Giovanni Stea

Tight end-to-end per-flow delay bounds in FIFO multiplexing sink-tree networks

Aggregate scheduling has been proposed as a solution for achieving scalability in large-size networks. However, in order to enable the provisioning of real-time services, such as video delivery or voice conversations, in aggregate scheduling networks, end-to-end delay bounds for single flows are required. In this paper, we derive per-flow end-to-end delay bounds in aggregate scheduling networks in which per-egress (or sink-tree) aggregation is in place, and flows traffic is aggregated according to a FIFO policy. The derivation process is based on Network Calculus, which is suitably extended to this purpose. We show that the bound is tight by deriving the scenario in which it is attained. A tight delay bound can be employed for a variety of purposes: for example, devising optimal aggregation criteria and rate provisioning policies based on pre-specified flow delay bounds.

EuQoS: End-to-End Quality of Service over Heterogeneous Networks

The EuQoS (End-to-End QoS over Heterogeneous Networks) IST Integrated European Project aimed to define a Next Generation Network architecture that builds, uses and manages end-to-end QoS across different administrative domains and heterogeneous networks (UMTS, xDSL, Ethernet, WiFi, Satellite and IP/ MPLS). The EuQoS architecture preserves the openness and the decentralized decision model of the actual Internet, runs on off-the-shelf hardware and network equipment, and allows end users to request various services without changing the Application Signaling protocol, while meeting regulators’ and users’ Net Neutrality requirements. This paper presents the key elements of the EuQoS architecture and describes the main results obtained in field trials performed on a fully-functional EuQoS system prototype developed over a pan-European testbed. Furthermore, the paper discusses the main strengths of the system and the issues related to its actually deployment on a large scale, from both technical and market points

Tradeoffs between low complexity, low latency, and fairness with deficit round-robin schedulers

Deficit Round-Robin (DRR) is a scheduling algorithm devised for providing fair queueing in the presence of variable length packets. The main attractive feature of DRR is its simplicity of implementation: in fact, it can exhibit O(1) complexity, provided that specific allocation constraints are met. However, according to the original DRR implementation, meeting such constraints often implies tolerating high latency and poor fairness. In this paper, we first derive new and exact bounds for DRR latency and fairness. On the basis of these results, we then propose a novel implementation technique, called Active List Queue Method (Aliquem), which allows a remarkable gain in latency and fairness to be achieved, while still preserving O(1) complexity. We show that DRR achieves better performance metrics than those of other round-robin algorithms such as Pre-Order Deficit Round-Robin and Smoothed Round-Robin. We also show by simulation that the proposed implementation allows the average delay and the jitter to be reduced.

An integrated framework for enabling effective data collection and statistical analysis with ns-2

The Network Simulator 2 (ns-2) is an open source tool for network simulation. When planning for large-scale simulation experiments, an efficient and flexible data collection and a statistically sound output data analysis are important aspects to keep in mind. Unfortunately, ns-2 offers little support for data collection, and statistical analysis of the simulation results is most often performed offline, using either home made code or available packages, which are not integrated with ns-2. In this paper we describe two complementary contributions: the first one consists of a set of C++ modules, that allow a flexible and efficient data collection; the second one is a software framework, which is fully integrated with ns-2, that performs all the operations required to carry out simulation experiments in a statistically sound way. Our framework allows a user to significantly reduce the postprocessing overhead and to save simulation time, especially with large-scale simulations. Our code is publicly available at [3].

SimuLTE - A modular system-level simulator for LTE/LTE-A networks based on OMNeT++

This paper describes SimuLTE, an open-source system-level simulator for LTE and LTE-Advanced (LTE-A) networks. SimuLTE is based on OMNeT++, a well-known, widely-used modular simulation framework, which offers a high degree of experiment support. As such, it can be seamlessly integrated with all the networkoriented modules of the OMNeT++ family, such as INET, thus enabling - among other things - credible simulation of end-to-end real-life applications across heterogeneous technologies. We describe the modeling choices and general architecture of the SimuLTE software, with particular emphasis on the MAC and scheduling functions, and show performance evaluation results obtained using the simulator.

Design and performance analysis of the Real-Time HCCA scheduler for IEEE 802.11e WLANs

This paper presents a new scheduling algorithm, called Real-Time HCCA (RTH), devised to support Quality of Service (QoS) at the flow level in an IEEE 802.11e network using the Hybrid Coordinator Function (HCF) Controlled Channel Access (HCCA) function. RTH separates online activities which take place at the frame transmission timescale, from offline activities which take place at the flow lifetime timescale. Complex computations are relegated to offline activities, while online tasks are kept as simple as possible. More specifically, at admission control time, RTH computes a periodic schedule based on the well-known Earliest Deadline First algorithm for 802.11e Traffic Streams (TSs). In doing so, the Stack Resource Policy is applied to account for non-pre-emptability of frame transmissions. Furthermore, the parameters are configured so as to reduce the MAC overhead due to polling uplink TSs. On the other hand, online scheduling is enforced simply by reading the pre-computed schedule, at little or no computational cost. RTH performance is assessed in terms of the admission control limit and of the amount of channel capacity that is left for contention-based access. Under both criteria, RTH is shown to outperform the sample scheduler proposed in IEEE 802.11e.

Aliquem: a novel DRR implementation to achieve better latency and fairness at O(1) complexity

Deficit round-robin is a packet scheduling algorithm devised to provide fair queueing in the presence of variable length packets (see Shreedhar, M. and Varghese, G., IEEE Trans. on Networking, vol.4, p.375-85, 1996). DRR can be implemented at O(1) complexity provided that each flow is allowed to transmit at least one maximum size packet on each round; however, under this constraint, DRR may exhibit high latency and poor fairness. We first generalize previous results known for DRR, related to its latency and fairness. We then introduce a novel DRR implementation technique, called active lists queue method (Aliquem), which allows the above constraint to be relaxed while preserving O(1) complexity, thus achieving better latency and fairness that are comparable to those of more complex algorithms, such as self-clocked fair queueing.

A methodology for computing end-to-end delay bounds in FIFO-multiplexing tandems

In this paper we address the problem of finding end-to-end delay bounds for single leaky-bucket shaped flows subject to FIFO multiplexing in tandems of rate-latency nodes. More specifically, we focus on a methodology, called the Least Upper Delay Bound (LUDB) method, which is based on Network Calculus. The latter has already been used for computing delay bounds in tandems in which the paths of the flows are subject to particular restrictions. In this paper we generalize and extend it to tandems traversed by flows following arbitrary paths. We show that such methodology yields better bounds than those obtained through both per-node analysis and comparable methods proposed in the literature.

An efficient cross layer scheduler for multimedia traffic in wireless local area networks with IEEE 802.11e HCCA

This paper proposes a scheduling algorithm, namely Wireless Timed Token Protocol (WTTP), for the Hybrid Coordination Function (HCF) Controlled Channel Access (HCCA) in IEEE 802.11e. WTTP provides traffic streams with a minimum reserved rate, as required by the standard, and it accounts for two types of traffic streams simultaneously, depending on the corresponding application: constant bit rate, which are served according to their rate, and variable bit rate traffic streams. The latter are guaranteed a minimum rate, but they are also allowed to exploit unused bandwidth, which preserves small access delays in case of bursty arrivals. Additionally, WTTP shares the capacity which is not reserved for QoS traffic streams transmissions among traffic flows with no specific QoS requirements. We also propose a strategy for the QoS Access Point to infer the idle/busy status of the uplink transmission buffers, based on cross layer information made available at the MAC by the application layer, which allows channel capacity to be saved. WTTP exhibits O(1) per packet computational complexity. We evaluate the performance of WTTP via simulation under different traffic conditions, and we investigate its resilience to variations of different system parameters.

Delay bounds for FIFO aggregates: a case study

In a DiffServ architecture, packets with the same marking are treated as an aggregate at core routers, independently of the flow they belong to. Nevertheless, for the purpose of QoS provisioning, derivation of upper bounds on the delay of individual flows is of great importance. In this paper, we consider a case study network, composed by a tandem of rate-latency servers that is traversed by a tagged flow. At each different node, the tagged flow is multiplexed into a FIFO buffer with a different interfering flow. The tagged flow and the interfering flows are all leaky-bucket constrained at the network entry. We introduce a novel methodology based on well-known results on FIFO multiplexing from Network Calculus, by means of which we derive an end-to-end delay bound for tagged flow traffic. The delay bound assesses the contribution to the delay due to the interference of other flows precisely, and to the best of our knowledge, it is better than any other applicable result available from the literature. Furthermore, we utilize the delay bound formula to quantify the level of overprovisioning required in order to achieve delay bounds comparable to those of a flow-aware architecture.