Tech – Roaming

30th July 2020

HOPPING FROM NETWORK TO NETWORK IN REAL-TIME

Authors
Dr. Christian Dombrowski – Xavier Bush – Jesper Lindström

10-minute read

In our series so far, we’ve demonstrated that existing wireless technology can easily meet performance requirements for time-critical industrial applications. That said, hurdles still remain regarding a key Industry 4.0 use case – flexible, reconfigurable shop floors enabled by mobile stations. Can existing methods provide these mobile stations with a stable, real-time connection over their entire operations area?

Currently, the two most popular techniques to ensure wireless coverage are roaming and mesh networks. This entry will analyze both methods and explain why roaming is the ideal solution for industrial automation.

Mesh Networks: A Clever Technique that Overuses Wireless Resources

Mesh networks have become increasingly popular in recent years. This is due to both their flexibility and current implementation in Wi-Fi and WirelessHART networks. The concept of mesh networks itself is simple and ingenious – all nodes connect dynamically and non-hierarchically to as many other nodes as possible, then route data to the receiver via the most direct path. Every mobile phone user has at some point found themselves in a room with a weak and constantly dropping Wi-Fi signal, impacting their user experience. Mesh networks provide an effective solution by harnessing other Wi-Fi-capable devices in the vicinity (since there are typically several of these devices in any given location) to create a clear path to the router.

A mesh Wi-Fi network can also increase its coverage area with the number of stations added. A baseline coverage area of 20m can increase to 100m simply by adding five new stations in strategic locations. This solution is highly flexible and versatile as all stations can both send and receive packets.

A mesh network comprised of AGVs, forklifts and a human operator (master)

That said, certain flaws are inherent in mesh networks that make them unsuitable for time-critical automation. To start, mesh network transmission latency can be long and unpredictable – if a transmitter is two “hops” away from a receiver (see figure above), the transmission’s latency is multiplied threefold. The longer it takes for a data packet to transmit, the longer this data occupies the channel and prevents other stations from transmitting. This latency increases further with the density of mobile stations in the network. Furthermore, each rerouting attempt requires an update to the network’s routing table, which can result in a lost connection. As cycle times in industrial applications are heavily optimized, mesh networks are not scalable beyond a certain threshold.

Mesh networks also struggle to guarantee constant service, especially when wireless stations are mobile. Consider automated guided vehicles (AGVs), which often operate in large shop floors with many obstacles between them and the central control unit (CCU) – typically a worker with a control tablet. While a large number of AGVs would increase the network’s coverage, all units could still end up in a single area out of range of the CCU, triggering an emergency stop. The CCU-holding worker must then walk over and reactivate them manually. It goes without saying that a setup with this kind of productivity risk is unsuitable for time-critical industrial requirements.  

All mobile stations have accidentally moved out of range of the CCU
(the operator)

These two key disadvantages – compounding “hop” latency and unpredictable connections – make mesh networks unsuitable for wireless automation, therefore other methods must be explored.

To learn more about roaming and EchoRing’s innovative wireless solution, please download the free PDF. 

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