[Hidden-tech] Network question

[Robert Heller]

At Tue, 29 Nov 2011 15:41:30 -0500 Daniel Lieberman <daniel at daniellieberman.org> wrote:

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> Have a perplexing situation. We have 20 megabit down internet service from
> Comcast coming in to the front building of our place, gets passed via Cat 5
> probably 150 feet to a patch panel, which distributes it to ethernet ports
> in 5 or 6 rooms in the back building.

"5 or 6 rooms" == How many ports?  How many computers are actually
connected to ports?  Are there wireless Access Points?  If so, how many
wireless devices (eg laptops, notbooks, iPads, IPod Touches, etc.) are
typically active?

> 
> Why would the connections in the rear building only provide 6-7 megabits?
> Could we replace switches or panels in the rear building to provide more
> bandwidth to the individual connections? Or is the splitting of the signal
> in the rear building the issue?
> 
> Any suggestions? I'm baffled.

20 megabits distributed over 3-4 *active* / *busy* computers yields like
5-6 megabits / computers.  A 20 megabit/sec connection means you have only
20 megabits for each second.  With one computer (assuming at least a
100BaseT Ethernet card) that *one* computer can sustain a 20 megabit/sec
download if (and only if) *no other computer* is doing anything on the
LAN.  If two computers are doing solid downloads, then they will 'share'
the bandwidth.  Given that the data is broken up into packets of no more
than about 1500 bytes each, the two machines will (more or less)
alternate packets.  The total packet stream will be about 20 megabit/sec
worth (assuming both machines are transfering data to/from 'outside'
(rather then with some third computer on the LAN). Roughly 50% of the
total packet stream will be for each computer, so each computer will
'see' an effective bandwidth of (about) 10 megabit/sec.  With 3
computers, about 33% of the total packet stream will be for each
computer, yelding about 6.6666666 megabit/sec per computer.  With 4
computers... I think you get the idea.

Yes, this assumes ALL of the computers are solidly busy with some big
download, all at once.  Computers that don't contribute to the total
packet stream don't count.  But all it takes is *three* totally flat out
downloads to bring things down to 6-7 megabits/sec.  If you have 10
computers (two per room?), chances are enough of them are busy enough to
use up that much bandwidth.


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> Daniel
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-- 
Robert Heller             -- 978-544-6933 / heller at deepsoft.com
Deepwoods Software        -- http://www.deepsoft.com/
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