Automation & Control Technical

A review of secure, fail-safe cable-less connectivity

Iby Loré Holtzhausen, Directech

In the realm of fail-safe, safety, mission-critical or even life-threatening systems, physical copper connections are traditionally regarded as the only acceptable connection method. This comes about since copper circuits can constantly be monitored for ohmic continuity when in the latent, inoperative state with the least complicated monitoring equipment.

Copper wire has therefore always been various systems and corresponding systems on microwave ovens, plastic welding machines, considered to be the only secure, fail-safe the ground. Look out of the window next time diathermy machines, etc. Access to these method of connecting mission-critical devices, and be assured that you will not see any trailing frequency bands is licence-free for users of such as sensors, actuators, “stop” buttons, etc. cables! approved equipment.

The theory underlying this assumption is that if the Modern wireless data communication These former military equipment manufacturers wire is broken or re-routed we will immediately connectivity has progressed a very long way realised that their stealth technology could be become aware of the fact through a change from the noisy reception we experience in the applied with minimal additional development in the continuity of the circuit. This is good and days of Springbok Radio. for general data communication purposes well while simple DC operated devices are in these ISM bands. Their stealth systems will The most important concern with “mission connected to this critical circuit. The picture happily operate in the presence of strong critical” radio connectivity, apart from the changes immediately if we pass an alternating interfering signals while providing error free obvious equipment quality specifications, is current through the circuit and then are suddenly data transmissions that are also very hard to catastrophic radio link failure resulting from “in exposed to the effects of induced interference intercept. band” or “co-located” radio interference. This and/or capacitive coupling effects from interference can be caused naturally or can There are presently a number of systems available other, “foreign” power sources. The continuous be man-made and is a very real concern, not with true stealth capabilities that are particularly monitoring or the presence of a “good” circuit unlike inductive surges and their effects on other suitable for robust industrial data connections on now suddenly becomes a lot more complicated connectivity methods. the 2,4 GHz ISM band. These systems are known since the monitoring devices are in themselves simply as IEEE 802.11 systems. Such systems active and in turn depend on their own power were installed by the author on a pair of bridge supplies and other active, failure-prone Modern wireless data cranes and basket cars at a steel works, stackers elements. communication and reclaimers at a coal loading plant as well Once a certain degree of active element content as a mine-wide network at a large colliery, where is considered acceptable the argument can be connectivity has they are still operating to this day. extended to examining alternative media such The most popular ISM band occurs at 2,4000 as fibre optic cabling instead of copper wires. progressed a very long to 2,4835 GHz and is therefore only 83,5 MHz Metal-free fibre optic cables are completely way wide. The frequency hopping, stealth systems immune to the effects of induced surges or occupies about 1MHz of this band while hopping capacitive coupling effects from adjacent high The military faced this exact dilemma ever since over all, or part, of the band in a pseudo random power AC sources. The terminal equipment at radio communication was first invented. The fashion. At each “hop” it dwells in that spot for either end of the fibre optic cable are however D-Day landing succeeded mainly as a result about 1mSec. The aggregate, usable data active devices that may or may not be capable of the Allies being able to selectively intercept throughput is <1Mbps. Many such systems of being employed to continuously monitor the and/or jam the German radio communications. can be co-located in the same “radio space” quality of the connection. However, during the Cold war of the 60s, 70s with minimal chance of interfering with each other since each system will hop according Beyond fibre optic cabling there is the wireless and 80s the Americans developed a new to a different sequence. Even if they are set to or radio connectivity medium. generation of “stealth” radio communications systems that finally proved to be totally immune the same sequence number they will typically The knee jerk reaction to the mention of radio to interception, interference or jamming by the have commenced the sequence at different connectivity is “ not secure enough, what about enemy, no matter how sophisticated the attack. times. IEEE 802.11 defines 79 such sequences interference, what about the effects of lightning, that statistically provide the minimal number of At the end of the cold war a small number of etc.” possible collisions. manufacturers of this stealth technology sought The proponents of this reaction have not new markets for their products. Fortuitously, there Frequency hopping systems also have the ability paused to think that every time they land in a were a number of frequency bands, known to recognise an interfering signal and then aircraft in inclement weather their very lives, as the industrial, scientific and medical (ISM) “remember” its position in the band, thereafter as well as everybody on the aircraft, depend bands, available internationally. These frequency it simply hops around it. From time to time it on radio connectivity between the aircraft’s bands are designated for applications such as will return to see whether the interference has

64 September 2006 - EngineerIT disappeared or not. It must also be recognised A totally unacceptable 3 dB fade margin will Conclusion. that the message exchange demands an provide a link with a 99,83% availability that will By using frequency hopping spread spectrum “acknowledgement” before proceeding with the also fail for ≥6160 minutes/year. Increasing the transceivers in a radio network, data can be remainder of the message. This means that even fade margin to an almost acceptable 12,5 dB transmitted across that network in a way that though transmission can be interrupted during will provide 99,995% availability and ≥25,35 is as secure as the most simple copper wire one or more successive hops, the message minutes/year “outage”. A mission critical link will connection. This network however, can transmit content will ultimately be delivered, error-free, all be designed with a fade margin of about 23 dB data at a far higher data rate, over longer the while maintaining an aggregate data rate of in which case the availability will be 99, 99999% distances while providing continuous and virtually about 1 Mbps. and the anticipated outage will be ≤0,06 mintes/year. instantaneous reports on the quality of the various The same basic technology is now being connections. applied with the well known “Bluetooth”. This is At the microwave frequencies of the ISM bands also a frequency hopping technology, not unlike a link’s respective antennae must “see” each Consideration must also be given to the other the original stealth systems. The major difference other, unlike transmissions at lower frequencies IEEE 802.11xxx systems that currently fill the data being that Bluetooth has a far higher hopping such as typical FM broadcasts that do not require sheets of all of the major vendors of “industrial rate of about 1 600 hops per second and is true line of sight transmission paths. Antennae wireless systems” typically limited to < 900 Kbps. It also has a are physically small in size at these frequencies As mentioned at the start of this article, the deliberate, severe limit on the distance between while providing considerable path gain. Higher 2,4 GHz ISM band is only 83,5 MHz wide and base and client. This distance limitation is tied to gain antennae also have the advantage that is license-free and available to anyone with the higher hopping rate as well as the original they are very “focussed”, not unlike the beam an approved piece to radio equipment. Be it requirements for very robust, short range, <10 m, of a torch. Since these antennae are highly laptop PC, cell-phones with Bluetooth headsets connections. “directional”, interfering signal sources must be or other industrial device such as bar code In South Africa a 802.11 frequency hopping within the “cone” of the torch beam in order to scanners, etc. It should also be kept in mind that radio link operating in the ISM band, can legally have any significant interfering effect. In order that data rate is directly related to bandwidth. be installed over a single, contiguous premises, to monitor whether a link is operational, some Or, if you like, the higher the data rate over your without a licence, providing the equipment is form of “test” message must continuously be wireless connection, the more of the precious type approved and the maximum legislated sent across the link. Failure to acknowledge the 83,5 MHz you are going to gobble up. output radio power is not exceeded. Distances test message will indicate an impending “soft” or up to 4 km are achievable. catastrophic “hard” link failure. The very popular IEEE 802.11d and g systems respectively provide 11 and 54 Mbps data rates The “reliability” of a radio link can be scientifically The most common connectivity protocol today is according to the sales blurb, but at the same calculated with a very high degree of certainty. TCP/IP Ethernet. With Ethernet terminal equipment time occupy 22 MHz of the 2,4 GHz ISM band “link” or “path” reliability is expressed as “fade the operation of these links are verified with a (typically NOT mentioned in the same blurb). margin”. Obviously, the higher the fade margin simple, widely accepted test procedure known the more reliable the link. The fade margin is the as “ping”. Each node on the network - radios Three carefully planned IEEE 802.11d or g difference between all the losses (free space included - is allocated unique IP addresses. At systems will therefore fit into the ISM band and loss, diffraction losses, terrain factor, climatic the master station a simple “ping” programme in a common radio space and operate as factors, cable and connector losses, etc) and all runs in the background and continuously “pings” specified. If however a 4th IEEE 802.11d/g or the gains (system gain and antenna gain). each and every node in the network. If a particular worse still Bluetooth, wanders into the same node fails to respond to a “ping” it means that radio space, some of the systems will fall over. the node could be busy, at that instance in time, This falling-over often occurs in a very sneaky with data traffic. It will then deliberately ignore the fashion. It is seldom obvious that the cause is “in- “ping” since “pings” are a low priority to the data band interference”, instead the blame is placed stack manager. The monitoring programme will on intermittent PLCs, dubious connections, now mark that node as having failed to respond incompetent vendors, irresponsible junior to the first “ping”. It will again “ping” that node engineers but almost never on the deliberate and it will either respond, having completed its overcrowding of the very scarce public resource data transfer or if it again fails will be marked as known as the ISM bands. “failing”. A third “ping” failure will mark the node The author if this article has spent most of his as ”failed” and sound an alarm accordingly. professional life involved in providing wireless Most Ethernet equipment is also equipped with connectivity solutions in the mining and industrial simple network management protocol (SNMP) arena of Southern Africa. capability. Each node’s SNMP capabilities can His first encounter with wireless data technology send “traps” to a central programme each was Motorola’s INTRAC system and from there time an operation occurs within the equipment on through radio load shedding, county-wide that has been designated as a “trap”. Traps HF SSB networks with digital “Sel-call”, Meteor could comprise simple occurrences such as Burst Communication for the old Dept of Water programmed, legal change-setting through to Affairs, surface and underground vehicle highly sophisticated events such as an attempt to managements systems, etc. enter an incorrect password through a particular port. A central monitoring programme can once Contact Loré Holtzhausen, Directech, again monitor, report and/or react to specific, Tel (011) 708-2202, pre-programmed “traps” or “events”. [email protected] ☐

66 September 2006 - EngineerIT