Space Priority Research Articles

NASA asks academy for space priorities

NASA asks academy for space priorities

New implementation of multi-priority pushout for shared memory ATM switches

In a shared memory ATM switch with output queueing, the output queues share space flexibly in a common cell buffer. Selective Pushout is a technique for efficiently sharing this cell buffer memory among these multiple queues in an environment where cells also belong to various space priority classes. With Selective Pushout, whenever the buffer is full, arriving cells make space for themselves by pushing out low priority cells from the longest queue. In earlier work, we explored the performance of Selective Pushout. In this paper we consider its implementation. First we describe a queue controller implementation for a system without pushout. Then we outline a straightforward implementation of a queue controller with Selective Pushout. Next we propose a novel implementation that requires less control memory for the queue controller's data structures. Finally, we compare these implementations in terms of control memory requirements, processing speed, and design complexity. The control memory savings of the new implementation are significant for switches with large shared cell buffers. Moreover, for a system with only two space priority classes, the new implementation has a particularly simple design that should run nearly as fast as the straightforward implementation.

Design and architecture of a new space priority mechanism for ATM network nodes

This paper presents the architecture of a new space priority mechanism intended to control cell loss in ATM switches. Our mechanism is a new generic concept called: the multiple pushout. It is based on the utilization of both AAL and ATM features and on a particular definition of the priority bit. Whenever one cell of a message overflows the buffer of an ATM switch, the algorithm causes the switch to discard other cells of the message (including later arrivals). Such discarding frees buffer spaces for cells of other messages that have a chance of arriving at their destination intact. Our objective is to emphasize that in case of overload, with most of proposed mechanisms, cells are discarded without any semantic information about the type of cells. Therefore, at the destination, all the fragments of the corrupted messages will be discarded anyway. Finally, we present simulation results comparing cell loss rates and message loss rates of several space priority mechanisms.

Exact fluid-flow analysis of single ON/OFF sourcefeeding an ATM bufferwith space priority

Formulae are developed to calculate the cell loss probabilities for high and low priority traffic feeding an ATM buffer with partial buffer sharing. This extends an exact fluid-flow method that models cells as discrete units rather than as a continuous fluid.

On protective buffer policies

Studies buffering policies which provide different loss priorities to packets/cells, while preserving packet ordering (space priority disciplines). These policies are motivated by the possible presence, within the same connection, of packets with different loss probability requirements or guarantees, e.g., voice and video coders or rate control mechanisms. The main contribution of the paper is the identification and evaluation of buffering policies which preserve packet ordering and guarantee high priority packets performance (loss probability), irrespective of the traffic intensity and arrival patterns of low priority packets. Such policies are termed protective policies. The need for such policies arises from the difficulty to accurately characterize and size low priority traffic, which can generate large and unpredictable traffic variations over short periods of time. The authors review previously proposed buffer admission policies and determine if they satisfy such "protection" requirements. Furthermore, they also identify and design new policies, which for a given level of protection maximize low priority throughput.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Simulation Study of Space Priorities in a Shared Memory ATM Switch

We study an ATM switch architecture in which the queues for all the switch output ports share space flexibly in a common buffer. Using a computer simulation of this switch under bursty traffic, we investigate various ways to manage space priorities in the shared memory. Our findings support one particular strategy which we call “Selective Pushout.” In this scheme, an arriving cell that finds the shared memory full overwrites a cell with priority less than or equal to itself from the longest output queue in the buffer (even if the arriving cell will be joining a different output queue). We simulated Selective Pushout as well as several simpler pushout and threshold schemes under a variety of load conditions. For each load pattern we studied, the Selective Pushout scheme performed at least as well and usually much better than its competitors. Selective Pushout offered a low overall cell loss rate, with very low losses for the high priority cells.

Optimal buffer control during congestion in an ATM network node

Study the problem of optimal buffer space priority control in an ATM network node. The buffer of a transmission link is shared among the cells of several traffic classes waiting for transmission through the link. When the number of cells to be stored in the buffer exceeds the available buffer space, certain cells have to be dropped. Different traffic classes have different sensitivities to cell losses. By appropriately selecting the classes of cells which are dropped or blocked in case of overflow, one can have the more sensitive classes suffering smaller cell losses. Depending on the control that on the system, three classes of policies are distinguished. In each one, policies that schedule the buffer allocation in some optimal manner are identified.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A novel congestion control strategy in ATM networks

In order to guarantee a committed Quality of Service (QoS) to the users of a Broadband Integrated Services Digital Network (B-ISDN), preventive congestion control becomes critical, and is implemented through Call Acceptance Control (CAC), Usage Parameter Control (UPC), and Space Priority functions. Currently, Asynchronous Transfer Mode (ATM) cells are equipped with a 1-bit Cell Loss Priority (CLP) field, which can be used for service-oriented and/or UPC marking. This creates a conflict, since these two marking approaches may have contradicting objectives, and are designed to operate independently. Moreover, by admitting excessive cells as marked traffic, this group is allowed to grow uncontrollably, thereby jeopardizing the QoS committed to other (low priority) marked cells. This paper presents a solution to these problems based on a new 4-class priority strategy that unifies the two marking approaches, by utilizing a 2-bit CLP field. This is implemented by expanding the CLP field with the adjacent 1-bit RES field in the ATM cell header. A new variant of the “marking Leaky Bucket” UPC mechanism, called the Forgiving Leaky Bucket (FLB) will then be positioned at the Network-Node-Interface (NNI) of interworking ATM-based Broadband ISDN subnetworks. The FLB additionally has the power of un-marking ( forgiving) previously marked ATM cells, whenever the network conditions are appropriate. As a result, the end-to-end survival probability of a cell is improved significantly. Furthermore, “forgiving” corrects access-point bias, a phenomenon observed in interworked ATM subnetworks of different congestion conditions. Through simulations it is shown that this new priority strategy provides a significant improvement over the traditional marking Leaky Bucket UPC mechanism.

Dimensioning of an ATM switch with shared buffer and threshold priority

A number of recent studies have addressed the use of priority mechanisms in Asynchronous Transfer Mode (ATM) switches. This investigation concerns the performance evaluation and dimensioning of a shared-buffer switching element with a threshold priority mechanism (partial buffer sharing). It assumes that incoming ATM ceils are distinguished by a space priority assignment, i.e., loss of a high priority cell should be less likely than loss of a low priority cell. The evaluation method is analytic, based on an approximate discrete-time, finite-state Markov model of a switch and its incoming traffic. The development focuses on the formulation of steady-state loss probabilities for each priority class. Evaluation of delay measures for each class is also supported by the model; results concerning the latter are illustrated without development. The analysis of loss probabilities is then used to dimension the buffer capacity and threshold level such that required maximum cell loss probabilities are just satisfied for each cell type. Moreover, when so dimensioned with respect to relatively stringent loss requirements, i.e., probabilities of 10 10 and 10 s for high and low priority cells, respectively, we find that both loss performance and resource utilization are appreciably improved over a comparable switch without such a mechanism.

ON DISCRETE-TIME SINGLE-SERVER QUEUES WITH MARKOV MODULATED BATCH BERNOULLI INPUT AND FINITE CAPACITY

This paper investigates a discrete-time system with Markov modulated batch Bernoulli process (MMBP) inputs, general service times, and a finite capacity waiting room (queue). The FIFa and space priority service rules are considered. The main motivation for studying this type of queue is its potential applicability to time-slotted communication systems in broadband ISDN (B-ISDN). Applying the supple­ mentary variable technique and the matrix analysis approach, we obtain the steady-state distributions of the number of customers in the system as well as loss probabilities under individual service rules.

Stochastic models of space priority mechanisms with Markovian arrival processes

In this paper, we study a single-server queue with finite capacity in which several space priority mechanisms are implemented. The arrival process is the general Markovian arrival process (MAP) which has been used to model the bursty arrival processes commonly arising in communication applications. The service times are generally distributed. These buffer mechanisms enable the Asynchronous Transfer Mode (ATM) layer to adapt the quality of the cell transfer to the quality of service requirements of the specific broadband ISDN services and to improve the utilization of the network resources. This is done by a selective discarding of cells according to the class they belong to. Computable expressions for various performance parameters are obtained. Numerical results are given for the case of a two-state Markov-modulated Poisson process (MMPP) and deterministic service times. The values derived can be used to evaluate the benefits of using priorities in an ATM network when the traffic is bursty and to make a comparative study of the buffer mechanisms. These results extend the models previously developed, which were limited to Poisson arrivals.

Space science: Who's at the helm?

Who should set priorities for space science research—the Space Studies Board of the National Academy of Sciences, NASA's Space Science and Applications Advisory Committee, or both? This was a basic theme discussed at a symposium on priorities in space research, held by the Space Studies Board January 24 in Washington, D.C. Louis Lanzerotti, chair of the board, commented that setting science priorities was “harder than solving differential equations.”The board released its report, “Setting Priorities for Space Research—Opportunities and Imperatives,” which stressed the fundamental need for ranking space initiatives. The forum itself was held to gather comments on a draft questionnaire developed to elicit research priorities from space scientists. Both documents were drawn up by a Space Studies Board task force on space priorities chaired by John Dutton of Pennsylvania State University.

Priority management in ATM switching nodes

Various space priority mechanisms and their detailed performance evaluation are described. A comparative performance study is given, indicating the excellent performance characteristics of a simple buffer management scheme called partial buffer sharing. The introduction of a second bearer capability provides a 10/sup -6/ cell loss rate instead of 10/sup -10/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A space priority queuing mechanism for multiplexing ATM channels

In an ATM network, it can be necessary to protect some cells against rejection when entering finite-capacity buffers. “Space priority” is an access mechanism intended to provide such a protection by allowing them to take the place of less important cells in case of congestion. We study an M/G/1 queue using this operating mode and we compute loss probabilities for both flows. The model is then compared with the usual finite capacity queue in order to qualify space priority: reduction in buffer size, and increase in the admissible load for a given Grade of Service.

Priorities in space for the USA

This article follows the story of Shuttle development, in the context of the history of the US space programme from Apollo to the Space Station. The Shuttle was chosen as one of a series of ‘space spectaculars’ and has proven to be prohibitively expensive and unreliable, practical only for a very limited number of specialized missions. The Space Station, too, cannot be economically supplied, even if the USA could afford to build it. The author concludes that NASA should cancel the Space Station and the replacement orbiter for Challenger , and engage on a major programme of launch vehicle development, independent of the US military. The aim should be a dramatic reduction of launch vehicle costs, making spaceflight practical, and a truly independent NASA which could restore the USA to space preeminence.

Priorities and policy analysis

Abstract This response to Alex Roland's article, ‘Priorities in space for the USA’, argues that his analysis and conclusions are based on shaky historical evidence. Professor Roland's interpretation of NASA's priorities since 1959 is challenged, and it is pointed out that the manned spaceflight programme has widespread support in the USA. The most important issue, raised by the article but not treated extensively enough, is whether the pursuit of the widely accepted emphasis on manned spaceflight is a large-scale societal mistake.

Troche Treatment of Oral Candidiasis

To the Editor. —Yap and Bodey, in theArchives(139:656-657, 1979), have shown the safety and value of troche application of clotrimazole for treating oral candidiasis in cancer patients, and they make a plea for this form of the drug to be available for the treatment of thrush. The important factor is the strictly topical application of an adequate concentration of a fungicide. Nystatin mouthwashes may fail dismally, but flat oval nystatin vaginal pessaries held comfortably overnight between the gums and cheek can be effective after two or three nights' treatment. Contributions to this section may include comments on articles published in the Archives, or reports of unique educational character. Communications will be published as space and editorial priorities permit. Contributions ordinarily should not exceed 500 words in length, with a maximum of five references; one Figure or Table can be printed .

Space, energy, pollution-which?

In its planning for a future space program, the National Aeronautics and Space Administration has decided to place all its eggs in one basket: the space shuttle. Designed to go into earth orbit and land like an airplane, this program will cost us $5 billion to $15 billion to develop during the 1970's and another $35 billion to use during the 1980's. It is the largest civilian technological program now under development in the United States and is so grand in scale that other space priorities would have to be drastically cut back to make room for the shuttle. The most severely threatened areas are in space science and applications which are those parts of the space program that really count—they support planetary exploration, scientific experiments in earth orbit and on the moon, all weather forecasting and earth resources satellites and some communications satellites. The Nixon Administration's cuts of the annual NASA budget to about ...

Priorities in Space

Priorities in Space

Astronomy Survey to Assess Ground and Space Priorities

Astronomy Survey to Assess Ground and Space Priorities