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Five performance metrics for wireless device networking

Tue, 09/06/2011 - 5:48am
Sourav Dey, Systems Engineer, On-Ramp Wireless
Sourav DeyThere are many positive use cases for enabling wireless device connectivity, including the smart grid, gas and water distribution systems, building energy management, shipping container tracking, and more. Only recently has technology been developed to address the particular needs of this segment. Though the communication link is one piece of a complete device network, it is the foundation—the base of the pyramid. As the foundation, it is an extremely important piece to get right. It is virtually impossible to correct inadequacies in the PHY/MAC at higher layers without sacrificing other fundamental aspects. For the utility space in particular, the stakes are very high. The selection of a non-suitable technology could threaten the existence of a substantially larger ecosystem that uses the foundational technology. Fortunately, the performance of these lower pieces of the pyramid can be analyzed objectively, and unlike higher layers, clear decisions based on objective criteria can be made. 

Figure 1. The PHY and MAC layers are the foundation of any wireless communication technology

Wireless Performance Metrics

There are five key performance metrics to consider when evaluating a wireless solution:

• Coverage: The range of a wireless link to reliably connect devices.

• Capacity: The amount of data from device endpoints an AP can simultaneously serve.

• Coexistence: The ability of a wireless technology to coexist with other devices that can cause significant and dynamic interference.

• Power Consumption: The ability to support a long battery life.

• Security: The ability of a wireless system to resist malicious attacks from cyber criminals.

Coverage
The primary driver of coverage is link budget. Link budget measures the ability of a wireless system to overcome obstacles to close a link. Link budget is driven by many factors, but at the highest level, there are three primary factors that contribute to link budget: transmit power, antenna gain, and receiver sensitivity.

In all radio frequency bands, especially unlicensed ones, regulation limits the Effective isotropic Radiated Power (EiRP), which ultimately limits transmit power and antenna gain. This leaves receiver sensitivity as the differentiator. Receiver sensitivity reflects the radio's ability to detect signals—the lower the sensitivity, the better. This is particularly important in the device networking space to ensure that the signal can penetrate into difficult radio environments. A good rule of thumb is to compare the receiver sensitivity to the thermal noise floor. A strong radio will have the ability to operate at negative SNRs, well below this floor.

Capacity
It is key to differentiate between the single-link throughput from a single endpoint to a gateway, and the aggregate capacity of the gateway serving many endpoints. The single-link throughput of a technology could be modest, but if there is a good multiple-access scheme, the aggregate capacity could be extremely high. Since most devices have low throughput requirements the aggregate capacity is the metric that matters.

Many wireless technologies quote a raw PHY-layer data rate as if it was their aggregate capacity. But that number often has no bearing on the aggregate capacity of interest – the application-layer data rate able to be sustained at the gateway. This number is often much lower because of MAC overhead, poor multiple access techniques, and required margin to account for unpredictable traffic fluctuations. It is important to account for these factors when assessing capacity.

Coverage and capacity must work in parallel to achieve a highly functioning network, and work in conjunction to drive the ratio of gateways to device endpoints. Providing coverage for many devices without proper capacity is pointless; conversely, excess capacity without coverage to reach devices is equally futile. It is the optimal mix of the two that delivers cost effective performance for a wireless network.

Coexistence
Device-networking radios must be robust to interference and a propagation environment that is in constant flux. This is particularly true in unlicensed bands, where transmitters are lightly regulated and traffic patterns are unpredictable. A strong radio will have interference mitigation techniques built in at every protocol layer. At the PHY-layer, techniques such as direct sequence spread spectrum (DSSS) that can operate at negative Signal to Noise Ratio (SNR) are particularly strong because they are designed to withstand far more co-channel interference. At the MAC, it is imperative that an acknowledged point-to-point data transfer between the endpoint and the gateway is implemented. A properly implemented protocol makes the underlying wireless medium, with all its interference and channel variation, seem like a wire to the higher protocol layers.

Power Consumption
A strong device networking technology should be configurable to provide a low latency, highly responsive connection for continuously powered devices and an extremely power-efficient connection for battery-powered devices. To be cost effective, applications such as water meters, gas meters, and distribution automation devices, should be able to achieve a battery life of greater than 10 years.

Power efficiency is driven by the wireless protocol. The key to efficiency is to minimize the time and power spent transmitting and receiving radio waves. This can be done using various techniques including fast acquisition – reducing the amount of time it takes to find the network and adaptive modulation – minimizing the amount of time needed to transmit the packet. In addition, the protocol should be designed such that the endpoint is in a low power “deep sleep” mode most of the time.

Security
Many of the target applications for device networking are critical infrastructure endpoints. Once the endpoint devices are connected, they can become the targets of hacking and cybercrime. They require a secure network that is built using proven algorithms. A strong network should use a comprehensive defense in depth strategy to deliver this information security, including:

• Prevention mechanisms: Provide access control, mutual authentication, confidentiality, and high availability.

• Detection mechanisms: Identify system breach attempts and alert operators.

• Recovery mechanisms: Ensure the system degrades gracefully and continued operation even while under attack.

A secure network should provide these key security guarantees preferably using NIST approved security algorithms that have greater than 20 years of life.

Conclusion
There is a confusing array of vendor choices when evaluating wireless device networking solutions including cellular, unlicensed wireless mesh radios, lightly-licensed narrowband radios, and unlicensed wideband radios. Many of these technologies were originally designed for applications that are significantly different from the requirements of device networking and have significant drawbacks adapting to the needs of this space. They are like a square peg in a round hole. Due to the unique challenges device networking presents, a successful wireless technology for this space must be purpose-built. Fortunately, there are a number of companies developing technologies purpose-built for this space. The important thing to remember, though, is to judge each technology objectively for your needs.

Sourav Dey is a Systems Engineer at On-Ramp Wireless. More information is available at www.onrampwireless.com.
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