Telecommunications

Telecommunications are an essential part of Road Network Operations and Intelligent Transport Systems. Over the past 40 years they have been developed piecemeal to support network operations – for example by linking control centres with roadside devices such as telephones, CCTV cameras, Variable Message Signs (VMS) and traffic signals. Today digital communications dominate the transmission of voice, video and data signals. Digital technology is inherently more reliable, flexible and manageable compared with previous generations of communications technology. Digital communications enable the development and operation of modern traffic management technology and the latest ITS applications – including connected vehicles and Active Traffic Management. CCTV is used increasingly and digital transmission of video images is possible over distances without the image being degraded.

Telecommunications networks resemble the nervous system in a human body. Specifically, the communications networks tie the different components of ITS together, allowing for a truly integrated system. For example, they provide a data link from the field devices (detection technologies, Dynamic Message Signs, signal controllers) to traffic operations centres – where the collected data is fused, analysed and acted upon. This is illustrated in the diagram below. Telecommunications are also needed to carry instructions and commands from control centres back to field devices for traffic control purposes. They are also the means for infrastructure operators (controllers) relaying information to travellers and stakeholders.

An ITS system will not function without an appropriately designed communications network that has adequate bandwidth and is capable of delivering an adequate level of service (in terms of message delivery, latency and drop-out rates). Decisions on the appropriate communication technology, the appropriate network topology and other communications design issues have to be made carefully. This is because the cost of the communication network typically constitutes a major component of the cost of a specific ITS system. In some cases, where a cable and transmission equipment infrastructure needs to be installed, it can be up to 50%.

There are a number of options for ITS professionals. Traffic operators need to decide how best to meet telecommunication needs and what they are capable of doing. Broadly speaking, the technologies can be divided into wired communications and wireless communications. The choice for roadside installations is often a trade-off between cost and functional capability.

Telecommunications architecture

The telecommunications network to support ITS needs to be carefully designed. A common architecture for such networks is known as a hierarchical or layered architecture, which exhibits many similarities to the hierarchical system of road networks themselves. Specifically, telecommunications networks may be regarded as consisting of four layers:

• backbone
• backhaul
• distribution
• access

Using highways as an analogy for telecommunications, the backbone layer is similar to the inter-state / inter-urban strategic roads. It enables moving (hauling) large amounts of data between a limited number of fixed distribution points. As with road networks, the different layers of a communication network are interconnected. Fibre optics cables are commonly used for this layer.

The function of the backhaul layer is to move (haul) large amounts of data (which still requires large bandwidth) from the backbone network to the Traffic Control Centre. It is often off the highway/road network – and can be provided by a service provider such as a telecommunications company (TelCo).

The distribution layer resembles the system of arterial roads in a road network. This layer typically does not handle large volumes of data. Its main purpose is to provide multiple points of presence to enhance accessibility.

Finally, the access layer resembles a residential or local street network or the lead/cable which connects the TV to the aerial socket – that provides local cabling to access the different devices on the network.

Leased Communications Systems

Another option for transport agencies is to lease wired communications services from a telecommunications company. In earlier years of ITS, some ITS applications (urban control systems) used leased telephone lines – their limitation is the very low bandwidth provided by telephone lines. Today it is becoming increasingly cost effective to use WiFi to replace leased telephone circuits, particularly in urban areas – or as an alternative to costly new cable networks for inter-urban roads when bandwidth requirements are modest.

More recently, new technologies have been developed to help improve the speed of communications on local telephone networks. In particular, Digital subscriber line (DSL) – which uses higher frequency bands for data – can offer speeds of up to 40 Mbits/second. Asymmetric Digital Subscriber Line (ADSL) is a type of digital subscriber line (DSL) technology that enables faster data transmission over copper telephone lines. Another technology is cable internet which uses cable television networks in much the same way as DSL uses telephone lines. Cable Internet could have speeds of up to 400 Mbits/second – and so can support most ITS applications that have demanding bandwidth requirements.

It is often more cost effective to lease dark fibres from a telecommunications operator or buy-in a service rather than install a dedicated system. Leased Communication Systems in ITS are widely used for urban traffic control systems and to provide the backhaul for connecting TMCs to the field communications on a motorway.There are two reasons for this:

• urban traffic control systems typically have very modest bandwidth requirements
• communication or telephone infrastructure is widely available in urban areas

Leased communications also provides a means for satisfying the communications needs of rural ITS applications where the installation of new communication lines may be too expensive.