Characteristics of IEEE 802.11n and 802.11ac deliver same performance for one quarter the cost of wired systems
We have discussed quite a bit of theory up until now. This week we’re shifting focus to talk about some of the things that have made IEEE 802.11n and the newest amendment, 802.11ac, viable contenders for the future of networking. The speed and reliability advantages afforded by each have resulted in widespread deployment of the technology, in many cases replacing wired networks.
Overall, there are several very good reasons for these changes. Wired networks require a considerable capital outlay for installation and configuration, and site conditions can make these costs prohibitive. Long-term administration, maintenance, and upgrade costs are high. Wireless technology can achieve the same results for about 25% of the cost and provide speed, reliability, and increased range of coverage over comparable wired systems. Flexibility is also a benefit often overlooked-adding an access point (AP) to your extended service set (ESS) adds additional coverage without the hassle and expense of buying new equipment and media, and dealing with electricians and the disruption to an operation.
Without the widespread use of wireless technology, newer technological movements, such as the Internet of Things (IoT), would not be feasible or even possible. Mobile smartphones would be greatly diminished in their usefulness. Home awareness and security would be unrealistic in the first case and expensive and invasive in the second. The introduction of low-cost APs and adding Wi-Fi capability to our phones and computers has been made possible by the development of fast and reliable wireless systems.
In the beginning, there was 802.11 prime. It was basically a science project, and it served its purpose well. It was slow, but it showed many possibilities. Later came the 802.11a/b amendments. Unfortunately, 5 GHz radio technology did not really exist on the scale required, and what did exist was expensive and difficult. 802.11a was not widely adopted. The 2.4 GHz ISM band was much more accessible, and 802.11b was deployed with some success, mostly in the commercial arena. While the 802.11a radios were capable of 54 Mbps, cost and complication conspired against it. At 11 Mbps, 802.11b was useful, even if it wasn’t competitive with wired systems. Added to this was the early discovery that the dominant security method, WEP, was easily compromised. It’s easy to see why the technology did not catch fire.
Enter 802.11g, which brought with it orthogonal frequency division multiplexing (OFDM) and speeds of 54 Mbps. Coupled with the newer WPA/WPA2 security standards, this amendment was immediately adopted into the mainstream and came into widespread use. The standard required the radios to be backwards compatible with earlier 802.11b radios, but not with 802.11a radios, which operate in the 5 GHz UNII band. 802.11g operates in the 2.4 GHz ISM band. It was the introduction of 802.11g that brought wireless to the masses and prompted further development into more advanced and faster technologies.
The first release of 802.11n equipment was problematic and did not live up to the promise of taming multipath interference. Many people did not have client devices that "spoke n," and many consumer routers were underutilized, defaulting back to 802.11g. This is changing rapidly.
What makes 802.11n so different from the previous, legacy type of wireless devices? The essence of 802.11n was its ability to use the multipath phenomenon to enhance received signals by using constructive algorithms to combine favorable signals (relatively in-phase) and rejecting unfavorable signals (out of phase). This vastly improved system reliability and reduced retransmissions. The genius of this approach was turning a negative into a solid positive. Legacy devices used combinational algorithms like maximal ratio combining (MRC) and antenna diversity (multiple antennas on half wavelength apart) to determine the best signal, eliminating the others.
The ability to bond multiple channels to a maximum of 40 MHz was introduced. This doubled the available bandwidth and the number of subcarriers from the previous channel bandwidth of 20 MHz. Subcarriers went from 56 in 802.11g OFDM to 114 subcarriers (108 data, 6 pilot) in 802.11n.
Another concept are modulation and coding schemes (MCS), which actively manage the channel quality. A favorable MCS is negotiated between the transmitter and receiver based on channel conditions. The MCS is a combination of the coding rate, modulation technique, number of data streams, and channel width. Other improvements are more arcane.
The ability to send multiple frames of data and acknowledge each as a block, instead of individually, allowed massive amounts of data to be sent in one burst. This works particularly well for tagged traffic like voice and video. Quality of service (QoS) enhancements that allow classification of traffic by priority tag vastly improved the delivery of audio and video to mobile devices.
With these improvements, data throughput on wireless LANs went from 54 Mbps in 820.11g to 650 Mbps in 802.11n.
802.11ac has taken these principles even further. While 802.11n supports four spatial streams, 802.11ac supports up to eight spatial streams at the AP; client devices are still limited to four. This allows the AP to transmit to multiple clients at the same time, a concept called Mu-MIMO, or multiple user-MIMO. The additional spatial streams increases overall network speed because of these multiple client connections. Aggregate throughput will approach 7 GBPS.
802.11ac adds the ability to bond channels to create 80 and 160 MHz channels, greatly increasing bandwidth and speed. Using an 80 MHz wide channel, the number of subcarriers goes to 242, with 234 usable for data. A 160 MHz wide channel uses 484 subcarriers, with 468 usable for data. Compare this to the 20 MHz channel width previously used, 80 and 160 MHz channels would increase capacity by 4.5 and 9 times, respectively.
802.11ac breaks new ground in that it does not support the 2.4 GHz frequency band. 802.11ac will operate in the 5 GHz spectrum only. One reason for this is the availability of channels for the enhanced channel bonding capability; channel bonding was an issue with 802.11n in the 2.4 GHz spectrum in that it was cumbersome to come up with a 40 MHz wide channel in the ISM band. Another reason is that the 5 GHz spectrum is not widely used at this time and is relatively quiet, in an RF sense.
802.11ac uses a specialized type of beamforming that will simplify and enhance the process. 802.11n required that two devices negotiate the most suitable beamforming options. Very few devices support this form of beamforming, called explicit beamforming. Implicit beamforming infers from the condition of the transmission, such as the amount of retransmissions or acknowledgments of frames. 802.11ac uses null data packet sounding, a technique that broadcasts a series of null data packets. The null data packets are analyzed by the receiver, or beamformee, which then calculates a feedback matrix that is sent back to the beamformer. The beamformer then uses this feedback matrix to calculate a steering matrix for that client, which steers the following transmissions toward the beamformee.
Aside from the standard modulation techniques used in previous versions, 802.11ac adds 256 QAM (quadrature amplitude modulation). Previous techniques topped out at 6 bits per symbol; this addition allows 8 bits per symbol. Modulation techniques will be discussed in the next segment.
While this discussion is not exhaustive, it is apparent that a once untrusted, slow, and expensive technology has been tweaked to produce a secure, lightning fast, and inexpensive technology that will replace the existing wired infrastructure in the years to come. 802.11ac is the end product of much hard work by dedicated researchers and industry professionals who had the vision and drive to develop wireless into the powerhouse it has become.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). He can be reached at email@example.com. Edited by Chris Vavra, production editor, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
Understanding modulation and coding schemes
OFDM: Orthogonal frequency division multiplexing
MIMO and spatial multiplexing
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.