Re: how reliable is ping?

[Stephen Samuel <samuel@bcgreen.com>]

Luca Ferrari wrote:
> Hi,
> in my company we installed a new DSL line, supposed to be faster than the 
> previous one. And in fact it seems to be, in interactive use, but if I use 
> ping to test connectivity against the old line I can see higher times. So my 
> questions is: why does ping show high times and then the line goes faster? Is 
> ping reliable enough to test network speed, or should I use another tool? (at 
> last, a chrono and a file transfer!)

There are many things that test network reactivity, and ping
only checks one or two of them.

Ping times only indicate how long it takes a packet to make it
from my box to yours, and back. The command is mostly intended
to just check to make sure that the machine is up.

  Note that some routers give ping packets lower priority, aince
they're (almost) never sending any real data.

Cable connections often have a beter intrinsic bandwidth
availability but they're more sporadic because they're shared
among some number of users in your neighborhood -- in other
words, you might get really good bandwith at 4AM and
sorse-than-modem bandwidth at 7PM.

In truth, ping times don't predict bandwidth. If you have two
machines at the same colo site, one which is bandwidth limited
to 18Kb/s and the other with an unlimited 10Mbit connection,
they may both give you the same ping times, but vastly different
download speeds.

Similarly, a modem user in your city may give a slightly better
ping time than a well-connect box across the country (or even
across the ocean), but still give far worse downloads.

Snother difference between cable and DSL is that, because
DSL is shared, the ping times are less *predictable*.  If
you're playing counterstrike, very low average latency with
short bursts of high-latency can be far more bothersome than
a slightly higher average latency that is more stable.


-- 
Stephen Samuel +1(604)876-0426                samuel@bcgreen.com
		   http://www.bcgreen.com/~samuel/
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