Communication asynchronous

Subsystems can interact with each other in many ways. This pattern defines a consistent scheme governing those interactions. For every subsystem, you may choose to uniformly have a distinguished head object that controls the connections between its children s ports and those in other subsystems based on a naming scheme. The head object also mediates all control and asynchronous communication between the subsystem and its parent system and coordinates the activities of its child components (see Figure 12.3). This arrangement gives a consistent structure for every subsystem a head object, a defined role relative to its children, and a consistent protocol regardless of actual subsystem function. 

When devices for asynchronous communication are configured, both devices must agree on the number of data bits and stop bits and on whether or not parity checking should be used. If both sender and receiver aren t set to the same values, communication can t take place. The values are usually set in the software of both the sending and receiving hardware. 

The only downside to modems is that this process is relatively inefficient. Because modem communications are so sporadic, they use asynchronous communications, which have their overhead of start and stop bits. Also, today s phone lines are limited to a maximum throughput of 56Kbps. 

When computers send data serially, they send it one bit at a time, one after another. The bits stand in line like people at a movie theater, waiting to get in. We ve already discussed serial (asynchronous) communication in Chapter 6. Just as with modems, you must set the communication parameters (baud, parity, start and stop bits) on both entities—in this case the computer and its printer(s)— before communication can take place. 

BPS (bits per second) A measurement of how much data (how many bits) is being transmitted in one second. Typically used to describe the speed of asynchronous communications (modems). 

To integrate synchronous and asynchronous communication tools into distributed work sessions, a framework was created which rnodularly incorporates different tools into one interface. In this context, synchronous communication comprises all those services which support multimedia conferencing. This includes not only the real-time transmission of audio/video data, but also the usage of locally installed design tools by the whole team. 

The user interface of KomPaKt only gives a unified way of accessing synchronous and asynchronous communications. This interface integrates some useful existing communication tools. For the considered application domain, also the development of some new tools was necessary. These tools are described in this subsection. 

Studies on Orbix (version 2.3 later a transition to a newer version was made) consisted of the analysis of the influence of different hardware architectures on communication in CORBA, the overhead produced by communication operations, a comparison between different modes of data transfer, the applicability of updating methods, and the realization of multicast communication. Synchronous, oneway, deferred synchronous, and the use of the CORBA Event Service for asynchronous communication were investigated. The data transferred in each experiment were strings, due to the lack of container objects in CORBA. 

The communication platform KomPaKt (Subsect. 3.3.2) was developed by IMPROVE project B3 and integrates both synchronous and asynchronous communication tools under a unique user interface. The aim is to satisfy the different cooperation and communication needs of a developer. Therefore, it allows access to the user information saved in the administration system AHEAD. 

Initial deployments will likely use asynchronous communication protocols, eliminating the need for a divider with its associated high-speed circuitry. 

The interlock mechanism of EP/3 uses both asynchronous and synchronous signals to control the components. This demonstrates that ITL is suitable for describing both synchronous circuits and asynchronous circuits. In (Cau and Zedan 1997) explicit constructs for both synchronous and asynchronous communication have been defined. 

The hierarchical network of ProcVhdl matches the component instantiation concept for structural descriptions in Vhdl, with communication carried out by simple Vhdl signals. This match would in fact be possible for most simulator-based languages, and is mainly a consequence of the choice of asynchronous communication protocols in the hardware model this model places very few restrictions on the actual low-level signal behavior. 

It should be noted that even as the asynchronous communication of ProcVhdl may be embedded in Vhdl s event-based semantics, it may also be built on top of synchronous hardware. In fact, a likely implementation of a ProcVhdl functional unit is a synchronous finite state machine with an attached datapath. 

The notion of combining sales with support services lies at the core of the marketing and operations interface, but it is not exclusive to the Internet (see e.g., Ak in and Harker, 1999). However, the Internet s customer-driven approach to product information, mass-customization capabilities, and asynchronous communication modes make it a better suited channel for that practice since it allows more accurate targeting and is less intrusive than other contact modes (e.g., Anderson, 2000 reported that among customers purchasing a new computer at IBM, 80% sign up voluntarily for newer product information emails as part of product support). 

An important problem in safety-critical software development results from its ever increasing complexity and from the fact that software functions often interact strongly with different contexts in event-based systems. This can be a physical context (e.g. a monitored and controlled device), human users in an organizational context, or other software and hardware (e.g. a device driver). Other factors increasing the complexity of this problem are asynchronous communication with and within the system, event-driven behaviour, complex data types, timing constraints, parallel execution and non-deterministic behaviour of the system under test. Testing event-driven software thus faces special challenges. In summary, the characteristics of event-driven, safety-critical software are ... 

Another problem with existing high-level description languages is that they usually have a built-in timing methodology that makes it difficult to express interfaces that include both synchronous and asynchronous behavior. These are quite common at the boundary between circuits, for example, when connecting two synchronous units that have different clocks or when connecting a synchronous device to an asynchronous communication bus. 

The implications of the unified representation for high-level specification seem to all be beneficial. One of these benefits is that the representation supports synchronous and asynchronous behavior equally well through the use of an asynchronous communicating processes model. Input data becomes available, is operated upon, and output data is made available. The use of specialized description methods for the interface and data-flow behavior is encouraged by the clean separation between the two portions of the graph. For example, an HDL with send and receive constructs can be used to describe the data-flow and timing diagrams can be used to represent the interface details. The interconnections between the two are made between the data event nodes on the interface and the send and receive operation nodes in the data-flow. Of course, the basic block structure of the description must be identical in the two representations. 

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