Introduction to Networking for NonStop S-Series Servers

Open Systems Interconnection (OSI) Network
Connections
Introduction to Networking for HP NonStop S-Series Servers—520670-005
8-4
A Brief Look at OSI
Each layer communicates with its corresponding (peer) layer on a connected system,
using a layer-specific protocol. Each layer of the OSI Reference Model uses the
services provided by the layer below it and provides services to the layer above it.
Interfaces between layers are defined in terms of primitives (sometimes called service
primitives); a primitive is the smallest unit of action that can be requested of the layer
or the smallest unit of response the layer can make. Higher-level operations can entail
the exchange of many request and response primitives.
The primitives are well-defined, and the internal workings of a layer are independent of
other layers. As a result, you can replace or modify any layer without changing layers
above or below.
Layers 1 through 4 deal with the communication, flow control, routing, and error-
handling needed to transport data end to end across the network. Below the Transport
Layer, there can be many different types of physical networks—for example, an X.25
packet-switched data network (PSDN) or a local area network (LAN).
Layers 5 through 7 deal with the coordination of applications across the network and
the way that information is presented to the applications. A wide range of application
programs providing various types of end-user services can be supported by a common
Transport Layer implementation.
The HP NonStop OSI product line includes five products: OSI File Transfer, Access,
and Management (OSI/FTAM), OSI Message Handling System (OSI/MHS),
ActionView.400, OSI/Application Services (OSI/AS), and OSI/Transport Services
(OSI/TS). Network and Data-Link Layer interfaces are provided by the HP X.25 Access
Method (X25AM) or the Port Access Method (PAM) subsystem.