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In contrast to photosynthetic reaction centers, which share the same structural architecture, more variety is found in the light-harvesting antenna systems of phototrophic organisms. The largest antenna system described, so far, is the chlorosome found in anoxygenic green bacteria, as well as in a recently discovered aerobic phototroph. Chlorosomes are the only antenna system, in which the major light-harvesting pigments are organized in self-assembled supramolecular aggregates rather than on protein scaffolds. This unique feature is believed to explain why some green bacteria are able to carry out photosynthesis at very low light intensities. Encasing the chlorosome pigments is a protein-lipid monolayer including an additional antenna complex: the baseplate, a two-dimensional paracrystalline structure containing the chlorosome protein CsmA and bacteriochlorophyll a (BChl a). In this article, we review current knowledge of the baseplate antenna complex, which physically and functionally connects the chlorosome pigments to the reaction centers via the Fenna–Matthews–Olson protein, with special emphasis on the well-studied green sulfur bacterium Chlorobaculum tepidum (previously Chlorobium tepidum). A possible role for the baseplate in the biogenesis of chlorosomes is discussed. In the final part, we present a structural model of the baseplate through combination of a recent NMR structure of CsmA and simulation of circular dichroism and optical spectra for the CsmA–BChl a complex. [ABSTRACT FROM AUTHOR]

Light-trail is proposed as a candidate to carry IP traffic over wavelength division multiplexing (WDM) optical networks given its capability of accommodating multi-granularity traffic by time-division multiplexing (TDM). Light-trail's unidirectional shared-media multicast nature makes it hard to implement distributed access control and restricts that at most one packet transmission can take place at any time. Recently, opportunistic optical hyperchannel was proposed to improve light-trail by permitting easy distributed access controL In this paper, we propose a set of distributed access control protocols, namely, 1-persistent protocols, for opportunistic optical hyperchannels to maximize the throughput and provide fair service among contending nodes by taking their inherent advantage of adaptive space-division multiplexing (SDM). We also point out a possible generalization of opportunistic optical hyperchannels by removing the restriction of linear structure, and demonstrate possible applications of such a generalization. [ABSTRACT FROM AUTHOR]

This paper employs optimization to improve the performance of the system the Canadian Army uses to give a Commander a picture of where his forces are. This system uses the Army's wireless battlefield communication network to send vehicle positions as data messages. The difficulty is that, when vehicles compete to send these reports, the messages sometimes collide and this wastes valuable network capacity. At the heart of the problem is the channel access control mechanism. We show how a simple change to this mechanism can improve the system's performance significantly. [ABSTRACT FROM AUTHOR]

IEEE 802.11 WLAN has high data rates (e.g., 11 Mbps for 802.11b and 54Mbps for 802.11g), while voice streams of VoIP typically have low-data-rate requirements (e.g., 29.2 Kbps). One may, therefore, expect WLAN to be able to support a large number of VoIP sessions (e.g., 200 and 900 sessions in 802.11b and 802.11g, respectively). Prior work by one of the authors, however, indicated that 802.11 is extremely inefficient for VoIP transport. Only 12 and 60 VoIP sessions can be supported in an 802.11b and an 802.11g WLAN, respectively. This paper shows that the bad news does not stop there. When there are multiple WLANs in the vicinity of each other—a common situation these days—the already low VoIP capacity can be further eroded in a significant manner. For example, in a 5 × 5, 25-cell multi-WLAN network, the VoIP capacities for 802.11b and 802.11g are only 1.63 and 10.34 sessions per AP, respectively. This paper investigates several solutions to improve the VoIP capacity. Based on a conflict graph model, we propose a clique-analytical call admission scheme, which increases the VoIP capacity by 52 percent from 1.63 to 2.48 sessions per AP in 802.11b. For 11g, the call admission scheme can also increase the capacity by 37 percent from 10.34 to 14.14 sessions per AP. If all the three orthogonal frequency channels available in 11b and 11g are used to reduce interferences among adjacent WLANs, clique-analytical call admission scheme can boost the capacity to 7.39 VoIP sessions per AP in 11b and 44.91 sessions per AP in 11g. Last but not least, this paper expounds for the first time the use of coarse-grained time-division multiple access (CoTDMA) in conjunction with the basic 802.11 CSMA to eliminate the performance-degrading exposed-node and hidden-node problems in 802.11. A two-layer coloring problem (which is distinct from the classical graph coloring problem) is formulated to assign coarse time slots and frequency channels to VoIP sessions, taking into account the intricacies of the carrier-sensing operation of 802.11. We find that CoTDMA can further increase the VoIP capacity in the multi-WLAN scenario by an additional 35 percent, so that 10 and 58 sessions per AP can be supported in 802.11b and 802.11g, respectively. [ABSTRACT FROM AUTHOR]

In this paper, we derive throughput of a threshold-based transmission policy, namely load-regulated carrier sense multiple access (CSMA), taking into account the propagation delay of the medium and the offered load at different probabilities of the fading channel. In case of the saturated load-regulated CSMA, a trivial relationship between deterministic offered load to the channel at a particular fading channel condition and the maximum possible offered load has been shown. We further extend the load regulation concept into multi-channel domain. Both single and multi-channel load-regulated CSMA improves the throughput of the system compared with the existing CSMA system, which does not consider channel fading to control the packet transmissions. Copyright © 2008 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: The structure of the chlorosome baseplate protein CsmA from Chlorobium tepidum in a 1:1 chloroform:methanol solution was determined using liquid-state NMR spectroscopy. The data reveal that the 59-residue protein is predominantly α-helical with a long helical domain extending from residues V6 to L36, containing a putative bacteriochlorophyll a binding domain, and a short helix in the C-terminal part extending from residues M41 to G49. These elements are compatible with a model of CsmA having the long N-terminal α-helical stretch immersed into the lipid monolayer confining the chlorosome and the short C-terminal helix protruding outwards, thus available for interaction with the Fenna–Matthews–Olson antenna protein. [Copyright &y& Elsevier]

Multi-hop wireless networks employing random access protocols have been shown to incur large discrepancies in the throughputs achieved by the flows sharing the network. Indeed, flow throughputs can span orders of magnitude from near starvation to many times greater than the mean. In this paper, we address the foundations of this disparity. We show that the fundamental cause is not merely differences in the number of contending neighbors, but a generic coordination problem of CSMA-based random access in a multi-hop environment. We develop a new analytical model that incorporates this lack of coordination, identifies dominating and starving flows and accurately predicts per-flow throughput in a large-scale network. We then propose metrics that quantify throughput imbalances due to the MAC protocol operation. Our model and metrics provide a deeper understanding of the behavior of CSMA protocols in arbitrary topologies and can aid the design of effective protocol solutions to the starvation problem. [ABSTRACT FROM AUTHOR]

This paper presents the design, implementation and performance evaluation of a hybrid MAC protocol, called Z-MAC, for wireless sensor networks that combines the strengths of TDMA and CSMA while offsetting their weaknesses. Like CSMA, Z-MAC achieves high channel utilization and low latency under low contention and like TDMA, achieves high channel utilization under high contention and reduces collision among two-hop neighbors at a low cost. A distinctive feature of Z-MAC is that its performance is robust to synchronization errors, slot assignment failures, and time-varying channel conditions; in the worst case, its performance always falls back to that of CSMA. Z-MAC is implemented in TinyOS. [ABSTRACT FROM AUTHOR]

In contemporary control nets, random access schemes like CSMA are commonly used and are often enhanced by fixed priorities for some message types to guarantee an upper bound for the transmission time of high-priority messages. The introduction of priorities has a significant impact on the overall quality of service, which means that messages with a low priority may have long transmission times or high loss rates if the whole network is not adequately dimensioned. Thus, capacity planning includes as one of the first steps a model-based analysis of the network. This paper presents an approach to analyze the quality of service of CSMA-based control nets. Based on open queueing networks, analytical formulas are derived that allow one to compute performance measures like mean throughputs, loss rates, and response times in an extremely efficient way and with limited information about the system load. Particular emphasis is spent on the analysis of priorities in networks using a variant of the CSMA access scheme which has not been analyzed analytically before. Furthermore, the integration of timeouts and multicast communication into the analysis approach is outlined. [ABSTRACT FROM PUBLISHER]

The most popular strategies for dealing with packet collisions at the Medium Access Control (MAC) layer in distributed wireless networks use a combination of carrier sensing and collision avoidance. When the collision avoidance strategy fails, such schemes cannot detect collisions and corrupted data frames are still transmitted in their entirety, thereby wasting the channel bandwidth and significantly reducing the network throughput. To address this problem, this paper proposes a new wireless MAC protocol capable of collision detection. The basic idea of the proposed protocol is the use of pulses in an out-of-band control channel for exploring channel condition and medium reservation and achieving both collision avoidance and collision detection. The performance of the proposed MAC protocol has been investigated using extensive analysis and simulations. Our results show that, as compared with existing MAC protocols, the proposed protocol has significant performance gains in terms of node throughput. Additionally, the proposed protocol is fully distributed and requires no time synchronization among nodes. [ABSTRACT FROM AUTHOR]

Abstract: We present a novel cross-layer design for improving energy efficiency in a wireless sensor network that utilizes a multi-channel non-persistent CSMA MAC protocol with adaptive MQAM modulation at the physical layer. Cross-layer interactions are achieved through joint, traffic-dependent adaptation of the backoff probability at the MAC layer and the modulation order at the physical layer. The joint optimization of the backoff probability and the modulation order is conducted subject to a constraint on the packet retransmission delay. Such an optimization is shown to produce a significant improvement in the per-bit energy requirement for successful packet delivery. Our analytical findings are verified through numerical results and computer simulations. [Copyright &y& Elsevier]

Hidden terminals can significantly lower the throughput of such competitive MAC protocols as CSMA. Therefore, a large number of MAC protocols that implement different hidden terminal strategies have been developed. In this paper we propose an algorithm for generating evaluation networks with different ratios of hidden terminals in order to evaluate the effect of hidden terminals on competitive MAC protocols. In addition, we use the same algorithm to report results that elucidate the characteristics of throughput as the ratio of hidden terminals that exist in a network is varied for both CSMA, a representative competitive MAC protocol, and MACA, a MAC protocol that makes use of RTS/CTS control, a representative hidden terminal strategy. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 90(4): 46–55, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20343 [ABSTRACT FROM AUTHOR]

Green sulfur bacteria possess two external light-harvesting antenna systems, the chlorosome and the FMO protein, which participate in a sequential energy transfer to the reaction centers embedded in the cytoplasmic membrane. However, little is known about the physical interaction between these two antenna systems. We have studied the interaction between the major chlorosome protein, CsmA, and the FMO protein in Chlorobium tepidum using surface plasmon resonance (SPR). Our results show an interaction between the FMO protein and an immobilized synthetic peptide corresponding to 17 amino acids at the C terminal of CsmA. This interaction is dependent on the presence of a motif comprising six amino acids that are highly conserved in all the currently available CsmA protein sequences. [ABSTRACT FROM AUTHOR]

In this work, we analyze and evaluate the maximum achievable throughput of split-channel MAC schemes that are based on the RTS/CTS (ready-to-send/clear-to-send) dialogue and that rely on pure ALOHA or on p-persistent carrier sensing multiple access (CSMA) contention resolution techniques. Our results show that, when radio propagation delays are negligible and when the pure ALOHA mechanism is used, then for a network with relatively large number of nodes, the maximum achievable throughput of the split-channel MAC schemes is lower than that of the corresponding single-channel MAC schemes. When the split-channel MAC schemes employ the p-persistent CSMA mechanism, then they out-perform the corresponding single-channel schemes when the maximum end-to-end propagation delays are at least 25% of the transmission time of the control packets on the single shared channel. [ABSTRACT FROM PUBLISHER]

Efficient transmission methods for fading radio channels often require an iterative decoder. This is for example the case for systems using turbo codes. Receiver decoder iterations could potentially lead to a latency problem which impacts the performance of the medium access control protocol. In this paper, we present modifications based on the carrier sense multiple access with collision avoidance (CSMA/CA) medium access control (MAC) protocol to accommodate the increased latency in the iterative processing. One area of applications is wireless local area networks (WLANs) with high data rate. The simulation results performed in the IEEE 802.11a WLAN environment by replacing the 802:11a's convolutional coding with turbo coding demonstrate that the proposed algorithm provides a throughput gain over the conventional method. [ABSTRACT FROM PUBLISHER]

Abstract: Contention-based medium access control (MAC) protocol is a key component for the success of wireless data networks. Conventional random access protocols like ALOHA and Carrier Sense Multiple Access (CSMA) suffer from packet collision which leads to low throughput. Aimed at improving the throughput performance, we propose to integrate erasure coding with contention-based MAC protocols for recovering collided packets. To demonstrate the effectiveness of this approach, we focus on combining erasure coding with slotted ALOHA and slotted non-persistent CSMA in this paper. The performances of the resulting protocols are evaluated by both analytical model and simulation. Simulation results match very well with analytical results and show that the system throughput is increased for low to medium traffic loading. Packet loss ratio is also improved considerably with our scheme when the maximum number of packet retransmission times is limited. However, the delay for our scheme is higher due to the longer waiting time in our scheme for recovering collided packets. It is also shown that delay can be significantly reduced if we choose appropriate coding parameters though throughput will be sacrificed. [Copyright &y& Elsevier]

The authors analytically derive the throughput performance of the media access control (MAC) protocol MACA, which has become the prototype of options used in IEEE802.11 for reducing the effects of hidden terminals. This derivation indicates that the MACA throughput performance basically is affected by the propagation delay and RTS recognition time. Also, by comparing MACA with CSMA, which is a typical contention mode MAC protocol, the authors show that (1) if the RTS recognition time is sufficiently small, MACA has better performance than CSMA even in a network in which no hidden terminal exists, and (2) when relatively large data packets are transmitted and received, MACA functions effectively. [ABSTRACT FROM AUTHOR]

We consider the Medium Access Control (MAC) protocol design for nodes in a wireless LAN that use a wide-azimuth switched beam smart antenna system comprised of a multiple beam antenna array. The one-hop performance of Carrier Sense Multiple Access (CSMA) as well as Slotted Aloha for such a system is presented analytically and through simulation. The problem of synchronization of multiple beams in CSMA is investigated in our analysis. Our results show that, under heavy offered load conditions, CSMA is a good choice with nodes that have multiple-beam smart antennas, despite the performance loss due to the beam synchronization, providing a stable throughput that approaches unity and is invariant to fluctuations in the offered load. Slotted Aloha, on the other hand, is capable of higher peak throughput in a narrow range of offered loads as more switched beams are employed, but performance drastically reduces beyond optimum offered loads. We also introduce a method, Expanded Receive Rule (ERR), whereby the tight synchronization among different beams of a receiver node in CSMA is relaxed, which is observed to provide better throughput. Finally, we also present performance results for a 4-way-handshake-type carrier sense mechanism using multiple beam antennas. [ABSTRACT FROM AUTHOR]

The ability to detect the presence or absence of optical signals on specific channels is used to prevent packet collisions in networks implementing the optical carrier-sense multiple-access with collision avoidance (CSMA/CA) protocol. This optical carrier-sense capability can be provided by a baseband carrier-sense circuit (BCSC), which directly detects the envelope of optical packets propagating in the network. A fraction of the optical power of packets passing through a node is tapped off for detection at the BCSC. It is therefore desirable to minimize the amount of optical power required at the circuit to perform reliable packet detection such that the additional insertion loss encountered by the in-transit packets is minimal. However, the performance of the BCSC is limited at low levels of received optical power. For a given topology and size of a practical optical CSMA/CA packet network, knowledge of this sensitivity limit is essential for determining the power budget while ensuring that collision avoidance and minimal insertion loss are achieved. This paper identifies the factors that contribute to the sensitivity of the BCSC. A theoretical description of a practical implementation of the BCSC is derived and an efficient method to evaluate the sensitivity is presented. The theory is validated using experiments, with results showing that the model presented in this paper is a useful tool for the design and performance analysis of highly sensitive BCSCs in future optical CSMA/CA networks. [ABSTRACT FROM PUBLISHER]

A comparative study of the performance characteristics of five multiple access configurations for a wireless communication network is presented and evaluated within the context of the medical field of application. A universal variable has been defined for comparison between the alternative configurations, embodying the data rate, number of channels, power consumption, and bandwidth requirements. Special medical specifications have been accommodated, while the broader applications of this approach have been addressed. CDMA is indicated as the preferred method, over ALOHA, slotted ALOHA, CSMA, and polling. [ABSTRACT FROM PUBLISHER]

The csmA gene of Aspergillus nidulans encodes a polypeptide that consists of an N-terminal myosin motor-like domain and a C-terminal chitin synthase domain. csmA null mutants showed marked abnormalities in polarized growth, hyphal wall integrity, and conidiophore development. Furthermore, the growth of the csmA null mutants was sensitive to low osmotic conditions. In an effort to investigate the intracellular behavior of the csmA product (CsmA) and the regulation of its production, we constructed strains that produced CsmA tagged with nine repeats of the hemagglutinin A (HA) epitope at its COOH terminus (CsmA–HA) instead of CsmA. Western blot analysis with anti-HA antibody showed that the entire coding region of csmA was translated as a single polypeptide with an approximate molecular mass of 210 kDa. CsmA–HA was produced during vegetative growth; however, its yield was significantly reduced under high osmotic conditions, suggesting that the role of CsmA in growth and morphogenesis is particularly important under low osmotic conditions. [Copyright &y& Elsevier]

Today, there are a number of basic networking technologies that can support data-centric home networking, including 802.11b, HomeRF, and Bluetooth. As with most new markets and technologies, what users want their home network to do is rapidly changing and evolving. Today's early-adopters want more from their home networks than just sharing PC peripherals; they want to be able to send their digital entertainment content (digital pictures, MP3 audio, and DV-based video) throughout their homes. As this "early-adopter market" rapidly changes into a mass market, today's home network must evolve to where it supports not only TCP/IP-based data, but also audio and video data that require QoS. This evolution will require changes in the architecture and implementation of wireless networks. Air5 is a solution for handling the demands of wireless home networking. Based on the lEEE802.11a PHY-layer specification, it includes enhancements for delivering range and throughput levels. Air5 also delivers triple-DES security and guaranteed QoS performance through its TDMA-based synchronous MAC. There are over 40 companies developing 5-GHz based networking technologies today, so this will be a highly competitive technology evolution. [ABSTRACT FROM PUBLISHER]

The authors begin by giving a brief overview of the physical layer, the data link layer, the network layer, the transport layer and the application layer, before going on to discuss the MAC sublayer (a subset of the data link layer) in more detail. Static allocation protocols are considered as are FDM, TDM, dynamic allocation protocols, ALOHA, CSMA, reservation protocols, wireless network protocols and MACA (multiple access with collision avoidance). The paper concludes by mentioning the IEEE 802.3 and IEEE 802.4 standards as examples [ABSTRACT FROM PUBLISHER]

The effects of inhibition of carotenoid biosynthesis by 2-hydroxybiphenyl on the photosynthetic growth, pigment composition and chlorosome structure of Chlorobium phaeobacteroides strain CL1401 were examined. At a concentration of 20 µg 2-hydroxybiphenyl ·ml–1, carotenoid synthesis was largely inhibited (85%), but the photosynthetic growth rate was almost unaffected (µcontrol=0.00525±0.00007 h–1 and µHBP-treated=0.00505±0.0005 h–1). Cells grown in the presence of the inhibitor were 5 µm–70 µm long, while control cells were between 2–5 µm long. Moreover, 2-hydroxybiphenyl-treated cells contained fewer, unevenly distributed chlorosomes per µm of cytoplasmic membrane with an irregular arrangement (2.5±1.5 vs of 9.1±1.9). This was concomitant to the 83% decrease in the content of bacteriochlorophyll (BChl) e in 2-hydroxybiphenyl-treated cells. Electron microscopy revealed that the shape of carotenoid-depleted chlorosomes changed from ellipsoidal to spherical, although the mean volume was similar to that of control chlorosomes. SDS-PAGE analysis of the chlorosome polypeptide composition showed that the amount of CsmA protein decreased by 60% in carotenoid-depleted chlorosomes. This was paralleled by a decrease in the baseplate BChl a content. The data suggest that carotenoids are close to the chlorosomal baseplate, where they carry out both structural and photoprotective functions. [ABSTRACT FROM AUTHOR]

CSMA protocols have been extensively studied under the assumption of an equilibrium condition. The results obtained in this manner provided useful insight for the development of packet radio and local area networks for the transmission of bursty data. The integration of isochronous and bursty traffic in the same network may, however, make the equilibrium assumption unrealistic, for example when fixed-length frames are defined, only a portion of which is devoted to the transmission of bursty data according to a CSMA protocol. In such cases, it is necessary to study the protocol behaviours in fixed-length time segments. This paper presents a technique for the analysis of slotted CSMA protocols in such an environment. Definitions for the transient throughput and transient average packet delay are introduced, and closed form expressions are derived for non-persistent and 1-persistent CSMA and CSMA/CD protocols with arbitrary distribution of the packet length. Numerical results help in the visualization of some interesting phenomena, peculiar to the CSMA operations in finite-length intervals. [ABSTRACT FROM AUTHOR]

HIPERLAN will provide a 23.5 Mbps data rate transmission to radio channel user. The medium access control is based on carrier sense (CSMA) and on active signaling. Active signaling is an original feature which makes it possible to merge priority contention and collision detection into an elegant and efficient channel access mechanism. In particular it is expected to boost the performance of HIPERLAN above the level of IEEE 8802.3 standard (Ethernet). It also gives the opportunity to mix time constraint traffics (like voice) with non-time constraint traffics (data) by making use of access priorities. [ABSTRACT FROM AUTHOR]

We present a novel architecture for an optical network, translucent smart pixel array (TRANSPAR), having smart pixel devices which effectively function in an optically translucent manner. The network protocol is similar to carrier-sense multiple-access/collision-detection (CSMA/CD) commonly used in Ethernet, but adapted to a ring configuration using optical parallel packets in free space. The TRANSPAR devices also function as a fine-grain mesh-connected parallel pipeline array for image and video signal processing. We designed, fabricated, and are currently testing the TRANSPAR smart pixel devices and network node hardware. This paper presents the network architecture, tradeoffs, and design decisions, the testing results to date, and ends with considerations on practicality and future scalability [ABSTRACT FROM PUBLISHER]

The limited self-healing ability of cartilage necessitates the application of alternative tissue engineering strategies for repairing the damaged tissue and restoring its normal function. Compared to conventional tissue engineering strategies, three-dimensional (3D) printing offers a greater potential for developing tissue-engineered scaffolds. Herein, we prepared a novel photocrosslinked printable cartilage ink comprising of polyethylene glycol diacrylate (PEGDA), gelatin methacryloyl (GelMA), and chondroitin sulfate methacrylate (CSMA). The PEGDA-GelMA-CSMA scaffolds possessed favorable compressive elastic modulus and degradation rate. In vitro experiments showed good adhesion, proliferation, and F-actin and chondrogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) on the scaffolds. When the CSMA concentration was increased, the compressive elastic modulus, GAG production, and expression of F-actin and cartilage-specific genes (COL2, ACAN, SOX9, PRG4) were significantly improved while the osteogenic marker genes of COL1 and ALP were decreased. The findings of the study indicate that the 3D-printed PEGDA-GelMA-CSMA scaffolds possessed not only adequate mechanical strength but also maintained a suitable 3D microenvironment for differentiation, proliferation, and extracellular matrix production of BMSCs, which suggested this customizable 3D-printed PEGDA-GelMA-CSMA scaffold may have great potential for cartilage repair and regeneration in vivo. [ABSTRACT FROM AUTHOR]

The clustering coefficient has been proposed to characterize complex networks. Hidden terminals may degrade the performance of the CSMA (carrier sense multiple access) protocol. This letter computes analytically the clustering coefficient and the quantity of hidden terminals for ad hoc networks. The former turns out to be a constant and the latter is proportional to n3 p2, where n is the number of nodes and p is the link probability. The connection between them has been established, and simulation results confirm our analytical work. [ABSTRACT FROM PUBLISHER]