wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Industrial Internet 56-5
about events generated in the automation layer. Typically, event sources in the field layer or the sensor/
actuator layer are using different techniques, addressing the real-time constraints, and the implementation
effort (e.g., mail servers need a lot of resources).
In addition, other protocols like File Transfer Protocol (FTP) [FTP85] and its derivates are quite often
used to transfer binary data between client and server. In Industrial Internet, such applications are useful
in the upper layers down to the automation layer or even the field layer. For example, huge data sets
in batch systems, large archives, programs, or parameter data are transferred using FTP-like protocols.
Especially in systems where Industrial Ethernet is used at the field layer, FTP-based solutions are easy
to implement.
56.3.1.1 HyperText Transfer Protocol
HyperText Transfer Protocol (HTTP) [HTTP99] is the most commonly known transport related protocol,
used in any Web application to connect between Web browser and Web server. HTTP is a stateless protocol.
The browser acts as a client and invokes the request (typically via HTTP-GET method), the server
responds with the document requested or with an error message. Besides HTTP-GET, other methods can
be used to transfer the header of the document only (HTTP-HEAD) or—if implemented at the server—to
upload documents (HTTP-PUT). The method HTTP-POST is widely used for the transfer of form data
from client to server and for invoking application logic at the server. The HTTPS protocol [HTTPS0] is a
secured version of HTTP, often combined with other security methods at server and/or client.
The application of HTTP in Industrial Internet requires a Web server at the component acting as the
server. The great variety of available Web server implementations for different purpose—ranging from
high-end, high-load server software like Apache Web Server or Microsoft Internet Information Server
(IIS) down to embedded Web servers with memory footprint of only a few kilobytes—enables a wide
range of applications. However, it has to be considered that the typical communication paradigm is pure
client–server. This means that transfer always starts upon client initiative. HTTP in its basic form is
problematic to transfer events, since a client would have to poll the server all the time.
56.3.1.2 Simple Network Management Protocol
Developed for network management functions, SNMP [SNMP90] realizes the communication model
and the information model part of OSI network management. Designed to be simple to use and to
implement, SNMP has become the de facto standard for network management. The communication
model part of SNMP can also be used as a transport protocol. It relies on the UDP of the TCP/IP protocol
suite. Besides methods for reading attributes of managed objects like SNMP-GET, SNMP-GET-
NEXT, or SNMP-GET-BULK, the protocol also provides SNMP-SET to set values of managed objects.
Additionally, SNMP provides TRAP methods to transfer events occurring in managed objects of an
agent to the corresponding manager. In contrast to client–server paradigms, this method can be used to
propagate events within a system without client initiative.
Especially, the TRAP method makes SNMP suitable as a transport protocol for industrial applications.
Automation systems using Industrial Ethernet typically provide SNMP implementations for the management
of the network-related information. However, the full integration of industrial solutions into network
management application requires using of the SNMP information model—the management information
base (MIB) for description of components—as well as the integration of the functional model into SNMP
applications. This is rather difficult, since both follow approaches different from those currently available
in industrial applications. The use of SNMP as a transport layer only, for example, for industrial Web
applications, is possible with limited effort.
56.3.1.3 Web Services Using Simple Object Access Protocol
In order to invoke functions of a server via the network, different systems have been developed. They
range from open, complex, fully featured solutions like Common Object Request Broker Architectures
(CORBA) [CORBA] to proprietary specifications like Microsoft’s Distributed Component Object
© 2011 by Taylor and Francis Group, LLC