Liaison point à point | EPFL Graph Search

vignette|Radio point à point en fonctionnement, Huntington Beach, Californie. Une liaison point à point est une liaison entre deux hôtes uniquement et qui n'est pas conçue pour être utilisée initialement dans un réseau. Il n'y a donc pas de notion native d'adresse réseau des deux hôtes, ni de contrôle avancé du flux. HDLC, PPP et SLIP sont des protocoles destinés à permettre l'utilisation d'une liaison point à point dans un réseau, en palliant ses limitations. Les liaisons Transfix, les lignes Numéris ou ISDN sont des exemples de liaison point à point. Grâce à un routeur à chaque extrémité de la liaison, il est possible de communiquer entre le point A et le point B au moyen d'une ligne numérique transmettant de la voix ou des données. Dans le domaine de l'aéronautique, un type de liaison utilisé par les compagnies aériennes à bas prix. En oppostion au mode en étoile, qui possède un hub (aéroport central), le point to point permet à des compagnies de relier toutes les lignes entre el

A multiple core mode for protocol independent multicast

Francesc Font Torre de Mer

Internet communications have traditionally been based on point-to-point connections. This technique is called unicast and allows one sender and one receiver to communicate. The Internet saw the appearance of multicast protocols in the early 90's. Multicast enables point-to-multipoint and multipoint-to-multipoint connections and is suitable for applications with several receivers. Multicast packets are transmitted from each sender to the receivers through a certain multicast tree. Multicast routing protocols are responsible for building such trees. There are different types of multicast routing protocols and each builds a specific kind of multicast tree. The main advantages of applying multicast in multiple-receiver applications are a more rational use of network resources and the minimization of processing in hosts. There are two types of multicast models: Source-Specific Multicast (SSM) and Any-Source Multicast (ASM). The former is suitable for single-source applications like Internet TV and news distribution. The latter supports multiple-source applications like videoconferencing, Distributed Interactive Simulations (DIS), multi-player gaming and interactive distance learning. The Internet multicast research community assumes that both models must co-exist. Protocol Independent Multicast - Sparse Mode (PIM-SM) is the ASM standard basis for intra-domain multicast routing in Wide-Area Networks (WANs). This protocol builds Shared Trees (STs) rooted at certain routers called Rendezvous Points (RPs). These types of trees bring some drawbacks, the most important being traffic concentration, sub-optimal forwarding of multicast packets and slow convergence when an active RP fails. Two proposals arose to alleviate these drawbacks. The first, Bidir-PIM, constructs Bi-directional Shared Trees (BSTs). However, this variant neglects the problem of having a single point of failure. The second assembles Multiple Core Trees (MCTs) and is based on two protocols, PIM-SM and Multicast Source Discovery Protocol (MSDP), used in combination with anycast. MSDP has been adapted from inter-domain multicast to enable PIM-SM to use several RPs per group. On the other side, anycast allows senders and receivers to send data and control packets to the closest RP. If a RP fails, anycast also provides an automatic backup to the next closest RP. However, this proposal is complex and is not suitable for bursty sources and the next generation Internet Protocol (IPv6). Furthermore, it suffers from high join latencies and poor multicast state scalability with the number of sources. The first purpose of this dissertation is to provide a comparison on the ìqualityî for the different sorts of multicast trees built in the ASM model by multicast routing protocols suitable to run in WANs. This study concludes that MCTs give the best performance in terms of traffic concentration and link cost of multicast trees in sparse groups and end-to-end delay. Furthermore,it is the only type of tree that does not suffer from a single point of failure, called core or RP. The main purpose of this dissertation is the design of an extension to PIM-SM that allows the building of MCTs without MSDP. The extension to the register mechanism in PIM-SM allows the construction of channels encompassing the Access Routers (ARs) and any number RPs. These channels enable sources to directly reach all the RPs, thus avoiding the exchange of data packets between RPs, which generates additional processing in the RPs and the suboptimal routing of multicast packets coming from new sources. Furthermore, these channels may improve the multicast routing state scalability with the number of sources. This approach, called Protocol Independent Multicast - Multiple Core Mode (PIM-MCM), enables PIM having the advantages of anycast RP without using MSDP. In addition, it conforms to the PIM-SM design philosophy and alleviates all the drawbacks of MSDP while minimizing the added complexity.

EPFL2004

Strategies for Delay-Limited Source-Channel Coding

Marius Kleiner

In point-to-point source-channel communication with a fidelity criterion and a transmission cost constraint, the region of achievable cost and fidelity pairs is completely characterized by Shannon's separation theorem. However, this is in general only true if coding of arbitrary complexity and delay is admitted. If the delay is constrained, the separation theorem only provides an outer bound to the achievable cost/distortion region, and the exact shape of this region is in general not known. The first part of this thesis studies source-channel communication when neither a required average fidelity nor a cost constraint are specified, but when the goal is to maximize the ratio of fidelity to cost. It is shown how the maximal ratio relates to existing quantities such as the capacity per unit cost. Finally, necessary and sufficient conditions are derived to test whether a given system operates at this maximal ratio and when this is possible using a single-letter code. The second part of the thesis studies communication of continuous-valued sources over the additive white Gaussian noise channel when only a single source symbol is to be encoded at a time. In particular, the case is considered where several uses of the channel can be made for each source symbol. Inspired by communication with feedback, a simple communication strategy combining quantization and uncoded transmission is derived and analyzed. It is shown that this strategy achieves a mean squared error that performs as well as any known communication strategy that encodes a single source symbol at a time. On the other hand, it is strictly suboptimal in the sense that the gap (in dB) between the achievable signal-to-distortion ratio (SDR) and the best SDR achievable without a delay limit grows with increasing signal-to-noise ratio. The thesis turns to a more practical subject in its last part. The case is made why object-oriented programming is particularly suited to implementing simulations. As a proof of concept, a complete implementation of an object-oriented simulator for source-channel coding is presented that allows for rapid development and analysis of arbitrary communication strategies.

EPFL2010

A Blockchain Consensus Protocol With Horizontal Scalability

Kelong Cong

Blockchain technology has the potential to decentralise many traditionally centralised systems. However, scalability remains a key challenge. A horizontally scalable solution, where performance increases by adding more nodes, would move blockchain systems one step closer to ubiquitous use. We design a novel blockchain system called CHECO. Each node in our system maintains a personal hash chain, which only stores transactions that the node is involved in. A consensus is reached on special blocks called checkpoint blocks rather than on all transactions. Checkpoint blocks are effectively a hash pointer to the personal hash chains; thus a single checkpoint block may represent an arbitrarily large set of transactions. We introduce a validation protocol so that any node can check the validity of any transaction. Since transaction and validation protocols are point-to-point, we achieve horizontal scalability. We analytically evaluate our system and show a number of highly desirable correctness properties such as consensus on the validity of transactions. Further, we give a free and open-source implementation of CHECO and evaluate it experimentally. Our results show a strong indication of horizontal scalability.

IEEE2018