WHITE PAPER

CBTC OVER WI-FI: GATHERING CLOUDS

Prepared by: Rodrigo Álvarez Practice Director, Telecommunications September 2014

www.railsystemsaustralia.com.au Summary unlicensed spectrum band meant that it was available This paper explores the challenges that modern for simple, low power, short range applications Communications-Based Train Control (CBTC) without involving the hassle (and the cost) of systems pose to IEEE 802.11 Wi-Fi radio networks. obtaining spectrum licenses from national regulators. We will describe how Wi-Fi, in spite having been designed as a “best-effort” system, became the radio Wi-Fi’s success exploded at the turn of the bearer of choice for a safety-critical and mission- millennium. Millions of laptop computers started critical application such as CBTC. We will explore the to incorporate Wi-Fi enabled network cards by limitations inherent in the IEEE 802.11 protocols with default. That brought Wi-Fi coverage into public regards to range, mobility, radio resource access, spaces, then into our offices, then into our homes. Quality of Service and interference. We will then briefly describe some potential mitigations, and we Nowadays, even a cursory inspection at will mention the latest development in the industry any urban location is likely to produce that are moving towards a replacement of the Wi-Fi- dozens of Wi-Fi Access Points within range. based CBTC radio fro an LTE system. Since Wi-Fi was such a convenient, cheap, familiar 1. Introduction radio system, it became an obvious candidate to At the turn of the Millennium, a great success become the broadband data bearer new CBTC story was brewing in the Mass Transit sector. systems were looking for. A number of successful The 1980s and 1990s had seen the deployment implementations around the world very soon meant of Transmission-Based Train Control (TBTC) that any CBTC system based on a more expensive, systems, which provided train control functions licensed band data radio technology was necessarily through on board computers that maintained at disadvantage against proven Wi-Fi solutions. All constant communication with wayside equipment suppliers were soon developing Wi-Fi-based CBTC through induction loops installed on the trackside. solutions.

The next technological steps led towards modern But could this marriage apparently made in heaven Communications-Based Train Control (CBTC). hid in its midst a flawed heart? Mass Transit signalling equipment suppliers started to provide moving block systems with high This paper will explore some of the in-depth technical grades of automation. At the same time, it became characteristics of Wi-Fi networks and the challenges clear that it was very desirable to reduce the these may present to CBTC systems in the future. We dependency on expensive, complex to install and will discuss how these challenges, if not addressed in difficult to maintain induction loops, and replace the short term, could potentially develop into a larger them with a reliable digital radio system as the issue. Finally, we will attempt to establish a dialogue main train-to-wayside means of communication. for the next steps in the CBTC systems road map.

All this drove the requirement for a new radio data 1.1 FROM TBTC TO CBTC bearer for new CBTC solutions. At the time, there 1.2 1980s – The Space Shuttle, Perestroika and… was a technology based on the IEEE 802.11 set TBTC of standards (soon marketed as “Wi-Fi”) that was After increasing city population density and the then on the brink of virtually conquering the world. 1973 OPEC Oil Crisis ended the Golden Age of the Automobile, public administrations around the world Wi-Fi had initially been designed to make use of the started to re-examine the train control systems in Industrial, Scientific and Medical (ISM) radio band their Mass Transit networks, and how they could made available by the U.S. Federal Communications cope with an increasing demand for fast, efficient Commission (FCC) around 1985. The goal had and reliable public transport. initially been to provide a wireless alternative to cabled Ethernet and Token-Ring Local Area Networks (LAN). The existence of these needs, coupled with advances In other words, it was a low-cost, short range radio in electronics and computer design, gave birth to a technology to link up desktop computers in a room. number of new proprietary Mass Transit train control systems. The first of these new systems may have In spite of these humble beginnings, Wi-Fi had within been the Automatic Train Operation (ATO) system it the seeds of one of the most successful radio deployed on Barcelona Metro Line 2 and on London technologies of all time. The fact that it could be Underground Victoria Line in the early 1960s [1]. deployed in what soon virtually became a worldwide

Copyright 2014 // Rail Systems Australia. All rights reserved. 2 Many of these systems used non-continuous or their scrap metal value. semi-continuous induction systems to provide a But maybe the critical problem presented by IL communications link between train and wayside. system is one of capacity. As long as train control Therefore, some systems were at the time was linked to trackside infrastructure and track labelled “Communications-Based Train Control” occupancy detection, the capacity limitations of systems. However, to differentiate them from more a signalling block system could not be physically modern systems that tend to use continuous radio overcome. The application of a moving block communications, we will use the ex post facto signalling principle would open the door to capacities label of Transmission-Based Train Control systems in excess of 30 trains-per-hour, but this would require (TBTC), reserving the term CBTC for those terms continuous train-to-wayside communications that that explicitly comply with standard IEEE 1474.1 [2]. was more easily achieved through a dedicated data radio system. TBTC systems, many of which are still operational at the moment of issue of this paper, are in most cases Because of these reasons, by the turn of the century, based on the installation of a near-field Induction what was once a group of proprietary products Loop (IL) in the four-foot transmitting information to and technologies almost exclusively based on antennas mounted under the train. Because of this induction loop train-to-wayside communications had reliance on induction loops, another frequent term to started to move very strongly towards radio-based refer to these systems is IL – CBTC. communications.

By the late 1980s, all major signalling suppliers 1.3 RF-CBTC where providing TBTC equipment. An in-depth In the early 1990s, the industry was ready to explore market research of al of these products falls beyond the possibility of using a radio bearer to provide the scope of this paper, but Table 1 below presents a the continuous communications channel needed to brief non-exhaustive list of some of these products, advance CBTC technology. The first radio-based RF- merely to aid the reader where further exploration of CBTC (Radio-Frequency) system implementation the subject is deemed necessary: seems to have been Bombardier’s CITYFLO 650 in February 2003 [3], namely the AirTrain Automated Supplier Product People Mover in San Francisco Airport. This was followed by a number of increasingly complex Alcatel SelTrac systems in the next years, to the point that all major Alstom URBALIS 300 signalling suppliers developed their own radio-based Bombardier CityFlo 650 CBTC solutions, as indicated in Table 2 below. CSEE OURAGAN GE CBTC / ITCS / AATC Supplier Product Siemens Meteor / SACEM / LZB 80 Alcatel SelTrac Westinghouse TBS 100 Alstom URBALIS 300 Table 1 - TBTC Products and Supplier Bombardier CityFlo 650 Induction loops, however, have a number of important CSEE OURAGAN drawbacks. Due to the need to position them on the GE CBTC / ITCS / AATC running line, between the rails, they are expensive to Siemens Meteor / SACEM / LZB 80 install and costly to maintain. Westinghouse TBS 100

Besides, induction loop systems consist of a leaky Table 2 - CBTC Products and Supplier coaxial cable placed in parallel to Over Head Lines All these proprietary, independent, unrelated or Third Rail traction systems. This, coupled with systems were designed to use a radio bearer as the use of a 56 kHz carrier signal, leaves train- the communications link between train borne and to-wayside communications exposed to Electro- wayside equipment. It could have been expected that Magnetic Compatibility (EMC) issues related to each one of them would have been built over its own their close proximity to high-voltage traction power proprietary, independent, unrelated radio technology. equipment. At the same time, the induction loop But hidden forces produced an unexpected result. can not be effectively screened without affecting the basic functionality of the TBTC system. In the early 1990s, a number of proprietary CBTC In addition to the problem outlined above, induction radio solutions did indeed start to proliferate. These loops are also frequently vandalized or resold for proprietary radios proved relatively expensive, and

Copyright 2014 // Rail Systems Australia. All rights reserved. 3 they ended up being so well adapted to a specific 1.4 Enter IEEE 802.11 (Wi-Fi) CBTC solution that their potential market was In the early years of the 21st century [4], at the reduced to supporting that single application, making same time as CBTC suppliers were looking for a them even more expensive to operate and maintain. better radio bearer solution, a new short-range wireless broadband technology was experiencing an In 2004, during the deployment of the Las Vegas unprecedented success. And, surprising as this may monorail, Alcatel (now Thales) decided to select a seem, the reasons for its success had initially more different data radio to support its SelTrac system. to do with governmental administrative decisions The choice fell upon a system based on standard than with technical or commercial ingenuity. IEEE 802.11 – generally marketed as “Wi-Fi”. The success story that is Wi-Fi started in 1985, The advantages that this choice brought upon were when the United States Federal Communications manyfold. The radio units themselves were much Commission (FCC) publicly issued a decision to cheaper and readily available than any proprietary – make unlicensed radio communications available in or even COTS – radio had been in the past. What is certain portions of the radio spectrum that had been more; the pool of potential suppliers was very, very previously reserved for Industrial, Scientific and large, so companies could select the product that Medical (ISM) purposes. best suited their systems at very competitive prices. The ISM frequency bands that were allocated for In addition, the existence of the “Wi-Fi Alliance” unlicensed radio usage were already in use by a ensured interoperability amongst different Wi-Fi number of non-communications related application suppliers for the consumer market, so choosing a s, including microwave ovens, car alarm sensors, given supplier did not necessarily create a long term cordless phones, baby monitors, industrial process dependency. And the small physical dimensions of machinery and medical equipment. So the only way access points let themselves very well to deployment to use these crowded areas of the spectrum for in tunnels, since they could be fixed to all sorts of radio communications purposes was to use spread existing structures. spectrum modulations.

Jumping onto the Wi-Fi bandwagon allowed the Spread spectrum modulations, due to their low industry to place itself onto a product with an power profile and their resilience to interference, excellent upgrade path, at very little cost. And on top had previously been developed for military radio of it all, the Layer 2 and 3 communications protocols applications. But the new unlicensed band opened these systems generally used (TCP, UDP and IP) the technology to equipment suppliers that, through were open standards, with the largest imaginable the late 1980s and early 1990s, developed radio pool of available expertise. equipment for Wireless Local Area Networks (WLAN). All this was just too good to miss. One by one, all the CBTC suppliers in the world produced versions Initially, all these different radios were proprietary of their CBTC products that supported some sort systems that could not interoperate. However, the of Wi-Fi interface. And with the relatively low cost tremendous success of Ethernet on the LAN space associated with that radio bearer, it was soon in the 1980s convinced suppliers that building their impossible for any CBTC supplier to compete in an systems to a common standard would actually make open tender with a system based on any other radio the technology much more attractive to customers, technology. and that the increase in sales of an open system could more than cover the loss of a locked-in There was a certain reticence to use a commercially- proprietary advantage. off-the-shelf, low-cost radio technology to support an application as critical as CBTC, but as successful In 1988, the industry sponsored the creation of references and case studies started to proliferate, a new committee within the Institute of Electrical the critics started to run out of arguments. By 2010, and Electronic Engineers (IEEE) to define a WLAN the happy marriage between CBTC and Wi-Fi standard. Committee 802.3 had defined the Ethernet seemed complete and destined to last. LAN standards a few years earlier, so the new But where had this irresistible radio technology committee came to be called 802.11. come from? What had it been initially designed for? And so, as it often happens in decisions drafted by And, more importantly, did it have any pitfalls that committees, negotiations took place in a protracted may only become more apparent over time? manner. It was not easy to get so many different suppliers to agree on the technical details, but a new

Copyright 2014 // Rail Systems Australia. All rights reserved. 4 standard was finally published in 1997. you get” does not apply? What are the limitations inherent to the IEEE 802.11 standard, and what Looking for a brand name with more attractive than impact could they have in future (and existing) “IEEE 802.11”, the industry organised itself in what CBTC applications? came to be called “the Wi-Fi Alliance”, and “Wi-Fi” (standing for “wireless fidelity”) products started to 2. The Gathering Clouds - Wi-Fi’s hit the market in 1999. Limitations Before we discuss the limitations of the different In 1999, Apple also started to incorporate a Wi-Fi systems based on the IEEE 802.11 standard family, network card in all its laptops, and the competition we need to understand which “flavours” of Wi-Fi followed suit. Very soon, all the laptops sold in the technology are there actually available, because world could connect to a Wi-Fi Access Point (or they will present slightly different challenges to “hotspot”) wherever one was to be found. And they CBTC applications. started to be found everywhere; first in university campuses, then coffee shops, business centres and The 802.11 standard actually defines a series of offices, and then in almost every home, all over the protocols to deliver a physical layer (PHY or OSI L1) world. and a media access control layer (MAC, included in OSI L2). Table 3 below presents a summary of the The late 2000s saw a turn of events that adds a local different 802.11 PHY standards and some of their Australian flavour to the history of Wi-Fi. Australia’s main characteristics. own Commonwealth Scientific and Industrial Research Organisation (CSIRO) filed a series of The initial 802.11-1997 standard defined two possible successful patent infringement lawsuits against some PHY multiplexing methods: Direct-Sequence Spread of the world’s major technology companies. The suit Spectrum (DSSS) and Frequency Hopping Spread was based in CSIRO’s 1996 patents for forward Spectrum (FHSS), both for the 2.4 GHz band. This error correction, frequency domain interleaving and original standard was soon (1999) replaced by two multi-carrier modulation techniques being used as new protocols: 802.11a, that specified an Orthogonal part of the IEEE 802.11 standard. Based on those Frequency Division Multiplexing (OFDM) encoding patents, it is often claimed Down Under that “CSIRO over the 5 GHz band, and 802.11b, that defined a invented Wi-Fi”. We must recognise that the truth DSSS modulation over the 2.4 GHz band. may be a bit stretched in statements of that nature, but Australia’s participation in the creation of the most Release Band Bandwidth Codification successful short-distance wireless transmission (GHz) (MHz) protocol in History is still something to be proud of. 802.11 2.4 20 DSSS / FHSS 802.11a 5 20 OFDM Now, over a decade after its commercial debut, Wi-Fi technology is still very much prevalent in the home, 802.11b 2.4 20 DSSS the office, the business centre and the coffee-shop. 802.11g 2.4 20 DSSS / OFDM If anything, its usage has only extended beyond 802.11n 2.4 / 5 20 / 40 OFDM domestic and business purposes, as more and more 802.11ac 5 40 / 80 / 160 OFDM devices use Wi-Fi to access the global Internet, and as more and more applications make use of cloud Table 3 – 802.11 PHY Standards computing technology to support mobility. 802.11b was the most successful protocol in the And, as we have seen in previous sections, Wi- family, and nearly all the Wi-Fi access points deployed Fi has also made its way into somewhere it was between 1999 and 2003 used the 802.11b protocol. not designed for and where it was never originally Part of the reason for the success of 802.11b over intended: mission-critical and safety-critical 802.11a was that the 2.4 GHz band provided much applications at the heart of many strategic industries. better propagation characteristics in office and domestic environments, where obstacles like walls and furniture would limit the effective range of 802.11a So what could be the implications of seeing this devices working on the 5 GHz band to a few meters. unlicensed spectrum, low-cost, commercially-off- Therefore, the first CBTC applications to make use the-shelf radio technology embedded into critical of a Wi-Fi radio bearer network were based on the systems like CBTC? Could this be the only proven 802.11b protocol. case where the expression “what you pay is what

Copyright 2014 // Rail Systems Australia. All rights reserved. 5 2003 saw the advent of 802.11g, which tried Standard Frequency Maximum Range to marry the higher data throughputs (up to 54 802.11a 5 GHz 120 m Mb/s) provided by 802.11a with the propagation characteristics of 802.11b by using OFDM 802.11b 2.4 GHz 140 m encoding in the 2.4 GHz band. It was adopted 802.11g 2.4 GHz 140 m very quickly by the industry, and 802.11b/g access 802.11n 2.4 / 5 GHz 250 m / 140 m points became readily available to all users. 802.11ac 5 GHz 250 m (amplified)

At this point in time, however, the ISM 2.4 GHz band Table 4 – 802.11 Maximum Range Values was becoming very crowded. It was being shared by millions of 802.11b and 802.11g access points, as But what is the practical impact of these range well as the microwave ovens and the medical and limitations? Why is effective range such an important industrial equipment that were already there before question? the advent of Wi-Fi. And other wireless communication devices were starting to be deployed in the same Most of the initial CBTC deployments were traditional band, like Blutetooth (IEEE 802.15). All this resulted “Metro” affairs. That is, they consisted of relatively in increasing levels of interference in the 2.4 GHz short lines (often less than 10 km in total), mostly in band, and in underperforming WLAN networks. underground tunnel environments.

In order to address these issues, the IEEE In a tunnel, the range of any radio system is limited incorporated the use of multiple antennas to both by the nature of the tunnel itself. Even in a broad, increase data rates and to provide better protection relatively straight tunnel, a slight curvature will against interference. This lead to the issue in 2009 mean that the walls of the tunnel will eliminate the of standard 802.11n, which makes use Multiple possibility for radio devices to have a line-of-sight Input – Multiple Output (MIMO) techniques in connection with a transmitting antenna in a few both the 2.4 GHz and 5 GHz bands, as well as hundred meters, at most. And that is even without increasing the maximum available bandwidth from taking into account the effects of tunnel topology on 20 MHz to 40 MHz in the less crowded 5GHz band. signal absorption, reflection, diffraction, etc.

This meant that, in the last five years, Wi-Fi nodes In those conditions, the fact that ISM band have effectively become dual band devices. They regulations limit the power of 802.11 transmitters can use 2.4 GHz band channels when available to about 100 mW in many jurisdictions did not in order to maximise range and power, but they represent the most limiting factor to the range of also have the ability to move to the 5 GHz band each access point. And, even if the system needed whenever the 2.4 GHz band becomes too crowded. to deploy one or two access points every 200 meters, the total number of access points was still In the last few years, the IEEE has been developing relatively small – tens or hundreds of access points. the 802.11 protocol family even further, with 802.11ac (January 2014) expected to provide 500 And, in those “Metro” tunnels, access points were Mb/s per single link in the 5 GHz band through 80 easy to install. They were often fixed to the tunnel MHz channel bandwidth, 8 MIMO spatial streams walls, so no supporting structures were needed. and high-density 256-QAM modulation, as well as Power and backbone fibre were often readily beamforming techniques. And standard specification, available, or it was relatively straightforward to with further IEEE protocol releases expected to move deploy them via sub-conducts fixed to the tunnel wall. into other frequency bands in the coming years. The situation, however, is evolving. As more and 2.1 Range more CBTC systems are being deployed, they Having seen all of the above, the first question we are coming out of the tunnels into overground need to consider is the maximum effective range sections of metropolitan suburban railways. achievable through the different 802.11 protocols. Table 4 below shows maximum range listed as part This means that the access points (or their antennas) of the different 802.11 PHY standards. need poles to be mounted on, as well as new equipment housing, additional fibre and new power connections in areas where it may not be easy to deploy those. Each access point and its associated infrastructure will be much more expensive to install than they were on a tunnel wall back in 2003.

Copyright 2014 // Rail Systems Australia. All rights reserved. 6 However, the range of those access points will Modifying the conditions that trigger the handover not improve dramatically once the limitations process in the train radio is also a critical factor. imposed by tunnel walls are removed. Due to the power limitations imposed by spectrum regulators The way most CBTC systems handle access point over the 2.4 GHz and 5 GHz bands, the radio roaming to date is by making use of the short signals generated by commercially off-the- distances between access points. Effectively, shelf Wi-Fi devices do not allow significant data trains are long enough to have one train radio in throughputs over distances much exceeding 100m. the front cab connected to one access point while the rear cab radio is connected to the previous The suburban open air railways were CBTC is access point. The on board CBTC system can expanding tend to actually be much longer than transmit through whichever radio is connected at the 10 km inner-city underground lines of old, any given time, while the other radio completes particularly in countries with large sprawling low- its roaming process to a new access point. density metropolis, such as Australia. And those longer distances (totalling hundreds of kilometres This very clunky mechanism, however, will not be for a complete metropolitan network) mean that well translated to longer, faster suburban railways. the number of access points necessary to provide As a train moves at higher speed through the same sufficient coverage to the track is in the thousands. density of access points per kilometre, train radios will spend a higher percentage of their time trying to Thousands of access points, thousands of break-back handle roaming processes. The result will necessarily poles with their concrete foundations, thousands of be an increase in the average packet delay. brackets, of FOBOTS, of antennas, of optical SFPs, of Ethernet transmission switches, thousands of Whether this increased delay will have an impact on assets to manage, operate and maintain all spread the operation of the railway will be an implementation- over hundreds of route kilometres… Wi-Fi may no specific and product-specific consideration. But the longer be the cheapest solution in those conditions. general idea is that Wi-Fi access point roaming will not get any better in faster CBTC lines. And all 2.2 Mobility this doesn’t even take into account the effects of The IEEE 802.11 protocols where not specifically Doppler frequency shifting at higher speeds, against designed with mobility in mind. Conventional which Wi-Fi has virtually no mechanisms. Doppler Wi-Fi access points act as isolated hubs for frequency shifting would also have a higher effect isolated WLANs. They do not negotiate roaming on 802.11g and 802.11n devices, due to their usage amongst each other, and there is no common of OFDM techniques that require very precise management entity to help two or more access reception of narrower sub-carrier frequencies. points allocate radio resources to users that fall within the range of more than one access point. 2.3 Radio Resource Access The way in which user devices access radio Mobility in Wi-Fi, therefore, is entirely left to the resources is defined in the Media Access Control user device. It is the Wi-Fi radio on board a CBTC (MAC) layer of all 802.11 protocols by a multiple train what decides with which access point it access method known as Carrier Sense Multiple is in communication at any given point in time. Access with Collision Avoidance (CSMA/CA) [6].

As a train radio moves along the track, it will According to CSMA/CA, when a node (either an see how the signal strength from the access access point or a user device) is ready to transmit point it has left behind decreases with distance. a Layer 2 frame, it first stops to ‘listen’ whether It will then probe nearby access points, select the radio channel (the section of the 2.4 GHz or one of them according to the perceived signal 5 GHz band it is planning to transmit on) is idle. strength, pass through an authentication process and re-associate with its new access point. If the channel is not idle – that is, if the signal from another device is detected – the All this process can take in the order of 500 ms, listening node in question waits for a random during which the flow of CBTC application layer period of time before trying to transmit again. data between train radio and access point is completely interrupted. Certain configurations – If the channel seems idle, the listening node will such as placing several access points to a single proceed to transmit. SSID – may speed the process up, but only slightly.

Copyright 2014 // Rail Systems Australia. All rights reserved. 7 of ‘contention window’ timers are then allocated to When two or more nodes decide to transmit at the different Access Category traffic streams, in order same time, a collision occurs, and the receiver will to provide a preferential Transmit Opportunity only receive random noise as the simultaneous to certain packets during reserved periods of transmitting signals interfere with each other. The time. It also allocates different back-off timers to Layer 2 frame will then be lost, and an upper layer different traffic streams to try to make high priority application will need to negotiate a retransmission. applications wait less time to transmit a packet.

Optionally, when planning to send large frames of Table 5 below shows the access categories data at once, IEEE 802.11 protocols use the Request defined by EDCA and their associated Arbitration to Send / Clear to Send (RTS/CTS) mechanism [7]. Inter-Frame Space Number (AIFSN) back- With RTS/CTS, the first frame transmitted by the node off period value before re-transmitting and does not contain user data, but is just a request to seize maximum Transmit Opportunity (TXOP). the channel for some time to transmit the large frame. Traffic Type Access Category AIFSN Max In either case, though, the key factor is that all the TXOP devices that make use of IEEE 802.11 protocols in Background AC_BK 7 0 a given area will equally use CSMA/CA to access Best Effort AC_BE 3 0 radio resources. From the point of view of IEEE 802.11, there is no way to differentiate between the Video AC_VI 2 3.008 ms mission-critical frame from a CBTC train radio, the Voice AC_VO 2 1.504 ms potentially-mission-critical frame from a train driver’s Legacy 2 0 personal communications device and the maybe- CSMA/CA not-so-critical frame from an on board PA system. As long as they try to access the same 2.4 GHz or Table 5 – EDCA Access Category Values 5 GHz channel radio resource, they all have the same level of priority as far as 802.11 is concerned. EDCA, however, is limited by the fact that these QoS mechanisms are controlled at different OSI layers, 2.4 Quality of Service ranging from the Application and Network Layers Wi-Fi’s ‘best effort’ approach to radio resource to Medium Access Control (MAC) and the Physical allocation creates an additional complication. (PHY) layers. Therefore, any application could Since there is no differentiation between users label its traffic as Access Category AC_VI, thus or application in the way the radio resource is pre-empting CBTC applications from reserving the accessed, there is no way to guarantee maximum highest QoS levels for critical communications. And, packet delay values to any given application. above all, EDCA is still a probabilistic mechanism; and cannot guarantee any level of Quality of Service So far, because CBTC systems have been built to a mission-critical application independently of over dedicated Wi-Fi networks, the impact of the presence of other applications in the network. this limitation may not have become apparent yet. But it means that railway companies are The obvious solution would be to build separate Wi-Fi limited in the extent to which they can leverage networks that made exclusive use of separate WLAN their radio communications infrastructure. channels for different applications. This, of course, means a duplication of the infrastructure: thousands If a different data application – say, live security of extra access points, thousands of power supply CCTV feeds from on board cameras – were to be upgrades, thousands of antennas… we would reach implemented over the same Wi-Fi network used for a point where selecting a radio system other than CBTC, there would be a significant impact on the Wi-Fi would actually represent significant savings. performance of the CBTC network, due to CBTC Wi-Fi frames having to compete in equal footing 2.5 Interference for radio resources with these new applications. The most critical limitation of all Wi-Fi systems is, however, the very same reason that has made them Standard IEEE 802.11e (and subsequent versions) so popular in the last decade – the use of unlicensed presented an optional QoS mechanism called bands of the spectrum. Enhanced Distributed Channel Access (EDCA). EDCA is based on assigning different Access People and business located in the proximity of Category values to different kinds of traffic. A series a railway line are nowadays extremely likely to

Copyright 2014 // Rail Systems Australia. All rights reserved. 8 set up Wi-Fi access points to link laptops and 3. Prospective Solutions tablet computers to domestic or business Internet We have already hinted at some potential solutions broadband connections. As conventional users of to some of these problems; for clarity’s sake, we are the ISM band, they are legally entitled to transmit going to summarise in this section the options that could on any 802.11 defined channels in the 2.4 GHz and alleviate some of the inherent flaws in Wi-Fi systems. 5 GHz bands, depending on whether they decide to use 802.11a,b,g,n or ac devices. They do not need a A move from the 2.4 GHz to the 5 GHz band could spectrum license from ACMA to do so. buy time against interference, given that most Wi-Fi devices are still transmitting on the 2.4 GHz band. Any user could configure their Wi-Fi devices to This, however, can only be a temporary measure any channel within the unlicensed bands, and – the 5 GHz band is already getting busier and they could transmit as much power as the nearby busier with every passing year, as more and more railways CBTC Wi-Fi network. They could even use devices escape the crowded 2.4 GHz segment. directional antennas and point them towards the In the case of 802.11b/g devices, this will also railway. mean a replacement for newer 802.11n/ac/ad equipment capable of operating on the 5 GHz band. And there would be absolutely nothing the railway operator could do to stop those legitimate ISM band Using EDCA instead of CSMA/CA to improve users from doing so. Quality of Service parameters could also work in the short term, provided that the number of Most residential users do not have anything to prove high bandwidth applications on the network is regarding Wi-Fi interference, so they may not set kept to a minimum – no CCTV and CBTC on directional antennae up. But even though, there are the same network. This would also require an hundreds, thousands, tens of thousands living in the upgrade to 802.11n/ac/ad from older versions. vicinity of railway lines. And there are also hospitals, university campuses, business centres, retail stores; In terms of mobility, roaming in small areas all with hundreds and thousands of Wi-Fi access (e.g. railway depots) could be improved points, all happily humming along at exactly the same through WLAN virtualization controllers. Large, frequencies currently used for CBTC Wi-Fi networks. network-wide roaming, however, will remain an impossibility with IEEE 802.11 technologies Moving CBTC Wi-Fi networks to the 5 MHz band and would probably require a replacement with may fend off the problem, but it is only a temporary technologies explicitly designed for mobility, the solution. Many residential access points are starting best candidate being the 3GPP-defined Long Term to experience problems due to interference from Evolution (LTE) 4G mobile telephony standard. nearby access points. It is just a matter of time until they also move to the 5 MHz band and start increasing As for interference in the ISM band, only a move the levels of interference on that band as well. to a licensed spectrum band could guarantee an interference-free radio medium in the long term. Either The only definitive solution to potential interference LTE or a series of proprietary radio solutions could then in the ISM band, bar a very unlikely political provide a viable radio bearer for a CBTC application. decision that banned Wi-Fi transmission in the vicinity of railway lines, would be to move the The latest developments in the CBTC space, Data Communications Networks that support however, seem to point very clearly in the direction CBTC applications to other, licensed bands. of LTE as the future alternative to Wi-Fi in CBTC deployments worldwide. This would represent a major complication to many railways around the world – 20 MHz of contiguous idle spectrum is not something someone finds every day – but, in Australia, railway operators have been lucky enough to be able to obtain ACMA licenses for 15 MHz in the 1800 MHz in all major capital cities. It would be good if this Australian peculiarity were exploited to the fullest by securing the future of CBTC systems in our cities against third party interference.

Copyright 2014 // Rail Systems Australia. All rights reserved. 9 4. Latest Developments 5. Conclusion During the preparation of this paper, several Once the IEEE 802.11 family is examined closely, major CBTC suppliers have made a number of it becomes evident that it was never designed to announcements regarding LTE as a potential support safety-critical, mission-critical applications replacement of Wi-Fi in CBTC deployments. such as CBTC. That it has effectively done so in so many places in the past is a testimony In February 2014 [8], Huawei announced that the new of the ingenuity of radio communications LTE network deployed in Zhengzhou’s metro Line 1 and signalling engineers in the industry. would be part of a trial to transmit CBTC over LTE. If the trial were successful, CBTC operations could Moving forward, however, it is clear that something be switched from Wi-Fi to LTE by the end of 2014. needs to be done. Wi-Fi will become less and less fit for the purpose of carrying CBTC data over the In August 2014 [9], Alstom and Huawei publicly next years as CBTC applications expand into the announced a joint project to test CBTC over suburbs and as the 2.4 GHz and 5 GHz bands LTE, together with voice communications and become more and more crowded. Its limitations CCTV transmission, at an Alstom test facility in in range, mobility, media access control, quality of Valenciennes, France. Alstom has stated that Urbalis service and resistance to interference pose a danger CBTC over LTE could be available by the end of 2014. to the future reliability of CBTC systems everywhere.

During the August 2014 Australasian Railway The supplier industry is already clearly showing its Association Telecommunications and Train Control willingness to replace Wi-Fi for LTE in coming years. Conference in Sydney [10], GE Transportation It is now up to the operators to realise that the clouds announced that the Tempo CBTC platform had are gathering over Wi-Fi, that these problems are been successfully trialled over a commercial only going to get worse, and that we all need to do LTE network in Hong Kong, using an Ericsson something to address them sooner rather than later. eNodeB and a Huawei on board data radio.

All these developments point very clearly at the supplier industry’s conviction that the CBTC market is ready for an alternative to Wi-Fi. They also seem to agree that the most viable alternative is an LTE network.

Copyright 2014 // Rail Systems Australia. All rights reserved. 10 6. References 7. About the Author [1] The Rail Engineer – 08/05/2014 – Communications Mr Rodrigo Álvarez Based Train Control – accessed 04/06/2014. Rodrigo has been involved in railway (http://www.therailengineer.com/2014/05/08/ communications in the UK and Europe for over communications-based-control/) ten years. His experience is especially centred [2] 1474.1-2004 - IEEE Standard for Communications- in the integration of advanced railway signalling Based Train Control (CBTC) Performance and systems and telecommunications technologies. Past Functional Requirements projects include Network Rail’s Cambrian ERTMS [3] Bombardier website – accessed 04/06/2014. Deployment and Crossrail Programme in London, as well as GSM-R deployment in ADIF’s High Speed (http://www.bombardier.com/en/transportation/ Network (Spain). He has also worked on railway products-services/rail-control-solutions/cbtc.html) research and development projects for the European [4] The Economist – Technology Quarterly – Q2 Commission. Since February 2013, Rodrigo has 2004 – A brief history of Wi-Fi been working for the Public Transport Authority of [5] Federal Communications Commission – Spectrum Western Australia in the planning phases of the Policy Task Force – Report of the Unlicensed Radio Replacement and ATP Replacement Projects. Devices and Experimental Licenses Working Group, November 15, 2002 Rodrigo’s technical background includes the design [6] IEEE 802.11, Wireless LAN MAC and Physical of GSM-R networks, as well as SDH and Carrier Layer Specifications. Editors of IEEE, June 1997. Ethernet networks. On the signalling side, Rodrigo [7] Karn, Phil: MACA – A New Channel Access has a wealth of experience in ERTMS systems, Method for Packet Radio, Proceeding of 9th ARRL and he is well acquainted with the transmission requirements of axle counters, modern Computer Computer Networking Conference, London, Ontario, Based Interlocking, SISS and DOO systems. In Canada, 1990 the last two years, he has had a very significant [8] International Railway Journal – 11/02/2014 – exposure to LTE, Wi-Fi and CBTC technologies. (http://www.railjournal.com/index.php/telecoms/lte- displays-potential-in-zhengzhou.html) [9] Railway Gazette – 06/08/2014 – (http://www.railwaygazette.com/news/technology/ single-view/view/alstom-and-huawei-announce- metro-lte-trial.html) [10] Official ARA web site – (http://www.ara.net.au/ ARA-events)

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