Hamed M. K. Alazemi

Stochastic modelling and analysis of 802.11 DCF with heterogeneous non-saturated nodes

In this paper we present a queueing model for a 802.11 wireless LAN under non-saturated conditions. Our model builds on previous work, but additionally takes into account heterogeneous arrival rates. In particular, we consider the case when there are two packet arrival rates in the network, with one set of stations generating packets at the lower rate and the rest generating packets at the higher rate.

Stochastic approach for modeling random early detection gateways in TCP/IP networks

This paper considers the random early detection (RED) active queue management algorithm in TCP/IP networks. A stochastic framework for modeling the performance of the RED algorithm is presented. We introduce a two-dimensional second-order discrete-time Markov chain to model the feedback effect of packet dropping on the incoming traffic. The mean system occupancy, packet drop probability, and system throughput are computed from the analytical model for both marking and dropping policies. The effects of RED parameters on a selected set of performance metrics are studied.

A Joint Transmission Grant Scheduling and Wavelength Assignment in Multichannel SG-EPON

In this paper, we investigate the problem of transmission grant scheduling in multichannel optical access networks using a scheduling theoretic approach. A novel cost-effective multichannel Ethernet passive optical network (EPON) is considered for our study. We show that the problem can be modeled as an open shop (OS) and we formulate the joint scheduling and wavelength assignment problem as a mixed integer linear program (MILP). Since the problem is shown to be NP-hard, we introduce a tabu-search-based heuristic for solving the joint problem. Different other heuristics are also considered and their performances are compared with those of tabu and MILP. Results indicate that by appropriately scheduling transmission grants and assigning wavelengths, substantial and consistent improvements may be obtained in the network performance. For example, tabu shows a reduction of up to 29% in the schedule length with substantial reduction in channel idle gaps yielding to both higher channel utilization and lower queueing delays. Additionally, when the number of channels in the network is not small, the benefits of performing appropriate wavelength assignment, together with transmission scheduling, are observed and discussed. We further perform a packet-level simulation on the considered network to study the benefits of efficient grant scheduling.

Delay-Aware Data Delivery in Vehicular Intermittently Connected Networks

The open literature encloses numerous studies on the efficiency of retransmission mechanisms used in typical data communication networks for the purpose of recovering from packet transmission errors or losses. This paper revolves around the design and analysis of a Delay-Aware Data Delivery (DADD) scheme for Vehicular Intermittently Connected Networks (VICNs). At the heart of DADD is a novel mechanism that allows a source stationary roadside unit (SRU) to carry out necessary bundle retransmissions to high speed vehicles newly entering its communication range. In turn, these vehicles will guarantee delay-minimal delivery of the retransmitted bundles to the destination SRU. A mathematical model is developed to characterize the operation of the source SRU under DADD as well as to evaluate the resulting bundle delivery delay. To verify the validity and the accuracy of the proposed model, extensive simulations are conducted where the performance of DADD is compared to that of two other existing schemes. Results show that DADD outperforms these two schemes by 14.28% to 36.84%.

Modeling and Delay Analysis of a Retransmission-Based Bundle Delivery Scheme for Intermittent Roadside Communication Networks

This paper proposes a novel bundle delivery scheme (BDS) aimed at achieving a delay-minimal bundle delivery in the context of an intermittent roadside network. The realization of this objective is challenging whenever network information is completely unavailable. The concept of virtual space (VS) presents itself as an efficient solution that allows the source to perform necessary data bundle retransmissions to a subset of arriving vehicles. In turn, these vehicles will secure earlier delivery of the retransmitted bundles to the destination. A thorough empirical performance evaluation of the BDS shows that this scheme exhibits a delay improvement of 22.6%-40% relative to other existing schemes.

Optimal Flexible Spectrum Access in Wireless Networks with Software Defined Radios

We investigate the problem of flexible spectrum access in multihop wireless networks. We assume radios that are capable of transmitting on channels of contiguous frequency bands and which do not require any sophisticated processing. Because these radios can flexibly configure their transmissions anywhere in the available frequency band, the spectrum becomes vulnerable to fragmentation and interference. We consider the joint problem of routing, link scheduling and spectrum allocation where scheduling feasibility is considered under the physical interference (SINR) constraint. We present a primal-dual decomposition for this complex optimization problem based on column generation. We show that obtaining the optimal solution to this problem is computationally not feasible, except for very small networks. We thus adopt a two-fold method to circumvent the complexity while yielding practical solutions. First, we relax the SINR constraint and use a simplified graph-based model for the interference. Second, we use a simulated annealing (SA) approach to solve the dual subproblem. Our SA approach however is augmented with an SINR feasibility check. Our results confirm that the primal-dual decomposition method using SA substantially reduces the computation time and achieves near optimal solutions. The results also reveal that substantial improvement in network performance is obtained with flexible spectrum assignment which results from its capability of better managing the interference in the network.

Joint Routing, Scheduling and Variable-Width Channel Allocation for Multi-Hop WMNs

Recent research results have shown that channel width is an important control knob that can be easily adapted through software and can be used for achieving higher system throughput and better energy efficiency. In this paper we address the problem of joint routing and transmission scheduling in a multichannel wireless mesh network with variable-width channel allocation. While narrower bands split the total available spectrum into more non-overlapping channels allowing more parallel concurrent transmissions, wider bands increase the capacity of the communication links. We present a cross-layer problem formulation which incorporates multi-path routing and link layer scheduling. We model this combinatorial complex problem as a mixed integer linear program and present a primal-dual decomposition method for solving it. Our method has always shown to strike a good balance between often conflicting objectives to achieve higher system performance. Numerical results revealed that up to 57% improvement in network performance is obtained when variable-width channel assignment is used against the best fixed-width channel assignment for larger networks; this is due to the capability of the former in achieving a good balance between higher concurrency and better control of interference.

Fair resource allocation and DF relay selection for multiuser OFDMA-based cooperative networks

We investigate the joint problem of power and subcarrier allocation with relay selection for a multi-user multi-relay cellular system with selective relaying. We first formulate the optimal problem using Boolean-convex optimization with the objective of improving the average network capacity while providing proportional rate fairness to all users. We show that except for very small network instances the model is computationally very complex to solve. Then, we present a sub-optimal model by decomposing the joint problem into two sub-problems. This model proves to be a simple one, however, the sub-optimal design of the first sub-problem restricts the model from achieving near optimal solutions. Therefore, to achieve near optimal solutions, we propose an iterative two step method where near-optimal solutions can be achieved by iterating between the two steps. Numerical results show that for smaller networks this two step iterative method obtains optimal solutions in much less time. We also show that a selective relaying technique always achieves better performance over the always relaying technique.

Optimization approach for multi-domain optical network provisioning

Multi-domain optical network provisioning is a key focus area as users continue to demand scalable bandwidth services across wider network regions. To date, a range of distributed schemes have been proposed to achieve lightpath routing across domain boundaries. In general, these solutions rely upon hierarchical routing and provisioning strategies and are mostly heuristics based. As such, it is difficult to gauge their true load-carrying capacity and effectiveness. Hence in order to address this concern, this effort proposes a formal optimization-based model for multi-domain lightpath setup pursuant to several key objectives, i.e., including throughput maximization, resource minimization, and load balancing. This model is then solved for some sample network topologies, and the results are compared versus existing heuristic strategies.

A Spatiotemporal Contention Resolution for Enhancing Spatial Reuse in Wireless Networks

In a multihop wireless network, it is important to control access to the channel so that concurrent transmissions do not interfere with each other. Access may effectively be controlled by properly tuning protocol parameters, e.g., the contention window CW and the carrier sensing threshold CSth. Although the former parameter aims to separate, in time, different transmissions, the latter parameter reduces interference by allowing senders separated by a safe margin to engage in concurrent transmissions. We propose a localized spatiotemporal algorithm that jointly controls the selection of these parameters to enhance the spatial reuse and optimize the overall network throughput. To efficiently decide which protocol parameter(s) to tune, we deem it necessary to first distinguish between the causes of frame loss. We propose an effective loss differentiation and establish accordingly appropriate reactive methods. We consider a realistic wireless network with fading channel conditions, where time-varying channel gains cause random frame losses that need to be distinguished from losses due to collisions and strong interference. Our protocol is extended with an adaptive request-to-send probe method to consider such dynamics, and significant performance gains are observed under different fading scenarios.