Showing papers on "Static routing published in 2010"

QELAR: A Machine-Learning-Based Adaptive Routing Protocol for Energy-Efficient and Lifetime-Extended Underwater Sensor Networks

Abstract: Underwater sensor network (UWSN) has emerged in recent years as a promising networking technique for various aquatic applications. Due to specific characteristics of UWSNs, such as high latency, low bandwidth, and high energy consumption, it is challenging to build networking protocols for UWSNs. In this paper, we focus on addressing the routing issue in UWSNs. We propose an adaptive, energy-efficient, and lifetime-aware routing protocol based on reinforcement learning, QELAR. Our protocol assumes generic MAC protocols and aims at prolonging the lifetime of networks by making residual energy of sensor nodes more evenly distributed. The residual energy of each node as well as the energy distribution among a group of nodes is factored in throughout the routing process to calculate the reward function, which aids in selecting the adequate forwarders for packets. We have performed extensive simulations of the proposed protocol on the Aqua-sim platform and compared with one existing routing protocol (VBF) in terms of packet delivery rate, energy efficiency, latency, and lifetime. The results show that QELAR yields 20 percent longer lifetime on average than VBF.

Routing without routes: the backpressure collection protocol

Abstract: Current data collection protocols for wireless sensor networks are mostly based on quasi-static minimum-cost routing trees. We consider an alternative, highly-agile approach called backpressure routing, in which routing and forwarding decisions are made on a per-packet basis. Although there is a considerable theoretical literature on backpressure routing, it has not been implemented on practical systems to date due to concerns about packet looping, the effect of link losses, large packet delays, and scalability. Addressing these concerns, we present the Backpressure Collection Protocol (BCP) for sensor networks, the first ever implementation of dynamic backpressure routing in wireless networks. In particular, we demonstrate for the first time that replacing the traditional FIFO queue service in backpressure routing with LIFO queues reduces the average end-to-end packet delays for delivered packets drastically (75% under high load, 98% under low load). Further, we improve backpressure scalability by introducing a new concept of floating queues into the backpressure framework. Under static network settings, BCP shows a more than 60% improvement in max-min rate over the state of the art Collection Tree Protocol (CTP). We also empirically demonstrate the superior delivery performance of BCP in highly dynamic network settings, including conditions of extreme external interference and highly mobile sinks.

Symbiotic routing in future data centers

Abstract: Building distributed applications that run in data centers is hard. The CamCube project explores the design of a shipping container sized data center with the goal of building an easier platform on which to build these applications. CamCube replaces the traditional switch-based network with a 3D torus topology, with each server directly connected to six other servers. As in other proposals, e.g. DCell and BCube, multi-hop routing in CamCube requires servers to participate in packet forwarding. To date, as in existing data centers, these approaches have all provided a single routing protocol for the applications.In this paper we explore if allowing applications to implement their own routing services is advantageous, and if we can support it efficiently. This is based on the observation that, due to the flexibility offered by the CamCube API, many applications implemented their own routing protocol in order to achieve specific application-level characteristics, such as trading off higher-latency for better path convergence. Using large-scale simulations we demonstrate the benefits and network-level impact of running multiple routing protocols. We demonstrate that applications are more efficient and do not generate additional control traffic overhead. This motivates us to design an extended routing service allowing easy implementation of application-specific routing protocols on CamCube. Finally, we demonstrate that the additional performance overhead incurred when using the extended routing service on a prototype CamCube is very low.

Routing for disruption tolerant networks: taxonomy and design

Abstract: Communication networks, whether they are wired or wireless, have traditionally been assumed to be connected at least most of the time. However, emerging applications such as emergency response, special operations, smart environments, VANETs, etc. coupled with node heterogeneity and volatile links (e.g. due to wireless propagation phenomena and node mobility) will likely change the typical conditions under which networks operate. In fact, in such scenarios, networks may be mostly disconnected, i.e., most of the time, end-to-end paths connecting every node pair do not exist. To cope with frequent, long-lived disconnections, opportunistic routing techniques have been proposed in which, at every hop, a node decides whether it should forward or store-and-carry a message. Despite a growing number of such proposals, there still exists little consensus on the most suitable routing algorithm(s) in this context. One of the reasons is the large diversity of emerging wireless applications and networks exhibiting such "episodic" connectivity. These networks often have very different characteristics and requirements, making it very difficult, if not impossible, to design a routing solution that fits all. In this paper, we first break up existing routing strategies into a small number of common and tunable routing modules (e.g. message replication, coding, etc.), and then show how and when a given routing module should be used, depending on the set of network characteristics exhibited by the wireless application. We further attempt to create a taxonomy for intermittently connected networks. We try to identify generic network characteristics that are relevant to the routing process (e.g., network density, node heterogeneity, mobility patterns) and dissect different "challenged" wireless networks or applications based on these characteristics. Our goal is to identify a set of useful design guidelines that will enable one to choose an appropriate routing protocol for the application or network in hand. Finally, to demonstrate the utility of our approach, we take up some case studies of challenged wireless networks, and validate some of our routing design principles using simulations.

The shortest path problem revisited: optimal routing for electric vehicles

Abstract: Electric vehicles (EV) powered by batteries will play a significant role in the road traffic of the future. The unique characteristics of such EVs - limited cruising range, long recharge times, and the ability to regain energy during deceleration - require novel routing algorithms, since the task is now to determine the most economical route rather than just the shortest one. This paper proposes extensions to general shortestpath algorithms that address the problem of energy-optimal routing. Specifically, we (i) formalize energy-efficient routing in the presence of rechargeable batteries as a special case of the constrained shortest path problem (CSPP) with hard and soft constraints, and (ii) present an adaption of a general shortest path algorithm (using an energy graph, i.e., a graph with a weight function representing the energy consumption) that respects the given constraints and has a worst case complexity of O(n3). The presented algorithms have been implemented and evaluated within a prototypic navigation system for energy-efficient routing.

TPGF: geographic routing in wireless multimedia sensor networks

Abstract: In this paper, we propose an efficient Two-Phase geographic Greedy Forwarding (TPGF) routing algorithm for WMSNs TPGF takes into account both the requirements of real time multimedia transmission and the realistic characteristics of WMSNs It finds one shortest (near-shortest) path per execution and can be executed repeatedly to find more on-demand shortest (near-shortest) node-disjoint routing paths TPGF supports three features: (1) hole-bypassing, (2) the shortest path transmission, and (3) multipath transmission, at the same time TPGF is a pure geographic greedy forwarding routing algorithm, which does not include the face routing, eg, right/left hand rules, and does not use planarization algorithms, eg, GG or RNG This point allows more links to be available for TPGF to explore more routing paths, and enables TPGF to be different from many existing geographic routing algorithms Both theoretical analysis and simulation comparison in this paper indicate that TPGF is highly suitable for multimedia transmission in WMSNs

GeoDTN+Nav: Geographic DTN Routing with Navigator Prediction for Urban Vehicular Environments

Abstract: Position-based routing has proven to be well suited for highly dynamic environment such as Vehicular Ad Hoc Networks (VANET) due to its simplicity. Greedy Perimeter Stateless Routing (GPSR) and Greedy Perimeter Coordinator Routing (GPCR) both use greedy algorithms to forward packets by selecting relays with the best progress towards the destination or use a recovery mode in case such solutions fail. These protocols could forward packets efficiently given that the underlying network is fully connected. However, the dynamic nature of vehicular network, such as vehicle density, traffic pattern, and radio obstacles could create unconnected networks partitions. To this end, we propose GeoDTN+Nav, a hybrid geographic routing solution enhancing the standard greedy and recovery modes exploiting the vehicular mobility and on-board vehicular navigation systems to efficiently deliver packets even in partitioned networks. GeoDTN+Nav outperforms standard geographic routing protocols such as GPSR and GPCR because it is able to estimate network partitions and then improves partitions reachability by using a store-carry-forward procedure when necessary. We propose a virtual navigation interface (VNI) to provide generalized route information to optimize such forwarding procedure. We finally evaluate the benefit of our approach first analytically and then with simulations. By using delay tolerant forwarding in sparse networks, GeoDTN+Nav greatly increases the packet delivery ratio of geographic routing protocols and provides comparable routing delay to benchmark DTN algorithms.

Energy-aware routing in data center network

Abstract: The goal of data center network is to interconnect the massive number of data center servers, and provide efficient and fault-tolerant routing service to upper-layer applications. To overcome the problem of tree architecture in current practice, many new network architectures are proposed, represented by Fat-Tree, BCube, and etc. A consistent theme in these new architectures is that a large number of network devices are used to achieve 1:1 oversubscription ratio. However, at most time, data center traffic is far below the peak value. The idle network devices will waste significant amount of energy, which is now a headache for many data center owners.In this paper, we discuss how to save energy consumption in high-density data center networks in a routing perspective. We call this kind of routing energy-aware routing. The key idea is to use as few network devices to provide the routing service as possible, with no/little sacrifice on the network performance. Meanwhile, the idle network devices can be shutdown or put into sleep mode for energy saving. We establish the model of energy-aware routing in data center network, and design a heuristic algorithm to achieve the idea. Our simulation in typical data center networks shows that energy-aware routing can effectively save power consumed by network devices.

Replication routing in DTNs: a resource allocation approach

Abstract: Routing protocols for disruption-tolerant networks (DTNs) use a variety of mechanisms, including discovering the meeting probabilities among nodes, packet replication, and network coding. The primary focus of these mechanisms is to increase the likelihood of finding a path with limited information, and so these approaches have only an incidental effect on such routing metrics as maximum or average delivery delay. In this paper, we present rapid, an intentional DTN routing protocol that can optimize a specific routing metric such as the worst-case delivery delay or the fraction of packets that are delivered within a deadline. The key insight is to treat DTN routing as a resource allocation problem that translates the routing metric into per-packet utilities that determine how packets should be replicated in the system. We evaluate rapid rigorously through a prototype deployed over a vehicular DTN testbed of 40 buses and simulations based on real traces. To our knowledge, this is the first paper to report on a routing protocol deployed on a real outdoor DTN. Our results suggest that rapid significantly outperforms existing routing protocols for several metrics. We also show empirically that for small loads, RAPID is within 10% of the optimal performance.

Global dynamic routing for scale-free networks

Abstract: Traffic is essential for many dynamic processes on networks. The efficient routing strategy [G. Yan, T. Zhou, B. Hu, Z. Q. Fu, and B. H. Wang, Phys. Rev. E 73, 046108 (2006)] can reach a very high capacity of more than ten times of that with shortest path strategy. In this paper, we propose a global dynamic routing strategy for network systems based on the information of the queue length of nodes. Under this routing strategy, the traffic capacity is further improved. With time delay of updating node queue lengths and the corresponding paths, the system capacity remains constant, while the travel time for packets increases.

Energy-Efficient Beaconless Geographic Routing in Wireless Sensor Networks

Abstract: Geographic routing is an attractive localized routing scheme for wireless sensor networks (WSNs) due to its desirable scalability and efficiency. Maintaining neighborhood information for packet forwarding can achieve a high efficiency in geographic routing, but may not be appropriate for WSNs in highly dynamic scenarios where network topology changes frequently due to nodes mobility and availability. We propose a novel online routing scheme, called Energy-efficient Beaconless Geographic Routing (EBGR), which can provide loop-free, fully stateless, energy-efficient sensor-to-sink routing at a low communication overhead without the help of prior neighborhood knowledge. In EBGR, each node first calculates its ideal next-hop relay position on the straight line toward the sink based on the energy-optimal forwarding distance, and each forwarder selects the neighbor closest to its ideal next-hop relay position as the next-hop relay using the Request-To-Send/Clear-To-Send (RTS/CTS) handshaking mechanism. We establish the lower and upper bounds on hop count and the upper bound on energy consumption under EBGR for sensor-to-sink routing, assuming no packet loss and no failures in greedy forwarding. Moreover, we demonstrate that the expected total energy consumption along a route toward the sink under EBGR approaches to the lower bound with the increase of node deployment density. We also extend EBGR to lossy sensor networks to provide energy-efficient routing in the presence of unreliable communication links. Simulation results show that our scheme significantly outperforms existing protocols in wireless sensor networks with highly dynamic network topologies.

Systems and Methods for Fractional Routing Redundancy

Abstract: Systems and methods for fractional routing are described. An exemplary method may include receiving, by a first router, data information regarding routing by a first portion of a third router, receiving, by a second router, data information regarding routing by a second portion of a third router, wherein the data information regarding routing by the first portion and data information regarding routing by the second portion is not the same, routing, by the first router, data associated with the routing by the first portion of the third router, and routing by the second router, data associated with the routing by the second portion of the third router.

Stateless, affinity-preserving load balancing

Abstract: The invention relates to an architecture that facilitates load balancing among a plurality of hosts and preserve session affinity to a given host. An incoming stream of data packets that include packet sessions is input to one or more forwarding mechanisms for forwarding to one or more hosts. The forwarders generate a routing function that takes into consideration host availability, and distributes session packets according to the routing function. A session is distributed to the same host to preserve session affinity. When host availability changes, a new routing function is generated, such that any new session is routed according to the new routing function and existing sessions are routed according to the old routing function. When the old routing function becomes irrelevant, it is phased out. An optimization utilizes a maximally backward compatible hash function to minimize the differences between the old and new routing functions.

Exact algorithms for routing problems under vehicle capacity constraints

Abstract: The solution of a vehicle routing problem calls for the determination of a set of routes, each performed by a single vehicle which starts and ends at its own depot, such that all the requirements of the customers are fulfilled and the global transportation cost is minimized. The routes have to satisfy several operational constraints which depend on the nature of the transported goods, on the quality of the service level, and on the characteristics of the customers and of the vehicles. One of the most common operational constraint addressed in the scientific literature is that the vehicle fleet is capacitated and the total load transported by a vehicle cannot exceed its capacity.

A tutorial on column generation and branch-and-price for vehicle routing problems

Abstract: This paper provides a tutorial on column generation and branch-and-price for vehicle routing problems. The main principles and the basic theory of the methods are first outlined. Some additional issues, including reinforcement of the relaxation or stabilization, complete the paper. For the sake of simplicity, this material is illustrated with the case of the vehicle routing problem with time windows.

Backpressure-based routing protocol for DTNs

Abstract: In this paper we consider an alternative, highly agile In this paper we consider an alternative, highly agile approach called backpressure routing for Delay Tolerant Networks (DTN), in which routing and forwarding decisions are made on a per-packet basis. Using information about queue backlogs, random walk and data packet scheduling nodes can make packet routing and forwarding decisions without the notion of end-to-end routes. To the best of our knowledge, this is the first ever implementation of dynamic backpressure routing in DTNs. Simulation results show that the proposed approach has advantages in terms of DTN networks.

Home Automation Routing Requirements in Low-Power and Lossy Networks

Abstract: This document presents home control and automation application specific requirements for Routing Over Low power and Lossy networks (ROLL). In the near future many homes will contain high numbers of wireless devices for a wide set of purposes. Examples include actuators (relay, light dimmer, heating valve), sensors (wall switch, water leak, blood pressure) and advanced controllers (RF-based AV remote control, Central server for light and heat control). Because such devices only cover a limited radio range, routing is often required. The aim of this document is to specify the routing requirements for networks comprising such constrained devices in a home control and automation environment.

Time Windows Based Dynamic Routing in Multi-AGV Systems

Abstract: This paper presents a dynamic routing method for supervisory control of multiple automated guided vehicles (AGVs) that are traveling within a layout of a given warehouse. In dynamic routing a calculated path particularly depends on the number of currently active AGVs' missions and their priorities. In order to solve the shortest path problem dynamically, the proposed routing method uses time windows in a vector form. For each mission requested by the supervisor, predefined candidate paths are checked if they are feasible. The feasibility of a particular path is evaluated by insertion of appropriate time windows and by performing the windows overlapping tests. The use of time windows makes the algorithm apt for other scheduling and routing problems. Presented simulation results demonstrate efficiency of the proposed dynamic routing. The proposed method has been successfully implemented in the industrial environment in a form of a multiple AGV control system.

Network coding-aware routing in wireless networks

Abstract: A recent approach--COPE, presented by Katti et al. (Proc. ACM SIGCOMM 2006, pp. 243-254)--for improving the throughput of unicast traffic in wireless multihop networks exploits the broadcast nature of the wireless medium through opportunistic network coding. In this paper, we analyze throughput improvements obtained by COPE-type network coding in wireless networks from a theoretical perspective. We make two key contributions. First, we obtain a theoretical formulation for computing the throughput of network coding on any wireless network topology and any pattern of concurrent unicast traffic sessions. Second, we advocate that routing be made aware of network coding opportunities rather than, as in COPE, being oblivious to it. More importantly, our model considers the tradeoff between routing flows close to each other for utilizing coding opportunities and away from each other for avoiding wireless interference. Our theoretical formulation provides a method for computing source-destination routes and utilizing the best coding opportunities from available ones so as to maximize the throughput. We handle scheduling of broadcast transmissions subject to wireless transmit/receive diversity and link interference in our optimization framework. Using our formulations, we compare the performance of traditional unicast routing and network coding with coding-oblivious and coding-aware routing on a variety of mesh network topologies, including some derived from contemporary mesh network testbeds. Our evaluations show that a route selection strategy that is aware of network coding opportunities leads to higher end-to-end throughput when compared to coding-oblivious routing strategies.

Clustering and cluster-based routing protocol for delay-tolerant mobile networks

Abstract: This research investigates distributed clustering scheme and proposes a cluster-based routing protocol for Delay-Tolerant Mobile Networks (DTMNs). The basic idea is to distributively group mobile nodes with similar mobility pattern into a cluster, which can then interchangeably share their resources (such as buffer space) for overhead reduction and load balancing, aiming to achieve efficient and scalable routing in DTMN. Due to the lack of continuous communications among mobile nodes and possible errors in the estimation of nodal contact probability, convergence and stability become major challenges in distributed clustering in DTMN. To this end, an exponentially weighted moving average (EWMA) scheme is employed for on-line updating nodal contact probability, with its mean proven to converge to the true contact probability. Based on nodal contact probabilities, a set of functions including Sync(), Leave(), and Join() are devised for cluster formation and gateway selection. Finally, the gateway nodes exchange network information and perform routing. Extensive simulations are carried out to evaluate the effectiveness and efficiency of the proposed cluster-based routing protocol. The simulation results show that it achieves higher delivery ratio and significantly lower overhead and end-to-end delay compared with its non-clustering counterpart.

Secure Routing in Wireless Sensor Networks.

Abstract: This research addresses communication security in the highly constrained wireless sensor environment. The goal of the research is twofold: (1) to develop a key management scheme that provides these constrained systems with the basic security requirements and evaluate its effectiveness in terms of scalability, efficiency, resiliency, connectivity, and flexibility, and (2) to implement this scheme on an appropriate routing platform and measure its efficiency. The proposed key management scheme is called Hierarchical Key Establishment Scheme (HIKES). In HIKES, the base station, acting as the central trust authority, empowers randomly selected sensors to act as local trust authorities, authenticating on its behalf the cluster members and issuing to them all secret keys necessary to secure their communications. HIKES uses a novel key escrow scheme that enables any sensor node selected as a cluster head to generate all the cryptographic keys needed to authenticate other sensors within its cluster. This scheme localizes secret key issuance and reduces the communication cost with the base station. The key escrow scheme also provides the HIKES with as large an addressing mechanism as needed. HIKES also provides a one-step broadcast authentication mechanism. HIKES provides entity authentication to every sensor in the network and is robust against most known attacks. We propose a hierarchical routing mechanism called Secure Hierarchical Energy-Efficient Routing protocol (SHEER). SHEER implements HIKES, which provides the communication security from the inception of the network. SHEER uses a probabilistic broadcast mechanism and a three-level hierarchical clustering architecture to improve the network energy performance and increase its lifetime. Simulation results have shown that HIKES provides an energy-efficient and scalable solution to the key management problem. Cost analysis shows that HIKES is computationally efficient and has low storage requirement. Furthermore, high degree of address flexibility can be achieved in HIKES. Therefore, this scheme meets the desired criteria set forth in this work. Simulation studies also show that SHEER is more energy-efficient and has better scalability than the secure version of LEACH using HIKES.

The Deployment of a Smart Monitoring System Using Wireless Sensor and Actuator Networks

Abstract: In this paper, we explore the implementation of a smart monitoring system over a wireless sensor network, with particular emphasis on the creation of a solid routing infrastructure through the Routing Protocol for Low- power and lossy networks (RPL), whose definition is currently being discussed within the IETF ROLL working group. Our framework is based on a very lightweight implementation of the REpresentational State Transfer (REST) paradigm by means of a binary web service, and on a publish/subscribe mechanism, whereby every node makes a set of resources (e.g., environmental sensors) available to interested parties. Limited to the effectiveness in creating the routing structure, we provide a performance evaluation of RPL through an experimental campaign, aimed at showing how RPLs key parameters affect the performance of routing in a smart grid scenario.

Energy-aware routing: A reality check

Abstract: In this work, we analyze the design of green routing algorithms and evaluate the achievable energy savings that such mechanisms could allow in several realistic network scenarios. We formulate the problem as a minimum energy routing optimization, which we numerically solve considering a core-network scenario, which can be seen as a worst-case for energy saving performance (as nodes cannot be switched off). To gather full-relief results, we analyze the energy savings in various conditions (i.e., network topology and traffic matrix) and under different technology assumptions (i.e., the energy profile of the network devices). These results give us insight into the potential benefits of different “green” technologies and their interactions. In particular, we show that depending on the topology and traffic matrices, the optimal energy savings can be modest, partly limiting the interest for green routing approaches for some scenarios. At the same time, we also show that the common belief that there is a trade off between green network optimization and performance does not necessarily hold: in the considered environment, green routing has no effect on the main network performances such as maximum link utilization.

Custom routing decisions

Abstract: With the advent of virtualization technologies, networks and routing for those networks can now be simulated using commodity hardware rather than actual routers For example, virtualization technologies such as those provided by VMWare, XEN, or User-Mode Linux can be adapted to allow a single physical computing machine to be shared among multiple virtual networks by providing each virtual network user with one or more virtual machines hosted by the single physical computing machine, with each such virtual machine being a software simulation acting as a distinct logical computing system that provides users with the illusion that they are the sole operators and administrators of a given hardware computing resource In addition, routing can be accomplished through software, providing additional routing flexibility to the virtual network in comparison with traditional routing As a result, in some implementations, supplemental information other than packet information can be used to determine network routing

DTN routing in body sensor networks with dynamic postural partitioning

Abstract: This paper presents novel store-and-forward packet routing algorithms for Wireless Body Area Networks (WBAN) with frequent postural partitioning. A prototype WBAN has been constructed for experimentally characterizing on-body topology disconnections in the presence of ultra short range radio links, unpredictable RF attenuation, and human postural mobility. On-body DTN routing protocols are then developed using a stochastic link cost formulation, capturing multi-scale topological localities in human postural movements. Performance of the proposed protocols are evaluated experimentally and via simulation, and are compared with a number of existing single-copy DTN routing protocols and an on-body packet flooding mechanism that serves as a performance benchmark with delay lower-bound. It is shown that via multi-scale modeling of the spatio-temporal locality of on-body link disconnection patterns, the proposed algorithms can provide better routing performance compared to a number of existing probabilistic, opportunistic, and utility-based DTN routing protocols in the literature.

A New Cluster Based Routing Protocol for VANET

Abstract: With the development of vehicles and mobile Ad Hoc network technology, the Vehicle Ad hoc Network (VANET) has become an emerging field of study It is a challenging problem for searching and maintaining an effective route for transporting some data information In this paper the authors designed a new routing protocol for VANET based on the former results, called CBR (Cluster Based Routing) Compared with other routing protocols, the new one has obvious improvement in the average routing overhead and small average end to end delay jitter with the increase of vehicles number The real-time traffic applications require data transmission delay time to be relatively stable, small average end to end delay jitter with the increase of vehicles number just meets the real-time application needs

Fair Dynamic Routing in Large-Scale Heterogeneous-Server Systems

Abstract: In a call center, there is a natural trade-off between minimizing customer wait time and fairly dividing the workload among agents of different skill levels. The relevant control is the routing policy, that is, the decision concerning which agent should handle an arriving call when more than one agent is available. We formulate an optimization problem for a call center with heterogeneous agent pools, in which each pool is distinguished by the speed at which agents in that pool handle calls. The objective is to minimize steady-state expected customer wait time subject to a “fairness” constraint on the workload division. We first solve the optimization problem by formulating it as a Markov decision process (MDP), and solving a related linear program. We note that this approach does not in general lead to an optimal policy that has a simple structure. Fortunately, the optimal policy does appear to have a simple structure as the system size grows large, in the Halfin-Whitt many-server heavy-traffic limit regime. Therefore, we solve the diffusion control problem that arises in this regime and interpret its solution as a policy for the original system. The resulting routing policy is a threshold policy that determines server pool priorities based on the total number of customers in the system. We prove that a continuous modification of our proposed threshold routing policy is asymptotically optimal in the Halfin-Whitt limit regime. We furthermore present simulation results to illustrate that our proposed threshold routing policy outperforms a common routing policy used in call centers (that routes to the agent that has been idle the longest).