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.
National Roads Transmission Network for England (Courtesy of Highways England. In this diagram MIDAS means Motorway Incident Detection Alert System.)
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.
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:
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.
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:
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.
Highways England (a Government owned company) established a public-private partnership to upgrade, operate, and maintain the communications systems that link the roadway communication devices (emergency telephones, CCTV, etc.) along the motorways and other strategic roads in England. A telecommunications consortium was selected for a 10 year project.
The consortium has end-to-end responsibility for voice, data and video transmission services that link the Highways Agency’s roadside devices to the control offices. The roadside devices and control centre applications themselves remain with the Agency. The consortium is responsible for monitoring the performance of the transmission services, for providing a resilient and reliable service and for providing additional local connections to support additional roadside devices.
Wired communications use fibre optic and copper cables to connect roadside equipment to control centres. Typically these cables run in ducts along the motorway or roadway with the necessary data transmission equipment housed in roadside cabinets. The control centres themselves are in strategically located buildings with cable connections to the main network.
Wired communications include a wide range of technologies that vary in performance, cost and bandwidth – meaning the volume of data that they are capable of communicating is variable. At one end of the spectrum there is fibre optic technology that provides the highest bandwidth of any communications system existing today. At the other end are the old-fashioned telephone lines with limited bandwidth for data transfer.
A fibre optics cable is a communications medium for light waves to carry a signal that transfers information from one point to another. The cable itself is very thin (slightly thicker than a human hair). For operations, an optical transmitter is needed at one end of the cable, and a receiver at the other – to convert electrical signals into light signals and back again at the receiving end.
Current fibre optics technology is capable of transmitting about 1.5 Gbits of information per second.
Another advantage of fibre optics communication is that it is not susceptible to magnetic interference or electrical resistance, since it uses light waves. On the downside, fibre optics communications are relatively expensive, although their widespread use nowadays has made them more affordable. A large portion of the cost of fibre optics technology relates to purchase of the -of-way, the termination equipment (converting electrical pulses to light and back again) – and the trenching needed.
A number of highway transport agencies have entered into an agreement with a telecommunications company:
Fibre optic cable is commonly used in ITS for applications where there large amounts of data transmitted. A good example is the connection between a Transportation Management Centre (TMC) and field devices such as video cameras. There is emerging interest in taking fibre cables direct to the end-devices – leading to roadside equipment now being specified with an optical fibre input or connector-socket. Fibre optic cables are expensive and challenging to fix when damaged.
Copper cabling is good for voice and data transmission – but increasingly cable systems need to transport high bandwidth signals associated with CCTV images and other video. Fibre optics are rapidly replacing copper for ‘main line’ telecommunications – but distribution within buildings and over the last mile often relies on copper coaxial cable. Copper cabling requires the use of line amplifiers to cover distance – with an increased risk of noise on the high bandwidth signals. With the spread of digital signalling and ADSL (see below) existing copper cables are having a new lease of life to provide the distribution and access layers.
Twisted wire pair (TWP) is amongst the most common communications media for ITS applications. It is made of two insulated copper conductors twisted together to cancel out electromagnetic interference. Recent advances have allowed the use of Ethernet over TWP in a number of ITS applications.
Twisted wire pairs are the most commonly used option for ITS communications for the access and distribution layers – especially since recent advancements in ADSL technology allows the use of Ethernet over TWP. This has also opened opportunities for the utilisation of legacy TWP infrastructure. ADSL is now widely used – following the practice of Telecommunications Companies – to make best use of their extensive existing copper cable networks.
Ethernet cable is used to create Local Area Networks (LAN) providing a physical data network – connecting devices together within a control centre. It carries data using the Ethernet protocol which is almost exclusively used for ICT applications in buildings/offices. The current most commonly used industry standard is Category 5 (CAT5), which contains four pairs of copper wire, and supports speeds of up to 100 Mbits/second. Newer standards are now allowing for faster speeds up to 1000 Mbits/second. CAT5 cable is limited to a maximum recommended length of only 328 feet.
An interesting development for ITS in recent years is the concept of Power over Ethernet (PoE), which allows a single cable to provide both the data connection as well as electrical power to ITS field devices. PoE allows for longer cable lengths.
Apart from the need for an Ethernet network within the TMC, Ethernet cables are commonly used in ITS to form the access layer to connect a field device (such as a CCTV camera) to a network or to an Internet access point. In this case the cables are there primarily as local device interconnects.
Advances in digital technology over the past two or three decades have made wireless communications an attractive option for road-based ITS in situations, such as control of VMS, car park counters, traffic signal communications, remote monitoring and CCTV. Specialised ITS applications also rely on a variety of wireless radio services for communication – principally:
Standardisation of Wireless Communications for ITS in Europe
In Europe efforts to standardise wireless communications for ITS are directed towards the creation of a continuous long-range / medium-range continuous air interface using a variety of communication media, including cellular, 5 GHz, 63 GHz microwave and infra-red links. The initiative is known as CALM – which stands for Continuous Air-interface Long and Medium (CALM). CALM will provide the communications platform for a range of applications, including vehicle safety and information, as well as entertainment for driver and passengers.
Some of the more common wireless communications media that are used for ITS include:
Point-to-point microwave communication uses ground-based transmitters and receivers resembling satellite dishes to provided dedicated backhaul links where landlines would impractical or prohibitively expenses to connect roadside networks to a control center. They are usually in the low-gigahertz range and limited to line of sight. Repeater stations can be spaced at approximately 48 km intervals to cover greater distances.
Wi-Fi has become a very popular technology for the exchange of data wirelessly using radio waves over a computer network. Wi-Fi can support non-critical ITS applications because it avoids delay – but does not have bandwidth guarantees. Wi-Fi operates using unlicensed frequencies – so are more susceptible to interference. The technology has potential for use in ITS as a means for connecting field devices to a Traffic Control Centre – for example, where a wired communications solution would be too expensive. In this case, a secure Wi-Fi connection, such as the wifi-mesh network shown in the figure below would need to be provided. This shows a multipoint to multipoint WiFi Mesh network suitable for VMS, car park counters, traffic signals, remote monitoring and non-enforcement CCTV.
Wi-Fi is based on the Institute of Electrical and Electronics Engineers’ (IEEE) (IEEE) Standard 802.11. It is designed to provide local network access over relatively short distances (between 50 – 100 meters) with speeds of up to 54 Mbits/second.
Figure 6 Diagram of a wifi-mesh network suitable for highway data transmission
(Figure courtesy of Barry Moore)
Bluetooth technology also supports data exchange over short distances. It uses short-wavelength ultra-high frequency (UHF) radio waves. Bluetooth technologies are used in many different applications such as smart phones, headsets, tablets and laptop computers. Recently, there has been increased interest in building on the presence, on-board vehicles, of Bluetooth devices in “discoverable” modes, to track and monitor traffic. These applications use roadside Bluetooth detectors to discover Bluetooth devices on-board the vehicles and detect their unique identifier. By tracking the same identifier through different location points, important traffic parameters can be determined such as travel time, speed, vehicle origin and destination.
Another group of communications technologies widely used in ITS is dedicated short-range communications (DSRC). DSRC was developed specifically for vehicular communications and is likely to witness a dramatic increase in use with the introduction of Connected Vehicle technologies. The technologies are used in a number of ITS applications including:
In the USA, DSRC generally refers to communications on a dedicated 5.9GHz frequency band reserved specifically for Wireless Access in Vehicular Environment (WAVE) protocols – defined in the IEEE 1609 standard and its subsidiary parts. These protocols are based on the widely-used IEEE 802.11 standard for Wi-Fi wireless networking.
Several DSRC technologies are used in transportation. These include:
Passive tags do not have an internal power supply. Instead, they use the very small electrical current induced in the antenna by the incoming radio frequency signal, to transmit a response. For this reason, the antenna has to be designed not only to collect power from the incoming signal, but also to transmit the outbound backscatter signal. The main advantage of passive microwave tags is that they can be quite small and have an unlimited life. Passive microwave was used in ITS for early types of electronic toll collection systems. Innovations in their use continue.
Active tags have their own internal power source which can generate the outgoing signal. Compared to passive tags, they may have a longer range and can store additional information sent by the transceiver. Active microwave is employed in many electronic toll collection systems. More expensive on-board units have batteries which need replacing.
Infra-red DSRC uses infra-red technology, as opposed to radio spectrum or microwave, for short-range communications. Infra-red DSRC can be used in ITS where it is difficult to secure a frequency spectrum license. The technology is also appropriate when the weather is generally rainy – but not foggy. Infra-red DSRC is less susceptible to security intercepts.
Bluetooth is a wireless technology designed to allow data exchange over short distances (a maximum of about 10 meters). Most cell phones on the market today have Bluetooth technology. They also have Wi-Fi wireless technology, which uses radio waves for connections for distances to a Wi-Fi base station of up to 90 meters. In recent years, several automotive manufacturers have been embedding Bluetooth technology into their vehicles to allow drivers to connect their phones or music devices to in-vehicle audio systems.
When activated, Bluetooth and Wi-Fi transceivers continuously broadcast “discovery” messages to allow other devices to find and connect with them. The discovery messages include a unique identifier that can be used for vehicle detection and tracking. Essentially, all that is needed is a Bluetooth or Wi-Fi sensor installed close to the roadway. These sensors record the time at which a a vehicle equipped with an on-board Bluetooth or Wi-Fi device drives past them. By utilising the unique identifiers recorded at successive monitoring points, information on travel times along a road segment – or the pattern of Origin-Destination flows through a network – can be derived.
The use of Bluetooth or Wi-Fi is ideal for crowd sourcing but the results have to be calibrated as:
not all vehicles report an identifier – since some will not be equipped with the technology or the equipment may be turned off – leading, in both cases, to no count being registered
or a single vehicle may have several active devices – leading to multiple counts.
When using this vehicle sampling technique a key challenge is to ensure that a sufficiently high proportion of vehicles are equipped with Bluetooth and Wi-Fi devices. In urban areas, this may not be a major concern, but in other regions low market penetration may limit the application of these detection technologies.
Communications over a wide area are often required in Network Operations – particularly in rural areas where the options for voice and data communications and the transmission of CCTV images are more limited.
WiMax stands for Worldwide Interoperability for Microwave Access. WiMax is designed to provide much higher bandwidth, compared to Wi-Fi, and at a much extended range. Recent years have seen increased interest from the ITS industry in integrating WiMax and Wi-Fi as an alternative communications medium to wired communications.
Under ideal conditions, WiMax could have a range of more than 40 kilometres, and offer speeds of up to 70 Mbits/seconds. It implements the IEEE 802.16 Standard, with newer standards designed for speeds of up to 1 Gbits/second. A typical WiMax network would consist of a base station connected to several client radios (Customer Premise Equipment or CPE).
Cellular networks were established primarily for voice communications, but there is steadily growing increased interest in their use for data communications as well. Two voice communication cellular technologies have evolved in this way:
Global System for Mobiles (GSM)
Code Division Multiple Access (CDMA)
They differ in the way they transfer data. GSM divides the frequency band into multiple channels for use by different users. CDMA digitises calls and unpacks them at the back end. Both GSM and CDMA have been refined and enhanced over the years to allow for increased speed. For example GPRS (for packet data communications) uses the GSM cellular network at speeds suitable for transmitting commands to VMS and car park counters.
The latest cellular data technology is Long Term Evolution (LTE) (also known as 4G LTE). Unlike GSM and CDMA, LTE is designed primarily for data communications, with voice as an override. It offers high bandwidth, low latency, and supports full data rates while travelling at high speeds (a feature which is important in the ITS environment with fast-moving vehicles). Cellular data communications can be used in ITS where wireline communications are not available or are cost-prohibitive.
Digital Radio Data (DRT) refers to the practice of transmitting digitised and compressed data over FM radio. This allows small amounts of digital information to be embedded in conventional FM radio broadcasts. A good example of a DRT application in ITS is the Radio Data System-Traffic Message Channel (RDS-TMC), where digitally encoded traffic information is made available for in-vehicle navigation devices. RDS-TMC is an early form of digital data transmission. It was developed in Europe to exploit the Radio Data System used by some broadcasting authorities. Travel information is transmitted digitally over FM radio frequencies – and a decoder, built into the car radio or navigation device, interprets the digital code for text or graphic display.
Spread spectrum radio is a radio network that has a “line-of-sight” requirement (unobstructed line of path between a subject and an object). In this network, one radio serves as the master and the other as slave. An example of the use of spread spectrum radio in ITS is the connection between a set of traffic controllers at signalised intersections and the Traffic Management Centre (TMC) needed for monitoring and signal timing purposes.
In some cases, it might not be possible to directly link the two radios because of distance or interference – in which case another radio (called a repeater) would need to be installed in-between the two radios. These networks are most commonly used to allow a number of traffic controllers to communicate with one another, or to communicate with a traffic operations centre. They can broadcast over the unlicensed frequencies of 900MHz, 2.4 GHz and 5.8 GHz. The 5.8 GHz frequency provides the highest bandwidth at about 54 Mbits/second, but is very susceptible to line-of-sight problems.
Whilst theoretically, spread spectrum radio can provide a communication range of up to 60 miles, in practice the range is much shorter because of line of sight attenuation or obstructions.