Ramesh Nagarajan

On optimal call admission control in cellular networks

Two important Quality-of-Service (QoS) measures for current cellular networks are the fractions of new and handoff “calls” that are blocked due to unavailability of “channels” (radio and/or computing resources). Based on these QoS measures, we derive optimal admission control policies for three problems: minimizing a linear objective function of the new and handoff call blocking probabilities (MINOBJ), minimizing the new call blocking probability with a hard constraint on the handoff call blocking probability (MINBLOCK) and minimizing the number of channels with hard constraints on both of the blocking probabilities (MINC). We show that the well-known Guard Channel policy is optimal for the MINOBJ problem, while a new Fractional Guard Channel policy is optimal for the MINBLOCK and MINC problems. The Guard Channel policy reserves a set of channels for handoff calls while the Fractional Guard Channel policy effectively reserves a non-integral number of guard channels for handoff calls by rejecting new calls with some probability that depends on the current channel occupancy. It is also shown that the Fractional policy results in significant savings (20-50\%) in the new call blocking probability for the MINBLOCK problem and provides some, though small, gains over the Guard Channel policy for the MINC problem. Further, we also develop computationally inexpensive algorithms for the determination of the parameters for the optimal policies.

Approximation techniques for computing packet loss in finite-buffered voice multiplexers

Three different approximation techniques are examined. The performance models studied differ primarily in the manner in which the superposition of the voice sources (i.e., the arrival process) is modeled. The first approach models the superimposed voice sources as a renewal process, and performance calculations are based only on the first two moments of the renewal process. The second approach is based on modeling the superimposed voice sources as a Markov modulated Poisson process (MMPP). The choice of parameters for the MMPP attempts to capture aspects of the arrival process in a more intuitive manner than previously proposed approaches for determining the MMPP parameters and is shown to compute loss more accurately. Finally, a fluid flow approximation for computing packet loss is evaluated. For all three approaches, a unifying example, the case of multiplexing voice sources over a T1-rate link is considered. The main conclusion is that both the MMPP model and the fluid flow approximation can provide accurate loss predictions for parameter ranges of practical interest. >

On defining, computing and guaranteeing quality-of-service in high-speed networks

Future high-speed networks are expected to support a wide variety of services such as voice and video, and to provide a guaranteed quality-of-service (QOS). The authors examine the issues of computing and guaranteeing QOS. Traditionally, the computation of user-oriented performance criteria such as the average delay has been carried out via steady-state analysis of queuing theoretic models of communication networks. It is shown that the steady-state computations are often not sufficient for QOS purposes in future high-speed networks. The authors provide mechanisms for computing and guaranteeing QOS criteria and consider the issue of approximate QOS criteria. It is argued that, for certain envisaged applications, traditional QOS criteria are not appropriate. A QOS criterion for such applications is proposed.<<ETX>>

Critical Infrastructure Analysis of Telecom for Natural Disasters

Critical national infrastructures for power, emergency services, finance, and other basic industries rely heavily on information and telecommunications networks (voice, data, Internet) to provide services and conduct business. While these networks tend to be highly reliable, severe, large scale outages do occur, especially at times of unfolding disasters, which can lead to cascading effects on other dependent infrastructures. This paper describes recent natural disasters in the USA, namely hurricanes Katrina, Rita, and Wilma, and the impacts they have had on power outages and flooding leading to failures on the dependent critical infrastructure. In particular we consider the impact of these disasters on the affected telecommunication networks. We quantify the level of availability of wireline and wireless services during these network failures. We also discuss studies that were performed in preparation for a hurricane impact. The studies have been performed using the network simulation modeling and analysis research tool (N-SMART), which has been developed to support detailed wireline and wireless network simulations under varying network conditions and degrees of failures. We analyze the levels of outages and recovery times for these disaster events as well as possible mitigations to prepare in advance for these and other potential future disasters

Rate control algorithms for the ATM ABR service

Over the last couple of years both the ATM Forum and ITU-T have converged on a closed-loop, feedback based rate control mechanism as the framework for the Available Bit Rate ATM Transfer Capability. In this framework, an ABR source adapts its transmission rate to changing characteristics of the ATM network by either testing periodically the state of the network or through direct feedback from the network. The state of the network is conveyed to the source either as a simple binary feedback signaling congestion or no congestion, or as an explicit rate indicating the rate at which the source can transmit. In this paper we review the detailed framework for the ABR service standardized by the ATM Forum, the underlying service philosophy and protocol design goals for ABR flow control, and the premises and high-level performance characteristics for the more promising ABR rate control schemes proposed so far.

First-order rate-based flow control with dynamic queue threshold for high-speed wide-area ATM networks

In this paper we present a new rate-based flow control scheme for ATM ABR services and analyze its performance. The proposed algorithm, which we refer to as first-order rate-based flow control (FRFC) is the most simple form of queue-length-based flow control. The asymptotic stability, the steady-state throughput, queue length and fairness, and the transient behavior are analyzed for the case of multiple connections with diverse round-trip delays. We also consider a novel approach to dynamically adjust a queue threshold in the FRFC according to the changes in the available bandwidth, and the arrival and departure of connections. Simulations show that the simple FRFC with dynamic queue threshold (DQT) effectively maintains high throughput, small loss and a desired fairness in these dynamic environments and is a promising solution for ABR flow control in ATM networks.

Designing stable ABR flow control with rate feedback and open-loop control: first-order control case

In this paper we present a control-theoretic approach to design stable rate-based flow control for ATM ABR services. The flow control algorithm that we consider has the most simple form among all the queue-length-based flow control algorithms, and is referred to as first-order rate-based flow control (FRFC) since the corresponding closed loop can be modeled as a first-order retarded differential equation. We analyze the equilibrium and the asymptotic stability of the closed loop for the case of multiple connections with diverse round-trip delays. We also characterize the asymptotic decay rate at which the stable closed loop tends to the equilibrium. The decay rate is shown to be a concave function of control gain with its maximum being the inverse of round-trip delay. We also consider an open loop control in which the queue control threshold is dynamically adjusted according to the changes in the available bandwidth and the number of connections. This open loop control is shown to be necessary and effective to prevent the closed loop from converging to an undesirable equilibrium point.

Fast, scalable, and distributed restoration in general mesh optical networks

Service providers are demanding transport network solutions that can accommodate exponential traffic growth and, at the same time, provide novel services such as point-and-click provisioning of very high bandwidth circuits, optical bandwidth service management, fast protection and restoration, and bandwidth on demand. It is becoming increasingly clear that the required scalability (terabits/s to petabits/s) and cost structure can only be provided by transparent optical cross connects (OXCs). The challenge, then, is to make the optical network consisting of OXCs, dense wavelength division multiplexers (DWDMs), and optical add/drop multiplexers (OADMs) dynamic and intelligent. A major aspect of this intelligence is fast provisioning and restoration. In this paper, we present a fast, scalable, and distributed solution for optical layer restoration in general mesh-type optical networks, which is being implemented as part of the Optical Navigator System (ONS) residing in Lucents LambdaRouter product. The key ingredients to our solution are a fast and scalable restoration strategy, a fast and scalable connection setup strategy, a contention-free wavelength assignment strategy, and a fast and reliable data communications network to exchange signaling messages. We also introduce novel concepts of demand bundling and optical virtual paths that ensure that restoration performance scales with network and traffic volumes. Together these components provide an intelligent optical networking solution that not only guarantees restoration times within few hundreds of milliseconds, but also achieves scalability.

Cost effectiveness of joint multilayer protection in packet-over-optical networks

The recent advances in generalized multiprotocol label switching/multiprotocol label switching (GMPLS/MPLS) and extensive study and understanding of packet-over-optical network architecture have made it possible to manage the resources of the packet network and underlying optical transport network in an interoperable manner. This opens up the possibility for coordinated actions across the two networking layers. In this context, we propose and evaluate a novel joint protection scheme for future packet-over-optical networks. There are two aspects that make the proposed joint scheme novel and, furthermore, cost effective: 1) It captures the best tradeoff between the finer grooming granularity of the packet layer and the cheaper port cost of the optical layer, and 2) it reuses network resources from both the primary paths and paths that exist for link failure protection by the optical layer. To examine the proposed approach, a mixed-integer optimization model was developed. In particular, we observe that overlaying joint packet-optical router protection with transport-layer link protection permits strong synergy of protection capacity reuse. Indeed, our case studies show that the proposed joint packet-optical router protection is low- or no-cost when it is overlaid on an existing transport-layer protection scheme.

Generalized MPLS-based distributed control architecture for automatically switched transport networks

Current circuit-switched transport networks, such as plesiochronous digital hierarchy (PDH) and synchronous optical network/synchronous digital hierarchy (SONET/SDH), have traditionally used centralized network management for connection control. To facilitate the value-added capabilities of todays networks — such as the rapid provisioning of services, dynamic setup of bandwidth requests, and fast mesh-based restoration — distributed connection control using signaling protocols has quickly gained industry momentum. Efforts have been initiated in various standards bodies to define the automatically switched transport network (ASTN). Although many architectural choices are now available, this paper describes a distributed control plane architecture that can be applied to various circuit-switching technologies and different network applications. This architecture adopts the concept of a generalized version of multiprotocol label switching (MPLS), which extends and modifies MPLS and other protocols on the Internet to make them applicable to various transport networks and also facilitates optical data networking. Four major functional components are incorporated in this architecture: element-level resource discovery, state information dissemination, path selection, and path control modules. Using these concepts, the transport network can be viewed as a virtual nonblocking, reconfigurable backplane of different network clients. This view represents a radical departure from the traditional data networking view of transport networks as providing fixed pipes and will have a dramatic impact on future network interworking and end-to-end traffic engineering (TE).