industrial wireless book special edition - Networking ...

More documents

Recommendations

Info

I n d u s t r i a l W i r e l e s s 44 Waiting in the wings – ISA-100.11a This article was written for last year's Embedded Systems conference and before ISA ratified its Wireless for Process Automation specification in September of this year. We feel that Joel Young's paper would not be complete without a suitable update. With Honeywell as its highest profile backer, the ISA-100.11a standard 'Wireless Systems for Industrial Automation: Process Control and Related Applications' is a mesh network built on an IEEE802.15.4 MAC and physical layer, TDMA mesh protocol (as championed by Dust Networks), uses the WirelessHART-style DSSS frequency hopping for interference mapping and avoidance. It also of course implements mesh routing. ISA describes the protocol as providing 'reliable and secure wireless operation for non-critical monitoring, alerting, supervisory control, open loop control, and closed loop control applications. The standard defines the protocol suite, system management, gateway, and security specifications for low-data-rate wireless connectivity with fixed, portable, and moving devices supporting very limited power consumption requirements.' The application focus is on applications such as monitoring and process control where latency in the order of 100ms can be tolerated, with optional behaviour for shorter latency. The declared aim is to provide reliable and secure wireless operation for non-critical monitoring, alerting, supervisory control, open loop control, and closed-loop control applications. While the first version requires network connection through a gateway and does not directly support IP-based address space at device level, other ISA working groups plan coexistence with other industrial wireless devices based on IEEE 802.11x, IEEE 802.15x, IEEE 802.16x, and other relevant standards. This plan includes ISA-100 proposals capable of direct network addressing using IPv6. Frank Ogden The ZigBee standard has evolved in three different versions: 2004, 2006 and 2007. ZigBee 2004 is no longer used and ZigBee 2006 had significant limitations. ZigBee 2007 includes key features for frequency agility, message fragmentation, and enhanced security associated with key management. The routing of messages follows Cluster Tree methodology where routes to all points are maintained at each cluster. This allows a very short routing time, but requires many routes. Discovery of routes uses the AODV algorithm where paths are explored between clusters. A ZigBee release using IPv6 addressing is presently under development. Network Architecture. The network consists of three specific device types. A ZigBee Coordinator (ZC) is required for each network to initiate network formation. The coordinator may act as a router once the network is formed. The ZigBee Router (ZR) is actually an optional network component, although a network without routers becomes a point to multipoint network. The router participates in multi-hop routing of messages. Finally, the ZigBee End Device (ZED) does not allow association and Fig. 4. ZigBee network example. End points sleep, routers don’t sleep and a coordinator is needed to start the network and to allow points to join the network. does not participate in routing. Figure 4 illustrates an example network. Strengths. End devices consume very low power. Cluster Tree routing provides quick knowledge of routes and thereby efficient routing. With ZigBee 2007, frequency agility skips interference-prone channels automatically. Long messages are allowed with message fragmentation support and security is flexible with support of separated keys. Finally, the network can scale to very large. Limitations. Routers must be powered at all times: no sleep mode. Cluster Tree routing creates heavy route discovery traffic on network changes. Heavy traffic volume produces collisions and potential message loss. A coordinator is needed to start and manage the network: if the coordinator goes down, the network can’t start. WirelessHART Key Characteristics. WirelessHART uses the Time Synchronised Mesh Protocol created by Dust Networks. Unlike other networks, the timebased system uses TDMA as access method. The network is optimised for low power and all nodes can be sleeping routers and every node is a router. A Gateway is required to implement the critical time synchronisation of sleeping and waking functions. Like ZigBee, it uses Fig. 5. WirelessHART network example. A Gateway is required to implement the critical time synchronisation of sleeping and waking functions. industrial ethernet book 802.15.4 DSSS, with the addition of a more advanced frequency hopping algorithm. Security includes encryption and authentication. Network Architecture. Note that all the nodes are routers. Figure 5 illustrates a typical network topology. The illustrated routes change dynamically based on visibility within specific time slots as it hops through the different DSSS channels. The relationship between any two nodes is negotiated to be in a specific time slot, thereby minimising the probability of any collisions. When sleeping, nodes awaken during their time slot and listen to see if there is any traffic. Clocks are kept synchronised by the gateway. Strengths. Every node provides a routing function with very low power consumption. Since transmissions occur only within the allocated time slot, retransmissions are minimised. Communications are reliable with every message acknowledged. Networks are able to scale to moderate level – around 1000 nodes. Frequency hopping minimises the probability of interference. Security includes encryption and appropriate authentication. Limitations. Because of the time slot approach, latency is long and non-deterministic. It takes a network some time to form, and for the participating nodes to negotiate their individual time slots. Because communications is slotted, the available 802.15.4 bandwidth is split up, meaning that throughput is minimised for burst traffic. A powered gateway (coordinator) is required to keep the network functioning – opening up a single point of failure if the gateway becomes unavailable for an extended period of time. Finally, the radios are expensive compared to the other available solutions. 6LoWPAN Key Characteristics. 6LoWPAN is an acronym for IPv6 over low power wireless personal area networks. Presently it is a proposed standard based on the IETF RCF 4944. It is designed to be used over 802.15.4 chips and radios. Unlike traditional IPv6, 6LoWPAN deals with packet size incompatibilities in message transport (128 bytes vs MTU of 1280 bytes in IPv6) and it is designed for a small memory footprint systems. Today it is a point to multipoint architecture but with proposed augmentation to a mesh routing scheme. Figure 6 provides an example of 6LoWPAN network topology. Note that for now it offers only point to multipoint. Unlike the other networks discussed in this paper, the figure shows an end to end IP-based link from a host computer to an end device. In this case it is illustrated by an electricity meter. The end device is directly addressable by the host computer on the far end of the network. The interworking function provided in the pictured box provides a transport change and repacketing at the data link level. Network Architecture. Typical ad hoc network topology. Unlike the Cluster Tree sponsored by Advantech
Fig. 6. IPv6 LoWPAN network example. Unlike the Cluster Tree method described in ZigBee, routes are only determined on an as-needed basis. This means that routes that are never used never get routing table entries and routes that are used frequently are continuously updated, thus optimising efficiency. method described in ZigBee, routes are only determined on an as needed basis. This means that routes that are never used never get routing table entries and routes that are used frequently are continuously updated, thus optimising efficiency. Strengths. Every node is a router with very low power consumption. Further, because every message is acknowledged and routes are determined on an as needed basis, the network is not overwhelmed with discovery traffic – important with sleeping battery powered routers. Efficient route discovery and routing means that the network only learns routes that actually get used (AODV). Frequency agility is supported and security involves both encryption and authentication. Reliability is projected at 99.99%. Finally, the system supports larger payloads with support for message fragmentation. Limitations. Efficient power management means long latency and non-determinism. Even though throughput is not limited by time slots, it is still limited depending on loading and discoveries. The network can scale to a moderate size of around 500+ nodes and can be very large if traffic is light and message flow doesn’t change much. System comparisons Using the criteria defined at the beginning of this document, they all do very well in security in that they have well defined encryption, authentication and authorisation schemes. ZigBee and 6LoWPAN have a slight advantage in that their key systems should be easier to implement and thus be more flexible. With respect to reliability, point to multipoint takes the biggest hit because it inherently has a single point of failure. Some schemes may have frequency agility options while others do not. Prior to the 2007 standard, ZigBee had a weakness in the frequency agility area which has now been fixed along with added support for message fragmentation. The others are similar – WirelessHART is designed to never lose a message while 6LoWPAN does well on the assumption that the existing TCP/IP protocol suite has class of service built in. While our own proprietary DigiMesh has a similar approach to WirelessHART, it is still somewhat unproven in large deployments. Power management will no doubt be hotly debated. WirelessHART defines systems where all nodes in the network, including routers, can sleep. Even though sleeping ZigBee end devices are most efficient when it comes to power, the fact that routers can’t sleep bumped the rating down. Until 6LoWPAN publishes a mesh and power management strategy, the rating will remain unknown. The scalability rating follows directly from the question of how big can the network get and still function. This is where the ZigBee 2007 Pro stack shines. The Cluster Tree architecture creates a hierarchy which enables scalability. WirelessHART scales well; particularly if most communication is kept local – however, the networks can tend to get very slow when they become too large. Finally, point to multipoint architecture has an obvious limitation in the number of nodes that can be attached to one central point. The best data mover is no doubt the simplest system – namely point to multipoint. The simple network design means that focus can be made on short, deterministic latency and high data throughput. There is a direct trade off here with power. WirelessHART rates lower because it is focused on minimising power and maximising reliability – this naturally leads to less deterministic latency and lower throughput. Of course, as a network gets bigger, these two networks will actually do better. However, this is represented in the high scalability ratings for these networks. ZigBee fits in the middle because the backbone of powered routers can move data very efficiently – but can get stuck if too many route discoveries are needed. Cost may create the most debate. The ratings here were based primarily on the cost of available chip set solutions under the assumption that the right architecture is chosen for the right job. If not, then the cost ratings become meaningless. For example, attempting to deploy a ZigBee solution where battery powered routers are desired means infrastructure costs will skyrocket. So, given this caveat, point to multipoint, ZigBee and our DigiMesh protocol have common costs because they all use similar chipsets. Each wireless mesh architecture has its respective benefits and there is no single approach that fulfils every wish list. Hence, it is important to match the needs of the application to the capabilities of the network. Joel K. Young is senior VP and Chief Technology Officer, Digi International Inc. First published in the industrial ethernet book November 2009 sponsored by Advantech industrial ethernet book
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 and 24: Simulcast RF wireless networks aid
Page 25 and 26: Fig. 2. WLAN controllers enable def
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
Page 35 and 36: For several reasons, including cost
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: Glossary • DSSS - Direct Sequence
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

industrial wireless book special edition - Networking ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?