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Abstract-The contribution of this paper is on the study of packet delays for the IEEE 802.11 wireless local area network DCF MAC protocol. A method is presented capable of taking into account retransmission delays with or without retry limits. We present an analytical model based on a Markov chain which allows us to derive closed form expressions for the packet delays, the probability of a packet being discarded when it reaches the maximum retransmission limit and the average time to drop such a packet for the basic and RTS/CTS access mechanisms. The results presented are for standard protocol parameters versus the number of contention stations. Finally, the accuracy of the analytical model is verified by simulations.

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IEEE 802.11 is worldwide established and the most used protocol for Wireless Local Area Networks (WLANs). In this paper, we propose an improved analytical model that calculates IEEE 802.11 DCF performance taking into account both packet retry limits and transmission errors for the IEEE 802.11a protocol. Validation of our new performance model analytical results is carried out by comparison with simulation results using the OPNET TM simulation package. We explore the effect of transmission errors, packet retry limits, data rate and network size on the performance of the basic access scheme, in terms of throughput, packet delay, packet drop time and drop probability.

This article presents an overview of optical wireless (OW) communication systems that operate both in the short-(personal and indoor systems) and the long-range (outdoor and hybrid) regimes. Each of these areas is discussed in terms of (a) key requirements, (b) their application framework, (c) major impairments and applicable mitigation techniques, and (d) current and/or future trends. Personal communication systems are discussed within the context of point-to-point ultra-high speed data transfer. The most relevant application framework and related standards are presented, including the next generation Giga-IR standard that extends personal communication speeds to over 1 Gb/s. As far as indoor systems are concerned, emphasis is given on modeling the dispersive nature of indoor OW channels, on the limitations that dispersion imposes on user mobility and dispersion mitigation techniques. Visible light communication systems, which provide both illumination and communication over visible or hybrid visible/ infrared LEDs, are presented as the most important representative of future indoor OW systems. The discussion on outdoor systems focuses on the impact of atmospheric effects on the optical channel and associated mitigation techniques that extend the realizable link lengths and transfer rates. Currently, outdoor OW is commercially available at 10 Gb/s Ethernet speeds for Metro networks and Local-Area-Network interconnections and speeds are expected to increase as faster and more reliable optical components become available. This article concludes with hybrid optical wireless/radio-frequency (OW/RF) systems that employ an additional RF link to improve the overall system reliability. Emphasis is given on cooperation techniques between the reliable RF subsystem and the broadband OW system.

Wireless technologies in the LAN environment are becoming increasingly important. The IEEE 802.11 standard is the most matnre technology for Wireless Local Area Networks (WLA"). The performance Of the Control (MAC) layer, which consists of Distributed Coordination Fuuetiou @cF) and Point (pcF), hasbeen examined Over the past yeprs, In this pPper, we present an analytical model to compute the saturated throughput of 802.11 protocol in the ahseoee of hidden stations and trans-ioo errom. A throughput analysis is carried out io order to study the performance of 802.11 DCF. Using the analytical model, we develop a frame delay analysis under tnfic conditions that hidden station problem occurs when a station is causing interference due to not been able to detect the existence of a transmission kom another station and thus that fie medium is free and available to transmit. As an example, lets assume that stations A and B are within communication range of each other and station C is within communication range of but not Of A. Therefore, it is possible that both Stations A and C could try to transmit to station B at.the same time causing a collision. The influence of hidden stations [4] on the performance of an IEEE 802.11 network has been shldied in Is1 --L.1 correspond to the maximum load that the network -0 SuppOrl in stable Conditions. The behaviour of the exponential backoff algorithm used in 802.11 is also examined. The performance of CSMA protocols over radio was investigated in 161, ~h~ M~C~W protocol was desiened to imorove wireless communication oerformance I. IWTRODUCTION Recent advances in wireless technology have equipped portable devices with wireless capabilities that allow networked communication even while a user is mobile. These devices include palmtop computers, personal digital assistants (PDAs), portable computers, digital cameras and printers. To deal with this wireless connectivity need, various wireless communication standards have been developed [I]. Two major projects have been involved in standardizing the physical and the medium access control (MAC) layers for wireless LANs, namely IEEE 802.1 1 [2] and ETSI HiperLAN [3]. This paper focuses on the analysis of the MAC protocol of the IEEE 802.1 1 protocol which is the most widely used WAN protocol today.The IEEE 802.11 standard for wireless networks incorporates two medium access methods. The mandatory Distributed Coordination Function (DCF) method and the optional Point Coordination Function (PCF) which provides Time Bounded Services (TBS). DCF is an asynchronous data , transmission function, which best suits delay insensitive data (e.g. email, Ap). It is available in ad-hoc or Mastmcture network configurations and can be either used exclusively or combined with PCF in an hhstiucture network. PCF, on the other hand, best suits delay sensitive data t"iS sions (e.g. real-time audio or video) and is only available in infrastructure environments. A common problem in wireless LAN systems is hidden stations. The presence of hidden stations may result i...

A new performance model of the IEEE 802.11 protocol is introduced that calculates throughput, average packet delay, packet drop probability and average packet drop time. This model is validated with simulation results using the OPNET TM simulation package. The proposed model predicts 802.11 protocol performance very accurately since it considers both transmission errors and packet retry limits. The effect of errors and network size on the performance of the basic access scheme, in terms of throughput, packet delay, packet drop time and drop probability is explored.Introduction: The IEEE 802.11 protocol [1] is the dominating standard for Wireless Local Area Networks (WLANs) and employs distributed co-ordination function (DCF) as the essential medium access control method. In DCF, a station transmits if the medium is idle. If the medium is sensed busy, the station waits until the current transmission is over. The station then defers for a randomly selected time interval to minimise collisions before transmitting. Each station maintains a retry count that indicates the number of retransmission attempts of a data packet. If the retry count reaches the specified limit, the packet is dropped.Since the release of the IEEE 802.11 standard many research efforts have been conducted on modelling IEEE 802.11 DCF. Bianchi in [2] uses a Markov chain to model the idealistic assumption that packet retransmissions are unlimited and a packet is being transmitted continuously until its successful reception. Wu et al. in [3] extends Bianchi's analysis to include the finite packet retry limits as defined in the IEEE 802.11 standard.In this Letter we introduce a mathematical model which extends the approach in [3] by taking into account both channel bit errors and packet retry limits for the basic access scheme. Our new performance model allows the calculation of the throughput efficiency, the average packet delay, the packet drop probability and the average time to drop a packet for the IEEE 802.11 protocol. Using OPNET simulation results, we validate our mathematical model and show that our model predicts DCF performance more accurately than Bianchi's model [2] that does not consider packet retry limits. Analytical results utilising our proposed model study the dependence of the protocol performance on the bit error rate (BER) and the network size.

A new packet delay analysis for the IEEE 802.11b protocol is developed by extending throughput analysis introduced by Bianchi. This analysis is validated by comparison with simulation results using the OPNET TM simulation package. Packet delay results are presented as a function of a number of stations and packet size for basic access and RTS=CTS medium access mechanisms. The proposed analysis identifies when the RTS=CTS mechanism achieves lower packet delay with respect to the basic access mechanism and becomes highly beneficial for the performance of IEEE 802.11 WLANs.