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 < [email protected]> 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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