november 2010 volume 1 number 2 - Advances in Electronics and ...

10 ADVANCES IN ELECTRONICS AND TELECOMMUNICATIONS, VOL. 1, NO. 2, NOVEMBER 2010
Fig. 5. Bidirectional transmission in a system containing hidden stations: a)
intensity of the operated traffic, b) coefficient of delivered packets
Fig. 6. System with exposed stations
collision appears, its duration will also be shorter, reducing its
influence on the channel transmission rate. The frames ACK
and DATA initiated by the coordinator are received by all
the nodes of the cluster, so the hidden stations have not any
influence on further part of the transaction. The transmission
directedtotheslave nodeis similarto atransactionconcerning
the reservation of channels with RTS and CTS frames, used in
IEEE 802.11 standard, and protecting WLAN network against
problems created by the hidden stations.
Irrespective of the status of the system, when the threshold
value of the intensity of offered traffic is exceeded, due to
the transmission realized by the coordinator, the coefficient of
delivered packets does not decrease to zero, as it was in the
previous case (Fig. 5.b). Its value gradually decreases because
the overfilling of the buffer in the coordinator’s emitter results
in the refusal of an increasing number of requests.
Fig. 7. Unidirectional transmission in a system containing exposed stations
C. Effect of the exposed station
Studyingtheeffectsoftheexposedstation,wehaveusedthe
modelpresentedin Fig. 6.Theintensity ofthe offeredtraffic is
evenly divided between nodes N1 and N3. The characteristics
obtained in these conditions are summarized in Fig. 7.
The obtained characteristics, as it concerns their shape and
values, are very similar to those observed for the system
consisting of two nodesand realizing the transmission towards
the coordinator(see Fig. 1). The total transmissionrate of both
clusters is slightly higher than the 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. Therefore, the presence of exposed stations
permits only a half of transmission resources of each cluster
to be used.
III. CONCLUSION
The main objective of the authors of the IEEE 802.15.4
standardwastocreateasystemthatcouldcontainanenormous
number of nodes (even 2 64 ) and at the same time using a
transmission protocol very simple to implement, guaranteeing
minimal energy consumption. The fulfilling of all the abovementioned
assumptionsprovesto be very difficult and – as the
realized studies have shown – leads to an important decrease
in the available transmission rate of the transmission channel.
Important problems result also from the presence of a hidden
station and exposed station.
REFERENCES
[1] A. Kouba, M. Alves, and Tovar, “A comprehensive simulation study of
slotted CSMA/CA for IEEE 802.15.4 wireless sensor networks,” [online],
IPPHURRAY Research Group, Polytechnic Institute of Porto, http://
www.iis.sinica.edu.tw/cclljj/publication/2006/06_WCNC_802.15.4.pdf.
[2] T. Sun, C. Ling-Jyh, H. Chih-Chieh, G. Yang, and M. Gerla, “Measuring
effective capacity of IEEE 802.15.4 beaconless mode,” in IEEE Wireless
Communications and Networking Conference, WCNC 2006, Las Vegas,
Apr. 2006, pp. 493–498.
[3] A. Herms, G. Lukas, and S. Ivanov, “Realism in design and evaluation
of wireless routing protocols,” in Proceedings of First international
Workshop on Mobile Services and Personalized Environments MSPE‘06,
2006.