Wireless Topologies

Feb. 8, 2012
There Are a Number of Considerations to Be Made With Wired Networks, but When Working With Wireless Networks, There Are Even Larger Range of Variables Considered
About the Author
Ian Verhappen is an ISA Fellow, Certified Automation Professional, and a recognized authority on industrial communications technologies with 25+ years experience predominantly in the process control field.It seems as if wireless is everywhere — at least everywhere with population density, or a cup of coffee. The trend is to 4G networks for cellular communications and IEEE 802.11 (Wi-Fi) networks in the coffee shop, and though the speeds of 4G keep improving, IEEE 802 networks keep growing.

Wireless networks tend to fall into one of two topologies. A star topology is a single-hop system in which all wireless nodes are end points and within direct communication range — usually 30–100 m — to the gateway that communicates data and commands to the endpoints.

A true mesh topology is a multi-hopping system in which all wireless nodes are routers and communicate with each other to hop data to and from the individual nodes and the gateway. Nodes in a mesh topology can hop messages among other router nodes without intervention of the gateway.

For most practical sensor network applications, especially in the industrial setting (ISA-100.11a and WirelessHART), the most common installation is a star-mesh hybrid that provides the inherent redundancy of the network between mesh nodes while managing the number of messages and "awake time" required for the star or end-point nodes.

In addition to the topology, people also need to determine whether they are designing a personal area network (PAN), which is typically used for a sensor network (ISA-100.11a or WirelessHART), or local area network (LAN), which will provide the backhaul network or infrastructure to connect the sensor network to the control system.

IEEE 802.11 is the most widely used license-free, wireless infrastructure in the business environment, with the most commonly available protocols being 802.11a/b/g. The backwards-compatible 802.11n  is the latest revision of this series of standards, and has many enhancements over its predecessors.

IEEE 802.11n delivers as much as a sixfold increase in effective bandwidth over 802.11a/b/g. This higher performance and increased reliability come about through the action and interaction of two key technologies: multiple in/multiple out (MIMO) transmit/receive capabilities, and channel bonding.

When you use only one antenna on the transmitter and one receiver, you are subject to multipath interference. MIMO uses the multipath reflections to gain significant signal strength and improve reliability. Improved reliability translates to a greater coverage area for a given data rate, or to higher data rates for a given coverage area.

MIMO allows multiple (two to four) transmitting and receiving antennas to simultaneously multiplex a message over separate transmitting antennas. The receivers digitally process the signal to identify separate spatial bit streams and reassemble them. This multiplexing of different data streams across multiple transmitters significantly increases the effective bandwidth.

Even if MIMO is used only in the access points, because MIMO receiving antenna technology handles multipath, the technology still delivers up to 30% performance enhancement over conventional 802.11a/b/g networks.

Channel bonding is a technique by which two adjacent, contiguous 20 MHz channels are combined into a wider 40 MHz channel. Because the bandwidth on both edges of a 20 MHz channel is typically not used at 100%, channel bonding allows use of both 20 MHz channels as well as this gap between channels, resulting in slightly more than double the bandwidth (Figure).

Channel Bonding

Four 5 GHz 20 MHz channels (left) are bonded to form two 40 MHz channels (right), providing slightly more than double the bandwidth.

Channel bonding can be done only in the 5 GHz band, which has the advantage of not being in the crowded 2.4 GHz band, though at the expense of transmission distance. Signal transmission distance is roughly proportional to frequency, so a 2.4 GHz signal should travel about twice as far as a 5.8 GHz signal, all things being equal. However, as we have just described with MIMO and channel hopping, it is rare that all things are equal.

The highest data rate for 802.11a or 802.11g is 54 Mbps for a single transmitter on a 20 MHz channel. The efficiencies of 802.11n such as MIMO increase the maximum data rate of a single channel to 65 Mbps. By "filling the gap" and multiplexing, a 40 MHz bonded channel on a single 802.11n transmitter results in an overall increased data rate of 135 Mbps.

When building a wireless LAN, it is important that no two access points (APs) operate on the same channel when they are in close proximity. Doing so causes co-channel interference. To avoid this, the APs need to change the channels they use so they do not interfere with each other. In the 2.4 GHz range, 802.11b/g has a 3:1 reuse pattern for usable channels. This 3:1 pattern is the very minimum number that can be used to build a non-interfering network. Using 802.11n with the 5 GHz band and channel bonding allows enough 20 MHz non-interfering channels to get to a 7:1 reuse pattern, or a 3:1 reuse pattern with 40 MHz channels. The actual bandwidth throughput is roughly 50% of the data rate.

One other relevant consideration is that many industrial protocols require support for multicast messages, and many wireless routers do not support this capability.

Just as there are a number of considerations to be made with wired networks, wireless networks have an even larger range of variables to be considered.

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