Wi-Fi “Secrets” Part 4 — Your Neighbours Are Using Your Wireless Bandwidth

In this next installment of the “guilty secrets” of Wi-Fi networking, we take a look at another facet of wireless connectivity that you may need to think about. Did you know that if you’re using a Wi-Fi network in your home or business, your neighbours may be eating up some of your potential wireless bandwidth? An interesting consideration, especially if Wi-Fi is a mission critical service for you or your organization…

In part two of this stroll through Wi-Fi secrets, we discussed how Wi-Fi networking is a contended medium, where every device that needs to send data needs to wait its turn. As more devices start to use the same channel on your network, the throughput enjoyed by each device starts to degrade as the time-slots available to send data become more scarce.

Part of the solution to this contended medium “problem” is to carefully plan how you configure your wireless network so that you can install more wireless access points on unique channels and add more capacity to your network. Note the word “carefully”, as it’s incredibly easy to cause unintended contention within your own wireless network and have zero overall increase in capacity when adding more access points.

Anyhow, back to your neighbours…

Although you may go to great lengths to ensure your own network is well planned and causing minimal self-interference, you have no control over nearby Wi-Fi networks. Maybe you have other organisations in the same building as your own organization? Maybe you even have co-located organizations or a service provider network inside your own work spaces? There are also a multitude of ‘mini’ Wi-Fi networks created by visitors and employees using their own devices (think smartphones, Mi-Fi devices) to tether other devices to give themselves unfettered Internet access at work. All of these networks have the potential to use the same channels as your carefully built, carefully planned Wi-Fi network.

All of these networks, large and small, use the same contention (sharing) mechanism as your Wi-Fi network. They will have to wait for your networks’ Wi-Fi devices to send data, and you network will have to wait for their clients to send data. There is no escape from this basic medium sharing requirement of Wi-Fi, which is built in to the 802.11 “Wif-Fi” standard. Due to this fundamental mechanism of Wi-Fi networking, you must share available airtime (and hence available bandwidth) with neighbours using the same channels as your network.

Fig 1 – Access Points on the same channel (even if they are a neighbor) must share available bandwidth (air-time)

The caveat to this issue is that the signal levels from neighbouring networks have to be strong enough to be detected in the areas occupied by your organization. However, if your neighbors have their access point powers cranked up to the max, or you have Mi-Fi devices in-use in your organization, this isn’t an uncommon scenario.

How can you mitigate this effect to maximize the throughput of your Wi-Fi network? Ultimately, you cannot stop nearby organizations using the same channels as your organization for their Wi-Fi, but you can lessen the impact by:

  • Careful design of your wireless network. Ensure your RF design takes account of neighbouring networks (in terms of your network channel planning)
  • Encourage employees to switch off their Mi-Fi devices and Wi-Fi phone tethering while at work
  • Monitor your RF environment over time to look at changes in usage by neighbouring networks and modify your network channel planning to suit
  • Perhaps look at using the built-in automatic channel planning capabilities of your Wi-Fi solution to automatically avoid neighbouring networks. Caveat: keep a close eye on your channel planning if using this option: you do not want to avoid neighbouring networks at the expense of causing self-interference on your own network

Fig 2 – Switch your access point to a new channel to avoid sharing available Wi-Fi bandwidth

References:

Share This:
Facebooktwittergoogle_plusredditlinkedinmail

Wi-Fi “Secrets” Part 3— Wired is Faster Than Wireless Networking

 

In my past couple of articles looking at the differences between wired and wireless networking, we’ve looked at the fact that Wi-Fi is a half duplex medium and that it is also a contended medium. Although Wi-Fi is a fabulously convenient network access method, understanding some of its technical limitations will help you get the best out of your Wi-Fi network.

There is another nuance of Wi-Fi networking, which is related to the contended nature of access to the RF medium. As devices using Wi-Fi have to take turns to send their data, there is an additional effect caused by the varying capabilities of the devices using network.

Different Wi-Fi devices have varying capabilities depending on when they were manufactured and the components used to construct them. Some may have the “latest and greatest” super-fast wireless cards, whereas others may be a few years older and may not have speed capabilities of newer equipment. Even the “latest and greatest” Wi-Fi equipment may have different performance characteristics dues to differences in numbers of antennas, antenna size, power availability and chip-set capabilities.

The best analogy to understand this subtlety of Wi-Fi networking is to think back to our example of a single lane highway from part one of this series. We discussed how only one vehicle can travel in each direction at any time, in the same way that half-duplex Wi-Fi connections operate.

We can extend this analogy a little further and consider the impact if we have a mix of vehicle types. Instead of having only fast vehicles using our road, we may have a mix of sports cars, family saloons, trucks, farm tractors and bicycles.

When sports cars are using the highway, they use the road at high speed and quickly complete their journey to make it available for other vehicles to use. But, when a bicycle decides to use the highway, it takes a lot longer to travel along its length. This means that all of the other vehicles (even the fast ones) are waiting longer to use the highway again.

Everyone has to wait, even the fast cars…

 

We have exactly the same issue with Wi-Fi networks. Devices which are “less capable” and connect at lower speeds will have an impact on faster devices that need to use the network. When a slower device sends its data, as it uses a slower connection speed, it will take longer to send its data than a faster device. As we add more slower devices that need to send data, then the more impact they will have on the faster devices that have to wait longer for their turn to use the RF medium and send their data.

What can we do about this?

The best solution is to ensure that our Wi-Fi is correctly designed to be as efficient as possible. If we ensure that all devices can operate at their highest possible speeds, we will at least ensure that we gain the maximum possible throughput available from all devices. This will maximize the air-time (i.e. throughout) available to all clients. A good RF design for your Wi-Fi network is critical — get an expert to design your Wi-Fi RF environment.

Another approach is to consider upgrading the devices that use the Wi-Fi network. This is a particularly important consideration if your wireless network is mission critical and provides the primary access method to your wired network. If you have any 802.11b, 802.11g or 802.11a devices on your network, seriously consider replacing them with 802.11n or 802.11ac devices.

Another useful approach is to relegate slower, legacy devices to the less-desirable 2.4Ghz band. By creating an SSID that is specific to only 2.4GHz and getting slower devices to use only that SSID, you can keep the higher performance 5GHz band for your higher-spec, mission critical devices. This approach obviously depends on the requirements of your particular network, but can be useful in some scenarios.

References:

(image attribution: Hans Deragon — hans@deragon.biz)

Share This:
Facebooktwittergoogle_plusredditlinkedinmail

Wi-Fi “Secrets” Part 2 — Wired is Faster Than Wireless Networking

Taking turns can slow you down

As discussed in my last article, when deploying Wi-Fi networks there are a number of considerations around wireless connectivity that we need to be aware of when comparing them to the more familiar world of wired networking.

In this article we’re going to look at another of Wi-Fi’s “guilty secrets” that few people seem to be aware of. Are you ready for it..?

“Wi-Fi is a shared medium”

What does this mean? All devices connected to each wireless access point (think of laptops, iPads, smartphones etc.) have to take turns to transmit and receive data. They have to share access to the access point. This means that as more devices that join an access point and have data to send, the longer each device may have to wait before it can send its data.

Wi-Fi is a “contended” medium: everyone using it has to take their turn to “talk” to the wireless access point. When things start to get busy on your Wi-Fi network, this can be a significant challenge.

Why is this an issue compared to wired (think Ethernet) networks? Well, each device connected to an Ethernet switch has its own dedicated chunk of network access bandwidth. If an Ethernet station is connected to a 1Gbps port, then it has a full 1Gbps available and does not share it with anyone else. Each device connected to an Ethernet switch port gets its own 1Gbps of access bandwidth to use.

In a Wi-Fi network, if we have a single device connected to an AP, it has access to the full bandwidth that the AP makes available. If we add a second device, the bandwidth available is now shared between both devices (50% bandwidth each) as they take turns to send data. If we add a third device to the AP, each device now has access to 33% of the bandwidth as three devices now take turns to send data. As more devices are added to an AP, the opportunities for each one to send data will decrease, reducing the bandwidth available to each one.

Effect of Multiple Wireless Clients Contending For Wi-Fi Access (Simplified)

The contention to access the wireless network described above applies only if multiple devices need to send data at similar times. However, if wireless forms a significant part of the access layer of your network, this will very likely become an issue very quickly as the number of users (who may have several Wi-Fi devices each) increases.

How do we mitigate the limitations of Wi-Fi’s contended access method? We must optimize our wireless design to achieve the maximum throughput available. This is achieved by designing each Wi-Fi network for capacity as well as wireless coverage. Careful design & planning of the wireless RF environment, together with taking account of client & application bandwidth requirements is the only way to ensure optimized performance.

Share This:
Facebooktwittergoogle_plusredditlinkedinmail