Martin Finlayson, Head of Visual Communications, avsnet

If you’ve ever experienced frustration because your video call goes out of sync, the picture freezes or worse still, the call drops, then let me explain the 3 biggest causes of these issues and how to remedy them.

In keeping with the motto of ‘a chain is only as good as its weakest link’, Wi-Fi could well be your downfall when it comes to UC so, the 3 causes then:

  1. Access point contention ratios
  2. Access point settings
  3. Roaming paths

Wi-Fi is very much a smoke and mirrors technology for most people. It is so much more than just getting 5 bars. For many years, I celebrated the fact that I’d remembered the password correctly, got a decent signal strength and then didn’t think any more about it.

It’s only when things don’t work as expected that you investigate further, only to have your happily ignorant illusions shattered by the reality of a technology. Now that I have taken the time to investigate a little further into the dark art of Wi-Fi, I am in awe that it works at all with UC in many environments!

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So, what’s the issue?

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Wi-Fi operates on one guiding principal; all devices are equal.

If a device wants to transmit data over the network it listens out to make sure no one else is transmitting and if it’s quiet it transmits its request to the recipient and waits for an acknowledgement.

If all goes well, the receiver transmits its acknowledgement and a window of opportunity is opened when all other devices refrain from transmitting, the duration of which is dependent on the traffic type being sent. If however an acknowledgment is not received – maybe because two devices transmitted at the same time – then the device backs off for a random period of time and tries again.

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So, what’s the problem, this all seems to work right?

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Well yes and no. This principal of operation means that wireless networks, unlike wired networks, can only receive or transmit, they cannot do both at the same time (this is called half-duplex).

This immediately means that the wireless network has to operate twice as fast to achieve the same throughput. Then you have the impact of collision detection and subsequent random timers, they also limit the maximum throughput of the system.

What’s worse is the more devices you have connected to your access point, the worse the problem becomes until everything effectively comes to a standstill because all the devices are constantly colliding and then waiting random amounts of time before retrying. In short, the channel becomes saturated.

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OK, so how do I avoid it?

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Almost all wireless access points (WAP) have the ability to limit the number of devices they allow to connect at any one time.

However, the default value is usually set to an unrealistically high number (it looks good on a datasheet). A WAP may have a default value of 200 but the throughput performance falls off a cliff when it’s above about 25 concurrent connections. Now you may not care much if these devices are just browsing websites and doing emails. But what if your users are reliant on these devices for their enterprise communications solution!?

Now your entire enterprise communication strategy may be in jeopardy because you haven’t deployed enough WAPs around the building! This ‘device to WAP’ relationship is referred to as the contention ratio, keep it to 25:1 or less.

So, I’ll have to deploy more WAPs.

Here is another potential trap for the unwary. Just because a device can connect to a WAP with a full 5 bars doesn’t mean everything is rosy. You see, the default power output for a WAP is 100%. So what’s wrong with that, surely we want the access point to cover a decent area with a good signal strength?

Well, actually, no.

You see while the WAP may be capable of microwaving your favourite brand of marshmallow at 100 paces, your personal device most certainly isn’t. And that’s a good thing, otherwise your battery would last 5 minutes and you would cook your grey cells every time you held the thing next to your head!

However, this asynchronous ability to transmit deceives the device into thinking it is much closer to the WAP than it really is.

This means that when it broadcasts its request to send some data it defaults to its maximum speed, only for the WAP to receive the barest whisper of a signal that it ignores as background noise.

Only after your personal device has tried and failed several times will it give up and try lower and lower speeds until it receives a successful acknowledgement from the WAP. As you can imagine, all this retrying and down speeding has a significant impact on the Wi-Fi throughput.

In order to avoid this the best practice is to match your WAP power output to the devices it has to support.

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But wait, we’re not finished yet…

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As if device contention and WAP power issues weren’t enough, there are the users themselves you have to contend with.

You see the problem with mobile devices is, well, they’re mobile. While a user may be content to sit stationary at their laptop to have a video call with a colleague, the moment their mobile rings they’re up, pacing around the office like a caged tiger.

If that call is being handled over your Wi-Fi then that user will be blissfully unaware of the explosion of traffic that they are causing by roaming the halls. You see all the time your device is connected to a WAP it is also constantly on the lookout for, and prioritising any alternative WAPs in the event that the current connection should be lost. A loss like when a user walks out of the reduced coverage area your WAP is now restricted to because it has to match the miniscule power output of the user’s mobile device.

So until this point you thought you only had to put enough coverage in for where users are sitting. But no! Now you have to also consider where they will be walking around. The halls, kitchen areas, office reception, the areas outside meeting rooms! You now even have to think about outside areas – gardens, the rooftop, smoking areas, car parks, joyfully the list is endless.

So now you know what I know. The good news is that you don’t actually need to worry about any of this. You just need to find a partner that understands this stuff properly and can design and deploy a wireless network that is fit for your purposes. All you need to do is clearly define what those purposes are.

As always with my blogs, if you want to know more, or maybe just want to drop by and toast some marshmallows, please get in touch. I like the pink ones best but don’t tell my daughter…

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