It is not a secret. Wi-Fi is increasingly becoming the preferred mode of internet connection all over the world. According to Gigaom, a blog-related media company, it is predicted that 24 billion devices will be connected to the Internet by 2020. A vast majority will use some form of wireless for accessing the internet.
Wireless technology has indeed widely spread lately. No matter what business you are in, wireless technology is causing disruption. According to Gartner, the top two strategic technology trends for 2014 are: Mobile Device Diversity & Management, and Mobile Apps & Applications. Concretely, if you are in the retail business, you can be sure your customers are wirelessly comparing products and pricing while shopping your store. If you are in the hotel business, you know that Wi-Fi is the most important amenity to your guests. Therefore, most businesses are planning to increase Wi-Fi capacity by at least 20% in 2014, according to Infonetics.
While everyone is familiar with Wi-Fi, it remains difficult to explain how it is actually working. Even among the IT experts community, certain facts about networking tend to be misunderstood, generating confusion and wrong assumptions. In this article, Mike Leibovitz, Director of Mobility Solutions, Extreme Networks discusses the top 10 aspects of Wi-Fi that are often misunderstood.
1) Shared medium
Each radio, on every Access Point (AP) is programmed to run on a single channel. Within this single channel, many clients can connect and communicate. All clients using it share this single channel medium.
The fundamental problem this creates in a radio system is that a station cannot hear while it is sending, and hence it impossible to detect a collision. Because of this, the developers of the 802.11 specifications came up with a collision avoidance mechanism called the Distributed Control Function (DCF). According to DCF, a Wi-Fi station will transmit only if it thinks the channel is clear. Consequently, the incidence of collisions will increase as the traffic increases or in situations where mobile stations cannot hear each other.
While there are elegant protocols that control the operation, Wi-Fi is very similar to legacy L2 hub technology from our wired-side counterparts. It is shared air our devices breathe in.
2) 802.11b and legacy protocols “slow” the medium
The effect of having older protocols running in the environment seems to “slow” all other clients. The truth is that legacy clients simply take more airtime to transmit the same amount of data as a newer client (11n or 11ac). Some solutions offer airtime fairness algorithms to account for this very effect.
3) 802.11 functions at L1/L2
It is commonly believed that Wi-Fi works with no physical wired connection between sender and receiver by using radio frequency (RF) technology. When a RF current is supplied to an antenna, an electromagnetic field is created that is then able to propagate through space.
The 802.11 protocol is an L2 technology, with underlying L1 handling of the OSI stack to function. The communication between client and AP is bridged across the air. Major aspects of over-the-air communication is handled at L2 such as QoS through 802.11e standard.
4) Downstream versus Upstream
There is a significant difference between downstream from the AP to the client versus upstream from client to the AP. Most of Wi-Fi’s over-the-air techniques only provide enhancements on the downstream flow.
5) Tx rate versus Rx rate
It is commonly accepted that the only rate is the transmit rate. However, asymmetric rates are typical where the transmit rate we see on our connected client does not necessarily represent the receive rate. This is also confirmed on the AP/infrastructure side as well, as it is proved that there are separate Tx and Rx rates in the Wi-Fi world.
6) Same TX power for all rates
Setting an AP radio to “Max power” does not mean that power is used for all rates. The default TX-Power of wireless is set to 20 dBm. Typically the higher the data rate in use the AP will be forced to lower its power of those frames (defined by FCC and ETSI). This notion has become even more prevalent with 802.11ac VHT rates.
7) Always set the radio to max power
Even if setting the radio to max power can look like a good thing to do, it might not be the best idea. It has been proved that many radios create signal distortion when they are “speaking” at their loudest. For a number of reasons, including cell size planning, it is not a defacto best practice to just simply set radio power to max.
8) Signal strength vs. Signal-to-noise ratio (SNR)
There is often a confusion between the measurement of the signal strength and the measurement of SNR. In fact, the performance of a Wi-Fi network depends partly on signal strength. Between a computer and wireless access point, the signal strength in each direction determines the data rate available on that connection. Therefore, the stronger is the signal, the better is the connection. The signal to noise ratio is best to be a bigger number. It signifies the difference between the strong signal you are receiving and the background noise that might impact the overall quality.
9) MIMO and Spatial streams
MIMO and Spatial streams are probably the most confused aspect of Wi-Fi since the 11n protocol.
Multiple-Input Multiple-Output (MIMO) technology is a wireless technology that uses multiple transmitters and receivers to transfer more data at the same time. All wireless products with 802.11n support MIMO, which is part of the technology that allows 802.11n to reach much higher speeds than products without 802.11n. MIMO relates to antennas and pathing where the numbers used denote how many antennas/paths can be used to Tx and Rx signal. For example, 3×3 means 3 antennas/paths Tx and 3 antennas/paths Rx.
Spatial streams relates to the actual data sent. For example a 3 spatial stream device can send 3 unique data streams to a receiving station where it’s rebuilt into one data set. In the end you add MIMO and Spatial Streams to equate to overall throughput that can be achieved – 3×3:3 means the sender has 3 available paths to Tx and Rx while sending it’s 3 data streams.
10) Access Point vs. Client capability
It’s pretty simple, yet often overlooked- the lowest common denominator always wins. In order to achieve the highest data rates, and real world throughput, both AP and client must be capable. It is important to know the client capability to really understand what the WLAN deployment will achieve in real world.
Mike Leibovitz is the Director of Mobility Solutions at Extreme Networks