802.11n channel bonding and 802.11ac backwards compatibility

I'm about to set a design with Cisco 1702 or 2702 APs (Cisco WLCs are already in place). The environment is a school. The majority of devices are smartphones then laptops and last tablets. 802.11n at 5 GHz and channel bonding is supported since Ihone 5 (sep 2012) and erlier in some cases with Android phones which makes me mainly focused at 5 GHz. Channel bonding seems like a "must-have" when the environment consists of many 1 spatial stream devices. 802.11n at 2,4 GHz will be more of best effort (since there are another 2,4 GHz network in the same building and avoiding interference will be tough).

My questions to you are:
1. What effect will channel bonding have when there are older clients online that doesn't support channel bonding? Will the AP be able to talk with a 20 MHz range to old clients and 40 MHz to newer clients or will they all be degraded to 20 MHz?
2. 1702 and 2702 APs can use 802.11ac. Can they be configured to use 802.11ac with 40 MHz bandwidth and still support 802.11n clients (both with and without channel bonding support).
3. In Europe we only have 4 non-DFS channels and they are all used by an adjacent network which leaves me with DFS channels only. I know Iphone 5 support all DFS channels, but can you give me some examples of devices that don't?
4. How near can a small airport be before I should get worried about DFS channel interference?
5. How would Cisco ClearAir help me with adjacent networks?
6. I've heard that 802.11ac networks won't be degraded to the speed of the slowest client, that it can send at different speeds to multiple clients. How does it work technically? What technologies are used? Anything Cisco proprietary? Have in mind that majority of the clients are smartphones with only 1 spatial stream (if that would make things different).
7. According to Cisco, 20 Mbps throughput could be expected with a PHY data rate of 75 Mbps in high density client environments. Would that 20 Mbps be regardless of encryption standard? The obvious choice feels like WPA2/AES.

I would deeply appreciate your answers - single question answers too.
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scorpius78Asked:
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Craig BeckCommented:
Simple answers...

1] Works on a per-client basis.  If a client supports 40MHz it'll use that.

2] Yes.

3] This is difficult to explain.  Some devices have full support for DFS channels while other devices support DFS but user preferences/locale dictate compliance, while others just don't support DFS channels at all.  The fail-safe is to use non-DFS only.  Not what you want to hear, I know!

4] Within a few miles will be an issue if you're using DFS channels.

5] CleanAir is to help mitigate non-802.11 interference, so it won't help if there are adjacent networks.  ED-RRM will help though and is a feature available on the WLC already - you just need to enable it.

6] It just works.  It's a myth that ALL clients 'connect' at the same data-rate as the slowest client.  They connect at the fastest speed possible within the cell that they can support individually.

7] WPA2/AES is the ONLY choice if you're wanting 802.11n or 802.11ac, unless you choose no encryption at all.  802.11n and 802.11ac REQUIRE AES where encryption is used.  Generally the PHY rate is halved instantly (as it's half-duplex), then you subtract at least 10% (for overheads and distance from the AP, etc) so that leaves a maximum of around 30Mbps in optimal conditions.  In high-density environments it'll be less, so 20dBm is a fair and accurate estimate.  Remember, you would never get a PHY rate of 75Mbps with TKIP - it has to be AES or none.
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scorpius78Author Commented:
Awesome. You just gave me the possibility of a good nights sleep. :)
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Darr247Commented:
I think for
1.
you'll find each channel width will achieve their maximum throughput only if '802.11n greenfield mode' is chosen for 5GHz.

That would not allow 802.11a clients to connect, though.
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Craig BeckCommented:
@Darr - [1] was talking about clients, not channels being used to their full potential.  If you allow .11a, clients will connect at .11a data-rates.
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