RTS TR-240 Especificaciones descargar pdf (Pagina 62)

802.11 “Non-Overlapping” Channels

Each of the RF channels in the 2.4GHz spectrum has a “22MHz

bandwidth” which allows channels 1, 6 and 11 to exist with no

overlap.

In reality, the 802.11 protocol for 2.4GHz does not actually

define a “width” of the channel. It does, however, define a

“spectral mask”, or channel shape, to which a transmitter must

conform. The specific requirements are that the signal must be at

least -30dB at ±11MHz from the center frequency. This defines

the “main lobe” and is where the “22MHz bandwidth” is derived.

The signal must also be at least -50dB at ±22MHz from the center

frequency and beyond.

Figure 10-5 shows that the “non-overlapping” RF channels

actually do overlap at lower signal levels. The reality is that even

though the overlap of “non-overlapping” RF channels is

measured at -30dB or -50dB, those levels are still strong enough

to cause interference, especially when in close proximity to each

other.

CSMA-CA

The reason devices can communicate without collisions in the

environment is a mechanism in the 802.11 protocol called Carrier

Sense Multiple Access-Collision Avoidance (CSMA-CA).

Before a device transmits an 802.11 packet over the wireless

channel, the device performs a Clear Channel Assessment (CCA)

which is a measurement of the amount of energy in the channel. If

the CCA fails, the device identifies the channel as busy and has to

back-off/wait to send it’s transmission until the channel is all

clear.

The CSMA-CA mechanism is also the reason why interference

must be avoided. When using overlapping channels, or

“non-overlapping” channels, portions of the spectrum are shared

and transmissions can be heard by all devices. The CCA of a

device will fail if it detects a strong enough interfering signal,

even if that it is not intended for that device.

Data vs. Real-time Audio

Interference can exist with any IEEE 802.11 WLAN. In many

areas, users may find several near-by access points to which they

can connect, all of them may even be on non-overlapping RF

channels. The difference is the application of data vs. real-time

audio.

For data applications in the presence of RF interference, the effect

may be slightly longer download / upload speeds while the

devices are waiting for the channel to become clear. This may

even go un-noticed to a user when browsing the internet or

downloading files.

For a real-time audio application, like the BTR-240 system,

devices cannot simply wait for extended periods of time for the

channel to become clear. If the channel is not clear after a specific

waiting period, the information packet is simply dropped. When

this happens, the result is a “tick” or “pop” in the received audio.

For speech applications, an occasional tick or pop may be

tolerable, but for systems with a lot of interference, consecutive

ticks and pops present themselves in the form of audio “break-up”

which may render the system unacceptable or unusable.

10-2

2.412 GHz

(Channel 1)

2.437 GHz

(Channel 6)

2.462 GHz

(Channel 11)

Figure 10-5

Actual Spectrum of “Non-overlapping” RF Channels

fc-

22 MHz

fc-

11 MHz

fc+

11 MHz

fc+

22 MHz

-30 dBr

0 dBr

Main Lobe

-50 dBr

Sideband Lobe

Figure 10-4

802.11 Transmitter Spectral Mask at 2.4GHz

1 2 ... 57 58 59 60 61 62 63 64 65 66 67 ... 85 86

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RTS TR-240 Especificaciones Pagina 62

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