José M. Barceló

Quality of service through bandwidth reservation on multirate ad hoc wireless networks

Achieving QoS (quality of service) in ad hoc wireless networks (AWNs) has been a research topic in the last years. In this paper we describe a QoS reservation mechanism for Multirate AWNs that allows bandwidth allocation on a per flow basis. By multirate we refer to those networks where wireless nodes are able to dynamically switch among several link rates. This allows nodes to select the highest possible transmission rate for exchanging data, independently for each neighbor.

A Cooperative Vehicular Network Framework

Vehicular Ad Hoc Networks are networks characterized by intermittent connectivity and rapid changes in their topology. This paper addresses car-to-road communications in which vehicles use Access Points (AP) in a Delay Tolerant Network architecture. Results show how the combination of a Delay-Cooperative ARQ mechanism reduces packet losses and in conjunction with a Carry-and-Forward cooperative mechanism improves performance parameters in terms of total file transfer delay and number of AP needed to download files.

An exact model for the multiplexing of worst case traffic sources

In this paper we analyse a multiplexer handling a number of identical and independent Worst Case Traffic (WCT) sources. Each WCT source produces a periodic stream of cells consisting of a constant number of back-to-back cells followed by a silent period of constant duration. The WCT can model the traffic produced by a “malicious” user who sends an ON/OFF traffic where a burst of back-to-back cells whose length is the largest compatible with the tolerance introduced in the control function alternates with an idle period whose length is the smallest compatible with the policed peak cell rate. WCT can also model, for example, the traffic produced by some ATM Adaptation Layer multiplexing schemes in the Terminal Equipment.

Study of internet autonomous system interconnectivity from BGP routing tables

Recently, several authors have proposed models based on power-laws to characterize Internet topologies. Most of these works use the BGP (Border Gateway Protocol) tables published by Oregon Route Views. The adjacency matrix containing AS (autonomous system) connectivity is built from a BGP table. Having access to BGP routing tables from several geographical sites gives a broader vision of ASs connectivity, since several ASs and links may be hidden for an AS due to routing processes policies, while they may be visible to other ASs. We compare BGP tables of different sizes and enrich the adjacency matrix with the union of them. This comparison is based on the AS degree connectivity, clustering coefficient and path length. Our initial results, published in [Proceedings of HPSR2003, Torino, Italy, June 2003], show an increment of 16% in the number of links and 0.7% in the number of ASs and show that a medium-size AS can get a vision similar to a repository such as Route Views. Most of the recent works analyze whether power-law distributions, also called scale-free models, arise on Internet topologies for the whole AS-level graph. However, few ones study whether the AS connectivity of the Internet core follows a power-law distribution. Here we use well-known heuristics to identify AS peeringship and split the Internet core from small regional ISPs and end customers. We will see that the Internet core does not fit a power-law well.

Subnet formation and address allocation approach for a routing with subnets scheme in MANETs

Due to MANET topological routing algorithms are not scalable respect to the number of nodes in the network, we evaluate the potential use of a subnet structure in MANETs. We show scenarios in which a MANET with subnet structure is applicable, present the main technical challenges for the application of this structure and show that a reduction of routing overhead from a factor of N2 to N2/k (being N the number of nodes and k the number of subnets) is reachable. In this paper we propose a Subnet Formation and Address Allocation mechanism and evaluate analytically the influence of this proposal on the overhead generated. This solution is only one of the many challenges to fix for putting in use a MANET with subnets structure.

Worst-case traffic in a tree network of ATM multiplexers

We study tree networks of discrete-time queues loaded with periodic traffic sources. By using the so-called Benes method, exact closed-form expressions are obtained for the queue length distributions. The models developed can be used to study the superposition of periodic sources emitting bursts of cells in ATM networks. The results obtained show the significant effect that this kind of traffic can have on the performance of these systems.

Multiplexing periodic sources in a tree network of ATM multiplexers

We obtain the queue length distribution in a tree of discrete time queues with constant service time whose input is periodic traffic. In the context of ATM the study could be applied to CBR sources. The tree consists of M-stages. To solve this system, we first solve a 2-stage tree network. Given this configuration, a more complex tree network can be easily solved making use of the properties of the discrete time queues with identical service times. We also give closed formulas for the average waiting time and average number of cells in any queue of the tree network.

Analytical evaluation of the overhead generated by a routing scheme with subnets for MANETs

Most of the current topological based routing algorithms used in MANETs treat all the nodes in the network like independent peers, making them not scalable with respect to the number of nodes in the network. Since hierarchical routing techniques have been known to afford scalability in large networks (e.g. Internet), and taking advantage of the existence of scenarios in which the nodes of a MANET can be aggregated in a natural manner, we propose a two level hierarchical routing scheme for MANETs. In this paper we present the main components and an analytical performance evaluation of our proposal, with the number of control packets per second as the metric of our interest. The evaluation shows a significant overhead reduction from Θ(N2) to Θ(N). However, a trade-off between the # of nodes in the network and the complexity of the system has to be achieved.

Inside BGP Tables: Inferring Autonomous System's Interconnectivity

In this paper we address the problem of Autonomous System (AS) interconnectivity. We analyze the union of six BGP Tables taken from different places. The union of BGP tables from several AS’s gives a richer perspective in terms of number of AS’s and inter-domain links discovered. We infer AS relationships using well-known heuristics. Once we have the AS relationships we can obtain the AS’s that form the regional area of the Internet eliminating the end customers. Pruning the heuristic we obtain AS’s that form the core of the Internet. We, then, can study whether the set of AS’s in the regional and core sub-graphs and the in-degree and out-degree distributions follow power-laws. Our results show that the In-degree CCDF fits well a power-law in all the graphs defined. However, the Degree CCDF only fits a power-law for the whole graph. For the regional and core sub-graphs the Degree CCDF is better fitted with a Go Model. The Out-degree CCDF for these sub-graphs are well fitted with a power-law only for the first hundred of neighbors.