It’s the latency, stupid!
01 Nov 2004While working on some bandwidth-related stuff (my bandwidth calculator), I came across an excellent article on “latency vs. bandwidth” by Stuart Cheshire. It was originally written in 1996, so focuses a lot on modems, but Fact 1, 2 and 4 are still valid.
His points:
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Fact 1: Making more bandwidth is easy: You can just put enough slow connections in parallel to get a fast one.</p>
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Fact 2: Once you have bad latency you’re stuck with it: Parallel devices, compression, … nothing helps! </p>
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Fact 3: Current consumer devices have appallingly bad latency: Modems are evil (but now, with cable and ADSL, this is less of an issue)</p>
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Fact 4: Making limited bandwidth go further is easy: Compression and caching help a lot. (This article was written about the time MP3 was invented, but long before it became hugely popular. DivX came later, in 1999)
The following calculation is eye-opening:
The distance from Stanford to Boston is 4320km.
The speed of light in vacuum is 300 x 10^6 m/s.
The speed of light in fibre is roughly 66% of the speed of light in vacuum.
The speed of light in fibre is 300 x 10^6 m/s * 0.66 = 200 x 10^6 m/s.
The one-way delay to Boston is 4320 km / 200 x 10^6 m/s = 21.6ms.
The round-trip time to Boston and back is 43.2ms.
The current ping time from Stanford to Boston over today’s Internet is about 85ms:
[cheshire@nitro]$ ping -c 1 lcs.mit.edu
PING lcs.mit.edu (18.26.0.36): 56 data bytes
64 bytes from 18.26.0.36: icmp_seq=0 ttl=238 time=84.5 ms
So: the hardware of the Internet can currently achieve within a factor of two of the speed of light.
Definitions of latency:
Latency, a synonym for delay, is an expression of how much time it takes for a packet of data to get from one designated point to another - techtarget.com
Latency is the time a message takes to traverse a system - wikipedia.org
