Advances in Internet of Things, 2011, 1, 51-54
doi:10.4236/ait.2011.13007 Published Online October 2011 (http://www.SciRP.org/journal/ait)
Copyright © 2011 SciRes. AIT
Advances in Intrusion Detection System for WLAN
Department of C om p ut er Science and En gi n eering, Beant C ol l ege of E ng i neering and Technolo gy,
Received July 8, 2011; revised July 28, 2011; accepted August 21, 2011
A wireless network is not as secure as compare the wired network because the data is transferred on air so
any intruder can use hacking techniques to access that data. Indeed it is difficult to protect the data and pro-
vide the user a secure information system for lifetime. An intrusions detection system aim to detect the dif-
ferent attacks against network and system. An intrusion detection system should be capable for detecting the
misuse of the network whether it will be by the authenticated user or by an attacker. Cross layer based tech-
nique help to make decision based on two layer physical layer where we compute RSS value and on MAC
layer where one compute RTS-CTS time taken. This will reduce the positive false rate. They detect attempts
and active misuse either by legitimate users of the information systems or by external. The paper has high-
lighted the advances in intrusion detection in wireless local area network.
Keywords: Reciever Signal Strength (RSS), Time Taken for RTS-CTS Handshake (TT),
Radio Frequency (RF)
A Wireless Local Area Network (WLAN) is a flexible
data communications system implemented as an exten-
sion to or as an alternative for, a wired LAN. Using radio
frequency (RF) technology, wireless LANs transmit and
receive data over the air, minimizing the need for wired
connections. Wireless LANs frequently augment rather
than replace wired LAN networks often providing the
final few meters of connectivity between a wired net-
work and the mobile user. Intrusion detection can be of
misuse detection and anomaly based d etection. In misuse
detection the decision by gathering the data of attacker
and then compare it with large database of attack signa-
ture. It looks for specific attack that has been already
documented. In anomaly detection the system adminis-
trator define the baseline or normal state of network like
packet size, protocol, traffic load. Then it monitor by
comparing network segment to normal behavior and look
for anomalies [1-10]. In cross layer based intrusions de-
tection the decision is based on the combine weight value
of two or more layer. So the decision is not based on
single layer, it will reduce false positive rate. Multi-hop
wireless networks are more unsafe as compared to wired
or single hop wireless networks. Multilayer security at-
tacks need to be considerate before the design of any
security mechanism or intrusion detection system [11-
2. Intrusion Detection System
2.1. Types of Intrusion Detection Systems
There are two types of intrusion detection system First,
Network Based Intrusion Detection System (NIDS) which
resides on network. Second, Host Based Intrusion Detec-
tion system (HIDS) which resides on host i.e. computer
2.2. Network Based Intrusion Detection System
Network based intrusion detection system resides on
network. It exists as software process on hardware sys-
tem. It changes the network interface card (NIC) into
promiscuous mode, i.e. the card passes all traffic on the
network to the NIDS software. The software includes the
rules which are used to analyze the traffic. It analyzes the
incoming packets against these rules to determine the
signature of the attacker. Whether this traffic signature is
of any attacker or not. If it is of interest then events are
generated. The data source to NIDS is raw packets. It
Copyright © 2011 SciRes. AIT
utilizes a network adapter which is running in promiscu-
ous mode to monitor and analyze the network. There are
four common techniques to identify attack.
1) Frequency or threshold crossing.
2) Correlation of lesser events.
3) Statistical anomaly detection.
4) Pattern, expression or byte code matching.
NIDS is not limited to read all the incoming packets
only. But also learn the valuable information on outgoing
traffic. With this feature the attacker form inside the
monitored network are identified .
2.3. Host Based Intrusion Detection System
Host based IDS are embedded on host computer. It exists
as a software process on a system. So it examines the log
entries in system for specific information. It identifies the
new entries and compares them to pre configured rules.
It also works on rule based, if th e entry match to the rule
then it will generate alarm that this is not legal user.
2.4. Anomaly Based Detection
Anomaly detection attempts to model the normal behav-
ior. Any occurring event which violates this model be-
havior is reflected to be suspicious. It aim is to detect the
patterns that do not conform normal behavior. The pat-
tern that does not conformed as normal are called as
2.5. Misuse Based Detection
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3. Cross Layer Based Technique
Cross layer based technique is used to make decision that
whether there is an attacker or not by combining the re-
sult of two or more layer in TCP protocol [12,13].
3.1. Monitoring Received Signal Strength (RSS)
A measure of energy which is observed by the physical
layer at the antenna of the receiver is called as Received
signal strength (RSS). In IEEE 802.11 networks, while
performing MAC clear channel measurement and in
roaming operations, the RSS indication value is used.
The radio frequency (RF) signal strength can be meas-
ured through absolute (decibel mill watts-dBm) or rela-
tive (RSSI) manner [8-10].
Exact RSS value from sender to receiver is not easy to
assume as mention above. To assume exact value of RSS
the attacker has to be present on the same location which
is not possible. The radio equipment used by the receiver
have to be same for identify exact value of RSS. More-
over there should be same level of reflection, refraction,
and interface. Even if the sender is fixed, RSS value
seems to vary a little and it is proved that it is almost not
possible to guess. This restricts the attacker from using
the radio equipment to spoof the RSS clearly by the re-
ceiver. A dynamic profile is build of the computer node
which are communicating depend upon the RSS value
from a server. Any sudden or unusual changes can be
marked as doubtful activity which indicates the possible
session of hijacking attack. Reason why we call RSS
profile dynamic is because during every session it is
build again and keeps on updating. Any sudden changes
in the RSS dynamic profile can be marked as doubtful
activity with a higher confidence level because BSs are
generally immobile. On the other hand, if the MS is mo-
bile, then its respective RSS values will vary quickly
which can be observed by the server. Therefore the un-
certainty of the wireless medium can be used in the favor
of intrusion detection, where the attacker is unable to
know what RSS values to spoof. Therefore it is effective
for the session hijacking attacks and it does not need any
additional bandwidth consumption.
For example, based on the observed RSS values at the
server it can develop a dynamic RSS profile for both
MS2 and BS when a valid MS2 has an active session
with a BS (Refer Figure 1). If an attacker MS1 hijacks
MS2 through isolating from the network and spoofing its
MAC address then the server will pick up the abrupt
changes in the RSS profile of MS2’s MAC and gives an
alert signal. Since they depend on the MS1’s actual loca-
tion, radio equipment and surrounding environment the
RSS values for the MS2’s MAC address will change.
In another situation, if the attacker MS1 spoofs the
base station BS then it will also get detected as the dy-
namic RSS profile for the BS undergoes sudden varia-
tions. Therefore this mechanism gives detection for both
session hijacking and man-in-the-middle attacks which is
targeted at either MSs or BSs.
3.2. Monitoring Time Taken for RTS-CTS
Virtual carrier sensing is created using RTS-CTS which
makes the transmission of data frames possible without
Copyright © 2011 SciRes. AIT
Figure 1. Received signal strength (RSS).
collision. The successful delivery of the CTS frame from
the receiver shows that the receiver is received the send-
ers RTS frame successfully and ready for receiving the
data. The time taken to complete the RTS-CTS hand-
shake between itself and receiver i.e. TT can be exam-
ined by the sender. This is the total time taken for the
RTS frame to travel from the sender to receiver and also
for the CTS frame to send an acknowledgement. RTS-
CTS handshake is free from collisions with any network
The TT values for a fixed transmission rate are not af-
fected because the size of RTS and CTS frames are fixed
and makes the TT between two nodes as an unspoofable
parameter. So this cannot be easily guessed by an at-
tacker when tracking the wave s. Since it is calculated by
the sender of the RTS-CTS handshake it is also protected
from snooping. Since it is a measurement related to the
entity measuring, the attacker should be exactly at the
same location as the sender. Also the attacker should use
the same radio equipment with the same attenuation and
antenna gain. In order to predict the values of TT be-
tween the sender and receiver as measured by the sender,
the attacker should receive the radio waves after the
same number of reflections and refractions. It can also be
calculated without any particular computational. From
the intrusion detection poin t of view, a mechanism which
is used to detect the session hijacking attacks uses the
quick and sudden changes in the TT between the two
nodes. Server can measure the time elapsed between
when it detects RTS frame from the sender to receiver
and when it detects a return CTS from the receiver back
to the sender i.e. TT. For understanding, this time can be
—time taken for a RTS frame to cover the dis-
tance between the sender and the server,
—time taken for a frame to cover the
distance between the server and the receiver,
—time taken for a handshake to
complete between a sender and receiver as observed by
But the server does not know these actual values.
values at the server pro-
vides a reliable passive detection mechanism for session
hijacking attacks since is an unspoofable parameter
related to its measuring entity. Also this cannot be gue-
ssed because its exact value depends on
1) The position of the receiver and the server
2) The distance between the server and receiver
3) The environment around the receiver and the server.
This is a property which cannot be measured or
spoofed by an attacker when tracking the network traffic
or using a specialized radio equipment.
We propose that changes in
between two com-
municating nodes can be observed by a passive server
and the sudden variations are marked as suspicious. This
helps to detect the attacker who tries to take over a re-
ceiver’s session by isolating it off the network and
spoofing its MAC address. On the other hand, the
handshake which originates from the re-
ceiver is used to detect the session hijacking attacks
which aims the senders. For example, the server can de-
velop a dynamic RSS profile which gets constantly up-
dated per session and it calculates thefor every
handshake from both MS2 and BS when a
valid MS2 has an active session with a BS (Refer
). If an attacker MS1 hijacks MS2 through spoofing its
MAC address then the server will observe abrupt
changes in the for MS2 and gives an alert signal.
Also to detect the man-in-the-middle attacks against BS,
values from handshakes between MS2
Figure 2. Round Trip Time (RTT).
Copyright © 2011 SciRes. AIT
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value and keep on updating it. RSS value is difficult to
assume because the attacker must use same equipment
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The author is thankful to Dr. Jatinder Singh Bal (Dean
and Professor, Computer Science & Engineering Desh
Bhagat Enggineering College, Moga) for critical discus-
sion as well as constant help during the present study.
The constant encouragement provided by Dr. H S Johal
as well as Mr. Dalwinder Singh and Deepak Prashar,
Lovely Professional University Jalandhar is also ac-
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