Wireless Sensor Network, 2010, 2, 7-17
doi:10.4236/wsn.2010.21002 anuary 2010 (http://www.SciRP.org/journal/wsn/).
Copyright © 2010 SciRes. WSN
Published Online J
Simulation Study of the Influence of the Hidden and
Exposed Stations for the Efficiency of IEEE 802.15.4
LR-WPAN Networks
AGH University of Science and Technology, Department of Electronics, Kraków, Poland
Email: koscieln@agh.edu.pl
Received August 14, 2009; revised Septemb er 20, 2009; accepted September 22, 2009
This article presents an analysis of wireless personal area networks with low transmission rate, utilized more
and more often in industrial or alarm systems, as well as in sensor networks. The structure of these systems
and available ways of transmission are defined by the IEEE 802.15.4 standard. The main characteristics of
this standard are given in the first part of this article. The second part contains the description of simulation
tests that have been realized. Their results make available an evaluation of the effective transmission rate of a
transmission channel, the resistance to the phenomenon of hidden station as well as sensibility to the problem
of the exposed station.
Keywords: Wireless Network, Sensor Network, Lr-Wpan, Csma/Ca, Slotted-Csma/Ca, Transmission Proto-
col, Hidden Station, Exposed Station
1. Introduction
In the past few years, the interest concerning the Wire-
less Personal Area Networks – WPAN, has been growing
in a very important way. These systems make possible a
free utilization of portable communication devices, a fast
creation of improvised networks as well as an automa-
tion of many repetitive everyday activities (updating of
prepared data, address books, notebooks and calendars).
All the WPAN networks must fulfill high expectations
concerning low energy consumption and limited trans-
mission range. However, in the recent period we can
clearly observe a clear distinction between solutions that
are currently created, resulting in a subdivision of the
IEEE802.15 standard defining WPAN networks into four
specialized thematic groups. The first of these groups
(IEEE802.15.1) concerns the development of the Blue-
tooth network, considered as a system of medium trans-
mission rate. The axis of the studies concerning the sec-
ond group (IEEE802.15.2) concerns the compatibility of
the WPAN and WLAN systems (mainly of the
IEEE802.11 standard). These works have been temporar-
ily suspended, because the recent studies indicate that—
at the current state of technology and standardization—
the required compatibility is impossible to achieve. The
third group (IEEE802.15.3) concerns WPAN networks
with high transmission rate—HR-WPAN (High Rate
Wireless Personal Area Network). The transmission in
these systems is realized with a rate achieving 54Mb/s –
there can be represented by the multimedia Wimedia
network with medium transmission rate of 20Mb/s. The
object of the IEEE802.15.4 standard concerns networks
with low transmission rate—LR-WPAN (Low Rate
Wireless Personal Area Network). The results of the
studies presented in the further part of this article apply
to the last one of the abovementioned systems.
2. LR-WPAN Networks
The IEEE802.15.4 standard has been created in 2003,
and its current form results of the modifications intro-
duced three years later. The specification defines the
physical layer—PHY (Physical Layer), the sublayer of
access to the transmission link—MAC (Medium Access
Control Sublayer), as well as the principle of their inter-
action with the higher layers.
The LR-WPAN networks are characterized by very
low energy consumption, simplicity of their structure
making possible to implement the transmission protocol
on 8-bit microcontrollers, as well as low costs of receiv-
ing and transmitting equipment. LR-WPAN networks are
designed to be used in different industrial, agricultural
and alarm systems, building automatics, monitoring, in-
teractive toys and in particular in sensor networks—
WSN (Wireless Sensor Network).
The IEEE802.15.4 standard defines three transmission
bands: 2.45GHz (16 channels), 915MHz (10 channels)
and 868MHz (1 channel). The frequency range, the ways
of modulation of the carrier wave and the detection and
correction code determine the final transmission rate,
equaling to: 20kb/s, 40kb/s, 100kb/s or 250kb/s. The
nodes realize the transmission in a discontinuous way,
trying to stay for the longest possible time in inactive
mode—this make possible to achieve low energy con-
sumption. The radiated power is less than 1mW, and the
transmission range, characteristic for the POS (Personal
Operating Space) class solutions, equals 10m.
A LR-WPAN network offers a high capacity of the
system and a very fast identification of equipment ap-
pearing in its range. The number of operating stations
can equal 216 or 264, dependent on the length of addresses,
whereas in general the time of registration of a new node
doesn’t exceed 30ms. Moreover, a precious advantage is
the automatic modification of connections with moving
2.1. Topology of the LR-WPAN Network
The IEEE802.15.4 standard defines two types of equip-
ment. The first one regroups fully functional terminals—
FFD (Full Functional Device). In the structure of the
network, these devices may realize any tasks (network
coordinator, cluster coordinator, coordinator of a slave
system). The second group contains devices with a re-
duced set of functions—RFD (Reduced Function De-
vice), working always as slave stations. In a given mo-
ment, the RFD node may be subject to only one master
device (FFD).
The main topology of the LR-WPAN network is the
star topology, in which the central node is always the
FFD device assuming the tasks of cluster head—CLH,
named simply coordinator. The tasks of the coordinator
include in particular connection and disconnection of
slave stations, synchronization of the transmission and
routing of packets.
The topology of direct (peer-to-peer) connections al-
lows the communication between FFD nodes located in
their reciprocal range. In this case, the routing realized
by the coordinator concerns only packets transmitted
between distant nodes.
The interconnection of many clusters creates a tree
topology. The operation of such a network is supervised
by the PAN coordinator, which is in the same time coor-
dinator of the main cluster. An alternative topology is a
mash structure, in which between not one, but many
routes may be defined between distant points.
The maximal range is achieved by systems with clus-
ter tree structure, still controlled and synchronized by a
single PAN coordinator. Theoretically, the area occupied
by several interacting clusters may have any dimensions
(Figure 1). Nevertheless, we must remember that the
realization of multi-stage transmissions results in impor-
tant delay of the transmitted data.
2.2. Access protocol to the Transmission Channel
The IEEE802.15.4 standard offers two ways of transmis-
sion: in non-synchronized (non-beacon) and in synchro-
nized (bacon enabled) mode. The first one defines only a
contention access, using a simple mechanism permitting
to identify the channel and avoid the collisions—unslot-
ted-CSMA/CA (Carrier Sense Multiple Access with Col-
lision Avoidance). In the second method, a less devel-
oped, slot contention protocol—slotted-CSMA/CA—has
been implemented, as well as a no-collision access mech-
Figure 1. Topology of a cluster tree.
Copyright © 2010 SciRes. WSN
Figure 2. Way of transmission: a) without acknowledgements, b) with acknowledgements.
Figure 3. Format of a transaction in non-synchronized
mode, directed to the coordinator.
Figure 4. Format of a transaction in non-synchronized
mode, directed to a slave node.
The non-synchronized networks are usually heteroge-
neous systems, in which the continue activity of the co-
ordinator requires supply from the power network. The
coordinator must be always ready for a transmission
from any node, activated by external events [1]. As the
same event can be registered simultaneously by more
than one node (ex. in the sensor network), the avoidance
of the collisions requires the anticipation of the transmis-
sion with a supplementary waiting period of random im-
portance. This duration includes a total number of peri-
ods BP (Backoff Period), within the range: 0 2
The BE (Backoff Exponent) value is increased by 1 after
each unsuccessful transmission attempt, and the duration
of the BP period equals to the period of transmission of
80 bits.
After the duration of transmission delay, the node veri-
fies the status of the channel and if it is empty, immedi-
ately starts the transmission. If the channel is occupied,
the value of the BE coefficient is increased, a new delay
time is randomly assigned and the next attempt of trans-
mission is executed. In the same time, the node incre-
ments the counter of unsuccessful attempts—NB (Num-
ber of Backoff). If its value exceeds a defined threshold,
further transmission attempts are stopped.
For the transmission of data are used the DATA
frames, with a maximal length of 1064 bits, including
transmission overhead. Optionally, the frames may be
acknowledged by ACK packets [2,3]. Between a DATA
frame and ACK there is always a short interval of tACK
length, allowing handing over the control of the channel
to the target node (Figure 2). The intervals between suc-
cessive transactions are longer and their duration de-
pends on the size of the transmitted DATA frame. If its
length does not exceed 18 bytes, the following transac-
tions may be started after a short inter-frame space—
SIFS, equaling to the period of transmission of 12 trans-
mission symbols. Otherwise, it is necessary to use a long
inter-frame space—LIFS, equaling to the period of 40
symbols (Figure 2).
Since the coordinator of a cluster operating in non-
synchronized mode always remains in active state [4],
the transactions directed to it can be started in any time
(Figure 3). The first transmitted frame is DATA, option-
ally acknowledges with the ACK packet. The remaining
nodes of the system generally remain in non-active state,
activating for a short period of time, usually not more
frequently than once a second or even once a minute [5].
Because of this, the frames directed to these stations
must be buffered by the coordinator until a given device
reports its activity and requests the transmissions waiting
to be received. Such a request is transmitted as a RE-
QUEST frame. If any non-delivered data exist in the
coordinator buffer, the coordinator sends an answer with
an ACK frame and successively transmits the DATA
packet. The target node may acknowledge the correct
transfer, sending an ACK (Figure 4).
In the synchronized mode, all the nodes of the cluster,
including its coordinator, activate in constant intervals,
named the intervals of the BEACON frame—BI (Beacon
Interval). Such a solution allows energy savings and
permits to create dedicated transmission channels with
guaranteed transmission rate and little delay [1].
The rhythm of the system is defined by a superframe.
Its beginning is indicated by the coordinator, which
opyright © 2010 SciRes. WSN
sends a BEACON packet. The superframe is divided into
active and non-active parts (Figure 5). Any transmissions
and related activities of single nodes take place only in
the first part of the superframe, containing 16 slots and
divided between contention access period—CAP and
contention free period—CFP.
The duration of the transmission of the BEACON
frame is defined by BO (Beacon Order) parameter in the
following way: BI = BSD*2BO, where the BSD (Base
Superframe Duration) constant defined the period of the
main superframe (the shortest possible), containing 16
slots with a length of 60 transmission symbols. In a
similar way is defined the size of the active part of the
superframe—SD (Superframe Duration), equaling to: SD
= BSD*2SO, where the values of: SO (Superframe Order)
and BO are interdependent: SO BO and 0 BO 14.
The contention access used in the CAP period uses a
somewhat more developed, slotted CSMA/CA (slot-
ted-CSMA/CA) mechanism. The node initiating a trans-
action synchronizes its activities to the start of the closest
BP period and begins the counting of the transmission
delay, defined randomly in the way described above.
Further away, the verification of the channel status be-
gins, during this time by the contention window duration
—CW. The contention window consists of several (by
default two) BP periods. The node can start a transmis-
sion only, if during the whole CW period the channel
remained unoccupied. Otherwise, after increasing by 1
the BE and NB values, a following attempt of transmis-
sion is started. However, before that the node must verify
if the counter NB does not exceed the threshold value
and the transaction does not exceed the contention period
of the superframe.
Before the transaction directed to the coordinator starts,
the source node must correctly receive a BEACON
frame and synchronize its activities within the limits of
appropriate time periods. Only then the contention pro-
cedure can be started. The procedure ends with the
transmission of the DATA frame (Figure 6). The coor-
dinator, being responsible for generating the BEACON
frame, must pass to the active state from the frequency of
redundancy of the superframe. In such a way, the trans-
actions directed to this node are not subject to any further
limitations. The other nodes of the network may activate
much more rarely. The coordinator must still buffer data
transmitted to it. The information concerning packets
waiting for delivery is sent in a BEACON frame, to-
gether with the network identification as well as its main
operation parameters. When a device activate, it receives
information indicating, if a transaction of reception
should be initiated (Figure 7). Therefore, also in this case
the source of transactions realized in both directions are
nodes not being coordinators (see Figure 3, Figure 4,
Figure 6 and Figure 7).
A supplementary function of the coordinator is the al-
location and maintenance of guaranteed time slots—GTS,
designed for applications that require channels with con-
stant transmission rate and little delay [2]. The coordina-
tor may prepare up to seven such structures, containing
Figure 5. Structure of a superframe.
Figure 6. Format of a transaction in synchronized
mode, directed to the coordinator.
Figure 7. Format of a transaction in synchronized
mode, directed to a slave node.
Copyright © 2010 SciRes. WSN
one or more superframe slots. The GTS elements fill the
ontention free period—CFP (Figure 5), and the t
plemented in the IEEE802.15.4 network was to define
Transmission Channel
The f the transmission chan-
el indicates a maximum number of user’s data transmit-
rst layers as well as because of the inactivity periods rel-
ated to the duration of transmission delay times as well as
the channel occupation during the contention [2].
nel operation time.
crans- testing of
mission realized by them use the channel commutation
mode. Since in realized simulations we did not take into
account this method of node communication, its details
will not be presented in a more detailed way.
3. Simulation Tests of the Contention
The main objective of the tests of the contention protocol
its efficiency and resistance to the appearance in the sys-
tem of hidden stations or exposed stations, named also
blocked nodes [4]. The simulation was realized using a
NetSim simulation system, created in the Department of
Electronics of the AGH University of Science and Tech-
nology. The NetSim software has been written in C++
language. The packet uses an event-planning technology
(event queue). Its mechanisms permit to correctly render
the reciprocal time interrelations, existing between sev-
eral simultaneous processes. The importance of simu-
lated time, as well as the number of stages of tested proc-
esses can be dynamically adapted to the following fac-
tors: the character of the observed events, the momentary
importance of the offered traffic, the size of the tested sy-
stem as well as the required precision of obtained results.
In the further part of this work we have presented the
results of tests relating to the evaluation of the efficiency
CSMA/CA protocol implemented in non-synchro-
nized and synchronized LR-WPAN network. In all the
studied cases the assumptions are as follows: transmis-
sion rate of 250kb/s, the DATA frames transmit data
fields with maximal permitted size, the node emitters are
equipped with buffers with a capacity of 50 packets and
every successful transaction ends with an ACK acknow-
ledgement. Moreover, we have admitted a zero-one
(two-ray ground) propagation model, signifying that the
nodes located within the emitter range correctly receive
its transmission with a probability equaling 1. The other
stations do not hear the transmission—their probability
of packet reception equals 0. In the simulation model, we
did not take into consideration the possible impact of any
external interference that might decrease the efficiency
of the transmission. Therefore, the only possible cause of
unsuccessful transfer can be a collision.
3.1. Effective Transmission Rate of
effective transmission rate o
ted within a time unit [2]. Usually, the value of this pa-
rameter is largely different of the used transmission rate,
because of the overhead introduced by the second and fi-
For the identification of the effective transmission rate
of the system we have used a model containing two
nodes, one of them working as coordinator (Figure 8).
The transmission is realized in only one direction—
towards the coordinator (node N0). Therefore, the net-
ork is free of collisions and the intensity of the oper-
ated traffic—the maximal possible.
The results obtained for both network operation modes
(non-synchronized and synchronized) are summarized in
Figure 9. The effective transmission rate in the non-syn-
chronized mode equals to about 116 kb/s, corresponding
to the utilization of 46% of the chan
The remaining transmission rate of the system is ab-
sorbed by the transmission overhead and by the dead
periods, related to the random delay of the moment of
starting the transmission. The effective transmission rate
of the synchronized network is even worse and equals to
about 98kb/s, corresponding to 39% of the assumed
transmission rate. The supplementary band losses result
from the necessity of the periodical transmission of the
BEACON frame, the increasing of the channel occupa-
tion test to the contention window size and the non-
utilization of the last fragment of the superframe, which
remains empty, because the transmitting node can not
manage to fit the entire transaction in it. The average
length of this section corresponds to the half of the
transaction time.
Figure 8. Unidirectional transmission in a system consisting
of two nodes.
Figure 9. Unidirectional transmission in a system consisting
of two nodes – intensity of the operated traffic.
opyright © 2010 SciRes. WSN
Figure 10. Unidirectional transmission in a system consist-
ing of two nodes – coefficient of delivered packets.
Figure 11. Unidirectional transmission in a system consist-
ing of two nodes – average transmission delay time.
Figure 12. Bidirectional transmission in a system consisting
of two nodes – intensity of the operated traffic.
The Figure 10 presents the relation between the coef-
ficient of delivered packets and the intensity of the of-
fered traffic. The losses of frames appear only during the
overloading of the system. The superiority of the traffic
offered over the traffic operated leads to the overfilling
of the emitter’s queue and the resulting refusal of a cer-
tain part of the requests.
A valuable feature of the tested system are permanent
and small average delay times, presented on Figure 11
and measured only for properly transmitted packets. This
situation changes only in a condition of congestion. In
this condition, generated data start to be gathered in the
transmitter queue whose length is quickly growing. As a
result, the average transmission delay time value grows
The same model of the system (Figure 8), loaded with
s are initiated by a single
nt of delivered packets, defined for the
a traffic directed in a symmetrical way to both nodes,
make possible to define the influence of the b
nsmission for the available transmission rate of the
network. The obtained results are summarizes in the
Figure 12. Their values are not significantly worse, even
if it could seem that the nodes should initiate a conten-
tion concerning the access to the common channel, lead-
ing to collisions. In the LR-WPAN network, the transac-
tions realized in both direction
ve station, so any contention is excluded. The de-
crease of the transmission rate of the transmission chan-
nel results from a worse efficiency of transmission di-
rected towards the slave node. Any such transaction must
start with a transmission of REQUEST and ACK frames
(Figure 3 and Figure 6 vs. Figure 4 and Figure 7), in-
creasing its duration.
The coefficie
scussed configuration, has slightly changed, because of
the decrease of the transmission rate of the network
(Figure 13). The form of both curves remains identical,
confirming a total operation of the requests directed to-
ward a system free of overloading.
Figure 13. Bidirectional transmission in a system consisting
of two nodes – coefficient of delivered packets.
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Copyright © 2010 SciRes. WSN
Figure 14. Bidirectional transmission in a system consisg tinFigure 16. Unidirectional transmission in a system contain-
ing hidden stations – intensity of the operated traffic.
of two nodes – average transmission delay time.
Figure 15. Example of a system with a hidden station.
Average delay time calculated for properly transmitted
packets is about three times longer then the one discus-
sed in example on Figure 14. Also this time, when reque-
sts are suppressed, queues in transmitter buffers begin to
grow quickly. As a result, the time when successful tran-
.2. Influence of a Hidden Station for the
Transmission Rate of the System
Usually, a station of a large WPAN network embraces by
its range only some closest nodes. More distant devices,
not receiving its signal, may have an important negative
impact for the transmission of data with the closest
neighbors. The simplest example
ted in Figure 15.
alization of the receiver of this transmis-
on, the range of the new transmission will include the
of both
fram of the trans-
actions will result unsuccessful.
The collisions caused by hidden stations are much
more troublesome for the system that those resulting
from the contention for the access to the radio channel. A
long time of emission of a single frame significantly in-
creases the probability of generating in this period a new
request directed to the hidden station [3]. Its immediate
realization will disturb the transaction being already in
progress with the distant node.
Studying the influence of the presence of a hidden sta-
tion for the operation of the LR-WPAN network, we have
used the model presented in Figure 15. A centrally places
coordinator (node N0) works with two slave nodes (N1
and N2), located out of their reciprocal range. The entire
offered traffic is evenly dividedween slave stations,
rs exclusively to the coordinator.
The results of simulation tests, summarized in Figure
16, indicate a radical decrease of the transmission rate of
e system—for both transmission modes it equals to
ervicing and to system overloading.
smission of the subsequent packet may begin increases
of this situation is pre-which direct their transfe
The node N1 transmits data to the node N0 located in
its range. The transmission is not „heard” by the node N2,
which is a hidden station for N1. However, the range of
N2 includes the N0 node. The hidden station does not
know about the transaction being realized. After receiv-
ing a transmission order, the node shall verify the chan-
nel occupation and start the transmission. Irrespective of
the physical loc
only 23% of the effective channel transmission rate.
Moreover, the network works with the efficiency close to
maximal only in certain, relatively narrow interval of the
intensity of the offered traffic. A further increase of the
number of requests results in an important worsening of
the quality of their s
e shape of obtained characteristics corresponds to the
panic curve, defining the operation of many systems with
collision access.
N0 node. In the area occupied by N0 a collision
es will occur and at least one (the first)
Figure 17. Unidirectional transmission in a system contain-
ing hidden stations – average collision probability.
Figure 18. Unidirectional transmission in a system contain-
ing hidden stations – coefficient of delivered packets.
igure 19. Bidirectional transmission in a system containing
idden stations – intensity of the operated traffic.
The reason of the decrease of the network transmis-
sion rate when the intensity of the offered traffic exceeds
of a given threshold value is the increase of the channel
occupation time, favorable to the appearance of colli-
sions with the hidden stations. The retransmissions acti-
vated by both nodes increase in an artificial way the in-
tensity of requests directed towards the system, leading
to its overloading. It is worth to mention that in a con-
gestion conditions the transmission rate of a non-syn-
chronized network decreases to zero, whereas a synchro-
nized system always guarantees a certain minimal level
of the servicing of transmission requests. Such an ad-
vantage is a side effect of the algorithm realized by the
node of the LR-WPAN network, verifying before the
start of each transaction, if its duration shall not exceed
ts the
verage collision probability, obtained for both network
operation modes (non-synchronized and synchronized).
The defined characteristics of the coefficient of deliv-
ered packets indicate that the loss of frames appears even
with a very little intensity of the offered traffic (Figure
18). Their reason is the cancellation of further retrans-
missions of these packets, not delivered with a pre-de-
fined admissible number of attempts. As the intensity of
the requests increases, this phenomenon appears more
and more often. In an overloaded system, the queues of
the single emitters become overfilled and a more signifi-
cant part of the offered traffic is refused.
The objective of the successive series of tests con-
sisted in verifying the influence of the hidden station for
the node located in the range of its signal. In a system
presented in Figure 15 this function is assumed by te
coordinator. We should remind that the transactions f
the limits of the finishing superframe. Thanks to that, the
hidden station rarely disturbs the last transmission that
can be fit into the superframe. The Figure 17 presen
the coordinator are initialized by other nodes of the clus-
ter, strongly influenced by the presence of the hidden
station. Based on this we can presume that the hidden
station shall also disturb the servicing of requests di-
rected towards the coordinator.
The diagrams presented in Figure 19 have been ob-
tained using the model given in Figure 15, in which the
offered traffic has been evenly divided between all the
nodes. Contrary to the assumptions, the presence of the
hidden station has only a limited influence for the trans-
actions realized by the coordinator. Moreover, the inten-
sity of traffic realized by this station is not suddenly de-
creased when the threshold value is exceeded, such as it
was the case for the other nodes.
The differences existing in the way of servicing the
transactions realized in each direction are connected with
the length of initiating frames. A transaction directed to
the coordinated starts with a long DATA packet, whereas
the transfer in the other direction is initiated with a much
shorter REQUEST frame (Figure 3 and Figure 6 vs. Fig-
ure 4 and Figure 7). Therefore, in the second case the
Copyright © 2010 SciRes. WSN
Copyright © 2010 SciRes. WSN
by hidden sta-
in system with bidirec-
ission rate of
ed in these conditions
probability of a collision caused by the hidden station imoment, only one of two independent channels can be
used—it signifies, that a half of the transm
uch lower. Moreover, if a collision appears, its duration
shall also be shorter, reducing its influence for the chan-
nel transmission rate. The frames ACK and DATA initi-
ated by the coordinator are received by all the nodes of
the cluster, so the hidden stations have not any influence
for the further part of the transaction. The transmission
directed to the slave node is similar to a transaction con-
cerning reservation of channels with RTS and CTS
frames, used in IEEE802.11 standard and protecting
WLAN network against problems created
ch channel is lost.
Studying the effects of an exposed station we have
used the model presented in Figure 22. The intensity of
the offered traffic is evenly divided between nodes N1
and N3. The characteristics obtain
Irrespective of the status of the system, the transmis-
sion realized by the coordinator make that when the
threshold value of the intensity of offered traffic is ex-
ceeded, the coefficient of delivered packets does not de-
crease to zero, as it was the case previously (Figure 20).
Its value gradually decreases, because the overfilling of
the buffer in the coordinator’s emitter results in the re-
fusal of an increasing number of requests.
Average collision probability
nal transmission (Figure 21) is very similar to the one
presented on Figure 17. The values of obtained points are
quit identical to results of simulation tests of the system
with unidirectional transmission.
3.3. Effect of the Exposed Station
The problem of the exposed station appears in a LR-WP-
AN network containing many ne
Figure 20. Bidirectional transmission in a system containing
hidden stations – coefficient of delivered packets.
ighboring clusters. In a
certain sense, this phenomenon is contrary to the above-
mentioned effect of a hidden station. In this case, nodes
belonging to different clusters and located in the range of
their respective signals disturb the reciprocal effective
utilization of independent, because physically separated
transmission channels. An exemplary fragment of a syst-
em containing exposed stations is presented in Figure 22.
When the N1 node starts a transaction directed to its
coordinator N0, the signal arrives also to the N3 station.
When N3 receives in the same time a transmission re-
quest, after verifying the status of the own channel it will
incorrectly conclude that the channel is occupied and the
start of the transmission will be unnecessary delayed. An
identical problem concerns the N1 node in the case if the
first transmission is started by the N3 device. In a given
Figure 21. Bidirectional transmission in a system containing
hidden stations – average collision probability.
Figure 22. Example of a system containing exposed stations.
Figure 23. Unidirectional transmission in a system contain-
ing exposed stations – intensity of the operated traffic.
Figure 24. Unidirectional transmission in a system contain-
ing exposed stations – coefficient of delivered packets.
are summarized in Figure 23.
The obtained characteristics, as it concerns their shape
and values, are very similar to those observed for the
smission rate of both clusters is slightly higher than
e effective transmission rate of a single channel. The
coefficients of delivered packets are also slightly higher,
thanks to a double capacity of the buffers of both nodes
(Figure 24) and the average transmission delay time is
little longer (Figure 25). Therefore, the presence of ex-
posed stations permits to use only the half of transmis-
sion resources of each cluster. Nevertheless we should
mention that—as opposed to the hidden station—the
presence of an exposed node does not result in a com-
plete stopping of the transmission in an overloaded sys-
tem (see Figure 16 and Figure 18).
system consisting of two nodes and realizing the trans-
mission towards the coordinator (see Figure 9). The
Figure 25. Unidirectional transmission in a system contain-
ing exposed stations – average transmission delay time.
4. Summary
The main objective of the authors of the IEEE802.15.4
standard was to create a system that could contain an
enormous number of nodes (even 264) and in the same
protocol very simple to im-
plement, guaranteeing minimal energy consumption. The
fulfilling of all the abovementioned assumptions proves
to be very difficult and—as the realized studies has
shown—leads to an important decrease of the available
transmission rate of the transmission channel. Important
problems result also of the presence of a hidden station.
One of several considered possibilities is the implemen-
tation of a reservation of the channel by RTS and CTS
frames, as it was made in the case of the IEEE802.11
standard [6,7]. Nevertheless, such a solution presents
also a significant disadvantage, consisting in an impor-
tant increase of the duration of different transactions, by
definition used for transmitting little portions of data and
using minimal energy. The reservation of the channel
becomes particularly difficult in non-synchronized s-
ng a
tarting transmission.
standard shall be subject
time using a transmission
tems, in which in any case the temporarily inactive hid-
den stations will not receive information concerni
Certainly, the IEEE802.15.4
to further modifications. We shall presume that its cur-
rent version shall be supplemented with mechanisms
preventing the effects of the presence of hidden stations,
exposed stations and hidden contention, related to the
reciprocal movement of the nodes. However, we should
answer the question if the implementation of these modi-
fications is really necessary. A further development of
the transmission protocol can lead to the loss of the idea
of a simple and cheap system, using low transmission
Copyright © 2010 SciRes. WSN
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opyright © 2010 SciRes. WSN