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Four Major Considerations for Wireless Network Planning

Wi-Fi is now present everywhere and people have become comfortable using it for just about any application. For example, many system designers are choosing to move their industrial applications from wired Ethernet to wireless utilizing Wi-Fi. The advantages of moving to a wireless design are numerous. Wireless allows for client devices to be located in difficult to reach areas, and it saves on installation and wiring costs in remote locations. The benefit of not having to re-wire your plant infrastructure when relocating machines on the plant floor is yet another advantage. Finally, it is easy to expand as additional devices are required. There are many different Wi-Fi technologies that can be used in industrial applications. Whether it is an existing application or a new design, an engineer must look at the application requirements to make informed decisions regarding which technology to use. When deciding to move from a wired to a wireless infrastructure, we have selected four major tips (access more tips at the end of the article) this month that will help ensure a successful wireless network design.

Tip 1: Know your application’s throughput requirements

Knowing the data rate that an application’s client devices will send and receive at is critical for planning the network. This information will be used to determine things like which Wi-Fi technology to use, and how many access points will be required to provide the desired throughput. In a Wi-Fi system, the actual throughput that is available to your application is generally about 50 to 60 percent of the overall bandwidth of the Wi-Fi technology selected. This is due to control and management overhead in Wi-Fi systems. If your application needs more than what is available, then the application will not work.

Spec Max. bit rate (Mbps) Approx. application throughput (Mbps)
802.11a 54 26
802.11b 11 6
802.11g 54 26
802.11n (2x2) 300 80 - 100

Tip 2: Plan for throughput

Knowing how much throughput your client application needs is critical in planning how many access points you need to install. Just because you have selected 802.11n and can support up to 300 Mbps of available bandwidth does not mean that every signal from every access point will transmit at 300 Mbps. The farther a client radio is from the access point the weaker the signal will be, and as the signal gets weaker, it gets closer and closer to the noise floor. The difference between the signal and the noise floor is the signal to noise ratio. A high signal to noise ratio means that a higher performing modulation and coding scheme can be used by the radios. As the signal to noise ratio degrades, the radios must utilize a lower performing modulation and coding scheme to deal with the interference on the channel. This is called dynamic rate switching. This means that the farther the client is from the access point the lower throughput it can achieve. If your application throughput needs are high, then you must design your wireless network with more access points in closer proximity to the client radio’s location, since doing so will ensure that the radio will be able to sustain higher data rates.

Throughput (Mbps) decreases as the client gets farther from the access point

Tip 3: Know how many clients will be accessing the Wi-Fi network

This tip goes hand in hand with the application throughput requirement discussed in Tip 1. Wi-Fi works in a manner similar to how Ethernet hubs work. That is, Wi-Fi uses a single collision zone for all the devices connected to the Wi-Fi network. This means that only 1 device can transmit at a time, and all the other devices must wait until the network is idle to transmit. When multiple devices try to transmit at the same time, there is a higher chance that collisions could occur. This causes latency in the network. If you plan to connect many devices to your industrial wireless network, then you should also plan to add multiple access points to allow different clients to use different access points, and thereby reduce the number of collisions on the wireless network. For typical wireless applications, it is generally not recommended to have more than 20 to 25 clients sharing the bandwidth of a single access point.

Additional access points increase the overall available throughput

Tip 4: Design your channel allocation plan

When using more than one access point or when there are other access points in the area, it is important to identify the channels that will be utilized on each adjacent access point. When doing this, it is important to avoid using the same channel as an adjacent access point. In the 2.4 GHz range only three channels do not overlap. These are channels 1, 6, and 11. If at all possible, make sure to stagger the access points in these channels. This will minimize a kind of interference known as co-channel interference. When using the 5 GHz range, each channel is non-overlapping if you’re using a 20 MHz channel width. This is known as HT20 mode. If the access point is using 802.11n with 40 MHz channel bonding (known as HT40), then you need to stagger access points every two channels to avoid overlap.

Use channels 1, 6, and 11 to avoid overlap in the 2.4 GHz spectrum

No overlap for 20 MHz channels; skip channels when using 40 MHz channel bonding in the 5 GHz spectrum

In addition to the convenience of mobile connectivity, benefits of wireless communication also include lower cost and faster deployment. Choosing a suitable wireless technology for a particular application is just the first step towards wireless connectivity, and building a reliable wireless network with optimized performance will require the right combination of antennas, RF cables, and connectors. For more useful tips on wireless network planning, please download the white paper here.

Moxa also offers an Industrial Wireless Guidebook to provide fundamental wireless knowledge and basic guidance for building a reliable wireless network with optimized capabilities. For more information on how to access the Industrial Wireless Guidebook, please visit here.

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