Dmitri Moltchanov

Cross-layer modeling of wireless channels for data-link and IP layer performance evaluation

To provide a tool for performance evaluation of IP-based delay- and loss-sensitive applications running over wireless channels we propose a novel cross-layer wireless channel modeling approach. We firstly develop simple and computationally efficient wireless channel modeling algorithm. For this purpose we adopt the special solution of the inverse eigenvalue problem and show that its complexity significantly decreases when the time-series is covariance stationary two-valued in nature. Our model explicitly takes into account autocorrelation and distributional properties of empirical data. Then, we extend this model to IP layer using the cross-layer mappings. The resulting model is represented by the IP packet error process and reflects memory properties of initial bit error process. We show that our approach allows to get accurate estimators of IP packet error probabilities in presence of FEC at the data-link layer eliminating the need for computationally expensive time-consuming bit level simulations. It also provides a way to choose the required correction capabilities of FEC codes resulting in best possible performance at the data-link and IP layers.

Modeling TCP SACK performance over wireless channels with completely reliable ARQ/FEC

SUMMARY In this paper, we propose an analytical cross-layer model for a Transmission Control Protocol (TCP) connection running over a covariance-stationary wireless channel with a completely reliable Automatic Repeat reQuest scheme combined with Forward Error Correction (FEC) coding. Since backbone networks today are highly overprovisioned, we assume that the wireless channel is the only one bottleneck in the system which causes packets to be buffered at the wired/wireless interface and dropped as a result of buffer overflow. We develop the model in two steps. At the first step, we consider the service process of the wireless channel and derive the probability distribution of the time required to successfully transmit an IP packet over the wireless channel. This distribution is used at the next step of the modeling, where we derive expressions for the TCP long-term steady-state throughput, the mean round-trip time, and the spurious timeout probability. The developed model allows to quantify the joint effect of many implementationspecific parameters on the TCP performance over both correlated and non-correlated wireless channels. We also demonstrate that TCP spurious timeouts, reported in some empirical studies, do not occur when wireless channel conditions are covariance-stationary and their presence in those measurements should be attributed to non-stationary behavior of the wireless channel characteristics. Copyright 2011 John Wiley & Sons, Ltd.

Performance response of wireless channels for quantitatively different loss and arrival statistics

In this paper we propose a cross-layer performance evaluation framework for wireless channels and subsequently explore the performance response at the FEC and ARQ enabled data-link layer in terms of frame losses and delays for different first- and second-order error and arrival statistics. For a wireless channel model to be appropriate for various FEC capabilities without the need for extensive measurements of the frame error process for each particular FEC code, the error process of the wireless channel is modeled at the physical layer. We assume weak stationary property for bit error observations and model them using a hidden Markov model. The associated parameters matching algorithm allows us to explicitly capture bit error rate and lag-1 autocorrelation of the bit error process. To explore the performance response of the wireless channel at the data-link layer, a cross-layer extension of the bit error model to the data-link layer is then developed. The performance of applications is evaluated using the queuing-theoretic approach that allows both arrival and error processes to be autocorrelated and still retains analytical tractability. The proposed methodology allows us to obtain estimators of frame loss and delay probabilities in the presence of FEC and ARQ procedures at the data-link layer eliminating the need for time-consuming simulations and extensive measurements of wireless channel characteristics for different error correction capabilities of the data-link layer. It is analytical in nature, efficient for small and moderate frame sizes and suitable for performance control purposes where fixed size frames are used at the data-link layer. Particularly, it provides a way to choose the required correction capability of the FEC code resulting in best possible performance at the data-link layer. Numerical results indicate that first- and second-order bit error and frame arrival statistics significantly affect performance parameters provided by a wireless channel and should be taken into account when choosing an appropriate correction capability of the FEC code for given wireless channel conditions.

Simple, accurate and computationally efficient wireless channel modeling algorithm

We propose simple and computationally efficient wireless channel modeling algorithm. For this purpose we adopt the special case of the algorithm initially proposed in [1] and show that its complexity significantly decreases when the time-series is covariance stationary binary in nature. We show that for such time-series the solution of the inverse eigenvalue problem returns unique transition probability matrix of the modulating Markov chain that is capable to match statistical properties of empirical frame error processes. Our model explicitly takes into account autocorrelational and distributional properties of empirical data. We validate our model against empirical frame error traces of IEEE 802.11b wireless access technology operating in DCF mode over spread spectrum at 2Mbps and 5.5 Mbps bit rates. We also made available the C code of the model as well as pre-compiled binaries for Linux and Windows operating systems at http://www.cs.tut.fi/~moltchan.

State description of wireless channels using change-point statistical tests

We consider the state of the wireless channel in terms of the covariance stationary signal-to-noise ratio (SNR) process and parameterize it using the probability distribution function of SNR and lag-1 autocorrelation coefficient of associated autocorrelation function (ACF). In order to discriminate the state of the wireless channel we apply methods of statistical process control. Particularly, we use exponential weighted moving average (EWMA) change-point statistical test to detect shifts in the mean of the SNR process. The proposed approach is verified using SNR measurements of IEEE 802.11b wireless channel.

Modeling TCP performance over wireless channels with a semi-reliable data link layer

Providing reliable data communications over wireless channels is a challenging task due to the time-varying wireless channel characteristics that often lead to bit errors. These errors propagate to the higher layers, causing loss of protocol data units. ARQ and FEC try to prevent this, providing a reliable service to the higher layers. Most analytical models that studied the effect of ARQ and FEC assumed that the ARQ scheme is fully reliable, meaning that a frame is successfully transmitted irrespective of the amount of retransmission attempts it takes. In this paper, we study the effect of a semi-reliable data link layer on performance experienced by TCP. We develop an analytical model for a TCP SACK connection running over a wireless channel with a semi-reliable ARQ, where the amount of retransmission attempts is limited by some number. The model allows to evaluate the effect of many parameters of wireless channels on TCP performance, making it suitable for performance optimization studies. These parameters include wireless channel characteristics, ARQ/FEC settings, size of protocol data units at different layers, buffer size at the IP layer, and the raw data rate of the wireless channel.

Modeling TCP SACK performance over wireless channels with semi-reliable ARQ/FEC

Providing reliable data communications over wireless channels is a challenging task because time-varying wireless channel characteristics often lead to bit errors. These errors result in loss of IP packets and, consequently, TCP segments encapsulated into these packets. Since TCP cannot distinguish packet losses due to bit corruption from those due to network congestion, any packet loss caused by wireless channel impairments leads to unnecessary execution of the TCP congestion control algorithms and, hence, sub-optimal performance. Automatic Repeat reQuest (ARQ) and Forward Error Correction (FEC) try to improve communication reliability and reduce packet losses by detecting and recovering corrupted bits. Most analytical models that studied the effect of ARQ and FEC on TCP performance assumed that the ARQ scheme is perfectly persistent (i.e., completely reliable), thus a frame is always successfully transmitted irrespective of the number of transmission attempts it takes. In this paper, we develop an analytical cross-layer model for a TCP connection running over a wireless channel with a semi-reliable ARQ scheme, where the amount of transmission attempts is limited by some number. The model allows to evaluate the joint effect of stochastic properties of the wireless channel characteristics and various implementation-specific parameters on TCP performance, which makes it suitable for performance optimization studies. The input parameters include the bit error rate, the value of the normalized autocorrelation function of bit error observations at lag 1, the strength of the FEC code, the persistency of ARQ, the size of protocol data units at different layers, the raw data rate of the wireless channel, and the bottleneck link buffer size.

Cross-layer modeling of TCP SACK performance over wireless channels with completely reliable ARQ/FEC

We propose an analytical model for a TCP SACK connection running over a wireless channel with completely reliable ARQ/FEC. We develop the model in two steps. At the first step, we consider the service process of the wireless channel and derive the probability distribution function of the time required to successfully transmit a single IP packet over the wireless channel. This distribution is used at the next step of the modeling where we derive the expression for TCP SACK steady state goodput. The developed model allows to quantify the effect of many implementation-specific parameters on TCP performance in wireless domain. We also demonstrate that TCP spurious timeouts, reported in many empirical studies, do not occur when wireless channel conditions are stationary and their presence in empirical measurements should be attributed to non-stationary behavior of wireless channel characteristics.

Performance Evaluation and Traffic Modeling

Mobile and wireless communication systems are becoming more and more complex, making understanding the interaction of different technologies on different layers a very difficult task. The introduction of sophisticated techniques on the physical layer that react to changes of the wireless channel on small timescales requires new paradigms for modeling, simulating, and analyzing current and future wireless networks. Investigating the relationship of new physical layer techniques, application-specific requirements and performance measures will become a major research topic for future wireless networks. A continuous change in the methodology for evaluating the network performance takes place in the Internet. In the past, network performance was mainly evaluated using concretely measurable values like packet loss rate, delay, or jitter. The current trend in the Internet goes toward application-specific quality measures that judge more the subjective experience of the end user than they do network parameters. In the terminology, this is expressed as the change from quality of service (QoS-Quality of Service (QoS)) to quality of experience (QoE). For wireless networks this leads to interesting consequences, as currently the traffic requirements for MAC layer connections are mainly formulated in terms of QoS parameters. Accomplishing the change from QoS to QoE also in the definition of connection parameters is a future challenge for wireless networks, for which the first approaches are presented.

The effect of data-link layer reliability on performance of wireless channels

Wireless channels are characterized by time-varying characteristics that often lead to incorrect reception of protocol data units. Automatic repeat request and forward error correction try to prevent it providing reliable service to higher layers. Most models proposed to date assumed that ARQ is fully reliable meaning that a frame is successfully transmitted irrespective of time it takes. In this paper we study the effect of ARQ reliability on performance experienced at higher layers. We propose a model for semi-reliable ARQ operation when a certain frame can be lost due to excessive amount of retransmissions. We also take into account IP packet losses due to limited buffer space. Using numerical results we show that the effect of maximum number of retransmissions depends on channel characteristics and other protocolspsila parameters. Dynamic tuning of this parameter might be beneficial for optimized performance of wireless channels.