Mesh networks on the factory floor

As an example, Gendreau described the Veracel pulp mill project, a greenfield, 900,000 metric ton/year bleached eucalyptus pulp mill under construction in Eunápolis in the State of Bahia, Brazil. Foxboro will provide a large I/A Series system with all stations linked via the mesh network configuration. More than 7,500 I/O points and 20,000 communication points will be integrated into the system. As of this writing, only one or two of the smaller units are up and running at the Veracel site.

Foxboro’s mesh system is based on COTS (commercial off-the-shelf ) Ethernet switches from IT world suppliers Enterasys, Allied Telesyn, and Cisco Systems. Foxboro developed a hardware and software interface between the I/A series and the COTS switches, so that its process control systems could talk via a mesh network.

Because the COTS switches have a maximum number of ports (8–24, depending on the switch), the wired system does not have the true peer-to-any-peer connectivity of a wireless mesh. However, it still provides the alternate paths and self-healing functions of a mesh network. By using a series of edge switches and chassis managed switches it can provide multiple paths between a control processor and a workstation.

Do They Really Work?
Until we actually see one of these control systems come to life and hear what the end user has to say about it, we have only the vendors’ opinions about how well they work.

Sauders says the NCS supports 10/100 Mbps over Cat 5/6 Ethernet cable, supports standard 10/100 Mbps switches and managed switches, and has a wireless link module (WLM) that can be used for distances up to 20 miles, line-of-sight. “To my knowledge,” claims Sauders, “so far everything is communicating just fine at the water plant installation.”
“Initial field acceptance tests and site acceptance tests indicated that the mesh network communications performance and switchover times are meeting, and often exceeding, our published specifications,” says Foxboro’s Gendreau.

Foxboro claims mesh networks actually work better than conventional systems. “The performance of mesh networks that have been designed, configured and installed correctly are fully suitable for all process control applications, and can offer users a much higher level of performance than was previously available with their conventional control networks,” says Gendreau.

Without user reports to back up vendor statements like that, we have to tread carefully. As you know, we like to talk to end users about their experiences, and not rely on marketing optimism.

Makes Sense Though
Nevertheless, what Foxboro and Moore say makes a lot of sense. It might very well be that hardwired mesh networks are superior to conventional networks. Gendreau offers five major reasons why switched mesh networks are better than conventional systems:

Reliability — A single switched-Ethernet network in a mesh topology provides a higher level of reliability and fault-tolerance than a conventional dual-redundant Ethernet network, he says. “At best, dual-redundant Ethernet networks can be configured so than no single fault will stop communications. In contrast, a ‘self-healing’ mesh network can maintain virtually non-stop communications even when faced with multiple faults.”

Determinism — Switched Ethernet industrial networks provide full-duplex communications with 100 megabit bandwidth between process control system stations and network switches and 1 gigabit between the switches themselves. “This eliminates the potential for collisions and ensures that packets arrive at the desired destination in real time,” Gendreau explains. “Unlike conventional networks, where typically only a fraction of the available bandwidth is used to avoid collisions, full-duplex communications makes the full bandwidth of the network available to every station.”

Security — Ethernet switches can provide an additional layer of security not available with conventional networks. For example, Enterasys switches can detect abnormal behavior on the network and intervene to quarantine the offending user or device, Gendreau says.

Scalability — A mesh network scales from relatively small point-to-point networks to mega-sized networks. “Up to 1,920 stations, including workstations, control processors, switches, etc.,” explains Gendreau, “can be accommodated on a mesh network by interconnecting Ethernet switches.”

Lifecycle Costs — Mesh process control networks are typically less expensive to install than conventional networks because redundancy of cabling media is not required and most components are standard COTS, rather than proprietary. “Furthermore, as the cost/performance ratio of commercial networking technology continues to improve dramatically, these hardware costs will continue to drop,” he says. “For example, a 16-port fiberoptic switch that we initially used is being superceded by a 24-port switch at a third less cost.”

Saunders offers a similar laundry list of advantages with the Moore system:

1.  Each node can communicate with any other node on the network without sending its information through a common node or communication gateway.

2.  Any I/O point at any node can easily be transferred or duplicated at any other node on the network.

3.  Each node can monitor the health of each independent communication link with all other connected nodes. If faults in these connections exist, the node can notify the outside world.

4.  When a communication link fails, the node and its associated output signals are user-configurable to hold the last good known output, or go to a pre-determined value.