Ka-Cheong Leung

An Overview of Packet Reordering in Transmission Control Protocol (TCP): Problems, Solutions, and Challenges

Transmission control protocol (TCP) is the most popular transport layer protocol for the Internet. Due to various reasons, such as multipath routing, route fluttering, and retransmissions, packets belonging to the same flow may arrive out of order at a destination. Such packet reordering violates the design principles of some traffic control mechanisms in TCP and, thus, poses performance problems. In this paper, we provide a comprehensive and in-depth survey on recent research on packet reordering in TCP. The causes and problems for packet reordering are discussed. Various representative algorithms are examined and compared by computer simulations. The ported program codes and simulation scripts are available for download. Some open questions are discussed to stimulate further research in this area

Transmission control protocol (TCP) in wireless networks: issues, approaches, and challenges

The Transmission Control Protocol (TCP) is the most popular transport layer protocol for the Internet. Due to the characteristics specific to wireless networks, such as signal fading and mobility, packets may be lost due to con- gestive and noncongestive losses. Substantial noncongestive losses violate the design principles of some traffic control mechanisms in TCP and thus pose performance problems. In this article we provide a comprehensive and in-depth survey on recent research in TCP for wireless communications. The taxonomy and characteristics of wireless networks, and problems for TCP in wireless communications are introduced. Various representative algorithms which preserve the end-to-end semantics are examined. Some open ques- tions are discussed in order to stimulate further research in this area.

Capacity Estimation for Vehicle-to-Grid Frequency Regulation Services With Smart Charging Mechanism

Due to various green initiatives, renewable energy will be massively incorporated into the future smart grid. However, the intermittency of the renewables may result in power imbalance, thus adversely affecting the stability of a power system. Frequency regulation may be used to maintain the power balance at all times. As electric vehicles (EVs) become popular, they may be connected to the grid to form a vehicle-to-grid (V2G) system. An aggregation of EVs can be coordinated to provide frequency regulation services. However, V2G is a dynamic system where the participating EVs come and go independently. Thus, it is not easy to estimate the regulation capacities for V2G. In a preliminary study, we modeled an aggregation of EVs with a queueing network, whose structure allows us to estimate the capacities for regulation-up and regulation-down separately. The estimated capacities from the V2G system can be used for establishing a regulation contract between an aggregator and the grid operator, and facilitating a new business model for V2G. In this paper, we extend our previous development by designing a smart charging mechanism that can adapt to given characteristics of the EVs and make the performance of the actual system follow the analytical model.

Generalized load sharing for packet-switching networks. I. Theory and packet-based algorithm

In this paper, we propose a framework to study how to effectively perform load sharing in multipath communication networks. A generalized load sharing (GLS) model has been developed to conceptualize how traffic is split ideally on a set of active paths. A simple traffic splitting algorithm, called packet-by-packet weighted fair routing (PWFR), has been developed to approximate GLS with the given routing weight vector by transmitting each packet as a whole. We have developed some performance bounds for PWFR and found that PWFR is a deterministically fair traffic splitting algorithm. This attractive property is useful in the provision of service with guaranteed performance when multiple paths can be used simultaneously to transmit packets which belong to the same flow. Our simulation studies, based on a collection of Internet backbone traces, reveal that PWFR outperforms two other traffic splitting algorithms, namely, packet-by-packet generalized round robin routing (PGRR), and packet-by-packet probabilistic routing (PPRR)

Joint Allocation of Resource Blocks, Power, and Energy-Harvesting Relays in Cellular Networks

Relaying is a promising technique in cellular networks for improving system capacity and coverage. To facilitate the deployment of relays in remote areas without ready access to the electrical grid, energy-harvesting relays may be deployed. Energy-harvesting has been studied extensively for sensor networks, but it is still an open problem for cellular networks. In this paper, we study the problem of the joint allocation of orthogonal frequency division multiplexing access resource blocks and transmission power to users in a cellular network with energy-harvesting relays. The energy-harvesting process is stochastically described by a time-varying Poisson process. We propose a new metric called survival probability as the selection criteria for an energy-harvesting relay to support data transmissions. We propose a survival probability-based resource allocation (SPRA) algorithm. The algorithm solves the joint problem of resource block allocation, power control, and associating relays to users in a cellular network. We show the achievable data rates of SPRA for different energy harvesting rates.

Optimal Scheduling With Vehicle-to-Grid Regulation Service

In a vehicle-to-grid (V2G) system, aggregators coordinate the charging/discharging schedules of electric vehicle (EV) batteries so that they can collectively form a massive energy storage system to provide ancillary services, such as frequency regulation, to the power grid. In this paper, the optimal charging/discharging scheduling between one aggregator and its coordinated EVs for the provision of the regulation service is studied. We propose a scheduling method that assures adequate charging of EVs and the quality of the regulation service at the same time. First, the scheduling problem is formulated as a convex optimization problem relying on accurate forecasts of the regulation demand. By exploiting the zero-energy nature of the regulation service, the forecast-based scheduling in turn degenerates to an online scheduling problem to cope with the high uncertainty in the forecasts. Decentralized algorithms based on the gradient projection method are designed to solve the optimization problems, enabling each EV to solve its local problem and to obtain its own schedule. Our simulation study of 1000 EVs shows that the proposed online scheduling can perform nearly as well as the forecast-based scheduling, and it is able to smooth out the real-time power fluctuations of the grid, demonstrating the potential of V2G in providing the regulation service.

Flow assignment and packet scheduling for multipath routing

In this paper, we propose a framework to study how to route packets efficiently in multipath communication networks. Two traffic congestion control techniques, namely, flow assignment and packet scheduling, have been investigated. The flow assignment mechanism defines an optimal splitting of data traffic on multiple disjoint paths. The resequencing delay and the usage of the resequencing buffer can be reduced significantly by properly scheduling the sending order of all packets, say, according to their expected arrival times at the destination. To illustrate our model, and without loss of generality, Gaussian distributed end-to-end path delays are used. Our analytical results show that the techniques are very effective in reducing the average end-to-end path delay, the average packet resequencing delay, and the average resequencing buffer occupancy for various path configurations. These promising results can form a basis for designing future adaptive multipath protocols.

Enhancing Wireless TCP: A Serialized-Timer Approach

In wireless networks, TCP performs unsatisfactorily since packet reordering and random losses may be falsely interpreted as congestive losses. This causes TCP to trigger fast retransmission and fast recovery spuriously, leading to under-utilization of available network resources. In this paper, we propose a novel TCP variant, known as TCP for non-congestive loss (TCP-NCL), to adapt TCP to wireless networks by using more reliable signals of packet loss and network overload for activating packet retransmission and congestion response, separately. TCP-NCL can thus serve as a unified solution for effective congestion control, sequencing control, and loss recovery. Different from the existing unified solutions, the modifications involved in the proposed variant are limited to sender-side TCP only, thereby facilitating possible future wide deployment. The two signals employed are the expirations of two serialized timers. A smart TCP sender model has been developed for optimizing the timer expiration periods. Our simulation studies reveal that TCP-NCL is robust against packet reordering as well as random packet loss while maintaining responsiveness against situations with purely congestive loss.

A resequencing model for high speed networks

In this paper, we propose a framework to study the resequencing mechanism in high speed networks. This framework allows us to estimate the packet resequencing delay, the total packet delay, and the resequencing buffer occupancy distributions when data traffic is dispersed on multiple disjoint paths. In contrast to most of the existing work, the estimation of the end-to-end path delay distribution is decoupled from the queueing model for resequencing. This leads to a simple yet general model, which can be used with other measurement-based tools for estimating the end-to-end path delay distribution to find an optimal split of traffic. We consider a multiple-node M/M/1 tandem network as a path model. When end-to-end path delays are Gaussian distributed, our results show that the packet resequencing delay, the total packet delay, and the resequencing buffer occupancy drop when the traffic is spread over a larger number of homogeneous paths, although the network performance improvement quickly saturates when the number of paths used increases. We find that the number of paths used in multipath routing should be small, say up to three. Besides, an optimal split of traffic occurs at paths with equal loads.

Optimal PMU placement for wide-area monitoring using chemical reaction optimization

The problem of optimal placement of phasor measurement units (PMUs) was first studied using graph theoretic and mathematical programming methods. However, due to the increase of renewable generations, PMUs may be required not only in transmission networks, but also in sub-transmission, or even distribution networks. Hence, the size of this PMU placement problem will become too big for mathematical programming approaches. As a result, in this paper, we investigated solving this problem using a novel metaheuristic technique, called chemical reaction optimization (CRO). CRO loosely mimics the interactions between molecules in a chemical reaction process. Based on the canonical CRO, we propose a simplified version of CRO (SCRO) for the optimal PMU placement (OPP) problem. Both canonical CRO and SCRO are tested on the full observability of OPP in two scenarios, i.e., considering and not considering zero injections. To investigate the effectiveness of the proposed methods, we evaluate their performances in the IEEE 14-bus, 30-bus, 57-bus, 118-bus, and 300-bus standard systems as well as a large-scale system with 1180 buses. Our simulation results show that, compared with other deterministic and metaheuristc algorithms, SCRO can find the optimal solutions in a shorter time for small-scale systems, and a near-optimal solution within a reasonable time even for a large-scale system.