Clustering Algorithm for Hidden Node Problem
In Infrastructure Mode IEEE 802.11 Wireless LANs
Woo-Yong Choi
Department of Industrial and Management Systems Engineering, Dong-A University
840 Hadan-2-dong, Saha-gu, Busan, KOREA
Tel: 82-51-200-7696, Fax: 82-51-200-7697, Email: wychoi77@dau.ac.kr
Abstract In this paper, we propose a clustering algorithm for
resolving the hidden node problem in infrastructure mode IEEE
802.11 wireless LANs by partitioning the STAs (Stations) in a
BSS (Basic Service Set) into the clusters, so that the STAs can
detect the transmission signal of the other STAs within the same
cluster, i.e., the hidden node problem does not exist within each
cluster. When M (. 1) clusters of STAs are derived by the
proposed clustering algorithm, a CP (Contention Period) is
divided into M SPs (Sub-Periods), each of which is
nonoverlappingly assigned to a cluster, so that the STAs within
each cluster contend for the access to the wireless channel with
the reduced possibility of hidden nodes within the cluster. Taking
into consideration the fairness among the clusters, we also
propose a fairness algorithm for dividing a CP into the SPs.
Keywords - Clustering Algorithm, Fairness Algorithm,
Hidden Node Problem, Wireless LANs
1. Introduction
To support the wireless internet services especially in the hot
spot areas such as schools, hospitals, homes, and conference
rooms, IEEE 802.11 wireless LAN systems have been widely
implemented as a strong alternative to wired LAN systems in
recent years. Wireless LAN systems have the advantages of
the mobility support and low implementation and maintenance
costs compared with wired LAN systems. [1]
IEEE 802.11 wireless LAN systems support the
infrastructure and ad hoc modes. [2] In the ad hoc mode, a BSS
(Basic Service Set) consists of a set of STAs (Stations) and the
STAs communicate directly with each other. In the
infrastructure mode, a BSS includes a set of STAs and an AP
(Access Point). The STAs can communicate with each other or
can communicate with an external network by first going
through the AP, which is connected to the external network
and relays the data from the STAs and the external network.
The STAs cannot directly communicate with each other in the
infrastructure mode.
IEEE 802.11 wireless LAN systems employ two MAC
functions ofthe DCF (Distributed Coordination Function) and
PCF (Point Coordination Function). The DCF based on
CSMA/CA (Carrier Sense Multiple Access with Collision
Avoidance) was designed to support the best-effort services,
and the PCF based on a round robin polling scheme was
designed for the real-time services. [2] IEEE 802.11 MAC
functions rely on the physical and virtual carrier sensing
mechanisms to avoid collisions. Especially, the virtual carrier
ISBN 978-89-5519-136-3
-1 335-
sensing mechanism using the RTS and CTS frame exchanges
was proposed to combat the well-known hidden node problem
in [3]. The RTS and CTS frame exchanges are performed
before the data frame transmissions to reserve the wireless
channel for safely transmitting the data frames only when the
data frames are longer than the RTS-CTS threshold, which is
called dot llRTSThreshold in IEEE 802.11 specification [2].
The use of the RTS and CTS frame exchanges before the data
frame transmissions has the advantage of shortening the
collision periods especially in the case of long data frame
transmissions because the length of the RTS frame (20 bytes)
is very small compared with the length of data frames. [7]
Thus, the RTS-CTS threshold can effectively control the
frequency of use of the RTS and CTS frame exchanges.
As explained in [7], as the retransmissions of data frames
due to the collisions among the STAs in a BSS occur more
frequently, the RTS and CTS frame exchanges need to be used
more frequently for the efficient use of radio bandwidth. The
hidden node problem can cause the collisions among the STAs
frequently. [3] Thus, if we can reduce the possibility ofhidden
nodes, we can use the RTS and CTS frame exchanges less
frequently and the efficiency of radio bandwidth can be
enhanced.
In the literature, the shortcomings ofthe RTS and CTS frame
exchanges in IEEE 802.11 ad hoc networks were discussed in
[4, 5], and the detection methods of hidden nodes in IEEE
802.11 ad hoc networks were proposed in [6]. But, the RTS
and CTS frame exchanges can truly resolve the hidden node
problem in IEEE 802.11 infrastructure networks because the
AP relays all data transmissions among the STAs, and the
wireless channel can be reserved free of hidden node problem
by the AP's broadcasting the RTS or CTS frame to all STAs in
the BSS. The dynamic use of the RTS and CTS frame
exchanges was investigated in [7] to increase the MAC
throughput in IEEE 802.11 infrastructure networks. The MAC
throughput can be further enhanced by reducing the possibility
of hidden nodes in IEEE 802.11 infrastructure networks.
In this paper, we propose a clustering algorithm for resolving
the hidden node problem in infrastructure mode IEEE 802.11
wireless LANs by partitioning the STAs (Stations) in a BSS
(Basic Service Set) into the clusters, so that the STAs can
detect the transmission signal of the other STAs within the
same cluster, i.e., the hidden node problem does not exist
within each cluster. When M (. 1) clusters of STAs are
derived by the proposed clustering algorithm, a CP
(Contention Period) is divided into M SPs (Sub-Periods), each
Feb. 17-20, 2008 ICACT 2008
of which is nonoverlappingly assigned to a cluster, so that the
STAs within each cluster contend for the access to the wireless
channel with the reduced possibility of hidden nodes within
the cluster. Taking into consideration the fairness among the
clusters, we also propose a fairness algorithm for dividing a
CP into the SPs.
2. Clustering Algorithm
Through the association/disassociation process between
STAs and the AP, the AP can maintain a list of STAs
associated with itself. N will denote the number of the STAs.
The set, S = I1, 2, ..., N} will represent the set of STAs. The
PCF coordinates the data transmissions during a CFP
(Contention Free Period). The CFP alternates with a CP during
which the DCF controls the data transmissions. During CFPs,
the AP grants the transmission opportunities to the STAs in the
BSS by transmitting the polling frames to the STAs in a round
robin order. The hidden node problem cannot disturb the
operation of the PCF because the STAs cannot transmit their
data frames without the receptions of the polling frames from
the AP. But, without the help of the RTS and CTS frame
exchanges, the DCF can suffer from the hidden node problem.
We will explain a simple scheme for the AP's collecting the
carrier sensing information among the STAs during the
operation ofthe PCF. Based on the carrier sensing information
among the STAs, we want to partition the STAs into the
clusters, so that the hidden node problem does not exist within
each cluster. After M (. 1) clusters of STAs are derived by our
proposed clustering algorithm, which will be explained in this
section, the next CP is divided into M SPs as shown in Figure 1.
And, the SPs will be nonoverlappingly assigned to the clusters,
so that the STAs within each cluster contend for the access to
the wireless channel with the reduced possibility of hidden
nodes within the cluster.
CFP
CP
i
1S P, I SP, j
Frame
Cont.
i
w,S
during a polling interval, which is defined as the time interval
from the time when STA i is polled in a CFP to the time when
STA i is polled again, Ui is updated as Ui e- Ui -{ STA j }. In
this manner, each STA can maintain the set, Ui. STA i sends to
the AP the MAC addresses of the STAs that are deleted from
or added to the set, Ui. Actually, each STA can piggyback the
M\AC addresses related with the change of the carrier sensing
information on the response null or data frames transmitted to
the AP. Using the MAC addresses sent from the STAs, the AP
can continuously update the carrier sensing information
among the STAs during CFPs.
If the symmetric carrier sensing holds (that is, if STA i can
detect the transmission signal of STA j . i, STA j can detect
the transmission signal of STA i), and no transmission error
occurs, the AP can easily collect the carrier sensing
information among the STAs in a polling interval by polling
each STA and receiving the MAC addresses, which are the
information of the carrier sensing between the STA and the
previous STAs polled in the interval, piggybacked on the
response null or data frames transmitted by the STA. And, the
AP can detect the change of the carrier sensing information
among the STAs in the next interval. If the symmetric carrier
sensing does not hold and no transmission error occurs, the AP
needs two polling intervals to collect the carrier sensing
information. Throughout this paper, the asymmetric carrier
sensing among the STAs is assumed.
Each STA can piggyback on the response frame the MAC
addresses related with the change of the carrier sensing
information between itself and other STAs during CFPs. We
need to change the format ofthe response data frame to add the
new field for this information as shown in Figure 2. In Figure 2,
Uc indicates the set of MAC addresses related with the change
ofthe carrier sensing information, and the other fields are from
[2]. (For convenience of presentation, the FCS (Frame Check
Sequence) field is omitted in Figure 2.)
Dur.ID
RA
TA
DA
SCq.
Cont.
|
Frame
Body
...
Fig. 2 Format of Response Frame
time
Fig. 1 CFP/CP alternation and CP structure
The method for the AP's collecting the carrier sensing
information among the STAs during CFPs is now explained.
Each STA i needs to maintain the set, Ui which is the set of
STAs of which the transmission signal can be detected by the
physical carrier sensing mechanism of STA i. At the initial
time when STA i joins the BSS newly, Ui is set to be empty.
When STA i detects the transmission signal of other STAj not
in the set, Ui, Ui is updated as Ui e- Ui u STA j (STA i can
obtain the MAC address of STA j from the polling frame
transmitted by the AP before the transmission of STA j.
According to IEEE 802.11 specification [2], the PHY layer of
STA i reports the channel state (busy or idle) to the MAC layer.
For this purpose, the polling frames from the AP should be
received by all N STAs in CFPs.) When STA i has not
detected the transmission signal of other STA j in the set, U1
Based on the carrier sensing information among the STAs,
which are collected during CFPs, the AP needs to partition the
STAs into the clusters, so that the hidden node problem does
not exist within each cluster. The constants, Fij for i and j E S
and i j, which specify the carrier sensing information among
the STAs, are defined as follows:
*
Fij: indicates whether STA j can detect the transmission
signal of STA i j or not. Fij = I when STA j can detect
the transmission signal of STA i. Otherwise, Fij = 0.
}.
ISBN 978-89-5519-136-3
-1 336-
The STAs in S need to be partitioned into as few clusters as
possible in such a way that no two STAs, i andj with Fij = 0 or
Fij = 0 are in the same cluster. Therefore, we can formulate the
problem for the optimal clustering as GCP (Graph Coloring
Problem) to minimize the number of clusters, where the STAs
in S are vertices and two STAs, i and j in S are connected only
when Fij = 0 or Fij = 0. [pp. 14 in 8] Although the carrier
Feb. 17-20, 2008 ICACT 2008
sensing information can be changed during the next CP after
the clusters are derived, we can expect the AP to reduce the
possibility ofhidden nodes during the CP by assigning a SP to
each cluster obtained by the heuristic for GCP, which is based
on the degree-descending order of the vertices. [pp. 14 in 8]
The AP needs to continuously perform the clustering
algorithm to partition the STAs into the clusters based on the
recently updated carrier sensing information among the STAs.
And, the AP needs to inform each STA of the updated cluster
number to indicate the cluster to which the STA belongs. To
inform the STAs of the updated cluster numbers, the AP can
piggyback the updated cluster numbers on the polling frames
transmitted to the STAs. For this purpose, we need to change
the format of the polling frame to add the new field for the
updated cluster number as shown in Figure 3. During CPs, at
the beginning of each SP, the AP needs to announce the start
of the SP by broadcasting the announcement frame to the
STAs. The announcement frame should include the cluster
number to indicate the cluster to which the SP is assigned for
the DCF channel access.
Frame
Control
Duration/ID
Drto D
RTA
Cer
CuRAstTA
Number
Fig. 3 Format of Polling Frame
The preceding signals correspond to the data transmission
attempts of the STAs and the AP, respectively. When the RTS
and CTS frame exchange is performed before a data frame
transmission, the separate RX or TX signals are generated for the
RTS, CTS and data frames. But, we want to regard the signals for
the RTS, CTS and data frames as related with a single data
transmission attempt. For this purpose, the signal for the CTS
frame is ignored, and the signal for the data frame is ignored
when the signal for the preceding RTS frame was counted. All
other signals are counted to obtain Wk's in the CP. The AP can
easily obtain the number of successful data transmissions in each
SP, which is Vk in (1), by counting the transmitted or received
ACK frames. Using Vk's and Wk's, we can obtain Gk's for M
clusters. We will denote the average rate of change of Gk by Hk
Gk/ Dk.
As explained in the previous section, the AP continuously
updates the clusters based on the recently collected carrier
sensing information among the STAs. The clusters to which the
next CP is assigned will be called clusters NC(1), NC(2), ..., and
NC(L). (L . 1. NC is the abbreviation of New Cluster.) When
cluster NC(p) is equal to cluster OC(q), the rate of change of
GNCO can be estimated to be HNCO = Hoc(c. (1 < p < L and 1 < q
< M.) Therefore, we can estimate the rate of change of GNCOJ) for
cluster NC(p) in the following set:
T = {NC(p)l there exists OC(q) such that NC(p) = OC(q)}.
3. Fairness Algorithm
The fairness algorithm in this section is aimed to distribute
the channel bandwidth to the clusters, which are derived by the
clustering algorithm in the previous section, in the manner that
allows the clusters attempting to transmit the data frames more
frequently to get more successful data transmissions in a CP.
That is, we want to keep the following measures, Gk's for the
clusters as equal as possible:
=Vkk
GkG =
Wk
The rate of change of GNc(P) for cluster NC(p) not in the set, T
cannot be estimated.
Because we have no information of GNC(P) for cluster NC(p)
not in T, the channel bandwidth is equally assigned to the
clusters not in T, that is, the durations of SPs assigned to the
clusters not in T are set to be D/L. The remaining channel
bandwidth ofthe CP is assigned to the clusters in T, so that the
clusters in T are expected to have the equal GNC(p)'SSpecifically, the duration of SP assigned to the cluster NC(p)
in T is obtained as
(1)
DNC(p) a:::(
where Vk is the number of successful data transmissions in a
total CP of cluster k, and Wk the number of data transmission
attempts per unit time of cluster k.
Assume that a CP is assigned to M (. 1) clusters. M clusters
will be called clusters OC(1), OC(2), ..., and OC(M). (OC is the
abbreviation of Old Cluster.) Let us denote the durations of SP's
assigned to clusters OC(1), OC(2), ..., and OC(M) by Doc(l),
Doc(2), .. ., and Doc(M), respectively. The total duration ofeach CP
is denoted by D. (At the initial time when the AP does not have
the information of Gk's, the channel bandwidth can be equally
assigned to M clusters, that is, the durations of SPs are set to be
D/M in Fig. 1.) According to [2], at the NIAC layer, the AP can
detect the data transmission attempts in each SP by receiving the
following signals from its physical layer:
* PHY-RXSTART.indication (RXVECTOR)
* PHY-TXSTART.confirm
ISBN 978-89-5519-136-3
(2)
-1337-
(aNC(cjp)
z-
iT
ic~
ai
(3)
where ITI is the number of clusters in T, DITI / L is the
remaining total duration of CP that can be assigned to the
clusters in T, and u, is Hi.
For example, assume that there are three old clusters, OC(1),
OC(2) and OC(3), the total duration of each CP is 2 ms, and
the rates of change of Goc(l), Goc(2) and GoC(3) are estimated to
be Hoc(l) = 0.5, HOC(2) = 0.2 and HOC(3) = 1.0, respectively.
When there are four new clusters, NC(1), NC(2), NC(3) and
NC(4), and T = {NC(1) = OC(2), NC(2) = OC(3)}, the
durations of SPs assigned to four new clusters are obtained as
* DNC(3): DNC(4) -= 2ms /4 = 0.5 ms
* DNC(l): 1 ms (5 / (5 + 1))= 0.833 ms
* DNC(2): 1 ms(1 / (5+1)) =0.167ms
Feb. 17-20, 2008 ICACT 2008
In the wireless service environment with the moderate mobility,
the carrier sensing information among the STAs does not change
frequently. Thus, the previous information of Gk's can be
efficiently used to fairly allocate CPs to the clusters.
4. Conclusions and Further Research
In this paper, we proposed a clustering algorithm for resolving
the hidden node problem in infrastructure mode IEEE 802.11
wireless LANs by partitioning the STAs (Stations) in a BSS
(Basic Service Set) into the clusters. Taking into consideration
the fairness among the clusters, we also proposed a fairness
algorithm for dividing a CP into the SPs. The further research is
needed to validate our proposed clustering and fairness algorithms
by numerical examples.
ISBN 978-89-5519-136-3
-1 338-
REFERENCES
[1] L. Brignol and L. Litzenburger, "Wireless LAN Technology and
HiperLAN/2", Alcatel Telecommunications Review, pp. 205-206, 2003.
[2] IEEE Std 802.11, Wireless LAN medium access control (MAC) and
physical layer specifications, 1999.
[3] F.A. Tobagi, and L. Kleinrock, "Packet switching in radio channels: the
hidden node problem in carrier sense multiple access modes and the busy
tone solution", IEEE Trans. Commun., vol. 23, pp. 1417-1433, 1975.
[4] K. Xu, and M. Gerla, "How Effective is the IEEE 802.11 RTS/CTS
Handshake in Ad Hoc Networks", IEEE GLOBECOM 2002, 2002.
[5] J.L. Sobrinho, R. Haan, and J.M. Brazio, "Why RTS-CTS is not Your
Ideal Wireless LAN Multiple Access Protocol", IEEE WCNC 2005,
2005.
[6] F.Y. Li, A. Kristensen, and P. Engelstad, "Passive and Active Hidden
Terminal Detection in 802.1 1-based Ad Hoc Networks", IEEE
INFOCOM 2006, 2006.
[7] W.-Y. Choi, "A Real-Time Algorithm for MAC Throughput
Enhancement by Dynamic RTS-CTS Threshold in IEEE 802.11 Wireless
LANs ", Journal of RF Engineering and Telecommunications, vol. 59, pp.
171-176, 2005.
[8] E. Falkenauer, "Genetic Algorithms and Grouping Problems", John
Wiley & Sons, 1998.
Feb. 17-20, 2008 ICACT 2008