March 2010

Web Exclusive

Special Section: Wireless & Ethernet

Dual-RF wireless redundancy

Reliability, availability of wireless Ethernet improves using redundant dual frequency configurations


  • Over 40% of wireless users are concerned about interference.
  • Wireless Ethernet redundancy can easily be achieved.
  • Redundant and non-redundant wireless nodes can coexist in one network.
By Ariana Drivdahl and Jackie Shi

According a recent VDC research report, more than 40% of wireless users are concerned about interference, and this issue is even more important for industrial and critical applications. Interference normally occurs at a particular frequency, so if we can use two or more different frequencies to communicate at the same time, data transmission will continue even if there is interference on one of the frequencies. Take a look at the traditional single radio frequency (single-RF) and dual radio frequency (dual-RF) wireless architectures and how these methods are used to improve wireless Ethernet communications reliability.

Single-RF wireless architecture-the traditional approach

In the standard architecture of a wireless infrastructure, access points (AP) are used to connect many clients to an Ethernet network. Since the AP and client are connected by a single-RF connection, if the RF connection fails, the system and network behind the client will be disconnected.  This is used where redundancy is generally not needed.

Standard architecture of a wireless infrastructure

Dual-RF wireless architecture-a new approach

One way to achieve network redundancy without needing to change the existing architecture of your wireless LAN is to use access points and clients that support dual radio frequencies (usually, the two radio frequencies are set to 2.4 GHz and 5 GHz to prevent interference). To ensure data can be delivered between the access points and clients even when there is interference on one of the frequencies, the devices provide seamless redundancy by supporting a special protocol with a switching time very close to zero. In essence, in a normal situation, wireless access points will receive two sets of data on two different frequencies.  If all is normal, the second one is discarded.  If there is interference on one of the two frequencies, only one packet of data is received, and the wireless network will still function normally. 

If your application requires more than just wireless redundancy, Ethernet redundancy can also be incorporated. Fast ring redundancy, such as Rapid Spanning Tree Protocol; IEEE 802.1w (RSTP) or Turbo Ring is important for the wired Ethernet side of the network.


Seamless redundancy with dual-RF


Dual-RF redundancy

Dual RF access points are used to achieve redundant wireless Ethernet.  The user needs to set up the redundant access points and redundant client on the client side, each with a different SSID for each RF frequency. The following figure shows a web console in which WLAN1 is set to SSID1 and WLAN2 is set to SSID2.

If both the existing Clients and dual-RF clients support redundancy on the same network, the access point can connect both types of clients to an Ethernet network. As shown in the figure below, enter SSID (in this example, Moxa_1_1) in the 2nd column for the AP to connect traditional wireless clients with this SSID to the AP.


Single-RF connection


In addition to wireless redundancy mode, advanced AP/client devices offer another dual-RF feature called "Wireless Bridge" mode. Wireless Bridge mode is designed to optimize Wireless Distributed System (WDS) mode overcoming WDS's throughput issues.

To those unfamiliar with WDS, it is a special type of wireless link that allows several access points to be connected to a central location.  A central location AP is configured as the partner Media Access Control (MAC), and the remote WDS AP is configured as the central AP MAC.  Due to the same frequency channel being used, the wireless link reduces to half bandwidth when adding more than one AP as per this equation:

The normal throughput is: Throughput = 25 Mbps/(n-1)

Where the variable n represents the number of WDS nodes (for example, the throughput is approximately 8 Mbps with 4 mesh nodes, resulting in poor performance). 

With Wireless Bridge mode, we can keep the throughput at 10 to 15 Mbps. Configuration is simple; just link the Wireless Bridge master to the Wireless Bridge slave. This type of technology allows, for example, several buildings on a campus to be connected to a central office. A central AP can be configured as a "master" device and all the remote stations as "slave" devices. 

Wireless Bridge mode can also connect wireless clients to another SSID, so it can be used in environments where APs cannot be wired.


Wireless Bridge mode



Bridge mode for extra APs

Mesh technologies

Mesh technologies utilize many of the redundancy aspects discussed here.  Mesh technologies are generally considered to be wireless communications systems interconnected with each other.  However, there are two distinct ways to build up a so-called mesh network: WDS and mesh routing. Both of these methods create Layer 2 connections to one or more bridges/mesh routers to allow data to be passed between them.

WDS differs from mesh routing in many ways. Generally WDS has the nature of a more static network configuration without significant demand for redundancy.  That is, a wireless bridge is configured to point to the adjacent bridge with a predefined MAC address.  So when a bridge fails and there is no adjacent bridge configured to serve as a backup path, the link will be lost.

On the other hand, a wireless mesh routing link can provide greater redundancy because it creates a redundancy path in the event of a node failure. In other words, the mesh router automatically detects a new node with the original node fails and dynamically determines the best path.

Mesh routing is often adopted in systems that require higher redundancy.  It often needs few manual confirmations for each node and provides greater expandability when more nodes are to be added in the future. It is also a more suitable choice when the connections are subjected to constant disruptions.

Roaming with redundancy

Newer technology allows users to also roam between access points extremely fast while still retaining redundancy. Roaming has always been available for cellular technologies (i.e., when traveling in a moving vehicle down the highway while constantly maintaining connection), but has only recently become available for wireless as well. As a client moves from one AP to another, the signal strength of the first AP will drop while the signal strength of the second AP will increase.  When the signal strength of the first AP drops below the signal strength of the second AP, we can say the client has roamed to the second AP.  Factors that affect the smoothness of roaming include the topology of the access points, the gain and coverage of the antennas, and the roaming threshold settings of the client. To ensure smooth roaming, users need to first take into consideration the route of the moving object, and carefully plan the wireless AP deployment configuration.

When considering redundancy within roaming, having two separate frequencies becomes more difficult to engineer, due to multiple connections needing to take place at the same time; however, having two separate frequencies allows more tolerance when connecting. For example, if one frequency is having a hard time connecting due to interference, the other will still connect and begin to transmit data immediately.  This type of technology allows for constant uninterrupted communications no matter how fast a client is traveling.  Crucial data (such as control information, updated track information, or ticketing information) can be counted on to arrive at its destination.


Ariana Drivdahl is the product marketing manager for Industrial Wireless at Moxa Americas, Inc. She has been in the industrial automation industry for more than four years and has focused mainly on networking, PLC programming, and motion control. Previous responsibilities included application engineering at Automation Controls and Cal Poly Pomona's IT department working with their wireless network infrastructure. Jackie Shi is with the Moxa Industrial Wireless Business Division.