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Network Strategies for Real-Time, Mission-Critical Distributed Systems
Publication: CI Press Release Issued: Date: 1996-01-01 Reporter: CI Systems

CI Press Release


Network-based architecture solutions offer considerable advantages over centralised star-wired or distributed point-to-point systems in Real-Time, mission-critical, distributed system environments.

This is one of the conclusions contained in an extensive study of network strategies for real-time, mission-critical distributed systems by Richard Young, Managing Director of CI Systems, a pre-eminent Cape Town-based systems integration and networking company.

Young's aim was to analyse the information management requirements of typical next-generation distributed control systems, with a view to synthesising an optimal solution using distributed computing elements and local area networks (LANs).

In addition to providing the best possible performance, dependability, transparency and flexibility, the ideal solution envisaged by Young had to exhibit a high degree of integration across its functional areas, an open systems architecture as well as compliance with international standards.

"To meet the ever increasing demands placed upon systems by users in commerce, industry, government, defence and other sectors, they are being designed to rely to a greater and greater extent on computers," Young comments.

"However, the incorporation of computer technology itself presents a number of possible problems. For example machines are susceptible to failure, with extensive implications in mission and/or safety-critical applications such as automatic teller machine (ATM) networks, process plants, fly-by-wire aircraft, medical-life support systems and air defence systems.

"In addition, the demands on software, especially those of modern high-level language implementations, are rapidly beginning to outstrip the capabilities of the most powerful computers.

"A solution to these apparent dilemmas is the implementation of distributed computer architectures, where a number of distributed, but connected, computers share the processing load.

"Such architectures allow sharing of information and resources over a wide geographical and organisational range. Furthermore they can exploit small, inexpensive computing elements, thereby achieving system reliability, expandability and affordability."

Young notes that while implementation of any distributed architecture is non-trivial, the requirements for such systems become complex when applied to critically real-time applications. These systems typically demand the capacity of handling high data rates and vast data volumes with low latency times in a reliable, deterministic and secure manner.

Other essential hallmarks of real-time, mission-critical distributed systems include :

  • Effectiveness - embracing fast access to accurate and informative information upon which to make quick and well-founded strategic and tactical decisions;
  • Coherency - achieved through full integration of sensors, decision support systems and actuators;
  • Dependability - achieved through fault-tolerance;
  • High survivability;
  • Ease of use and maintenance;
  • Upgradability and more.

Given these numerous and complex parameters, the ideal solution proposed by Young incorporates the following elements and concepts :


The system architecture should be network-based, as this offers considerable advantages over centralised star-wired or distributed point-to-point systems. These advantages include fault-tolerance, survivability, flexibility, expandability and upgradability. A further benefit is that a higher degree of integration is possible without the inappropriate mixing of diverse data types on the same LAN.

LAN Profile 

The recommended LAN profile incorporates the RSA SAFENET (Survivable Adaptable Fibre Optic Embedded Network) model. This is derived from the USA SAFENET model, which is prescribed for the new generation of US Navy shipborne, airborne and ground facility operations.

RSA SAFENET offers a flexible, practical and achievable implementation of the ISO OSI model, and is capable of cost-effective, real-time performance as well as maximum interconnectivity. A further advantage is that RSA SAFENET provides all the layers required for a complete system.

Implementations (hardware and software) for all the layers exist or are readily implementable.

Another reason for recommending RSA SAFENET is that it can be tailored to meet specific user requirements in terms of functional performance, dependability and afordablity.


IP (Internet Protocol) at the network layer and XTP (Xpress Transport Protocol) at the transport layer are the protocols recommended by Young, as these provide the required interoperability, internetworking, real-time performance and reliability.

Application Interface Services 

The upper three layers of the OSI 7-layer Basic Reference Model detract significantly from the required performance of real-time, mission-critical, distributed systems, Young states. Therefore implementors of such systems should dispense with strict adherence to the provision of the upper three layers and, in their place, provide appropriate lightweight support services capable of real-time performance.

The APIS (Application Interface Services) network communications protocol is considered most suitable for a number of reasons. Firstly, it supports programme constraints such as providing standard high-performance interfaces to specified real-time operating systems and network interface hardware.

APIS also affords complete abstraction of the network system to the application user. It does so by furnishing a small set of software service calls that establish the required transparency between devices on the network. This makes it possible to replace or upgrade any of the network infrastructure without affecting the application user.

Furthermore, APIS allows for the transfer layer protocols and network cable plant to be modified without impacting on the performance of the system, thereby achieving the key user requirement of obsolescence management.

Technology Issues

Asynchronous Transfer Mode (ATM) 

ATM, though it has many advocates, will not become the standard next-generation high speed network in the short to medium term owing to high cost, complexity and lack of progress in the standardisation effort.

Young therefore argues that the current maturity of ATM standards and technology does not support real-time, mission-critical, distributed systems. Neither does it intrinsically provide fault-tolerance, which means that users will have to employ special techniques - probably at considerable cost - to ensure the required degree of system dependability.

Standards and Standard Building Blocks 

Young recommends that all solutions be strictly standards based to maximise their life cycles. Only where no existing, usable standards exist should proprietary standards or products be developed - and then only with the aid of formal methodologies and documentation standards.

In conclusion, Young notes: "Only rigorous system engineering, including cost/benefit analysis, can determine precisely the appropriate choice of LAN technology and topology for the particular application.

"Therefore it is advisable that implementors of real-time, mission-critical, distributed systems work with experienced and knowledgeable suppliers such as CI Systems to design and install the most effective and cost-efficient solutions."