A Spectrum of Possibilities

After the spectrum analysis is complete, you are able to determine which frequency bands are available to you. This should help you decide whether to deploy a licensed or unlicensed solution (for larger networks, it is common to deploy a combination of the two). FCC licensed frequency bands for PtMP and Mesh networks have limited availability and can often be very expensive due to the high demand from many of the large cellular carriers. Therefore, it is practical to deploy most PtMP and Mesh solutions using unlicensed frequency bands.

Licensing a PTP radio solution can be a fairly painless process. If you’re unfamiliar with it, there are many organizations that specialize in this service. When licensing a PTP link, you must plan ahead a minimum of six to eight weeks to allow for paperwork to b
e filed, completed and approved by the FCC.

Winning the Bandwidth Battle

Now that you have completed your site survey and outlined you network topology, how do you know which radio manufacturer to choose? All radio manufacturers highlight their individual feature sets, price points and differentiating factors that make their products unique. There is another key component to deploying a successful system: bandwidth. Understanding the bandwidth requirements of your surveillance application and knowing how much bandwidth the radio will actually provide in a real-world deployment (not what the marketing department puts on the datasheet) is essential to providing a high quality video stream.

Calculating bandwidth requirements for a camera is relatively easy. There are many free bandwidth calculators available online (search “camera bandwidth calculator”). The combination of frame rate, resolution and compression will determine how much bandwidth the camera(s) will use. However, understanding the actual data rate of a wireless system is not as clearly identifiable. The reason is simple: Most radio manufacturers advertise the aggregate throughput of their radios, not the throughput that their radio is capable of pushing in one direction at any given time. Since bandwidth consumption for video is primarily used for uploading, it is crucial to understand how much bandwidth is available in each direction.

This brings up the differences between Timed Division Duplex (TDD) and Frequency Division Duplex (FDD) and why they are important to consider in any system design. TDD is like a one-lane road where data can only travel in one direction at a time. Most unlicensed PTP, PtMP and Mesh solutions are TDD. FDD can be viewed as a two-lane road where data traffic is simultaneously transmitted and received. Most licensed PTP radio solutions are FDD.

Deciphering the Datasheet

To demystify actual throughput versus advertised data rates, let’s look at your run-of-the-mill 54Mbps TDD radio as an example. There are several manufacturers that advertise their radios will operate at this speed. This number of 54Mbps is the aggregate throughput of the radio. What does aggregate mean? Aggregate speed is adding together the upload AND the download speed of the radio. For example, if you are traveling on a freeway at a speed of 50 mph, then turn around and maintain that speed of 50 mph heading back the way you came, most radio manufacturers’ datasheets would say that you are traveling at 100 mph! A little misleading right? Of course, there’s a little more to it than that.

Since a TDD radio can only transmit or receive at any given time, we can start by taking our 54Mbps and cutting it in half (since we are only traveling in one direction at a time). We also need to take into consideration packet overhead, which is the time it takes to transmit data from one point to the other in a switched network. In a TDD radio, packet overhead can impact the actual throughput by as much as 10%-20%. This doesn’t take into consideration any advanced QoS settings or advanced security encryption that could easily account for another 10%-20% reduction in throughput.

Starting with 54Mbps, cut that in half and you have 27Mbps in each direction. Subtract the assumed packet overhead of 20%, or 5.4Mbps, that leaves you with 21.6Mbps actual data rate in one direction. That is 40% of the bandwidth that is advertised on the datasheet! Could you imagine the quality of video had you designed the network under the assumption of having 54Mbps of total streaming data?

On the other hand, you have FDD radios. The datasheets for these radios tend to be much more accurate. Historically, FDD radio manufacturers advertised their throughput speeds in one direction (the number you care about when designing a system), but recently a number of manufacturers have begun advertising the aggregate throughput, so be sure to ask. Since FDD radios have separate transmitters and receivers, they can send traffic simultaneously in both directions. This prevents them from having the switching packet overhead delays common to TDD radio solutions. Therefore, the packet overhead is typically less than 2% for most FDD manufacturers. Once you confirm whether the advertised data rate is the aggregate data rate or the data rate in one direction, you can easier determine the actual throughput of a FDD radio.

Ready to Earn Your Wireless Wings

When engineered, designed, and deployed properly, a wireless system can provide the same quality and reliability as a wired system. In many cases a wireless solution may be your only option. The topics outlined here are a few of the key concepts needed to design and deploy an effective and reliable wireless system. Use this information as talking points to help better understand your customers’ wants and needs, have more pertinent discussions with your wireless vendor and, most importantly, make sure your next wireless project is a success. 

Daniel Redmond is CSO for Moonblink Communications. He can be reached at (408) 701-5605.

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