Cisco Cisco Aironet 3700i Access Point Libro blanco

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Market and Technology Overview

As three-spatial-stream 802.11ac devices come to market, and as iPads and other one- and two-spatial-stream

802.11ac devices proliferate, it is critical to maximize performance for all devices. Not doing so could result in a

slower network and in slower application performance for all devices.

The Cisco Aironet 3700 Series Access Points were designed to give best-in-class performance to all devices,

allowing optimal network performance and investment protection so enterprises can effectively accommodate any

device allowed on the network, regardless of how many spatial streams it has.

The keys to the best-in-class performance of the Aironet 3700 Series are:

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The first enterprise-class four-transceiver 802.11ac multiple-input multiple-output (MIMO) design (4x4:3)

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Cisco ClientLink 3.0 beamforming that works with all 802.11ac and 802.11n clients

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Cisco CleanAir

technology to manage interference

How does it all work? It’s worth starting with a bit of history. The first generation of 802.11n devices that came to

market, beginning a few years ago, were able to support a maximum data rate of 300 Mbps. This data rate could

be achieved by running two spatial streams, each carrying 75 Mbps of data per 20 MHz of spectrum, over a

double-wide 40-MHz channel. The formula was 75 Mbps x 2 streams x 2 channels = 300 Mbps. Note that in order

to support two spatial streams bidirectionally, a minimum of two MIMO transceivers were required at both ends of

the link (access point and client).

Next, a newer generation of 802.11n devices emerged in the market, and these devices supported up to three

spatial streams. This means that the theoretical maximum data rate that can be achieved is now 75 Mbps x 3

streams x 2 channels = 450 Mbps. To achieve this maximum data rate bidirectionally, both ends of the link (client

and infrastructure) must support three spatial streams, which in turn requires that both ends have at least three

MIMO transceivers.

Very recently, a new generation of 802.11ac devices have emerged. Not only can these devices support three

spatial streams, but they also support an 80-MHz channel (twice that of 802.11n and four times that of 802.11a)

and may also support up to 256-QAM (up to a 30 percent improvement over the modulation capabilities of

802.11a/n). This means that the theoretical maximum data rate that can be achieved is now 75 Mbps x 3 streams x

4 channels x 1.3 = 1170 Mbps. It is also worthwhile to note that 1170 Mbps also assumes an 800-nanosecond (ns)

guard interval (GI). If one assumes a 400-ns GI, the theoretical maximum data rate is 1300 Mbps.

Maximum speed is important, but we also need to consider how often it will be achieved. It turns out that getting to

the 1300-

Mbps maximum data rate is not easy and requires careful design, as we’ll see later in this white paper.

For now, keep in mind that current solutions that use 3x3:3 architecture (three transceivers, three spatial streams)

on both sides of the link will rarely achieve 1300 Mbps in real-world applications and will have significant

performance degradation beyond a short distance. In real-world scenarios, in which clients are not all 10 feet from

the access point, the fourth transceiver on one end of the link is required to provide the necessary reliability for

three spatial streams to work. And the logical place to put the fourth transceiver is naturally on the access point,

where size and power are of less concern than on battery-powered clients.