Ethernet utilization formula per RFC2819

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Hi folks,

Usually not being so clumsy about maths, I don't get the clue in this
case though it MUST be simple, I guess... RFC2819 presents a formula
that can be used to approximate the utilization of an 10 Mbps Ethernet
link as follows:

                          Pkts * (9.6 + 6.4) + (Octets * .8)
          Utilization = -------------------------------------
                                  Interval * 10,000

What I don't get now seems to be two-fold:

(1) Why do we use both pkts and octets in the formula? Won't octets,
bandwidth, and an interval be enough? What do the pkts help here?

(2) What about the 'magic numbers' 9.6, 6.4, and 0.8? What might be the
source of these figures and what's their purpose? There must be some
obvious one I guess because otherwise one would have summed up 9.6+6.4
in the equation, at least. However, I don't get it anyway.

Is there anybody who would point me to what I am missing apparently, or
just explain the entire thing I am struggeling with? Any idea will be
appreciated!


Thanks,
/Thomas.

--
Thomas Bahls               | "Wege entstehen dadurch, daß man sie geht."
Greifswald, Germany        |                              -- Franz Kafka

ICQ #119230485  +++  PGP Key  0x6E70B6AE  +++  http://solitaryhiker.net /


Re: Ethernet utilization formula per RFC2819


Thomas Bahls wrote:
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Ugrgh.  Someone cancelled out a factor of ten.

rewrite as:

                           Pkts * (96 + 64) + (Octets * 8)
           Utilization = -------------------------------------
                                   Interval * 100,000


96 is the number of bit times (minimum) for the interframe gap.  64 is
the number of bits in the preamble + SFD.  (The octet count doesn't
include these, so we have to add them in for each packet.)
8 is the number of bits in each octet.

Utilization will be in %, so rewrite as:

                           Pkts * (96 + 64) + (Octets * 8)
           Utilization = ------------------------------------- * 100%
                                   Interval * 10,000,000

The 10000000 in the denominator is the raw bit rate, in bits per
second.

In other words, Utilization (sic) is the number of bits transmitted,
divided by the number of bits that could have been transmitted,
multiplied by 100%.

RFC2819 lists its author as Steve Waldbusser.  Perhaps you'd like to
contact him and ask him why he chose to make this hard to understand.
There could be a valid reason!

Regards,
Allan


Re: Ethernet utilization formula per RFC2819


Hi Allan,

On 20 Jan 2006 02:31:02 -0800, allanherriman@hotmail.com wrote:

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This makes it quite clear, so clear that one wonders why I didn't get it
myself. I apparently overlooked the fact that we transfer more than the
raw octets starting with dstMAC. The point was nicely disguised by one of
those tricks mathematicians seems to enjoy: They almost 'randomly' divide
and expand entire equations by 'magic' factors. We loved that at
university, kind of. ;-)

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I think I'd better not bore Steve with this now I see what the parts of
the equation represent. Instead, I will carefully keep a copy of your
explanation and enjoy the understanding.


Thanks for your prompt f'up (also to Robert),
/Thomas.

--
Thomas Bahls               | "Wege entstehen dadurch, daß man sie geht."
Greifswald, Germany        |                              -- Franz Kafka

ICQ #119230485  +++  PGP Key  0x6E70B6AE  +++  http://solitaryhiker.net /


Re: Ethernet utilization formula per RFC2819


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Because there's a certain amount of overhead per packet (src
& dest MAC, inter frame gap, etc).  A network might be very
heavily loaded if all the data is in very small packets.

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As the other response, someone has divided top & bottom by
ten.  Octets are 8 bits.  96 + 64 would correspond to packet
overheads,  IIRC 96 for preamble & MACs, 64 for IFG.

-- Robert


Re: Ethernet utilization formula per RFC2819


This is for 10 mbps Ethernet. For 100 and 1000 mbps Ethernet do I just
divide by 100 and 1000? Since they use different encoding schemes something
tells me it is not that simple.

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Re: Ethernet utilization formula per RFC2819


Noah Davids wrote:
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No, it is that simple.  10Gbps Ethernet also follows the same rule.
The line encodings are different (and have different overhead) but that
is made up for by changing the symbol rate on the line to make the
effective usable bit rate 10M, 100M, 1G, 10G, etc.
The IFGs and preambles also stay the same (nominal) number of bits.

Regards,
Allan


Re: Ethernet utilization formula per RFC2819


allanherriman@hotmail.com wrote:
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Not counting frame extension for half duplex gigabit ethernet.

And maybe someday the useless preamble on continuous signaling
systems will be removed.

-- glen

Re: Ethernet utilization formula per RFC2819


Thanks.

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