• By Peter Szyszko
  • IIoT Insight

Modern factories heavily rely on artificial intelligence–driven processes to optimize every step of production and also on precise data collected by numerous sensors. Historically, factories used slow, cable-driven serial protocols, for example, RS-232 cables or twisted pairs for RS-422/485. With the development of new technologies, there has been a transition to Ethernet-based communication. Two main factors play a key role in this process. The price of Ethernet nodes went down with the arrival of cheap microcontrollers that include fully integrated Ethernet communication hardware in one chipset, and the sophisticated sensors that provide the flow of data are not compatible with old serial buses.

Wi-Fi communication (wireless Ethernet) is a key technology for delivering all the necessary metrics from sensors. It provides freedom from cables, allowing unrestricted three-dimensional movement. Typical 802.11ac wireless communication extends to 100 m. Commonly, this distance is not sufficient, and multiple access points (APs) are installed to cover a large operational area. In a scenario of moving wireless, the client (vehicle or robot) needs to switch over communication to the next strong signal AP. The best solution is the implementation of 802.11r across the infrastructure. It manages the switch-over mechanism with transitions of less than 50 ms. In some places, existing Wi-Fi infrastructure does not support 802.11r, but the need for moving installation demands fast transition times.

In this situation, one answer is an enhanced Wi-Fi client that can actively monitor surroundings and prepare new possible AP connection opportunities before classic die-down and drop-off connection processes take place, which cause a drop in communication for a minute or longer. Rapid roaming takes an active approach and seeks a new AP when communication is still healthy. It ensures good throughput and fast transitions with a switch time less than 150 ms.

IEEE 802.11r wireless roaming

Roaming has been a desired feature in wireless devices for decades, and in 2002, the IEEE 802.11r standard was introduced. It is still under heavy development with major fundamentals published in IEEE 802.11r-2008. The main goal of 802.11r was to hand over wireless connections between numerous APs along a client travel path without significant delay. It has been particularly important for voice over Internet Protocol (VoIP) applications where human conversation requires transmission times of 50 ms or better to avoid noticeable interruptions. 

The new 802.11r standard allowed speed with secure and seamless handoffs where authentication and quality of service (QoS) configurations were preconfigured ahead of switching to the next AP. It allowed a stable data throughput without delays caused by the regular authentication process (figure 1).

Figure 1
Figure 1. The 802.11r standard allows speed with secure and seamless handoffs, where authentication and QoS configurations are preconfigured before switching to the next AP. This allows stable data throughput without delays caused by the regular authentication process.

Fast roaming steps

  • Authentication and QoS: In this step, two technologies are properly transitioning. Not only devices are connected to one AP, but the AP has the same privileges in respect to communication priority. It is important for VoIP scenarios when delays could affect the human-to-human conversation.
  • Exchange 802.11r (2a – cable, 2b – radio): This special protocol allows the exchange of all necessary information ahead of the travel path of a client, making the transition smooth and fast.
  • Travel path: This is the way the client travels along the available APs.

Fast roaming is particularly useful in older installations when existing infrastructure does not support 802.11r. However, it is not as efficient as having 802.11r; it is attempting to close the gap with systems that do not have the roaming technology where conventional disconnecting and reconnecting occurs with new APs. This process may take a long time—sometimes even longer than a minute. It is frustrating when slow-moving clients operate with extremely weak AP signals when there is another AP with a strong signal level readily available in range (figure 2). 

Figure 2
Figure 2. AP with a strong signal level available in range.

Requirements for the rapid roaming infrastructure include:

  • Same service identifier (SSID)
  • Same password
  • Same security mode
  • Same band
  • Same channel width.

For the rapid roaming technology to work correctly, it is necessary to use an AP with the same SSID and security key. When rapid roaming is enabled, the client device will be configured to scan for the surrounding APs. It is necessary to set slow scan time intervals to specify relatively slow scans when received signal strength indication (RSSI) signal levels are relatively high and the client device can comfortably concentrate on delivering the maximum data throughput. Then, it is necessary to specify the RSSI threshold level that indicates an imminent need for a new connection. When this level is reached, the client device will perform fast scans to look for a new AP. When it is detected, it will authenticate and auto-connect to the new AP while simultaneously dropping the current connection.

This active process eliminates weak signals deprived of links and prepares a new connection ahead when needed. In addition, there are two channel modes for scanning. One mode is “standard,” and it works when all the channels are scanned. The other mode is “intelligent,” and it works when a client device, for example, goes back and forth along the same APs. In this scenario, the device is smart and can learn those APs channels and look for them automatically. This further accelerates the reconnection process.

To implement 802.11r, the wireless infrastructure must support this standard. This typically requires significant additional investment, as most systems that support 802.11r must have a wireless local area network (LAN) controller in addition to the APs, which are then controlled by the wireless LAN controller. In applications where necessary infrastructure does not exist or where there are cost restrictions, rapid roaming technology can provide many of the same advantages at a much lower cost.

Rapid Roaming Application

An example of a place where this scenario has played out is in a warehouse application with autonomous robots. The robots move about the warehouse stocking shelves and fulfilling orders. Here, a legacy Wi-Fi network was already in place to support employees connecting their PCs, tablets, and phones, but the network did not have the necessary equipment to support 802.11r. The solution was fitting each of the robots with a wireless router that could implement rapid roaming technology at a fraction of the cost of installing an entirely new wireless network (figure 3).

Figure 3
Figure 3. Fitting each robot with a wireless router enables rapid roaming technology.

All images courtesy of Antaira Technologies

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About The Authors

Peter Szyszko is director of engineering at Antaira Technologies.