98 Communication Networks
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APDU into smaller PDUs, each of which is marked by a confirmed minor synchronization point. The transfer of an APDU is managed as one session activity.
RTSE uses the ACSE to establish an association: its RT-OPEN service primitive maps to the A-ASSOCIATE primitive. RT-TRANSFER is a confirmed primitive
and manages the transfer of an APDU as segments. It maps to a sequence of P- ACTIVITY, P-DATA and P-SYNC-MINOR primitives. RT-CLOSE maps to A-
RELEASE and gracefully terminates the transfer. In case of errors, RTSE provides appropriate recovery procedures. When possible, a transfer activity which has been
disrupted by an error is restarted; otherwise, it is entirely repeated.
ISO 9066.1 and CCITT X.218 standards describe the RTSE service. The RTSE protocol is described in ISO 9066.2 and CCITT X.228 standards.
8.2.3. Remote Operations
The Remote Operations Service Element ROSE serves the needs of distributed applications in invoking remote operations. An example is an application which
requires access to a remote database. ROSE enables a requester AE to submit an operation to a replier AE, then waits for a response, and finally delivers the response
to the application. The response may indicate the result of successful completion of the operation, an error condition, or complete rejection of the operation.
ROSE supports two modes of operation: i synchronous, whereby the requester AE waits for a result before submitting the next operation, and ii asynchronous,
whereby the requester may submit further operations before waiting for earlier ones to be completed.
ROSE provides a remote operation notation which other ASEs may use to define their operation interface. The ROSE protocol specifies how its four APDUs
are transferred using the presentation service which maps them to P-DATA primitives or the RTSE service which maps them to RT-TRANSFER primitives.
ISO 9072.1 and CCITT X.219 standards describe the ROSE service. The ROSE protocol is described in ISO 9072.2 and CCITT X.229 standards.
8.3. Specific Application Service Elements
This sections describes three widely-used SASEs.
8.3.1. Virtual Terminal
There are many different makes and types of character-based terminals in use throughout the world. Very few of them use the same set of commands to control the
display or obtain input from keyboards or other input devices. Because of these incompatibilities, terminal dependency has been a common problem in applications.
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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.
