# Bandwidth Calculations

Does anyone have a good way to remember all of the bandwidth calculations required for the exam (in your head or with some quick paper scratch)? For instance, calculating the required bandwidth for a G729 call for LBCAC vs. RSVP vs. GK vs. LLQ, etc. And for G711 as well...

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## Comments

write it down. The best way I know to memorize something is to write it down over and over again from short term memory till it goes into long term memory.

I'm doing that with QOS formulae .

Takes 10 minutes > immediately upon waking write them down.

I figure about 2 weeks of doing it and then once every now and then i'll have em down.

HTH

Tox!

Tox,

Thanks for the advice. I agree, it's best to use repetition. But what are you writing down?

-Brandon

you just gave me a reason for a quick review

for WFQ >>

Virtual Scheduling time = Queue Tail time + (packet size X weight)

Queue Tail time = Previous virtual scheduling time

weight = 32384/(IP precedence +1)

For Random early Detect.

New average queue depth = Old average Queue depth X (1 - 2 ^ -N) + Current Queue depth ( 2 ^ -N)

N = exponential weighting constant = 9 by default

the smaller the value of N the more closely the Average Q depth resembles the actual queue depth and vice versa.

When queue depth is above min threshold but below max threshold

drop probability = (Average Q depth - mim queue depth)/ (max - min) X (1 / Mark probability demonimator)

When q depth is above Max threshold you drop at 1/Mark probability deminimator

For Flow based RED

you get the average q depth for each flow by dividing total Q size by number of flows

Avg depth = Queue size/ # of flows

As long as

Current Flow Q size < Average Q Queue Depth X scaling factor we are good

if current flow Q size gets larger than the Scaled Average then decrease max threshold to

New max threshold = min threshold + (max-min)/2

For now those are it....i think....

Edit<i just re read this and i've misspelled DENOMINATOR 6 different ways it seems...for the record..i do know how to spell that particular word...i just dont know how to type!>

Tox!

edit / add

cisco doc however says that once you get over max threshold you do a Full drop not at mark probability

the above furmulae are from WB1 for RnS

Tox!

Tox,

This isn't quite what I was asking, although, I genuinely appreciate you taking the time to write it all down. [:)]

I'm looking more for CUCM LBCAC vs. Gatekeeper vs. RSVP bandwidth utilization. Does that make sense?

-Brandon

a review is a review regardless

in anycase as i said before..i'm studying for the RnS ...what i didn't say is..i know nothing about VOIP except how to interleave it and sent it along a priority Q!

sorry man...my advice about writing it down works but as to the WHAT to write down of the matter, I have no clue [*-)]

Tox!

Understood. Very good information though [;)]