Cisco Cisco Aironet 3700i Access Point 백서

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Why Optimized Roaming?

Optimized Roaming is basically an extension of Assisted Client Roaming. Optimized Roaming is a technique used

to intelligently evaluate the RF performance of a client device as it approaches the edge of coverage, and to

provoke client scanning/roaming if necessary. In other words, the access point is continuously evaluating the

quality of the Wi-Fi connection for each associated device and can actively disconnect a device upon detecting that

the device is moving into a region of poor coverage. This detection is accomplished by measuring data frames

received from the client device.

As an example, if the access point receives too much traffic from a client that is below a particular signal level

threshold during a predetermined measurement interval, the access point disconnects the client. The result of

actively disconnecting the client via Optimized Roaming is that it forces the client to enter its scanning mode much

earlier than it ordinarily would. Some may think this method is extreme, but in reality it is merely terminating a

connection that is becoming essentially unusable.

Therefore, prompting the client to begin scanning for a better point of attachment (such as another access point or

possibly a cellular connection) much sooner than it ordinarily would helps provide an improved user experience.

Furthermore, Optimized Roaming can also improve the overall performance of the WLAN, since it effectively

eliminates clients that may consume excessive airtime (low-rate data frames and/or retransmissions). Therefore,

the benefit is not only to the end user but to the overall performance of the access point and therefore to other

users as well. If you are not convinced, consider that a client that is gradually degrading in performance (and

possibly degrading faster than it can dynamically rate-adapt to or compensate for) may cause a large number of

retransmissions (either frames it is receiving from the access point or frames it is transmitting to the access point).

This consumes airtime. If enough clients are being sticky, this phenomenon can degrade the performance of the

access point. Therefore, sticky clients that maintain poor connections can easily affect well-behaved clients. In a

high-density environment, helping make sure that clients are not consuming more than their fair share of airtime

and can connect to the best possible access point is essential.

The advantages of relying on data traffic are twofold. First, more reliable measurements can be made for active

clients that are in motion. Second, idle clients that could have satisfactory coverage are not disconnected (it would

not be advisable to penalize a client and disconnect it simply for being quiet). Furthermore, the Cisco access point

can perform this detection before the device goes into a dead zone, where typically the device can hear only

beacons. Note that beacons are typically transmitted at the lowest data rate and the highest power and therefore

have the greatest range. Therefore, and unfortunately, many devices make their scanning decision based solely

upon beacons, thereby creating a sticky client.

Optimized Roaming has been enhanced from previous generations to include BSS Transition Management (from

the 802.11v amendment for Wireless Network Management). Using BSS Transition Management can enable the

Wi-Fi infrastructure to inform the client of an imminent disconnection prior to the disconnection occurring.

Furthermore, the information includes a list of neighboring access points as candidates the client to connect to.

In other words, the Wi-Fi infrastructure provides a request (or a recommendation) that advises the client to

reconnect. This is a less harsh method for prompting the client to begin scanning for a better point of attachment

(such as another access point or possibly a cellular connection) much sooner than it ordinarily would. The result is

that the user will get a much improved experience.