MMS

Control Eng. Practice, Vol. 4, No. 6, pp. 825-829, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0967-0661/96 $15.00 + 0.00

Pergamon

PII:S0967-0661(96)00074-3

I N T E G R A T I O N OF W I R E L E S S M O B I L E N O D E S IN M A P / M M S P. Morel and J.-D. Decotignie Swiss Federal Institute of Technology, Computer Engineering Department, EPFL-DI-LIT, CH-IO15 Lausanne, Switzerland ([email protected])

(Received October 1995; in final form March 1996)

A b s t r a c t . In industrial networking, wireless communication can be justified by a functional requirement (e.g., communication with mobile nodes) or by convenience (e.g., physical reconfiguration). The latter case is important in flexible manufacturing, where reconfigurations can be costly. On the other hand, the control of mobile devices (autonomous vehicles, mobile robots, etc.) is a fundamental requirement, especially in flexible manufacturing. This paper presents a way of managing mobile wireless nodes as a distributed application using the MMS Object Model. K e y w o r d s : Mobile robots, wireless network, distributed systems, M A P / M M S , IEEE 802.11

1. I N T R O D U C T I O N

manipulate MMS objects on the MMS server (the mobile node) which contains at least one VMD.

Mobile robots, the main application target of this work, are expensive devices which have embedded computers for their control. They communicate with their (fixed) local command computer as intelligent stations. The commands are highlevel ones, such as 9o to that place or take this object. The robot sends acknowledgements in report form, such as I am at that place. On the other hand, it must be possible to download the embedded software automatically, and read journals and internal status tables for technical support. This calls for two types of services, both of which can be handled by the industrial Manufacturing Message Specification MMS (ISO 1989).

This paper presents a way of extending the M A P / M M S protocol to allow mobility. The integration of the IEEE 802.11 wireless protocol in the MAP stack is also shown. The changes made are transparent to the upper layers of the OSI model. In this context, the issues of addressing and routing in mobile environments are explored. The paper is organized as follows. Section 2 presents the M A P / C N M A , MMS and IEEE 802.11 standards. Section 3 is a presentation and analysis of the proposed architectures. Section 4 shows the integration of a wireless mobile node in a local industrial network MAP (Pleinevaux and Decotignie 1993).

The mobile node can be modelled in an abstract way known as a virtual manufacturing device (VMD). Inside the VMD, MMS objects are used to represent physical entities associated with the mobile device. Each MMS object has a set of MMS services to manage it, and it is through these services that actions are conducted in the MMS environment. Typically, the MMS client (in this case, the cell supervisor) uses MMS services to

2. T E C H N I C A L B A C K G R O U N D 2.1 M A P and C N M A The Mamlfacturing Automation Protocol (MAP) was defined by General Motors with the goal of reducing the cost of installation and achieving independence in the choice of its suppliers. It.s 825

826 architecture standard.

P. Morel and J.-D. Decotignie

is based on the seven-layer ISO

MAP started with the use of the protocol I E E E 802.4 - token bus. Since 1986, a superset of MAP, called CNMA (Communications Network for Manufacturing Applications) (CNMA 1991), has been specified and implemented by European companies and research institutes in an Esprit II project. The I E E E 802.3 (Ethernet) protocol at the MAC level and the R e m o t e Database Access protocol at the application layer have also been added. The remainder of this paper refers to the MAP Ethernet version.

2.2 MMS The Manufacturing Message Specification (MMS) (ISO 1989) is an international standard that defines a set of services (as well as a corresponding communication protocol) that constitute a part of the application layer of MAP. MMS was designed to standardise and facilitate the remote control and monitoring of industrial devices made by different vendors. MMS serves as a c o m m o n language that forms a foundation for the interconnectivity of industrial devices. MMS is based on a client-server model of communications. In m a n y a u t o m a t i o n systems, the controlling application, called the MMS client, is responsible for directing the operations of the individually controlled machines, called the MMS servers, distributed throughout the a u t o m a t e d system, possibly on different subnets. This paper examines the case in which an MMS client application node or an MMS server application node residing in a mobile device moves from one subnet to another.

2.3 Wireless L A N (IEEE 802.11) A wireless LAN is conceptually different from a wired LAN (Lessard and Gerla 1988). The most i m p o r t a n t differences are the shared medium, increased error rates and a dynamic topology. In wireless environments, direct interaction between mobile stations is less reliable than communication through a base station and therefore often prohibited. Another difference is the meaning of addressing: for a wireless network the address is not equivalent to a physical location. Since 1990, the P802.11 Working Group has been developing standards for all kinds of wireless communications. Their initial goal was

to Develop a medium access control (MAC) and

Physical Layer (PHY) specification for" wireless connectivity for" fixed, portable, and moving stations within a local area (IEEE 1994). 2.3.1. At the MAC Level. Networks with more than 1000 nodes are allowed by the standard, and it handles d a t a transmission speeds up to 20 Mbps. 802.11 uses a contention mechanism to allow stations to access a shared channel, in the spirit of 802.3. Due to the fact that a station cannot simultaneously listen on the same channel on which it is transmitting, it is not able to determine that a collision has occurred until the end of a packet transmission. A special collisionavoidance mechanism has therefore been added to the CSMA protocol to reduce the probability of collision. The MAC protocol uses Carrier Sense Multiple Access with Collision Avoidance ( C S M A / C A ) (Desimone and Nanda 1995). The 802.11 MAC-layer protocol is tied to the I E E E ' s 802.2 Logical Link Control layer. This makes 802.11 LANs easier to integrate with CNMA, which conforms to the 802.2 LLC standard. 2.3.2. At the Phgsical Level. The draft, standard I E E E 802.11 defines three different physical media: direct sequence spread-spectrum (DSSS) in the 2.54 GHz ISM band, fl'equency-hopping spread-spectrum (FHSS) in the 2.54 GHz ISM band, and baseband IR. A 1 Mbps transmission speed has been specified for DSSS LANs.

3. N E T W O R K A R C H I T E C T U R E S Before discussing routing between mobile nodes and fixed stations, it is interesting to define how wireless subnetworks interconnect with the wired factory network. In this respect., it makes sense to consider two schemes of interconnection, which can be called "basic" and "extended" architectures. The evMuation of these two topologies will pinpoint, the problem areas arid help to define new elements that improve the existing structure.

3.1 Wireless Cell Structure Within a given area or "cell", mobile stations operate in the same physical and logical channel, and form a wireless LAN segment. Mobile stations cannot reach each other directly, but have access to only one central station, the hub, which is also the access point (gateway) to the wired network. Mobile stations can roam from cell to cell by

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Integration of Wireless Mobile Nodes registering with another access point; this process is called "handover".

3.2 Basic N e t w o r k Architecture In the simple architecture shown in Fig. 1, the roaming area of the mobiles is covered by multiple wireless cells, centred around wireless access points (WAP) interconnected by a single MAP subnetwork.

Any system using mobile MAP protocols must remain compatible with existing hosts. This means that the base MAP protocols cannot be modified in either the existing routers or the hosts. It is therefore not possible to specify any change above the Network Layer. 3.3.1. Requirements.

Existing distributed applications must continue to work without interruption when a mobile host moves between adjacent cells. Furthermore, the fact that a node is mobile should be hidden, by the network, from other systems which wish to communicate with it. In this extended architecture, it is necessary to enhance the MAP protocols and develop a method for routing a mobile MAP system.

4. ROUTING

Fig. 1. Base Network Architecture The MAP subnetwork carries the communication between all stations, whether wired or wireless, and in addition the special traffic between WAPs, for example during handover procedure. 3.2.1. Evaluation. The main advantage of this approach is that it is possible for a mobile node to roam between wireless cells without modifying the MAP protocols.

The first problem encountered with the introduction of mobility is that protocols like IP assume that a computer network address encodes its physical location. The issue of handling mobility in office networks such as those using IP have been published (Perkins et al. 1994, Ioannidis et ell. 1991, Reichert 1994). Based on the above research, Younger (Younger et al. 1993) has proposed a model for the integration of wireless nodes in OSI networks. It is now proposed to adapt this model to the local industrial network, MAP.

Another advantage is that there is no need to update the routing table of the subnetwork gateway, or to introduce special mobile controller nodes, because the movement is confined to a single subnet. If, however, a mobile needs to roam across several subnetworks, this architecture is no longer applicable and it is necessary to enhance the MAP protocols as outlined in the next section.

3.3 Extended N e t w o r k Architecture The architecture, shown in Fig. 2, represents an example of a factory floor divided into fabrication cells. Each fabrication cell has its own subnetwork with one or more wireless cells. A mobile can roam between fabrication cells, but logically "belongs to" one particular fabrication cell.

Fig. 2. Extended Network Architecture

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P. Morel and J.-D. Decotignie

4.1 Design

Old AP

Each mobile host is assigned a constant NSAP address on a home subnet, known as its "home address". Interacting hosts m a y always use the home address in order to address packets to a mobile host. Each mobile host has a home agent (for example, the cell controller) that maintains a list which identifies the mobile hosts that it is configured to serve, and indicates the current location of each of these mobile hosts. To become operational in the network, the mobile host must establish a relationship with a WAP. This process comprises two phases: authentication and association, see Fig. 3.

Successful AuthentJcati°n~k~ f ' ' ~Authenticated. 'a t e ~

","~, . . . . . .

/ DeAuthenticati°n

Association

Class 1,2 & 3 Frames Fig. 3. Authentication and Association Phases Each WAP maintains a "visitor list" of its currently registered mobile hosts. When the registration process is performed, the new WAP notifies the new location to the mobile host's home agent.

~.1.1. Tunnelling. The home agent exchanges packets with an autonomous vehicle's current WAP using "tunnelling". Tunnelling involves the use of an encapsulation protocol. The original destination address is moved into the packet's body. The new destination address corresponds to the mobile host's WAP, or to the vehicle controller. Once delivered to that host, the packet is be handled by the enhanced MAP protocol software and sent eventually to the wireless host.

New AP

New AP

Mobile

Node

Handov=REQ

- AP-H=mdovorIND J

.2..=.REo I

H~dov~RESP H~mdov~CONFD

Fig. 4. Handover Procedure 4.2 Routing within Subnets If movement is confined to a single subnet, as in Fig. 1, the routing is done by the WAPs. The updating of the W A P ' s routing tables and the home agent's routing tables is done during the handover procedure. If the mobile is in its home subnet, its in-coming packets are routed directly by the WAP to the mobile host without the help of the home agent. Such movements require no modification to tile MAP protocols because they are invisible l.o the subnetwork independent convergence sublayer (SNIC), which provides the subnet-independent ISO network service to the transport layer, and includes internetwork routing and switching.

4.3 Routing Between Subnets If a mobile host is able to move between subnets, then the movement is visible to the SNIC layer.

.~.3.1. Location of Functionality.

In computer integrated manufacturing, the control of autonomous vehicles is done hierarchically by a single host. It is therefore most efficient, in terms of network traffic, to group the functions of home agent, and vehicle controller in the same station. Interacting hosts will always communicate with the mobile through this same fixed home agent. This means that only the vehicle controller and the WAP's MAP protocols need to be modified to handle the mobile packet traffic.

4.4 Discussion.

4.i.2. Handover.

The handover protocol is used by a mobile station that has found another WAP giving substantially better RF communication quality than the current WAP. The handover procedure is initiated by the mobile host. It can be seamless if WAP coverage areas overlap.

This solution assumes that the m a n a g e m e n t of the autonomous vehicles is undertaken by an enhanced MAP protocol host, the home agent which must coincide with the vehicle controller, which knows where the mobile host is, and the whereabouts of its WAP.

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Integration of Wireless Mobile Nodes

The main advantage of this approach is that the intermediate routers need not understand the tunnelling protocol since, after encapsulation, the packet is simply a normal MAP packet addressed to the WAP or to the home agent. The main drawback is that messages sent to mobiles must be encapsulated and re-directed whenever the mobiles are roaming away from their home subnetworks.

5. CONCLUSION This paper has considered a problem related to the distribution of a control application in a mobile automated system. It has outlined two architectures that allow the integration of mobile nodes into a MAP industrial network, and has shown that new functionalities must be added to enable mobile hosts to maintain network connections, while moving from one subnet to another. However these functionalities only need to be added on special nodes. The MAP protocol stack on other nodes remains unchanged, thus ensuring compatibility. Future work consists of implementing a field test, and the evaluation of other wireless standards, like DECT, in the framework of industrial communication networks.

Acknowledgements The authors would like to thank Alain Croisier for his helpful insights, provided during the editing of this paper.

6. REFERENCES CNMA, Esprit Project (1991). Implementation guide 5.0. (available on FTP server litsnn.epfl.ch). Desimone, A. and S. Nanda (1995). Wireless data: Systems, standards, applications. MOBIDATA an interactive journal of mobile computing. IEEE (1994). IEEE Draft Standard 802.11 Document P802.11/D1. USA. Ioannidis, J., D. Duchamp and G. Q. Maguire Jr. (1991). IP-based protocols for Mobile Networking (ACM SIGCOMM 91). Computer Communication Review 21(4), 235-245. ISO (1989). Manufacturing Message Specification. Service Definition. Lessard, A. and M. Gerla (1988). Wireless Communication in the automated factory environment. IEEE Network Magazine 2(3), 64-69. Perkins, Charles, A. Myles and D. B. Johnson (1994). IMHP: A mobile host protocol for the Internet. Computer Networks and ISDN Systems 27(3), 479-491. Pleinevaux, P. and J.-D. Decotignie (1993). A survey on industrial communication networks. Annales des Tdldcommunications 48(9-10), 435-448. Reichert, F. (1994). The walkstation project on mobile computing. In: Wireless Networks (IEEE/ICCC conference). Vol. 3. pp. 974978. Younger, E.J., K.H. Bennett and R. HartleyDavies (1993). A model for a broadband cellular wireless network for digital communications. Computer Networks and ISDN Systems 26(4), 391-402.