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 right-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.