Marcelo M. Carvalho

Delay analysis of IEEE 802.11 in single-hop networks

This paper presents an analytical model to compute the average service time and jitter experienced by a packet when transmitted in a saturated IEEE 802.11 ad hoc network. In contrast to traditional work in the literature, in which a distribution is usually fitted or assumed, we use a bottom-up approach and build the first two moments of the service time based on the IEEE 802.11 binary exponential backoff algorithm and the events underneath its operation. Our model is general enough to be applied to any type of IEEE 802.11 wireless ad hoc network where the channel state probabilities driving a nodes backoff operation are known. We apply our model to saturated single-hop ad hoc networks under ideal channel conditions. We validate our model through extensive simulations and conduct a performance evaluation of a nodes average service time and jitter for both direct sequence and frequency-hopping spread spectrum physical layers.

Modeling energy consumption in single-hop IEEE 802.11 ad hoc networks

This paper presents an analytical model to predict energy consumption in saturated IEEE 802.11 single-hop ad hoc networks under ideal channel conditions. The model we introduce takes into account the different operational modes of the IEEE 802.11 DCF MAC, and is validated against packet-level simulations. In contrast to previous works that attempted to characterize the energy consumption of IEEE 802.11 cards in isolated, contention-free channels (i.e., single sender/receiver pair), this paper investigates the extreme opposite case, i.e., when nodes need to contend for channel access under saturation conditions. In such scenarios, our main findings include: (1) contrary to what most previous results indicate, the radios transmit mode has marginal impact on overall energy consumption, while other modes (receive, idle, etc.) are responsible for most of the energy consumed; (2) the energy cost to transmit useful data increases almost linearly with the network size; and (3) transmitting large payloads is more energy efficient under saturation conditions

Modeling Wireless Ad Hoc Networks with Directional Antennas

Modeling Wireless Ad Hoc Networks with Directional Antennas Marcelo M. Carvalho Computer Engineering Department University of California Santa Cruz Santa Cruz, CA 95064 USA carvalho@soe.ucsc.edu Abstract— This paper presents the first analytical model of wireless ad hoc networks that considers the impact of realistic antenna-gain patterns on network performance. As such, our modeling approach allows the study of ad hoc networks in which nodes are equipped with directional antennas. This modeling capability stands out from all previous analytical models, which have only dealt with omnidirectional or over-simplified antenna gain patterns, and which have not addressed the specific mecha- nisms of the medium access control (MAC) protocols used (e.g., the backoff mechanism). A new analytical model for the IEEE 802.11 DCF MAC is introduced that allows the study of different carrier-sensing mechanisms, such as the directional virtual carrier sensing (DVCS) protocol that we use to validate our analytical model and show its applicability. Our numerical results show that our new analytical model predicts the results obtained by discrete-event simulations very accurately, and does it with a processing time that is orders of magnitude faster than the time required by simulations. I. I NTRODUCTION Many protocols for wireless ad hoc networks with direc- tional antennas have been proposed, mostly on the medium access control (MAC) layer (e.g., [1], [2], [3], [4]). The vast majority of the performance-evaluation work on MAC proto- cols that exploit directional antennas has used discrete-event simulations to model protocol behavior. Only a few works have attempted to model ad hoc networks with directional antennas analytically. Section II summarizes this prior work, which has been very limited in that all the analytical models proposed to date have assumed (a) simplistic ways in which packets are offered to a shared channel, ignoring the specific mech- anisms used in MAC protocols (e.g., backoff mechanisms), and (b) the use of over-simplified antenna gain patterns, like the “pie-slice” and “cone-plus-ball” antenna models [5], [6], [7], [8]. In the simplistic antenna models used to date, all directions within a certain angle sector have constant gain, while no power is radiated/absorbed along the other directions (“pie-slice”), or a lower constant gain is assumed for the directions outside the angle sector to represent the back and side lobes of the antenna pattern (“cone-plus-ball”). In reality, no physical antenna can provide such constant gain for a given angle sector, and real antenna patterns are far more complex than “pie-slices” or “cone-plus-ball” models. In fact, real antenna patterns have non-negligible gains in all directions, and often have significant side and back lobes that J. J. Garcia-Luna-Aceves Palo Alto Research Center 3333 Coyote Hill Road Palo Alto, CA 94304 USA jj@soe.ucsc.edu can contribute considerably to the amount of perceived noise, leading to performance degradations such as the ones observed by Ramanathan et al. [9] in a real-life ad hoc network testbed. Consequently, conclusions about capacity improvements based on such over-simplified antenna models (e.g., [7] and [10]), may not necessarily reflect the true potentials or limitations of the use of directional antennas in ad hoc networks. In this paper, we present the first analytical modeling of wireless ad hoc networks that considers the impact of realistic antenna gain patterns on network performance. In particular, we focus on the modeling of wireless ad hoc networks with directional antennas. Section III summarizes the operation of the directional virtual carrier sensing (DVCS) protocol [11], and outlines the main problems that arise in developing an analytical model for this and similar schemes based on directional antennas. Section IV presents our new analytical model, which is derived from our previous work [12], and captures the in- teractions between the physical and the MAC layers while taking into account the radio connectivity among nodes. The extensions and modifications of our new analytical model include: (a) taking into account the impact of packet flow distribution among multiple receivers; (b) expressing the im- pact of frame size distribution; (c) modeling the impact of the carrier sensing mechanism in carrier-sensing MAC protocols; and (d) providing richer interference matrices that explicitly model the impact of a node’s transmission on the SINR degradation of every other node, i.e., the effect of capture with respect to every potential interferer is treated individually. We attain a linear approximation for the probabilities of successful handshakes among transmitters and receivers by taking advantage of the fact that any MAC protocol must attempt to avoid having interfering transmissions around the recipient of a frame transmission. Section V introduces a new model for the IEEE 802.11 DCF MAC that allows the study of different carrier-sensing mechanisms, including DVCS. We apply our analytical model to DVCS to compare its accuracy against results obtained with discrete-event simulations using realistic antenna patterns. The numerical results presented in Section VI indicate that our analytical model predicts simulation results very accurately, with processing times that are orders of magnitude faster

Modeling single-hop wireless networks under Rician fading channels

An analytical model for single-hop ad hoc networks is introduced that considers the impact of the physical layer on the operation and performance of saturated IEEE 802.11. Using the bottom-up approach, aspects of the physical layer are explicitly incorporated in the dynamics of the events governing the operation of the IEEE 802.11 binary exponential backoff algorithm leading to a more realistic computation of the nodes average service time and throughput. We study the impact of frequency-nonselective slowly time-variant Rician fading channels on the performance of single-hop ad hoc networks using the IEEE 802.11 distributed coordination function in saturation. We validate our model through simulations and study of the throughput performance of the four-way handshake mechanism under direct sequence spread spectrum (DSSS) with differential binary phase shift keying (DBPSK) modulation.

Reversing the IEEE 802.11 backoff algorithm for receiver-initiated MAC protocols

To date, a significant body of work has already been done in the design of receiver-initiated medium access control (MAC) protocols for wireless ad hoc networks. Most of these works, however, have not appropriately dealt with the fundamental issues inherently related to this class of protocols, such as the rate at which polling is triggered by a node, and the way in which nodes are selected for polling. Moreover, previous works on receiver-initiated protocols have not considered the impact of physical layer aspects on network performance. We analyze the effects of considering such aspects and then we show how to improve these issues with ideas based on the operation of the IEEE 802.11. In addition, we present an algorithm that prioritizes the transmission of polling nodes to nodes that are more likely to have a successful handshake with them. This way, we are actually improving the network overall performance. Our analytical results confirm that even when physical layer aspects are considered, receiver-initiated protocols outperforms sender-initiated protocols, but with a margin significantly lower than that obtained in previous works.

Analytical Modeling of Ad Hoc Networks that Utilize Space-Time Coding

This paper presents the first analytical model for ad hoc networks equipped with multiple-input multiple-output (MIMO) radios using space-time coding (STC) that considers the impact of the underlying radio-based topology on network performance. In particular, we consider the space-time block coding (STBC) technique known as the “Alamouti scheme.” We derive the effective signal-to-interference-plus-noise density ratio (SINR) of the Alamouti scheme under multiple access interference (MAI), and we propose the moment generating function (MGF) method to derive closed-form expressions for its symbol error probability under different modulation schemes when fading paths are independent but not necessarily identically distributed. The impact of the Alamouti scheme on IEEE 802.11 ad hoc networks is studied by introducing a new analytical model for the IEEE 802.11 DCF MAC. The model we introduce takes into account the impact of errors in both control and data frames, the carrier-sensing activity, and the finite-retry limit of frame retransmissions. Both PHY- and MAC-layer analytical models are incorporated into our previously-designed, general analytical model for ad hoc networks based on interference matrices. We apply the Alamouti scheme to different antenna system configurations and compare their performance with respect to the basic single-input-single-output (SISO) IEEE 802.11 DCF MAC.

RIMAP: Receiver-initiated MAC protocol with Adaptive Polling Discipline

This work introduces the Receiver-Initiated MAC with Adaptive Polling Discipline (RIMAP), a unicast MAC protocol that dynamically selects a polling discipline according to channel contention and link quality homogeneity to all neighbors. For that, two polling disciplines are considered: one that prioritizes nodes according to the likelihood of successful handshake (LSH), and another that targets throughput fairness among nodes, the proportional fair (PF) discipline. The adaptive behavior is controlled by two switching parameters that can be tuned to trade fairness with throughput-delay performance. To control its polling rate, RIMAP uses a reversed version of the binary exponential backoff (BEB) algorithm of the IEEE 802.11 DCF. Additionally, it implements a frame reordering technique that relax the need for data frames to reach the head of the queue in order to be transmitted, and a Nothing-To-Send (NTS) control frame to speed up polling rounds. RIMAP performance is evaluated with discrete-event simulations under topologies with hidden terminals, concurrent transmissions, and saturated traffic. Also, its performance is compared with the same BEB-based MAC protocol under fixed polling disciplines (LSH or PF only), as well as with the IEEE 802.11 DCF MAC, a representative of sender-initiated paradigms.