1、AEROSPACE INFORMATION REPORTAIR4271Issued 1989-11Reaffirmed 2011-11Handbook of System Data CommunicationsFOREWORDThis document has been prepared by the Society of Automotive Engineers (SAE) AS-2A Sub Committee, which is part of the AS-2 Committee of the Avionic Systems Division (ASD). Since this is
2、a “living“ document, the officers of the Committee to which correspondence may be addressed are:George Cotter, Chairman, AS-2Grumman Aerospace CorporationSystem Development CenterPlant 14Bethpage NY 11714Tel: (516) 575 6552John Bicknell, Technical Chairman, AS-2Lucas Aerospace, Electronic Systems Di
3、visionYork RoadGreen HallBirminghamB28 8LNUKTel: (44)21-777-3222Terry Martin, Chairman, AS-2AFerranti InternationalWestern RoadBracknellBerkshireRG121RAUKTel: (44)34-448-3232SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engin
4、eering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising therefrom, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may
5、be revised, reaffirmed, stabilized, or cancelled. SAE invites your written comments and suggestions.Copyright 2011 SAE International All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, p
6、hotocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-4970 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.orgSAE values your input. To provi
7、de feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/AIR4271Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4271- 2 -TABLE OF CONTENTS1. SCOPE .42. INTRODUC
8、TION . 43. SYSTEM ISSUES 53.1 System Concepts. 53.1.1 Introduction .53.1.2 Data Communication Architectures.63.1.3 The High Speed Data Communication System.93.2 System Design133.2.1 Media Sharing.133.2.2 Message Labels153.2.3 Communications Management .153.2.4 Message Delay .163.2.5 System Monitorin
9、g and Control.183.3 System Synchronization193.4 OSI Applicability213.4.1 Technical Background.213.4.2 Fault Tolerance223.4.3 Time Criticality.233.4.4 Distributed Control 253.4.5 SAE Protocol Mapping onto OSI Seven Layers253.4.6 OSI and Real Time System Commonality.263.4.7 Future Development263.5 Com
10、munication Security (COMSEC) and Red/Black Multilevel Security303.5.1 Security through Physical Isolation .313.5.2 Security Through Time Share .313.5.3 Security Through Trusted Systems.323.5.4 Implementation Aspects of Security334. APPLICATION ISSUES 374.1 Audio.374.1.1 Audio Bandwidth .384.1.2 Digi
11、tal Coding Techniques.394.1.3 Difficulties with Compression Techniques .404.1.4 Acceptable Delay 404.2 Video .414.2.1 Standards414.2.2 Unprocessed Data Rates 414.2.3 Data Compression 424.2.4 Synthetic Images.424.3 Closed Loop Control .44Copyright SAE International Provided by IHS under license with
12、SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4271- 3 -TABLE OF CONTENTS (Continued)5. IMPLEMENTATION ISSUES 495.1 Gateways 495.2 Interoperability.506. ACRONYMS AND DEFINITIONS.507. REFERENCES .52Copyright SAE International Provided by IHS under license
13、 with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4271- 4 -1. SCOPE:This Aerospace Information Report (AIR) has been prepared by the Systems Applications and Requirements Subcommittee of SAE Committee AS-2. It is intended to provide guidance primarily
14、, but not exclusively, for specifiers and designers of data communication systems for real time military avionics applications within a platform. The subject of high speed data transmission is addressed from two standpoints: (1) the influence of developments in technology on avionics architectures a
15、s a whole and (2) the way in which specific problems, such as video, voice, closed loop control, and security may be handled. While the material has been prepared against a background of experience within SAE AS-2 relating to the development of a family of high speed interconnect standards, referenc
16、e to specific standards and interconnect systems is minimized. It should be noted, however, that many of the concepts described require interconnect systems with advanced operational and performance characteristics, such as those developed by SAE AS-2.2. INTRODUCTION:The data communications requirem
17、ents of future systems, coupled with the capability of emergent data communication technology, have increased significantly the importance of data communications and its influence on overall system performance. This influence extends to the avionics system architecture, where high performance data c
18、ommunication technology offers the opportunity for radical departures from traditional concepts, leading to increased system performance, increased mission effectiveness, and reduced life-cycle cost.Within the AS-2 Committee of SAE, standards have been developed for high speed data buses and a messa
19、ge orientated backplane. Work continues on a variety of data communications and system related topics. The high speed data bus development is of considerable importance since it is seen as the cornerstone of future avionics systems. Significant effort has been devoted to the real time data transfer
20、requirement in future systems, which is a recurrent theme throughout this document.Real time data communication systems are characterized by the presence of messages for which there is a requirement to guarantee their delivery within a specific time. Such a requirement arises in one of two ways: (1)
21、 either as a consequence of operational requirements, as in the case of control systems, or (2) as a result of the nature of the data itself, as in the case of sources of periodic data. The influence of both of these aspects and the factors that need to be considered for various data sources are inc
22、luded in this document.Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4271- 5 -3. SYSTEM ISSUES:A data communication system is a shared communication medium and, as such, it should not be con
23、sidered a direct substitution for dedicated point-to-point links in a system design. For example, a data communication system utilizing dedicated point-to-point links may be predicated on word-by-word handshakes. Such handshakes would be very difficult, if not impossible, on a shared medium. Analysi
24、s of these data communication system designs usually determines that such handshakes are not necessary, but were used to simplify the interface. On a shared medium, handshakes can be provided, but only on a message basis. Therefore, a data communication system utilizing a shared medium should be des
25、igned with that shared medium in mind and the system concepts must be changed to accommodate the shared medium.Developments in data processing technology, allied with the operational requirements of future generations of military aircraft, give rise to the need for high-performance data communicatio
26、n systems. Performance parameters, such as message latency and throughput, are significant factors here, but fault tolerance, flexibility, and logical interfaces to support vendor independence are also required. New approaches are under discussion toward sparing policies and the probable requirement
27、 for federated or distributed systems in which process-to-process communication is a key element. Advances in semiconductor technology also have an impact; whereas, in the past, functions have been self-contained, it is likely that more than one function will be contained within a cabinet. As a resu
28、lt, a new approach is required to data communications in the form of:- Overall architecture- Low latency, high throughput systems with fault tolerance.The application of the ISO OSI model is also an issue due to the militarys desire for interoperability. However, the OSI model is precisely that - a
29、model. Slavish adherence to the model, rather than using it as a guide, is detrimental in real time systems.The use of a shared medium also requires the consideration of mixing classified and unclassified (Red and Black) data. It is not desirable to have separate buses for each because of the cost a
30、nd weight penalties, but such data sharing requires careful consideration in overall system design.3.1 System Concepts:In these paragraphs the importance of the data communication system in the total integration of all the subsystems is presented. The need for data communication requirements, such a
31、s distributed access control, message priorities, message labels, and data driven operation, are also explained.3.1.1 Introduction: One of the parameters affecting the choice of data communication system is the type of data that is being passed. Data communication systems are concerned with real tim
32、e and resource sharing. For real time systems, the transfer of data from one subsystem to another must be carried out within a defined latency, and the latency must be deterministic. For these reasons, the point-to-point link has been used widely in the past for real time systems since the data may
33、be transmitted as soon as it is produced.Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4271- 6 -3.1.1 (Continued):In resource sharing systems, the requirement is for many subsystems to acces
34、s common resources that are to be used. The latency in passing data between the subsystem and the resource is less stringent and data buses provide an excellent way of connecting everything together. The extra cost of the bus interface is offset by the simple interfacing of many types of equipment f
35、rom different vendors.Within the military systems arena, there has lately been a move to incorporate the data bus advantages, while still maintaining the attributes of point-to-point links enabling real time systems to be implemented. This move has been spearheaded by the use of the MIL-STD-1553B bu
36、s to connect subsystems together. This bus uses a centralized controller to schedule the passing of data between the subsystems allowing the integration of the subsystems into a single system to be carried out more easily.The trend of the military systems integration is now being carried further to
37、implement a real time system using a high speed data bus that has a distributed access control. To achieve this end, a radical approach to the design of system architecture is indicated.3.1.2 Data Communication Architecture: A typical present-day data communication architecture is shown in Fig. 1. P
38、rocessing functions are self-contained and are housed in individual cabinets connected via a 1553B bus. This configuration provides adequate redundancy and lends itself to a sparing policy in which each cabinet supports a dedicated function, and functions are completely replicated. It may be the cas
39、e, however, that the capacity of the 1553B bus is inadequate, particularly where cooperation between functions is required or where rapid recovery from failure is needed; the 1553B bus is unlikely to be adequate for substantial data base transfers. For reasons of space and thermal considerations, an
40、d bearing in mind the increasing levels of integration in semiconductor technology, it would seem logical to progress to the configuration shown in Fig. 2, where discrete functions are contained on a card, and functions communicate over a backplane bus (such as a PI bus). This approach would seem to
41、 support a dynamic sparing policy in which uncommitted spares are provided in support of a number of functions; but it suffers, however, from two major weaknesses. First, backplane buses are vulnerable to fault propagation; a single fault on a backplane or its interface can lead to failure of the co
42、mplete cabinet, including the spares which are intended to provide fault tolerance. While the backplane bus can be replicated, this results in a considerable increase in complexity and, due to the active nature of the connections to the bus, is unlikely to provide the level of integrity required. Th
43、e second major weakness of the message orientated backplane bus is the set-up time for transfers which places severe restrictions on the latency and throughput characteristics. Moreover, the length of backplanes is limited, and communication outside an enclosure would require a store-and-forward gat
44、eway for which message latency is considerable. For these reasons it is considered that message orientated backplanes are of little use in future avionic systems except in certain particular circumstances, such as fully federated systems in which a complete set of processes must always be operationa
45、l to carry out the function of the subsystem.Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4271- 7 -FIGURE 1 - Typical Present-Day ConfigurationCopyright SAE International Provided by IHS un
46、der license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-,-SAE AIR4271- 8 -FIGURE 2 - Use of Message Orientated BackplaneCopyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license
47、 from IHS-,-,-SAE AIR4271- 9 -3.1.2 (Continued):Requirements are anticipated in which sensor data will be processed in two stages. In the first stage, data from each sensor would be processed separately, and in the second stage, processed data from a number of sensors would be combined for further p
48、rocessing. In such a configuration it is necessary that data be available at the first stage, both for operational reasons and for reconfiguration in the event of second stage processor failure. Two examples of the need for second stage processing, but not at the expense of reduced availability of t
49、he prime processing functions, are multispectral signal analysis and the use of AI processing; while it is important that messages between the two stages of processing should not be subject to significant delay, it is also important they not be directly in-line.An approach in which process-to-process communication can be carried out both in
