[cisco-voip] QoS Policing Bandwidth Question

[Anthony Holloway]

Oops!  Sorry folks, but I have an error in my explanation of my selection.
However, for the sake of keeping my reply hidden from view, I cannot easily
correct it via the mailing list (a benefit of a web forum).  When you spot
it, you'll know, and you may even be able to guess why it's wrong.  Hint: I
originally typed 64Kbps but then edited the value without editing the
explanation.

On Sat Nov 22 2014 at 3:14:33 PM Anthony Holloway <
avholloway+cisco-voip at gmail.com> wrote:

> Here's a little Saturday afternoon thought provoking question for the
> group.
>
> If you were asked to police your access ports to allow only a single g711
> call, which of the following values is correct?  Assume you run an Ethernet
> based LAN on Cisco switches for this question.
>
> A.  80Kbps
> B.  87.2Kbps
> C.  93Kbps
> D.  95.2Kbps
> E.  96.8Kbps
> F.  None of the above
>
> Yes, I am studying QoS at the moment.  ;)
>
> So, here's my answer and why I think that it's right...Stop reading if you
> haven't picked your answer yet.
>
> Only read on after selecting your own answer, so that my answer does not
> influence your choice.
>
> The same would go for any replies to this discussion, pick your answer
> first, then read what others are saying.
>
> Ok, here we go...
>
> I choose F. None of the above
>
> Here is why...also, if you click the links, it will take you to a
> reference for each value and why they were chosen as choices to this
> question.
>
> A.  80Kbps
> <http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/admin/7_1_2/ccmsys/accm-712-cm/a02cac.html#wp1033346>
> is the bitrate of the codec, so that's plain wrong.  It's lacking all L2 -
> 4 headers.
>
> B.  87.2Kbps
> <http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/7934-bwidth-consume.html#topic1>
> does consider the L2 - 4 headers, but it fails to account for Ethernet CRC
> and IPG.
>
> C.  93Kbps
> <http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book/QoSIntro.html#29651>
> also considers the L2 - 4 headers, while ignoring the CRC+IPG, but it does
> account for a 5% overhead buffer which is nice.
>
> D.  95.2Kbps
> <http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/ipv6/ipv6srnd/netstruc.html#wp1057244>
> considers L2 - 4 headers, CRC+IPG, but an IPG value only valid for Gig
> ports.  Actually, you could argue a swap for CRC and IPG because they're
> both 4 Bytes.  It's not like you define which headers you're covering,
> rather it's the final value that matters.
>
> E.  96.8Kbps <http://www.bandcalc.com/> considers L2 - 4 headers,
> CRC+IPG, and an IPG value for 100meg ports, which covers Gig ports as
> well.  I have two complaints with this value:
>
> 1) The value isn't a clean increment of 500 bps or 1Kbps, so when you type
> in: police 96.8k, the switch will change it to 96.5k, forcing you to go
> with 97k.  Then how would your value look if you needed to police for more
> than one call (think BIB media forking
> <http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cust_contact/contact_center/mediasense/105_SU1/Design_Guide/CUMS_BK_MC36D963_00_mediasense-srnd_105_SU1/CUMS_BK_MC36D963_00_mediasense-srnd_10-5_chapter_01110.html#CUMS_RF_PE0FD1E8_00>)?
>  97k * 2 = 194k.  97k * 2.5 = 242.5k  I don't know about you, but I would
> have to double check with a calculator to find out if those values were
> correct.
>
> *SW1(config-pmap-c)#police 96.8k 8000 exceed-a drop*
>
> *SW1(config-pmap-c)#do sh run | in police 96*
> *  police 96500 8000 exceed-action drop*
>
> 2)  The value doesn't consider any overhead or "wiggle room."  I'd rather
> allow an extra 5% of traffic and carry the burden on the network versus
> impact a voice call.
>
> Since all choices come with a caveat, I'd like to go with 100Kbps.
>
> I have three main reasons for this value:
>
> 1)  It's enough to cover some wiggle room, "all preambles, headers, flags,
> cyclic redundancy checks, and padding"*, the highest IPG, but not so large
> that it weakens the integrity of the goal to police traffic to a single
> call.
>
> 2)  It's a nice round increment and multiplying the value for 2 or 2.5
> calls (mediasense) is easy work.  100k * 2 = 200k.  100k * 2.5 = 250k.
>
> *A modified wording from the following document
> <http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book/QoSIntro.html#pgfId-46536>
> .
>
> 3)  Since picking anyone of the alternative answers leaves you defending
> your choice anyway ("I saw it in the SRND", "It's what Auto QoS
> configures", "My Magic 8 Ball Said I Could Rely On It
> <http://en.wikipedia.org/wiki/Magic_8-Ball#Possible_answers>," etc.), why
> not go with something original and creative and defend your answer with a
> well thought out explanation?  Who knows, you might provoke the next big
> revolution in QoS.  Oh wait, that's MediaNet, isn't it?
>
> So, now that you have my answer, I'd be really interested in reading
> your's.
>
> Have a great Saturday everyone.
>
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