With the prevalence of mobile devices today, the need for faster Wi-Fi has and continues to become more important. 802.11ac promises faster, more efficient wireless communications, but there are a few important things to remember. (The 802.11ac protocol is being delivered in two waves. Wave 1 of 802.11ac is discussed here.)

There are many factors to consider during an 802.11ac deployment. The following five considerations can be nuances if not planned for accordingly:

  • Range
  • Gigabit Throughput
  • Additional Infrastructure Required
  • Channels and Available Frequencies
  • Backward Compatibility

Range

One of the biggest differences with 802.11ac is that it only operates in the 5GHz frequency. This is important because the 5GHz wavelength is shorter than the 2.4GHz wavelength. The majority of Wi-Fi devices that have been produced to date operate in the 2.4GHz range on 802.11b, g or n. Due to the shorter wavelength, 802.11ac will have a smaller range than what has been observed with previous Wi-Fi.

The shorter wavelength also means that the fastest data rates will only be available close to the access point. In most cases, the Gigabit Ethernet speeds that 802.11ac advertises will be realized in the same room as the access point. As is the case with other 802.11 protocols, the further that a wireless device drifts away from the access point the lower the associated data rate will be.

To ensure the best possible performance and range from 802.11ac, perform a wireless site survey. The survey should be performed with the exact equipment (access points and clients) that will be deployed. Use a throughput measuring application on the mobile device and measure from various locations to help determine the optimal location for the access point(s).

Gigabit Throughput

An appealing aspect of 802.11ac is the advertised Gigabit Ethernet speeds over Wi-Fi. While a wireless client may be able to achieve an associated data rate that resembles Gigabit Ethernet link speeds, the reality is Wi-Fi is a half-duplex medium with Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) connectivity. When these inefficiencies are taken into account, the realized data throughput is about one third of the associated data rate. Which means that a wireless client associated at 1.3Gbps will likely see data transfer rates around 0.43Gbps.

In order to get a wireless client attached at Gigabit Ethernet speeds several requirements need to be satisfied. The client and access point need to support three spatial streams – most 802.11ac handheld devices support only a single spatial stream. Another requirement for Gigabit Ethernet speed is 80GHz bonded channels must be configured.

While 802.11ac wireless clients will see the fastest connection speeds and throughput, all wireless clients connected to an 802.11ac access point will see improved connectivity. This is because of advancements in the access point radio technology and improved efficiency in radio communications. Overall channel utilization will be lower as devices spend less time transmitting and receiving, which results in greater battery life and increased Wi-Fi cell capacity.

As previously noted, 802.11ac connected data rates can be as high as 1.3Gbps. With these higher data rates and a dual radio access point, it’s becoming imperative to have the access point connected to a Gigabit Ethernet switch. What’s more, Wave 2 of 802.11ac will be able to provide connected data rates approaching 7Gbps. This is important because it impacts the wired side of network planning.

Additional Infrastructure

Many vendors currently offer access points with dual band radios, 802.11ac on 5GHz and 802.11n on 2.4GHz, and a single gigabit wired interface. This is the first time that there is the potential for the combination of radio interfaces on an access point to significantly over-subscribe the gigabit bandwidth on the wired interface. It is crucially important to plan accordingly for the combined throughput from both radios on the access point and note that the aggregate wired throughput might be the limiting factor.

When implementing new 802.11ac access points and/or planning for their use, consider pulling two Cat 5e or Cat 6 cables to each access point location. Early indications are that in order to achieve the advertised Gigabit Ethernet speeds in Wave 2, at least two Gigabit Ethernet or one 10 Gigabit Ethernet connections are going to be required. An unfortunate nuance to Power over Ethernet is at this time there is no support for POE on 10 Gigabit Ethernet.

Not only is additional cabling a potential future expense, budgets should also include line items for closet switches with 10 Gigabit Ethernet ports for the 802.11ac access points. With 802.11ac Wave 1 products, existing closet switches will need a minimum of Gigabit Ethernet ports to avoid the wired network being the bottleneck.

Channels and Available Frequencies

In theory, there are about 24 channels available in the 5GHz Wi-Fi space. This frequency range is broken into four UNII bands: UNII-1, UNII-2, UNII-2 Extended and UNII-3. 5GHz has been used for many years as an alternative to 2.4GHz to avoid the ever-present interferers like microwave ovens, Bluetooth and cordless phones. Depending on which regulatory domain the access point is operating in, the available and allowable channels will vary. In practice, only three or four 80MHz bonded channels will be capable.

An unfortunate circumstance of the slow adoption of the 5GHz frequency range is some wireless client adapters don’t work in all of the channels. When an access point is using a channel that is not supported by a wireless client (like those in the UNII-2 Extended range), it creates a “dead spot” for the wireless clients that don’t work on those channels. In order to achieve the gigabit throughput promised in 802.11ac, it is assumed that these channels will be used. To ensure connectivity for all devices and maximize the 802.11ac coverage, any legacy 5GHz wireless clients that don’t support all of the available channels should be upgraded.

Another drawback to the 5GHz frequency range is the requirement to follow dynamic frequency selection (DFS) rules in certain regulatory domains (FCC and ETSI). DFS impacts the UNII-2 and UNII-2 Extended frequency ranges, which are shared with radar and military devices. Basically, a Wi-Fi device has to abandon a DFS channel if it senses a radar signal and not reuse that channel for a considerable amount of time after it senses the signal. The access point also has to send notification to any attached wireless clients that it is about to change channels.

Due to DFS, some access point manufacturers simply avoid or don’t use these channels. Depending on personal preference and where the 802.11ac installation may be, like near an airport, the UNII-2 and UNII-2 Extended channels may not be a good idea to use. This is significant because there are a combined fifteen 20MHz channels subject to the DFS rules.

While 802.11n introduced channel bonding, 802.11ac takes channel bonding to a whole new level. 802.11ac Wave 1 allows up to four 20MHz channels to be bonded, which creates an 80MHz bonded channel. The number of channels that get bonded is a configurable parameter.

Determining the optimal number of channels to bond is not a trivial task. This decision will be based on the following factors:

  • Wireless client capabilities
  • The number of spatial streams supported on the wireless clients
  • The desired throughput for each wireless client
  • The bonding strategy doesn’t need to be pervasive, it is perfectly acceptable to have more than one channel bonding plan implemented
  • Match the channel bonding strategy to the needs of the users in any given area
    • Bond more channels in areas with higher throughput needs, like where there are power users
    • Bond less channels in areas with lower throughput needs, like where there are bring-your-own-device or BYOD users

Taking the time upfront to design a channel bonding plan will help ensure a positive user experience at deployment time. Be flexible and revisit the channel bonding plan at regular intervals to make sure that it still fits the users’ needs.

Backward Compatibility

Taking a holistic and realistic view of the network is an important exercise in expectation setting. Implementing an 802.11ac wireless network will improve wireless network efficiency, but the biggest gains will be recognized on 802.11ac clients. Legacy 802.11a and 802.11n wireless clients will still operate at their respective data rates and throughputs.

What’s more, an 802.11ac access point that is connected to a 100Mbps Ethernet connection will not provide gigabit wireless speeds to network services. The wired-side connection is every bit as important as having 802.11ac wireless clients. As previously mentioned, having Gigabit Ethernet or 10 Gigabit Ethernet network connections for the 802.11ac access points is imperative to fully realize the benefits of 802.11ac Wi-Fi.

Summary

While there may be a few nuances at the present time with implementing an 802.11ac Wi-Fi network, the improved spectral efficiency is well worth the time, effort and money spent. Wireless clients will work faster on an 802.11ac access point, but be careful when thinking that Gigabit Ethernet throughput is achievable. Start budgeting and securing extra funding, between switch ports, cabling, upgraded wireless clients and Wave 2 products. Now is the time to start planning for these additional costs. In the end, a properly planned and implemented 802.11ac Wi-Fi network will put smiles on the faces of users.

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