Today, most mines run several distinct networks to perform several communications tasks.
Twisted-pair is commonly used for fixed telephony, while VHF and UHF radios are used for mobile voice communications and a twisted pair-based fieldbus (such as Profibus or DeviceNet) is installed for data collection.
Recently in some mines, leaky feeder has been introduced to provide more widespread voice-radio coverage and allow communications between radios when the distance exceeds their normal range. This coverage is extended up to surface to allow communications between underground and surface personnel.
Somes mines have recently begun to investigate the use of Ethernet underground. Access to data underground is usually restricted to human-machine interface display screens connected to PLCs. However, in the last few years some PLC installations have been done using Ethernet, which has encouraged some mines to examine the possibility of using Ethernet for more than just data for the PLC networks. Several mines have installed limited systems themselves to bring the company LAN underground to a few fixed computers.
Twisted-pair was perhaps the first technology used for voice and as a result it is well understood and fairly inexpensive.
It offers just enough bandwidth for voice, however special modems are required to send data as these connections are point-to-point via pairs of modems. Although there is some extension of the validity of point-to-point wiring with the advent of DSL, it is still relatively slow and needs many repeaters and phone equipment to send it over long distances.
Twisted-pair is still useful where you want a cheap limited distance PBX for phones, such as in a small office, but even above ground, PBX for telephony is being replaced by VoIP (Voice over IP) equivalents as prices come down.
The only reason twisted pair will not be obsolesced in the near future is because it is already installed and difficult to replace physically.
Leaky-Feeder is, on the other hand, an analogue push-to-talk communication system where one person talks and everybody else listens. The system divides the cable bandwidth into very small chunks and, like twisted pair telephony, it needs analogue modems to carry data. It is popular because it provides fairly cheap wireless voice coverage. However, the ability of leaky-feeder to carry data is very limited and unreliable. The RF signal from the cable has limited propagation and while this is not a problem in a small tunnel, it can be an issue where larger coverage is needed.
Leaky feeder is still useful when a location already has a radio investment and data applications are not required.
Northern Light Digital, by contrast, is a robust, modular Ethernet network with a suite of related applications designed specifically for underground mining.
The applications are integrated into a common software framework and presentation that is web-based, so underground personnel can access them via wireless or fixed connection.
Some applications use miners’ cap lamps and power source as a platform for communications or tracking.
Current applications include: tracking of personnel, vehicles and equipment using RFID technology; gas monitoring and airflow sensing; fixed and mobile voice communications using VoIP; fixed and wireless data access underground.
The network runs on a true, open-standard Ethernet backbone, which may be over twisted pair cable (CAT5) at 100Mbs multimode fibre at 100Mbs or single mode fibre at 100Mbs.
The central units of the backbone are run in a redundant ring over 1Gb fibre and the network can take advantage of fibre cable already installed.
The backbone is configured for Virtual Local Area Network (VLAN) functionality and true quality of service to assure reliable data transfer and connectivity, and to increase bandwidth. This helps to run multiple applications (voice, data and video) over the same network.
This backbone is accessed by a series of rugged, modular nodes and can contain components for 802.11 wireless communications for voice and data; tracking and tagging of personnel and machinery; telemetry units for gas monitoring, temperature and airflow sensing; and proximity detection.
The components are modular, so they can be added or removed at any time. In this way a node only has the equipment needed for the applications supported in that area of the mine. The nodes are field-serviceable by electricians and do not require dismounting and remounting of enclosures.
The nodes are easy to install and lightweight, and are rated to NEMA 4X (IP55) to prevent dust and moisture ingress.
The units can run from 110V or 240Vac power and can supply power to adjacent nodes up to 300m away via a dc power cable when ac power is not available.
All units contain a power supply and a battery backup that lasts for a minimum of eight hours. Cutover to battery and back again is transparent to all applications and causes no service interruption.
Maintenance is much simpler and the knowledge of several different protocols and wiring systems is not required because all the applications and services run over the same network,.
Applications for this system include VoIP for fixed and mobile voice communications, “Messenger”, which is a two-way 802. 11 text messaging system using backlit LCD screens embedded into cap lamps, vehicle telemetry for monitoring of vital statistics and run time, fixed gas and airflow monitoring, RFID tracking of vehicles and personnel and real-time video. Of course the mine’s PLC network and IT network can also run over the same infrastructure.
The system can incorporate technology advances and its modularity ensures there is no need for an entire redesign of the system.
Importantly, investment is not tied up to the development schedule of a single vendor.
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