Michael D. Logothetis

Connection-dependent threshold model: a generalization of the Erlang multiple rate loss model

In this paper first, we review two extensions of the Erlang multi-rate loss model (EMLM), whereby we can assess the call-level quality-of-service (QoS) of ATM networks. The call-level QoS assessment in ATM networks remains an open issue, due to the emerged elastic services. We consider the coexistence of ABR service with QoS guarantee services in a VP link and evaluate the call blocking probability (CBP), based on the EMLM extensions. In the first extension, the retry models, blocked calls can retry with reduced resource requirements and increased arbitrary mean residency requirements. In the second extension, the threshold models, for blocking avoidance, calls can attempt to connect with other than the initial resource and residency requirements which are state dependent. Secondly, we propose the connection-dependent threshold model (CDTM), which resembles the threshold models, but the state dependency is individualized among call-connections. The proposed CDTM not only generalizes the existing threshold models but also covers the EMLM and the retry models by selecting properly the threshold parameters. Thirdly, we provide formulas for CBP calculation that incorporate bandwidth/trunk reservation schemes, whereby we can balance the grade-of-service among the service-classes. Finally, we investigate the effectiveness of the models applicability on ABR service at call set-up. The retry models can hardly model the behavior of ABR service, while the threshold models perform better than the retry models. The CDTM performs much better than the threshold models; therefore we propose it for assessing the call-level performance of ABR service. We evaluate the above-mentioned models by comparing each other according to the resultant CBP in ATM networks. For the models validation, results obtained by the analytical models are compared with simulation results.

Medium-term centralized virtual-path bandwidth control based on traffic measurements

The paper presents a centralized virtual path bandwidth (VPB) control scheme for ATM networks which satisfies mainly three specifications: a) optimality during a medium-term control interval, b) fast time response so as to absorb the medium-term traffic fluctuations, and c) easy implementation. The paper mainly points at the impact of direct, on-line traffic measurements on bandwidth control. The control objective is to rearrange the installed bandwidth of the virtual paths according to the offered traffic so as to minimize the maximum call blocking probability of the whole network. Network simulation shows that a sophisticated VPB controller which relies on simple measurements of the offered traffic can substantially improve the performance of an ATM network. The necessary bandwidth rearrangement time is also examined by simulation. >

Performance Analysis of OCDMA PONs Supporting Multi-Rate Bursty Traffic

Optical Code Division Multiple Access (OCDMA) provides increased security communications with large dedicated bandwidth to end users and simplified network control. We analyse the call-level performance of an OCDMA Passive Optical Network (PON) configuration, which accommodates multiple service-classes with finite traffic source population. The considered user activity is in accordance with the bursty nature of traffic, so that calls may alternate between active (steady transmission of a burst) and passive states (no transmission at all). Parameters related to multiple access interference, additive noise, user activity and number of traffic sources are incorporated to our analysis, which is based on a two-dimensional Markov chain. An approximate recursive formula is derived for efficient calculation of call blocking probability. Furthermore, we determine the burst blocking probability; burst blocking occurs when a burst delays its returning from passive to active state. The accuracy of the model is completely satisfactory and is verified through simulation. Moreover, we reveal the consistency and necessity of the proposed model.

On the End-to-End Delay Analysis of the IEEE 802.11 Distributed Coordination Function

The IEEE 802.11 protocol is the dominant standard for wireless local area networks (WLANs) and has generated much interests in investigating and improving its performance. The IEEE 802.11 medium access control (MAC) is mainly based on the distributed coordination function (DCF). DCF uses a carrier sense multiple access with collision avoidance (CSMA/CA) protocol in order to resolve contention between wireless stations and to verify successful transmissions. In this paper we present an extensive investigation of the performance of the IEEE 802.11b MAC protocol, in respect of end-to-end delay. The end-to-end delay analysis of the IEEE 802.11b has not been completed, because no adequate queuing delay is provided. Our delay analysis is based on Bianchis model for the DCF, while a more comprehensive model could be used as well. We use z-transform of backoff duration to get mean value, variance and probability distribution of MAC delay. From the mean value and the variance of the MAC delay we determine the mean queuing delay in each station. Our analysis is validated by simulation results for both the Basic and RTS/CTS access mechanisms of the DCF. The accuracy of the analysis found to be quite satisfactory. We assume data rates of 1, 5.5 and 11 Mbps, in order to highlight the effect of the bit rate on delay performance for both access mechanisms.

Call-Level Performance Modelling of Elastic and Adaptive Service-Classes with Finite Population

The call-level performance modelling is a challenge in the highly heterogeneous environment of modern telecom networks, due to the presence of elastic traffic. In this paper, we review existing teletraffic loss models and propose a model for elastic traffic of service-classes with finite population (quasi-random call arrival process). Upon arrival, calls have contingency alternative bandwidth requirements that depend on thresholds which indicate the available/occupied link bandwidth (state dependent model). Calls are admitted under the complete sharing policy, and can tolerate bandwidth compression, while in-service. We prove a recurrent formula for the efficient calculation of the link occupancy distribution and consequently the call blocking probabilities and link utilization. The accuracy of the proposed model is verified by simulation and is found to be quite satisfactory. Comparative results with other existing models show the necessity and the effectiveness of the proposed model. Its potential applications are mainly in the environment of wireless networks.

Performance behaviour of IEEE 802.11 distributed coordination function

The authors present an extensive investigation of the performance of the IEEE 802.11 medium access control (MAC) protocol, with respect to throughput and delay. For the protocol analysis, a new model, which describes the protocols behaviour to a great extent by incorporating and extending the existing models, is proposed. The authors also present a detailed analysis of the end-to-end delay through the study of the MAC delay and the queueing delay. The authors use the Z-transform of backoff duration to obtain the mean value, the variance and the probability distribution of the MAC delay. For the queueing analysis, first the authors consider an M/G/l queue in order to provide a first look at the queueing delay. Second, the authors modify the input process of the queue so that the packet arrival process is described by an ON- OFF model, which expresses the bursty nature of traffic. In the investigations, data rates of 1, 5.5 and 11 Mbps are assumed to highlight the effect of the bit rate on network performance for both Basic and request-to-send/ clear-to-send access mechanisms. The throughput and delay analyses are validated by simulating the distributed coordination function, whereas the models are compared with the existing models based on their results. The accuracy of the analyses was found to be quite satisfactory.

Congestion probabilities in a batched Poisson multirate loss model supporting elastic and adaptive traffic

The ever increasing demand of elastic and adaptive services, where in-service calls can tolerate bandwidth compression/expansion, together with the bursty nature of traffic, necessitates a proper teletraffic loss model which can contribute to the call-level performance evaluation of modern communication networks. In this paper, we propose a multirate loss model that supports elastic and adaptive traffic, under the assumption that calls arrive in a single link according to a batched Poisson process (a more “bursty” process than the Poisson process, where calls arrive in batches). We assume a general batch size distribution and the partial batch blocking discipline, whereby one or more calls of a new batch are blocked and lost, depending on the available bandwidth of the link. The proposed model does not have a product form solution, and therefore we propose approximate but recursive formulas for the efficient calculation of time and call congestion probabilities, link utilization, average number of calls in the system, and average bandwidth allocated to calls. The consistency and the accuracy of the model are verified through simulation and found to be quite satisfactory.

Call-burst blocking of ON-OFF traffic sources with retrials under the complete sharing policy

In this paper we calculate both call and burst blocking probabilities of ON-OFF traffic sources with retrials. Calls of service-classes arrive to a single link according to a Poisson process and compete for the available link bandwidth under the complete sharing policy. Blocked calls may immediately retry one or more times to enter the system, with reduced bandwidth and increased mean service time requirements. Call blocking occurs when a call cannot enter the system with its last bandwidth requirement, due to lack of bandwidth. Accepted calls enter the system via state ON and may alternate between states ON and OFF, or remain always in state ON. When a call is transferred to state OFF it releases the bandwidth held in state ON, so that this bandwidth becomes available to new arriving calls. When a call tries to return to state ON, it re-requests its bandwidth. If it is available a new ON-period (burst) begins. Otherwise burst blocking occurs and the call remains in state OFF. The proposed ON-OFF retry models do not have a product form solution and therefore the calculation of call and burst blocking probability is based on approximate formulas. The formulas we propose for the call blocking probabilities are recursive, whereas for the burst blocking probabilities are robust. Simulation results validate our analytical methodology. For further evaluation, the results of the ON-OFF retry models are compared with those of the ON-OFF model without retrials. We also discuss the extension of the proposed formulas in the case of a fixed-routing network.

An Analytical Approach for Dynamic Wavelength Allocation in WDM–TDMA PONs Servicing ON–OFF Traffic

Optical access systems are now considered a feasible alternative to the predominant broadband access technologies, while, at the same time, passive optical networks (PONs) are viewed as an attractive and promising type of fiber access system. In this paper we present and analyze three basic dynamic wavelength allocation scenarios for a hybrid wavelength division multiplexing-time division multiple access (WDM-TDMA) PON. We propose new teletraffic loss models for calculating call-level performance measures, like connection failure probabilities (due to unavailability of a wavelength) and call blocking probabilities (due to the restricted bandwidth capacity of a wavelength). The PON accommodates bursty service-classes of ON-OFF traffic. The proposed models are extracted from one-dimensional Markov chains, which describe the wavelength occupancy in the PON, and two-dimensional Markov chains, which describe the bandwidth occupancy inside a wavelength. The accuracy of the proposed models is validated through simulation and is found to be quite satisfactory. Moreover, these models are computationally efficient because they are based on recursive formulas.

The Wireless Engset Multi-Rate Loss Model for the Handoff traffic analysis in W-CDMA networks

The call-level performance modelling and evaluation of 3G cellular networks is important for the proper network dimensioning and efficient use of the network resources, such as bandwidth and power. In the case of the wideband code division multiple access (W-CDMA) cellular networks, this task is complicated, not only because of the so called soft blocking, the multiple access interference (MAI) and the heterogeneous traffic, but also due to the handoff process. In this paper, we propose a new teletraffic model for the call-level analysis of handoff traffic in the uplink of W-CDMA networks where multiple services with finite number of traffic sources are accommodated. Parameters related to the soft blocking and MAI are included into the model. The proposed model is described by a discrete time Markov chain. An efficient recurrent formula for the calculation of the system state probabilities is derived. Based on this formula, blocking probabilities of both new and handoff calls are determined. The accuracy of the proposed model is evaluated through simulation and is found to be very satisfactory. Moreover, we show the necessity of distinguishing the handoff traffic from the new traffic, in respect of blocking probabilities.