Selective Rejection Research Articles

Suspension-feeding behaviour in tropical bivalve molluscs:Perna viridis, Crassostrea belcheri, Crassostrea iradelei, Saccostrea cucculata and Pinctada margarifera

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 166:173-185 (1998) - doi:10.3354/meps166173 Suspension-feeding behaviour in tropical bivalve molluscs: Perna viridis, Crassostrea belcheri, Crassostrea iradelei, Saccostrea cucculata and Pinctada margarifera A. J. S. Hawkins1,*, R. F. M. Smith1, S. H. Tan2, Z. B. Yasin2 1Plymouth Marine Laboratory, Natural Environment Research Council, West Hoe, Plymouth PL1 3DH, United Kingdom 2Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia *E-mail: a.hawkins@pml.ac.uk Filter-feeding behaviours of the mussel Perna viridis and the oysters Crassostrea belcheri, Crassostrea iradelei, Saccostrea cucculata and Pinctada margarifera were compared during natural tidal variations in the concentration (6 to 40 dry mg total particulate mass l-1) and organic content (6 to 22%) of seston available in the Merbok mangrove system, Malaysia. In P. viridis, C. belcheri and S. cucculata, declining retention efficiencies for the largest available particle size classes of more than 6 to 12 µm diameter were associated with an overall organic enrichment of filtered relative to available matter. As seston availability increased, a minimum average of 71% of the additional filtered matter was rejected by each species as pseudofaeces prior to ingestion. And in all 5 species, preferential rejection as pseudofaeces of filtered particles with higher average inorganic content resulted in the net organic enrichment of ingested relative to filtered matter. In P. viridis and C. belcheri, the efficiency of that net organic selection declined for seston of lower organic content. Combining all data from each species, absorption efficiencies from ingested organics increased exponentially with the organic content of ingested matter. Collective findings suggest that key interrelations between component processes of nutrient acquisition were similar to those that have been established for the modeling of feeding, growth and environmental impact among filter-feeding bivalves from temperate latitudes. However, especially fast growth in P. viridis stemmed from higher average (± 2 SE) clearance rates (13.2 ± 7.0 l h-1 g-1 when particulate organic matter < 1 mg l-1), greater average organic enrichment of filtered relative to available matter (0.99 ± 0.67) and greater average organic enrichment of ingested relative to filtered matter (0.63 ± 0.03) than have hitherto been recorded in any species of bivalve filter-feeder. In contrast, P. margarifera was least well adapted for the pre-ingestive selection of organic matter, with no differential retention on the gill and little selective rejection as pseudofaeces, representing at least part of the physiological basis for characteristically slow growth in pearl oysters. Filter-feeding behaviour · Particle selection · Seston composition · Perna viridis · Crassostrea belcheri · Crassostrea iradelei · Saccostrea cucculata · Pinctada margarifera Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 166. Publication date: May 28, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

Mechanisms for the selective rejection of solutes in nanofiltration membranes

Two tight nanofiltration membranes of 100 and 400 Da were tested at different operating conditions. The results show that the selective properties of nanofiltration membranes come from a combination of several physicochemical mechanisms. Mass transport is mainly diffusive in the 100 Da membrane, while two mechanisms of segregation exist in the 400 Da membrane. Large molecules are retained at the membrane surface by electrostatic and steric interactions and small, weakly charged ions can enter the pores. In these pores, the segregation is due to surface forces (electrostatic and friction) and the solvent can transfer by a convective mechanism.

Rationing Capacity Between Two Product Classes

In this paper, we study the capacity allocation problem faced by make-to-order manufacturing firms encountering expected total demand in excess of available capacity. Specifically, we focus on firms' manufacturing short-life-cycle or seasonal products such as high fashion apparel. Using a decision-theory-based approach, we develop a capacity allocation policy that allows such firms to discriminate between two classes of products (one yielding a higher profit contribution per unit of capacity allocated to it than the other), resulting in selective rejection of orders for the class with the lower unit contribution. The effectiveness of capacity rationing is investigated under a wide array of conditions characterized by variations in factors such as the ratio of unit profit contributions of the two product classes, the ratio of total available capacity to expected total demand, the ratio of expected demands between the two classes, and the variability in demand for each product class. The results indicate that capacity rationing is very effective in increasing the total profit, and could therefore serve as a valuable decision tool for managers in such firms.

A data link control protocol for LEO satellite networks providing a reliable datagram service

A data link control protocol for low earth orbit satellite networks has been developed to overcome throughput limitations inherent in event-based positive acknowledgment automatic-repeat-request class protocols. This was accomplished by relaxing a reliability constraint, specifically, the in-sequence delivery constraint. This relaxation results in a new class of link layer service, that of reliable datagram, and permits a fresh approach to link layer protocol design. The authors present one such protocol, the low altitude multiple satellite data link control (LAMS-DLC) protocol. They derive the throughput efficiency of LAMS-DLC and compare the results with HDLC (selective reject). The analysis is verified using an event-based simulation. Measurements suggest that LAMS-DLC provides near optimal throughput efficiencies in the target environment, while using significantly less buffer space than that required for HDLC-SREJ. >

Reintroducing group selection to the human behavioral sciences

AbstractIn both biology and the human sciences, social groups are sometimes treated as adaptive units whose organization cannot be reduced to individual interactions. This group-level view is opposed by a more individualistic one that treats social organization as a byproduct of self-interest. According to biologists, group-level adaptations can evolve only by a process of natural selection at the group level. Most biologists rejected group selection as an important evolutionary force during the 1960s and 1970s but a positive literature began to grow during the 1970s and is rapidly expanding today. We review this recent literature and its implications for human evolutionary biology. We show that the rejection of group selection was based on a misplaced emphasis on genes as “replicators” which is in fact irrelevant to the question of whether groups can be like individuals in their functional organization. The fundamental question is whether social groups and other higher-level entities can be “vehicles” of selection. When this elementary fact is recognized, group selection emerges as an important force in nature and what seem to be competing theories, such as kin selection and reciprocity, reappear as special cases of group selection. The result is a unified theory of natural selection that operates on a nested hierarchy of units.The vehicle-based theory makes it clear that group selection is an important force to consider in human evolution. Humans can facultatively span the full range from self-interested individuals to “organs” of group-level “organisms.” Human behavior not only reflects the balance between levels of selection but it can also alter the balance through the construction of social structures that have the effect of reducing fitness differences within groups, concentrating natural selection (and functional organization) at the group level. These social structures and the cognitive abilities that produce them allow group selection to be important even among large groups of unrelated individuals.

Multislotted ALOHA scheme for high‐efficiency transmission of variable‐length packets

AbstractThe paper proposes the multislotted ALOHA (MS‐ALOHA) as a new random access scheme which can improve the throughput in variable‐length packet signal transmission. This scheme is to improve the throughput in the variable‐length packet signal transmission by controlling the transmission timing according to the packet‐signal length.The throughput performance of the proposed scheme for the variable‐length packet transmission is formulated for the infinite station model and the finite station model. The proposed scheme is compared to the traditional slotted ALOHA with the single transmission timing by computer simulation. It is shown that the throughput of the proposed scheme is larger independently of the signal length and the performance is stable against the variation of the signal‐length distribution.When the signal length is distributed uniformly, the maximum throughput is improved by approximately 20 percent compared to the traditional scheme. The application of the retransmission slot‐control scheme to the proposed scheme is considered. Here, the transmission slot for the retransmission packet is assigned to each transmitting station. As a result of analysis by computer simulation, it is shown that a higher throughput is obtained than the ALOHA scheme with the selective rejection retransmission protocol.

A Crisis of Party?*

The debate about the (future) role of political parties in modern democracies suffers from generally unacknowledged normative preconceptions as well as a tendency to reason in terms of ‘inevitable’ social processes, rather than precise theoretical reasoning or empirical analysis. At least four distinct bodies of thought affect the discussion about the assumed crisis of party: (1) the view that parties are a danger to the good society, leading to the denial of parties as legitimate actors; (2) the belief that some types of parties are ‘good’ and others ‘bad’, causing a selective rejection of parties; (3) the proposition that certain party systems are ‘good’ and others ‘bad’, resulting in a selective rejection of party systems; and (4) the affirmation that parties are becoming redundant. The latter suggestion is shown in a variety of approaches ‐e. g. the idea that parties are transient agents of democratization, the analysis of parties as mere market‐forces, the assumption that parties do not matter in policies, and the view, as exemplified by neocorporatism, that parties inevitably lose their functions to other political actors. The pervasive presence of aprioristic views suggests the need for a conceptual house‐cleaning, the importance of distinguishing normative from empirical arguments, and the need for more detailed empirical research, giving due weight to differences between countries, party systems, parties and periods instead of postulating inexorable trends.

Natural sediment as a food source for the cockle Cerastoderma edule (L.): effect of variable particle concentration on feeding, digestion and the scope for growth

Cockles Cerastoderma edule (L.) from a mud-flat estuarine population were exposed to variable concentrations of suspended bottom sediment to determine the physiological components of the energy budget and scope for growth, as a function of particle concentration. Associated processes of feeding and digestion were given particular attention, involving quantification of selective rejection of material in pseudofaeces and estimations of gut passage time of food and gut contents. Although clearance rate declined with increasing particle concentration, regulation of ingestion rate at high particle load was mainly achieved by pseudofaeces production. Two mechanisms contributed to maintain absorption efficiency of filtered material approximately constant throughout changing food concentration: (a) Preingestive selection of organically rich particles, resulting in adjustments of organic content in the ingested ration; and (b) reduced effects of ingestion on gut passage time of food, since changing gut contents provide an important mechanism for the adjustment of variations in ingestion rate. Scope for growth increased to reach an asymptotic value at food concentrations which correspond to a maximum of ingestion. Constraints on feeding rates thus appear as the main limiting factor for growth.

Development and performance evaluation of highly efficient retransmission scheme for high‐speed data transfer via satellite

AbstractSatellite communication has the following problems. The propagation delay is larger than in the terrestrial channel. When the carrier frequency exceeds 10 GHz, the bit‐error rate tends to increase due to the effect of rain and snow, which degrades the transmission efficiency. To cope with such problems, a highly efficient error control protocol has been proposed to prevent the degradation of the transmission efficiency.This paper describes a new large‐capacity, highly efficient retransmission scheme for satellite communication (extended SREJ scheme), which was devised considering the compatibility to the terrestrial network. The method is based on the SREJ (selective reject) scheme, which is specified in the HDLC (high‐level data link control) protocol, employed widely in terrestrial data transmission. To evaluate quantitatively REJ, SREJ and extended SREJ schemes, a satellite communication control equipment was developed.The result of the data transmission experiment is described in detail, indicating that the extended SREJ scheme is efficient. Then, through the discussion of practical usefulness, it is shown that the extended SREJ scheme has no problem from the viewpoint of the number of buffers in transmitting and receiving stations. Furthermore, to realize the efficient processing as far as possible, even if the bit‐error rate is higher than 1.0 × 10−4, the extended SREJ/REJ scheme is discussed by combining the proposed scheme with the REJ scheme.

Calculation of message length transition probabilities in selective reject ALOHA channels

A method is described for the exact calculation of message length transition probabilities in asynchronous selective reject (SREJ) ALOHA channels with Poisson message arrivals. The formulas derived are used for exact calculation of the throughput characteristics of example SREJ-ALOHA channels which were previously analyzed using bounds or approximate methods.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Peirce and evolution: Comment on O'Hear

After stressing the shortcomings of Darwinian accounts of self‐consciousness and knowledge ‐ i.e. in terms of their survival value ‐ Anthony O'Hear presents Peirce's metaphysical hypotheses on cosmic evolution as an alternative approach that avoids those shortcomings. Although O'Hear does not straightforwardly defend Peirce's views, his argument suggests that only some teleological account of self‐consciousness and knowledge is reasonable. The argument, though correct, is not enough to establish the metaphysical point O'Hear defends. Before developing his metaphysical ideas, Peirce's rejection of natural selection as an explanation for every phenomenon brought him to consider the more appropriate question of how natural selection could give rise to a different kind of evolution. This involved outlining an evolutionary account of the origin of self‐consciousness and of the mechanisms of belief fixation. The point is not one about Peirce, of course, but about the relationship between, on the one hand, biolo...

Dynamic performance of ALOHA-type VSAT channels: a simulation study

A simulation study is presented of the dynamic behavior of ALOHA-type protocols used in interactive-data very small aperture terminal (VSAT) networks. A simulation model for quantitative evaluation of random access channel stability in terms of the transient response to a traffic overload pulse is described, and the usefulness of a single performance measure called backlog fall time is established. The variation of backlog fall time with selectable protocol parameters (such as average retransmission delay for nonadaptive systems or retransmission backoff policy parameters for adaptive systems) is investigated, for example, for ALOHA and selective reject ALOHA channels. A methodology for joint optimization of steady-state and dynamic performance based on obtaining contours of average delay versus backlog fall time over the variation range of selectable protocol parameters is outlined and demonstrated. It is shown that, generally speaking, a suitable operating point is easily identified from these contours because they exhibit a characteristic knee region in which both delay and fall time are close to the minimum values that can be independently obtained. >

Selective reject ALOHA/FCFS: An advanced VSAT channel access protocol

AbstractThis paper proposes and evaluates selective reject (SREJ) ALOHA/FCFS, a highly efficient, flexible and robust multiaccess protocol for very small aperture terminal (VSAT) applications. SREJ‐ALOHA with locally synchronous FCFS retransmission scheduling is a ‘semi‐compatible’ enhancement of asynchronous SREJ‐ALOHA, which has earlier been shown to be an attractive satellite channel sharing protocol for low volume bursty terminals with variable length messages. The FCFS retransmission scheduling policy proposed here broadens the applicability of SREJ‐ALOHA by providing higher maximum throughput, particularly in the important source traffic scenario with bimodal message length distributions. Improved channel‐sharing efficiency and delay are achieved by exploiting time‐of‐arrival feedback naturally embedded in an unslotted SREJ‐ALOHA channel to develop an implicit time‐of‐arrival based (FCFS) ordering for conflict‐free, locally synchronous retransmission of most contending messages. An analytical procedure for evaluation of stability, throughput and delay for a general finite source, variable message length traffic model is described. The model is used to demonstrate that random access SREJ‐ALOHA/FCFS typically achieves a maximum throughput in the region of 0·4–0·45 along with excellent stability and delay properties. In addition, a natural extension of the protocol which supports implicit reservation for long messages is described and shown to offer a high maximum throughput (&gt;0·6), comparable to that achieved by highly co‐ordinated slotted systems, for example mixed interactive/file‐transfer traffic scenarios.

Verified data transfer protocols with variable flow control

We present and verify a sliding window protocol which uses modulo- N sequence numbers to achieve reliable flow-controlled data transfer between a producer and a consumer connected by unreliable channels. The consumer's data needs are represented by a receive window whose size can vary with time. The producer entity sends segments of data words that lie within the consumer's receive window. The consumer entity sends acknowledgment, selective acknowledgment, and selective reject messages that inform the producer entity of the current receive window size, the data word next expected, and the reception (or lack of reception) of out-of-sequence data segments. Our protocol is, therefore, a proper extension of existing transport and data link protocol standards such as TCP, IS0 TP, HDLC, ADCCP, and so forth. We consider two types of unreliable channels. The first type, referred to as transport channels, can lose, duplicate, and reorder messages to an arbitrary extent, but impose an upper bound on message lifetimes (which can be very large, e.g., days). The second type, referred to as data link channels, can only lose messages. For both types of channels, we obtain the minimum value of N that ensures safe operation without imposing any constraints on the retransmissions of messages or on the data segment sizes. Thus, any retransmission or acknowledgment policy that optimizes the protocol's performance can be used. For transport channels, this value of N is a function of the maximum message transmission rate, the maximum message lifetime, and the maximum receive window size. For data link channels, this value of N is a function only of the maximum receive window size. We verify progress under three different liveness assumptions: retransmissions initiated by both entities, only by the producer entity, and only by the consumer entity. The protocol also satisfies a convenient noninterference safety property between the acknowledgement, selective acknowledgment, and selective reject messages. The protocols are specified as event-driven systems and verified hierarchically in two major stages. First, we verify that correct flow-controlled data transfer results if the sequence numbers in the channels satisfy certain correct interpretation bounds , irrespective of the types of errors that the channels may have. Second, for both transport and data link channels, we verify that the correct interpretation bounds are enforced by the corresponding minimum values of N . For the verification of the transport channel case, we use a system model with continuous measures of time in which real-time constraints can be formally specified and verified.

Bateson's two Toronto addresses, 1921: 2. Evolutionary faith.

William Bateson's plenary address to the American Association for the Advancement of Science meeting in Toronto in 1921 was titled "Evolutionary Faith and Modern Doubts." In it he expressed his deep-seated skepticism about the causes of evolution (and in particular, his dissatisfaction with Darwinian natural selection) while reaffirming his belief in the reality of evolution itself. The address led to controversy at both the scientific and popular levels. Scientific criticism centered on Bateson's rejection of natural selection; popular controversy, as evidenced by contemporary newspaper clippings, was very widespread, not least because religious fundamentalists misrepresented Bateson in their campaign against evolution. I draw attention to the forgotten case of F.E. Dean, a superintendent of schools at Fort Sumner, New Mexico, who in 1922 was forced to resign his post for merely challenging a resolution of the local school board banning the teaching of evolution in schools. Dean deserves to be remembered, along with John T. Scopes, as an early hero of the continuing fight for the right to teach evolution in U.S. schools.

A multiple address data communication system via satellite link

AbstractOne of the features of satellite communications is broadcasting ability (multiple address communication). Since a satellite has lengthy transmission delay, transmission errors at each link accumulate and the number of retransmissions increases, resulting in decrease in throughput efficiency if a one‐to‐one transmission control procedure (such as HDLC) is applied to a multiple address communication. This research proposes a multiple address communication system via a satellite link. In this system, information sequence is divided into N blocks based on the Chinese Remainder Theorem and the N blocks are sent with the SREJ (selective reject) procedure. At a receiving station the original information sequence can be restored with at least K(N) received frames. Therefore, the number of frames re‐sent from a transmitting station decreases and a decrease in throughput efficiency is prevented. The transmission characteristics of the proposed system are examined under the assumption that an ideal SREJ procedure is used. As a result, it is revealed that the proposed multiple address communication system can achieve higher throughput efficiency than the iterations of individual one‐to‐one communication.

Channel access protocols for Ku-band VSAT networks: a comparative evaluation

The performance of candidate protocols for first-generation Ku-band very small aperture terminal (VSAT) satellite networks is compared. The goal is to assist the process of selecting a protocol. The performance comparison is carried out over a range of possible traffic, channel and satellite parameters, leading to an understanding of the appropriate regime for each of the protocols under consideration. The protocols considered are unslotted Aloha, slotted Aloha, selective-reject (SRE) Aloha, and demand-assigned time-division multiple-access (TDMA) or DAMA. >

Simulation models for performance evaluation of satellite multiple access protocols

Simulation models are described for the performance evaluation of satellite multiple-access protocols used in VSAT (very small aperture terminal) data networks. Since the design of VSAT-based wide-area networks for interactive applications is critically dependent on the delay-throughput characteristics of the channel access protocol used, detailed simulation models have been developed to augment analytical performance-evaluation methods. Simulation models for several candidate VSAT access protocols, including random-access techniques such as Aloha, selective reject Aloha and slotted Aloha and demand-assignment multiple-access based techniques, are discussed in terms of traffic-source models, access algorithms, and performance-evaluation criteria. Numerical results for a realistic interactive data VSAT network environment are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stability, Throughput, and Delay of Asynchronous Selective Reject ALOHA

An analytical performance evaluation model is presented for selective reject (SREJ) ALOHA, a recently proposed high-capacity protocol for unslotted channels with high propagation delay. SREJALOHA approaches the theoretical asynchronous multiaccess throughput limit of 0.368 for fixed or variable length message traffic, using subpacketization of messages in conjunction with a selective reject retransmission policy. A finite user flow equilibrium-based model for SREJ-ALOHA with variable length messages is derived and used to characterize stability, throughput, and delay. Particular attention is given to the development of a stability analysis methodology for unslotted ALOHA-type protocols with variable length transmissions. Numerical results based on a satellite channel example are presented to demonstrate the significant performance advantages offered by SREJ-ALOHA, even after accounting for realistic subpacket overheads.

Ku‐Band satellite data networks using very small aperture terminals—part I: Multi‐access protocols

AbstractThis two‐part paper presents a comprehensive overview on the technology of very small aperture terminal (VSAT)‐based satellite data networks, which have received increasing attention in recent years because of a combination of technical and economic factors. Ku‐band networks with customer‐premise earth‐stations can overcome the inherent local access bottleneck in wide‐area terrestrial data networks by suitably exploiting the unique multi‐access and broadcast features of satellite communication. In this part of the paper, the issue of multiple access, which is an important critical path technology in the ongoing evolution of VSAT data networks, is considered in detail. Satellite multi‐access protocols are classified in terms of channel synchronization (slotted and unslotted) and the qualitative nature of message access (fixed assigned, contention and reservation). In addition to well known techniques such as ALOHA, slotted ALOHA, tree CRA, packet CDMA and demand assigned (DAMA) TDMA, a number of new approaches to efficient unslotted access, including selective reject (SREJ) ALOHA, time‐of‐arrival CRA and locally synchronous reservation, are discussed. The general review is followed by a detailed performance comparison of a number of candidate first‐generation VSAT protocols, namely ALOHA, slotted ALOHA, SREJ‐ALOHA, DAMA with TDMA reservation access and DAMA with slotted ALOHA reservation access. The performance evaluation, based on detailed simulations with VSAT equipment and transaction traffic parameters, is summarized by curves of average delay and throughput vs. number of VSATs per channel for each of the protocols considered. Higher‐order differences in delay performance are illustrated with appropriate delay distribution results and the sensitivity of performance with respect to key equipment and traffic parameters is investigated. For the transaction traffic model under consideration, the results suggest the use of SREJ‐ALOHA among contention techniques or DAMA with slotted ALOHA reservation among controlled access alternatives. The trade‐off between contention and reservation access is shown to be more complex, depending on the combination of average delay, peak delay, transmission cost and equipment complexity suited to the particular application.In Part IITo published in the next issue of this paper on Ku‐band VSAT networks, the issue of overall system design is considered.