The information-theoretic capacity of discrete-time queues

The information-theoretic capacity of continuous-time queues was analyzed recently by Anantharam and Verdu (see ibid. vol.42, p.4-18, 1996). Along similar lines, we analyze the information-theoretic capacity of two models of discrete-time queues. The first model has single packet arrivals and departures in a time slot and independent packet service times, and is the discrete-time analog of the continuous-time model analyzed by Anantharam and Verdu. We show that in this model, the geometric service time distribution plays a role analogous to that of the exponential distribution in continuous-time queues, in that, among all queues in this model with a given mean service time, the queue with geometric service time distribution has the least capacity. The second model allows multiple arrivals in each slot, and the queue is modeled as serving an independent random number of packets in each slot. We obtain upper and lower bounds on the capacity of queues with an arbitrary service distribution within this model, and show that the bounds coincide in the case of the queue that serves a geometrically distributed number of packets in each slot. We also discuss the extremal nature of the geometric service distribution within this model     

Efficient scheduling of transmissions in optical broadcast networks

All-optical networks (AONs) with a broadcast-star based physical topology offer the possibility of transmission scheduling to resolve channel and receiver conflicts. This paper considers the problem of scheduling packet transmissions in a wavelength-division multiplexed (WDM) optical network with tunable transmitters and fixed-tuned receivers. The scheduling problem is complicated by tuning latency, a limited number of channels, and arbitrary traffic demands. We first analyze scheduling all-to-all packet transmissions and obtain a new lower bound for the schedule length. The lower bound is achieved by an algorithm proposed by Pieris and Sasaki (1994). We then extend the analysis to the case of arbitrary traffic demands and obtain lower bounds for the schedule length. Two constructions for scheduling algorithms are provided through list scheduling and multigraphs. The upper bounds so obtained not only provide performance guarantees with arbitrary demands, but also nearly meet the lower bound in simulations     

Parallel-machine rescheduling with machine disruptions

In this study we consider a rescheduling problem on identical parallel machines. The rescheduling is undertaken because of a period of unavailability on one of the machines. We consider the total flow time as an efficiency measure and stability is gauged in terms of the number of jobs processed on different machines in the original and new schedules. We show that all efficient schedules with respect to efficiency and stability measures can be generated in polynomial time.     

Optical CDMA via temporal codes

The authors provide an analysis of the performance of optical orthogonal codes in an optical code division multiple access (CDMA) network by considering the probability distribution of the interference patterns. It is shown that the actual performance is close to a previous estimate. A less structured temporal code in which the code words are allowed to overlap at two pulse positions is also considered. The bit error probability for this class of codes is obtained for two cases: with and without optical hard-limiting at the receivers. It is shown that this code may increase the number of users in the network considerably without a significant loss in the performance     

Scheduling parallel machines to minimize weighted flowtime with family set-up times

We describe and evaluate several branch-and-bound algorithms for an identical parallel machine scheduling problem with family set-up times and an objective of minimizing total weighted flowtime. The algorithms differ by choice of lower bound method. Computational results suggest conditions favourable to a particular algorithm as well as the range of problem sizes that can be optimally solved in reasonable CPU time.     

Performance analysis of generalized selection combining withswitching constraints

Generalized selection combining (GSC), in which the best L_c out of L independent diversity channels are linearly combined has been previously proposed and analyzed for Rayleigh fading channels. GSC is a less complex alternative to maximal ratio combining (MRC) that provides a performance gain over traditional selection combining (SC). Both MRC and SC are special cases of CSC, with L_c=L and L_c=1, respectively. We show that CSC also dramatically reduces the impact of switching rate constraints, whereby a selection must be held for the duration of a packet     

Impact of antibiotic stress on acid and heat tolerance and virulence factor expression of Escherichia coli O157:H7

This study was conducted to determine the effect of antibiotic stress on the virulence factor expression, simulated gastric fluid (SGF; pH 1.5) survival, and heat tolerance (56 degrees C) of Escherichia coli O157:H7. The MIC for three antibiotics (trimethoprim, ampicillin, and ofloxacin) was determined for two E. coli O157:H7 strains (ATCC 43895 [raw hamburger isolate] and ATCC 43890 [fecal isolate]) by the dilution series method. Subsequently, cells were stressed at the MIC of each antibiotic for 4 h, and poststress tolerance and virulence factor production were evaluated. Heat tolerance (56 degrees C) was determined by the capillary tube method, and SGF (pH 1.5) survival was used to assess acid tolerance. Virulence factor expression (stx, hlyA, and eaeA) was evaluated by the creation of lacZ gene fusions and then use of the Miller assay (a beta-galactosidase assay). Stressed and control cells were evaluated in triplicate. The MIC for trimethoprim was 0.26 mg/liter for both strains; for ampicillin, it was 2.05 mg/liter for both strains; and for ofloxacin, it was 0.0256 and 0.045 mg/liter for each strain. Heat tolerance and SGF survival following antibiotic stress decreased when compared with control cells (P < 0.05). Exposure to ofloxacin increased stx and eaeA expression (P < 0.05). Exposure to ampicillin or trimethoprim increased eaeA expression (P < 0.05). hly expression increased following trimethoprim stress (P < 0.05). Antibiotics can increase E. coli O157:H7 virulence factor production, but they do not produce a cross-protective response to heat or decreased pH.     

Scheduling a batch processing machine with incompatible job families

The problem of scheduling batch processors is important in some industries and, at a more fundamental level, captures an element of complexity common to many practical scheduling problems. We describe a branch and bound procedure applicable to a batch processor model with incompatible job families. Jobs in a given family have identical job processing times, arbitrary job weights, and arbitrary job sizes. Batches are limited to jobs from the same family. The scheduling objective is to minimize total weighted completion time. We find that the procedure returns optimal solutions to problems of up to about 25 jobs in reasonable CPU time, and can be adapted for use as a heuristic for larger problems.     

Progressive transmission of images using MAP detection overchannels with memory

We propose a new maximum a posteriori (MAP) detector, without the need for explicit channel coding, to lessen the impact of communication channel errors on compressed image sources. The MAP detector exploits the spatial correlation in the compressed bitstream as well as the temporal memory in the channel to correct channel errors. We first present a technique for computing the residual redundancy inherent in a compressed grayscale image (compressed using VQ). The performance of the proposed MAP detector is compared to that of a memoryless MAP detector. We also investigate the dependence of the performance on memory characteristics of the Gilbert-Elliott channel as well as average channel error rate. Finally, we study the robustness of the proposed MAP detector's performance to estimation errors.     

All-optical networks with sparse wavelength conversion

Unlike broadcast-and-select networks, wavelength-routing networks offer the advantages of wavelength reuse and scalability and are thus suitable for wide-area networks (WANs) We study the effects of topological connectivity and wavelength conversion in circuit-switched all-optical wavelength-routing networks. A blocking analysis of such networks is given. We first propose an analytical framework for accurate analysis of networks with arbitrary topology. We then introduce a model for networks with a variable number of converters and analyze the effect of wavelength converter density on the blocking probability. This framework is applied to three regular network topologies that have varying levels of connectivity: the ring, the mesh-torus, and the hypercube. The results show that either a relatively small number of converters is sufficient for a certain level of performance or that conversion does not offer a significant advantage. The benefits of conversion are largely dependent on the network load, the number of available wavelengths, and the connectivity of the network. Finally, the tradeoff between physical connectivity, wavelength conversion, and the number of available wavelengths is studied through networks with random topologies