Configure RTS/CTS on Moxa AWK Series Radios To Solve Hidden Node Problem

When setting up wireless networks, there are many factors that can contribute to connection issues such as line of sight, radio interference, network range, firmware upgrades, etc. Although the ones listed here are fairly generic, there are other wireless issues not as apparent which require much more in-depth planning and configuration. In order to troubleshoot IEEE 802.11 wireless networks, it helps to have a general understanding of the rules and standards of 802.11 wireless communication. First reminder, wireless networks operate using half-duplex communication. This simply means that a device cannot transmit data to an Access Point (AP) and receive data at the same time. While this was the standard for a long time in wired ethernet networks, the industry has since supplanted this technology with full duplex capabilities which allows simultaneous transmission and reception of data. Whenever half duplex is in use with wireless networks, a technology known as Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) is always in play. This technology is in use today with wireless networks as a means to prevent data transmission collisions which can degrade wireless performance.

With that basic principle in mind, let's get into a fairly complex issue we came across recently with one of our applications and discuss the solution. The issue involved the use of a wireless network using Moxa AWK-4131 (802.11n) Access Points (APs) and AWK-3131 client radios for an IP video system. This surveillance application was used on a bus system for the purposes of monitoring activity on various buses throughout the city. Moxa client radios were installed on each bus and connected to an IP camera/DVR system. Once the buses returned within a certain range of the access points located throughout the bus yard, data was instantaneously transmitted to the access points and uploaded to a video server. With the appropriate wireless surveys, planning, and device configurations this seems like a straightforward application. However, as with many wireless implementations, new challenges and obstacles can arise.

With this particular implementation, the customer discovered from the Moxa logs that the bus radios were connecting and disconnecting from one AP to the next continuously. Another indicator is observed high signal strength but low transmission rate on the client. What Moxa engineers diagnosed was a behavior known as the "hidden node problem." A hidden node problem occurs when a node is visible from a wireless AP, but not from other nodes communicating with the same AP (see graphic below). The node at the far edge of the access point's range can see the access point, but it is unlikely that the same node can see a node on the opposite end of the access point's range. These nodes are known as hidden. The problem occurs when nodes start to send packets simultaneously to the access point. Since nodes cannot properly sense the carrier, the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) does not work and as a result, collisions occur scrambling the data.

In order to resolve the issue, IEEE 802.11 uses a function called RTS/CTS acknowledgment and handshake packets to partly overcome the hidden node problem. A node wishing to send data initiates the process by sending a Request to Send frame (RTS). The destination node replies with a Clear To Send frame (CTS). Any other node receiving the RTS or CTS frame should refrain from sending data for a given time (solving the hidden node problem). On the Moxa radios, we effectively enable the RTS/CTS function (reference screenshot below) by lowering the RTS/CTS threshold to 256 (smallest packet size). After RTS/CTS was enabled, we saw both connections stay stable and share the resource reliably. 

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