引言
本文轉載自Cisco的802.11ac: The Fifth Generation of Wi-Fi的技術白皮書；未附插圖，詳參原文。內文
1. Executive summary
802.11ac, the emerging standard from the IEEE, is like the movie The Godfather Part II. It takes something great and makes it even better. 802.11ac is a faster and more scalable version of 802.11n. It couples the freedom of wireless with the capabilities of Gigabit Ethernet.Wireless LAN sites will see significant improvements in the number of clients supported by an Access Point (AP), a better experience for each client, and more available bandwidth for a higher number of parallel video streams. Even when the network is not fully loaded, users see a benefit: their file downloads and email sync happen at low lag gigabit speeds. Also, device battery life is extended, since the device’s Wi-Fi interface can wake up, exchange data with its AP, and then revert to dozing that much more quickly.
802.11ac achieves its raw speed increase by pushing on three different dimensions:
• More channel bonding, increased from a maximum of 40 MHz with 802.11n up to 80 or even 160 MHz (for speed increases of 117 or 333 percent, respectively).
• Denser modulation, now using 256 Quadrature Amplitude Modulation (QAM), up from 64QAM in 802.11n (for a 33 percent speed burst at shorter, yet still usable, ranges).
• More Multiple Input, Multiple Output (MIMO). Whereas 802.11n stopped at four spatial streams, 802.11ac goes all the way to eight (for another 100 percent speed increase).
The design constraints and economics that kept 802.11n products at one, two, or three spatial streams haven’t changed much for 802.11ac, so we can expect the same kind of product availability, with first-wave 802.11ac products built around 80 MHz and delivering up to 433 Mbps (low end), 867 Mbps (mid-tier), or 1300 Mbps (high end) at the physical layer. Second-wave products, or 802.11ac Wave 2 as they are sometimes referred to, support more channel bonding and spatial streams, with plausible product configurations operating at up to 3.47 Gbps.
802.11ac is a 5-GHz-only technology, so dual-band APs and clients will continue to use 802.11n at 2.4 GHz. However, 802.11ac clients operate in the less crowded 5-GHz band.
Second-wave or Wave 2 products also come with a new technology, multiuser MIMO (MU-MIMO). Whereas 802.11n is like an Ethernet hub that can transfer only a single frame at a time to all its ports, MU-MIMO allows an AP to send multiple frames to multiple clients at the same time over the same frequency spectrum. That’s right: with multiple antennas and smarts, an AP can behave like a wireless switch. There are technical constraints, and so MU-MIMO is particularly well suited to Bring-Your-Own-Device (BYOD) situations in which devices such as smartphones and tablets have only a single antenna.
802.11ac-enabled products are the culmination of efforts at the IEEE and Wi-Fi Alliance pipelines. IEEE 802.11ac delivered an approved Draft 2.0 amendment in January 2012 and a refined Draft 3.0 in May 2012, with final ratification in December 2013. In parallel, the Wi-Fi Alliance adopted an early but very stable and mature IEEE draft, namely Draft 3.0, and used that as the baseline for an interoperability certification of first-wave products in mid-2013. The Wi-Fi Alliance split its 802.11ac certification into two parts, in order to include testing of the more advanced features. This second-wave certification includes features that were not in the earlier certification (Wave 1). These features include channel bonding up to 160 MHz, four spatial streams, and MU-MIMO. Overall, this arrangement closely follows how 802.11n was rolled out, with 802.11ac Wave 1 coming out first and Wave 2 products being released about 18 months later.
Enterprise networks considering an investment in infrastructure Wi-Fi have the choice to move from an older technology such as 802.11n while also delivering a remarkable level of performance. If you still have an older standard like 802.11n deployed today, upgrading to the latest 802.11ac Wave 2 technology will provide better performance for bandwidth-intensive applications like streaming video and collaboration. The newer technology will also address a common issue that most networks are seeing today, providing consistent performance in the face of higher density or more wireless clients accessing the network. Cisco offers a broad portfolio of products that support 802.11ac Wave 2, as well as the right innovation and features designed for a variety of different network sizes and needs.
When it comes to interoperability with older standards, 802.11ac will have a few effects on existing 802.11a/n deployments, even if the deployment is not upgraded to 802.11ac immediately: (1) the wider channel bandwidths of neighboring APs require updates to radio resource management, or RRM (and in particular the dynamic channel assignment algorithm), and (2) 802.11a/n Wireless Intrusion Protection Systems (WIPS) can continue to decode most management frames such as beacon and probe request/response frames (that are invariably sent in 802.11a format) but do not have visibility into data sent in the 802.11ac packet format.
One thing not to worry about is compatibility. 802.11ac is designed in a deep way to coexist efficiently with existing 802.11a/n devices, with strong carrier sense, a single new preamble that appears to be a valid 802.11a preamble to 802.11a/n devices, and extensions to Request-To-Send/Clear-To-Send (RTS/CTS) to help avoid collisions with users operating on slightly different channels.
2. What is 802.11ac?
First, 802.11ac is an evolution of 802.11n. If you want to learn more about 802.11n, jump to the Appendix. If you are already familiar with the channel bonding, MIMO, and aggregation introduced by 802.11n, and you don’t need a refresher, read on.2.1 Drivers for 802.11ac
802.11ac is an evolutionary improvement to 802.11n. One of the goals of 802.11ac is to deliver higher levels of performance that are commensurate with Gigabit Ethernet networking:• A seemingly “instantaneous” data transfer experience
• A pipe fat enough that delivering a high Quality of experience (QoE) is straightforward
In the consumer space, the target is multiple channels of High-Definition (HD) content delivered to all areas of the house. The enterprise has different challenges:
• Delivering network with enterprise-class speeds and latencies
• High-density environments with scores of clients per AP
-This density is exacerbated by the BYOD trend, such that one employee might carry two or even three 802.11 devices and have them all consuming network resources at the same time
-The proliferation of wireless IoT devices also increases the density of networks
• The increased adoption of video streaming, collaboration and other bandwidth-intensive applications
802.11ac is about delivering an outstanding experience to each and every client served by an AP, even under demanding loads.
Meanwhile, 802.11 is integral to a hugely broad range of devices, and some of them are highly cost, power, or volume constrained. One antenna is routine for these devices, yet 802.11ac must still deliver peak efficiency.
The one thing that 802.11ac has in its favor is the evolutionary improvement to silicon technology over the past half-dozen years: channel bandwidths can be wider, constellations can be denser, and APs can integrate more functionality.
2.2 How does 802.11ac go so fast?
Wireless speed is the product of three factors: channel bandwidth, constellation density, and number of spatial streams. 802.11ac pushes hard on the boundaries on each of these, as shown in Figure 1.For the mathematically inclined, the physical layer speed of 802.11ac is calculated according to Table 1. For instance, an 80-MHz transmission sent at 256QAM with three spatial streams and a short guard interval delivers 234 × 3 × 5/6 × 8 bits/3.6 microseconds = 1300 Mbps.
Immediately we see that increasing the channel bandwidth to 80 MHz yields 2.16 times faster speeds, and 160 MHz offers a further doubling. Nothing is for free: it does consume more spectrum, and each time we’re splitting the same transmit power over twice as many subcarriers, so the speed doubles, but the range for that doubled speed is slightly reduced (for an overall win).
Going from 64QAM to 256QAM also helps, by another 8/6 = 1.33 times faster. Being closer together, the constellation points are more sensitive to noise, so 256QAM helps most at shorter range where 64QAM is already reliable. Still, 256QAM doesn’t require more spectrum or more antennas than 64QAM.
The speed is directly proportional to the number of spatial streams. More spatial streams require more antennas, RF connectors, and RF chains at transmitter and receiver. The antennas should be spaced one-third of a wavelength (3/4 inch) or more apart, and the additional RF chains consume additional power. This drives many mobile devices to limit the number of antennas to one, two, or three.
Collectively, these three speed increases are significant. As shown in Figure 2 and Table 2, the minimum allowed 802.11ac product is 4.4 times faster than the corresponding 802.11n product, and the mid-tier and high-end Wave 1 products are nearly 3 times faster, reaching 1.3 Gbps PHY data rates. Actual throughput will be a function of MAC efficiency (rarely better than 70 percent) and the capabilities of the devices at each end of the link.
2.3 How do we make 802.11ac robust?
The sticker on the box that shows the maximum data rate doesn’t help us much in the real world, where devices have to contend with interference from non-802.11 devices, preexisting APs that might only use 20 or 40 MHz, multipath fading, few antennas on mobile devices, weak signals at range, and so forth. What makes the raw speed of 802.11ac so valuable are the extensions that help to deliver reliable throughput under realistic conditions.2.3.1 Technology overview
By design, 802.11ac is intended to operate only in the 5-GHz band, as shown in Table 3. This avoids much of the interference at 2.4 GHz, including Bluetooth headsets and microwave ovens, and provides a strong incentive for users to upgrade their mobile devices (and hotspot APs) to dual-band capability so that the 5-GHz band is more universally usable. This choice also streamlines the IEEE process by avoiding the possibility of contention between 802.11 and 802.15 proponents. And there is barely 80 MHz of bandwidth at 2.4 GHz anyway.As we’ve already seen, 802.11 introduces higher-order modulation, up to 256QAM; additional channel bonding, up to 80 or 160 MHz; and more spatial streams, up to eight. There is an alternative way to send a 160-MHz signal, known as “80+80” MHz, discussed later (see Section 2.3.6).
802.11ac continues some of the more valuable features of 802.11n, including the option of a short guard interval (for a 10 percent bump in speed) and an incrementally better rate at range using the advanced low-density parity check (LDPC) forward error-correcting codes. These LDPC codes are designed to be an evolutionary extension of the 802.11n LDPC codes, so implementers can readily extend their current hardware designs.
Various Space Time Block Codes (STBCs) are allowed as options, but (1) this list is trimmed from the overrich set defined by 802.11n, and (2) STBC is largely made redundant by beamforming. 802.11n defined the core STBC modes of 2×1 and 4×2 and also 3×2 and 4×3 as extension modes, but the extension modes offered little gain for their additional complexity and have not made it to products. Indeed, only the most basic mode, 2×1, has been certified by the Wi-Fi Alliance. With this experience, 802.11ac defines only the core 2×1, 4×2, 6×3, and 8×4 STBC modes, but again only 2×1 is expected to make it to products: if you had an AP with four antennas, why would you be satisfied with 4×2 STBC when you could - and should - be using beamforming?
What 802.11ac also gets right is to define a single way of performing channel sounding for beamforming: so-called explicit compressed feedback. Although optional, if an implementer wants to offer the benefits of standards-based beamforming, there is no choice but to select that single mechanism, which can then be tested for interoperability.
Because of the wider channel bandwidths of 802.11ac, it is much more likely that an 80-MHz AP will overlap with another 20- or 40-MHz AP - and similarly an 80- or 160-MHz AP - or even several of them, all potentially on different channels. To enable reliable operation amid this complexity, 802.11ac mandates extensions to the RTS/CTS mechanism, stronger Clear-Channel Assessment (CCA) requirements, and new primary channel selection rules. See Section 2.3.4.
802.11ac also introduces a valuable new technology called multiuser MIMO. This is challenging to get right, so it is deferred until the second wave of 802.11ac products and will likely be optional. More on this later in Section 2.3.9.
2.3.2 Differences between 802.11ac and 802.11n
802.11ac has avoided the battles of 802.11n and instead has focused on extending the tremendous advances made in 802.11n to deliver the next generation of speed and robustness.For instance, 802.11n pioneered aggregation through the selective use of A-MPDU, A-MSDU, and A-MPDU of A MSDU (see the Appendix). 802.11ac actually requires every 802.11ac transmission to be sent as an A-MPDU aggregate. This is due in part to the intrinsic efficiency of A-MPDU, as well as to some other factors (see Section 2.3.5).
In a further example, 802.11ac extends the 802.11n channel access mechanism: virtual carrier sense and backoff occur on a single 20-MHz primary channel; CCA is then used for the remaining 20-MHz subchannels immediately before transmitting on them.
Given the power of A-MPDU and the 802.11n channel access mechanism, 802.11ac actually didn’t need to innovate much in the MAC. Indeed, extensions to the RTS/CTS mechanism are the only new mandatory MAC feature.
802.11n does include many options with reduced value, and 802.11ac takes a very pragmatic approach to them. If a “useless” option is used and affects a third-party device, 802.11ac typically forbids an 802.11ac device (operating in 802.11ac mode) from using the option. If a “useless” option has not been used in 802.11n products or affects only the devices that activate the option, the feature is not updated for 802.11ac but is instead “left to die.”
For instance, there is no 802.11ac version of the “802.11n greenfield” preamble format. 802.11ac defines only one preamble format, which, to legacy 802.11a/n devices, will look safely like an 802.11a preamble followed by a payload with a bad CRC. This means that legacy devices won’t try to transmit over the top of the 802.11ac transmission, nor will they attempt to send a bad payload up the stack.
802.11n introduced Reduced Interframe Spacing (RIFS), which reduces overheads between consecutive transmissions, but experience has shown that A-MDPU solves much the same problem even more efficiently. 802.11ac devices operating in 802.11ac mode are not permitted to transmit RIFS (as of Draft 3.0).
802.11n features that are not updated for 802.11ac (or are explicitly forbidden for 802.11ac devices operating in 802.11ac mode) include all the 802.11n sounding options, including extension LTFs, the calibration procedure, antenna selection, PCO, L-SIG TXOP protection, unequal modulation, 4×3 and 3×2 STBC modes, MCS32, and dual CTS protection. Don’t worry if you don’t know these terms; you’ll almost certainly never need to understand them.
2.3.3 Standards-based beamforming
Any device (with multiple antennas) can beamform to any other device at any time. What 802.11ac adds is the opportunity for the receiver to help the beamforming transmitter to do a better job of beamforming. This is called “sounding,” and it enables the beamformer to precisely steer its transmitted energy toward the receiver. 802.11ac defines a single, though optional, protocol for one 802.11ac device to sound other 802.11ac devices. The protocol selected closely follows the 802.11n explicit compressed feedback protocol, as follows.A device, typically an AP, sends a “Very High Throughput (VHT) Null Data Packet (NDP) Announcement” frame. Its only purpose is to contain the address of the AP and of the target recipients. The VHT NDP Announcement frame is immediately followed by a “VHT Null Data Packet” (VHT NDP) intended for those target recipients. Each intended recipient measures the RF channel from the AP to itself using the preamble of the VHT NDP and compresses the channel. The first intended recipient responds with the compressed channel information in a VHT Compressed Beamforming frame immediately, and other recipients respond when they are polled by the AP. The VHT NDP Announcement frame, the VHT NDP, and the VHT Compressed Beamforming frame are all similar to features in 802.11n. However, because of some subtle differences, the 802.11ac sounding is not backward compatible with 802.11n devices.
Also, to support the new MU-MIMO feature (see Section 2.3.9), the channel feedback can contain an extra level of detail.
Explicit Compressed Feedback (ECFB) is known to provide the most precise estimate of the channel, taking into account all the imperfections at transmitter and receiver.
However, ECFB comes with a lot of overhead: the VHT NDP Announcement frame, the VHT NDP itself, and the frame carrying the compressed feedback. For an AP with four antennas, the compressed feedback varies from 180 to 1800 bytes, depending on the number of client antennas and level of compression. Sounding just one single antenna 80-MHz client takes about 250 microseconds. When devices can transmit at 433 Mbps, this is expensive, since that same time could instead have been used to send an extra 13,000 bytes.
And so technologies that solve the problem of sounding without depending on client assistance (such as Cisco ClientLink technology) continue to add genuine value. They (1) still help legacy 802.11a/n clients, (2) still help those 802.11ac clients that do not support 802.11ac sounding, (3) still help clients at 2.4 GHz, and (4) can avoid the overhead of standards-based explicit sounding when it is not actually necessary.
2.3.4 RTS/CTS with bandwidth indication
An 802.11ac AP operating on 80 MHz (or 160 MHz and so on) should still be capable of allowing 802.11a or 802.11n clients to associate. Thus, beacons are sent on one 20-MHz channel, known as the primary channel, within that 80 MHz. The AP and all clients associated with the AP receive and process every transmission that overlaps this primary channel and extract virtual carrier sense from the frames they can decode.However, the AP could be near other uncoordinated APs. Those APs could be preexisting 802.11a or 802.11n APs, and their primary channels could be any 20 MHz within the 80 MHz of the 802.11ac AP. The different APs and their associated clients then have a different virtual carrier sense and so can transmit at different times on the different subchannels, including overlapping times. With the wide 802.11ac channel bandwidths, this scenario becomes much more likely than with 802.11n.
For this reason, 802.11ac defines an enhanced RTS/CTS protocol. RTS/CTS can be used to determine when channel bandwidth is clear and how much, around both the initiator and the responder, as shown in Figure 3.
First, when an 802.11ac device sends an RTS, (1) this initiating device has to verify that the 80-MHz channel is clear in its vicinity, (2) the RTS is normally sent in an 802.11a Physical Protocol Data Unit (PPDU) format, and (3) the basic 802.11a transmission, which is 20 MHz wide, is replicated another three times to fill the 80 MHz (or another seven times to fill 160 MHz). Then every nearby device, regardless of whether it is an 802.11a/n/ac device, receives an RTS that the device can understand on its primary channel. And every device that hears the RTS has its virtual carrier sense set to busy (see Figure 3a). To make the protocol robust, the replication bandwidth of the RTS is reported inside the 802.11a PPDU1.
Second, before the device addressed by the replicated RTS responds with a CTS, the recipient device checks to see if anyone is transmitting near itself, on its primary channel or on any other 20 MHz within the 80 MHz. If a portion of the bandwidth is in use nearby, the recipient responds with a CTS only on the available and “usable” 20 MHz subchannels and also reports the bandwidth of the replicated CTS inside the CTS’s PPDU. Here “usable” subchannels means the subchannels on which the initiating device is allowed to send something, such as a 20-, 40-, or 80-MHz (but not 60-MHz) transmission. This is shown in Figure 3b.
Third, the CTS is sent, like the RTS, in an 802.11a PPDU format, replicated in 20-MHz chunks across the available and useful bandwidth. Again, every nearby device receives a CTS that the device can understand on its primary channel.
There are other variations on this protocol, for when the initiator is incapable of switching to a narrower bandwidth on the fly and so forth, but the previous description captures the essence of the enhancement: the recipient can say, “These subchannels are busy - don’t use them.”
1 Since the 802.11a PPDU format doesn’t contain a bandwidth indication, 802.11ac has to play some tricks to maintain backward compatibility. The bandwidth indication is encoded in the scrambling sequence, and also the individual/group bit in the transmitter MAC address in the RTS frame is changed from “individual” to “group.” This last change will be visible in sniffer traces.
2.3.5 All A-MPDUs
802.11 defines that every 802.11 PPDU transmission is an A-MPDU, yet the A-MPDU might contain only a single MPDU. Why? The short answer is that it’s complicated.Here’s the long answer: There are three reasons: (1) In 802.11a/n, the duration of the transmission is set by the number of octets and the data rate for the transmission. But a maximum-length 5.5-ms transmission at 6.93 Gbps could contain over 4 million bytes, and this takes 23 bits to represent. These bits would be sent at the lowest Modulation and Coding Scheme (MCS) rate at the start of every 802.11ac transmission and so practically would add 4 microseconds each time. Instead, the length of an 802.11ac transmission is constrained to be a multiple of the number of data bits per Orthogonal Frequency-Division Multiplexing (OFDM) symbol, and then only the number of OFDM symbols needs to be signaled. Moreover, the number of (assumed to be) 4-microsecond-long OFDM symbols is already implicitly available in the legacy portion of the preamble, so this signaling comes almost for free2. Then we need a way to completely fill even the last OFDM symbol with data. A-MDPU makes this easy: send the data as MDPUs within MDPU subframes in an A-MDPU, then pad the A-MDPU with enough null MDPU subframes to fill up the last OFDM symbol. (2) This same padding mechanism will come in handy for the new MU MIMO feature. (3) A-MDPU is in general a good idea to increase reliability for long payloads.
2 Just a single bit is needed to disambiguate the number of actual OFDM symbols present if the transmission instead uses the short guard interval and the OFDM symbols are actually 3.6 microseconds long.
2.3.6 Channelization and 80+80 MHz
802.11ac adopts a keep-it-simple approach to channelization. Adjacent 20-MHz subchannels are grouped into pairs to make 40-MHz channels, adjacent 40-MHz subchannels are grouped into pairs to make 80-MHz channels, and adjacent 80-MHz subchannels are grouped into pairs to make the optional 160-MHz channels, as shown in Figure 4. A BSS (that is, AP plus clients) uses the different bandwidths for different purposes, but the usage is principally governed by the capabilities of the clients.In the United States, there are 20 to 25 20-MHz channels, 8 to 12 40-MHz channels, 4 to 6 80-MHz channels, and 1 or 2 160-MHz channels. These numbers are ranges because of the evolving regulatory issues surrounding the different spectrum noted in Figure 4.
What if most clients at a deployment are still 802.11n clients with 40 MHz maximum? Does deploying 802.11ac APs mean fewer channels and more interference? As you would expect from an IEEE standard, the answer is a resounding “no.” It is entirely allowed for two 80-MHz 802.11ac APs to select the same 80-MHz channel bandwidth but for one AP to put its primary 20-MHz channel within the lower 40 MHz and the other AP to put its primary 20-MHz channel within the upper 40 MHz. What this means is that 802.11n clients associated with the first AP can transmit 20 or 40 MHz as usual, at the same time that 802.11n clients associated with the second AP transmit 20 or 40 MHz in parallel. What is new in 802.11ac is the ability for any 802.11ac client that sees the whole 80 MHz as available to invoke a very high-speed mode and to transmit across the whole 80 MHz. This is shown in Figure 5.
The ability to have overlapped APs but different primary channels is made possible by:
• The enhanced secondary CCA thresholds mandated by 802.11ac, which are up to 13 dB more stringent than the secondary CCA thresholds defined by 802.11n
• The addition of a bandwidth indication to the RTS/CTS exchange (see Section 2.3.4)
Over time, clients will transition from 802.11n to 802.11ac, so that 80 MHz is used more and more. In this environment APs should change to align their primary 20-MHz channels.
The capability of 80-MHz channels is markedly increased over narrower bandwidths. This offers a lot of value in many typical scenarios: a few clients transferring a lot of traffic associated with a 40-MHz AP are limited to 802.11n’s 300 or 450 Mbps. This is true even if the APs on the adjacent 40 MHz are all lightly loaded. With the wider channel, more clients get to transfer their data more quickly and can complete their transmissions that much sooner. Overall, less battery energy is consumed, and other clients don’t have as long to wait (for better Quality of Service [QoS]). This discussion comes under the umbrella of “statistical multiplexing,” in which more multiplexing is more efficient for bursty traffic.
Since the number of 160-MHz channels is tiny, 160 MHz is unsuited to typical enterprise use. In the home, every 160-MHz channel is subject to difficult radar detection regulatory requirements. Thus, 802.11ac also introduces a noncontiguous 80+80 MHz mode. As can easily be imagined from the name, it is the 160-MHz waveform but is transmitted in two separate 80-MHz segments, each of which can lie on any allowed 80-MHz channel. To make this feasible, it is still a time-division-duplex system, in that APs and clients only ever transmit on 80+80 or receive on 80+80; they are never expected to transmit on one 80-MHz segment and receive on the second 80-MHz segment.
In lightly and moderately used spectrum, this appears to provide vastly more flexibility to avoid interference, as shown in Figure 6. 80+80 MHz has 13 options versus the 2 options for 160 MHz (ignoring regulatory issues). Unfortunately, an 80+80 MHz device is much more complicated than a 160-MHz device, since the 80+80 MHz device needs twice as many RF chains. A device might operate either as a two-spatial-stream 80-MHz device or as a one-spatial-stream 80+80 MHz device. In this case, 80+80 MHz allows the use of more spectrum but uses that spectrum only half as efficiently.
Neither 160 MHz nor 80+80 MHz are recommended for typical enterprises, given the currently available spectrum.
As shown in Figure 4, RRM becomes a much more complicated task. It must:
• Avoid channels with radar (if present).
• Uniformly spread the channel bandwidth used by each AP and preferably spread the AP’s primary 20-MHz channel too.
• Avoid a channel that overlaps with other 20-, 40-, 80-, 160-, or 80+80 MHz APs nearby.
• Within an 80-MHz channel bandwidth (for example), decide whether to align primary 20-MHz channels with other APs or deliberately not align primary channels. This is not a clear-cut choice:
-If the primary channels are aligned, virtual carrier sense works completely, yet all 20- and 40-MHz traffic (including broadcast, multicast, and data traffic to 802.11a/n devices) is sent in series. During these times, 40 or 60 MHz of bandwidth is unused. Still, if the clients are predominantly 802.11ac, this is generally the best approach in terms of throughput and airtime fairness.
-Conversely, if one AP’s primary channel is assigned to the lower 40 MHz and another AP’s channel is assigned to the upper 80 MHz, 20- and 40-MHz traffic can be parallelized (as shown in Figure 5). If clients are predominantly 802.11a/n, this is the better choice. And when the whole 80 MHz is free, as measured by physical carrier sense and/or RTS/CTS with bandwidth indication, 80-MHz communication between 802.11ac devices is still allowed.
Certainly it is difficult to get the most out of 802.11ac without coordination of AP channel assignment, typically under the aegis of an effective centralized RRM algorithm.
2.3.7 Rate at range
As well as offering higher speeds, 802.11ac also delivers greater robustness than 802.11a or 802.11n.Consider that, to deliver 450 Mbps, 802.11n has to use three spatial streams maxed out at the sensitive 64QAM constellation, and with little multipath immunity: short guard interval and very little coding gain (a rate 5/6 code, so 20 percent allocated to redundancy). Yet by going from 40 to 80 MHz, 802.11ac achieves 530 Mbps using only a long guard interval, 16QAM, and rate 3/4 coding (that is, 33 percent redundancy).
We see this improvement in Figure 7, where 80-MHz links offer higher data rates close in and farther out. In Wave 1, different product configurations offer different benefits, but all are a marked step up from 802.11n.
Meanwhile Wave 2, and particularly 160 MHz, potentially offers still greater speeds. However, this improvement is not immediately useful, especially in the enterprise, due to the very limited number of 160-MHz channels that are available.
2.3.8 Regulatory considerations
Regulatory considerations and 802.11ac intersect in five respects:• In some regulatory domains, new rules are needed for devices to transmit 80-, 160-, and/or 80+80 MHz waveforms at all:
-Effective March 2012, greater than 40-MHz operation is allowed in the United States, the European Union, Australia, New Zealand, Brazil, and South Africa, and no obstacle is expected in numerous other countries.
-A few countries might allow 80-MHz or 802.11ac operation only after it is ratified by IEEE.
• In some regulatory domains, new tests are needed for devices that generate 160- and/or 80+80 MHz waveforms across adjacent subbands, where the present rules allow this (for example, 5.15 to 5.25, 5.25, and 5.35 GHz).
• In some regulatory domains, new rules are needed to allow the transmission of waveforms across adjacent subbands where the rules presently don’t allow this (for example, below and above 5.725 GHz, also known as channel 144).
• 802.11ac devices (and other unlicensed devices) suffer from reduced access to spectrum containing time-domain weather radars in and around 5.6 to 5.65 GHz.
• Due to the wider bandwidth of 802.11ac, there are strong market desires to open up new spectrum, for instance, in the 5.35- to 5.47-GHz band (which enables two new 80-MHz channels and one new 160-MHz channel):
-See, for instance, U.S. Act of Congress HR 3630, which empowers the National Telecommunications and Information Administration (NTIA) to study opening up this band to unlicensed use.
Due to the fact that regulations around the world are continually evolving, it is difficult to comment on this topic in detail in this white paper.
2.3.9 MU-MIMO
With 802.11n, a device can transmit multiple spatial streams at once, but only directed to a single address.For individually addressed frames, this means that only a single device (or user) gets data at a time. We call this Single-User MIMO (SU-MIMO). With the advent of 802.11ac, a new technology is defined, called multiuser MIMO (MU-MIMO). Here an AP is able to use its antenna resources to transmit multiple frames to different clients, all at the same time and over the same frequency spectrum. If 802.11n is like a hub, 802.11ac can be thought of as a wireless switch (on the downlink).
However, MU-MIMO is a challenging technology to implement correctly and won’t be available in the first wave of AP products. And even when available, MU-MIMO does come with caveats.
Figure 8 shows one piece of the puzzle. To send data to user 1, the AP forms a strong beam toward user 1, shown as the top right lobe of the blue curve. At the same time the AP minimizes the energy for user 1 in the direction of user 2 and user 3. This is called “null steering” and is shown as the blue notches. In addition, the AP is sending data to user 2, forms a beam toward user 2, and forms notches toward users 1 and 3, as shown by the red curve. The yellow curve shows a similar beam toward user 3 and nulls toward users 1 and 2. In this way, each of users 1, 2, and 3 receives a strong copy of the desired data that is only slightly degraded by interference from data for the other users.
For all this to work properly, especially the deep nulls, the AP has to know the wireless channel from itself to all of the users very accurately. And since the channel changes over time, the AP has to keep measuring the channel, which adds overhead. Some APs might use the higher-overhead 802.11ac sounding protocol only, but the greatest benefit of MU-MIMO comes if the AP can minimize the number of explicit sounding exchanges, such as with the ClientLink mechanisms.
Meanwhile, the client is receiving its desired signal distorted by some interference from the signals intended for other users. This interference makes the highest constellations such as 256QAM infeasible within an MU-MIMO transmission.
In summary, MU-MIMO allows an AP to deliver appreciably more data to its associated clients, especially for small form-factor clients (often BYOD clients) that are limited to a single antenna. If the AP is transmitting to two or three clients, the effective speed increase varies from a factor of unity3 (no speed increase) up to a factor of two or three times, according to wireless channel conditions.
3 If the speed-up factor drops below unity, the AP uses SU-MIMO instead.
2.3.10 802.11ac project authorization request
The 802.11ac Project Authorization Request (PAR) that kicked off 802.11ac includes some throughput numbers: 500-Mbps single-user throughput and 1-Gbps multiuser throughput. These numbers are requirements in the 802.11ac amendment (that is, the document), not in individual products. The amendment defines that the minimum product allowed to call itself 802.11ac can operate at 290 Mbps for a single user and not support multiuser at all.3. How does 802.11ac affect me?
3.1 Compatibility
One issue not to worry about is compatibility.802.11ac is carefully designed to be maximally forward and backward compatible with 802.11a/n devices. In fact, the 802.11ac design is even simpler and more thorough than 802.11n compatibility with 802.11a devices, as shown in Table 4.
An 802.11ac device must support all the mandatory modes of 802.11a and 802.11n. So an 802.11ac AP can communicate with 802.11a and 802.11n clients using 802.11a or 802.11n formatted packets. For this purpose it is as if the AP were an 802.11n AP. Similarly, an 802.11ac client can communicate with an 802.11a or 802.11n AP using 802.11a or 802.11n packets. Therefore, the emergence of 802.11ac clients will not cause issues with existing infrastructure.
Furthermore, the preamble of the 802.11ac formatted packet is identical to an 802.11a formatted packet, so the CCA mechanism kicks in for third-party 802.11a and 802.11n devices. As soon as these third-party devices see the 802.11ac preamble, they know the duration of the packet and know not to transmit during that time. Also, since the packet is typically followed by an Ack or Block Ack frame sent in an 802.11a frame, the third-party devices can correctly receive the Ack or Block Ack and then can continue to try to transmit as usual. In the worst case, a third party device hears the 802.11ac frame but is out of range of the transmitter of the Ack or Block Ack. But even here the third party must wait for an extended duration (called EIFS) to allow time for the Ack or Block Ack to be transmitted without fear of collision.
Because of this preamble-level compatibility, there is no need for 802.11ac devices to precede their 802.11ac transmissions by CTS-to-self or RTS/CTS. The kinds of inefficiencies associated with sending 802.11g packets in the presence of 802.11b devices are completely avoided at 5 GHz.
3.2 Radio resource management and WIPS effects
802.11a/n deployments not upgraded to 802.11ac still have to consider the effect of 802.11ac introduced by neighbors in the normal way and the additional exploits available to attackers.802.11ac affects RRM, since overlapped devices can now transmit over 80 or even 160 MHz. With a software upgrade, it is possible for existing RRM systems to detect the presence of 802.11ac APs from the new 802.11ac fields in beacon frames and extract the affected bandwidth. With this knowledge, the RRM system can mitigate the effect from nearby 802.11ac APs. The RRM system has to work harder since (1) a single overlapped 802.11ac AP affects a wider bandwidth, and (2) the effect on any 20-MHz subchannel depends on whether or not the primary 20-MHz subchannels of the in-network and overlapped APs are aligned (see Section 2.3.6).
Users should verify that their APs are capable of using all the available channels, even those subject to radar detection (dynamic frequency selection, or DFS) requirements. (Many consumer-grade and some enterprise-grade APs are not certified by regulators to operate on DFS channels. This is unfortunate, since in the United States, for example, 63 percent of 20-MHz channels are DFS channels.)
In general, the Wireless Intrusion Protection System (WIPS) of an 802.11a/n deployment can detect and mitigate many attacks by 802.11ac devices, particularly when conducted by naïve attackers. This is because the 802.11ac device communicates using 802.11a/n format packets when communicating with 802.11a/n devices, and 802.11ac devices invariably continue to transmit beacons, probe requests, and probe responses at 802.11a rates. However, packets sent using 802.11ac format cannot be decoded by 802.11a/n devices. The recommended countermeasure against such attacks is to provide a sprinkling of 802.11ac APs operating full-time WIPS (for example, one 802.11ac WIPS AP for five or six serving 802.11a/n APs) or a full upgrade of all APs.
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