86 research outputs found
Specification and Analysis of the DCF Protocol in the 802.11 Standard using Systems of Communicating Machines
The 802.11 specification is an emerging standard for WLANs. In this paper,
we propose a formal model for a section of the 802.11 MAC protocol using
systems
of communicating machines. We model the ad-hoc mode of the DCF, i.e.,
CSMA/CA protocol
and the MACA\footnote{The 802.11 standard does not refer to RTS/CTS
exchanges as MACA. However,
the paper which introduced this idea called it the MACA, and we use this
name.} using RTS/CTS sequences.
Each station is modelled as a finite state machine which has a set of
local variables, and the Wireless Medium
is modelled as a shared variable. Analyses show that the 802.11 MAC CSMA/CA protocol and
the MACA using RTS/CTS exchanges are free from state deadlocks and
non-executable transitions.
However, the MACA protocol has a potential livelock,
though it is unlikely it will come to pass in normal operation.
(Also UMIACS-TR-2002-37
Investigation of quality of services (QoS) support for real-time or mission critical services over IEEE 802.11e wireless networks.
Multimedia application is currently making much impact in this technological era. It has been thekey driving force behind the convergence of fixed, mobile and IP networks. Furthermore, real-timeapplications are making head way in vehicular networks, mission critical applications which usededicated short range communications (DSRC). 802.l i e standards support quality of services(QoS) guarantees in these applications. This is opposed to the problem with 802.11 legacy whichis based on distributed coordination function (DCF), and its inability to prioritized applications forservice differentiation. Simulation was done on various 802.l i e networks which use enhancedDCF (EDCF). In these simulations, it was observed that controlling low priority applicationsenhances the effectiveness of high priority applications. Different MAC and traffic generationparameters were used in various scenarios. It was actually observed that high priority applicationshave greater impacts on the performance of the network and hence performs better when itcomes to delay and throughput requirements. Even when the number of high priority applicationswere reduced, the results obtained was still able to satisfy QoS requirements for each traffic type.Results for different scenarios were taken and discussed. Also, differentiated values of delay,throughput and packet loss were recorded when same and different values of MAC and trafficgeneration parameters were used. In all results the International Telecommunications Union (ITU-T) values of these metrics parameters were kept low. These make the network design suitable forroad safety application where very low delay is required for emergency messages and tolerabledelay in routine messages. The results obtained show th at, this network can be applicable inroad safety, simply because of the low delay, and low loss which implies , messages to cars canbe successfully delivered and also good throughput. 802.11 legacy standard lacks servicedifferentiation that limits QoS support for real-time applications. These simulations were able tohandle the drawback associated with this standard and prefer a better standard which is 802.l i ethat provides differentiated access to the metrics that was used in analyzing QoS in this research
Performance modelling and enhancement of wireless communication protocols
In recent years, Wireless Local Area Networks(WLANs) play a key role in the data communications and networking areas, having witnessed significant research and development. WLANs are extremely popular being almost everywhere including business,office and home deployments.In order to deal with the modem Wireless connectivity needs,the Institute of Electrical and Electronics Engineers(IEEE) has
developed the 802.11 standard family utilizing mainly radio transmission techniques, whereas the Infrared Data Association (IrDA) addressed the requirement for multipoint
connectivity with the development of the Advanced Infrared(Alr) protocol stack. This work studies the collision avoidance procedures of the IEEE 802.11 Distributed
Coordination Function (DCF) protocol and suggests certain protocol enhancements aiming at maximising performance. A new, elegant and accurate analysis based on Markov chain modelling is developed for the idealistic assumption of unlimited packet retransmissions as well as for the case of finite packet retry limits. Simple equations are derived for the through put efficiency, the average packet delay, the probability of a packet being discarded when it reaches the maximum retransmission limit, the average time to drop such a packet and the packet inter-arrival time for both basic access and RTS/CTS medium access schemes.The accuracy of the mathematical model is validated by comparing analytical with OPNET simulation results. An extensive and
detailed study is carried out on the influence of performance of physical layer, data rate, packet payload size and several backoff parameters for both medium access
mechanisms. The previous mathematical model is extended to take into account transmission errors that can occur either independently with fixed Bit Error Rate(BER) or in bursts. The dependency of the protocol performance on BER and other factors related to independent and burst transmission errors is explored. Furthermore, a simple-implement appropriate tuning of the back off algorithm for maximizing IEEE 802-11 protocol performance is proposed depending on the specific communication requirements. The effectiveness of the RTS/CTS scheme in reducing collision duration at high data rates is studied and an all-purpose expression for the optimal use of the RTS/CTS reservation scheme is derived. Moreover, an easy-to-implement backoff algorithm that significantly enhances performance is introduced and an alternative derivation is developed based on elementary conditional probability arguments rather than bi-dimensional Markov chains. Finally, an additional performance improvement scheme is proposed by employing packet bursting in order to reduce overhead costs such as contention time and RTS/CTSex changes. Fairness is explored in short-time and long-time scales for both the legacy DCF and packet bursting cases. AIr protocol employs the RTS/CTS medium reservation scheme to cope with hidden stations and CSMA/CA techniques with linear contention window (CW) adjustment for medium access. A 1-dimensional Markov chain model is constructed instead of the bi-dimensional model in order to obtain simple mathematical equations of the average packet delay.This new approach greatly simplifies previous analyses and can be applied to any CSMA/CA protocol.The derived mathematical model is validated by comparing analytical with simulation results and an extensive Alr packet delay evaluation is carried out by taking into account all the factors and parameters that affect protocol performance. Finally, suitable values for both backoff and protocol parameters are proposed that reduce average packet delay and, thus, maximize performance
IEEE 802.11 Wireless LAN Security Performance Using Multiple Clients
IEEE 802.11 Wireless Networks have gained popularity, providing users mobility and
flexibility in accessing information. Existing solutions for wireless LAN networks have been
exposed to security vulnerabilities. Previous study has evaluated the security performance of
IEEE 802.11 wireless networks using single server-client architecture. This research
investigated the effect of multiple security mechanisms on the performance of multi-client
congested and un-congested networks. The effect of different TCP and UDP packet sizes on
performance of secure networks was also studied. The results showed that WEP encryption
significantly degrades the performance of congested wireless networks. Network performance
degradation increased as the number of clients was increased under all security mechanisms
Optimization of the interoperability and dynamic spectrum management in mobile communications systems beyond 3G
The future wireless ecosystem will heterogeneously integrate a number of overlapped Radio
Access Technologies (RATs) through a common platform. A major challenge arising from the
heterogeneous network is the Radio Resource Management (RRM) strategy. A Common RRM
(CRRM) module is needed in order to provide a step toward network convergence. This work
aims at implementing HSDPA and IEEE 802.11e CRRM evaluation tools.
Innovative enhancements to IEEE 802.11e have been pursued on the application of cross-layer
signaling to improve Quality of Service (QoS) delivery, and provide more efficient usage of
radio resources by adapting such parameters as arbitrary interframe spacing, a differentiated
backoff procedure and transmission opportunities, as well as acknowledgment policies (where
the most advised block size was found to be 12). Besides, the proposed cross-layer algorithm
dynamically changes the size of the Arbitration Interframe Space (AIFS) and the Contention
Window (CW) duration according to a periodically obtained fairness measure based on the Signal
to Interference-plus-Noise Ratio (SINR) and transmission time, a delay constraint and the
collision rate of a given machine. The throughput was increased in 2 Mb/s for all the values of
the load that have been tested whilst satisfying more users than with the original standard. For
the ad hoc mode an analytical model was proposed that allows for investigating collision free
communications in a distributed environment.
The addition of extra frequency spectrum bands and an integrated CRRM that enables spectrum
aggregation was also addressed. RAT selection algorithms allow for determining the gains obtained
by using WiFi as a backup network for HSDPA. The proposed RAT selection algorithm
is based on the load of each system, without the need for a complex management system. Simulation
results show that, in such scenario, for high system loads, exploiting localization while
applying load suitability optimization based algorithm, can provide a marginal gain of up to
450 kb/s in the goodput. HSDPA was also studied in the context of cognitive radio, by considering
two co-located BSs operating at different frequencies (in the 2 and 5 GHz bands) in the
same cell. The system automatically chooses the frequency to serve each user with an optimal
General Multi-Band Scheduling (GMBS) algorithm. It was shown that enabling the access to
a secondary band, by using the proposed Integrated CRRM (iCRRM), an almost constant gain
near 30 % was obtained in the throughput with the proposed optimal solution, compared to a
system where users are first allocated in one of the two bands and later not able to handover
between the bands. In this context, future cognitive radio scenarios where IEEE 802.11e ad hoc
modes will be essential for giving access to the mobile users have been proposed
Low power radio networks
Low power radio networks are the networks which depend upon wireless radio links and consume very low energy for their operation. These networks suit best for applications where frequent renewal of power supply is not possible. Power supply has always remained a major concern in radio networks. An efficient low power consuming network is always recommended for greater mobility and lifetime of the network. This thesis introduces low power radio networks, their features and applications. Energy concerns and various techniques that can be used for energy conservation are discussed, along with the security techniques that can be used to make the system reliable. Different technologies available in the market and their features and applications are considered. Included is a detailed study of the IEEE 802.15.4 standard. A simulation study of the CSMA/CA algorithm and topology discovery algorithms is presented
IMPROVING QoS OF VoWLAN VIA CROSS-LAYER BASED ADAPTIVE APPROACH
Voice over Internet Protocol (VoIP) is a technology that allows the transmission of
voice packets over Internet Protocol (IP). Recently, the integration of VoIP and
Wireless Local Area Network (WLAN), and known as Voice over WLAN
(VoWLAN), has become popular driven by the mobility requirements ofusers, as
well as by factor of its tangible cost effectiveness. However, WLAN network
architecture was primarily designed to support the transmission of data, and not for
voice traffic, which makes it lack ofproviding the stringent Quality ofService (QoS)
for VoIP applications. On the other hand, WLAN operates based on IEEE 802.11
standards that support Link Adaptive (LA) technique. However, LA leads to having a
network with multi-rate transmissions that causes network bandwidth variation, which
hence degrades the voice quality. Therefore, it is important to develop an algorithm
that would be able to overcome the negative effect of the multi-rate issue on VoIP
quality. Hence, the main goal ofthis research work is to develop an agent that utilizes
IP protocols by applying a Cross-Layering approach to eliminate the above-mentioned
negative effect. This could be expected from the interaction between Medium Access
Control (MAC) layer and Application layer, where the proposed agent adapts the
voice packet size at the Application layer according to the change of MAC
transmission data rate to avoid network congestion from happening. The agent also
monitors the quality of conversations from the periodically generated Real Time
Control Protocol (RTCP) reports. If voice quality degradation is detected, then the
agent performs further rate adaptation to improve the quality. The agent performance
has been evaluated by carrying out an extensive series ofsimulation using OPNET
Modeler. The obtained results of different performance parameters are presented,
comparing the performance ofVoWLAN that used the proposed agent to that ofthe
standard network without agent. The results ofall measured quality parameters hav
SNR-Based OLSR Routing Protocol for Wireless Mesh Networks
Wireless Mesh Networks (WMNs) consist of a collection of mobile and fixed nodes that
form a network. Nodes are capable of communicating with each other either with
infrastructure, or infrastructureless, or in a hybrid mode. The major advantages of WMNs
over the other wireless networks are the low-cost, self organization, self configuration,
last mile internet solution, scalability, and reliability. These advantages have attracted the
researcher over the last five years. WMNs technology is gaining an increased attention
from the Institute of Electrical and Electronics Engineers (IEEE) community. This led the
IEEE organization to emerge a special working group (IEEE 802.11s) in charge of the
issues deriving from a completely wireless distribution system used to interconnect
different Basic Service Sets (BSSs) through secure and performing links.
In a multi-hop networks, like WMN, one of the main factors that influences the
performance is the routing protocol. Generally speaking, routing protocols can be
classified based-on the routing metric to 1) hop count-based routing protocols, like Adhoc
on demand distance vector (AODV) where the optimum path is defined as the path
that goes through the minimum number of nodes, 2) the link quality-based routing
protocols, like OLSR where some metrics such as the bandwidth and the packet error rate
are considered to define the optimum path to the destination.
In this work the performances of a three commonly used routing protocols are compared.
The main goal of this stag is to study the influence of different routing protocols in
WMNs. The comparison is conducted with two scenarios of networks; a high mobility network and a low mobility network. (Open network) OPNET 11.5 modeler is used to
build the WMNs. The performance of the network and the routing protocols has been
studied in means of network throughput, End-to-End delay, routing protocol overhead
and the mobility. The obtained results show that the Optimized link state routing protocol
(OLSR) has the highestthroughput overDSR andAODVrouting protocols in WMNs.
The unpredictable behavior of the wireless medium in WMNs environment demands the
need for a routing protocol that is aware of the link conditions. Unfortunately the routing
protocols used such as AODV and Dynamic source routing (DSR) are hop count-based;
where the routing algorithm uses the number of nodes to determine the optimum path to
the destination.
In the second stage of this work a new routing technique for WMNs based-on Signal to
noise ratio (SNR) as a new metric for OLSR routing protocol, is developed. The new
metric has been implemented on the OLSR routing protocol module using OPNET
simulator. The modified OLSR routing protocol is implemented in the comparison
scenarios. The obtained results show that, when SNR is used as a routing metric in the
OLSR routing protocol, the OLSR is getting the significantly higher network throughput
over the DSR and AODV routing protocols. In the same time, the modified OLSR
implemented with the SNR metric is showing a high improvement over the OLSR with
the traditional hop-count metric. This thesis also studies the affect of different amounts of
mobility in WMNs performance.
VI
Smart Metering Technology and Services
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