In the paper, an improved link adaptation scheme is proposed for the WiMedia MB-OFDM UWB system, in which quality of service (QoS) support is provided. The proposed scheme consists of three functional blocks: link quality indicator (LQI) calculator, frame error rate (FER) estimator, and transmitter (TX) parameter selector. Instead of using the average receive SNR (ASNR) as LQI, a new LQI metric is defined based on a union bound analysis to capture the effects of both path loss and frequency selectivity of an instantaneous UWB channel. How to calculate LQI for each rate mode is investigated by analyzing the distribution of soft bit information. With the calculated LQI, the FER performance of each rate mode can be accurately estimated with a look-up table method, which is suitable for practical implementation. Using the estimated FERs, TX parameter selector can optimize TX rate mode to improve the communication throughput under QoS constraints. It is shown the proposed scheme can significantly improve the throughput while maintaining the required QoS compared to conventional ASNR based link adaptation schemes.

This paper presents a theoretical study of the throughput of mobile content distribution (MCD) in vehicular ad hoc networks (VANETs). Since VANET is well-known for its fast-changing topology and adverse wireless channel environments, various protocols have been proposed in the literature to enhance the performance of MCD in a vehicular environment, using packet-level network coding (PLNC) and symbol-level network coding (SLNC). However, there still lacks a fundamental understanding of the limits of MCD protocols using network coding in VANETs. In this paper, we develop a theoretical model to compute the achievable throughput of cooperative MCD in VANETs using SLNC. By considering a one-dimensional road topology with an access point (AP) as the content source, the expected achievable throughput for a vehicle at a certain distance from the AP is derived, for both using PLNC and SLNC. Our proposed model is unique since it captures the effects of multiple practical factors, including vehicle distribution and mobility pattern, channel fading and packet collisions. Through numerical results, we provide insights on optimized design choices for network coding-based cooperative MCD systems in VANETs.

The task of estimating path available bandwidth is difficult but paramount for QoS routing in supporting bandwidth-demanding traffic in multirate and multihop wireless ad hoc networks. The multirate capability and the impact of background traffic has not been carefully studied for the problem of estimating path available bandwidth in prior works. In this paper, we develop a theoretical model for estimating the available bandwidth of a path by considering interference from both background traffic and traffic along the path. We show that the clique constraint widely used to construct upper bounds does not hold any more when links are allowed to use different rates at different time. In our proposed model, traditional clique is coupled with rate vector to more properly characterize the conflicting relationships among links in wireless ad hoc networks where time-varying link adaption is used. Based on this model, we also investigate the problem of joint optimization of QoS routing and link scheduling. Several routing metrics and a heuristic algorithm are proposed. The newly proposed conservative clique constraint performs the best among the studied metrics in estimating available bandwidth of flows with background traffic.

In this paper, we derive a theoretical model to calculate the available bandwidth of a path and study its upper and lower bounds with background traffic. We show that the clique constraint widely used to construct upper bounds does not hold any more when links are allowed to use different rates at different time. In our proposed model, traditional clique is coupled with rate vector to more properly characterize the conflicting relationships among links in wireless sensor networks where time-varying link adaption is used. Based on the model, we also investigate the problem of joint optimization of QoS routing and propose several routing metrics. The newly proposed conservative clique constraint performs the best among the studied metrics in estimating available bandwidth of flows with background traffic.

Multi-channel multi-radio technology offers a great space of resource diversity. In this paper, we propose an opportunistic multiradio MAC (OMMAC) protocol to improve system performance by utilizing multiradio diversity. The proposed OMMAC collects the physical layer feedback over multiple radios simultaneously and schedules multiple transmissions on the available channels accordingly. The channel based packet scheduling leverages the channel quality information to jointly select data rates, channels and packets to increase the overall spectral usage of multiple radios which have not been exploited in previous literature. Extensive results from both analysis and ns2 simulations demonstrate that OMMAC significantly improves the network throughput in both single- and multi-hop wireless multirate networks in terms of both aggregate and per-radio throughput.

Routing in multi-hop wireless networks presents a great challenge mainly due to unreliable wireless links and interference among concurrent transmissions. Recently, a new routing paradigm, opportunistic routing (OR), is proposed to cope with the unreliable transmissions by exploiting the broadcast nature and spatial diversity of the wireless medium. Previous studies on OR focused on networks with a single channel rate. The performance of OR in a multi-rate scenario is not carefully studied. In addition, although simulation and practical implementation have shown that OR achieves better throughput performance than that of traditional routing, there is no theoretical results on capacity enhancement provided by OR or network capacity bounds of OR. In this paper, we bridge these gaps by carrying out a comprehensive study on the impacts of multiple rates, interference, candidate selection and prioritization on the maximum end-to-end throughput or capacity of OR. Taking into consideration of wireless interference, we propose a new method of constructing transmission conflict graphs - we propose transmitter based conflict graph in contrast to link conflict graph. Then, we introduce the concept of concurrent transmitter sets to represent the constraints imposed by the transmission conflicts of OR, and formulate the maximum end-to-end throughput problem as a maximum-flow linear programming problem subject to the transmission conflict constraints. We also propose a rate selection scheme, and compare the throughput capacity of multi- rate OR with single-rate ones. We validate the analysis results by simulation, and show that OR has great potential to improve end- to-end throughput and system operating at multi-rates achieves higher throughput than that operating at any single rate.

Broadcast scheduling is a fundamental problem in wireless ad hoc networks. The objective of a broadcast schedule is to deliver a message from a given source to all other nodes in a minimum amount of time. At the same time, in order for the broadcast to proceed as predicted in the schedule, it must not contain parallel transmissions which can be conflicting based on the collision and interference parameters in the wireless network. Most existing work on this problem use a limited network model which accounts only for conflicts occurring inside the transmission ranges of the nodes. The broadcast schedules produced by these algorithms are likely to experience unpredictable delays when deployed in the network. This is because they do not take into consideration other important sources of conflict in parallel transmissions, namely the interference range and the carrier sensing range. In this paper we develop a conflict-aware network model, which uses these parameters to increase the probability of scheduling conflict-free transmissions, and thereby improve the reliability of the broadcast schedule. We present and prove correctness of a constant approximation algorithm for minimum-latency broadcast scheduling under this network model. We also present a greedy heuristic algorithm for the same problem. Experimental results are provided to evaluate the performance of our algorithms. In addition, the algorithms are analyzed to justify their performance trends.

Opportunistic scheduling has been known as one of the possible ways of exploiting multi-user diversity, which comes from the time-varying propagation characteristics and broadcast nature of wireless radios, to improve communication efficiency and reliability. Multi-channel multi-radio technology has offered great opportunity in designing wireless network MAC protocol to utilize all kinds of resource diversities. Other than multi-user diversity, we identify the unique multi-radio diversity which can be exploited in multi-radio wireless network to improve network throughput performance. The proposed opportunistic multi-radio MAC (OMMAC) takes advantage of multi-radio diversity to increase the spectrum usage as much as possible. Extensive NS2 simulation results demonstrate that OMMAC significantly improves the network throughput in both single and multi-hop wireless networks.

Streaming and multimedia services are key applications in UWB-based high-rate short-range wireless personal area networks (WPANs). In this paper, we study how to support the quality of service (QoS) requirement of these services over the distributed TDMA-based WiMedia UWB MAC protocol. We develop an integer-linear programming model to solve the path available bandwidth problem. To reduce the computation complexity, a lower bound, an upper bound and an estimation of the path available bandwidth are derived. They are also used as routing metrics to develop a novel QoS routing algorithm to find paths with high available bandwidth. The performance results demonstrate that the lower bound is close to the upper bound and they are even equal to each other in most cases. When used as routing metrics, they can find paths with larger available bandwidth and accommodate more traffic flows than existing ones.

In wireless sensor networks, path breakage occurs frequently due to node mobility, node failure, and channel impairments. It is challenging to combat path breakage with minimal control overhead, while adapting to rapid topological changes. Due to the Wireless Broadcast Advantage (WBA), all nodes inside the transmission range of a single transmitting node may receive the packet, hence naturally they can serve as cooperative caching and backup nodes if the intended receiver fails to receive the packet. In this paper, we present a distributed robust routing protocol in which nodes work cooperatively to enhance the robustness of routing against path breakage. We compare the energy efficiency of cooperative routing with noncooperative routing and show that our robust routing protocol can significantly improve robustness while achieving considerable energy efficiency.

Opportunistic routing (OR) copes with the unreliable transmissions by exploiting the broadcast nature of the wireless medium and spatial diversity of the multi-hop wireless networks. In this paper, we carry out a comprehensive study on the impacts of multiple rates, interference, candidate selection and prioritization on the maximum end-to-end throughput or capacity of OR. Taking into account the wireless interference and unique properties of OR, we introduce the concept of concurrent transmitter sets to represent the constraints imposed by the transmission conflicts of OR, and formulate the maximum end-to-end throughput problem as a maximum-flow linear programming subject to the transmission conflict constraints. We also propose two multi-rate OR metrics: expected medium time (EMT) and expected advancement rate (EAR), and the corresponding distributed and local rate and candidate set selection schemes, one of which is least medium time OR (LMTOR) and the other is multi-rate geographic OR (MGOR). We compare the capacity of multi-rate OR with single-rate ones under different settings. We show that our proposed multi-rate OR schemes achieve higher throughput bound than any single-rate GOR. We observe some insights of OR: 1) although involving more forwarding candidates increases the end-to-end capacity, the capacity gained from involving more forwarding candidates decreases; 2) there exists a node density threshold, higher than which 24 Mbps GOR performs better than 12 Mbps GOR, and vice versa.

Using multi-channel MAC protocols in mobile ad hoc networks (MANETs) is a promising way to improve the through-put performance. Channel assignment, which directly determines the efficiency of the frequency utilization, is the critical part of multi-channel schemes. Current 802.11-like schemes of multi-channel MAC do not efficiently use the multiple channels due to the overhead caused by channel assignment. Moreover, the control channel saturation problem limits the number of channels of these previous schemes. In this paper, we propose a new scheme called SAM-MAC (Self-Adjustable Multi-channel MAC), which features with one common channel and two half-duplex transceivers for each node. A method called self-adjustment is used to reassign the channels and balance the traffic on different channels. Due to less contention in common channel and smaller channel assignment overhead, this scheme increases the throughput compared with previous approaches. Control channels are free from saturation problem and can furthermore be used for data transmission.

Finding a path with enough throughput in multihop wireless ad hoc networks is a critical task of QoS Routing. Previous studies on routing algorithms focused on networks with a single channel rate. The capability of supporting multiple channel rates, which is common in wireless systems, has not been carefully studied in routing algorithms. In this paper, we first carry out a comprehensive study on the impacts of multiple rates, interference and packet loss rate on the maximum end-to-end throughput or path capacity. A linear programming problem is formulated to determine the path capacity of any given path. This problem is also extended to a joint routing and link scheduling optimization problem to find a path with the largest path capacity. We show that interference clique transmission time is inversely proportional to the upper bound of the path capacity, and hence we propose to use it as a new routing metric. Moreover, we evaluate the capability of various routing metrics such as hop count, expected transmission times, end-to-end transmission delay or medium time, link rate, bandwidth distance product, and interference clique transmission time to discover a high throughput path. The results show that different routing metrics lead to paths with significantly different path capacity, and the interference clique transmission time tends to discover paths with higher throughput than other metrics.

Abstract — Physical carrier sensing is an effective mechanism of medium access control (MAC) protocols to reduce collisions in wireless networks, and the size of the carrier sensing range has a great impact on the system performance. Previous studies have shown that the MAC layer overhead plays an important role in determining the optimal carrier sensing range. However, vari-able transmission ranges and receiver sensitivities for different channel rates and the impact of multihop forwarding have been ignored. In this paper, we investigate the impacts of these factors as well as several other important factors, such as SINR (signal to interference plus noise ratio), node topology, hidden/exposed terminal problems and bidirectional handshakes, on determining the optimum carrier sensing range to maximize the throughput through both analysis and simulations. The results show that if any one of these factors is not addressed properly, the system performance may suffer a significant degradation. Furthermore, considering both multirate capability and carrier sensing ranges, we propose to use bandwidth distance product as a routing metric, which improves end-to-end throughput by up to 27% in the simulated scenario. I.

One of the most promising approaches to improving communication efficiency in wireless communication systems is the use of multiuser diversity. Although it has been widely investigated and shown feasible and efficient in cellular networks, there is little work for the ad hoc networks, especially in real protocol and algorithm design. In this paper, we propose a novel scheme, namely, the Opportunistic Medium Access and Auto Rate (OMAR), to efficiently utilize the shared medium in IEEE 802.11-based ad hoc networks by taking advantage of diversity, distributed scheduling, and adaptivity. In an ad hoc network, especially in a heterogeneous ad hoc network or a mesh network, some nodes may need to communicate with multiple one-hop nodes. We allow such a node with a certain number of links to function as a clusterhead to locally coordinate multiuser communications. We introduce a CDF-based (Cumulative Distribution Function) K-ary opportunistic splitting algorithm and a distributed stochastic scheduling algorithm to resolve intra and intercluster collisions, respectively. Fairness is formulated and solved in terms of social optimality within and across clusters. Analytical and simulation results show that our scheme can significantly improve communication efficiency while providing social fairness.

Recent studies have shown that the performance of wireless multihop ad hoc networks is very poor. In this paper, we first demonstrate that one important reason of the poor performance is the close coupling between medium contention and network congestion. Therefore, we present a framework of distributed flow control and medium access control to address both medium contention and network congestion. The proposed scheme utilizes the MAC layer control frames to efficiently conduct the network layer's flow control function and only allows the upstream nodes to forward enough packets to make it possible for the downstream nodes to fully utilize the shared channel but never introduce severe MAC collisions and network congestion. Extensive simulations illustrate that the proposed scheme well controls congestion and greatly alleviates medium collisions. It achieves up to 12 times the end-to-end throughput of IEEE 802.11, maintains a short delay and a low control overhead, and improves the fairness regardless of the hop count and the traffic load.

In the emerging IEEE 802.11e MAC protocol, the enhanced distributed channel access (EDCA) is proposed to support prioritized QoS; however, it cannot guarantee strict QoS required by real-time services such as voice and video without proper network control mechanisms. To overcome this deficiency, we first build an analytical model to derive an average delay estimate for the traffic of different priorities in the unsaturated 802.11e WLAN, showing that the QoS requirements of the real-time traffic can be satisfied if the input traffic is properly regulated. Then, we propose two effective call admission control schemes and a rate control scheme that relies on the average delay estimates and the channel busyness ratio, an index that can accurately represent the network status. The key idea is, when accepting a new real-time flow, the admission control algorithm considers its effect on the channel utilization and the delay experienced by existing real-time flows, ensuring that the channel is not overloaded and the delay requirements are not violated. At the same time, the rate control algorithm allows the best effort traffic to fully use the residual bandwidth left by the real-time traffic, thereby achieving high channel utilization

IEEE 802.11 MAC protocol has been the standard for wireless LANs and is also implemented in many simulation software for mobile ad hoc networks. However, IEEE 802.11 MAC has been shown to be quite inefficient in the multihop mobile environments. Besides the well-known hidden terminal problem and the exposed terminal problem, there also exists the receiver blocking problem, which may result in link/routing failures and unfairness among multiple flows. Moreover, the contention and interference from the upstream and downstream nodes seriously decrease the packet delivery ratio of mulitihop flows. All these problems could lead to the "explosion" of control packets and poor throughput performance. In this paper, we first analyze these anomaly phenomena in multihop mobile ad hoc networks. Then, we present a novel effective random medium access control (MAC) protocol based on IEEE 802.11 MAC protocol. The new MAC protocol uses an out-of-band busy tone and two communication channels, one for control frames and the other for data frames, and can give a comprehensive solution to all the aforementioned problems. Extended simulations demonstrate that our protocol provides a much more stable link layer, greatly improves the spatial reuse, and works effectively in reducing the packet collisions. It improves the throughput by up to 20% for one-hop flows and by up to 5 times for multihop flows under heavy traffic comparing to the IEEE 802.11 MAC

Abstract Mobilead hocnetworks (MANETs) are useful in environment where fixed network infrastructure is unavailable. To function normally, MANETs demand an efficient and distributed medium access control (MAC) protocol. However, characteristics of MANETs such as radio link vulnerability, mobility, limited power pose great challenges on MAC design. This paper surveys the recent advances in MAC design for MANETs. We first identify the challenges that are facing MAC in MANETs. Then we discuss the proposed MAC schemes according to their design goals, focusing on some critical design issues, and tradeoffs. Finally, we point out some future research directions. Copyright © 2006 John Wiley & Sons, Ltd.

Despite its support of prioritized services, the IEEE 802.11e enhanced distributed channel access (EDCA) cannot guarantee strict QoS required by real-time services such as voice and video without proper network control mechanisms. To overcome this deficiency, we first build an analytical model to derive upper bounds for both delay means and variations for services of different priorities in the non-saturated 802.11e WLAN, showing that the QoS requirements of real-time services can be satisfied if the input traffic is properly regulated. Based on the analysis, we then propose a call admission control scheme and a rate control scheme to ensure that QoS requirements of real-time services are statistically guaranteed and that best effort services can efficiently use the residual bandwidth.

The traditional congestion control mechanism, i.e., TCP, when applied in mobile ad hoc networks (MANET), encounters a number of new challenges, such as wireless link error, medium contention and frequent route failures. Very poor performance of TCP in MANET has been reported in many recent studies. We focus on the problems resulting from medium contention and propose a novel rate based end-to-end congestion control scheme (RBCC). We first illustrate that, under the impact of medium contention, a window based congestion control algorithm is unstable and hence may not be appropriate for MANET because the optimum congestion window size is very small and may even be less than one, i.e., the source should send less than one packet in one round trip time (RTT). Based on the novel use of channel busyness ratio, which, we show, is an accurate sign of the network utilization and congestion status, a new rate control scheme has been proposed to support the transport service in MANET efficiently and reliably. In RBCC, a sublayer consisting of a leaky bucket is added under TCP to control the sending rate based on the network layer feedback at the bottleneck node. Extensive simulations show that our scheme significantly outperforms traditional TCP in terms of channel utilization, delay and fairness.

This paper studies an important problem in the IEEE 802.11 distributed coordination function (DCF)-based wireless local area network (WLAN): how well can the network support quality of service (QoS). Specifically, this paper analyzes the network's performance in terms of maximum protocol capacity or throughput, delay, and packet loss rate. Although the performance of the 802.11 protocol, such as throughput or delay, has been extensively studied in the saturated case, it is demonstrated that maximum protocol capacity can only be achieved in the nonsaturated case and is almost independent of the number of active nodes. By analyzing packet delay, consisting of medium access control (MAC) service time and waiting time, accurate estimates were derived for delay and delay variation when the throughput increases from zero to the maximum value. Packet loss rate is also given for the nonsaturated case. Furthermore, it is shown that the channel busyness ratio provides precise and robust information about the current network status, which can be utilized to facilitate QoS provisioning. The authors have conducted a comprehensive simulation study to verify their analytical results and to tune the 802.11 to work at the optimal point with maximum throughput and low delay and packet loss rate. The simulation results show that by controlling the total traffic rate, the original 802.11 protocol can support strict QoS requirements, such as those required by voice over Internet protocol (VoIP) or streaming video, and at the same time achieve high channel utilization.

Many rate adaptation schemes at the medium access control (MAC) layer have been proposed to utilize the multirate capability offered by the IEEE 802.11 wireless MAC protocol through automatically adjusting the transmission rate to best match the channel conditions. In this paper, we present the Opportunistic packet Scheduling and Auto Rate (OSAR) protocol to exploit the channel variations. The basic idea of OSAR is as follows: rather than just matching the channel condition for a node pair in communications, our protocol takes advantage of the multiuser diversity as much as possible and adapt the rate accordingly, i.e., based on the channel conditions to its neighboring nodes, the sender chooses the neighboring node with channel quality better than certain level to schedule the transmissions of packets in its queue, then the overall system throughput may be increased. The key mechanisms of OSAR are channel aware media access, rate adaptation and packet bursting. We carry out several sets of ns-2 simulations and evaluate the impact of various factors such as channel condition, network topology and traffic load on the throughput of OSAR. Simulation results show that our proposed protocol can achieve much better performance than other auto rate schemes.

In this paper, we proposed a novel call admission and rate control (CARC) scheme. Unlike previous research works that are focused on providing service differentiation in the contention-based 802.11 DCF, we aim to support stringent QoS requirements of real-time and streaming traffic. The key idea of this scheme is to regulate the arriving traffic of the WLAN such that the network can work at an optimal point. We first show that the channel busyness ratio is a good indicator of the network status in the sense that it is easy to obtain and can accurately and timely represent channel utilization. Then we propose two algorithms that function upon the use of the channel busyness ratio. The call admission control algorithm is used to regulate the admission of real-time or streaming traffic and the rate control algorithm to control the transmission rate of best effort traffic. A comprehensive simulation study in ns-2 has verified the performance of our proposed CARC scheme, showing that the original 802.11 DCF protocol can statically support strict QoS requirements, such as those required by voice over IP or streaming video, and at the same time, achieve a high channel utilization.

In the wireless LANs or mobile ad hoc networks, a node with multi-packets in its queue waiting for delivery to several neighboring nodes may choose to schedule a candidate receiver with good channel condition for transmission. By choosing a receiver with good channel condition, the Head-of-Line (HOL) blocking problem can be alleviated and the overall system throughput can be increased. Motivated by this observation, we introduce the Opportunistic packet Scheduling and Media Access control (OSMA) protocol to exploit high quality channel condition under certain fairness constraints. We base our design on CSMA/CA so that it can be simply incorporated into the 802.11 standard. The key mechanisms of OSMA protocol are multicast RTS and priority-based CTS. In the OSMA protocol, RTS includes a list of candidate receivers. Among those who are qualified to receive data, the one with the highest order would be granted to catch the channel by replying CTS in the first place. The ordering list is updated dynamically according to certain scheduling policy such as Round Robin (RR) and Earlier timestamp First (ETF), and also other performance metrics, ex., fairness and timeliness, can be enhanced. To the best of our knowledge, this is the first paper to exploit the multiuser diversity in the CSMA/CA based wireless networks. We evaluate the OSMA using ns-2 and our simulation results show that this protocol can improve the network throughput significantly.

In wireless multihop ad hoc networks, nodes need to contend for the shared wireless channel with their neighbors, which could result in congestions and greatly decrease the end-to-end throughput due to severe packet loss. Several recent papers have indicated that the IEEE 802.11 fails to achieve the optimum schedule for this kind of contentions. In this paper, we present a framework of multihop packet scheduling to achieve maximum throughput for traffic flows in the shared channel environment. The key idea is based on the observation that in the IEEE 802.11 MAC protocol the maximum throughput for chain topology is 1/4 of the channel bandwidth and its optimum packet scheduling is to allow simultaneous transmissions at nodes which are four hops away. The proposed fully distributed scheme generalizes this optimum scheduling to any traffic flows which may encounter intra-flow contentions and inter-flow contentions. Extensive simulations indicate that our scheme could perform well and achieve high throughput at light to heavy traffic load while the performance of the original IEEE 802.11 MAC protocol greatly degrades when the traffic load becomes heavy. Moreover, our scheme also achieves much better and more stable performance in terms of delay, fairness and scalability with low and stable control overhead.

Abstract IEEE 802.11 MAC protocol is the de facto standard for wireless local area networks (LANs), and has also been implemented in many network simulation packages for wireless multi‐hop ad hoc networks. However, it is well known that, as the number of active stations increases, the performance of IEEE 802.11 MAC in terms of delay and throughput degrades dramatically, especially when each station's load approaches its saturation state. To explore the inherent problems in this protocol, it is important to characterize the probability distribution of the packet service time at the MAC layer. In this paper, by modeling the exponential backoff process as a Markov chain, we can use the signal transfer function of the generalized state transition diagram to derive an approximate probability distribution of the MAC layer service time. We then present the discrete probability distribution for MAC layer packet service time, which is shown to accurately match the simulation data from network simulations. Based on the probability model for the MAC layer service time, we can analyze a few performance metrics of the wireless LAN and give better explanation to the performance degradation in delay and throughput at various traffic loads. Furthermore, we demonstrate that the exponential distribution is a good approximation model for the MAC layer service time for the queueing analysis, and the presented queueing models can accurately match the simulation data obtained from ns‐2 when the arrival process at MAC layer is Poissonian. Copyright © 2004 John Wiley & Sons, Ltd.

IEEE 802.11 MAC protocol has been the standard for wireless LANs, and also adopted in many network simulation packages for wireless multihop ad hoc networks. However, it is well known that, as the traffic goes up, the performance of IEEE 802.11 MAC drops dramatically in terms of delay and throughput, especially when the station approaches its saturation state. To explore the inherent problems in this protocol, it is important to characterize the probability distribution of the packet service time at the MAC layer. In this paper, by modeling the exponential backoff process as a Markov chain, we can use the signal transfer function of the generalized state transition diagram to derive the probability distribution of the MAC layer service time. We then present the discrete probability distribution for MAC layer packet service time, which is shown to accurately match the simulation data from network simulations. Finally, based on the probability model for the MAC layer service time, we are able to analyze a few performance metrics of the wireless LAN and give better explanation to the performance degradation in delay and throughput at various traffic load.