Руководство По Проектированию для Cisco Cisco Aironet 350 Mini-PCI Wireless LAN Client Adapter
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Enterprise Mobility 4.1 Design Guide
OL-14435-01
Chapter 3 WLAN Radio Frequency Design Considerations
Radio Resource Management (Auto-RF)
Dynamic Channel Assignment
The 802.11 MAC layer uses Carrier-Sense Multiple Access/Collision Avoidance (CSMA/CA). With
CSMA/CA, two APs on the same channel (in the same vicinity) get approximately half the capacity of
two APs on different channels because of the shared wireless channel. This is caused by the 802.11 MAC
sensing that the channel is busy, and deferring sending frames until the channel has become free. If the
802.11 MAC defers traffic that is not part of its own AP cell, this is considered interference. Interference
from another AP on the same channel is commonly called co-channel interference, and is to be expected
in most 2.4 GHz 802.11 deployments, because there are insufficient non-overlapping channels to prevent
some channel overlap from occurring. One of the goals of design, planning, and dynamic radio
management is to minimize the amount of co-channel overlap, which minimizes co-channel interference
and therefore maximizes AP traffic capacity. The Cisco Unified Wireless Network addresses this
problem and other co-channel interference issues by dynamically allocating AP channel assignments to
avoid conflict. Because the WLC, or a designated WLC (RF Group Leader), has system-wide visibility,
it can control how channels are “reused” to minimize co-channel interference.
CSMA/CA, two APs on the same channel (in the same vicinity) get approximately half the capacity of
two APs on different channels because of the shared wireless channel. This is caused by the 802.11 MAC
sensing that the channel is busy, and deferring sending frames until the channel has become free. If the
802.11 MAC defers traffic that is not part of its own AP cell, this is considered interference. Interference
from another AP on the same channel is commonly called co-channel interference, and is to be expected
in most 2.4 GHz 802.11 deployments, because there are insufficient non-overlapping channels to prevent
some channel overlap from occurring. One of the goals of design, planning, and dynamic radio
management is to minimize the amount of co-channel overlap, which minimizes co-channel interference
and therefore maximizes AP traffic capacity. The Cisco Unified Wireless Network addresses this
problem and other co-channel interference issues by dynamically allocating AP channel assignments to
avoid conflict. Because the WLC, or a designated WLC (RF Group Leader), has system-wide visibility,
it can control how channels are “reused” to minimize co-channel interference.
The WLC examines a variety of real-time RF characteristics to efficiently handle channel assignments,
including the following:
including the following:
•
Noise—This limits signal quality at the client and AP, and can vary in range and periodicity. There
are numerous potential types and effects of interference. An increase in noise reduces the effective
cell size. The WLC, at regular intervals, reassesses the RF environment of an AP, and optimizes
channel selection to avoid noise sources while still maintaining overall system capacity. Channels
that become unusable because of excessive noise can be avoided. If other wireless networks are
present, the WLC shifts its channel usage to complement the other networks. For example, if one
network is on Channel 6, the adjacent WLAN is assigned Channel 1 or 11. This increases the
capacity of the network by limiting the sharing of frequencies. If a channel is used so much that no
capacity is available, the WLC might choose to avoid this channel.
are numerous potential types and effects of interference. An increase in noise reduces the effective
cell size. The WLC, at regular intervals, reassesses the RF environment of an AP, and optimizes
channel selection to avoid noise sources while still maintaining overall system capacity. Channels
that become unusable because of excessive noise can be avoided. If other wireless networks are
present, the WLC shifts its channel usage to complement the other networks. For example, if one
network is on Channel 6, the adjacent WLAN is assigned Channel 1 or 11. This increases the
capacity of the network by limiting the sharing of frequencies. If a channel is used so much that no
capacity is available, the WLC might choose to avoid this channel.
•
Client load—Client load is taken into account when changing the channel structure to minimize the
impact on the clients currently on the WLAN system. The WLC periodically monitors the channel
assignment in search of the best assignments. Change occurs only if it significantly improves the
performance of the network or corrects the performance of a poorly performing AP.
impact on the clients currently on the WLAN system. The WLC periodically monitors the channel
assignment in search of the best assignments. Change occurs only if it significantly improves the
performance of the network or corrects the performance of a poorly performing AP.
The WLC combines the RF characteristic information to make system-wide decisions. The end result is
an optimal channel configuration in a three-dimensional space, where APs on the floor above and below
factor into an overall WLAN configuration.
an optimal channel configuration in a three-dimensional space, where APs on the floor above and below
factor into an overall WLAN configuration.
Interference Detection and Avoidance
Interference (as it pertains to a Cisco Unified Wireless deployment) is defined as any 802.11 traffic that
is not part of the Cisco WLAN system; this includes a rogue AP or a neighboring WLAN. It can also
include non-802.11 sources such as certain microwave ovens or many cordless phones. It can in certain
instances also include various sources of electromagnetic interference (EMI) such as arc welders or
federal/military radar facilities. APs are constantly scanning all channels looking for major sources of
interference.
is not part of the Cisco WLAN system; this includes a rogue AP or a neighboring WLAN. It can also
include non-802.11 sources such as certain microwave ovens or many cordless phones. It can in certain
instances also include various sources of electromagnetic interference (EMI) such as arc welders or
federal/military radar facilities. APs are constantly scanning all channels looking for major sources of
interference.
If the amount of 802.11 interference hits a predefined threshold, the WLC attempts to rearrange channel
assignments to optimize system performance in the presence of the interference. This might result in
adjacent APs being on the same channel, but logically this represents a better scenario than staying on a
channel that is otherwise totally unusable because of an interfering AP.
assignments to optimize system performance in the presence of the interference. This might result in
adjacent APs being on the same channel, but logically this represents a better scenario than staying on a
channel that is otherwise totally unusable because of an interfering AP.
For example, the WLC can respond to a rogue AP on channel 11 by shifting nearby APs to channel 1 or
channel 6.
channel 6.