Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
3-9
Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 3 WLAN Radio Frequency Design Considerations
RF Basics
IEEE
802.11a OFDM Physical Layer
IEEE 802.11a defines requirements for the physical layer of the OSI model, operating in the 5.0 GHz
UNII frequency, with data rates ranging from 6 Mbps to 54 Mbps. It uses Orthogonal Frequency
Division Multiplexing (OFDM), which is a multi-carrier system (compared to single carrier systems).
OFDM allows subchannels to overlap, providing a high spectral efficiency. The modulation technique
allowed in OFDM is more efficient than spread spectrum techniques used with 802.11b.
UNII frequency, with data rates ranging from 6 Mbps to 54 Mbps. It uses Orthogonal Frequency
Division Multiplexing (OFDM), which is a multi-carrier system (compared to single carrier systems).
OFDM allows subchannels to overlap, providing a high spectral efficiency. The modulation technique
allowed in OFDM is more efficient than spread spectrum techniques used with 802.11b.
IEEE 802.11a Channels
The 802.11a channel shows the center frequency of the channels. The frequency of the channel is 10
MHz on either side of the dotted line. There is 5 MHz of separation between channels, as shown in
MHz on either side of the dotted line. There is 5 MHz of separation between channels, as shown in
.
Figure 3-3
Channel Set Example
For the US-based 802.11a standard, the 5 GHz unlicensed band covers 300 MHz of spectrum and
supports 12 channels. As a result, the 5 GHz band is actually a conglomerate of three bands in the USA:
5.150-to-5.250 GHz (UNII 1), 5.250-to-5.350 GHz (UNII 2), and 5.725-to-5.875 GHz (UNII 3).
supports 12 channels. As a result, the 5 GHz band is actually a conglomerate of three bands in the USA:
5.150-to-5.250 GHz (UNII 1), 5.250-to-5.350 GHz (UNII 2), and 5.725-to-5.875 GHz (UNII 3).
Table 3-6
802.11g Modulation and Transmission Types
Modulation
Transmission Type
Bits per Subchannel
Data Rate (Mbps)
BPSK
DSSS
NA
1
QPSK
DSSS
NA
2
CCK
DSSS
NA
5.5
BPSK
OFDM
125
6
BPSK
OFDM
187.5
9
CCK
DSSS
NA
11
QPSK
OFDM
250
12
QPSK
OFDM
375
18
16-QAM
OFDM
500
24
16-QAM
OFDM
750
36
64-QAM
OFDM
1000
48
64-QAM
OFDM
1125
54
30 MHz
30 MHz
Lower Band Edge
Upper Band Edge
5150
5180
5200
5220
5240
5260
5280
5300
5320
5350