Security considerations for wireless implementation

Whether specifying a new wireless installation, expanding an existing implementation, or evaluating wireless protocols, the security issues are similar. Understanding them can improve the security and signal quality.

11/29/2011


Considerations for successful wireless implementations include RF technology, security, interference rejection, sensitivity, power management, and the ability to embed wireless into existing OEM technologies. However, of the factors in applying any wireless technology to an industrial environment, security is widely seen as the most significant barrier to industrial wireless adoption.

The most common bands used for industrial wireless are in the ranges 902-928 MHz, 2.4-2.4835 GHz, and 5.725-5.85 GHz. The two most common spread spectrum modulation methods used in these bands are frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS).

Rather than transmitting over a static spectral segment, FHSS radios pseudo-randomly vary carrier frequency, quickly hopping through multiple channels while sending data. Interference is avoided by hopping over different frequencies, each of which has a different interference effect or characteristic. This provides FHSS with collision-free access by allocating a specific time slot and frequency for its transmission.

DSSS spreads a narrow-band source signal by multiplying it with a pseudo-random noise signal. The resulting signal is then spread over a large range of continuous frequencies. This introduces redundancy into the transmission, enabling a receiver to recover the original data even if parts of it are damaged during transmission.

Overall, spread spectrum has three significant advantages over fixed frequency licensed radio transmissions. The first is that the user needs no FCC license. The second, specifically relating to FHSS, is its lack of susceptibility to interference. This is crucial in an industrial plant environment where machinery and other equipment generate interference over a very broad spectrum of frequencies. The third, also relating to FHSS, is better security.

Since, as the radios communicate, their communication frequency is changing rapidly (as much as 1,000 times per second), FHSS provides an additional layer of security by making transmissions very difficult to detect. To outside listeners, transmissions simply look like noise spread over the spectrum, with only a small signal at any one given frequency. Additional data security is gained through a 128- and 256-bit advanced encryption standard (AES).

High data integrity

In addition to security, data integrity is paramount. As with existing data transmission over wire, packet protocol acknowledgment is supported by error checking. Error checking is designed to ensure the data received by any spread spectrum radio is not forwarded from its buffer until it is acknowledged as a correct transmission, guaranteeing what is received is identical to what is sent. In order to accomplish this, a cyclic redundancy check (CRC) is generated, giving the packet a unique digital signature.

The specification of a receiver’s sensitivity is also important. The more sensitive the receiver, the weaker the transmitted signal can be and still get through. In other words, the distance and obstructions between a transmitter and receiver can be greater. One reason that receiver sensitivity may be confusing is that it is expressed in a unit of measure known as a decibel (dB). A decibel is a ratio expressed on a logarithmic (exponential) scale. A 10:1 ratio is 10 dB, and a 2:1 ratio is 3 dB. A 1:1 ratio is 0 dB, while ratios of less than 1:1 are expressed as negative numbers. For example, a 1:2 ratio equals -3 dB. Because receiver sensitivity indicates how faint a signal can be successfully received by the radio, the lower power level, the better. This means the larger the absolute value of the negative number, the better the sensitivity.

Line of sight

Is a clear line of sight required for safe and secure radio links? No, but it doesn’t hurt. Radio waves travel through a variety of objects with different levels of attenuation. The area over which the radio waves propagate from the antenna is known as the Fresnel zone. Like the waves created by throwing a rock into a pool of water, radio waves are affected by the presence of obstructions and may be reflected, refracted, diffracted, or scattered, depending on the properties of the obstruction and its interaction with the radio waves. This is often how the signal gets to the receiver when there is no line of sight. However, this effect attenuates the signal, and affects how a radio will operate without line of sight. Proper use of antennas and the ability to adjust output power can provide a great degree of assistance in overcoming these issues and getting messages through. Industrial quality directional and high-gain omni-directional antennas allow radio communications at long distances through a crowded industrial facility. At the same time, the use of low-gain antennas can be used to keep radio signals from straying unwanted distances or directions.

Flexibility is another important consideration, not least because wired networks lock users into one particular protocol. With a well-planned architecture, it is possible to operate several protocols operating over the same communications layer, resulting in greater user flexibility. Any wireless device needs to tie into existing control systems. Getting information into a myriad of existing control systems is no small task. Switch closures and 4-20 mA signals are universally translatable. Digital input allows more data flow at significantly lower cost, but generally adds a level of complexity to any system. Modbus and OPC servers offer degrees of acceptance where large data flows are required.

Note also that products intended for industrial applications should use industrially rated components and therefore operate reliably over typical industrial temperature ranges (-40 to 75 °C). Temperature extremes are commonplace in many applications. In addition, these products are generally better constructed than consumer devices and continue reliable operation under shock and vibration conditions.

Finally, intrinsic safety certification is required for operation in some hazardous environments. Most industrial wireless modems carry appropriate certification to allow radio operation in the presence of flammable or explosive gases, fluids, or vapors. Having such certification is also beneficial because a user can standardize on a single type of device and use it for many applications, regardless of the environment.

Understanding the parameters for security and integrity in a wireless implementation will help new and current wireless users capitalize on the main application areas for wireless: wireless I/O, safety, site security, workforce mobility, mobile asset and material tracking, and increasing the scope of on-site networks.

Gary Williams is principal consultant and practice lead, wireless EMEA, for Invensys Operations Management.



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