Auto Channel Selection in 2.4GHZ Using Overlap GOPI MOHAN Assistant Vice President, Technology, Aricent
www.aricent.com
AUTO CHANNEL SELECTION IN 2.4GHZ USING OVERLAP
The ever-growing size of the Wireless LAN (WLAN) market bears testimony to the popularity of the technology among enterprises and mobile operators. Most WLANs are based on IEEE 802.11 standards that operate in an unlicensed spectrum, and are therefore prone to interference. This paper documents Aricent’s development of a simple auto channel selection algorithm that would enable the selection of an interference-free channel, thereby enhancing the overall customer experience.
Introduction
access point itself based on parameters such as signal strength, signal-to-noise ratio, regional settings, etc. The latter mechanism
The WLAN market generated a whopping $4 billion in revenue
is called Auto Channel Selection. It is relatively easy to implement
in 2012, a new high, according to the market research firm
a channel selection algorithm in simple environments with just
Infonetics Research. The sheer size of the WLAN market
one access point. For example, in the home environment, a
confirms that WLAN is a popular technology. With the new
single AP functions as a router/switch to connect wirelessly to
wave of smart devices hitting the market, individuals and
the Internet. It may passively listens to neighboring beacons
enterprises are increasingly relying on WLAN technology and
and allocates itself a non-overlapping channel. It also avoids
WiFi to remain connected. Service providers are also seriously
low-signal-noise-ratio channels. However, in enterprise and other commercial deployments, the number of interfering
pressure on their cellular networks. The majority of modern WLANs are based on the IEEE 802.11 standard and operate in the unlicensed radio spectrum. The downside of operating in the unlicensed spectrum is the potential for a large number of wireless devices to be competing for
devices increases drastically and a robust channel selection algorithm is required to ensure a high level of service. This paper addresses Aricent’s development of a simple auto channel selection algorithm for devices operating in complex environments over the 802.11 2.4 GHz band.
bandwidth in a particular location, often resulting in interference and degraded user connectivity in terms of throughput, link quality, and range. In addition, many household appliances such
IEEE 802.11
as, microwave ovens, cordless phones, remote car keys, and
IEEE 802.11 is a set of standards for implementing WLAN. The
wireless cameras operate in the same unlicensed spectrum
first 802.11 standard to be developed was 802.11b (2.4 GHZ
and cause interference to the WiFi devices. WLANs also face
ISM band, DSSS, max 11 Mbps), followed by 802.11a (5 GHz
non-802.11 interference from Zigbee, Bluetooth, and Near Field
UNII, OFDM, max 54 Mbps), 802.11g (2.4 GHz ISM, OFDM, 54
Communication (NFC) devices that work in the ISM bands.
Mbps), 802.11n (2.4 GHz ISM, OFDM, max 200 Mbps and 5 GHz,
For optimal operation, the wireless Access Point (AP) must select a channel or frequency that is least prone to both 802.11 and non-802.11 interference. The selection of this optimal channel
OFDM, 200 Mbps), and 802.11ac (5 GHz UNII, 256 QAM, 1-7 Gbps) with each standard maintaining backward compatibility with preceding standards within the same spectrum.
can be made either statically by the user or dynamically by the
Auto Channel Selection in 2.4Ghz Using Overlap
1
WLAN AC-AP Architecture
interfere with other 802.11 or non-802.11 sources. An auto
Access point is an intermediate device located between the
non-interfering 802.11 channel. Figure 1 below shows a typical
WiFi clients and the wired infrastructure that provides the radio
WLAN AC-AP architecture.
channel selection algorithm can be implemented to select the
interface to the WiFi clients. It can operate in a single band or dual band. A single band operates at either 2.4 GHz or 5 GHz. A dual-band device, on the other hand, has multiple antennas and can operate simultaneously in the 2.4 GHz and 5 GHz band. A variation of the single channel device is the Multiple Input Multiple Output device (MIMO) that operates at a single frequency but has multiple antennas.
Channel Allocation in the IEEE 802.11 ISM Band The IEEE 802.11 standard divides the 2.4 GHz spectrum into 14 channels with each channel, having a bandwidth of 20 MHz/ 22 MHz/25 MHz. The 14 channels can be visualized as shown
An Access Controller (AC) is a device that manages a group of
in Figure 2. The first band center starts at 2.412GHz and has
APs in an enterprise, campuswide, or public WiFi deployments.
13 channels, each separated from the center of the preceding
The function of the AC is to ensure that each AP operates on
channel by 5 MHz. The last band channel CH14 is separated
non-overlapping channels. Selected channels should not
from CH13 by 12 MHz.
Wired World Enterprise Gateway DHCP Internet/Intranet
AAA
DNS
Access Controller
WLAN-Aware Intermediate Switch
Access Points
Ever-Growing WiFi Clients
Figure 1: WLAN AC-AP architecture
2
22 MHz CH6
CH12 CH11
CH5
CH10
CH4
CH9
CH3 CH2 CH1 2.402
2.412
CH14
CH8 CH13
CH7
2.422
2.432
2.442
2.452
2.462
2.472
2.482
2.492
GHz
83.5 MHz 2.4 GHz
2.483.5 GHz Figure 2: 802.11 2.4 GHz channel allocation
There are country-specific restrictions on channel usage. For
overlap with channel CH1 and therefore has the lowest priority.
example, North America uses only CH1 to CH11, while most of
For APs, each channel is assigned in such a way that the sum
Europe uses CH1 to CH13, with some exceptions and Japan
of all weights is maximized.
uses CH1 to CH14, with CH14 used only for 802.11b.
From the table below, it can be concluded that CH1, CH6, and CH11 are the completely non-overlapping channels. If a less
Static Channel Relationship Table
than 25 percent overlap is acceptable, then more options are
Each channel in 2.4 GHz ISM band overlaps with other channels
except for minimum overlap between CH11 and CH14.
available. Channels CH1, CH6, CH11, and CH14 do not overlap,
as show in Figure 2. The relative overlap for each channel is
When CH14 is unavailable, CH1, CH5, CH9, and CH13 become
shown in Table 1. The extent of overlap is inversely proportional
minimum overlapping channels.
to its priority. This table is used for assigning the least overlapping channel based on its neighbors. For example, for channel CH1,
In the event CH12, CH13, and CH14 are not available, CH1, CH4,
CH6 to CH14 are the non overlapping channels and are therefore
CH7, and CH11 provide a workable set of channels with minimum
assigned the highest priority. Channel CH2 has maximum
overlap.
Priority CH1
P1
P2
P3
P4
P5
Wt: 100%
Wt: 75%
Wt: 50%
Wt: 25%
Wt: 10%
6-14
5
4
3
2
CH2
7-14
6
5
4
3, 1
CH3
8-14
7
6
5, 1
4, 2
CH4
9-14
8
7, 1
6, 2
5, 3
CH5
10-14
9, 1
8, 2
7, 3
6, 4
CH6
1, 11-14
10, 2
9, 3
8, 4
7, 5
CH7
1, 2, 12-14
11, 3
10, 4
9, 5
8, 6
CH8
1, 2, 3, 13, 14
12, 4
11, 5
10, 6
9, 7
CH9
1-4, 14
13, 5
12, 6
11, 7
10, 8
CH10
1-5
14, 6
13, 7
12, 8
11, 9
CH11
1-6
7, 14
8
13, 9
12, 10
CH12
1-7
8
9, 14
10
11, 13
CH13
1-8
9
10
11, 14
12
CH14
1-9
10
11
12
13
Table 1: Static 802.11 2.4 GHz channel relationship table
Auto Channel Selection in 2.4Ghz Using Overlap
3
Auto Channel Selection Solution Using Overlap Channels for the 2.4 GHz Spectrum In crowded venues such as conferences or in stadiums, WiFi deployment is a challenge. In such situations, one AP might have multiple overlaps with adjacent APs (as shown in Figure
> Signal strength and signal noise ratio are also taken into consideration when determining channel overlap. > Other non-changeable factors, including other 802.11 interference sources and non-802.11 interference sources, must also be considered. > APs may not all share the same capabilities, so some channels might be inaccessible or disallowed for some APs. > New APs might power up and existing APs might power
3). Manual allocation of non-overlapping channels and management of allocated channels may not be practical in
down during operation, at which time channel adjustments
such a scenario. An auto channel selection mechanism is useful
have to be made with the least impact on the existing
in allocating non-overlapping channels to APs, of which there are multiple mechanisms to assign channels to WiFi APs in the 2.4 GHz ISM band.
allocated channel. > Country-specific settings might allow certain channels to be available or unavailable.
Solving the auto channel selection problem is similar to solving the color mapping problem, with each area of arbitrary shape
AUTO CHANNEL SELECTION MECHANISM
colors. Auto channel selection is similar but not identical
are APs densely packed into a small space. The region of
because a minimum of four channels are required to assign
influence of each AP is shown in a uniquely colored circle.
unique neighbors. Other important considerations include:
It is assumed that all APs operate in the 2.4 GHz ISM band
> Non-overlapping channels but in situations where it is not
and use channels CH1 to CH11.
Figure 3 is an example of a dense WiFi deployment. APa to APi
possible, channels are preferred, with minimum overlap is chosen, with bandwidth and power levels adjusted.
APh
APb
APg
APc
APi
APa APf
APd
APe
Figure 3: Dense AP deployment
4
Neighbor Access Points APa
APb
APc
APd
Ape
APf
APg
APh
APi
APa APb APc Access Points
APd Ape APf APg APh APi Table 2: Access point and its neighbors
The table above can also be represented using the graph theory
nodes. The goal is to assign channels (equivalent in color) to
modeling shown in Figure 4. Each AP is represented by a node,
nodes in such a way that there is no frequency (color) overlap
and its neighbor relationship is shown via the links between the
with the adjacent nodes.
H
C
B
I
A
D
G
F
E
Figure 4: Representation of AP and its neighbors using graph theory
Auto Channel Selection in 2.4Ghz Using Overlap
5
The AP sends a periodic beacon on its chosen channel to
and reports the information to the controlling AC and
indicate specific information, including MAC address, SSID,
assigns a channel number to the AP based on the information
channel number, and other parameters. The auto channel
provided by the AP and the neighboring active APs.
mechanism requires only the MAC address and channel
In this example, let us assume that all nodes are powered up
number fields.
and each AP comes up one by one in the following sequence.
The AC has the auto channel selection algorithm running on
A -> B -> C -> D -> E -> F -> G -> H -> I
it. Each AP passively scans all available channels for beacons
H
H C
B
I
A
G
E
E
Figure 5
Figure 6
B A CH1
I
E
Figure 7
Figure 8
C A CH1
G
E
Figure 9
Figure 10
H
C CH11 I
D
F
B CH6
H
AP C comes up and declares its neighbors as AP A (CH1) and AP B (CH6). The first non-overlapping channel CH11 is allocated to AP C.
I
AP D comes up and declares its neighbors as AP A (CH1) and AP C (CH11). The first non-overlapping channel CH6 is allocated to AP D.
C CH11
A CH1
G
I
C CH11
D
F
E
A CH1
G
H
A CH1
G
D
F
B CH6
I
AP B comes up and declares its neighbor as AP A with CH1. The first non-overlapping channel CH6 is assigned to AP B.
D
E
B CH6
I
C
F
H
Any Available channel can be taken for AP A. The first available channel CH1, is assigned to AP A.
H
A CH1
G
D
F
B CH6
B CH6
C
I
D
F
H
G
A CH1
G
D
F
C
B
D CH6
F
E
E
Figure 11
Figure 12
6
B CH6
H
A CH1
G
F E
B CH6
C CH11 I
H
D CH6
F
F
E CH11
B CH7
C CH11 I
H
D CH6
F CH4
E CH11
F CH4
B CH7
C CH11 I D CH6
G CH1 F CH4
Figure 17
B CH7
H
F CH4
E CH11
B CH7
C CH11 I D CH6
G CH1 F CH4
Figure 19
B CH7
H
F CH4
E CH11
AP F comes up and declares its neighbors as AP A (CH1), AP B (CH6) and AP E (CH11). There is no non-overlapping channel available. The AC invokes overlapping mode if enabled. AC reassigns CH7 to AP B. AC assigns CH4 to AP F.
I
AP G comes up and declares its neighbors as AP B (CH7) and AP F (CH4). The first non-overlapping channel CH1 is assigned to AP G.
I
AP H comes up and declares its neighbors as AP B (CH7) and AP C (CH11). The first non-overlapping channel CH1 is assigned to AP H.
I CH1
AP H comes up and declares its neighbors as AP B (CH7) and AP C (CH11). The first non-overlapping channel CH1 is assigned to AP H.
C CH11
E CH11
D CH6
H
C CH11
A CH1
E CH11
D CH6
H CH1
C CH11
A CH1
E CH11
D CH6
Figure 20
B CH7
C CH11
A CH1
G
H
Figure 18
A CH1
G
I
D CH6
Figure 16
A CH1
G
AP E comes up and declares its neighbors as AP A (CH1) and AP D (CH6). The first non-overlapping channel CH11 is allocated to AP E.
A CH1
G
Figure 15
B CH7
E CH11
I
Figure 14
A CH1
G
C CH11
A CH1
G
Figure 13
B CH6
H
I D CH6
G CH1 F CH4
Figure 22
H CH1
A CH1
E CH11
C CH11
D CH6
Figure 23
In this manner, this auto channel selection algorithm could be implemented to assign channels to APs with minimum overlap.
Auto Channel Selection in 2.4Ghz Using Overlap
7
Conclusion
GOPI MOHAN
Assigning non-overlapping channels is the first priority, but in
Technology at Aricent. He has
dense AP deployment situations where it is not feasible to
17 years of experience in the
assign completely non-overlapping channels, a workable
networking industry, spanning
configuration can be achieved by assigning few overlapping
multiple technologies, including
channels with minimal disruption to existing configuration.
broadband access, carrier
is Assistant Vice president,
ethernet, enterprise routing, and ARICENT WLAN SWITCHING SOLUTION Aricent’s WLAN Switching Solution (WSS) is a software
infrastructure WLAN. gopi.mohan@aricent.com
framework for access controllers and access points. It implements a comprehensive set of RF management and auto channel selection features for 802.11a/b/g/n WLANs.
For more information visit aricent.com/software/wireless-local-area-network-wlan.html REFERENCES http://en.wikipedia.org/wiki/Four_color_theorem
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