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The Optimal Architecture for Real-Time, Mission-Critical, Distributed Control Systems
Publication: CI Press Release Issued: Date: 1996-01-01 Reporter: CI Systems

CI Press Release

1996-01-01

SA network builders looking to construct real-time, mission-critical, distributed systems, should follow the Survivable Adaptable Fibre Optic Embedded Network (SAFENET) model to attain cost-effective, real-time performance and maximum interconnectivity.

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).

The ideal solution had to provide the best possible performance, dependability, transparency and flexibility, as well as exhibit a high degree of integration across all its functional areas. Further, the optimal solution had to incorporate an open systems architecture and comply with international standards.

The study found that the most appropriate means to meet numerous, diverse and stringent user requirements is a distributed system architecture integrated by means of local area networks (LANs).

Such an architecture is best able to satisfy high-level (or allocated) requirements, such as system coherency, dependability and operability, as well as lower-level (or derived) requirements such as reliability maintainability, reconfigurability and electromagnetic compatibility.

To develop an appropriate communication model (or LAN Profile), Young matched both allocated and derived system requirements to potential solutions for each layer of the OSI model. Of specific significance to the choice of appropriate options was their ability to support dependable, real-time performance.

After comparing all the potential candidates, Young concluded that the optimal LAN architecture is SAFENET, a model prescribed for the new generation of US Navy shipborne, airborne and ground facility operations.

SAFENET, the study found, offers a flexible, practical and achievable implementation of the OSI model that is capable of cost-effective, real-time performance as well as maximum interconnectivity.

In addition, SAFENET provides all the layers required for a complete system and is achievable because all implementations (hardware and software) for all of the layers exist or are readily implementable.

Another reason for its applicability is that, while SAFENET is a US Navy standard, the same framework can be used in other environments such as commercial and industrial without requiring full adherence to the SAFENET model.

This is significant in South Africa as full adherence would have extensive - and possibly prohibitive - cost implications. The SAFENET Standards Suite and the various options for each appropriate level are shown in Table 1.

In dealing with the ideal network topology for real-time, mission-critical, distributed systems, the study noted that this should be derived from a formal process such as the System Engineering Process (SEP).

Though the possibilities are extensive, the recommended approach is a topology based on logical system segmentation into principal functional areas. This offers flexibility in terms of reconfigurability and survivability, and provides for management of bandwidth allocation and the achievement of acceptable levels of bandwidth utilisation.

The study also looked at the various implementation options and concluded that, as regards the Physical Layer, fibre optic media are the only technology able to support all the main desired attributes, from electromagnetic compatibility to cost-effectiveness to supportibilty.

Requirements pertaining to the Data Link Layer are split into the Media Access Control (MAC) sub-layer and Logical Link Control (LLC) layer.

Regarding the MAC sub-layer, the study noted that it is highly desirable to use a commercial MAC standard that meets the following criteria:

  • Supports present and future performance requirements
  • Supports data transfer determinism requirements
  • Has significant reserves of data transfer capacity
  • Supports present and future functional requirements
  • Supports appropriate network topologies
  • Meets the cost-effectiveness requirements

Various alternatives were considered including commonly used technologies such as Ethernet and Token Ring, as well as FDDI (Fibre Distributed Data Interface) and ATM (Asynchronous Transfer Mode). The study concluded that Ethernet et al as well as ATM suffer from serious deficiencies (such as lack of fault tolerance and self-healing). As a result, FDDI emerged as the network technology of choice as it is the only standard which meets all the requirements.

Moving on to the Network Layer Protocol, the study noted that there are material advantages to utilise a commercial standard that

  • Meets present and future functional and performance requirements
  • Provides a connectionless service to the Transport Layer
  • Supports special services such as priority routing
  • Is available as a production-quality software product
  • Minimises implementation risk by finding widespread application

Theoretically, both the OSI-compliant Connectionless Network Protocol (CLNP) and Internet Protocol (IP) are suitable candidates for real-time, mission-critical, distributed systems. However, due to IP's extensive installed Internet (ARPAnet) base and consequent legitimacy among many network users, the recommended option for the network layer protocol is IP.

The choice of the optimal Transport Layer Protocol was influenced by a wide array of requirements. These ranged from the ability to meet present and future functional and performance requirements, to minimising implementation risk.

Several alternatives were considered, including TCP (Transmission Control Protocol), TP4 (the ISO Class 4 Transport Protocol) and XTP (Xpress Transport Protocol). The later is a recently developed standard prescribed as the real-time option at Layer 4 for the SAFENET protocol suites.

TP 4.0 (the current release) is a true OSI transport layer protocol providing error, flow and rate control, multi-level priority message scheduling, optimised inter-network addressing and reliable multicast support.

Other attributes include reliable multigram, effective connection management, selective error/flow control, selective retransmission and acknowledgement, maximum transmission unit detection, out-of-band data, alignment, traffic descriptors and more.

A major advantage of XTP is its orthogonal approach to transfer policy and mechanism in a real-time environment. Within the orthogonal approach, protocol definition and implementation differentiates between policies regarding real-time LAN issues (addressing, error/flow/rate control, etc.), mechanisms by which these policies are actually implemented and interfaces between policies and the user application. XTP provides the implementer with options for almost every protocol feature.

Though XTP is not without some deficiencies, it has sufficient positive attributes to make it the recommended transport layer protocol for real-time systems, with TCP the recommended maximum interconnectivity protocol.

The study found that none of the current commonly used higher layer protocols will meet all of the requirements of the next generation of real-time, mission-critical, distributed systems. It therefore proposed that an extended protocol with functionality spanning the session, presentation and application layers be defined to provide both generic and specific features.

An example of an extended protocol is APIS (Application Interface Services), developed by Young and colleagues for the exchange of information between functionally independent applications incorporated into a distributed, real-time system. APIS can be considered as Message Oriententated Middleware (MOM).

APIS conceptually encompasses Layers 5 - 7 of the ISO Reference Model. It interfaces below to Layer 4 (Transport Layer) and above to the APIS Service User (ASU), which is normally a collaborative, networked software application.

A prime advantage of APIS is its data driven approach to dataflow management. This provides a higher level of flexibility than the traditional point-to-addressed-point facilities provided by current LAN protocols.

A set of auxiliary time services, the Network Time Services (NTS), is also proposed in order to nullify the effects of the latencies inherent in standard network technologies. The Network Time Protocol (NTP) implements timing mechanisms between all participating sub-systems over the network and provides basic functionality such as synchronisation and timestamping to NTS. NTS in turn provides user-level timing services to the application.

Finally, the study considered the security requirements for real-time, mission-critical, distributed systems. It noted that, with a high degree of data and information integration, especially in military and certain commercial applications such as banking, there are critical requirements for security features to be implemented throughout the information management system.

The network infrastructure therefore must be able to offer security services to the system, and support the implementation of the user's security policy. Thus the design and implementation of the network must offer the following security services :

  • Confidentiality
  • Integrity
  • Accountability
  • Access Control
  • Availability