SHADOWING EFFECTS ON ROUTING PROTOCOL OF MULTIHOP AD HOC NETWORKS

International Journal of Ad hoc, Sensor & Ubiquitous Computing ( IJASUC ), Volume 1, Number 1, March 2010
DOI : 10.5121/ijasuc.2010.1102 12
SHADOWING EFFECTS ON ROUTING
PROTOCOL OF MULTIHOP AD HOC
NETWORKS
Md. Anwar Hossain, Mohammed Tarique and Rumana Islam
Faculty of Engineering, American International University-Bangladesh
anwar94113@aiub.edu
ABSTRACT
Two-ray ground reflection model has been widely used as the propagation model to investigate the
performance of an ad hoc network. But two-ray model is too simple to represent a real world network. A
more realistic model namely shadowing propagation model has been used in this investigation. Under
shadowing propagation model, a mobile node may receive a packet at a signal level that is below a
required threshold level. This low signal level affects the routing protocol as well as the medium access
control protocol of a network. An analytical model has been presented in this paper to investigate the
shadowing effects on the network performance. The analytical model has been verified via simulation
results. Simulation results show that the performance of a network becomes very poor if shadowing
propagation model is used in compare to the simple two-ray model. Two solutions have also been proposed
in this paper to overcome the effects of shadowing. One solution is a physical layer solution and the other
one is a Medium Access Control (MAC) layer solution. Simulation results show that these two solutions
reduce the shadowing effect and improve network performance.
KEYWORDS
Ad hoc networks, shadowing, link distance, probability, delivery ratio, signal-to-interference-plus-noise
power ration (SINR), MAC layer, and transmission power.
1. INTRODUCTION
Mobile Ad hoc Network (MANET) is a highly appealing means of providing network support to
a group of people without the aid of any infrastructure support. MANET consists of a group of
mobile nodes that may not be within the range of each other. Necessary controls and networking
functions are performed by using of a distributed control algorithm. MANET is characterized as a
multihop communication network unlike a cellular mobile network where each mobile node is
connected to a nearest base station based on a single hop connection. In MANET, a mobile node
forwards packet for other mobile nodes in addition to transmit its own packet. Dynamic topology
is another important characteristic of MANET [1]. Mobile nodes may join or leave a network at
any time. In order to maintain network connectivity under a dynamic topological condition an
efficient routing protocol is very essential for MANET. Existing routing protocols like
Destination Sequence Distance Vector (DSDV) [2], Dynamic Source Routing (DSR) [3] and Ad-
hoc On-Demand Distance Vector (AODV) [4] consist of two main mechanisms namely route
discovery and route maintenance. A mobile node discovers a route or a set of routes to a

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destination mobile node by using a route discovery mechanism. On the other hand, a mobile node
detects any network topology change by using the route maintenance mechanism. While using
any of these two mechanisms, a routing protocol relies on mobile radio channel. Mobile radio
channel places a fundamental limitation on the performance of a MANET. The transmission path
between a transmitter and a receiver can vary from simple line-of-sight to one that is severely
obstructed by buildings, trees, road signs, mountains, and other objects. Hence mobile radio
channel is extremely random unlike its wired counterpart. Different propagation models have
been proposed in the literatures to predict the signal attenuation that occurs between two mobile
nodes separated by a distance. The ground reflection model or two-ray propagation model is
widely used in the test bed and also in the simulation model [10-17]. Tworay propagation model
assumes that there is a line-of-sight path and a ground reflected propagation path between a
transmitter and a receiver. This model has been found to be reasonably accurate for predicting the
large-scale signal strength over distances of several kilometers for mobile radio system. This
model characterizes signal propagation in an isolated area with few reflectors such as rural road
or highways. It is not typically a good channel model for a real world mobile communication
system especially when the system is deployed in an urban area. Because this model does not
consider the fact that the surrounding environment of a network is always changing. In a real
world situation, the surrounding environmental cluster is always changing. This leads to a signal
level that can vary vastly across a given distance. The short-term variations in the signal strength
can be as high as 10-20 dB from the mean value of the signal. Measurements have shown that at
any given distance, the path loss at a particular distance is random and distributed log-normally
about a mean value [5]. The log-normal distribution describes the random shadowing effects
which occur over a large number of measurements locations that have the same separation
distance. This adverse effect of shadowing on the routing protocol has been investigated in this
paper.
One of the first papers related to connectivity issues in wireless multihop network was [18]. The
authors studied the percolation of a broadcast in a multihop radio network modeled by a spatial
Poisson process. The effect of station density and transmission radius on the extent of broadcast
percolation was examined. The results presented in this paper shows that in optimizing
transmission radius as a function of communication performance measures, the choice of radius
may be bounded from below by the need to maintain a desired network connectivity. Another
early paper [19] addressed connectivity issues for mobile nodes that are randomly distributed
according to a uniform probability distribution on a one-dimensional line segment. More recently
another work [20] performed a fundamental study on the connectivity of uniformly distributed
mobile nodes on a circular area. According to [20] mobile nodes should adjust transmission
power to a level that is just enough to maintain connectivity in a network provided a mobile node
cooperates with other mobile node to route packet. Further analytical investigations of the
connectivity in bounded areas were made in [21- 22]. In [21] the authors analyzed the critical
transmission range for connectivity in wireless ad hoc networks. The authors first considered the
connectivity problem for stationary network and provided an upper and lower bound on the
critical transmission range for one dimensional network. The authors evaluated the relationship
between the critical transmission range and the minimum transmission range that ensured
formation of a connected component containing a large fraction (i.e., 90%) of the nodes. The
authors then extended this work to a mobility condition where mobile nodes were allowed to
move during a time interval. In [22] the authors considered a ddimensional region, with 1=d=3
and they presented an analysis to determine the transmission range that ensure the resulting
network is connected with a high probability. Based on the bounds of node density the authors

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concluded that, as compared to the deterministic case, a probabilistic solution to this range
assignment problem achieves substantial energy savings. A framework for the calculation of
stochastic connectivity properties of wireless multihop network has been presented in [23]. In
fact, the connectivity problem has been solved for the general case of a k-connected network
accounting for the robustness against node failures. These issues were studied for uniformly
distributed nodel, Gaussian distributed nodes, and nodes that move according to the commonly
used random waypoint mobility model. A large scale network with low node density has been
investigated in [25]. The author studied the connectivity for both purely ad hoc network and
hybrid network. In hybrid network, base stations were placed in a network. Mobile node
communicates with other mobile node through the base stations. The authors obtained an
analytical expression for the probability of connectivity in one dimensional network. They
showed that bottlenecks are unavoidable in a low density sparse network. Most of the works
mentioned so far assume idealized radio propagation model without considering fading and
shadow effects. One of the earliest papers that considered fading and shadowing is [26]. The
authors claim that many well designed protocols will fail simply because of fading and
shadowing experienced in a realistic wireless environment. The authors have shown that fading
and shadowing can have significant influence on network performance. They studied three
different systems namely (1) a multichannel CDMA system, (2) a pure CDMA system, and (3) a
contention based system. They also show that the multichannel CDMA system outperform the
pure CDMA system as well as the contention based system under fading and shadowing
environments. The connectivity of multihop radio networks in a log-normal shadow fading
environment has been investigated in [27]. Assuming the mobile nodes have equal transmission
capabilities and are randomly distributed according to a homogenous Poisson process, the authors
provided a tight lower bound for the minimum node density that is necessary to obtain an almost
surely connected subnetwork on a bounded area of given size. The authors also provided an
insight into how fading affects the topology of multihop networks. The connectivity of a network
from a layered perspective has been investigated in [28]. The authors first pointed out how the
transmission range affects the end-to-end connection probability in a long-normal shadowing
model and compared the results to theoretical bound and measurements in the path loss model.
The authors then showed how connectivity issues behave in 802.11 and IP based networks if
fading effects increases. The authors came up with an analytical model for the link probability in
log-normal shadowing environments as a function of the number of nodes, network area,
transmission range, path loss and shadowing deviation.
A probabilistic analysis of the shadowing effects on the signal level variations has also been
presented in this paper. While investigating the impact of shadowing on the network
performance, the delivery ratio has been considered as a performance metric. The delivery ratio is
defined as the ratio between the number of packets received at a destination and the number of
packets that was sent by a source to that destination. Hence the packet delivery ratio indicates
how many packets were lost in a network. An analytical model to estimate packet losses in a
given network has been presented in this paper. This analytical model has also been verified via
simulation results. In order to investigate the shadowing effects on a routing protocol, we selected
Dynamic Source Routing (DSR)[3] protocol as the candidate. A brief description of the DSR
protocol has been provided in the following section for the completeness of this work. The effects
of shadowing on the DSR protocol have been explained in the section III. The shadowing has
grave effects on a medium access control scheme. Since IEEE 802.11 MAC layer protocol has
been used in this paper, section IV contains a brief description of IEEE 802.11 MAC layer
protocol and section V shows the effects of shadowing on IEEE 802.11 MAC layer protocal.

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Using a derived probability density function based on the results published in [6], the mean value
of the link distances has been derived in section VI. The probability that the received signal level
will be greater than a threshold level has also been derived in the same section. Simulation
models and results have been presented in section VII. Two solutions of shadowing problem have
been presented in the same section. Finally, we conclude this paper in the last section.
2. THE DSR PROTOCOL
The DSR [3] protocol consists of two basic mechanisms: (1) route discovery, and (2) route
maintenance. Route discovery is the mechanism by which a source node discovers a route to a
destination. When a source node wants to send a data packet, it first looks into its route cache to
find a route. If a source cannot find a route in its route cache, it initiates a route discovery
mechanism by broadcasting a request packet to its neighbours. When a neighbour of a source
receives a request packet, it first checks whether the request packet is intended for it or not. If a
neighbour discovers that it is the destination, it sends a reply back to the source after copying the
accumulated routing information contained in the route request packet into a route reply packet.
If a neighbour discovers that it is not the destination of this request packet, it checks for a route in
the route cache for that destination. If this neighbour is neither a destination nor it has a route in
the route cache to that destination, it appends its address in the route request packet and re-
broadcasts the route request packet to its neighbours. This process continues until a route request
packet reaches at the destination node. Then the destination node replies all route requests that it
receives. When a source node receives a route reply packet, it starts sending data packets using
the route indicated in the reply packet. If multiple paths are discovered, it chooses a path that is
the shortest one.
Figure1. Route discovery mechanism of DSR protocol
A typical route discovery mechanism of DSR protocol is illustrated in Figure 1. In this scenario,
mobile node ‘1’ is the source and mobile node ‘5’ is the destination. Mobile node ‘1’ does not
have any route in its cache to this destination. Hence it initiates the route discovery mechanism
by broadcasting a request packet. When some neighbouring nodes like mobile node ‘2’ and ‘6’
receive the request packet, they put their addresses in the request packet and re-broadcast that
request message. Similarly mobile nodes ‘3’,’4’ and ‘7’ also rebroadcast that request packet until
the destination node ‘5’ receives it. The destination node ‘5’ then replies back to the source node
‘1’ by using a route reply packet. The route reply packet contains the information of the route that
has been just discovered. After receiving the route reply packet, mobile node ‘1’ records all the
routes in its route cache. In this simple scenario, two routes namely ’1-2-3-4-5’ and ‘1-6-7-5’ are
discovered. According to the routing algorithm, mobile node ‘1’ should select the shortest path

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between these two paths. In this case, the shortest path is ‘1-6-7-5’. After selecting this shortest
path, the source node starts sending data packet by using the route ‘1-6-7-5’.
Figure 2. Route maintenance mechanism of DSR protocol
Route maintenance is the mechanism by which a node is able to detect any change in the network
topology. When a node detects a ’broken’ link, for example, by using missing MAC layer
acknowledgments, it removes the link from its route cache and sends a route error message to
each node that has sent packets over that link. A typical route maintenance mechanism is shown
in Figure 2. Let us assume that the link between mobile node ‘3’ and ‘4’ is ‘broken’ due to
battery exhaustion of mobile node ‘4’. Mobile node ‘3’ detects that the mobile node ‘4’ is
unreachable by using a MAC layer mechanism. Mobile node ‘3’ then creates a route error
message and sends it to the source. A route error message contains the information of the faulty
link (i.e., 3-4). After receiving the route error message, the source mobile node ‘1’ marks the
route ‘1-2-3-4-5’ as invalid route and tries to find an alternative route from its route cache. Since
an alternative route ‘1-6-7-5’ is there in the route cache, the source mobile node should select that
route and start using this new route.
3. EFFECTS OF SHADOWING ON ROUTING PROTOCOL
In the route discovery and route maintenance operation of the DSR protocol, it is assumed that
the link between two nodes is stable and the variation of signal level only depends on the distance
between them. That means a neighbouring node is always ‘reachable’ unless this neighbour is not
out of battery or it has moved out of the reach. But shadowing effect assumes that the signal level
can vary widely for a given distance between two nodes. This increases the probability that the
signal level may go below a certain required level called a ‘threshold’ level. In this case, a
receiving mobile node may not successfully receive a packet. Hence the following problems may
arise:
(a) The route request packet may not reach all the neighbours. Hence there is a probability of an
unsuccessful route discovery. That means the route request packet may not reach to a destination.
(b) The route discovery mechanism may not be an efficient one. The route discovery mechanism
of the DSR protocol aims to discover as many paths as possible. The reason is that if one path
fails, a source can selects an alternative path instantly. But this kind of flexibility may be lost
under shadowing condition. Since only a few number of paths are discovered, there is a
probability that a source may not find any other alternative route once a current route fails.

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(c) Once some routes are discovered and a source mobile node starts sending data packets using
one of the discovered routes, a neighbouring node may not receive that data packet because of the
wide variation of signal. Hence a data packet may be lost at an intermediate node.
(d) The route maintenance operation of the DSR protocol also may not work properly because
there is a probability that a route error message may be lost during its way to a source mobile
node. If a source does not receive the route error message, it cannot detect a link breakage. The
source continues sending data packet using the route that contains the broken link. All these data
packets will be lost at the broken link.
The shadowing not only affects a routing protocol but also makes problems for a medium access
control scheme. In all the simulation results presented in this paper, IEEE 802.11 has been chosen
as the medium access protocol. A brief description of IEEE 802.11 MAC layer has been
described in the following section.
4. IEEE 802.11 MAC LAYER
Like other IEEE 802.x protocol, the IEEE 802.11 protocol defines the Medium Access Control
(MAC) and physical layers. The IEEE 802.11 MAC layer defines two different access methods
namely Distribution Coordination Function (DCF) and Point Coordination Function (PCF). We
will now only describe DCF based MAC protocol since PCF based MAC protocol cannot be used
in an ad hoc network. The basic access mechanism of DCF based MAC protocol is basically a
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The main mechanism of
CSMA/CA protocol is as follows: a mobile node senses the medium before transmitting its
packet. If it finds that the medium is free, it transmits its packet. But if the medium if busy, a
mobile node defers its transmission to a later time which is chosen randomly. IEEE 802.11
protocol includes both physical carrier sensing and virtual carrier sensing.
Physical carrier sensing is used to detect other mobile nodes operating in the same network by
analyzing all detected packets. It also helps to detect activities in the channel via relative signal
strength from other sources. Virtual carrier sensing is performed by sending MPDU duration
information in the header of Request-to-Send (RTS), Clear-to-Send (CTS), and data frames as
shown in Figure 3. In addition to header information, payload and a 32-bits CRC, MPDU also
contains a duration field. The duration field contains information of the time duration that will
take this data or management frame to complete transmission. Other mobile stations in the same
area use this information to adjust their Network Allocation Vector (NAV), which indicates the
amount of time that must elapse until the current transmission session is completed and the
channel can be sampled again for idle status. The channel is marked busy if either the physical or
virtual carrier sensing mechanism indicates the channel is busy. Priority access to the wireless
medium is controlled through the use of Inter-Frame Space (IFS) time intervals between the
transmissions of frames. Two IFS intervals are specified in IEEE 802.11 standard (a) Short Inter-
frame Space (SIFS) and, (b) DCF Inter-Frame Space (DIFS). The SIFS is the smallest IFS after a
DIFS as shown in Figure 3. When a station senses the medium and finds it is idle, it has to wait
for a DIFS period to sense the medium again. If the channel is still idle, the mobile station
transmits its MPDU. Upon receiving the MPDU, a receiver checks the checksum to determine
whether a packet was received correctly or not. If a packet is received correctly, a receiver waits
for a SIFS time period and sends a positive acknowledgement (ACK) to the transmitting station
to indicate that the transmission was successful. If an acknowledgement is not received within a

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given time period, another attempt is made to send a packet again. The number of times a source
attempts to send a packet is determined by an important parameter named ‘Long Retry Limit’.
Figure 3. Carrier sensing in IEEE 802.11
When the number of attempts exceeds this limit, a source discards a packet permanently. Since
each mobile node has a limited buffer to store packet, this ‘Long Retry Limit’ parameter helps a
mobile node to operate with a limited packet buffer size. In order to reduce collision and save
channel bandwidth, Request-to-Send (RTS) and Clear-to- Send (CTS) are also used in IEEE
802.11 layer. RTS and CTS control frames are used by a station to reserve channel bandwidth
prior to the transmission of MPDU. The timing diagram of RTS and CTS packets are shown in
Figure 4. The RTS control frame is first transmitted by a source mobile. All other stations in a
given area read the duration field and set their NAVs accordingly. The destination station
responds to the RTS packet with a CTS packet after an SIFS idle period has elapsed. Mobile
stations hearing the CTS packet look at the duration field and again update their NAV. Upon
successful reception of the CTS, the source station is virtually assured that the medium is stable
and reserved for successful transmission of the MPDU. In this way, the mobile stations update
their NAVs based on the information contents of RTS and CTS packets, which helps to combat
‘hidden terminal’ problem. The collision avoidance portion of CSMA/CA is performed through a
random back-off procedure. If a mobile station with a frame to transmit senses the channel and
finds the medium is busy, then the mobile node waits for a DIFS period. At the end of the DIFS
period, the mobile station computes a random period of time called back-off period. In IEEE
802.11, time is slotted in time periods that corresponds to a time unit called slot time. The
random back-off time is an integer value that corresponds to a number of time slots. Initially, the
mobile station computes a back-off time in the range of 0-31. This period is called the contention
window as shown in figure 4. After this contention period, a mobile node again senses the
medium. If it finds the medium busy again, it goes for longer contention period. On the other
hand, if it finds the medium free, it waits for DIFS period to make sure no other mobile node is
transmitting. If no other mobile node transmits during that period, a mobile node starts
transmitting its own packet.

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Figure 4. Transmission of MPDU with RTS and CTS
5. EFFECTS OF SHADOWING ON MAC LAYER
The CSMA/CA based multi-access technique assumes that the medium is an intermittent
synchronous multi-access bit pipe on which idle periods can be distinguished from packet
transmission periods. If nodes can detect idle periods quickly, it is reasonable to terminate idle
periods quickly and to allow nodes to initiate packet transmission after such idle detections. This
is the philosophy of a CSMA/CA based multiple access technique. Shadowing effects may put
hurdle on the normal operation of IEEE 802.11 MAC layer operation in the following ways: (a)
as mentioned in the earlier section that IEEE 802.11 carrier sensing is performed at both the air
interface, which is referred to as physical carrier sensing and at the MAC sub-layer referred to as
virtual carrier sensing. Physical sensing may not work properly due to shadowing because signal
level may go below a threshold level so that it cannot be detected. Hence both physical carrier
sensing and virtual carrier sensing may not work properly, (b) a source station performs virtual
carrier sensing by sending MPDU duration information in the header of RTS, CTS and data
packets. Stations in a given area use this information in the duration field to adjust their Network
Allocation Vector (NAV). The NAV indicates the amount of time that must elapse until the
current transmission session is complete and the channel can be sampled again for idle status.
The stations in a given area may not be able to recover the bit duration information from the
packet because of the poor signal level arisen from the shadowing effects. Hence the adjustment
of NAV cannot work properly, (c) The RTS and CTS frame exchanging between a source station
and a destination station may not be successful due to variation of signal level. If the destination
station does not receive RTS packet due to signal variation, it does not reply that RTS by sending
a CTS packet. Since a source station cannot send a data packet unless it receives a CTS packet
from the destination, a source station has to keep the data packet in the buffer for longer period of
time. Similarly, if the CTS packet is not successfully received by a source station due to signal
variation, a source station also delays its transmission. Hence unsuccessful reception of RTS and
CTS packets can cause unnecessary delaying of a packet transmission, (d) according to IEEE
802.11 after receiving a data packet, a source station sends an acknowledgement. A destination
station may not receive a data packet successfully due to variation of the signal strength. Hence a
source station has to resend a data packet several times. These redundant packets will occupy
channel unnecessarily and waste scarce channel bandwidth. On the other hand, a destination may
successfully receive a packet, but the acknowledgement packet sent by that destination may be
lost due to shadowing effects. Since a source node does not receive an ACK packet, it keeps
sending the data packet repeatedly. According to IEEE 802.11 MAC layer, a source has the
opportunity to resend a packet for seven times after that it will assume that the destination is

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unreachable and it drops the packet. Hence there will be a large number of packet losses in a
network due to unsuccessful reception of an ACK.
6. DERIVATION OF EXPRESSION
If we do not consider the shadowing effects, then the quality of each link should remain constant
during the operation of the network. But this is not a valid assumption under a shadowing
condition. Shadowing causes a wide range variation of the received signal. It increases the
probability that the received signal goes below a threshold level. Hence a packet may not be
correctly received by a receiver. The analytical model presented in this section is based on the
following assumptions: (1) all link distances are uniform within a rectangular service area, (2) the
characteristic of the channel is almost same over the whole rectangular service area (i.e., the
whole network has shadowing effect), (3) the node density (i.e., number of nodes per square

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meter) is kept constant when network size is varied, and (4) once a packet is successfully
received, the receiving mobile node re-transmits that packet at the same power level at which the
previous node has transmitted the same packet to itself. Shadowing effect states that at any given
distance d from a transmitter, the path loss PL(d) at a particular location is random and
distributed log-normally (normal in dB) about the mean distance-dependent value [5]. Therefore,
path loss and received power at d meter from the transmitter is modelled as follows:
Figure 6. Comparison results of probability density functions of path loss at d = 4 , 40, 80, and 186m
The probability density functions for path loss using Equation (4) is a Gaussian log-normal
distribution. The distance dependent mean values of path losses at d = 4,40,80, and 186 meter far
from the receiver is 50, 80, 88, and 100 dBm respectively shown in the Figure 6.

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Figure 7. Probability of received power greater than threshold power level with respect to distance d and
circular area πd2
Based on the analysis presented in [5], the probability that the received signal level will be
greater than the threshold power Pth can be expressed as
7. SIMULATION MODEL AND RESULTS
To investigate the effects of shadowing on the performance of an ad hoc network, a network
consists of 30 mobile nodes was created and tested via Network Simulator (NS-2) [9]. These
nodes were placed randomly over an area of 400m´ 300m area. Five connections were randomly
set up in the network. While setting up each connection of the DSR protocol was used as the
routing algorithm. Once a connection is set-up, Constant Bit Rate (CBR) agent was used to

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generate packets. Each CBR connection started at random period of time. Once a CBR
connection started, it continued generating packet till the end of the simulation.
Table 1: Simulation parameters
The packet generation rate was 1 packet per second. Each simulation was tested for 250 seconds
simulation time. IEEE 802.11 MAC layer was used as the MAC layer. The network size was then
increased to 400m x 400m, 500m x 400m, 500m x 500m, 600m x 500m, 600m x 600m, 700m x
600m and 700m x 700m by keeping the node density constant so that the network connectivity is
not affected. That means there are 40, 50, 62, 75, 90, 105 and 122 mobile nodes were deployed
over the network area when the network sizes were 400m x 400m, 500m x 400m, 500m x 500m,
600m x 500m, 600m x 600m, 700m x 600m and 700m x 700m respectively. The total number of
data packets generated in the network at the source mobile nodes and the total number of data
packets delivered to the destination mobile nodes were monitored during each simulation. The
delivery ratio is the ratio defined by . For a given area, ten different topologies
were created and tested by using different seeds. Ten simulation results were then averaged. The
other simulation parameters are shown in Table I. The transmission range is 250 meter with the
given transmission power level of 24.50 dBm and the threshold power of -64.38 dBm. These are
the default values of these two parameters in Network Simulator (NS-2) [9].
The two-ray model has been used as the propagation model. As mentioned in the previous section
that two-ray model is too simple to represent a real world scenario. The two-ray reflection model
assumes that there are two paths between a source and a destination. One path is the line-of-sight
path and the other one is the reflected path from the ground. The variation of the signal strength
follows the following rule [5]
where Gt and Gr are the transmitting antenna gain and receiving antenna gain, ht and hr are the
transmitting and receiving antenna heights, Pt is the transmitting power and Pr is the receiving
power, and d is the distance between the transmitter and the receiver. Based on the two-ray model
the received signal variation is illustrated in the upper graph of Figure 8.

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Figure 8. Delivery ratio of simplest two-ray channel model with respect to rectangular area
The figure shows that the received power level remains above a threshold power level Pth unless
the distance between a transmitter and a receiver is 160 meter. After that the received signal level
goes below a threshold level. Hence there will be packet losses in the network if a network is
large enough to have average link distance greater than 160 meter. Since each mobile node uses
the same power level to transmit all kinds of packets (i.e., route discovery, route maintenance and
data packets), the link up to 160 meter is considered stable in this case. That means once a route
is discovered, the qualities of all links lying along that route do not change over the time. Hence
there is almost no packet loss. The lower graph of Figure 8 also depicts the simulation results. It
shows that the delivery ratio for two-ray model is almost 100%. That means a negligible number
of packets has been lost. This figure shows that the delivery ratio is 100% under different
network size. Because the average link distance is those simulated networks is less than 160
meter. The above mentioned simulations were repeated with shadowing propagation model by
keeping other parameters mentioned in Table I same. The simulation results with that of the
probabilistic analytic model are shown in the Figure 9. It is depicted that the delivery ratio is
almost 100% when network area is less than or equal to 1.6 x 105
square meter.
Figure 9. Comparison of simulated delivery ratios of two-ray channel model and shadowing model and
theoretical probabilistic analytic result of shadowing model
After that the delivery ratio decreases exponentially as the network gets larger. The delivery ratio
decreases exponentially to almost 15% when the network size was 700m x 700m. This

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exponential drop in the delivery ratio complies with the theoretical model derived in section VI
where it was shown that the probability of the received power is greater than the threshold power
level. In order to successfully receive a packet, the received signal level should be greater than
the threshold value defined by the parameter ( i.e. Pth ), which is listed in Table 1. The decision of
successfully received a packet is determined by the probability defined as Pr ( Pr (d) > Pth) since
successfully received packets solely depend on the received power that is a random variable for
shadowing model. This probability is directly proportional to the delivery ratio within the same
rectangular area where link distance is almost linearly increased with the rectangular area, i.e.,
Delivery ratio = k Pr (Pr (d) > Pth ) (7)
where k is a scaling factor constant. Both the theoretical and simulation results of shadowing
model are shown in Figure 9. In addition to these results, simulation result of two-ray model is
also plotted in the same graph to show the difference between the network performance under
two-ray model and shadowing model. Two methods that can reduce the shadowing effects
suggested in this paper are: (1) by increasing the transmission power, and (2) by increasing the
retry-limit of MAC layer protocol. The first solution is a physical layer solution. This solution of
reducing shadowing effects is to increase the transmission power level. As shown in Equation (3)
that the received signal level at a given distance will increase if the transmission power Pt is
increased.
To investigate how a higher transmission power can reduce the shadowing effect the previous
simulations have been repeated here. But the transmission power is increased to 0.58432 watts
(or 27.67 dBm). This transmission range of a mobile for this new transmission power is 300
meter (if two-ray model is used). The network performance in terms of delivery ratio under a
shadowing condition but with the increase in transmission power is shown in Figure 10 labelled
as high transmission power.
Figure 10. Comparison results of delivery ratio in shadowing environment using different Techniques
The figure shows that the delivery ratio can be improved if a higher transmission power is used.
For example, when the network area is 500m x 500m, the delivery ratios are 40% and 55% for
the transmission power of 24.5 dBm and 27.67 dBm respectively. But the improvement in
delivery ratio is less when the network size is small. The higher transmission power level not
only increases the probability of improving the signal level. But it also helps to reduce the

26
number of hops that a packet travels from a source to a destination. Hence there is less
probability of packet loss. Although the higher transmission power improves delivery ratio, it
may not be a good choice to increase the transmission power to reduce shadowing effect. High
transmission power will increase interference level in a network. Another alternative solution to
reduce the shadowing effect is to modify the MAC layer protocol. As mentioned previously that
an important parameter of IEEE 802.11 MAC layer is ‘Long Retry Limit’. By default the ‘Long
Retry Limit is set to 7. That means after 7 attempts, a mobile node discards a packet permanently
by assuming that the next hop is unreachable. But under a shadowing condition 7 attempts are not
enough. A mobile node may not successfully receive a packet from its previous hop due to poor
signal level in 7 attempts. In order to increase the probability of successfully receiving a packet
the ‘Long Retry Limit’ was increased to a higher value of 12. That means a mobile node has 5
additional attempts to send a packet successfully to its next hop. The simulation result for this
MAC layer solution is shown in shown in Figure 10 labelled as ‘MAC Layer’. This figure shows
that delivery ratio is improved compared to that of a shadowing. Although the amount of
improvement in MAC layer solution is less in compare to its higher transmission power counter
part, but still the packet loss can be reduced in a network. The figure shows that an average
almost of 10% packet loss can be reduced in a network if MAC layer is modified as mentioned
above.
8. CONCLUSIONS
In this paper, the shadowing effects on the performance of an ad hoc network have been
investigated. Although two-ray model is widely used as a propagation model in ad hoc network
simulation, but this model does not represent a real world network propagation model because the
surrounding environment of a network is always changing. It is shown that the performance of a
network deteriorate very quickly if the shadowing effects are taken into account. The main
reasons for this degraded performance resulted from the fact that there will be a large variation of
the received signal level for a given link distance. Hence a packet (routing packet or MAC
packet) may not be received successfully by a mobile node due to poor signal level. This causes
problem to the normal operations of a routing protocol as well as the MAC protocol. One of the
solutions of reducing showing effect suggested in this paper is to increase the transmission power
level. The simulation result shows that the delivery ratio can be improved by almost 40% on
average if the transmission power is increased from 24.5 dBm to 27.67 dBm. But higher
transmission power increases the interference level in a network. Another alternative MAC
solution has been suggested in this paper. This alternative solution is based on a modification of
MAC layer protocol parameter. In this solution the number of packet transmission attempts was
increased. The simulation result shows that this solution also improves the network performance.
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AUTHORS
Md. Anwar Hossain received the B.Sc. degree in Electrical and Electronic
Engineering from Rajshahi University of Engineering & Technology, RUET in 2001
and M.Sc. Engineering degree in Information and Communication Technology from
Asian Institute of Technology, AIT in 2006. He had worked as a lecturer in
Electrical and Electronic Engineering Department at Rajshahi University of
Engineering & Technology, RUET. Recently, he is working as an Assistant
Professor at American International University-Bangladesh. His research interests
are in Ad hoc networks, CDMA, OFDM and MIMO-OFDM Systems.
Mohammed Tarique received B.Sc. degree in Electrical and Electronics from
Bangladesh University of Engineering and Technology (BUET) in 1992. He has
worked in Beximco corporation for 6 years as a Senior Engineer. He received his
Master of Business Administration (MBA) degree from the Institute of Business
Administration (IBA), Dhaka. He also received his Master of Science (MS) degree
from Lamar University, Texas, USA and Ph.D. degree from University of Windsor,
Ontario, Canada. He is currently working in American International University-
Bangladesh as an assistant Professor. His research interests are in wireless
communication, sensor network, ad hoc network and mesh networks. He has presented his research works
in the conferences held in Europe, USA and Canada. He has published several papers in the top tier
journals.
Rumana Islam completed her Master of Science (M.Sc.) in Biomedical
Engineering from Wayne State University, Michigan, USA in 2005 and her B.Sc.
Engineering degree in Electrical and Electronic Engineering from Bangladesh
University of Engineering and Technology (BUET) in 1995. She is currently with
the Department of Electrical and Electronic Engineering of American International
University-Bangladesh. She has 7 years of professional experience in reputed
public and private sectors. Her research interests are in Biomedical sensor design,
sensor networks, ad hoc networks and Signal Processing.

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