For more than 10 years, the Department of Defense through the Open Systems Joint Task Force (OSJTF) has been under a directive to use Open Systems standards and specifications for technology acquisitions for weapon systems. The OSJTF defines Open Systems as those that con-form to consensus based standards on all key interfaces, such as hardware, software and user interfaces. The Alliance for Telecommunications Industry Solutions defines Open Systems as layered systems that allow each layer to provide a set of functions that can be controlled by the layers above, allow each layer to be implemented without affecting the implementation of the other layers, and allow the alteration of system performance by the modification of one or more layers without altering the other layers.

Given these definitions of an Open System, it is important for an Open System to have a layered structure, as well as conform to open standards. Open standards are typically those that are set by a committee of industry partners, such as IEEE, PCI-SIG, or ISO. An open standard must be freely available to any who wish to implement a system conforming to said standard. Use of the standard may have anominal licensing fee, but it should not be so excessive as to be a barrier to implementation. Some examples of open standards are PCI (Peripheral Component Interconnect), USB (Universal Serial Bus), RS232, IEEE1394, and GNU Linux. Some key motivating factors for Open Systems are reduction of implementation and design costs, increased interoperability, decreased development time, and modularity of design. An Open System can be any type of system ranging from a DSP (Digital Signal Processor) based communications system to a microcontroller-based control system.

Example One: PC
A desktop PC consists of hierarchical layers of open standards such as the operating system/environment, the BIOS (Basic Input/Output System) firmware, and the underlying hardware.

The hardware consists of a processor with a processor bus interface across which the processor communicates with all peripheral components such as the memory management unit, the PCI bus, the AGP port and all other devices such as the RS232 serial port, or the PS/2 ports for the keyboard and mouse. As shown in Figure 5-1, most of the peripheral interfaces in a typical PC reside in the chipset which consists of two chips which are called the north and south bridges. The north bridge typically handles high-speed peripherals, and the south bridge typically handles the slower interfaces such as the keyboard, mouse, and RS232 serial ports. Since all of theses peripheal interfaces have well-defined open standards there are many manufacturers of these chipsets.

The BIOS configures and manages the peripheral devices at a low level and creates standard interfaces through which the Operating System (OS) can communicate with them. For example the BIOS will manage the basic configuration of the hard-disk drives, the memory map to the PCI and AGP peripheral devices, and serial communications to the key-board and mouse. These simplified interfaces provide a layer of abstraction for the operating system designers to utilize so they do not need to deal with low level access and control of the devices. The OS provides yet another level of abstraction from that which the BIOS supplies. It provides standard control interfaces through which user applications can utilize the hardware, further abstracting the peripheral device control from the user.

The POSIX standard is a good example of the application level interface to system devices. In a POSIX-based system an application can access the disk drive for reading and writing through simple system calls to open, read, write, and close a file. This is a well-defined, open standard which has been around for decades and is utilized in many Unix-based systems such as Linux, though can be applied to any OS.

Example Two: OSI Model
The OSI (Open Systems Interconnect) is a model for computer networks, which can be applied to all digital communications. This model consists of seven layers; application, presentation, session, transport, network, data link, and physical. A network can be implemented with any number of these layers. The physical layer is the lowest and most implementation specific layer, followed by the data link, network, transport, session, presentation, and application layers in order from lowest to highest level (see Figure 5-2).

This model can be applied to the RS232 communications port. The voltage levels that define the logic level of the data stream are the physical level. The data link layer defines the signaling protocol, 1 start bit at a low logic level followed by 8 data bits, and finished by 1 stop bit at a high logic level. The RS232 interface is only a 2 layer protocol. Ethernet is another layer 2 protocol, as it defines the datalink layer. Internet Protocol (IP) is a layer 3 protocol and is most commonly used in conjunction with Ethernet, however it can be applied to the layer 2 protocols, for example it could also utilize the RS232 protocol. A layer 4 protocol would be TCP (Transmission Control Protocol), which is most commonly used with IP. As shown each layer is independent of the others, allowing implementation of each layer independently of layers above and below.

Example Three: Software-Defined Radio

The SDR, OSI reference model, and PC architecture are good examples of Open Systems, providing many possibilities for different configurations and implementations. It is this modularity and ease of implementation that are making Open Systems desirable over proprietary systems. The Department of Defense has been pushing for future defense systems to be implemented as Open Systems, specifically to take advantage of this modularity, the ease of upgrade, and cost reductions that can be achieved with this type of system over a closed proprietary system.