Reliable Transfer Common Application Service Elements

www.pragsoft.com Chapter 8: The Application Layer 99 The aim of the Virtual Terminal VT standards is to facilitate terminal independency by providing a model for connecting applications and terminals which hides the device specific information from applications. VT consist of two standards: ISO 9040 describes the VT services, and ISO 9041 describes the VT protocol. VT employs a model in which terminal access is provided through a Conceptual Communication Area CCA. CCA provides data abstractions for the terminal screen, keyboard, etc., in form of objects, of which there are three types: • Display Object . All terminal data is routed through a display object. The display object reflects the state of the terminal display andor its related input devices. • Device Object . The device object specifies the physical characteristics of a device. Naturally, the information provided by a device object is device dependent and outside the scope of the standard. • Control Object . A control object manages a specific VT function. There are generally many control objects responsible for functions such as interrupts, character echoing, and field definition. Figure 8.85 illustrates the role of the CCA and its objects. Using the device objects, the display and control objects are mapped to the actual terminal device. VT maintains a copy of CCA at both the terminal and the application end, and ensures that these two copies reflect the same picture by exchanging updates between them as they take place. VT supports synchronous called S-mode and asynchronous called A-mode communication between terminals and applications. In the S-mode the same display object is used for input and output paths. In the A-mode two display objects are employed, one for the input device and one for the output device. The VT service contains a set of facilities for managing the communication process. These are used to establish and terminate VT associations, negotiate VT functional units, transfer data, exchange synchronization and acknowledgment information, and manage access rights. 100 Communication Networks Copyright © 2005 PragSoft Figure 8.83 CCA and its objects. Control Objects CCA Display Object Lower ISO Layers Lower ISO Layers VT Association Terminal Application Mapping Device Objects Control Objects CCA Display Object Mapping Device Objects

8.3.2. Message Handling Systems

Electronic mail e-mail represents one of the most successful classes of network applications currently enjoyed by many users. Early e-mail systems were network dependent, and their use was limited to the private networks of individual organizations. The CCITT X.400 and the ISO 10021 series of standards for Message Handling Systems MHS have paved the way for standardized and widely-available e-mail services. Figure 8.85 illustrates the X.400 view of MHS architecture. At the center of MHS is a Message Transfer System MTS which handles the delivery of messages. The system consists of the following components: • A Message Transfer Agent MTA is responsible for the routing of complete e-mail messages called envelopes through the MTS. MTAs handle envelopes in a store-and-forward fashion. • A User Agent UA manages a user’s mailbox. It enables the user to create, submit, and receive messages. The UA may serve an application or provide a user interface for direct interaction. UAs typically run on multi-user systems e.g., mainframes. • A Message Store MS acts on behalf of a UA running on a system which may not be available on a continuous basis e.g., personal computers. MSs are typically used within a LAN environment, serving a collection of personal computers.