Mesh Radio Network Performance in Cargo ContainersMarch 1, 2005 By: Robert Lau, Dr. Peter Fuhr, Oak Ridge National Laboratory Sensors
Wireless sensors installed inside a cargo container? Aren't they metal? Then the radio signal can't get out . . .or can it? Here's how mesh-networked 2.4 GHz, 802.15.4-compliant RF transceivers performed when placed within cargo containers.
Each year $12.5 trillion of merchandise is traded worldwide using more than 20 million intermodal containers; 90% of these shipments are between seaports. As has been reported and documented in numerous studies and observations, such unsecured freight represents a global security threat in terms of both potential for lost or damaged merchandise and the crippling of the global trading economy. Within the more immediate (or tangible) realm of homeland security, such containerized freight could be used to transport harmful biological, chemical, and radioactive materials into the United States and its allied countries. The implications of a security "event" involving a cargo container are enormous. A Brookings Institute study estimated the GDP impact of a shipment, via container, of weapons of mass destruction to a major port: "This will cause extended shutdown in deliveries, physical destruction and lost production in contaminated areas, massive loss of life, and medical treatment of survivors. Potential cost: up to $1 trillion." Add to this the apparently endless variety of goods packed into the containers, and the parameters that sensors could measure seems almost boundless.
The reality is that a cargo container is nothing more than a large metal box with bolts, locks, and doors. Many companies have been working with various governmental agencies to try to define a single technical and logistical solution for the cargo container sensing problem. As corporate entities get involved, disagreements arise over which is the best sensing technology or wireless implementation to use. Yet within the realm of RF engineering lie tangible problems requiring realistic technical answers, the most fundamental being, "Can I put a radio into the container and still have it communicate reliably with the outside world?" It is this specific question that these presented measurements address.
First, What Frequency?
As with any wireless communication system, we must address the matter of which frequency to use and whether to operate in the licensed or unlicensed band. Atmospheric attenuation indicates that the system should operate at lower frequencies (see Figure 1). However, the situation is clouded by worldwide frequency allocations. Given that the cargo containers may be used throughout the world, for worldwide regulatory compliance, operation at (nominally) 2.4 GHz is chosen.
Figure 1. This is a comparison of attenuation levels presented to three common RF frequencies.
Implementation Implications and Network Topologies
While various networking topologies may be used in configuring systems of wireless sensors, a mesh networking architecture conveys certain key advantages for our particular application: wireless sensor systems installed inside containers. While the containers themselves are relatively low-cost metal boxes (~$1500), any installed "solution" must be capable of operating (including both sensing and data transmission) for years while running on a simple battery power source. While advances in low-power RF and sensor design are reported seemingly daily, the reality is that the RF section of the module typically consumes a few milliamperes of current while in the transmitting mode and microamperes to milliamperes of current while in various sleep states. The level of power consumption substantially increases when the RF transmit power is increased.
If the wireless sensor unit installed within a cargo container is to communicate with the outside world, the network topology used for system deployment is critical. For example, if the ever-present star (hub and spoke) topology exemplified by most 802.11 (Wi-Fi) networks is used in this application, each wireless sensor module must be able to communicate directly with the base station. Radio link performance for each node is characterized by the point-to-point radio communication between node and base station. Classic communication theory (and reality) dictates that if the attenuation present in the node-base station channel increases—due to an increased separation distance or some container cargo that absorbs more of the RF signal—system performance will degrade unless transmitted power is increased. However, increasing the radiated power consumes more of the battery, thereby decreasing the installed system's lifetime. It is a vicious circle.
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