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An Introduction to 802. 11 (Wi-Fi) Technologies
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| bet | 2/3 | | Sana | 02.10.2024 | | Hajmi | 35,6 Kb. | | #273168 |
Bog'liq ADABIYOTLARInfrastructure Mode is used when there is at least one Wireless Access Point and client. The client connects to the network through the Access Point to gain internet access.
Ad Hoc Mode is used when wireless clients want to directly communicate with each other without going through an Access Point. This is also called peer-to-peer mode.
Note: The vast majority of 802.11 WLANs that you will have encountered will have been operating in Infrastructure Mode.
Further information on application methods for 802.11 Wireless Networks can be found in the Introduction to 2.4GHz Technology and Introduction to 5GHz Technology articles respectively.
Popular 802.11 Standards 802.11a
The 802.11a protocol standard uses the same data link layer protocol and frame format as the original 802.11-1997 standard but instead uses an OFDM air interface for the physical layer. Its operating frequency range is in the 5GHz band and has a maximum bandwidth of 54mbps. Due to the 2.4GHz range becoming overcrowded using the 802.11a standard has a significant advantage. However, the effective range of the 5GHz 802.11a is lower than that of the 802.11b, g and n protocols as, in theory, the signal is more easily absorb by solid objects such as walls due to the smaller wavelength which reduces its penetration. Nevertheless, due to the lack of interference in the 5GHz range, 802.11a often has a similar or even greater range in practice.
802.11b
802.11b was the first widely adopted standard wireless networking products. It has a maximum bandwidth of 11Mbps and uses the same media access method as that of the original 802.11-1997 standard. Working on the 2.4GHz frequency range, 802.11b suffers greatly from interference with other consumer items such as Bluetooth devices, DECT & VoIP Wireless phones, wireless keyboards and mice and also microwave ovens etc.
802.11g
802.11g was the next step on from 802.11b still operating a 2.4GHz but using OFDM based transmission. It has a maximum bandwidth of 54Mbps and is backwards compatible with 802.11b hardware. It was the next industrial standard and was, again, widely adopted for WLAN applications due to the increased data transfer rates. Much similar to 802.11b, 802.11g devices can suffer badly from interference from other 2.4GHz consumer products. OFDM is enabled at speeds above 20Mbps which greatly increases NLOS (Non-Line-of-Sight) capabilities.
802.11n
802.11n is a revision that improves on the previous standards by adding MIMO (Multiple-In-Multiple-Out) capabilities, can operate not just on the 2.4GHz frequency range but also at 5GHz, 40 MHz channels to the physical layer and frame aggregation to the MAC layer. 802.11 is backwards compatible with 802.11a, b and g. OFDM is enabled across the whole speed range which greatly increases NLOS (Non-Line-of-Sight) capabilities.
MIMO uses multiple antennas to intelligently resolve a larger amount of data than possible when using a single antenna. It does this by using SDM (Spatial Division Multiplexing) which uses multiple but independent data streams that are transferred simultaneously inside one channel of bandwidth. This increases the throughput bandwidth as the number of resolved streams is increased.
Doubling the channel size from 20MHz to 40MHz can be enabled on 802.11n compatible equipment which allows for twice the data rate on the physical layer relative to a 20MHz channel.
These two features combined give the 802.11n standard increased bandwidth capabilities when compared to 802.11g at 2.4GHz and 802.11a at 5GHz of up to 600Mbps (in theory) when using 4 spatial streams and a 40MHz channel and an increase in range over previous standards when put in practice. It seems that with 802.11 Wi-Fi chip manufactures applying market pressure to move onto this protocol, 802.11 should become the new standard over the next few years.
802.11ac
802.11ac operates at 5GHz using OFDM based modulation. The specification expects WLAN throughput of at least 1Gbp/s and a single link throughput of at least 500Mbp/s. This is achieved by extending concepts from the 802.11n standard such as wider RF bandwidth (up to 160MHz, mandatory 80MHz), more MIMO spatial streams (up to 8), mulit-user MIMO and high-density odulation (up to 256-QAM).
Shown below is a comparison table of the characteristics of the five standards mentioned above:
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