[mrtg] Re: Utilization graph uncertainty

[Soden, David]

Thanks Steve and you are right from the perspective of a Full duplex 10
MBps LAN connection; though, I typically see 10 MBps connections setup
as half duplex which is the case in my scenario.

As I understand it, the T1 provides symmetric communications, meaning
that any one of the 24 channels in the circuit can be sending or
receiving, but not both, at any given point in time.  So, if we were
only using 50% of the T1 to receive data, it is possible for the other
50% of available bandwidth to be utilized for sending data.  If we are
using 100% of the bandwidth for sending data there are no pathways left
for receiving.

So with that said, I'm still left uncertain how the combined total was
over 100%.

-----Original Message-----
From: Steve Shipway [mailto:s.shipway at auckland.ac.nz] 
Sent: Wednesday, February 11, 2004 4:41 PM
To: Soden, David
Subject: RE: [mrtg] Utilization graph uncertainty


If you have a 10Mb interface (for example), this means 10Mb inbound and
10Mb outbound.  Therefore you can get up to 100% inbound and 100%
outbound simultaneously -- provided you have full duplex, of course,
which most things are nowadays.

Your link's inbound and outbound counts are independent, ie, you don't
add the percentages together to get a total usage figure.  Better would
be to average them.  This is why you can have 97% incoming and 50%
outgoing at the same time, because the incoming traffic does not prevent
outgoing traffic for using the outgoing line.

If you want an analagy, think of a road.  A normal netowrk link is a
2-way road -- no matter how much traffic goes north, it doesn't prevent
traffic from simultaneously travelling south.  The exception to the rule
is a half-duplex ethernet link.  This is like a one-way road that
changes direction -- some commuter tunnels have lanes like this that are
inbound in the morning and outbound in the evening.  So, a lot of
northern traffic prevents the southern traffic from having much time to
flow.

Hope this helps,

Steve

---
Steve Shipway: ITSS, University of Auckland
Email: s.shipway at auckland.ac.nz  Web: http://www.steveshipway.org/  
** We can only discover new oceans when we have the **
** courage to lose sight of the shore.              **

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