Mining Network Functions
MineNET, Becker’s network technology for use in the underground mine environment, combines device functionality so devices can be daisy-chained in order to reduce installation effort and to make the network literally follow the mine’s layout. Functionality, reliability and safety require an extraordinarily consequent network design, in many cases meeting the needs of three different generic end uses: automation (which is real-time critical), safety (very high demands on survivability and reliability) and a general IT “underground intranet” system (incalculable bandwidth behavior). In many cases, a separate network will be designed for each of these uses, running in parallel on separate hardware if required or using different virtual LAN’s with a strict management of the relevant Quality-of-Service (QoS) parameters. For redundancy reasons the network hardware can be set up as ring structures.

The Automation network is used for all real-time critical communication, such as in machine remote control or the communication of timing sensitive information between distributed automation systems. The Automation network is typically run on a limited bandwidth philosophy so the real time behavior can be statistically assured (“quasi-deterministic” behavior).

The Safety network runs all communication which may be subject to local safety approvals by authorities, such as voice communication (telephony, PA systems, etc), tracking of underground personnel or forwarding of information from ventilation and gas sensors. If an underground emergency and related power loss occurs, the safety network at least is designed to survive for the amount of time required by the relevant authorities.

The IT network covers all remaining network traffic including file transfer, access to the Intranet, and mobile personal IT applications for miners using PDAs, Pocket PCs or notebook computers. In this network a certain bandwidth is available for all IT traffic. However, since the network behavior is not strictly controlled, latency times and throughput may vary depending on the current network use.

Network Design
Wireless LAN is increasingly used underground for nearly all wireless communication, including wireless telephony, tracking, machine communication and mobile data access. But special care has to be taken with the coverage design of the wireless network.

First, the mine must decide which areas should be completely covered and which areas can be served using a “hotspot” type of layout. As the coverage per access point (AP) is highly dependent on the mine’s tunnel cross sections, its intended applications and the choice of antennas, the final coverage plan is usually established during an on-site evaluation. RF disturbances in tunnels are common as a result of the multi-path feeding effect— where the signal is received a number of times because the signal bounces back and forth in the tunnel—and it has been shown that a direct line of sight between the client device and the AP will give the best possible performance. In earlier applications usable coverage of up to 300 m per AP in straight tunnels has been achieved. The different networks for Automation, Safety and IT can be mirrored on the wireless network by assigning different RF channels to different networks or by using different frequency bands.

In order to integrate the WLAN base stations into the MineNET backbone, they can be set up as universal networking devices (NetNodes) consisting of:
• An Ethernet switch with direct hook-up to a fiber optic or copper-based backbone;
• One or two independent WLAN interfaces for interference-free, parallel use of different network types;
• A central CPU for switch and WLAN management and extended application functionality (tracking, etc);
• Optional mining-specific add-ons (such as sensor interfaces and battery backup);
• An optional extension unit with up to six additional fiber ports so that up to two Ethernet rings can be linked at the device.

Using this principle of “daisy chaining,” where the WLAN-capable network nodes link to the wired infrastructure, there is no need for the dedicated APs to be connected to the next switch via potentially long cable lines in a star network layout. At the same time, the network nodes provide a (wired or wireless) entry point to the network for any additional distributed information available throughout the mine. So this distributed networking component has an important additional function for the acquisition of, for example, ventilation and gas sensor information, and it provides this functionality at marginal additional cost compared to alternative solutions.

When mobile machinery operates underground, it seldom moves within the coverage area of one single WLAN access point. If a machine moves into a different drift its data traffic typically has to be transmitted via another WLAN base station or access point. The process of moving from one AP to another is called roaming. As the WLAN standard was not originally designed to be used for mobile equipment, the roaming algorithm used by most implementations is quite simple: As soon as the client (machine) loses its connection to one AP it starts searching (scanning) for another that is within reach. Having located an available AP, the client connects to it and data traffic is ready to continue. But this handover may take up to several seconds, far too long for mobile units that may be remote controlled with an operator watching the machine via video.

To solve this problem, a reliable special roaming function has been developed which does not need the time-consuming search procedure and optimizes the handover itself. A handover latency of 2-5 msec can be achieved, so that roaming is invisible even in video streams. This “ROAMEO” function needs no modification of the access point infrastructure nor does it require any alterations to the WLAN standards.

Telephony and Public Address Integration
One goal of installing a universal underground network is to make telephone and public address systems available throughout the mine. Voice-over-IP (VoIP) technology, nowadays standard for Internet telephony as well as for nearly all private and public digital telephony systems, can be used for this purpose: both wireless handheld VoIP telephones and stationary Ethernet phones are now available for underground use. In the future, the use of wireless handheld devices underground, together with a VoIP gateway above ground, will be combined with VoIP based PA systems installed, for example, along conveyor belts or in fixed underground installations. This approach does not require a separate PA infrastructure while the speech quality is excellent—irrespective of cable length.

The system developed by Becker Mining Systems also uses the fiber optic Ethernet infrastructure and can ideally be combined with the underground MineNET WLAN infrastructure. It integrates PA systems with modern wired and wireless VoIP telephony. Phone calls directly to the PA system will be possible as well as contact with centralized dispatcher units located anywhere in the company’s Intranet. This fully digital system eliminates all of the cable length restrictions imposed by traditional PA systems.

Network Management
In MineNET, network and infrastructure supervision and administration are regarded as an integral part of mine operation and overall mine process control. This provides a large amount of additional functionality and information that is vital for forward looking mine operation and mine process optimization: The benefits include:
• Immediate issue identification: using a 3-D visualization tool like MineVIEW to check the network’s online status;
• Rapid recognition of the true causes of malfunctions: for example, switching to the power supply layer in the 3-D mine visualization tool may show that the suspected network node is out of power so an electrician is needed rather than a network specialist;
• As the network can be “mirrored” mathematically in the MineVIEW program, “missing link status” information can be used to identify underground hazard locations;
• Automatic reconfiguration if extraordinary operational situations arise; and
• Automatic checking for configuration consistency before sending a configuration to a device.

Integrating network administration and management into the mine process operations in this way benefits mine operations significantly. For example, visualization of all tracking information appears in the mine’s up-to-date 3-D model. As this software is also used for network node administration, tracking information can be linked to the local network status to assess tracking data relevance (for instance, when network nodes are temporarily switched off or out of power). The visualization can then also oblige the network nodes to reconfigure into specific “safety modes,” or to reconfigure the surviving network after an underground emergency.

Especially, Müller says, extended safety functions will in the future become a crucial part of any location-based central network administration. The network nodes can be instructed to send emergency messages to the miners’ personal devices and the central visualization display will be able to calculate individual and dynamic escape routes for the miners in order to lead them to the safest exit. For this purpose the underground network nodes can also be used as navigational aids to guide people safely to the exit even if they are not familiar with the underground layout. The safety network runs all communication which may be subject to local safety approvals by authorities. Becker Mining Systems will dedicate a major development effort to network-related mining safety functionality in the years to come.

Application Examples
The world’s largest underground WLAN network has been built up in hard coal mines operated by RAG Deutsche Steinkohle AG (RAG) in Germany. RAG currently uses more than 200 APs for logistics applications and material tracking as well as for telephony and for connecting fully automated underground monorail trains with the control room. The units have been in use since September 2005, have performed as intended and have confirmed the reliability of the design.

The installations use an intrinsically safe WLAN AP purpose designed by Embigence, now a subsidiary of Becker Mining Systems. This AP consists of the access point itself plus an integrated switch and media converters to attach the unit directly to the mine’s fiber optic network. Two fiber optic ports are provided to enable easy installation in the drifts by simply chaining the APs along the fiber optic network. A third fiber port is provided as an option to enable both the addition of branch lines and the connection of stationary underground PC’s. The AP has an ATEX type approval certificate as an intrinsically safe system.

Recent developments include the use of WLAN technology in longwall installations. Working with the German shearer manufacturer Eickhoff, Embigence designed the first application for a longwall at a Slovenian coal mine in 2006. This installation has been working since August 2006 and has functioned reliably despite the extremely harsh environment.

Other applications that have been installed include remote control and video applications via WLANs for market-leading equipment manufacturers.

One particularly complex application involved full automation of a monorail train used for underground material transport. The entire system consists of a mobile local area computer network on the machine and central IT components in a control room. All communication is carried out via Ethernet technologies using a wireless LAN and a fiber optic backbone. An additional challenge here was the fact that the train runs in a potentially hazardous coal mine environment so all equipment used was subject to “Ex” approval.

The system on the train consists of four computers used for different application purposes. Two combined video server / communication gateways are located at either end of the train, close to the traditional operator cabins. One machine web server acts as an application server for the IT clients accessing the train and as an ITlevel machine application controller. One machine controller bridge communicates with the train's proprietary electronic controller and provides the process image to all other computers in the machine's local network. Additionally, an application server is used in the control room to physically separate the monorail network from other networking infrastructures and to coordinate the traffic of multiple machines.

Communication in this system is solely carried out via Ethernet and underground wireless LAN. Communication between the train's network and the above ground application server is run via a VPN that assures network security and data compression. A separate Multi Path Routing Application is being used to enable fully redundant application level communication from both ends of the train to the stationary network. Video information sent via the WLAN is used for remote machine supervision as video-on-demand functionality.

In this project a large number of new components and technologies were integrated into a fully automated underground mobile machine and the advanced nature of the project was recognized by an innovation award in 2007.

Müller concludes that Ethernet-based networks are the technology of choice for up-to-date and future oriented universal underground communication. Networks should be designed in such a way that network status and configuration information can be integrated into future process optimized mining operations. This is especially relevant when the network infrastructure and its active components can be used to provide additional benefit to the mining operations as well as performing their basic networking tasks. For example, using the network to track information generated by the mining infrastructure saves the investment in a separate tracking system and no separate client devices are needed.

The same applies to integrative, location- based status visualization and network management from within 3-D mine process visualization software: This software can be used for infrastructure management and to display tracking information with one single tool which, moreover, can be used for overall mining process management. Numerous application examples show that investment in a dedicated value added mining network pays off in daily operation.

Acknowledgement
This article is based on the paper Adding Mining Specific Value to Underground Network Communications that Müller presented at the Massmin 2008 conference in Luleå, Sweden.