Cisco Cisco Aironet 1310 Access Point Bridge Libro blanco
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Absent from this discussion thus far is the 802.11a standard. While ratified at the same time as 802.11b, 802.11a has achieved a small fraction of
the unit sales of 802.11b or even the much more recent 802.11g standard---despite the fact that 802.11a provides the same 54-Mbps data rate of
802.11g. This relative lack of acceptance is due in large part to the standard’s lack of backward compatibility, a critical capability provided by
both 802.11b and 802.11g.
802.11a’s lack of backward compatibility is due to its operation in the 5-GHz, not 2.4-GHz, portion of the radio frequency spectrum. The 5-GHz
band brings with it both regulatory advantages and physical disadvantages relative to the 2.4-GHz band. While regulatory bodies like the FCC and
ETSI have provided a small amount of license-free spectrum in the 2.4-GHz band, they have provided a great deal of license-free spectrum at 5 GHz.
Today, the FCC provides 300 MHz of license-free spectrum at 5 GHz, with an additional 250 MHz planned in 2005. In Europe, a similar amount of
license-free spectrum is already provided in the 5-GHz band. With the same 22-MHz channel width as other 802.11 technologies, 802.11a operating
on 5 GHz provides for as many as 19 nonoverlapping channels compared to the three channels available in 2.4 GHz.
On the other hand, devices operating in the 5-GHz band have a fundamental disadvantage relative to 2.4 GHz in terms of range and resulting
coverage area. There is an inverse relationship between frequency and signal propagation---as frequency increases, range decreases. Early 802.11a
devices suffered from both the laws of physics and the shortcomings inherent in any first-generation product, resulting in range significantly less
than 802.11g devices that provide the same nominal data rates (see Table 1).
Table 1.
Comparison Between Different Wireless LAN Standards
Standard
Maximum Data
Rate (Mbps)
Typical
Throughput
(Mbps)
Operating Band
Maximum
Available
Channels
Capacity (Mbps)
Compatibility
802.11b
11
5
2.4-GHz
3
33
802.11b
802.11g
54
14
2.4-GHz
3
162
802.11b/g
802.11a
54
25
5-GHz
19
1,242
802.11a
802.11a/g
54
25 and 14
2.4- and 5-GHz
22
1,404
802.11a/b/g
Note:
Only devices that support both 802.11a and 802.11g provide backward compatibility and a substantial increase in network capacity.
THE EVOLVING WIRELESS LAN CLIENT---THE FUTURE
The limitations of 802.11b and 802.11g are already being encountered, as IT professionals deploy infrastructure in multitenant and multifloor
buildings. Channel reuse patterns are complicated, and interference from adjacent access points is inevitable given the small number of available 2.4-
GHz channels. As the number of wireless users increases, the available bandwidth is taxed, which limits performance and the sorts of applications
that can be supported.
Few organizations are willing to abandon the investment they’ve already made in these technologies, making 802.11a nonviable on its own. To
address this seeming conundrum, the wireless LAN community has learned from both radio and television. In much the same way as radio added
the FM band to the legacy AM band and television added the UHF band to the legacy VHF band, the wireless LAN industry is set to make a 2.4-
and 5-GHz architecture the norm. This dual-band architecture provides both backward compatibility with associated investment protection and a
large number of available channels with associated high capacity.
Already, this migration to a dual-band architecture is underway. Client adapters that support 802.11a, 802.11b, and 802.11g are becoming the norm.
Typically, these client devices prioritize their 802.11a radio, meaning that the device will first scan the 5-GHz band for an available 802.11a access