• By (Deleted User)

    Hello again CWNP forums. We're having the same discussions here in the North West and I haven't been able to find a real documented answer.

    The new plan that we have designed is 9 APs running at 11.5dbm EIRP, which is considered a dense deployment. It seems to meet the coverage and speed needs of our floors. The other group trying to fix the poor design that was already deployed to another floor is working on a 20 AP plan. Their plan includes power down in the 5.5dbm range. That equates to roughly 3.16 mW and with people actually being on the floor with ~3db attenuation through the human body we're looking at 1.5mW of power on their plan.

    Needless to say this low of a power setting worries me, especially when you add the fact that the phones are currently set to mirror the power settings of the network. Does anyone have, or has anyone seen a recommended minimum for functional wireless?

    As always, thanks in advance for any help you can provide.

  • I always look at it from a client devices point of view.

    For example, if a client device has a certain sensitivity (say -72 dBm) at a particular rate, then I would add about 17 dB to that to give -72+17 = -55 dBm as the minimum power level I need throughout my installation.

    What's maybe more important is the packet retry rate which will be affected by your channel plan, and the level of interference in each particular location.

  • Mr Reardon:

    Good to see on on the forums. :) I was doing a little research for the CWDP and saw your post. I like where Wlanman is going basing this on the client. There are a lot of factors at play here, but at the highest of levels I'd say:
    + look at the devices you're supporting and understand their max Tx power at the higher PHY rates. Usually manufacturers say "we do 100 mW", but that is only at DSSS and not OFDM. Even at higher OFDM rates, Tx power can drop significantly. Usually assume 25 mW max Tx power for high mobile devices like VoWiFi phones.
    + AP placement is key. APs placed in the hallways down a hospital corridor, for example, will see each other very strongly and usually only adjust their power based on how they hear each other. That might decrease your usable coverage at the edges of the patient rooms where you are downlink limited (AP isn't transmissing with enough juice to get a signal back to the client). In other words, it's really not that different than your colleagues setting their APs down to 5 dBm.
    + DTPC (dynamic transmit power control) isn't supported on all client devices and client may not reciprocate by powering down their transmissions back to the AP. Remember, it's not just AP transmissions we're concerned about; think about the clients, too. Also, sometimes infrastructure is configured to not advertise its transmit power and clients therefore have no clue how to set their Tx power even if they did support DTPC.
    + uplink receive sensitivty never changes regardless of transmit power. Putting APs in a highly dense fashion is never good regardless of how low you set the Tx power to. Unless you're also putting attenuators on the receive inputs of the AP you're going to get a lot of RF contention at 2.4 GHz especially.

    I generally assume VoWiFi support is a current or future application and survey AP placements based on 12.5 or 25 mW depending on the facility and other factors. Less than that has diminishing returns in my opinion. Going more than 25 mW usually results in link budget imbalances and some devices will struggle.

    I'm doing more of the 12.5 mW (11 dBm) lately because I'm giving the automatic RF management algorithm a +/- 3 dB window to make adjustments. The survey is how APs are placed and what determine what types of antennas are used.

    Does that help?

  • By (Deleted User)

    Hey Jackman,
    First, I'm not even sure my dad was Mr Reardon, much less me. ;-) That was some helpful info. I used 11.5 dbm EIRP for my design. Based on the floor layouts and everything we needed to support, it seemed to be sufficient. Also seeing as how ARM only adjusts in 3db incriments, we decided to start with 9db out of the controller with the 2.5db gain from the AP125.

    Our main objective was a good VoWiFi design. It also has to support RTLS. Past that we really weren't too worried about the data, as we've seen them be pretty resilient and the network is the least of your worries when we're in Oregon and the EPIC datacenter is in Virginia. Luckily Aruba's AirMonitors also forward location information (at least according to their verified design docs say they do), so we can add AMs for location needs without increasing the actual coverage and interference

    I'll have to look into the other portions you mentioned. I know our VoWiFi provider told us at one point that the handsets will go to 100mW, but I guess it's long past time to look up the specifics.

    As for placement, that was a big part of figuring out my design. As for previous placement, yes. All of the APs were placed in the hallways. As you can imagine, when you get to 13-20 APs on a 30 patient room floor, that makes for a lot of co-channel interference.

    Now for the good news. The design that a co-worker and I designed has been approved for roll out on the top floor of our building. We have also recommended reducing power on the floor below to limit floor to floor bleed through we are seeing around the elevators. Our wiring company is planning on having the moves done by the end of next week, so we'll be able to see if our theoretical work matches the real world.

    The optimistic me was hoping someone would have some resource that said, you never really want to go under X mW on a deployment because it causes these problems. I understand it isn't that cut and dry, but we should see at the end of next week.

    Thanks for all the help and the other items to drill down on.

  • Well, just to be clear, towards the end of my late night post above, I was trying to say that you don't really get value in your use case of going below ~10-15mW in my opinion. Recall the uplink situation mentioned above. I don't usually like to see APs going below that value in a typical deployment scenario. And, nothing more than 25mW in a high density, VoWiFi deployment.

    Ascom reports to go upwards of 100 mW, but I'm betting Jon's paycheck that if you look into the chipset specs, as you go to higher and higher PHY rates, Tx power lowers substantially as with just about every other client product.

  • By (Deleted User)

    Ok I'm a bit clearer on it now. My concern when looking at higher power was cell size and user limit per AP. When going through AirMagnet and letting it plan where it wanted everything it came back with a 5 AP per floor design. The concern there was RTLS and user load.

    As we started with the ICU, I'll start there. On our 30 patient room ICU floor (approx 38k sqft), we have 50 ASCOM phones, 1 computer in motion (CIM) cart per room (=30), usually no less than 1 respritory specialist (laptop+ASCOM phone), 3 Nutritionists with ASCOM phones, Hospira pumps running on b (just to give it a number we'll say 10), and various individuals on laptops and ASCOM phones as well as blackberry's and such.

    Counting just the devices we know about we're looking at 95+ units on the floor at any one time. Given, that we assume the units are spread evenly across the floor we're still looking at 20+ units per AP at any one time. With Aruba telling us that we should plan for no more than 35-40 units on any one AP, a 5 AP plan leaves us little wiggle room. Add to that fact that about 10% of our units are b clients (and apparently irreplaceable with different technology), it gets me a bit nervous.

    Granted I am not yet a design guru, and that's why I came here for info. Originally my question was posed about it RTS/CTS was SSID or AP based for this reason. Big cell size high power and units that say they match the power settings of APs could cause the cells to be incredibly large. Thinking of course of a VoWiFi handset that is at the edge of one cell, also broadcasting at 25mW then increases the interference range on that channel by a significant margin. I guess this is one of the reasons you told me a while back that WLAN designing for VoWiFi isn't for the feint of heart.

    It is nice to see that the ~14mW our plan calls for is in a range you mention, even if that range is the low end. We'll see how it goes and I'll post back on here as we start getting some data.

  • By (Deleted User)

    Hi Brad.

    Please let me know about your progress with your VoWiFI isntalaltion.
    Do you have our configuration manuals, recomendations etc.

    Ascom Wireless
    International VoWiFi Trainer

  • By (Deleted User)

    Hi Martin,
    We have tried to encompass a myriad of documents and information in our design. Specific to ASCOM, we spent our time on TD 92408GB titled: Considerations for ASCOM VoWiFi System Planning. The other information we have received from TAC and our various discussions.

  • By (Deleted User)

    Hi Kevin.

    How did this turned out for you?
    Just curious since I am woking with oversseing those documents.

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