VERTICAL INTEGRATION OF TTP/A FIELDBUS SYSTEMS USING

WEB SERVICES

Volker Turau, Marcus Venzke, Christoph Weyer and Yesenia Vigil

Technische Universit

at Hamburg-Harburg

Department of Telematics, Schwarzenbergstraße 95, 21073 Hamburg, Germany

Keywords:

Fieldbus system, vertical integration, Web service, TTP/A

Abstract:

This paper presents a generic technique to expose data and control of ﬁeldbus systems to applications located

at the level of operational management. To provide a high degree of interoperability between the operational

level and different ﬁeldbus systems we utilize standardized techniques such as XML, HTTP, and Web services

which can be deployed independently of the platform. The proposed service is based on the interface ﬁle

system for TTP/A smart transducers. The deﬁned interface provides high level abstractions appropriate for the

integration into business applications and considers the latency introduced by the internet protocols.

1 INTRODUCTION

In industrial automation systems the network link

among different devices such as sensors, actuators

and programmable logic controllers is provided by

ﬁeldbus systems. Examples of such systems are

Proﬁbus, CANbus and BITbus. In practice very of-

ten ﬁeldbus systems from different vendors have to

be combined to build a complete system, this task is

called horizontal integration. In recent years there

has been a growing interest in the use of informa-

tion rooted in automation systems at the level of op-

erational management to enhance production and its

control (e.g. for supply chain management, enterprise

resource planning and manufacturing execution sys-

tems). The provision of data controlled by ﬁeldbus

systems at this level is called vertical integration. It

enables a large variety of applications outside the

realm of the factory shop ﬂoor such as condition mon-

itoring, fault diagnosis and predictive maintenance.

Research in this area has been spurred by the suc-

cess of the internet, XML and related web technolo-

gies. We begin to see a tendency that organiza-

tions provide functionality over the web, no more em-

ploying wire protocols such as those of DCOM or

CORBA. XML has become a preferred format for

encoding and exchanging data in an open, system-

independent way. Web services apply XML for re-

mote software integration. To avoid not being caught

in the trap of a hype, a careful analysis of the applica-

bility of Web services for the task of vertical integra-

tion is required.

The main contribution of this paper is a require-

ments analysis for the usage of Web services for

vertical integration of ﬁeldbus systems and the con-

crete design and implementation of such a service for

TTP/A ﬁeldbus systems. The proposed service makes

use of the recently approved OMG Smart Transducer

Interface speciﬁcation and especially of the concept

of the Interface File System (IFS) (OMG, 2003).

2 INTEGRATION TECHNIQUES

The internet provides the ideal infrastructure to ac-

complish the vertical integration of ﬁeldbus systems.

Currently there is a shift towards web-enabling the

industrial Ethernet protocols rather than enhancing

them with real-time support. This stems from the per-

ceived need by end users to implement homogeneous

vendor environments even when a common higher-

level protocol is employed. Companies are concerned

that devices from different vendors may not interop-

erate, even those supporting the same protocol.

Integration of applications on the enterprise level

is very often achieved through technologies based

on remote procedure calls, examples are DCOM and

CORBA. These technologies have also been deployed

to the domain of vertical integration (OMG, 2003;

OPC, 2003a) and for monitoring and conﬁguration

390

Turau V., Venzke M., Weyer C. and Vigil Y. (2004).

VERTICAL INTEGRATION OF TTP/A FIELDBUS SYSTEMS USING WEB SERVICES.

In Proceedings of the First International Conference on Informatics in Control, Automation and Robotics, pages 390-393

DOI: 10.5220/0001129503900393

 SciTePress

(Obermaisser et al., 2001). The problem is, that

most higher-level applications do not implement the

DCOM or CORBA interfaces necessary to communi-

cate with servers. The success of the World Wide Web

opened new opportunities for the vertical integration

problem. Some projects exploit traditional Web ap-

plication technology (Ferrari et al., 2002; Topp et al.,

2003). Either through an embedded Web server or

a gateway users can access ﬁeldbus systems using a

Web browser. While this approach is appropriate for

monitoring applications, it is not sufﬁcient for the in-

tegration problem where applications and not human

users require access to ﬁeldbus systems.

Web services are accessible over the Web and in-

tended to be used by other applications. The output of

a Web service is an XML document, a format that is

easy to use for the client application and in fact, many

vendors of our target applications are in the process

of providing interfaces to integrate Web services. The

inherent mechanism to handle faults of the underlying

protocol SOAP enables an intelligent error handling.

Descriptions of the services can be published into reg-

istries, which can then form the basis for service dis-

covery tools.

XML has been used in association with ﬁeldbus

systems, but mainly to provide interoperability be-

tween different ﬁeldbus systems (Eberle and G

ohner,

2003). Use cases for Web services to access ﬁeldbus

systems are described in (Venzke and Pitzek, 2003).

The most advanced usage of Web services in this area

up-to-date is the OPC XML-DA speciﬁcation by the

OPC Foundation (OPC, 2003b). The proposed Web

service resembles our approach, but there are also sig-

niﬁcant differences. While XML-DA is built on the

OPC-COM interface, our work is based on the ﬁeld-

bus protocol TTP/A and puts forward a platform in-

dependent open architecture. We also include mech-

anisms that we believe are indispensable for the area

of vertical integration such as authentication and au-

thorization.

Summarizing, we believe that Web services are an

appropriate technology for the problem of vertically

integrating ﬁeldbus systems.

3 WEB SERVICES FOR

VERTICAL INTEGRATION

There are many issues to be solved when designing

a Web service for vertical integration. In this section

we collect requirements and discuss alternatives. The

design has to consider the environment in which ver-

tical integration is carried out and the characteristics

of the communication technique Web service.

It is commonly agreed that the goal of Web services

is to provide high level abstractions. Because of the

high latencies of calls to a service, they should have

coarse grain interfaces. Querying the status of a regis-

ter storing few bytes is certainly not a suitable task for

a Web service. Also due to the characteristics of the

protocols used for web services real time guarantees

cannot be provided. This is not a severe restriction,

because applications making use of vertical integra-

tion usually have no real time demands. We regard

the following operations as candidates for a web ser-

vice accessing a ﬁeldbus system:

• Reading or writing lists of data items

• Reading aggregated or accumulated data

• Executing a sequence of operations that implement

a semantically higher concept

Allowing access over the internet demands for a se-

curity concept to protect ﬁeldbus systems. Only au-

thenticated and authorized users should be accepted.

In most cases authorization schemes will be derived

from an enterprise wide security policy. To carry over

this scheme to a web service demands for an interface

providing an appropriate granularity.

Accessing ﬁeldbus systems over the internet may

fail due to a variety of reasons (ill posed requests, net-

work failures, unavailable services etc.). Applications

making calls to such services should be given a maxi-

mum amount of information in an error case to handle

failures gracefully. If an operation has failed entirely,

it should be answered with a SOAP fault message.

Instead of application data it contains a well-known

structure for error information.

For periodically reading the same set of data items,

as required for monitoring, callbacks appear advis-

able. A client subscribes to a set of data items. The

Web service then calls a client’s operation for passing

the data items whenever new values get available or

in ﬁxed time intervals. Using callbacks avoids polling

by the client. However it makes clients more complex

due to the need of server functionality. It cannot be

applied if network address translation (NAT) or ﬁre-

walls only allow connection setup in one direction.

If polling is used it can be optimised using time-

outs. Calling a blocking poll operation allows a client

to wait until new values get available. A maximum

call time may be provided after which the operation

is aborted with an error code, to ensure that control

is given back to the client after a deﬁned time. Pro-

viding a minimum call time makes the poll operation

collect new values before returning them in bulk.

Web services can be accessed by many users in par-

allel, hence implementations of such services have to

deal with concurrency problems. To guarantee the

atomicity of bulk operations either requests have to

be serialized or a more subtle transaction mechanism

has to be implemented.

VERTICAL INTEGRATION OF TTP/A FIELDBUS SYSTEMS USING WEB SERVICES

391

4 TTP/A FIELDBUS SYSTEMS

Fieldbus systems considered in this paper consist of

smart transducers connected by a TTP/A bus. A smart

transducer (ST) comprises hard- and software, con-

sisting of a small, compact unit containing a sensor

or actuator element, a micro-controller, a communi-

cation controller and the associated software. The

TTP/A bus allows real time and non real time data

exchange between STs.

A key feature of TTP/A ﬁeldbus systems is the con-

cept of an Interface File System (IFS) (Kopetz et al.,

2000). It is a conceptual model for addressing data

stored distributedly in the STs. STs have ﬁles consist-

ing of 4 byte records. Records are identiﬁed with hi-

erarchical names composed of identiﬁers for the ﬁeld-

bus system, ST, ﬁle and record.

IFS supports three operations. With Read and

Write a record’s value can be retrieved or changed.

Execute performs a node speciﬁc operation identiﬁed

by the record’s name.

According to (Kopetz, 2001) STs should support

three interfaces. Using the real time services inter-

face (RS) time sensitive information is accessed. The

diagnostic and management interface (DM) is used

for diagnostic purposes and to monitor STs. The con-

ﬁguration and planning interface (CP) allows conﬁg-

uring STs.

5 WEB SERVICE FOR FIELDBUS

SYSTEMS

In this section we describe the core of our research:

a Web service for the vertical integration of ﬁeldbus

systems based of the IFS speciﬁcation. The design of

the Web service interface follows the categorization

of interfaces outlined in section 4.

5.1 General Architecture

The requirements analysis from section 3 and the de-

sign of the Interface File System led to the overall ar-

chitecture depicted in ﬁgure 1 and presented in more

detail in (Vigil, 2004). The core of the architecture

consists of a generic implementation of the Web ser-

vice. The Web service accesses the ﬁeldbus system

via a proprietary interface of the gateway.

The operations of the service are implemented in

the service processor. The generic SOAP function-

ality is provided by a SOAP processor. It receives

the calls from the Web Server, processes the SOAP

messages and forwards the content to the service pro-

cessor. To decouple the implementation of the web

service from a particular gateway implementation, the

concept of pluggable adapters is applied.

A cache is introduced for data periodically read by

the client. The data can thus be returned immediately

without accessing the ﬁeldbus system.

To accommodate the fact that gateways may not

support concurrent access to the ﬁeldbus system,

requests are serialized. The serializer component

queues all requests and handles them in order.

TTP/A Node

Master

Node

Interface

Proprietary

Interface

Cache

Request

Handler

SOAP

Internet /

Intranet

Monitoring

Configuration

Management

Production

Planning

Gateway

TTP/A Bus

Serializer

SOAP Processor

Service

Processor

Web Service

CCD

TTP/A Fieldbus System

Figure 1: Overall architecture

5.2 Security and Sessions

Since the proposed service will be deployed in an en-

terprise environment, security is an important require-

ment. Therefore encryption based on the protocol

SSL is used along with authentication combined with

state-full sessions.

In order to access the TTP/A cluster the client must

have authenticated itself at the Web service. The au-

thentication is done via the operation Login. The

client provides credentials identifying itself and spec-

iﬁes the desired conceptual interface (RS, DM, or

CP). A state-full session is started and a session iden-

tiﬁer is returned, which must be provided by the client

in all future requests. Implementing a session mech-

anism in this style is independent of protocol speciﬁc

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techniques such as cookies and thus improves the web

service’s interoperability.

5.3 Operations

The conceptual interface chosen for a session con-

strains the operations available for a client. Only

those operations are available that are meaningful for

the client’s viewpoint and for which the user is autho-

rized. All operations allow accessing several records

with a single call providing a coarse granularity.

The operations Read, Write and Execute have sim-

ilar meaning as the respective IFS operations.

The two operations Subscribe and ReadSubscrip-

tion allow optimizing periodic reads of the same set of

records. A client subscribes a set of records. The Web

service returns a subscription identiﬁer. It is provided

in subsequent requests for the operation ReadSub-

scription that returns a response similar to the op-

eration Read. Subscriptions increase efﬁciency be-

cause record names only need to be checked once and

records can be cached and preloaded inside the ser-

vice processor.

The operations WriteConﬁguration and ReadCon-

ﬁguration allow conﬁguring a ﬁeldbus system and re-

trieving its conﬁguration respectively. A new con-

ﬁguration is provided in the request to the operation

WriteConﬁguration as XML data structure explained

in (Pitzek, 2002). It is used for conﬁguring the ﬁeld-

bus system and stored in the Web Service for later

retrieval via the operation ReadConﬁguration.

Operations accessing several records (Read, Write,

Execute, ReadSubscription) can fail partially. A

record, ﬁle or ﬁeldbus node might not exist or be inac-

cessible. In this case the operation still continues with

the others. The SOAP response contains a sequence

with one data structure per accessed record. To report

the failure the data structure contains an error code.

5.4 Implementation

The described Web Service has been implemented

as prototype based on Java and Apache Axis (Vigil,

2004). To be self-contained the prototype uses a sim-

ulation instead of a physical ﬁeldbus system, imple-

mented as pluggable adapters. The hierarchical ad-

dressing of IFS is mapped to a hierarchy of ﬁle system

directories.

6 CONCLUSION

We have presented a requirements analysis for the us-

age of Web services for the vertical integration. This

results in a loose coupling of all parties concerned and

allows to adapt to new requirements gradually. Based

on the requirements analysis we discussed a concrete

design and implementation of such a service.

Currently we are investigating several ways to ex-

tend our research. Available metadata about the ﬁeld-

bus system can be further exploited to enhance the

quality of the service. It could be used to provide de-

tailed information about the data returned by the oper-

ations. Furthermore, we plan to implement operations

providing higher abstractions dedicated to a particular

ﬁeldbus system on this basis.

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