From Roader's Digest: The SABRE Wiki
Urban Traffic Control is a centralised computer system running on either VAX, Compaq VMS or, for one manufacturer's latest release, Microsoft Windows server. It uses fixed time plans or SCOOT, to control traffic signal junctions by means of second-by-second communications.
The two main providers of Urban Traffic Control systems in the UK are Siemens Traffic Controls and PEEK Traffic, whose forerunners were Plessey and GEC respectively. The two systems – while different in terms of how data is organised and slight differences in how communications are structured – share many fundamental similarities. In this article, we shall use the Siemens UTC system as an example.
A typical Urban Traffic Control system comprises three elements:
- The instation: this comprises a server, such as a Compaq Alpha Server or VAX upon which the operating software runs and a Tele-12 cabinet which deals solely with the communications;
- The communications system, which mainly comprises EPS42/45 leased communications lines from BT but can be anything from analogue or digital fibre optic systems or wireless media;
- The outstation transmission unit: basically a modem in the traffic signal controller which translates the incoming information and commands from the UTC server and replies with the status of the signal junction.
The MMI (Man Machine Interface) for the UTC system comprises a desktop layout with facilities accessed by drop down menus, or entered directly in to the command entry window, with the look and feel of very early GUI desktops. While the PEEK and Siemens systems offer similar functionality they also have many widely different features. A prime example would be system graphics; the PEEK system is able to handle full graphics, with .jpeg images of junctions as well as more traditional .dxf drawing files whereas the Siemens system can only handle basic .dxf imports for the junction layout, with additional graphics being added in a rudimentary "paint" style sub-program.
It is possible to look at individual junctions by means of a plan monitor window, which shows which stage is being forced by the plan, whether the controller is in the stage or moving to that stage and whether any faults exist on the site.For more complex analysis of data coming from/going to a particular junction there is a monitor function which shows each of the control/reply bits being sent to and from the site.
The communications systems for UTC are relatively simple compared to modern data transmission examples.
When UTC is controlling a junction, it is sending out "force bits". Essentially this is as it sounds:
- UTC takes over the site by sending a force bit to move to or hold the stage it is currently running – let's say the main road stage "A". It continues transmitting this bit to the controller on street to maintain this stage.
- When the plan dictates a move to another stage, let's say the side roads "B", UTC stops transmitting the bit for A and starts transmitting the force bit for B. Within one second the controller starts the move to that stage.
Meanwhile the controller is continuously replying with its status, second by second. When the controller is running stage A, it is sending the stage A reply bit to UTC.
When the controller is forced to another stage it will drop the Stage A reply bit and won't transmit anything until it is in the next stage – this is known as the interstage period.
Because of the architecture of the communications for UTC, there is an upper limit on the number of sites you can have connected to any line before it becomes full. This is dependant on a number of factors including how much information needs to be transmitted to each site and is complicated further by whether the site is running SCOOT or not.
The data needs to be sent to each site in turn each second and the data has to be replied in that same second – listening in on a BT line this data transmission can be heard as a series of chirps every second. Each chirp is the control and reply data being passed between UTC and each outstation on that line.
Outstation Transmission Unit
This is the modem which sits in the junction or pedestrian controller. The outstation consists of a modem card which connects to the BT line; the outstation is also hard-wired in to the controller I/O board to transmit the control and reply bits.
In a free-standing OTU, SCOOT detectors are fitted in to the unit. These provide the SCOOT traffic volume information replies to the UTC system and are independent of the controller. In a Siemens ST800 and ST900 controller, the OTU is integral to the controller in most instances when working with a Siemens UTC system. That way the controller is doing the most of the work and means that additional detectors and functions can be added over a conventional OTU.