the 747 could do it in three trips where the 737 would take ten, so you might say the Boeing 747 can transport large numbers of people three times faster than a Boeing 737, but you would never say that a Boeing 747 is three times faster than a Boeing 737.

That’s the problem with communications devices today. Manufacturers say “speed” when they mean “capacity”. The other problem is that as far as the end-user is concerned, the thing they want to do is transfer large files quicker. It may seem to make sense that a high-capacity slow link might be the best thing for the job. What the end-user doesn’t see is that in order to manage that file transfer, their computer is sending dozens of little control messages back and forth. The thing that makes computer communication different from television is interactivity, and interactivity depends on all those little back-and-forth messages.

Thanks for reading and let’s continue to be good network citizens.

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