Opening the closed-loop to realise ITS benefits
First publishedin ITS International
The traditional on-street master can be converted from closed-loop to ATMS
Jim Leslie, manager of ITS applications engineering at the Econolite Group looks at practical steps in transitioning from closed-loop masters to a centralised ATMS.
Not many years ago the standard method of coordinating signalised intersections in local areas was to install an on-street master – each of which monitored and controlled a limited number of signal controllers or intersections as a closed-loop system. And, to a certain extent, each closed-loop system was autonomous from others deployed by the agency. The on-street masters would typically be connected to a centralised computer workstation for monitoring functions and uploading of timing plans, often over analogue dial-up modems.
Today, however, transportation agencies around the world are experiencing and documenting the exceptional benefits of Intelligent Transportation Systems (ITS) including real time monitoring and control, instant reporting and data retrieval. In some cases, these new ITS capabilities are providing active traffic management, contributing to reductions in traffic congestion and vehicle emissions. At the heart of these ITS deployments is a centralised Advanced Transportation Management System (ATMS) which can control signals centrally over a wide geographical area in order to achieve the optimum timing to keep traffic flowing.
There is a huge legacy of on-street master closed-loop systems which at some point will have to be transitioned to a centralised ATMS as agencies identify upgrade opportunities on an incremental or complete system replacement basis. The transition from on-street master closed-loop systems to a centralised ATMS poses two primary system challenges: the replacement of the on-street master with a centralised ATMS and the switch from the dial-up communications that the majority employ to the latest Internet Protocol (IP) which is necessary if the full benefit of the changeover is to be realised.
The main system change or upgrade when transitioning from a traditional closed-loop system to an ATMS will be the communications infrastructure. When an on-street master is running in a closed-loop system, the communications infrastructure is partitioned into two major segments: the infrastructure connecting the local controllers to the on-street master and and that in turn to connect the on-street master to the central traffic operations centre.
Fortunately, standardised communication technologies, such as the National Transportation Communications for ITS Protocols (NTCIP) are readily available to help agencies upgrade and quickly realise the full benefits of ITS. This allows agencies to replace most of the old dial-up communications to the field-based, closed-loop or on-street masters to the latest IP with minimal risk.
The advances in IP-based communications and equipment make it possible to move beyond dial-up to secure digital communications which provide a gateway to new ITS capabilities, without the need for major utility works.
CLS converted to ATMS: with connection to central system across VPN tunnel, the OSM was removed and replaced by a communications server, dial-up has been replaced by a VPN connection over ADSL for an always-on connection
With today’s IP-based applications, the infrastructure is multipoint-to-multipoint – each and every device can communicate simultaneously with all the other devices on the network. For instance, the ATMS or a central server can communicate directly with all of the local controllers to provide the foundation for a network of connected intersections to form the foundation of a comprehensive ITS program.
Another, perhaps more important benefit of IP communications is that it is application agnostic or independent. This means the infrastructure can be shared by many ITS applications (such as CCTV video, adaptive signal control, automated data collection…) which opens up a range of control and monitoring options for traffic authorities.
The most common closed-loop communications systems use a traditional dial-up phone service through the public switched telephone network to the on-street master (Image 1) with onward serial communication to the local controllers via twisted pair copper wire or fibre-optic cable.
Various forms of communication equipment may have been deployed for each part of this network at various locations. These include combinations of dial-up modems over public telephone systems, frequency-shift keying over leased line (Telco) or twisted pair copper and serial over fibre or wireless.
Converting local controllers to enable IP/Ethernet communications solves a significant portion of the challenge in the transition from a closed-loop system to a centralised ATMS.
The easiest portion to convert is the connection between the on-street master and the local controller. Converting this portion to IP first involves replacing the frequency-shift keying serial modems with a Very high speed Digital Subscriber Line (VDSL) switch, utilising the existing twisted pair wiring (Image 2). If the VDSL switch has an integral terminal server capability allowing it to convert to serial, it can be directly connected to the controller –albeit that this wouldn’t provide a complete conversion to IP/Ethernet. A better option is to use the VDSL switch’s Ethernet output and then convert the local controllers to IP/Ethernet protocol.
Upgrading the communications between the traffic operations centre and the master cabinet(s) will be considerably more involved but will result in substantially improved capabilities compared with the legacy system. These legacy communications systems vary widely, so individual advice from a reputable transportation and systems engineering services consultant is highly recommended. When working with the consultant the opportunity should not be missed to create an incremental transition plan that not only provides a system that addresses the requirements for the immediate upgrade but also allows for the future needs of an evolving transportation system. Planned upgrade over time also provides the potential to demonstrate incremental results, which can be used to help secure additional measures-based funding.
Incremental TransitionIt might seem counterintuitive, but upgrading communications from the outer edges of the closed-loop system inward (the on-street master to controllers portion first) is the simplest and provides the foundation for future IP communications.
Although one of the aims of the upgrade may be to add features such as CCTV, it is recommended to hold off adding such high-density data applications and connecting to a municipality-wide area network (WAN) until the entire system is upgraded. This is because such applications would be detrimental in the limited data handling capabilities of an interim or partially upgraded system and could result in a lot of time being spent tracing and overcoming ‘problems’ that will disappear anyway as soon as the full upgrade has been completed.
Taking advantage of two state-of-the-art technologies – namely a central ATMS system and the latest in communications infrastructure, IP/Ethernet at the same time can be accomplished relatively quickly. Today, cellular communications, along with thorough technical considerations, can also be a viable and cost-effective solution option.
Back to the original example: converting communications from the traffic operations centre to the on-street master locations would look similar to the central system across a virtual private network (VPN) tunnel in Image 3. While this example addresses replacing dial-up communications over the public telephone network with some variant of digital subscriber line, upgrades are also routinely done with combinations of wireless and fibre. Consultants will be able to provide additional details and guidance on each particular network to help smooth the upgrade to an ATMS by replacing the on-street masters with the appropriate bridging equipment. Upgrading the legacy closed-loop systems to a centralised ATMS is often the last hurdle for transportation agencies to realise newer and more powerful ITS capabilities – real time control, management, monitoring and measurement. It’s important to identify the high-level steps to make appropriate communication infrastructure upgrades. By working with a systems consultant, transportation agencies will have a better picture of the options available for communications upgrades that will be suitable for both today’s and future needs, helping smooth the CLS-to-ATMS transition.
Prior to 2009, the city of Centennial in Colorado worked on multiple communication projects, trying to upgrade its leased line communications to a mixed fibre optic and radio system. During this time, the City started working with a consultant to gather information on a new transportation system to upgrade its CLS to ATMS.
In 2011, Centennial started working with supplier Econolite and Aegis ITS as the engineering services consultant to design, integrate and install a Centracs ATMS. As the city had more than 45 ASC/2S controllers that were capable of being upgraded to Ethernet, it was a straightforward upgrade to the communications and saved the authority thousands of dollars.
For more than 14 years, Garden Grove in California operated and managed a closed-loop system to control 110 intersections using eight masters. The communication configuration was frequency-shift keying over copper wiring and leased phone lines.
As Garden Grove’s traffic management needs grew, the City updated its traffic signal equipment, including new controllers, but still had to deal with its legacy communications infrastructure. The City was able to secure funding through the traffic light synchronisation program that helped to upgrade the system to Ethernet communication and deploy an ATMS. It now manages nearly 100 intersections in real time with its ATMS, as well as operating a citywide network of CCTV cameras.