Good Wireless Design Smoothes Path to Benefits

Despite Limitations, Wireless Fits the Bill in Many Industrial Applications

By Jim Montague

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Wireless can save cabling and installation costs, deliver previously unreachable signals and data, and reduce infrastructure and maintenance expenses. But many potential users are still hesitant to evaluate and implement it, according to Rob Snyder, Rockwell Automation, product manager, for Stratix 5100 wireless access points, and Scott Friberg, senior applications engineer in Cisco's Internet of Things (IoT) group.

To help answer users' questions and alleviate their concerns, the pair walked attendees of this week's TechED conference in Orlando, Florida, through a comprehensive application manual recently published by Rockwell Automation and Cisco. Wireless Design Considerations for Industrial Applications is a free, 97-page guide intended to help users apply wireless appropriately, safely and securely in their machines, applications and production processes. It includes details of wireless local area network (WLAN) implementations, test details and results, and links to Cisco documentation.

"The main advantages of wireless are that it can reduce installed costs due to less cable and labor; reduce operations costs by eliminating cable failures; allow connections to hard-to-reach and remote areas; and improve productivity by making equipment and people more mobile. However, these savings sometimes aren't enough because there are some challenges to using wireless, such as dealing with interference and other issues," said Snyder. "For instance, one of the main ways wireless is different is that wireless is a half-duplex, shared medium. So, while wired can communicate in both directions simultaneously, only one wireless device can communicate at a time. Also, wireless signals can vary with time and direction; wireless coverage areas can't be precisely defined; wireless signals may reach beyond intended area; wireless can lose more data packets than wired Ethernet; and wireless is less protected from interference."

Wireless 802.11 Complements Ethernet

Though there are several major wireless technologies, Snyder stressed his and Friberg's presentation focused primarily on those governed by the IEEE 802.11a/g/n standard, which covers I/O components, peer-to-peer and safety controls, and mobile HMIs.

"IEEE 802.11 has many benefits for industrial automation control systems (IACSs), such as widely adopted, standard-based technology, convergence with the enterprise wireless local area networks (WLANs), WLAN mobility and fast-roaming capabilities, high throughput and reliability for real-time applications, 5-GHz spectrum availability with more bandwidth and less interference, and direct transmission of Ethernet protocols such as EtherNet/IP," explained Snyder. "There are many types of wireless access points (APs), including embedded adapters, universal bridges and work group bridges (WGBs) that are very useful for adding wireless to machines. They can handle multiple wired clients, but are viewed as one wireless client on the network. For example, our Stratix 5100 can handle up to 19 devices."

Meanwhile, Friberg reported that WLANs come in two main flavors: autonomous WLAN that has lower initial costs, requires less expertise and allows more control of some parameters, and unified WLANs that centralize management and control, support larger deployments and offer enhanced security and services.

"There are WLANs in most plants today, and they probably extend from the business to the plant," said Snyder. "We typically think of wireless being used to monitor and control large gantry cranes, but smaller and smaller machines and devices are using wireless because it's so capable. In fact, wireless is enabling new manufacturing techniques, such as allowing newly mobile machines to move around products they're working on, connecting formerly separate islands of automation and securing big data to aid optimization."

Look Out for Latencies

The fastest of these mobile devices are called "fast roaming" because they operate and move between APs at the same time. Snyder said it's important for users to make sure this switching doesn't cause roaming delays and application timeouts greater than 50 milliseconds. If there's a risk of longer delays, then a unified WLAN will be needed to pre-check the next AP that the device is switching to. Because of this and other possible latency sources, Rockwell Automation and Cisco's wireless application guide recommends the users employ the 5-Ghz band for their wireless networks, don't exceed 20 wireless nodes per wireless AP and also keep 20% of network's bandwidth capacity in reserve to handle increased data traffic.

"Determining the right packet size, speed and interval is probably the most important metric for wireless implementation," added Snyder. "In general, we found it's important not to exceed 2,200 data packets per second (PPS) in a wireless channel, but further reducing packet rates is important in environments with radio frequency (RF) issues or other interference."

To prepare an application and facility for a WLAN, Snyder and the guide recommend that users assess their site requirements and identify:

  • Number of wireless channels available and in use;
  • IT policy regulating wireless spectrum in the facility;
  • Existing and potential sources of wireless interference in the area;
  • Locations, dimensions, material compositions of required coverage areas;
  • Environmental characteristics of the site;
  • Obstructions that may enter and leave the coverage areas;
  • Installation limitations for the antennas, APs and cabling;
  • The site survey. Check to see if it has been done before and determine what survey equipment and parameters were used.

"Performing a comprehensive site survey is crucial. No wireless system should be installed without one," said Snyder. "Also, IT personnel know a lot about wireless technologies and methods, so it's also very important for automation and control staffs to collaborate with their IT departments on existing wireless infrastructures and new projects. And an accurate site survey is necessary to determine appropriate antenna type and placement."

Once the site survey is done, preparing to install a WLAN also requires identifying network requirements. These include:

  • Picking an autonomous or unified WLAN architecture;
  • Evaluating existing WLAN and switch infrastructure;
  • Deciding who is responsible for managing the WLAN;
  • Settling on required WLAN security and required network redundancy;
  • Determining IP addressing, DHCP and VLAN requirements.

Next, individual application requirement must be determined. These include:

  • Number and type of wireless and wired devices;
  • Type of CIP (Common Industrial Protocol) and non-CIP protocols required by the application;
  • Packet intervals, size and rate for each type of traffic;
  • Directional flow of the traffic per protocol;
  • Application timeout requirements per protocol;
  • Maximum tolerable latency and jitter per protocol;
  • Handling of lost or late data packets by the application;
  • Time synchronization requirements;
  • Equipment mobility requirements, such as fast roaming;
  • If multiple identical applications need to operate throughout the plant, the number of installations and distance between each operation area.

In addition, Snyder reported that several CIP-based application protocols can be used via 802.11 wireless networks, while others have limited wireless applicability. For example, CIP Class 1 for control and I/O communications, CIP Class 3 for HMI and peer-to-peer messaging, and CIP Safety can all go wireless. However, CIP Sync protocol can only use wireless for sequence of events (SoE) and event logging, and CIP Motion should not try to use wireless for motion control.

Finally, the application guide recommends using WPA 2 with AES encryption to secure wireless network, which doesn't affect application performance. "Components like our Stratix 5100 have these security functions built-in, but users must enable them," concluded Snyder. "Security is organic to the wireless standards, so use them."