Fascinating. While we are (we are engineers after all) beating the horse senseless, this is interesting to see how others arrive at their decisions.
I have always worked for schools where the budget is X. Spend it on spares or get gear out the students, it is on us to make that decision. I have always erred on the side of less spares more for the students…. However, don’t forget the basics –environmental variables. Before I moved to Florida, I would have maybe 2-3 switches (we had 200 switches in use) and ~5 AP’s for the 1200 in use fail. I never ran out of spares and had an exposure… Until I moved to Florida. Now, a good storm can blow up 2-3 switches in a day and 6-10 ap’s. Worst storm to date was 2 chassis blades, 12 switches and 32 AP’s. So all things being equal never discount the environment you live in and your mileage may vary, you know your environment better than anyone (or will know it after a year ;) ) Also don’t forget the product, when I was a Cisco wifi shop (in Florida) during the Wave 2 refresh (which people are still talking about) I had a huge failure rate. Since I went to Aruba I am down to (lightening hits aside) about 1-2 failures a year out of 1500 aps. Conversely, I do not lose Cisco switches at all (lightening hits aside) Ian. From: The EDUCAUSE Wireless Issues Constituent Group Listserv [mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU] On Behalf Of Frans Panken Sent: Wednesday, February 28, 2018 3:14 AM To: WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU Subject: Re: [WIRELESS-LAN] Offline/Spare Gear Inventory Size I have a background in mathematics and addressed this question scientifically. The number of spares depend on the mean-time-between-failures of an AP (provided by vendors; e.g., Cisco’s 3702 APs have a MTBF of 325000 hours), the number of days it takes to order a new AP (k) and the risk you are accept to take that you have no spare AP available. If n is the spare factor (1 spare AP for every n active APs) and [cid:image005.png@01D3B072.CCF5AF90] is the risk you accept that you have no spare AP available, this number can be computed as follows: [cid:image006.png@01D3B072.CCF5AF90] Here “ln” is the natural log, ln(2,71828…) = 1. So, in the case of Cisco AP3700 and you accept an availability of 0.95 (5% of the time you have n spare AP) and it takes 6 days to order a new AP, you need one spare on every 116 APs. -Frans P.S. I was reluctant to spam you with the derivation; please send me an e-mail if you are interested in how derived this formula. From: The EDUCAUSE Wireless Issues Constituent Group Listserv <WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU>> on behalf of Greg Briggs <brigg...@plu.edu<mailto:brigg...@plu.edu>> Reply-To: The EDUCAUSE Wireless Issues Constituent Group Listserv <WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU>> Date: Wednesday, 28 February 2018 at 00:06 To: "WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU>" <WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU>> Subject: Re: [WIRELESS-LAN] Offline/Spare Gear Inventory Size I solve replacement items (of many kinds) with a formula that works for small and large deployments. It works for everything from switches to toilet paper so I have used it for more than my day job. It can and can be expressed in a spreadsheet like this "=CEILING(N1*M1,1)" where N is a column with the calculated need and M is a rate at which you need spares, or it is consumed before your next purchase. The formula is sometimes calculated on a spreadsheet and sometimes it is just a rough estimate in my head. Here are some example starting values for the M column. Optionally you can add a field for the ceiling value to reflect things that are only available in quantities greater than one. For example 10 for hotdogs and 8 for hotdog buns. The formalization of this formula is adapted from one my manager used, so credit is due to David Allen. * 1.1 for something like a new line of APs you don't already have a deployment of. This allows for 10 spares in a deployment of 100 in case some arrive DOA or I find a flaw in my plan after the order/ post install. I would be more conservative if I didn't think I was ever going to need that model of APs elsewhere, and just take the heat if I end up short. * 1.01 for the subsequent deployment of APs if spares are already on hand. * 1.1 for something that is mission critical but we only have a few of. * 1 for expensive things that have a high availability feature and are under a reasonably quick turnaround service/replacement contract. * 1.2 for items that we ran out of quickly last time we made an annual purchase. * 1.05 for inexpensive things that would save some time to have spares of, but are only a minor inconvenience if you run out. Modify the value for subsequent orders based on current inventory or if you find that the failure (or consumption) rate is higher than expected. Another reason to modify the value is if replacements can be found locally, or if you can count a similar model of item that you have spare of as a replacement. MTBF is another variable to consider. Using this formula also helps determine the support level because you can calculate what you need to make the cheaper support levels a better value with an acceptable and/or similar risk. Greg Briggs Network Manager Pacific Lutheran University On Tue, Feb 27, 2018 at 8:23 AM, Jeffrey D. Sessler <j...@scrippscollege.edu<mailto:j...@scrippscollege.edu>> wrote: * Look at the turn-around time for warranty replacement. The free limited-lifetime may take longer than if the AP is under an extended contract. * Evaluate your deployment plan. If your deployment is coverage-based, where the loss of a single AP could be devastating to clients, then keep more spares. If you have a dense deployment where the loss of one or more APs is of little consequence, keep less. * Spares are technology collecting dust with the same life-cycle as those in production. If you have 5000 APs and spare 2%, that’s 100 APs that would likely cover a moderately sized building, and provide a lot of in-fill. * If you keep spares, make sure to cycle them into production i.e. always install them into a new project, and put new APs back on the spare shelf. * When you upgrade controller code, pull those spares out and let them upgrade too, then test that they still work. Jeff From: "wireless-lan@listserv.educause.edu<mailto:wireless-lan@listserv.educause.edu>" <WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU>> on behalf of "Trinklein, Jason R" <trinkle...@cofc.edu<mailto:trinkle...@cofc.edu>> Reply-To: "wireless-lan@listserv.educause.edu<mailto:wireless-lan@listserv.educause.edu>" <WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU>> Date: Monday, February 26, 2018 at 10:21 AM To: "wireless-lan@listserv.educause.edu<mailto:wireless-lan@listserv.educause.edu>" <WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU<mailto:WIRELESS-LAN@LISTSERV.EDUCAUSE.EDU>> Subject: [WIRELESS-LAN] Offline/Spare Gear Inventory Size Hi All, I’m curious to know the size of your spare gear inventories. Do you keep a percentage of each model of AP in inventory, and what is your reasoning? Storms? Last minute/emergency wireless coverage needs? What percentage of your live gear do you keep as offline inventory? (100 live APs with 1 inventory AP = 1% offline inventory). With Xirrus, we had an offline inventory of more than 10% of live inventory. We kept that inventory to cover the high failure rate of the equipment, the incidence of hurricanes and lightning strikes in our area, the broad range of AP models on campus, and last minute large events in low coverage areas. We are evaluating the minimum offline inventory for our new Aruba gear as we finish up the vendor switch. I have been thinking 1-2%, but I want to see what you guys do first, and why. Thank you, -- Jason Trinklein Wireless Engineering Manager College of Charleston 81 St. Philip Street | Office 311D | Charleston, SC 29403<https://maps.google.com/?q=81+St.+Philip+Street+%7C+Office+311D+%7C+Charleston,+SC+29403&entry=gmail&source=g> trinkle...@cofc.edu<mailto:trinkle...@cofc.edu> | (843) 300–8009<tel:(843)%20300-8009> DID YOU KNOW? The Princeton Review selected the College of Charleston as one of 50 schools focused on providing students with practical experiences that take their academics to the next level. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/discuss. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/discuss. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/discuss. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/discuss. ********** Participation and subscription information for this EDUCAUSE Constituent Group discussion list can be found at http://www.educause.edu/discuss.
