Xinghua Sun

A Unified Analysis of IEEE 802.11 DCF Networks: Stability, Throughput, and Delay

In this paper, a unified analytical framework is established to study the stability, throughput, and delay performance of homogeneous buffered IEEE 802.11 networks with Distributed Coordination Function (DCF). Two steady-state operating points are characterized using the limiting probability of successful transmission of Head-of-Line (HOL) packets p given that the network is in unsaturated or saturated conditions. The analysis shows that a buffered IEEE 802.11 DCF network operates at the desired stable point p=pL if it is unsaturated. pL does not vary with backoff parameters, and a stable throughput can be always achieved at pL. If the network becomes saturated, in contrast, it operates at the undesired stable point p=pA, and a stable throughput can be achieved at pA if and only if the backoff parameters are properly selected. The stable regions of the backoff factor q and the initial backoff window size W are derived, and illustrated in cases of the basic access mechanism and the request-to-send/clear-to-send (RTS/CTS) mechanism. It is shown that the stable regions are significantly enlarged with the RTS/CTS mechanism, indicating that networks in the RTS/CTS mode are much more robust. Nevertheless, the delay analysis further reveals that lower access delay is incurred in the basic access mode for unsaturated networks. If the network becomes saturated, the delay performance deteriorates regardless of which mode is chosen. Both the first and the second moments of access delay at pA are sensitive to the backoff parameters, and shown to be effectively reduced by enlarging the initial backoff window size W.

Backoff design for IEEE 802.11 DCF networks: fundamental tradeoff and design criterion

Binary Exponential Backoff (BEB) is a key component of the IEEE 802.11 DCF protocol. It has been shown that BEB can achieve the theoretical limit of throughput as long as the initial backoff window size is properly selected. It, however, suffers from significant delay degradation when the network becomes saturated. It is thus of special interest for us to further design backoff schemes for IEEE 802.11 DCF networks that can achieve comparable throughput as BEB, but provide better delay performance. This paper presents a systematic study on the effect of backoff schemes on throughput and delay performance of saturated IEEE 802.11 DCF networks. In particular, a backoff scheme is defined as a sequence of backoff window sizes {Wi}. The analysis shows that a saturated IEEE 802.11 DCF network has a single steady-state operating point as long as {Wi} is a monotonic increasing sequence. The maximum throughput is found to be independent of {Wi}, yet the growth rate of {Wi} determines a fundamental tradeoff between throughput and delay performance. For illustration, Polynomial Backoff is proposed, and the effect of polynomial power x on the network performance is characterized. It is demonstrated that Polynomial Backoff with a larger x is more robust against the fluctuation of the network size, but in the meanwhile suffers from a larger second moment of access delay. Quadratic Backoff (QB), i.e., Polynomial Backoff with x=2, stands out to be a favorable option as it strikes a good balance between throughput and delay performance. The comparative study between QB and BEB confirms that QB well preserves the robust nature of BEB and achieves much better queueing performance than BEB.

Throughput Optimization of Heterogeneous IEEE 802.11 DCF Networks

This paper presents the throughput analysis of an M-group heterogeneous IEEE 802.11 DCF network where nodes in different groups have distinct input rates and initial backoff window sizes. An explicit expression of the network steady-state operating point is obtained based on the fixed-point equation of the limiting probability of successful transmission of Head-of-Line (HOL) packets given that the channel is idle, which is shown to be closely dependent on the backoff parameters of saturated groups and the input rates of unsaturated groups. Both the network throughput and the group throughput performance are further characterized, and the maximum network throughput is derived as an explicit function of the holding times of HOL packets in successful transmission and collision states. The analysis reveals that to achieve the maximum network throughput, the optimal set of input rates of unsaturated groups and initial backoff window sizes of saturated groups should satisfy a constraint that is determined by the group sizes of saturated groups. Given the input rates of unsaturated groups, for instance, the initial backoff window sizes of saturated groups should linearly increase with their group sizes, and those with higher increasing rates achieve lower group throughput.

IEEE 802.11e EDCA Networks: Modeling, Differentiation and Optimization

Enhanced distributed channel access (EDCA) is an extension of the distributed coordination function to support quality-of-service for IEEE 802.11 wireless local area networks. By assigning distinct backoff parameters to each access category (AC), differentiated throughput performance can be achieved when the network is saturated. Although it has been long observed that the network throughput with the current EDCA standard setting may significantly degrade as the network size grows, how to properly tune the backoff parameters to optimize the network throughput under a certain differentiation requirement remains largely unknown. In this paper, a new analytical model is proposed to address this open issue. Specifically, we focus on an M-AC IEEE 802.11e EDCA network where nodes in the same AC have identical backoff parameters, including the initial backoff window sizeW(g), the cutoff phaseK(g), and the arbitration interframe spaces (AIFS) numberA(g), g = 1, . . . , M. The network steady-state operating point in saturated conditions, i.e., pA, is characterized by using the steady-state probability of successful transmission of head-of-line (HOL) packets given that the channel is idle, based on which explicit expressions of node throughput and network throughput are further obtained. For given target ratios of node throughput of ACs, the optimal initial backoff window sizes and AIFS numbers to maximize the network throughput are derived and verified by simulation results. The analysis reveals that the maximum network throughput is solely determined by the holding time of HOL packets in successful transmission and collision states. To achieve the maximum network throughput, the initial backoff window size of each AC should be linearly increased with the network size. In the meantime, the increasing rate of the initial backoff window size, or the AIFS number, of each AC should be also carefully set according to the target ratios of node throughput. Although the maximum network throughput with pre-specified target ratios of node throughput of ACs can be achieved in both ways, the backoff window size differentiation could be a more preferable option as it requires fewer tuning parameters and provides better precision than the AIFS differentiation.

A comparative study of Quadratic Backoff and Binary Exponential Backoff in IEEE 802.11 DCF networks

In this paper, a unified analytical model is established to analyze the performance of backoff schemes in IEEE 802.11 DCF networks under saturation condition. It is shown that BEB suffers from deteriorated queueing performance due to a large second moment of access delay, and a key to reduce the second moment lies in the growth rate of the backoff window size. Quadratic Backoff (QB) is further proposed, with which the backoff window size is quadratically increased upon collisions. Both the theoretical and simulation results show that QB can achieve a comparable throughput with that of BEB, but with much better queueing performance.

Performance Optimization of CSMA Networks With a Finite Retry Limit

A retry limit is usually adopted in practical carrier sense multiple access (CSMA) networks, where a packet is discarded if the maximum number of retransmission attempts is reached. Despite extensive studies, the effect of retry limit on the performance optimization of CSMA networks has remained largely unknown. This paper focuses on a CSMA network with a finite retry limit M , and aims to address the following open issues. First, for a given retry limit M , how should the backoff parameters be adaptively tuned to achieve the optimal network performance? Second, how does the optimal network performance vary with M ? Specifically, in this paper, the explicit expressions of the network steady-state points, the network throughput, and moments of access delay of successfully transmitted packets are all obtained as the functions of the retry limit M , based on which the optimal network performance is further characterized. It is revealed that a CSMA network with a finite retry limit M may have three steady-state points, and the retry limit M has distinct effects on the throughput and delay performance at these steady-state points. The maximum network throughput is found to be independent of M . Yet to achieve the maximum network throughput, the aggregate input rate and the initial transmission probability of each node should be set according to M in unsaturated and saturated conditions, respectively. To optimize the mean access delay, on the other hand, the initial transmission probability should be carefully selected in saturated conditions. The minimum mean access delay can be greatly reduced by choosing a smaller retry limit M , which, nevertheless, leads to a significant throughput loss. The analysis sheds important light on performance optimization of practical CSMA-based networks such as IEEE 802.11 networks.

Throughput optimization of heterogeneous IEEE 802.11 DCF networks

This paper presents the throughput analysis of an M-group heterogeneous IEEE 802.11 DCF network where nodes in different groups have distinct input rates and initial backoff window sizes. An explicit expression of the network steady-state operating point is obtained based on the fixed-point equation of the limiting probability of successful transmission of Head-of-Line (HOL) packets given that the channel is idle, which is shown to be closely dependent on the backoff parameters of saturated groups and the input rates of unsaturated groups. Both the network throughput and the group throughput performance are further characterized, and the maximum network throughput is derived as an explicit function of the holding times of HOL packets in successful transmission and collision states. The analysis reveals that to achieve the maximum network throughput, the optimal set of input rates of unsaturated groups and initial backoff window sizes of saturated groups should satisfy a constraint that is determined by the group sizes of saturated groups. Given the input rates of unsaturated groups, for instance, the initial backoff window sizes of saturated groups should linearly increase with their group sizes, and those with higher increasing rates achieve lower group throughput.

Performance Evaluation of Multi-channel CSMA for Machine-to-Machine Communication.

Machine-to-machine (M2M) communication aims to exchange information among a large number of devices without human interference. When more and more devices are connected, nevertheless, serious delay and energy efficiency problems may emerge due to massive access. In this paper, we apply a multi-channel Carrier Sense Multiple Access (CSMA) protocol for M2M communications where the frequency band is divided into several sub-bands. It is found that whether the band partitioning offers performance gains in terms of the delay and energy efficiency performance is critically determined by the traffic load. When the traffic load exceeds certain thresholds, a larger number of sub-channels is preferable. Moreover, it is found that the packet size and the signal-to-noise ratio (SNR) have a crucial effect on the thresholds. Based on this, the number of sub-channels can be optimally chosen accordingly to make sure that the system operates in the optimum working zone.

Fairness-Constrained Maximum Sum Rate of Multi-Rate CSMA Networks

This paper presents the sum rate analysis of a saturated

Delay-Aware Dynamic Barring Scheme for Massive Access in NB-IoT Network

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