Implementation of Throughput Enhancing
Client-AP Association Scheme On a WLAN
Controller
Vikram Ravindra∗ , Aditya Prakash∗ , S.V.R. Anand∗ , Malati Hegde∗
∗
Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore, India
Abstract—In a typical enterprise WLAN, a station has
a choice of multiple access points to associate with. The
default association policy is based on metrics such as Received Signal Strength(RSS), and “link quality” to choose
a particular access point among many. Such an approach
can lead to unequal load sharing and diminished system
performance. We consider the RAT (Rate And Throughput)
policy [1] which leads to better system performance. The
RAT policy has been implemented on home-grown centralized WLAN controller, ADWISER [2] and we demonstrate
that the RAT policy indeed provides a better system
performance.
I. I NTRODUCTION
In IEEE 802.11 wireless local area networks
(WLANs), there are various issues such as client-AP
association, multi-rate unfairness and upload-download
unfairness [3] [4]. We have addressed these problems
in ADWISER, which is described in [2]. In this paper,
we implement the RAT algorithm proposed in [1] and
integrate it with ADWISER to solve issues pertaining to
client-AP association.
In a WLAN, whenever the station enters the network,
it scans the radio channel to detect the available APs.
If there are multiple APs present in the network, the
station by default associates with that AP for which
the Received Signal Strength Indicator (RSSI) or “Link
Quality” is the highest. This association policy is based
only on the “rate” metric because the station can then
associate at a higher rate if the station’s RSSI value
corresponding to an AP is high. This sort of association
leads to unequal load sharing and degradation in the
performance of the network [1], [5].
The “selfish” policy described in [1] only considers the
throughput that the entering station would receive. The
entering station associates to that AP for which it would
receive the maximum throughput. It is shown in [1] that
although the optimal choice for an entering station seeing
almost equal throughputs from multiple APs would be
This work is supported by the Department of Information Technology (DIT), Ministry of Communications and Information Technology,
India.
c 2012 IEEE
978-1-4673-0298-2/12/$31.00 to associate with an AP providing a higher physical rate,
the selfish policy makes the wrong choice based only on
the throughput metric.
A heuristic which considers both Rate And Throughput (RAT) is proposed in [1]. We have implemented the
RAT algorithm and demonstrate that it is a good solution
to the client-AP association problem.
ACCESS
POINT
STATION
ADWISER
Details of the
Stations associated
with the AP
AP MAC ID
Received Signal Strength
Process data for each station,
RAT algorithm is applied
Best AP is sent to the station
If station asked to dissociate,
it sends DISSOCIATE REQ
to current AP, ASSOCIATE REQ
to new AP.
Else station continues associated
with current AP.
Fig. 1.
Flow Diagram
II. R ELATED LITERATURE
In [6], a decentralized AP selection algorithm has
been proposed which uses signal strength from each
AP, number of STAs currently associated with each AP,
and packet error rates of STAs communicating with
each AP. The authors of [7] present a load balancing
technique based on cell breathing which requires the
ability to dynamically change the transmission power
of the AP. This reduces the load on the congested APs
by decreasing their cell size, forcing the users near the
boundaries of congested cells to move to neighbouring
less congested cells. The flip side is that this scheme can
potentially hamper the wireless access to those users that
are already associated earlier. In [8] a technique known
as beacon stuffing has been presented where beacons
are overloaded with information such as the number of
users connected to that particular AP. It however requires
modification to the AP firmware which is impractical. On
the contrary, the RAT being a centralized approach does
not require any modification to the AP. Also, several of
the previous papers provide solutions to the client-AP
association problem only for the static scenario whereas
the RAT considers the dynamic scenario where STAs
come and go.
III. RAT
RAT uses both rate and throughput parameters to
calculate the metric that is used to decide the best AP
for a station to associate with.
The formula used by RAT to calculate the metric is
g(i)
= αT (i) + βr(i)
policy is applied. (ii) The observation of the throughputs
after the RAT algorithm is implemented.
(1)
where α and β are non-negative constants. T (i) is
the throughput received by the particular STA from the
AP i and r(i) is the rate at which the particular STA is
associated with AP i. For each available AP, the station
computes g(i) and associates with that AP for which
g(i) is the highest.
The throughput T (i) is given by [5] as
T (i) =
1
Ni1
r1
+
Ni2
r2
+ ... +
(2)
NiL
rL
where Ni1 is the number of stations associated with
AP i at a rate r1 , and similarly for Ni2 , . . . , NiL and
L denotes the number of possible physical rates.
For the default SNR based policy, the values are α =
0 and β = 1. For the selfish policy, the values are α = 1
and β = 0. In our implementation of the RAT algorithm
we have considered α = 1 and β = 0.2, as suggested in
[1].
IV. I MPLEMENTATION
The RAT algorithm has been implemented in ADWISER. The software architecture of the implementation
is shown in Figure 1. The newly associated STA will
send the scanned results containing the list of APs
and the corresponding signal strengths to ADWISER.
Based on this information, ADWISER selects the AP
with the highest metric as computed in Eq.(1) and is
communicated to the STA for re-association.
AP1
SWITCH
ADWISER
SERVER
ON LAN
STA−2
AP2
STA−1
Fig. 2.
Demo Setup
V. D EMONSTRATION
The demonstration setup is shown in Figure 2. The
setup consists of the ADWISER box, two Netgear Access Points, two laptops as stations, a server and a switch.
The demonstration will include (i) The observation of
the throughputs obtained by the stations when the default
Fig. 3.
Throuhput vs time
VI. R ESULTS
We consider two wireless STAs associated at the
physical rate of 54 Mbps downloading large files from
the server and the throughputs observed by the stations
with the default client-AP association are plotted as
shown in Figure 3. The individual throughputs obtained
by the stations and the aggregate throughput after the
RAT policy is applied show considerable improvement.
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