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Archive for September 2014

What’s New (and Missing) in the WiFi for iPhone 6

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Two years ago tomorrow Apple introduced the iPhone 5.  It was a big deal.  It was a big deal for gadget folks who wanted a bigger iPhone.  It was a big deal for wireless LAN folks who wanted users to use smartphones with speedier WiFi.  

Now the iPhone 6 has been announced and it appears to be more of the same.  Gadgeteers get their bigger iPhone.  Wireless folks get their faster speeds.  Problem is, the faster wireless speeds likely won’t mean anything for high capacity wireless deployments.

The big news about the iPhone 6 is 802.11ac.  Yippee!  Apple has finally adopted the latest and greatest WiFi standard in a mobile device.

802.11ac has data rates as high as 6.9 Gbps in the standard, but wireless LAN folks know that’s not what happens in real life.  Real 802.11ac devices top out at a 1.3 Gbps data rate when multiple input-multiple output (MIMO) antenna systems are supported, while non-MIMO devices top out at 433 Mbps.

The iPhone 6 and iPhone 6 Plus are non-MIMO 802.11ac devices.  That means a top rate of 433 Mbps.  That is higher than the top rate of the 802.11n-supporting iPhone 5 and iPhone 5S, which is 150 Mbps.  And that is where Apple gets the nerve justification for putting this on their site:

You see, 433/150 = almost 3.  That means three times faster wireless!  Except it doesn’t.
802.11ac is basically the same thing as 802.11n.  I know that 433 and 150 seem like very different numbers, but in most real world cases, they’re actually the same.
Here’s how it works:
802.11n = 150 Mbps –>

–> Normal 802.11ac = 150 Mbps –>

–> 802.11ac with clear line of sight and a distance less than 30ft/10m = 200 Mbps –>
(That’s because when you’re that close and there are no obstructions, then 802.11ac can use a technology called 256-QAM, which allows waves to carry 8 bits of data rather than 6 bits.  8/6 = 200/150, so that means that adding 256-QAM boosts the top data rate to 200 Mbps.)
–> 802.11ac with clear line of sight and a distance less than 30ft/10m and 80 MHz channels enabled = 433 Mbps
(80 MHz channels are no good for high capacity WiFi.  Instead of being able to split users up amongst 9 [if disabling dynamic frequency selection {DFS}] or 21 [if enabling DFS] channels, an 80 MHz wireless network only has 2 [non-DFS] or 4 [DFS] channels.  And here in the U.S. of A., our beloved FCC regulatory commission just made it a whole lot harder to use DFS channels because of Item 67 in the 2014 updates to UNII rules.)

Think about the average high-capacity wireless network.  Do the users have a line of sight to the APs?  Usually, No.  Are the users within thirty feet (ten meters) of the APs?  Often, No.  Is it better to spread users out among four or five times as many channels?  Definitely, Yes.  If you agree with these answers, then 802.11ac in the iPhone 6 and iPhone 6 Plus reverts to the 150 Mbps data rates used in the iPhone 5 and iPhone 5S.

A year ago, yours truly wrote that non-MIMO devices were going to be around for a while.  MIMO drains battery life faster and it can cause a device to heat up.  So, the lack of MIMO in the new iPhones is no surprise.  But it is disappointing.  And it comes down to this:

802.11n w/ MIMO > 802.11ac w/o MIMO

The iPad Air, which has been out for about a year now, is a mobile device from Apple that supports MIMO.

In a typical high capacity WiFi environment, the iPad Air or iPad Mini is going to get as high as 300 Mbps in the 5 GHz band or 144 Mbps in the 2.4 GHz band.  The iPhone 6 or iPhone 6 Plus is going to get as high as 150 Mbps in 5 GHz, or 72 Mbps in 2.4 GHz.  That’s big.  It’s big for users of the gadgets and it’s even bigger for the WLAN folks who have to manage those networks.  Apple may have done a solid for people who want to be able to see five Tweets on their screen at a time instead of four, but for wireless folks who have to deal with high capacity networks they didn’t help us out as much as they could have.

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Twitter: @Ben_SniffWiFi


Written by sniffwifi

September 17, 2014 at 7:04 pm

Capacity or Coverage or Neither?

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In the beginning, there was Coverage.  And so it was that 802.11 and his only begotten Son, WiFi were blessed upon PCMCIA cards who doth receiveth adequateth Coverage.

And then as Coverage grew and the lands of Tablets wereth discovered, so came Capacity.  And thus did Capacity grow to represent all that was good and great about deploymenteths upon this fruitful land.

And now, my Sons and Daughters, things have changeth again.   For Coverage and Capacity will both leave the Higheth of Densiteth WiFi wanting.  And so we shun them both.  For it is Neither — Coverage nor Capacity — that will taketh thy to the WiFi promised land.

In case it was unclear, designing wireless LANs for Capacity has become an article of faith in some circles.  Keep it to 40 devices per AP.  Or 50.  Or 150.  Whatever the number is, the whole concept is misguided.

WiFi uses radio frequency as its physical layer, and there is a finite amount of radio frequency in any given location.  If every radio frequency channel gets used up, then adding more access points to fulfill a Capacity requirement becomes counter-productive.  It becomes the equivalent of adding a Hub to handle a high density wired LAN.  A hub doesn’t add additional access.  It just splits up the existing access; often degrading the quality of the network.  Same thing for adding APs to occuppied channels.

A lot of people understand that having more than one AP on the same channel is a problem, and that’s good.  The problem is that some of those people go about designing a wireless network for one-AP-per-channel in the wrong way.

Imagine that the diagram below represents the APs that are on channel one in my high density area:

See that nice, green check mark?  That means you’ve surveyed the room and confirmed that your two APs on channel one don’t interfere with one another.  You’ve (theoretically) added Capacity to your wireless LAN.  Now twice as many users can get on channel one (theoretically).
The problem with the above picture is that it is often accomplished by turning down the power on APs.  DON’T DO THAT.  It usually results in a Capacity-based design failing once put under stress.
Here’s what happens when a Capacity design with low AP transmit power starts to get crowded:
Consumer devices (smartphones, tablets, laptops, etc.) typically don’t lower their transmit power to the level of APs.  That ends up ruining your previously-thought-to-be-solid design.  Users start to connect.  Users’ devices start transmitting data.  The radio frequency from that data bleeds into another APs’ coverage area.  And the end result is that users in the bleed area get interfered with.  As the room fills up with users, this interference starts to happen so often that the wireless becomes subpar.
When designing WiFi, it’s best to ignore both Coverage and Capacity.  Instead, just stick to radio frequency principles.  Keep AP transmit power around 12 to 15 dBm (because that’s the range that most consumer devices transmit within).  Make sure that no more than one AP on a given channel is covering any area.  And with those two principles in mind, try to make sure that your signal is strong, your coverage only goes where your users are and you create enough overlap if users expect to stay connected while moving from place to place.  Just like the 802.11 Creators intended it.
If you like my blog, you can support it by shopping through my Amazon link or donating Bitcoin to 1N8m1o9phSkFXpa9VUrMVHx4LJWfratseU

ben at sniffwifi dot com

Twitter: @Ben_SniffWiFi

Written by sniffwifi

September 5, 2014 at 8:22 pm

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