Signal distribution / Control

Phil HIppensteel on 802.11 Wireless (Wi-Fi) and Video

6/11/2017 6:37 PM Eastern
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Customers who are implementing IP video systems often ponder the role wireless might play. It seems to offer some significant advantages. Two of the most promising are mobility and avoiding new cabling. But, can wireless provide the required level of video quality and security? That’s the contention we’ll investigate in this newsletter.

First, a primer on 802.11 wireless. Commonly referred to as Wi-Fi or wireless Ethernet, neither term is really accurate. The first is a trade-mark of the Wi-Fi Alliance and the second is used because 802.11 wireless nearly always extends a wired Ethernet network. I’ll consistently refer to this technology as 802.11 wireless because it is specified by the IEEE as the 802.11 series of specifications.

By now, most of us have learned that there are several versions of 802.11 wireless. 802.11 b came first and had a nominal speed of 11 Mb/sec. 802.11g came next and operated at a nominal speed of 55 Mb/sec. This was followed by 802.11n with a nominal speed of 100 Mb/sec and variations that advertised 150, 300 and 600 Mb/sec. The latest version to be ratified is 802.11 ac, nominally operating at 1 Gb/sec. Currently the most widely deployed wireless is probably a mixture of g, n, and ac.   The devices used by consumers tend to be mostly n or ac.

However, these are theoretical maximum speeds and should never be considered a real and practical operating speed for your application. Table 1 shows the results of a small test I conducted with my own 802.11n router/access point (AP). It’s nominal speed is 100 Mb/sec . I merely moved my notebook computer to different positions in the office.

There are several lessons we can learn from this table. But an explanation of the terms is necessary. The signal level is measured relative to 0 dB, which is 1 mW. Each 10 dB the signal is lowered, the power level is cut to 1/10 of the previous level. So, the table shows that moving the computer by five feet to a position 10 feet away decreased the signal strength to nearly a tenth of the previous level.

                                                                                                                                                                                    Table  1    

Distance  to    AP     (feet)  

Resulting  Speed     (Mb/s)  

Received    Signal    level     (dBi)  

5     

72  

‐32  

10  

72  

‐40  

15  

58  

‐52  

20  

58  

‐57  

30  

43  

‐62  

At 30 feet from the AP, the signal level was 1/1000 the level when it was five feet away. So, the distance between the device and the access point is very significant. You can also see, moving the computer to a distance of thirty feet away from the AP decreased the operating speed (bandwidth) to less than half the nominal speed of the access point.

I had no other wireless devices in operation, no competing neighbor devices, and I chose the best position for the antenna in my computer. These are ideal results, not what you would typically expect in a classroom, workspace, or restaurant. The air is a shared medium. So, if there are five people simultaneously using the wireless AP, cut the speed rates in the table by approximately a factor of 1/5. If there is competing web traffic using HTTP/TCP, it will burst in spurts to using 60% or more of the available bandwidth.

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