Cisco Aironet 2702i AIR-CAP2702I-E-K9 Folheto

 

 

© 2014 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. 

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80 

3 

256QAMr5/6 

Short 

1300 

910 

80 

8 

256QAMr5/6 

Short 

3470 

2400 

160 

1 

256QAMr5/6 

Short 

867 

610 

160 

2 

256QAMr5/6 

Short 

1730 

1200 

160 

3 

256QAMr5/6 

Short 

2600 

1800 

160 

4 

256QAMr5/6 

Short 

3470 

2400 

160 

8 

256QAMr5/6 

Short 

6930 

4900 

*

Assuming a 70 percent efficient MAC, except for 802.11a, which lacks aggregation. 

+

Assuming that 40 MHz is not available due to the presence of other APs. 

The sticker on the box that shows the 

maximum data rate doesn’t help us much in the real world, where devices 

have to contend with interference from non-802.11 devices, preexisting APs that might only use 20 or 40 MHz, 

multipath fading, few antennas on mobile devices, weak signals at range, and so forth. What makes the raw speed 

of 802.11ac so valuable are the extensions that help to deliver reliable throughput under realistic conditions. 

By design, 802.11ac is intended to operate only in the 5-GHz band, as shown in 

. This avoids much of the 

interference at 2.4 GHz, including Bluetooth headsets and microwave ovens, and provides a strong incentive for 

users to upgrade their mobile devices (and hotspot APs) to dual-band capability so that the 5-GHz band is more 

universally usable. This choice also streamlines the IEEE process by avoiding the possibility of contention between 

802.11 and 

 proponents. And there is barely 80 MHz of bandwidth at 2.4 GHz anyway. 

As we’ve already seen, 802.11 introduces higher-order modulation, up to 256QAM; additional channel bonding, up 

to 80 or 160 MHz; and more spatial streams, up to eight. There is an alternative way to send a 160-MHz signal, 
known as “80+80” MHz, discussed later (see 

). 

802.11ac continues some of the more valuable features of 802.11n, including the option of a short guard interval 

(for a 10 percent bump in speed) and an incrementally better rate at range using the advanced low-density parity 

check (LDPC) forward error-correcting codes. These LDPC codes are designed to be an evolutionary extension of 

the 802.11n LDPC codes, so implementers can readily extend their current hardware designs. 

Various space time block codes (STBCs) are allowed as options, but (1) this list is trimmed from the overrich set 

defined by 802.11n, and (2) STBC is largely made redundant by beamforming. 802.11n defined the core STBC 

modes of 2×1 and 4×2 and also 3×2 and 4×3 as extension modes, but the extension modes offered little gain for 

their additional complexity and have not made it to products. Indeed, only the most basic mode, 2×1, has been 

certified by the Wi-Fi Alliance. With this experience, 802.11ac defines only the core 2×1, 4×2, 6×3, and 8×4 STBC 

modes, but again only 2×1 is expected to make it to products: if you had an AP with four antennas, why would you 

be satisfied with 4×2 STBC when you could - and should - be using beamforming? 

What 802.11ac also gets right is to define a single way of performing channel sounding for beamforming: so-called 

explicit compressed feedback. Although optional, if an implementer wants to offer the benefits of standards-based 

beamforming, there is no choice but to select that single mechanism, which can then be tested for interoperability.