White Paper: 802.11ac Survey | NETSCOUT

White Paper: 802.11ac Survey

Gigabit WiFi has arrived.
Big promises with high expectations – yours only if 802.11ac is optimized and implemented correctly.


802.11ac was developed and is now increasingly available on the market, driven by the rising demands on the WiFi infrastructure. The BYOD explosion has not only increased the number of devices per user connecting to the environment, it has brought with it a new type of use, including voice, HD video, and other bi-directional bandwidth guzzling applications such as Microsoft Lync, FaceTime, WebEx and more.

Delivery of these high-capacity, low-latency applications is further complicated by the fact that there are more devices per user accessing them.One survey by Cisco Systems estimates this number to grow to 3.5 devices per user by 2015. These highly mobile devices are not alone in bringing a new level of strain to WiFi. Laptop connectivity, usage, and application throughput on wireless networks is steadily increasing. Carriers are beginning to adopt WiFi as a last-mile delivery method to their customers. These trends push IT departments to provide a level of signal quality, coverage, and two-way capacity like never before.

To meet these demands, the IEEE got to work and released 802.11ac, perhaps the largest evolution of wireless delivery since, well, wireless. Successfully implementing 802.11ac in an environment will require more than simply buying a few new AP’s, plugging them in, and purchasing a few client-end radios. Achieving the expected coverage and improved data rates will require a clear understanding of how 802.11ac works versus a/b/g/n, as well as best practices for migrating to this new technology.

  • TABLE OF CONTENTS
  • Introduction
  • Planning and Site Evaluation
  • Deployment and Validation
  • Troubleshooting and Optimization

Introduction

Improving WiFi Technology – 802.11ac

The wireless standards we have grown accustomed to have several limitations in delivering high-bandwidth applications. As shown in the chart below, 802.11n has a Max PHY rate of up to 600Mbps, with user throughput realistically landing at 200Mbps. This data rate is only deliverable when the environment is ideal, and only with one or two clients connected. In real WiFi hybrid environments where clients are sharing the space, throughput over 802.11n may plummet to sub-10Mbps levels, which will not support the present or future user demand.

PHY Data Rates for Select Configurations

802.11ac is a backwards-compatible technology, allowing for seamless migration with present 802.11a/n environments. It operates only on the 5 GHz band and supports potential data rates in excess of 1Gbps. The 5 GHz band typically suffers less contention, less interference, and offers more channels than 2.4GHz, enabling the higher throughput provided by 802.11ac. The introduction for 802.11ac into the market was planned in two phases – phase one delivering PHY rates up to 1.3Gbps, and phase 2 up to 6.9Gbps. Today’s measured user rates for phase one may reach up to 800Mbps, making delivery of high bitrate applications like HD and UHD video over WiFi a possibility to multiple users simultaneously. With this level of performance, it is possible to support more users, more devices, and more capacity to the environment as a whole, while ensuring backward compatibility with legacy technologies.

Existing 802.11a/n hardware is not upgradable to 802.11ac. New hardware is required to support the underlying changes needed to achieve the high data rates provided by 802.11ac.

Like 11n, 802.11ac makes use of MIMO (Multiple Input/Multiple Output) antenna scheme and multiple spatial streams for high capacity delivery. Up to an 8x8 antenna scheme is possible, but most initial rollouts will use 3x3, much like 11n. In the case of 802.11ac, 80MHz channels are created by grouping four 20MHz channels together, which allows for higher data rates to the user. This is due to the fact that the wider the channel, the more sub-carriers for bit transmission, which results in higher throughput. The tradeoff with using wider channels is that fewer bonded channels are available – reducing the 5 GHz band to five available 80 MHz channel selections. Only two of these channels are available if DFS channels are to be avoided. Seamless coverage with low overlap may seem impossible when only two channels are available. However, the ability is built-in to the technology to have two adjacent AP’s configured to the same 80MHz channel, falling back to different 40MHz or 20MHz channels when co-channel interference occurs.
 

The phase two rollout, beginning in 2014, will introduce 160MHz channels, which will further increase potential user throughput to 6.9Gbps. This gives us a picture of what 802.11ac can provide, if we build it correctly from the ground up.


Best Practices in Deploying 802.11ac

Understanding more about the underlying technology of 802.11ac is critical when considering a deployment. Despite the tremendous benefits of 802.11ac, it is still susceptible to the standard performance-killers that impact all WiFi environments – non-Wi-Fi interference, co-channel interference, poor signal quality, noise, and channel sharing with slower legacy clients. These challenges can be successfully met only when a solid plan is in place for deploying this breakthrough technology. Resist the urge to buy a few 802.11ac APs, light them up and let the users come on board.

 

  1. Thorough Planning and Site Evaluation
  2. Validating the Installation
  3. Troubleshooting and Optimizing

 

We will describe the considerations and best practices for each stage, along with recommendations to achieve the best capacity and signal quality.


Planning and Site Evaluation
 

It is expected that new 802.11ac implementations will be done in parallel with legacy a/b/g/n systems. Since 802.11ac is backward-compatible with a/n deployments that use the 5 GHz band, there is no need to completely remove these older AP’s. However, it is critical to understand which devices are already competing for RF space, and how 802.11ac can complement the environment to achieve the project performance goals. The planning stage will include a pre-deployment survey to determine present device configuration, noise levels, interference sources, signal coverage, and capacity.


Initial Site Survey

Before purchasing and installing any 802.11ac equipment, or removing any legacy AP’s, determine the present state of the WiFi environment. Identify interference sources, signal coverage, channel availability in the 5 GHz range, and present configuration of all installed 802.11a/n devices. This can be followed by performing an AP-On-A-Stick survey, where a single 802.11ac AP is lit up and deployed, while noting the impacts of the environment both in coverage and in throughput.

 

For example, if in a certain area we expect five users to connect with a maximum of 15 devices (three per user), depending on how many will need voice, video, or only web services, we may estimate the necessary bandwidth to be somewhere around 30Mbps. This of course will depend on the applications in use and how many users will be simultaneously connecting. To support the user density, generally plan for not more than 20 active devices per AP.

 


Channel Allocation Considerations

802.11ac allows for 80MHz channels in the 5 GHz band, which will effectively bond four 20 MHz channels together. Each AP will be configured to a single 20 MHz primary channel, 36 for example, which will act as a beacon and fallback channel. If a legacy radio desires to connect to the AP, it can use this primary 20MHz channel to connect and operate. However, since this single channel falls within the overall 80 MHz bonded channels, this will halt transmission of a pure 802.11ac client to the AP while the 20MHz primary channel is in use.

The best practice with deploying 802.11ac APs is to stagger them between the two to five 80 MHz channels available, one AP bonding channels 36-48 and the other 52-64. If it becomes necessary to overlap these channels in a given area, configure them to different primary channels 36, 44, 52, and 60 respectively. This allows enough of a gap between channels to support legacy devices that need to connect on 20MHz channels, without inducing co-channel crosstalk.


Deployment and Validation
 

After carefully determining the capacity needs and the coverage area, configure and deploy the 802.11ac APs according to the design plan. This does not mean simply removing the old APs and connecting in the new 802.11ac APs in the same locations. There are several considerations when planning AP configuration and location.

 

 

 

After calculating user requirements, the AirMagnet Planner software can be used to create a virtual WiFi environment before rolling out the APs physically. The AP count and layout can be simulated to model adequate coverage and capacity in the environment, while taking into account wall materials and interference sources. Using this data, APs can be physically deployed in the planned areas.

A post-deployment validation survey is critical to determine if the environment is providing the expected coverage and capacity as planned. To validate this, both an active survey that measures user throughput as well as a passive survey to measure signal, noise, interference, channel overlap and other important parameters of the entire WLAN environment is recommended. The active survey should include both an upstream and downstream throughput test from an 802.802.11ac tool. This test should be performed during peak traffic times to be sure that all normal parameters are in place when the test is run.

This active survey can be run using the AirMagnet Survey Pro iPerf survey, which will measure and map real-world user throughput in the environment while visualizing areas with low throughput. Using a multi-adapter survey to simultaneously run both the passive and active survey is recommended, allowing the tool to measure the data points needed in only one pass.

Troubleshooting and Optimization
 

If any of the requirements in user throughput are not achieved by the survey, adjustments can be made to ensure that performance goals are met. Within AirMagnet Survey Pro, the AirWise Policy check feature can be used to determine what wireless factors in the environment contributed to the reduced performance. A guided workflow is provided to assist in making the right adjustments in the right locations to achieve the desired goals.

Adjustments may include changing AP placement, installing additional APs, adjusting the channel plan, eliminating sources of interference, or adjusting transmit power to affect the cell size. Following the adjustments recommended by AirWise, validate the environment with another multi-adapter, active and passive survey to ensure the performance goals are achieved.

Finally, a final pass with the iPerf feature of Survey Pro will provide proof that the network is successfully built to satisfy the user need.