Leonidas Georgiadis

Resource allocation and cross-layer control in wireless networks

Information flow in a telecommunication network is accomplished through the interaction of mechanisms at various design layers with the end goal of supporting the information exchange needs of the applications. In wireless networks in particular, the different layers interact in a nontrivial manner in order to support information transfer. In this text we will present abstract models that capture the cross-layer interaction from the physical to transport layer in wireless network architectures including cellular, ad-hoc and sensor networks as well as hybrid wireless-wireline. The model allows for arbitrary network topologies as well as traffic forwarding modes, including datagrams and virtual circuits. Furthermore the time varying nature of a wireless network, due either to fading channels or to changing connectivity due to mobility, is adequately captured in our model to allow for state dependent network control policies. Quantitative performance measures that capture the quality of service requirements in these systems depending on the supported applications are discussed, including throughput maximization, energy consumption minimization, rate utility function maximization as well as general performance functionals. Cross-layer control algorithms with optimal or suboptimal performance with respect to the above measures are presented and analyzed. A detailed exposition of the related analysis and design techniques is provided.

A management and control architecture for providing IP differentiated services in MPLS-based networks

As the Internet evolves toward the global multiservice network of the future, a key consideration is support for services with guaranteed quality of service. The proposed differentiated services framework is seen as the key technology to achieve this. DiffServ currently concentrates on control/data plane mechanisms to support QoS, but also recognizes the need for management plane aspects through the bandwidth broker. In this article we propose a model and architectural framework for supporting DiffServ-based end-to-end QoS in the Internet, assuming underlying MPLS-based explicit routed paths. The proposed integrated management and control architecture will allow providers to offer both quantitative and qualitative services while optimizing the use of underlying network resources.

Optimal buffer sharing

We address the problem of designing optimal buffer management policies in shared memory switches when packets already accepted in the switch can be dropped (pushed-out). Our goal is to maximize the overall throughput, or equivalently to minimize the overall loss probability in the system. For a system with two output ports, we prove that the optimal policy is of push-out with threshold type (POT). The same result holds if the optimality criterion is the weighted sum of the port loss probabilities. For this system, we also give an approximate method for the calculation of the optimal threshold, which we conjecture to be asymptotically correct. For the N-ported system, the optimal policy is not known in general, but we show that for a symmetric system (equal traffic on all ports) it consists of always accepting arrivals when the buffer is not full, and dropping one from the longest queue to accommodate the new arrival when the buffer is full. Numerical results are provided which reveal an interesting and somewhat unexpected phenomenon. While the overall improvement in loss probability of the optimal POT policy over the optimal coordinate-convex policy is not very significant, the loss probability of an individual output port remains approximately constant as the load on the other port varies and the optimal POT policy is applied, a property not shared by the optimal coordinate-convex policy. >

Broadcast erasure channel with feedback - Capacity and algorithms

We consider the two-user broadcast erasure channel where feedback in the form of ack messages is fed back to the transmitter. We provide an upper bound to the capacity region of this system. We then present two algorithms whose rate region (information bits per transmitted bit) becomes arbitrarily close to the upper bound for large packet sizes. The first algorithm relies on random coding techniques while the second relies only on XOR operations between pairs of packets. Complexity and feedback information tradeoffs for the two algorithms are discussed. For the case where, in addition to traffic destined exclusively to either one of the users there is additional multicast traffic, we present an algorithm that shows that the rate region of the system can be increased by allowing intersession coding. Finally, for the case where there are random arrivals to the system we present an algorithm, based on the previous algorithms, whose stability region gets close to the capacity region for reasonably large packet sizes. The latter algorithm operates without knowledge of arrival process and channel statistics.

Efficient support of delay and rate guarantees in an internet

In this paper, we investigate some issues related to the efficient provision of end-to-end delay guarantees in the context of the Guaranteed (G) Services framework [16]. First, we consider the impact of reshaping traffic within the network on the end-to-end delay, the end-to-end jitter, as well as per-hop buffer requirements. This leads us to examine a class of traffic disciplines that use reshaping at each hop, namely rate-controlled disciplines. In this case, it is known that it is advantageous to use the Earliest Deadline First (EDF) scheduling policy at the link scheduler [8]. For this service discipline, we determine the appropriate values of the parameters that have to be exported, as specified in [16]. Subsequently, with the help of an example, we illustrate how the G service traffic will typically underutilize the network, regardless of the scheduling policy used. We then define a Guaranteed Rate (GR) service, that is synergetic with the G service framework and makes use of this unutilized bandwidth to provide rate guarantees to flows. We outline some of the details of the GR service and explain how it can be supported in conjunction with the G service in an efficient manner.

Multiuser Broadcast Erasure Channel With Feedback—Capacity and Algorithms

We consider the N-user broadcast erasure channel with N unicast sessions (one for each user) where receiver feedback is regularly sent to the transmitter in the form of ACK/NACK messages. We first provide a generic outer bound to the capacity of this system; we then propose a virtual-queue-based inter-session mixing coding algorithm, determine its rate region, and show that it achieves capacity under certain conditions on channel statistics, assuming that instantaneous feedback is known to all users. Removing this assumption results in a rate region that asymptotically differs from the outer bound by 1 bit as L → ∞, where L is the number of bits per packet (packet length). For the case of arbitrary channel statistics, we present a modification of the previous algorithm whose rate region is identical to the outer bound for N = 3, when instant feedback is known to all users, and differs from the bound by 1 bit as L → ∞, when the three users know only their own ACK. The proposed algorithms do not require any prior knowledge of channel statistics.

A Collision Resolution Protocol for Random Access Channels with Energy Detectors

In this paper, we consider the random accessing of a single slotted channel by a large number of packet-transmitting, bursty users. We assume that feedback broadcasting is available where some different information, in addition to the information assumed by the Capetanakis, Gallager, Massey, etc., models, is included in the feedback. In particular, we assume that the existence of energy detectors permits the broadcasting of the number of collided packets within each collision slot, whenever this number is below a certain limit. We first consider this limit to be infinity, and then a finite small number. For the model considered, we propose and analyze a collision resolution protocol (CRAI), whose implementation is simple. For Poisson input traffic and infinite number of energy detectors, we found that the CRAI is stable for input rates below 0.53237. For finite number of energy detectors, we propose a modified version of the CRAI (MCRAI). We found that the MCRAI reaches the throughput 0.53237, through the utilization of only about eight energy detectors. These protocols, like the ones introduced by Capetanakis, Gallager, Massey, etc., have good delay properties.

Stability of token passing rings

A sufficient stability condition for the standard token passing ring has been “known” since the seminal paper by Kuehn in 1979. However, this condition was derived without formal proof, and the proof seems to be of considerable interest to the research community. In fact, Watson observed that in the performance evaluation of token passing rings, “it is convenient to derive stability conditions ... (without proof)”. Our intention is to fill this gap, and to provide a formal proof of thesufficient and necessary stability condition for the token passing ring. In this paper, we present the case when the arrival process to each queue is Poisson but service times and switchover times are generally distributed. We consider in depth a gatedl-limited (l≤ ∞) service discipline for each station. We also indicate that the basic steps of our technique can be used to study the stability of some other multiqueue systems.

Exploiting wireless channel state information for throughput maximization

The problem of scheduling packets over a number of channels with time varying connectivity is considered. Policies proposed for this problem either stabilize the system when the arrival rates are within the stability region, or optimize an objective function under the assumption that all channel queues are saturated. We address the realistic situation where it is not known a priori whether the channel queues are saturated or not, and provide a scheduling policy that maximizes the weighted sum of channel throughputs. We employ a burstiness-constrained channel model that allows us to dispense of statistical assumptions and simplifies the proofs.We consider the problem of scheduling packets over channels with time-varying quality. This problem has received a lot of attention lately in the context of devising methods for providing quality of service in wireless communications. Earlier work dealing with this problem considered two cases. One case is that the arrival rate vector is in the throughput region and then policies that stabilize the system are pursued. The other case is that all packet queues are saturated and then policies that optimize an objective function of the channel throughputs are investigated. In this paper, we address the case where no assumption on the arrival rates is made. We obtain a scheduling policy that maximizes the weighted sum of channel throughputs. Under the optimal policy, in the general case, the system may operate in a regime where some queues are stable, while the other become saturated. If stability for the whole system is at all possible, it is always achieved. The optimal policy is a combination of a criterion that gives priorities based on queue lengths and a strict priority rule. The scheduling mechanism switches between the two criteria based on thresholds on the queue lengths and is modulated by the availability of the channels. The analysis of the operation of the system involves the study of a vector process which in steady state has some of its components stable while others are unstable. We adopt a novel model for time-varying channel availability that dispenses with the statistical assumptions and makes a rigorous description of system dynamics possible.

Any work-conserving policy stabilizes the ring with spatial re-use

We consider the ring network with spatial reuse. Traffic streams may enter and exit the network at any node. We adopt an arrival traffic model with deterministic constraints on its sample paths, which conforms to the output traffic of a leaky bucket rate control mechanism. A transmission policy specifies each time at which the traffic stream will be transmitted at the outgoing link by each node. We provide an upper bound on the asymptotic backlog of the ring that holds for all work-conserving policies and is independent of the initial conditions. This bound remains finite as long as the maximum load of every link is less than one. The latter condition is also necessary for the existence of an asymptotic bound that is independent of the initial conditions.