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.