This paper presents the design and evaluation of Pastry, a scalable, distributed object location and routing substrate for wide-area peer-to-peer ap- plications. Pastry performs application-level routing and object location in a po- tentially very large overlay network of nodes connected via the Internet. It can be used to support a variety of peer-to-peer applications, including global data storage, data sharing, group communication and naming. Each node in the Pastry network has a unique identifier (nodeId). When presented with a message and a key, a Pastry node efficiently routes the message to the node with a nodeId that is numerically closest to the key, among all currently live Pastry nodes. Each Pastry node keeps track of its immediate neighbors in the nodeId space, and notifies applications of new node arrivals, node failures and recoveries. Pastry takes into account network locality; it seeks to minimize the distance messages travel, according to a to scalar proximity metric like the number of IP routing hops. Pastry is completely decentralized, scalable, and self-organizing; it automatically adapts to the arrival, departure and failure of nodes. Experimental results obtained with a prototype implementation on an emulated network of up to 100,000 nodes confirm Pastry's scalability and efficiency, its ability to self-organize and adapt to node failures, and its good network locality properties.

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Pastry: Scalable, Decentralized Object Location, and Routing for Large-Scale Peer-to-Peer Systems

Controlled/living radical polymerization: Features, developments, and perspectives

Due to their abundance, high strength and stiffness, low weight and biodegradability, nano-scale cellulose fiber materials (e.g., microfibrillated cellulose and bacterial cellulose) serve as promising candidates for bio-nanocomposite production. Such new high-value materials are the subject of continuing research and are commercially interesting in terms of new products from the pulp and paper industry and the agricultural sector. Cellulose nanofibers can be extracted from various plant sources and, although the mechanical separation of plant fibers into smaller elementary constituents has typically required high energy input, chemical and/or enzymatic fiber pre-treatments have been developed to overcome this problem. A challenge associated with using nanocellulose in composites is the lack of compatibility with hydrophobic polymers and various chemical modification methods have been explored in order to address this hurdle. This review summarizes progress in nanocellulose preparation with a particular focus on microfibrillated cellulose and also discusses recent developments in bio-nanocomposite fabrication based on nanocellulose.

Microfibrillated cellulose and new nanocomposite materials: a review

This paper proposes COPE, a new architecture for wireless mesh networks. In addition to forwarding packets, routers mix (i.e., code) packets from different sources to increase the information content of each transmission. We show that intelligently mixing packets increases network throughput. Our design is rooted in the theory of network coding. Prior work on network coding is mainly theoretical and focuses on multicast traffic. This paper aims to bridge theory with practice; it addresses the common case of unicast traffic, dynamic and potentially bursty flows, and practical issues facing the integration of network coding in the current network stack. We evaluate our design on a 20-node wireless network, and discuss the results of the first testbed deployment of wireless network coding. The results show that using COPE at the forwarding layer, without modifying routing and higher layers, increases network throughput. The gains vary from a few percent to several folds depending on the traffic pattern, congestion level, and transport protocol.

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XORs in the air: practical wireless network coding

23 Stereocontrol of Lactide ROP 6164 231 Isotactic Polylactides 6164 232 Syndiotactic Polylactides 6166 233 Heterotactic Polylactides 6166 3 Anionic Polymerization 6166 4 Nucleophilic Polymerization 6168 41 Mechanistic Considerations 6168 42 Catalysts 6169 421 Enzymes 6169 422 Organocatalysts 6169 43 Stereocontrol of Lactide ROP 6170 44 Depolymerization 6170 5 Cationic Polymerization 6170 6 Conclusion and Perspectives 6171 7 Acknowledgments 6173 8 References and Notes 6173

Controlled ring-opening polymerization of lactide and glycolide.

The performance of the IEEE 802.11b wireless local area networks is analyzed. We have observed that when some mobile hosts use a lower bit rate than the others, the performance of all hosts is considerably degraded. Such a situation is a common case in wireless local area networks in which a host far away from an access point is subject to important signal fading and interference. To cope with this problem, the host changes its modulation type, which degrades its bit rate to some lower value. Typically, 802.11b products degrade the bit rate from 11 Mb/s to 5.5, 2, or 1 Mb/s when repeated unsuccessful frame transmissions are detected. In such a case, a host transmitting for example at 1 Mb/s reduces the throughput of all other hosts transmitting at 11 Mb/s to a low value below 1 Mb/s. The basic CSMA/CA channel access method is at the root of this anomaly: it guarantees an equal long term channel access probability to all hosts. When one host captures the channel for a long time because its bit rate is low, it penalizes other hosts that use the higher rate. We analyze the anomaly theoretically by deriving simple expressions for the useful throughput, validate them by means of simulation, and compare with several performance measurements.

/pdf/performance-anomaly-of-802-11b-keprtquk1p.pdf

Performance anomaly of 802.11b

We consider wireless LANs such as IEEE 802.11 operating in the unlicensed radio spectrum. While their nominal bit rates have increased considerably, the MAC layer remains practically unchanged despite much research effort spent on improving its performance. We observe that most proposals for tuning the access method focus on a single aspect and disregard others. Our objective is to define an access method optimized for throughput and fairness, able to dynamically adapt to physical channel conditions, to operate near optimum for a wide range of error rates, and to provide equal time shares when hosts use different bit rates.We propose a novel access method derived from 802.11 DCF [2] (Distributed Coordination Function) in which all hosts use similar values of the contention window CW to benefit from good short-term access fairness. We call our method Idle Sense, because each host observes the mean number of idle slots between transmission attempts to dynamically control its contention window. Unlike other proposals, Idle Sense enables each host to estimate its frame error rate, which can be used for switching to the right bit rate. We present simulations showing how the method leads to high throughput, low collision overhead, and low delay. The method also features fast reactivity and time-fair channel allocation.

/pdf/idle-sense-an-optimal-access-method-for-high-throughput-and-4gogjfrj5z.pdf

Idle sense: an optimal access method for high throughput and fairness in rate diverse wireless LANs

Following our previous papers on the mechanism of cyclic esters polymerization induced by tin(II) octoate (Sn(Oct) 2 ) and particularly papers on ∈-caprolactone (CL), the present work shows that L,L-dilactide/Sn(Oct)2 does not differ mechanistically from the CL/Sn(Oct) 2 system. Sn atoms bonded through alkoxide groups to macromolecules were also observed by MALDI-TOF mass spectrometry. Formation of the actual initiator from Sn(Oct) 2 and a hydroxy group-containing compound (ROH) was envisaged by kinetic arguments. The appropriate experiments were carried out to show that some mechanisms put forward during the past few decades by several research groups were not sufficiently substantiated. Eventually, we conclude that L,L-dilactide/Sn(Oct)2 polymerization proceeds by simple monomer insertion into the ...-Sn-OR bond, reversibly formed in the reaction ...-SnOct + ROH ⇄ ...-Sn-OR + OctH, where ROH is either the low molar mass co-initiator (an alcohol, hydroxy acid, or H 2 O) or a macromolecule fitted with a hydroxy end group. These interconversions take place throughout the whole polymerization process. Sn(Oct) 2 itself does not play an active role in the polymerization.

Kinetics and mechanism of cyclic esters polymerization initiated with Tin(II) octoate. 3. Polymerization of L, L-dilactide

A general kinetic treatment of the system with intermolecular chain transfer followed by fast reinitiation is given. It leads to the broadening of the molecular weight distribution (MWD), the number of growing chains being invariable. Thus, this system can be considered as a special case of living polymerization. A general method has been elaborated allowing the determination of the ratio of the rate constant of propagation (kp) to the rate constant of the bimolecular transfer (k(2)tr) from the dependence of the MWD on monomer conversion. Numerical values of kp/k(2)tr equal to ≈ 102 and 25 were thus determined for the polymerization of L, L-lactide (L, L-dilactide) initiated with aluminium tris(isopropoxide) trimer ({Al(OiPr)3}3) and tributyltin ethoxide (nBu3SnOEt), respectively.

Intermolecular chain transfer to polymer with chain scission : general treatment and determination of kp/ktr in L,L-lactide polymerization

New opportunities, provided by the 2,2‘-[1,1‘-binaphthyl-2,2‘-diylbis(nitrylomethilidyne)]diphenol (SB(OH)2)/Al(OiPr)3/racemic lactide (rac-LA) polymerization system, employing a combination of stereoelection with (S) and (R) ligand-exchange mechanism at Al-alkoxide active centers were explored. The stereoelectivity was comparable to that determined for the process with an additional synthetic step of isolation and purification of the SBO2Al−OiPr alkoxide. The resultant poly(rac-LA) had a gradient stereocopolymer structure and exhibited enhanced thermal stability due to a stereocomplex formation (Tm = 210 °C). This is the highest melting temperature reported until now for poly(lactide) (PLA) prepared directly from rac-LA.

Andrzej Duda

Papers

Performance anomaly of 802.11b

Idle sense: an optimal access method for high throughput and fairness in rate diverse wireless LANs

Kinetics and mechanism of cyclic esters polymerization initiated with Tin(II) octoate. 3. Polymerization of L, L-dilactide

Intermolecular chain transfer to polymer with chain scission : general treatment and determination of kp/ktr in L,L-lactide polymerization

Stereocontrolled polymerization of racemic lactide with chiral initiator: combining stereoelection and chiral ligand-exchange mechanism.