Industrial Automation Tech Note 15 - TNIN15
Introduction
As the global experts in communication, monitoring and control for Industrial Automation applications, Red Lion Controls has been delivering innovative solutions to customers for a variety of applications. A key product for supporting these applications is the SN & RAM Cellular routers. Many of the targeted systems are in remote, rugged environments that do not have Internet or WAN access. The SN & RAM series of routers solves this by providing reliable Internet and WAN access using cellular technology.
Introducing cellular into the automation world is still in its infancy stage. The convergence of these two worlds allow for many benefits and some limitations, that can be challenging, even for experienced professionals. Some of the benefits of using cellular in automation environments include, but are not limited to:
- Wireless transmission of Modbus data to remote SCADA systems
- Generous data transfer speeds using LTE and 3G wireless technologies
- Data security
The goal of this guide is to assist in achieving the above by highlighting some “best practice” guidelines on device installation, antenna selection and placement, cellular network usage, etc. to ensure the best possible signal and cellular data speeds. The guidelines illustrated in this guide do not have to be followed verbatim, but any deviation may result in degraded network speeds and signal quality so it is highly recommended to follow the guidelines as closely as possible for your specific environment.
Antennas and Signal
This section contains information regarding antennas. Antennas are a crucial piece of the equation when implementing cellular into a network. The wrong antenna can quickly bring data transfers to a crawl or cause data loss because the signal strength is too low or unusable. Not only is selecting the proper antenna important, knowing where to place the antenna and at which angle (depending on the environment), must also be considered for best performance. The purpose of this section is to guide you through these factors in order to ensure the cellular installation has the best signal possible.
Antenna Selection
Red Lion Controls offers a selection of high-quality antennas that are best suited for our portfolio of cellular devices. Each antenna choice was based on upon options available on the cellular device such as Wi-Fi, GPS, etc. Some of these antennas are “all in one” antennas so one antenna is capable of providing all the needed services eliminating the need to purchase multiple antennas. This sometimes is not ideal depending on the device’s location. For example, a Wi-Fi antenna might achieve better signal if placed in a different location than the cellular antenna or the need for an antenna with a higher dBi gain to improve the overall signal. In this case, choosing another antenna than what RLC offers might be a better option.
We do not require that you purchase an antenna from Red Lion however; there are aspects of the antenna to consider should you choose to purchase a different antenna from another vendor. For the cellular main and diversity connections, the antenna must be SMA male. The threads of these antennas will be on the inside with the connector pin in the middle, as shown in Figure 1.
Figure 1.
Wi-Fi antennas are slightly different. They also have the threads on the inside however the connector pin is missing, as shown in Figure 2. The contact pin is found on the SN and RAM device’s SMA connector instead of the antenna. This is known as reverse polarity.
NOTE: The term "reverse polarity" here refers only to the gender of the connector's contact pin, not in any way to the signal polarity. Make sure to understand this distinction before purchasing antennae for your installation.
Figure 2.
Signal and the Environment
The environment plays a significant role in a cellular or Wi-Fi installation. A user must account for many facets beyond their control. Water, glass, hills, and mountains all affect the behavior of a signal. They can cause the signal to reflect or bounce off an object causing the signal to take a different path to the receiver than the original, arriving out of sync, causing data loss. Reflection can occur indoors as well. Metal is highly reflective and if a router is being placed onto a warehouse floor, reflection is almost guaranteed. This is where using multiple antennas can help with the multiple signal paths arriving at the cell router.
Another environmental factor to consider are walls, especially if placing the cell router in an interior wall of a building. The signal propagated from the tower has to go through these walls to get to the cell router. Every wall the signal must travel through will absorb some of that signal causing it to weaken by as little as -4 dBm for drywall and as much as -12 dBm for concrete.
Walls can also cause the signal to refract or bend in a slightly different angle from the original signal causing the signal to, again, take a different path. The signal can scatter in several different directions after coming in contact with rough surfaces or in especially dusty or sandy environments. Even the human body can have an effect on the signal. Humans are made mostly of water. Human bodies can attenuate or weaken the signal.
A user must also take into account other equipment in and around the cell. Most machines give off EMI (Electro-magnetic interference). The cell routers antennas detect this interference and view it as “noise”. This noise can have a significant effect on a signal to the point of being completely unusable.
There are ways to help mitigate these issues so they will have less of an impact on performance and signal quality. Using a higher gain antenna is one such way. Another way is by adding height to the antenna, which can significantly improve the received signal when coupled with a low loss coax cable. Many times, it can come down to simple trial and error. There might be areas on a plant floor that are better suited for receiving signal than others. Sometimes simply moving the antenna to a different location or the device itself can be the difference between a weak or strong signal.
Antenna Gain - dBi
By itself, an antenna does not generate any amount of power. In other words, an antenna generates no power when it is disconnected. An antenna’s gain simply focuses the radiated RF into narrower patterns such that there appears to be more power coming from the antenna in the required direction. In other words, “gain” is a measure of how an antenna can effectively focus RF energy in a particular direction. A perfect antenna, called an isotropic antenna, will radiate power equally in all directions, however; no antenna can physically do that. What an antenna can do is decrease RF radiating from one direction and focus that energy in another direction, increasing the range in that particular direction.
For example, using a 9 dBi gain antenna vs. a 3 dBi gain antenna would add 6 dB to the link budget in each particular direction thus increasing the received signal strength (RSSI). In Figure 3, notice how the higher the gain, the more narrow the signal becomes and the longer the range.
Figure 3.
Antenna gain may be required for installation in other situation. One such example is when a cable is needed between the antenna port on the cell router and the antenna itself, especially if the antenna needs to be placed in a different location than the cell router like near an open window or outside of a building. A will need a low loss cable in these cases. These cables adversely affect the signal by attenuating or weakening the signal as it travels down the cable. The signal attenuation increases along with cable length. Adding a higher gain antenna can help offset that attenuation by adding additional dB to the link budget.
It is tempting to install a signal booster in low signal areas, as well. These can end up being more problematic because they not only amplify the signal but the noise as well and could cancel out any potential increase in signal. Red Lion does not recommend using signal boosters.
WARNING: When increasing gain for any transmitter, whether with boosters or alternate antennas, be sure to have your design checked for FCC compliance. Consult an RF Engineer to see if unapproved situations violate FCC limits.
Directional vs Omnidirectional
Most traditional antennas are omnidirectional. In other words, they radiate RF in multiple directions in a 360-degree plane, as shown in Figure 4. This way, no matter where you might be in a room or area, the signal has a greater likelihood of reaching your host device. In most cases, omnidirectional antennas will meet your installation needs, however, there will be times when a directional antenna may be a better choice. For example, suppose there is more than one cellular tower within a small geographic area. Due to using the omnidirectional antenna, it is going to receive a signal from both towers. This can be problematic if the cellular module keeps switching between the two as the signal fluctuates over time.
In this case, using a directional antenna that can focus the RF by pointing towards one specific tower will receive that tower’s signal while ignoring the second tower’s signal. There are some caveats to consider when choosing this path. Using a directional antenna will mean, if that tower should have problems or become congested with users, the overall received signal will decrease significantly causing the cellular module to possibly disconnect from that tower altogether. With the omnidirectional, the cellular module would scan for a stronger signal and connect to the other cellular tower.
Figure 4.
MIMO vs Diversity
Every SN and RAM cellular router comes equipped with two antenna ports. On the LTE devices, the ports labeled “Antenna” and “MIMO/Diversity”. The meaning between the two is dependent upon the technology that the device is connecting.
As the signal propagates from a cellular tower, it is arriving from all directions and via multiple paths. This causes the signal to fluctuate. These multi-paths causing the overall received signal to decrease because the RF signal received is signal that is bouncing or reflecting off buildings, trees, and other structures and arriving at different times than the main signal, which affects the data transmission and performance, can negatively affect Many times the main antenna’s signal. By adding a diversity antenna and properly spacing that antenna, the signal that was negatively affecting the main antenna is positively affecting the diversity thus achieving better signal, as shown in Figure 5. The user can find the formula for spacing the two antennas in our hardware guide.
Figure 5.
Regardless if using one or two antennas, only one antenna will transmit the signal and, since the signal has to go through environments beyond someone’s control causing lots of multipath reflections, the diversity antenna increases the likelihood that one of the two antenna’s will receive the data correctly. Red Lion highly recommends using two antennas in all installations.
LTE uses a technology called MIMO. This means that the RF signal can be received and transmitted in multiple paths. With only a single signal path (SISO), data can only be transmitted and received, but not at the same time, same as half-duplex in Ethernet. With MIMO, the signal can be received and transmitted simultaneously, as shown in Figure 6 comparing SISO, SIMO (diversity), and MIMO.
Figure 6.
It is highly recommended to use two antennas when connecting to LTE to take full advantage of MIMO. When installing the two antennas, it is very important to point both antennas in opposite directions in relation to each other. For example, if one antenna is vertical, it is important to tilt the second antenna 90 degrees from the other, as shown in Figure 7, in order to take full advantage of the oscillating signal. For paddle and yagi antennas, both can be mounted/pointed in the same direction but turning the MIMO antenna 90 degrees still applies, as shown below.
Figure 7.
RSSI (3G) vs RSRP (LTE)
Measuring signal strength differs depending on the technology used. 2G, 3G, and Wifi use RSSI to measure the strength of a received signal, measured in dBm. The higher the dBm displayed, the stronger the signal. For example, an RSSI of -75 dBm is considered a strong signal with a -60 dBm being even stronger. The lower the detected dBm; the weaker and more unreliable the signal is for data transfers. For example, a detected signal of -98 dBm is borderline poor with -110 dBm being no signal at all. In these situations, it is a good practice to move the antenna(s) to different positions or areas to try to improve the received signal.
Table 1: RSSI Signal Levels
As mentioned earlier, RF signals detected from other devices (noise) can adversely affect RSSI significantly. This noise is called EC/IO. EC/IO is a little different from RSSI in how it is measured. The closer to zero, the less noise detected. You can never fully eliminate signal noise but you can mitigate it by moving antennas around and installing antennas with higher gain to lower the noise floor.
Table 2: EC/IO Levels
LTE does not use RSSI to measure the signal. LTE, instead, uses RSRP to measure signal strength. RSRP is an RSSI type of measurement but, instead of measuring all received signals from all sources, it measures the average received power. With RSRP, a signal of -100 dBm is a decent signal where, with RSSI, it would have been unreliable for data transfers. Table 3 shows the RSRP ranges from excellent to poor.
Table 3: RSRP Signal Levels
Just as with 3G, LTE has a way to measure noise and signal quality. It uses Received Signal Reference Quality or RSRQ. RSRQ is a negative value in dBm and used as a measure for tower handoff when more than one tower is detected. For example, if there are two LTE towers nearby, the two towers calculate the RSRQ and send it back to the serving cell to determine the handoff between the towers.
Just as in EC/IO, if the RSRQ value becomes too high, the quality of the signal starts to degrade, and could result in significant data losses. All of the values discussed in this section are located in the cell routers wireless.cardstats file under Network – Cell Connection – Status under the Card Stats window.
Table 4: RSRQdB Levels
When reviewing signal strength, regardless if using RSSI or RSRP, it is possible that having too strong of a signal can cause just as much adverse effect on communication stability as too little signal. Communication and packet loss issues can be a direct result of having a too strong of a signal. If experiencing such issues, it might be worth exploring adding a signal attenuator to the antenna. An attenuator can bring a signal’s overall strength down by as much as 20 dB, which can greatly improve stabilizing a cellular connection.
Lightning and Surge Protection
If there is one thing that is unpredictable, it is Mother Nature. Lightning storms can spring up at any time and could cause irreparable damage to a cell router if struck by lightning, especially when installed outdoors or if there is an antenna connected on a roof or side of a building. If the cell router installation is in an area where lightning storms are prevalent, it might be wise to consider installing a lightning arrestor. There are different types you can choose from, depending on your preference. One such arrestor is the gas tube suppressors that connect between the antenna and the cell router. The gas inside the arrestor acts as a conductor when a surge occurs by ionizing the gas inside it. The gas then passes the voltage through the ground line until things normalize. After a lightning strike occurs, replace the gas tube, as it will offer no further protection.
High Pass Filter is another lightning arrestor that is often used. One such recommended for cellular use is the DSXL-NS from PolyPhaser. This arrestor has an SMA connection in the inside of the box on the protected side with 400mil diameter lower loss coax cable like NFC 400 or LMR 400 cabling with an N connector at the protector end. Manufacturers do not make a filter based unit with SMAs on both sides so an SMA female adapter for the antenna to connect to on the other end would need to purchased separately. More information about the arrestor can be found at the below link.
http://www.smithspower.com/brands/polyphaser/products/rf-surge-protection/dsxl-ns
Red Lion does not carry any lightning arrestors in our portfolio for use with cellular but many distributors sell them. Proper grounding is required, regardless of the kind of arrestor used.
Cellular Network
Cellular can be a fast and reliable medium for accessing devices and retrieving data if used properly. All carriers have a set of guidelines that they publish that explains how devices should access their networks but also network usage. Too often users expect the reliability of a wired medium from cellular when that is simply not a feasible expectation. Regardless of the agreements made with a carrier, cellular is still a shared medium. A single tower can only handle so many concurrent users and they are all competing for access to the network while sharing bandwidth. Locking down a data channel by constantly streaming data is highly frowned upon by carriers; doing so may violate the usage agreement. This section will explain the ins and outs and recommended cellular network usage and the types of networks available.
Public vs Private Networks
Each carrier has two available IP spaces that they offer for access to their networks: public and private. The public space means exactly that. The IP given can be publically accessible from anywhere with Internet access. Using a public IP makes setting up IPsec and OpenVPN tunnels easy as well as getting access to devices behind the SN/RAM. However, there are some downsides as well, in relation to security, with having a public IP address. Unauthorized users can potentially try to hack into the device by scanning for open ports or attempting to guess passwords to gain access to the router. The SN/RAM’s firewall does an exceptional job of mitigating these hack attempts by giving the ability to block IP’s, allow only certain IP’s, setting complex passwords, or ignore ping requests altogether.
The SN/RAM can keep unauthorized users out but it cannot stop them from attempting, which means data usage can increase quickly if someone attempts to hack into a device. Most attempts are automated scripts that will keep trying hundreds and hundreds of times, which could last for hours. Carriers consider each attempt, even if unsuccessful, data usage. Even if the device is not responding to those attacks, the data usage can quickly increase to gigabytes of data over the course of time, which may go unnoticed until the next billing cycle. Our event engine can send out an alert for excessive data usage, if needed, to keep track of such events in real time.
If the potential for unauthorized access is simply too great of a risk, the other option is to request a private IP. Every carrier has a private IP space for cellular use. When the cell router connects to a cellular tower and requests an IP, instead of receiving one from the public space, it will receive a private IP like 10.x.x.x or 192.168.x.x. With this IP, the SN/RAM can typically access public IP’s on the internet, however, due to the nature of the IP, users cannot directly access the SN/RAM from the public side. From a security perspective, this is ideal because unauthorized users cannot even attempt access, let alone gain it. This also means, without some other method like tunneling, neither will you.
Also, with private networks, if two devices that have private IP’s need to exchange data, that communication will also be blocked by the carrier. For example, if two SN/RAM’s are VPN tunnel endpoints, they will not be able to set up a tunnel without the carrier explicitly allowing the exchange. This is the inherent nature of private networks. The last thing a carrier wants to do is to give someone access to their private network and, by doing so, gives them the same freedoms as they would with a public IP. There are options that the carrier can do to allow two private IPs to talk to each other such as a custom APN. Consult your carrier for the best options available.
Network Usage
Cellular is a shared medium, which means every device, from cell phones to M2M devices to tablets, are all fighting for bandwidth. A cell tower can only handle so many concurrent connections so carriers provide guidelines as to how they want devices interacting with their network and the frequency of that interaction. Carriers do not constantly monitor their networks to ensure these guidelines are met, however, if they detect a device is abusing the network by, for example, locking down a data channel by constantly streaming high bandwidth data, they can and will, disconnect that device from their network. This can result in communication issues and potential data loss. The guidelines below will help minimize or avoid disconnections and interruptions.
Polling:
When polling data, limit the frequency to no more than once every 5 minutes for automated or timer based polls. Poll data when needed and to set up events and alerts for alarm based polls. For manual polling, use an I/O data concentrator like the RAM cellular RTU so all data is polled once from one central location when needed. Carriers do allow for more frequent polling intervals as long as the data is immediately used and the burst of data usage is temporary. In other words, polling for 10 minutes straight is OK as long as there is ample “down” time between the next polling session.
Keep Alive Timers:
Ping Alive, a connection keep-alive service, is also subject to these limits. Set Ping Alive attempts to no less than every 5 minutes. The SN/RAM Ping Alive service offers the option to only ping when an interface is inactive for the ping timer interval. For example, if the ping timer interval is set for 15 min and the idle interface is wwan0; the SN/RAM will send a ping only if traffic is idle for at least 15 min on the wwan0 cellular interface. There is no need to test for an active connection if traffic is flowing, so it is highly recommended to use this idle feature.
IPsec DPD timers:
When using VPN tunnels for secure data transfers, an option called Dead Peer Detection that will periodically send a “hello” packet to its peer (VPN tunnel endpoint) to ensure that peer is still active and responding. Do not send DPD polls more than once every 5 minutes.
OSPF/BGP hello timers:
The SN/RAM cellular routers do come equipped with zebra, a Linux daemon that mimics OSPF and BGP configurations of a Cisco router. Both OSPF and BGP use hello timers to test for communication among its “neighbors”. If a neighbor does not respond, the SN/RAM updates the routing tables for best path selection. Do not exceed one keep alive/hello timer per 5 minutes.
Device Firmware Updates:
Red Lion Controls releases new firmware for the SN/RAM cell routers typically four times a year. Do not schedule over the air firmware updates more than once every 90 days.
Device resets/reboots:
To ensure things are running smoothly a router may need rebooted for unknown reasons or perhaps schedule a periodic reboot. In either case, if there is an active cellular connection, it is recommended to issue the reboot through the SN/RAM’s web UI under Admin – Factory Defaults/Reboot. Doing so sends the appropriate disconnect signals to the cellular network. If a device is unresponsive and GUI access is not available then this reboot procedure is not possible, however, if GUI access is possible, then this is the recommended process for rebooting the device.
SNMP:
When enabling SNMP on the SN/RAM for the purpose of performing an SNMP walk of the device, it is recommended to only run a full SNMP walk a maximum of once every hour and individual GET requests to once every 5 minutes.
Acronyms
| BGP | Border Gateway Protocol |
| dBm | Decibels – milliwatts |
| dBm | Decibels – isotropic |
| LTE | Long Term Evolution |
| MIMO | Multiple In/Multiple Out |
| OSPF | Open Shortest Path First |
| RF | Reference Frequency |
| RSRP | Received Signal Reference Power |
| RSRQ | Received Signal Reference Quality |
| RSSI | Received Signal Strength Indicator |
| SINR | Signal to Interference Noise Ratio |
| SIMO | Single In/Multiple Out (Diversity) |
| SISO | Single In/Single Out |
| SNMP | Small Network Management Protocol |
Disclaimer
It is the customer's responsibility to review the advice provided herein and its applicability to the system. Red Lion makes no representation about specific knowledge of the customer's system or the specific performance of the system. Red Lion is not responsible for any damage to equipment or connected systems. The use of this document is at your own risk. Red Lion standard product warranty applies.
Red Lion Technical Support
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