Cisco Cisco Aironet 3700i Access Point 백서

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In summary, beamforming is most practical using techniques that don't expect assistance from the client, such as

Cisco ClientLink 3.0 (and its predecessors). ClientLink 3.0 solves the problem of channel estimation without

depending on client assistance and continues to add genuine value. The ClientLink family still helps provide

optimal performance for (1) legacy 802.11a/n clients, (2) those 802.11ac clients that do not support 802.11ac

sounding, and (3) clients at 2.4 GHz. Finally, it avoids the overhead of standards-based explicit sounding.

A Few Details on ClientLink 3.0

How does ClientLink 3.0 work? To answer that, it’s worth starting with a bit of history. The first generation of

802.11n devices that came to market beginning a few years ago were able to support a maximum data rate of 300

Mbps. This data rate could be achieved by running two spatial streams, each carrying 75 Mbps of data per 20 MHz

of spectrum, over a double-wide 40-MHz channel. The formula was 75 Mbps x 2 streams x 2 channels = 300

Mbps. Note that in order to support two spatial streams bidirectionally, a minimum of two multiple-input, multiple-

output (MIMO) transceivers were required at both ends of the link (access point and client).

Next, a newer generation of 802.11n devices emerged in the market, and these devices supported up to three

spatial streams. This means that the theoretical maximum data rate that can be achieved is now 75 Mbps x 3

streams x 2 channels = 450 Mbps. To achieve this maximum data rate bidirectionally, both ends of the link

(client and infrastructure) must support three spatial streams, which in turn requires that both ends have at least

three MIMO transceivers.

Very recently, a new generation of 802.11ac devices has emerged. Not only can these devices support three

spatial streams, but they also support an 80-MHz channel (double that of 802.11n and four times that of 802.11a)

and may also support up to 256-QAM (up to a 30 percent improvement over the modulation capabilities of

802.11a/n). This means that the theoretical maximum data rate that can be achieved is now 75 Mbps x 3 streams x

4 channels x 1.3 = 1170 Mbps. It is also worthwhile to note that 1170 Mbps also assumes an 800-ns guard interval

(GI). If one assumes a 400-ns GI, the theoretical maximum data rate is 1300 Mbps.

Maximum speed is important, but we also need to consider how often it will be achieved. It turns out that getting to

the 1300-Mbps maximum data rate is not easy and requires careful design.

But for now, keep in mind that current solutions that use 3x3:3 architecture (three transceivers, three spatial

streams) on both sides of the link will rarely achieve 1300 Mbps in real-world applications and will have significant

performance degradation beyond a short distance.

In real-world scenarios, Cisco realized that a fourth transceiver on one end of the link is required to provide the

necessary reliability for three spatial streams to work. Of course, the logical place to put the fourth transceiver is on

the access point, since this can benefit all type of clients with varying capabilities.

In the end, the fourth transceiver on the access point gives extra decibels (gain) of link margin, which translates to

better performance.

In the uplink direction (client to access point), the extra receiver allows for MIMO equalization gain (via diversity

and redundancy). This can greatly improve receiver sensitivity. Cisco leapfrogs products that depend on pure

spatial multiplexing and delivers hybrid spatial multiplexing and diversity with the addition of a fourth receiver.

Cisco continues this uncompromising approach in HDX.