Application‐level differentiated services for Web servers

The current World Wide Web service model treats all requests equivalently, both while being processed by servers and while being transmitted over the network. For some uses, such as Web prefetching or multiple priority schemes, different levels of service are desirable. This paper presents three simple, server‐side, application‐level mechanisms (limiting process pool size, lowering process priorities, limiting transmission rate) to provide two different levels of Web service (regular and low priority). We evaluated the performance of these mechanisms under combinations of two foreground workloads (light and heavy) and two levels of available network bandwidth (10 Mb/s and 100 Mb/s). Our experiments show that even with background traffic sufficient to saturate the network, foreground performance is reduced by at most 4–17%. Thus, our user‐level mechanisms can effectively provide different service classes even in the absence of operating system and network support. This revised version was published online in August 2006 with corrections to the Cover Date.     

Remote detection of bottleneck links using spectral and statistical methods

Persistently saturated links are abnormal conditions that indicate bottlenecks in Internet traffic. Network operators are interested in detecting such links for troubleshooting, to improve capacity planning and traffic estimation, and to detect denial-of-service attacks. Currently bottleneck links can be detected either locally, through SNMP information, or remotely, through active probing or passive flow-based analysis. However, local SNMP information may not be available due to administrative restrictions, and existing remote approaches are not used systematically because of their network or computation overhead. This paper proposes a new approach to remotely detect the presence of bottleneck links using spectral and statistical analysis of traffic. Our approach is passive, operates on aggregate traffic without flow separation, and supports remote detection of bottlenecks, addressing some of the major limitations of existing approaches. Our technique assumes that traffic through the bottleneck is dominated by packets with a common size (typically the maximum transfer unit, for reasons discussed in Section 5.1). With this assumption, we observe that bottlenecks imprint periodicities on packet transmissions based on the packet size and link bandwidth. Such periodicities manifest themselves as strong frequencies in the spectral representation of the aggregate traffic observed at a downstream monitoring point. We propose a detection algorithm based on rigorous statistical methods to detect the presence of bottleneck links by examining strong frequencies in aggregate traffic. We use data from live Internet traces to evaluate the performance of our algorithm under various network conditions. Results show that with proper parameters our algorithm can provide excellent accuracy (up to 95%) even if the traffic through the bottleneck link accounts for less than 10% of the aggregate traffic.     

二羟乙酸~*与胶原的反应及其参与形成稳定的胶原铬交联键的机理

本文研究了二羟乙酸与胶原的反应及其与三价铬及胶原之间交联的机理,为此采用了合成模型化合物,通过高效液相色谱及离子交换液相色谱等方法分离提纯产物,并用核磁共振等方法对产物进行了分析。结果表明二羟乙酸与胶原肽链中的精氨酸的胍基发生了反应,在铬鞣过程中,二羟乙酸与胶原胍基及羧基构成了铬络合物。作者推测,新的稳定交联键是在一般情况下铬不可能与胶原形成交联键的胶原某些区域内构成,这可能是二羟乙酸能提高铬鞣效率的主要原因,因为在每个新的交联键内,只需要一个铬离子,从而为生产实际中有可能减少50%的铬鞣剂用量提供了理论依据。     

Ensemble analyses by Minkowski-operations for spatially resolved structural and optoelectronic features of Cu(In,Ga)(Se2,S2) absorbers

Polycrystalline chalcopyrite semiconductors like Cu(In,Ga)(Se₂,S₂) as physically and technologically promising thin film solar cell absorbers show a considerable degree of spatial inhomogeneity of structural, optical and optoelectronic properties above and below the length scale of grain sizes. The dilution of magnitudes such as splitting of quasi-Fermi levels with a certain distribution of lower energies, introduced e.g. by local fluctuations, introduce an additional reduction of the free energy of the photon field and drop thus the solar light conversion yield. For a comprehensive interpretation of the influence of such inhomogeneity effects on the solar cell efficiency we perform lateral scans of various magnitudes recorded with a confocal setup with high lateral resolution (≤ 1 µm), like splitting of quasi-Fermi levels, AFM-surface contours with different scan sizes and at different sample positions. We compare these sets of magnitudes by their respective Minkowski-opening-operations, extract correlation coefficients, determine average values and their respective statistical momenta (variances), and extrapolate minimum scan sizes for the collection of statistically representative data.     

Design and analysis of a propagation delay tolerant ALOHA protocol for underwater networks

Acoustic underwater wireless sensor networks (UWSN) have recently gained attention as a topic of research. Such networks are characterized by increased uncertainty in medium access due not only to when data is sent, but also due to significantly different propagation latencies from spatially diverse transmitters—together, we call these space-time uncertainty. We find that the throughput of slotted ALOHA degrades to pure ALOHA in such an environment with varying delay. We therefore propose handling this spatial uncertainty by adding guard times to slotted ALOHA, forming Propagation Delay Tolerant (PDT-)ALOHA. We show that PDT-ALOHA increases throughput by 17–100% compared to simple slotted ALOHA in underwater settings. We analyze the protocol’s performance both mathematically and via extensive simulations. We find that the throughput capacity decreases as the maximum propagation delay increases, and identify protocol parameter values that realize optimal throughput. Our results suggest that shorter hops improve throughput in UWSNs.     

Medium access control with coordinated adaptive sleeping for wireless sensor networks

This paper proposes S-MAC, a medium access control (MAC) protocol designed for wireless sensor networks. Wireless sensor networks use battery-operated computing and sensing devices. A network of these devices will collaborate for a common application such as environmental monitoring. We expect sensor networks to be deployed in an ad hoc fashion, with nodes remaining largely inactive for long time, but becoming suddenly active when something is detected. These characteristics of sensor networks and applications motivate a MAC that is different from traditional wireless MACs such as IEEE 802.11 in several ways: energy conservation and self-configuration are primary goals, while per-node fairness and latency are less important. S-MAC uses a few novel techniques to reduce energy consumption and support self-configuration. It enables low-duty-cycle operation in a multihop network. Nodes form virtual clusters based on common sleep schedules to reduce control overhead and enable traffic-adaptive wake-up. S-MAC uses in-channel signaling to avoid overhearing unnecessary traffic. Finally, S-MAC applies message passing to reduce contention latency for applications that require in-network data processing. The paper presents measurement results of S-MAC performance on a sample sensor node, the UC Berkeley Mote, and reveals fundamental tradeoffs on energy, latency and throughput. Results show that S-MAC obtains significant energy savings compared with an 802.11-like MAC without sleeping.     

Fruchttod beim Reh

Zusammenfassung Es wird über eine Beobachtung von Fruchtmazeration in der Vagina beim Reh berichtet.

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Summary A case of maceration of a fetus in the vagina of roe deer is reported.

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Résumé Il est décrit la maceration d'un fetus dans la vagine chez le chevreuil.

     

Structural studies of collagen-like sequential polypeptides

Sequential polyhexapeptides, synthesised by combination of sequences from collagen type Gly-X-Y (X = Ala, Pro, Ser; Y = Ala, Gly, Lys, Pro), were characterized by the temperature dependence of circular dichroism spectra. Under comparable conditions these studies revealed that alternating triplets of Gly-Pro-Pro or Gly-Pro-Ala combined with Gly-Pro-Lys or Gly-Pro-Glu exhibit collagen-like structures in aqueous solutions. In case of unstructured chains of (Gly-Pro-Ala) ≈ 12 it can be shown that N-terminal crosslinking of three chains produces a similar ordered structure.