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Industrial Wireless Controllers to centralise the management of big WLANs The use of Wireless LAN has gained increasing acceptance in automation. Hardware manufacturers have accordingly designed products for use in line with industrial requirements. This radio technology mostly finds employment within application islands; additional requirements have to be fulfilled if it is to be used over a wider area. Achieving coordinated wireless operation requires a degree of centralised management. A new design take on the WLAN controller enables the creation of extended networks comprising hundreds or thousands of access points says Hirschmann’s Olaf Schilperoort 24 IN WHAT SEEMS like an age ago back in 2002, a few entrepreneurs linked to American universities set out to turn the WLAN world on its head. Before then, radio networks consisted of one or more access points connected to a wired network which enabled mobile clients such as laptops and PDAs to gain network access. These access points, which were used predominantly in the office sector, had just the basic functions necessary for a Layer 2 infrastructure. Only few of them included any sort of routing or firewall mechanisms. The start-up companies initially considered adding extensive Layer 3 functions. They wanted to take WLAN beyond local applications in order to construct and centrally administer campus and company-wide networks. These networks could extend to several thousand access points. There was also a question of the software needed to manage this number of devices. But the promise was to administer large networks with a server-like device. The original idea This device was initially known as a WLAN switch, because it was meant to control all the access point functions centrally. The access points were given the commercially attractive name Thin AP. Their task was limited to that of providing an interface between cable and radio link. Every packet sent and received – including all control and management information – was initially sent to the central WLAN switch, before the latter relayed the user data to the local area network (LAN). In order to make this possible, the switch and access points were inter-connected by a Layer 2 tunnel. This central approach was actually advantageous for large installations since WLAN clients that move around an extended site had to remain within the same IP sub-network at all times. The reason for this was to avoid new IP addresses having to be assigned when switching to other sub-networks. The inevitable interruption of the network connection would otherwise last several seconds, causing VoIP for example to suffer service interruptions. WLAN switches were therefore regarded as a solution to cut down roaming handover time. Roaming in this context is the term given to the change by a client from one access point or sub-network to another. However WLAN switches turned out to be a bottleneck as each data packet first had to be transported from the receiving access point across the entire network to the central device which only then dispatched it to the assigned destination in the LAN (Fig. 1). This led to the switch’s bandwidth quickly becoming exhausted. This problem was further exacerbated with the advent of WLAN Standard IEEE 802.11n, which hiked the data rate possible with each access point. With bandwidth continuing to be an important sales argument in the office sector, the development of WLAN switches looked as though it was going to reach an impasse. Decentralised improvement For such reasons of bandwidth management, it became necessary to work around some aspects of the centralised approach. WLAN switches became WLAN controllers, which now only exchange control data with the access points. Many functions, which had previously been centralised, migrated back to the access points. In addition, the CAPWAP Standard (Control and Provisioning of Wireless Access Points) had since been defined, which describes how the Layer 2 tunnel between controller and access industrial ethernet book point should be structured; this standard was later also included in IEEE 802.11w. This subsequently led to a symbiosis of the centralised and decentralised approaches. In other words, it combined the flexibility of the local, standalone access point with broadband network connection and the benefits of a high-performance controller solution. A WLAN controller locates all available access points when the network is first configured, provided a cable connection exists to the Fig. 1. WLAN switches were regarded as a solution to cut down roaming handover time. However WLAN switches turned out to be a bottleneck as each data packet first had to be transported from the receiving access point across the entire network to the central device which only then dispatched it to the assigned destination in the LAN sponsored by Advantech PHOTO: BELDEN
Fig. 2. WLAN controllers enable defective access points to be replaced rapidly and also support and replace each other controller's network. The controller then checks whether the access points have the right firmware and also sends out configuration data assigning a logical network position. This saves the administrator time and effort in putting a functional network together. During operation, the controller provides all the necessary information via the network. For instance, if an access point fails, the controller immediately recognises the alternative device as soon as it is replaced and connects it to the network. The management software therefore no longer has to call up the data from each individual access point. This reduces network load and information is available more quickly. In addition, further functions can be provided which would not be possible without a complete overview of the network. For example, controllers can be used to set up the WLAN network in such a way that channel distribution results in the fewest possible overlaps and problems between the access points. As the controller represents the central access to the WLAN network, it also becomes the gate keeper between the wired and wireless part of the network. The controller then provides a firewall and security function. The devices can also serve as VPN gateways that can interconnect several WLAN networks. Moreover, controllers can also be set to provide redundancy if one of them fails (Fig. 2). Controller-compatible access points WLAN controllers are not economic for small network use. In this case, there is still a need for stand-alone access points designed for industrial applications. But if 50 or more of these devices are used, this investment will certainly be worthwhile. Industrial access points that can be managed by WLAN controllers are an important consideration since enhanced devices provide a ready made upgrade path should controller-based operation prove necessary. In the office sector, controller-supported networks are currently used mainly for VoIP where roaming across several sub-networks is the main benefit. In automation, a WLAN network rarely has to handle several applications simultaneously for all the reasons that one might expect: diagnostics are too complex and the possible error sources too great. The main purpose behind the use of a controller for industrial applications would be to ease network administration. In addition, client localisation is also desirable for process automation. This enables the administrator to see at any time where personnel with a portable WLAN client are physically located, whether they are completing their work in the right place. Controller-based networks are especially suitable for such tasks, as they provide the necessary data centrally. And finally… There is an increasingly wide range of WLAN applications for automation within the context of networks with a growing complexity. With the WLAN controllers, it becomes possible to manage networks with hundreds or thousands of access points from a central location. Olaf Schilperoort is with Belden, Inc First published in the industrial ethernet book April 2010 Industrial Wireless sponsored by Advantech industrial ethernet book 25
Page 1 and 2: sponsored by Special Issue ISSN 147
Page 3 and 4: GET CONNECTED… www.iebmedia.com
Page 5 and 6: Infrastructure. By using routers as
Page 7 and 8: IEEE 802.11n: the next step for ind
Page 9 and 10: What is Spatial Multiplexing? SM pr
Page 11 and 12: Fig. 2. 802.11 relative rate and ra
Page 13 and 14: Lessons learned from IEEE802.11n En
Page 15 and 16: IEEE 802.11p MAC protocol for vehic
Page 17 and 18: efficacy of a CBTC system is highly
Page 19 and 20: Explicit Messaging application: Wir
Page 21 and 22: Frequency hopping sequencing Freque
Page 23: Simulcast RF wireless networks aid
Page 27 and 28: RFID technology Active RFID tags ha
Page 29 and 30: Extended WLAN is a single network A
Page 31 and 32: Fig. 1. Field device meshing and in
Page 33 and 34: Internet Protocol for wireless conn
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Page 37 and 38: Flow-based compression techniques a
Page 39 and 40: 6LoWPAN node Router solicitation Ro
Page 41 and 42: This happened in the early days of
Page 43 and 44: Glossary • DSSS - Direct Sequence
Page 45 and 46: Fig. 6. IPv6 LoWPAN network example
Page 47 and 48: With Diversity Path Mesh (left) all
Page 49 and 50: Two case studies Wireless solves pr
Page 51 and 52: 60GHz Industrial Wireless: perfect
Page 53 and 54: Wireless avoids cable trouble on el

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