Understanding latency vs. network speed

High network speed doesn't always imply low latency and vice versa.

A popular joke from 1980s and 1990s:

Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.
-- Andrew S. Tanenbaum

(Wikipedia: Sneakernet)

Jokes aside, such a method was used in practice:

Observational data were recorded on 2-terabyte SATA hard disk drives fed from the Arecibo Telescope in Puerto Rico, each holding about 2.5 days of observations, which were then sent to Berkeley.[20] Arecibo does not have a broadband Internet connection, so data must go by postal mail to Berkeley.

(Wikipedia: SETI@home)

Data will be recorded on high-density tapes at the Arecibo telescope in Puerto Rico, filling about one 35 Gbyte DLT tape per day. Because Arecibo does not have a high bandwidth Internet connection, the data tape must go by snail-mail to Berkeley. The data is then divided into 0.25 Mbyte chunks (which we call "work-units"). These are sent from the Seti@Home server over the Internet to people around the world to analyze.


Not that Puerto Ricans was so poor. But radio telescopes are built in such deserted areas to eliminate any possible electronic noise:

A radio quiet zone is an area where radio transmissions are restricted in order to protect a radio telescope[1] or a communications station[2] from radio frequency interference.


Quiet zones are located in areas that are sparsely populated, and may be enforced by government legislation.[1] A radio quiet zone is often divided into two zones: an exclusion zone where all radio emissions are prohibited, and a larger coordination zone of up to 100 km2 where the power levels of radio transmissions are suitably limited so as not to interfere with the radio telescope.[3] 

(Wikipedia: Radio quiet zone)

So when the data from Arecibo was transfered to Berkeley by postal mail, speed was high, but latency (delay) was also very high. But acceptable for scientific analysis. (Nothing urgent here, aliens will wait...)

The opposite example: Morse code over radio link. Speed is painfully slow, but latency (delay) is also low -- you can hear the signal almost instantly on any place of Earth.

Latency is what you see running the 'ping' command. This is not speed or bandwidth.

 % ping google.com
PING google.com ( 56(84) bytes of data.
64 bytes from waw02s22-in-f14.1e100.net ( icmp_seq=1 ttl=117 time=14.7 ms
64 bytes from waw02s22-in-f14.1e100.net ( icmp_seq=2 ttl=117 time=14.7 ms
64 bytes from waw02s22-in-f14.1e100.net ( icmp_seq=3 ttl=117 time=14.7 ms

Another example: SSD vs HDD. SSD is faster, but latency is also smaller, because SSD has no moving head.

(the post first published at 20230217. Updated in Dec-2023.)

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