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

Códigos do produto

AIR-CAP2702I-E-K9

Page 5 of 25

Figure 1. How 802.11ac Accelerates 802.11n

2.2 How Does 802.11ac Go So Fast?

Wireless speed is the product of three factors: channel bandwidth, constellation density, and number of spatial

streams. 802.11ac pushes hard on the boundaries on each of these, as shown in

Figure 1

For the mathematically inclined, the physical layer speed of 802.11ac is calculated according to

Table 1

. For

instance, an 80-MHz transmission sent at 256QAM with three spatial streams and a short guard interval delivers

234 × 3 × 5/6 × 8 bits/3.6 microseconds = 1300 Mbps.

Table 1.

Calculating the Speed of 802.11n and 802.11ac

PHY

Bandwidth (as Number of
Data Subcarriers)

Number of Spatial
Streams

Data Bits per
Subcarrier

Time per OFDM
Symbol

PHY Data
Rate
(bps)

802.11n or
802.11ac

56 (20 MHz)

1 to 4

Up to 5/6 × log

(64) =

3.6 microseconds
(short guard interval)

108 (40 MHz)

4 microseconds (long
guard interval)

802.11ac
only

234 (80 MHz)

5 to 8

Up to 5/6 × log

(256) ≈

6.67

2 × 234 (160 MHz)

Immediately we see that increasing the channel bandwidth to 80 MHz yields 2.16 times faster speeds, and 160
MHz offers a further doubling. Nothing is for free: it does consume more spectrum, and each time we’re splitting

the same transmit power over twice as many subcarriers, so the speed doubles, but the range for that doubled

speed is slightly reduced (for an overall win).

Going from 64QAM to 256QAM also helps, by another 8/6 = 1.33 times faster. Being closer together, the

constellation points are more sensitive to noise, so 256QAM helps most at shorter range where 64QAM is already
reliable. Still, 256QAM doesn’t require more spectrum or more antennas than 64QAM.

The speed is directly proportional to the number of spatial streams. More spatial streams require more antennas,

RF connectors, and RF chains at transmitter and receiver. The antennas should be spaced one-third of a

wavelength (3/4 inch) or more apart, and the additional RF chains consume additional power. This drives many

mobile devices to limit the number of antennas to one, two, or three.