An MDS White Paper
A Revolution in Radio
We live in a world in which wireless data transmission has become an indispensable part of our common experience in business, in government, and in private life. Consider that radio and television signals have been omnipresent for decades; wireless telephones have become widespread within the past several years. The list is endless. Devices are now so ubiquitous that one is tempted to minimize or disregard the enormous technical complexity lurking beneath the glossy surface of modern industrial design.
Make no mistake; wireless data transmission is a complex matter. The wireless industry operates at the intersection of physics, statistics, computer networking, cartography, and meteorology. Each of these disciplines introduces its own set of challenges and limitations to wireless transmission; the result is a set of tradeoffs involving speed, distance, sensitivity, power consumption, frequency, flexibility, regulatory concerns, and cost. Some of these tradeoffs can be addressed by world class engineering, but others have remained as roadblocks to effective wireless systems design, resulting in compromises that lowered the overall performance of the system, or dramatically increased its cost.
Today, there is a new wireless future on the horizon.
Modern technologies in computer networking and radio frequency design are evolving exponentially. Developments occur so rapidly, in fact, that it has become increasingly difficult to harness a cohesive set of technologies that work synergistically to solve specific wireless challenges. This fact is diametrically opposed to the real-world requirements of the industrial sector, which dictate that systems must function reliably for decades, and yet meet demands that grow in tandem with changes in those discrete technologies.
The MDS answer to this challenge is Purestream - a revolutionary framework for technology integration that guarantees robust performance, outstanding flexibility, exceptionally long operational life, and maximum platform extensibility.
The Purestream framework harnesses five interdependent technologies that together govern the operational capabilities and characteristics of an RF device. These are:
RF Robustness - Signals are highly resistant to environmental factors, multipath fading, and interference.
Transmission Optimization - Variable modulation techniques ensure optimal transmission efficiency, regardless of the spectral environment.
Real Time Services - Sophisticated Quality of Service (QoS) and prioritization mechanisms ensure that latency-sensitive data are given the bandwidth they need, when they need it.
Advanced Security - Robust, multi-layered security mechanisms protect your network against attack.
Power Management - Low power consumption options offer solar-friendly installation and operation.
Industrial Design - Industrial grade specifications and high Mean Time Between Falire (MTBF) ensure reliable operations measured in decades, even under the most demanding and extreme conditions.
Wireless environments are chaotic. The further a wireless signal must travel, the more susceptible it becomes to interference from nearby or very strong signal sources, and the weaker the signal becomes. ‘Robustness’ in this context, then, refers to the ability of a signal to travel long distances, and to withstand the RF environmental factors that are normally present. This means exceptional noise resilience, the ability to effectively handle signal fading, and the ability to capture weak signals, even when in motion.
Purestream specifies three specific technology ‘building blocks’ that address the need for RF Robustness. OFDM and FHSS address RF transmission, while Diversity addresses RF reception. Let’s look at these in detail.
Orthogonal Frequency Division Multiplexing (OFDM)
OFDM is a sophisticated technology for using spectrum that permits reliable, accurate, high-speed RF transmission in a ‘noisy’ or crowded RF environment.
OFDM defines the ‘landscape’ of a wideband channel, and provides mathematical rules for the use of that landscape. Essentially, OFDM divides a wideband channel into many narrowband sub-channels that each transmit data independently. In and of itself, this is not particularly noteworthy. But the magic of OFDM is that it does so with narrowband channels that actually overlap without interfering with each other. (Hence the use of the term ‘orthogonal.’)
OFDM is far more reliable and accurate than other types of over-th-air protocols, particularly in the domains of interference, efficiency, and mobility.
Interference Robustness: In crowded RF environments with many signal sources competing for adjacent or overlapping bandwidth, OFDM signals can carry data successfully, even at high data rates. OFDM signals are more resistant to noise, meaning that they can be successfully transmitted over longer distances. As a practical matter, this means that fewer transmitters (access points) are needed to cover a given amount of geography, and the overall cost of deploying a system decreases.
Spectral Efficiency: OFDM permits very high data rates to be achieved within allocated spectrum. In short, it’s fast. Really fast. Transmission of real-time services such as video and voice, and high-speed data services become practical.
Mobility: Mobile receivers greatly exacerbate the challenges of multipath fading. Essentially, when a receiver is in motion, the spectral environment changes constantly. This is also true to a lesser extent for fixed wireless transmissions, but in a mobile environment the rate of change can each become very large, as anyone with a history of cell phone usage will immediately recognize. When the rate of change exceeds the modulated data rate (also called the symbol time), the signal environment becomes hopelessly chaotic, and no data transmission is possible. No modulation scheme can control the rate of change, as that is an unpredictable environmental factor; OFDM, therefore, addresses the problem by lengthening the symbol time. As long as the symbol time remains substantially longer than the rate of spectral change, successful transmission is possible.
Frequency Hopping Spread Spectrum (FHSS)
OFDM technology maximizes data throughput, minimizes the effects of interference, and mitigates the influence of noise within the channel. For a variety of reasons, however, an entire wideband channel will, from time to time, become unavailable. Frequency Hopping is one particularly effective method of addressing this challenge.
FHSS works by continually changing the wideband frequency in use according to a predetermined pattern that has been agreed upon in advance by the sending and receiving radios. That is, the transmitter transmits in one wideband channel for a period of time, and then hops to an entirely different wideband channel, where it transmits for a while, and then hops to a third, and so on. If the frequencies on any one wideband channel are saturated or otherwise unavailable, only a small amount of transmission time is lost, and successful data transmission can resume after hopping to a clear channel.
An important side benefit of FHSS is that it is highly secure. That is, unless you know the exact hopping pattern and the timing of the hops in advance, it is impossible to listen to and decode the transmission. FHSS also permits the simultaneous use of unregulated RF spectrum by many radio transmission sources without interference, and is thus a highly efficient technique for allocating a limited amount of spectrum among many users.
As a practical matter, radio signals multiply. In a typical real-world scenario, what begins at the transmitter as a single signal will end at the receiver as many signals that arrive at slightly different times with varying intensity. This is because radio signals, like sound and light, are waves, and behave according to well-understood properties of wave mechanics. Waves can be reflected, refracted, or scattered by environmental obstacles such as buildings and trees. From the point of view of the receiver, therefore, a typical transmission arrives from many sources, rather than from one; from the point of view of the transmitter, the transmission takes not a single path to the receiver, but many. This phenomenon is called multipath interference.
The presence of multipath interference means that at a given moment in time, at a specific physical location, a specific radio frequency might fade. If the fade is too large, the signal becomes too weak to be reliably received. The point at which this occurs is called the fade margin. In practice, the lower the fade margin, the better; a low fade margin provides for a highly sensitive radio that can receive and understand even very weak signals.
Purestream specifies three techniques that work to lower the fade margin of a radio. Collectively, these techniques fall under the rubric of diversity. Diversity techniques function by introducing redundancy into the transmission and reception of a radio signal. Remember that multipath fading is a function of frequency, location, and time. Diversity techniques, therefore, can be applied to the same three areas in order to combat multipath fading.
Frequency Diversity - Multipath fading is frequency-specific. Therefore, at a given moment in time, if one frequency experiences a fade, it is likely that another frequency will not. Frequency diversity, therefore, is the technique of sending the same data on different frequencies simultaneously. The frequencies chosen are far enough apart to minimize the likelihood that both will fade at the same time.
Spatial Diversity - Another component of multipath fading is location. The combinations and effects of wave intersections change from location to location; therefore a frequency that is faded at one location is likely to be visible at other. Spatial diversity is the technique of using more than one antenna to receive an RF transmission. The antennas are physically separated by a distance that is large enough to minimize the probability that both antennas would experience signal fades simultaneously.
Temporal Diversity - The last factor in multipath fading is time. The fading characteristics of a spectral environment will change with time, particularly if the receiver is in motion. Temporal diversity, then, involves repeating a transmission over a specified period of time until an acknowledgment is received.
In order to send data wirelessly, a carrier signal must be altered, or modulated to encode data on the carrier in a way that both the sending and receiving radio understand. AM and FM radio are simple examples; AM encodes information by modulating the amplitude (strength) of a carrier signal; FM does so by modulating the frequency of signal. Phase modulation is also possible (for you math types, phase is the integral of frequency, and can be thought of, therefore, as a slightly different form of frequency modulation). These three domains - amplitude, frequency, and phase - constitute the sum total of available options for modulating a carrier signal. It sounds simple, but tremendously sophisticated modulation schemes involving one, two, or even all three of these methods have been developed in order to maximize the robustness of the signal.
Every type of carrier modulation has strengths and weaknesses. Typically, these involve tradeoffs between spectral efficiency (the ability to send larger amounts of data with smaller amounts of bandwidth) and interference resilience. Higher-order modulation schemes such as 64 QAM typically require a greater signal to noise ratio for successful data transmission. As transmission distance increases, their performance will decrease. Lower-order modulation schemes, such as QPSK, do well over long distances, but at the cost of slower speed and reduced throughput.
Purestream offers the ability to sample an RF transmission environment in real time, and then dynamically select and adjust to the best modulation scheme for that set of conditions.
In any given spectral environment, there is an ‘optimal’ transmission rate, which can be defined, roughly, as the fastest sustainable data rate at given reliability or bit-error threshold. Many factors contribute to this rate, including bandwidth efficiency, the modulation technique in use, error correction techniques, receiver sensitivity, antenna gain, and transmission power. Purestream permits dynamic adjustment of these factors according to real-time analysis of the RF environment in evidence. For example, if exceptionally noisy conditions prevail, QPSK modulation might be selected, rather than a higher-order technique such as 64 QAM in a given set of narrowband channels. QPSK has a lower symbol rate than 64 QAM, but better noise resilience; the result is that a slower signal gets through in conditions where a faster signal would not.
Purestream also makes use of a variety of Forward Error Correction (FEC) techniques in order to maximize the reliability of the signal and minimize the need for retransmission. The purpose of any error correction mechanism is to ensure that the data that is received is identical to the data that was transmitted. FEC techniques do this by placing small amounts of additional data into the transmission stream. This additional data is designed to describe the data stream in such a way as to identify transmission errors when they occur, and to assist the receiver in reconstructing the transmitted data stream accurately when errors do occur. No FEC mechanism is foolproof, and different types of mechanisms excel at handling different types of data streams. For this reason, Purestream makes use of several techniques, including Verbisi, Reed Solomon, and retransmission, according to the needs of the data stream being transmitted.
Thus far, we have been concerned with how Purestream facilitates robust RF transmission. In modern networking and transmission environments, however, it turns out that we must be cognizant of what is transmitted as well. Certain types of data, such as video or voice transmissions, can require large amounts of bandwidth, because they are extremely time sensitive. That is, even small variations in the smooth delivery of the data (a phenomenon known as jitter) can cause problems for the end user. Other types of data transmission may be bandwidth intensive, but not jitter-sensitive. A good example of this would be the transmission of a high-resolution static image. Still other types - messaging services, email, polling, etc., are neither bandwidth-intensive nor jitter sensitive.
In serving the growing data needs of modern public and private sector organizations, wireless transmission systems must accommodate each of these broad categories of data with equal facility. Purestream provides two mechanisms to do so -- Quality of Service (QoS) and Media Access Control (MAC).
Quality of Service
In the absence of QoS capabilities, access to the transmitter is more or less democratic. Broadly speaking, data is transmitted on a first come, first served basis, and retains the channel until transmission is complete. QoS, by contrast, permits a system administrator to prioritize data stream transmission - that is, to give preferred access to certain data streams over others under certain conditions. Priority can be allocated on the basis of a variety of factors, including IP address and IP port.. As a practical matter, this might mean that wireless video requests are always given large allocations of bandwidth, or that transmissions to and from Chief of Police are always given priority access to the transmitter. QoS, then, can be thought of as the ‘rules’ that govern access to the wireless system.
Media Access Control
Media Access Control (MAC) implements those rules in real time. The Purestream MAC provides scheduling and allocation services to data streams requesting access to the wireless transmitter. It is responsible for analyzing the source, destination, and type of data, and for deciding which streams get access, when, and for how long. The MAC, therefore, is an indispensable component of providing QoS functionality to a wireless transmission system. It helps to reduce packet latency and minimize jitter for time-sensitive transmissions, while providing comprehensive support for traditional polling networks.
By definition, wireless transmissions should be easy to detect. They’re supposed to be - that’s the point. The trick has always been to facilitate reception only by those radios permitted to do so. Security in wireless transmissions, therefore, has always been of paramount concern, and it has not been uncommon for the wireless industry to lead the development of security mechanisms that then find their way into network security standards. Industrial-grade security adds a further dimension to the problem, and industrial wireless vendors have often been constrained to offer proprietary solutions for over-the-air security for wireless transmissions.
In recent years, however, publicly available security and encryption standards have become sufficiently robust that expensive proprietary solutions are no longer deemed necessary, although they continue to provide capabilities above and beyond the standards.
Purestream specifies, at minimum, the use of 128-bit AES encryption algorithms using the Wi-Fi Protected Access (WPA) standard, and extends these capabilities through the use of key management and RADIUS authentication. Purestream also specifies physical-layer protections to prevent unwanted access through physical interfaces on the radio itself.
The sum total of these measures is the strongest security available anywhere in the wireless industry. Period. Your system will not be compromised.
Purestream offers options for low power consumption operation. This capability provides solar-friendly installation options where external power sources may be prohibitively expensive to tap, or simply unavailable. This is a critical requirement for remote installations without ready or convenient access for inspection and maintenance.
What is an ‘industrial’ solution? The short answer is that industrial capabilities are about robustness -- in basic capability, in performance, in survivability, in security, and in deployment and operational costs. Think of Industrial solutions as consumer-grade solutions on steroids. They offer superior operational capabilities, over a wider range of operational environments, than their consumer-grade counterparts.
Industrial enhancements such as additional reliability and security may be unimportant to the casual Wi-Fi surfer in a local coffee shop, but they are critical for business and government agencies with time-sensitive, mission critical, or highly classified data to transmit.
Industrial packaging is intended to ensure that individual units can withstand tremendous mistreatment by the elements and/or the operational environment. Frequently, wireless transmission must be accomplished in conditions that consumer-grade packaging cannot tolerate. The extreme heat of a desert in summer; the bitter cold of a alpine mountaintop in winter; highly variable, dangerous conditions found on the factory floor in heavy industrial environments. High-grade metal enclosures and robust internal circuit designs ensure operational resilience, as well as resistance to accidental mistreatment by humans.
Industrial packaging results in some important capabilities for wireless infrastructure:
Radio infrastructure can be installed and operated even in remote or environmentally unfriendly locations that consumer-grade electronics cannot tolerate. This means greater coverage capacity, as well as enhanced flexibility in designing and engineering a robust wireless infrastructure.
In order to facilitate extremely remote installations, industrial technologies must often be solar friendly, due to the absence of an available electrical grid for power. Radios must be therefore be designed to consume extremely low amounts of power while simultaneously providing excellent range and sensitivity. Low-power operations is rarely, if ever, an important consideration for consumer-grade equipment.
Mean Time Between Failure (MTBF)
Industrial packaging provides much lower Mean Time Between Failure (MTBF) rates than consumer-grade options. Commercial MTBF for wireless equipment averages about five years; typical industrial MTBF is closer to 35 years - a seven-fold difference.
In practical terms, this is an enormous difference. Consider that it is not uncommon for an industrial radio infrastructure to contain many thousands of radios. MTBF rates can mean the difference between an manageable and and unmanageable infrastructure. In an installation of 1500 radios, for example, commercial MTBF would generate as many as 300 service failures per month. With industrial-grade MTBF, that figure falls to about 20. Operational costs, therefore, are dramatically and directly impacted by MTBF rates.
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