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Wireless Networking Limits - Part 2 of 3 - Throughput

By: John Fredrickson on September 10th, 2014

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Wireless Networking Limits - Part 2 of 3 - Throughput

wireless  |  radio  |  ethernet  |  5.8 GHz  |  RF  |  300 Mbps  |  point-to-point  |  point-to-multipoint  |  throughput  |  data rate  |  tech support

At AvaLAN Wireless, we have done our best to design our products in such a way as to push the limits of possibility, sometimes even venturing into the realms of the unfathomable. However, the Laws of Physics and the Universe still govern much of what we can and cannot do. We still have limits. The trick to making the most of our products is to know those limits. Knowing your limitations is not just beneficial to your project, it is crucial to its success. Knowing your limitations will save you time, money, effort, and especially frustration.

In Part 1 we looked at the line of sight limitations of the 300Mbps 5.8GHz Radios (AW58300HTA, AW58300HTS, and AW58300HTP-PAIR). These include line of sight, throughput and Gigabit cable lengths. In Part 2, we will discuss the throughput limitations.

Throughput 

AW58300HTPThroughput is an important aspect, and limitation, that needs to be kept in mind when designing any network, especially a wireless network. We at AvaLAN are excited to be able to provide radios that can provide up to 300Mbps radio data rate, but it is important to recognize what that entails. Unlike wired transmission, only a fraction of the radio data rate is usable as Ethernet throughput. The rule of thumb is typically only 60% of the radio data rate can be turned into actual Ethernet throughput, and so with the 300Mbps radios, you can get up to 180Mbps of throughput. 180Mbps is great and all, but where does the other 40% of the radio data rate go? Well, a lot of that data simply makes up the “vehicle” in which the Ethernet data travels. Let’s say I am an Ethernet data packet. I live in a city with roads that connect location to location. These roads are kind of like the wired network. An ethernet packet can jump in a car and drive quickly to any location that the roads interconnect. But if I want to cross an ocean, there are no roads. I need a more complex vehicle, like an airplane, to take me across the ocean. There are a lot of complicated parts to that airplane, and a lot of protocols and safety equipment built into it to help me arrive at my destination safely. The same goes for wireless. The Ethernet packets must be padded, and protected, and stamped with an itinerary as to where and when they need to go to their destined location. So if you think of the radio data rate as the airplane, with the Ethernet data as its cargo or passengers, you can somewhat visualize why you aren’t getting 300Mbps worth of Ethernet throughput.


Next, we need to determine whether or not you are planning on using a point to point, or point to multipoint wireless configuration. It is important to remember that, for the 300Mbps 5.8GHz radios (and for all wireless radios for that matter), your aggregate throughput is calculated
per Access Point. This includes transmit and receive. So, for example, if you have one access point, and 4 subscribers, the 180Mbps Ethernet throughput would be share between the 5 radios, giving them an average of 36Mbps throughput each. Thankfully, these radios will auto negotiate to balance an uneven load. They access and shift the throughput to wherever it needs to be, but nonetheless, you still have a total maximum aggregate Ethernet throughput of 180Mbps.

I would like to emphasize that all of the numbers are based on ideal conditions. These are the maximum throughputs. There are a few more factors that will affect the data rate that you ultimately get. These include your RF environment, broadcast distances, and cable lengths. The invisible world of the RF environment can be a very busy and crowded place. If there are too many other devices shouting on that frequency, it will be harder to get data through. Also, the farther the data has to travel, the less data will get across in a timely manner (to get a better understanding of how the distance affects radio data rate, see this chart on Data Rate Vs. Distance.).

In Part 3 we will explore the limitations of Ethernet Cable Lengths, specifically with regard to the 300Mbps 5.8GHz Radios (AW58300HTA, AW58300HTS, and AW58300HTP-PAIR) which can negotiate at speeds up to 1Gbps.


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Written by John Fredrickson