Introduction to Networking for NonStop S-Series Servers

Communications Product Concepts and
Components
Introduction to Networking for HP NonStop S-Series Servers—520670-005
2-9
Interfaces
Interfaces
HP communications subsystems provide several kinds of user interfaces:
•
Most subsystems provide application program interfaces (APIs) through which
users gain access to subsystem services. These APIs let you develop applications
that communicate with devices or across communications lines controlled by the
subsystem. These interfaces are described in more detail in Section 3, Application
Programming With Communications Products.
•
Some subsystems, such as the X.25 Access Method (X25AM), also provide
interactive interfaces to their services.
•
All subsystems provide interfaces for managing subsystem resources, for example,
defining device configurations and starting and stopping lines. Such management
interfaces can be interactive or programmatic, as described in Management
Interfaces on page 2-10.
Very High-Level, High-Level, and Low-Level Interfaces
There are, generally speaking, three levels of APIs to communications subsystems:
very high-level interfaces, high-level interfaces, and low-level interfaces.
Most programmers who write applications that use terminals or communications lines
do not need to be concerned about data communications: they use very high-level
interfaces. For example, most programmers using the Pathway environment or the
SNAX/High Level Support (SNAX/HLS) product need to know very little about
communications. Programmers using the Expand network need to know nothing at all
about communications; in fact, applications communicate across Expand networks
without explicitly invoking the Expand subsystem. Other very high-level interfaces
include the Exchange remote batch emulation system and the mail, file transfer, and
terminal emulation services of NonStop TCP/IP.
Most products discussed in this manual demand slightly more of the application and
the application programmer. For example, if you use an HP Open Systems
Interconnection (OSI) product, a device-specific access method such as AM3270, or
SNAX/Advanced Program Communication (SNAX/APC), which implements the SNA
LU 6.2 protocol, your application need not be concerned with low-level
communications protocol, but it will have various options for setting parameters and for
handling errors. Therefore, to use these products, you must be familiar with some
aspects of the protocol they support.
Finally, those products and interfaces intended for custom device support typically
require you to understand the target protocol and its data stream—for example, its
message formats—because you implement part of the protocol, and possibly format
the messages, in your application. Products and interfaces in this group include the
SNAX Application Logical Unit (SNALU) interface; the Envoy, EnvoyACP/XF, and
CP6100 input/output processes (IOPs); and the Ethernet port interface of the Port
Access Method (PAM) subsystem, among others. Some of these products also require
that you understand the underlying communications hardware.