Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter Guia Do Desenho
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Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 2 Cisco Unified Wireless Technology and Architecture
Roaming
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LWAPP APs periodically send out neighbor messages over the air that includes the WLC’s IP
address and a hashed message integrity check (MIC) derived from a timestamp and the BSSID of
the AP.
address and a hashed message integrity check (MIC) derived from a timestamp and the BSSID of
the AP.
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The hashing algorithm uses a shared secret (the RF Group Name) that is configured on the WLC and
is pushed out to each AP. APs sharing the same secret are able to validate messages from each other
using the MIC. When APs belonging to other WLCs hear validated neighbor messages at a signal
strength of -80 dBm or stronger, their WLCs dynamically become members of the RF group.
is pushed out to each AP. APs sharing the same secret are able to validate messages from each other
using the MIC. When APs belonging to other WLCs hear validated neighbor messages at a signal
strength of -80 dBm or stronger, their WLCs dynamically become members of the RF group.
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Members of an RF group elect an RF domain leader to maintain a “master” power and channel
scheme for the RF group.
scheme for the RF group.
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The RF group leader analyzes real-time radio data collected by the system and calculates a master
power and channel plan.
power and channel plan.
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The RRM algorithms:
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Try to achieve a uniform (optimal) signal strength of -65 dBm across all APs
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Attempt to avoid 802.11 co-channel interference and contention
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Attempt to avoid non-802.11 interference.
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The RRM algorithms employ dampening calculations to minimize system-wide dynamic changes.
The end result is dynamically calculated, near-optimal power and channel planning that is
responsive to an ever changing RF environment.
The end result is dynamically calculated, near-optimal power and channel planning that is
responsive to an ever changing RF environment.
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The RF group leader and members exchange RRM messages at a specified update interval, which is
600 seconds by default. Between update intervals, the RF group leader sends keep alive messages
to each of the RF group members and collects real-time RF data. Note that the maximum number of
controllers per RF group is 20.
600 seconds by default. Between update intervals, the RF group leader sends keep alive messages
to each of the RF group members and collects real-time RF data. Note that the maximum number of
controllers per RF group is 20.
Roaming
Roaming in an enterprise 802.11 network can be described as when an 802.11 client changes its AP
association from one AP within an ESS to another AP in the same ESS. Depending on network features
and configuration, several events can occur between the client, WLCs, and upstream hops in the network,
but at the most basic level, roaming is simply a change in AP association.
association from one AP within an ESS to another AP in the same ESS. Depending on network features
and configuration, several events can occur between the client, WLCs, and upstream hops in the network,
but at the most basic level, roaming is simply a change in AP association.
When a wireless client authenticates and associates with an AP, the corresponding WLC (to which the
AP is connected) creates an entry for that client in its client database. This entry includes the client MAC
and IP addresses, security context and associations, QoS context, WLAN, and associated AP. The WLC
uses this information to forward frames and manage traffic to and from the wireless client.
AP is connected) creates an entry for that client in its client database. This entry includes the client MAC
and IP addresses, security context and associations, QoS context, WLAN, and associated AP. The WLC
uses this information to forward frames and manage traffic to and from the wireless client.
When the wireless client moves its association from one AP to another, the WLC simply updates the
client database with information about the new AP. If necessary, new security context and associations
are established as well.
client database with information about the new AP. If necessary, new security context and associations
are established as well.
A Layer 2 roam occurs when a client leaves one AP and re-associates with a new AP, in the same client
subnet. In most cases, the 'roamed to' AP is connected to the same WLC as the original AP.
subnet. In most cases, the 'roamed to' AP is connected to the same WLC as the original AP.
The description above represents the simplest roaming scenario because a single WLC database
maintains all information about the client. Network elements upstream from the WLC are unaffected by
the client moving from one AP to another as illustrated in
maintains all information about the client. Network elements upstream from the WLC are unaffected by
the client moving from one AP to another as illustrated in
.