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Cisco 第五代WiFi 802.11ac 白皮書 (802.11ac: The Fifth Generation of Wi-Fi Technical White Paper)
White paper Cisco public Second-wave or Wave 2 products also come with a a remarkable level of performance. If you still have an new technology, multiuser MIMO(MU-MIMO). Whereas older standard like 802 1 1n deployed today, upgrading 802 1 1n is like an ethernet hub that can transfer only a to the latest 802 1 1ac Wave 2 technology will provide single frame at a time to all its ports, MU-MIMo allows better performance for bandwidth-intensive applications an aP to send multiple frames to multiple clients at like streaming video and collaboration. The newer the same time over the same frequency spectrum technology will also address a common issue that That's right: With multiple antennas and smarts, an AP most networks are seeing today, providing consistent can behave like a wireless switch There are technical performance in the face of higher density or more constraints, and so MU-MIMO is particularly well suited wireless clients accessing the network. Cisco offers a to Bring-Your-Own-Device(BYOD) situations in which broad portfolio of products that support 802 1 1ac Wave devices such as smartphones and tablets have only a 2, as well as the right innovation and features designed gle antenna for a variety of different network sizes and needs 802.1 1ac-enabled products are the culmination of When it comes to interoperability with older standards efforts at the iEee and Wi-Fi Alliance pipelines. IEEE 802 1 1ac will have a few effects on existing 802.1 1a/n 802.1 1ac delivered an approved Draft 2.0 amendment deployments, even if the deployment is not upgraded to in January 2012 and a refined Draft 3.0 in May 2012, 802 1 1ac immediately: (1)the wider channel bandwidths with final ratification in December 2013. In parallel, the of neighboring APs require updates to radio resource Wi-Fi Alliance adopted an early but very stable and management, or RRM(and in particular the dynamic mature IEEE draft, namely Draft 3.0, and used that as channel assignment algorithm), and(2)802.11a/n the baseline for an interoperability certification of first Wireless Intrusion Protection Systems(WIPS)can wave products in mid-2013. The Wi-Fi Alliance split its continue to decode most management frames such 802.1 1ac certification into two parts, in order to include as beacon and probe request/response frames (that testing of the more advanced features. This second are invariably sent in 802. 1 a format) but do not have wave certification includes features that were not in the visibility into data sent in the 802.1 1 ac packet format earlier certification(Wave 1). These features include channel bonding up to 160 MHz, four spatial streams, One thing not to worry about is compatibility 802 1 1ac and MU-MIMO. Overall, this arrangement closely follows is designed in a deep way to coexist efficiently with how 802.1 1n was rolled out, with 802.1 lac Wave 1 existing 802.11a/n devices, with strong carrier sense, coming out first and Wave 2 products being released a single new preamble that appears to be a valid about 18 months later 802 1 1a preamble to 802.1 1a/n devices, and extensions to Request-To-Send/Clear-To-Send(RTS/CTS)to Enterprise networks considering an investment in help avoid collisions with users operating on slightly infrastructure Wi-Fi have the choice to move from an different channels older technology such as 802 1 1n while also delivering c2018 Cisco andyor its affiliates. All rights reserved White paper Cisco public 2. What is 802.lac? First, 802.1 1ac is an evolution of 802.11n. If you want to This density is exacerbated by the BYOD trend learn more about 802.11n, jump to the Appendix. If you such that one employee might carry two or even are already familiar with the channel bonding, MIMO, and three 802.11 devices and have them all consuming aggregation introduced by 802.11n, and you don't need network resources at the same time a refresher, read on The proliferation of wireless loT devices also increases the density of networks 2.1 Drivers for 802.lac The increased adoption of video streaming, collaboration 802 1 1ac is an evolutionary improvement to 802.11n and other bandwidth-intensive applications One of the goals of 802 11 ac is to deliver higher levels of performance that are commensurate with gigabit 802 1 lac is about delivering an outstanding experience Ethernet networking to each and every client served by an AP, even under demanding loads A seemingly " instantaneous data transfer experience Meanwhile, 802 11 is integral to a hugely broad range A pipe fat enough that delivering a high Quality of of devices, and some of them are highly cost, power, experience(QoE)is straightforward or volume constrained. One antenna is routine for these In the consumer space, the target is multiple channels of devices, yet 802 1 1ac must still deliver peak efficiency High-Definition(HD) content delivered to all areas of the house. The enterprise has different challenges The one thing that 802. 11 ac has in its favor is the evolutionary improvement to silicon technology over the Delivering network with enterprise-class speeds past half-dozen years: channel bandwidths can be wider and latencies constellations can be denser, and APs can integrate High-density environments with scores of clients per AP more functionality Figure 1. How 802.lac accelerates 802.11n per Subcarrier 802.11acAP 256QAM@5/6 64QAM@r5/6 802.11nAP Channel 40 80 160 Spatial streams c2018 Cisco andyor its affiliates. All rights reserved White paper Cisco public 2.2 How does 802.lac go so fast? Wireless speed is the product of three factors: channel bandwidth, constellation density, and number of spatial streams. 802.1 1ac pushes hard on the boundaries on each of these, as shown in Figure 1 For the mathematically inclined, the physical layer speed of 802 1 lac 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 x 3 x 5/6 8 bits/3.6 microseconds=1300 Mbps Table 1. Calculating the speed of 802.11n and 802.llac PHY Bandwidth Number Data bits per Time per PHY (as number of of spatial subcarrier OFDM symbol data rate data subcarriers) streams (bps) 802.11n 56(20MHz) 1 to 4 Up to 5/6 3.6 og2(64)=5 microseconds 802.11ac (short guard interval) 108(40MHz) 4 microseconds (long guard interval) 80211ac234(80MHz) 5 to 8 Up to 5/6 ly onl g2(256)≈6.67 2×234(160 Immediately we see that increasing the channel 256QAM helps most at shorter range where 64QAM bandwidth to 80 MHz yields 2.16 times faster speeds is already reliable. Still, 256QAM doesn't require more and 1 60 MHz offers a further doubling. Nothing is for spectrum or more antennas than 64QAM free: it does consume more spectrum, and each time were splitting the same transmit power over twice as The speed is directly proportional to the number of many subcarriers, so the speed doubles, but the range spatial streams. More spatial streams require more for that doubled speed is slightly reduced antennas, RF connectors and rf chains at transmitter for an overall win and receiver. The antennas should be spaced one third of a wavelength (3/4 inch) or more apart, and Going from 64QAM to 256QAM also helps, by another the additional rF chains consume additional power. 8/6=1.33 times faster. Being closer together, the T his drives many mobile devices to limit the number of constellation points are more sensitive to noise, so antennas to one, two, or three c2018 Cisco andyor its affiliates. All rights reserved White paper Cisco public Collectively, these three speed increases are significant. As shown in Figure 2 and Table 2, the minimum allowed 802.1 1ac 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 Figure 2. Evolution of Cisco APs with 802.11 physical layer amendments 69006900 3500* 1300 1730 600) 870 450 300 290 290 65 54 (24) VHT VHT HT 802.11ac802.11ac 802.11 802.11b 80211a/g 802.11n Wave 1 Wave 2 ASsuming 160 MHz is Available and suitable c2018 Cisco andyor its affiliates. All rights reserved White paper Cisco public Table 2. Important data rates of 802. 11a, 802.11n, and 802.lac No omina Band- Number Constellation Guard PHY data Throughput configuration width spatia size and rate interval rate(Mbps)(Mbps)* (MHz) streams 802.11a All 20 64QAMr3/4 Long 54 24 802.1Tn Amendment min 20 64QAMr5/6 Long 65 46 oW-end product(2.4 64QAMr5/6 Short 51 GHz only+ Mid-tier product 300 210 Max product 40 234 64QAMr5/6 Short 64QAMr5/6 Short 450 320 Amendment max 40 64QAMr5/6 Short 600 420 802.11ac80MHz Amendment min 64QAMr5/6 Long 293 210 LOW-end product 80 256QAMr5/6 Short 433 300 Mid-tier product 80 256QAMr5/6 Short 867 610 High-end product 80 238 256QAMr5/6 Short 1300 910 Amendment max 80 256QAMr5/6 Short 3470 2400 802.11ac160MHz LOW-end product 160 256QAMr5/6 Short 867 610 Mid-tier product 160 256QAMr5/6 Short 1730 1200 High-end product 160 Ultra-high-end product 160 2348 256QAMr5/6 Short 2600 1800 256QAMr5/6 Short 3470 2400 Amendment max 160 256QAMr5/6 Short 6930 4900 Assuming a 70 percent efficient MAC, except for 802.1 1a, which lacks aggregation +Assuming that 40 MHz is not available due to the presence of other APs c2018 Cisco andyor its affiliates. All rights reserved White paper Cisco public 2.3 How do we make 802.lac robust? Various Space Time Block Codes(STBCs)are allowed The sticker on the box that shows the maximum data as options, but(1) this list is trimmed from the overrich rate doesn't help us much in the real world, where set defined by 802. 11n, and(2) STBC is largely made devices have to contend with interference from non redundant by beamforming. 802 1 1n defined the core 802.11 devices, preexisting APs that might only use 20 STBC modes of2x1and4x2 and also3×2and4×3 or 40 MHZ, multipath fading, few antennas on mobile as extension modes, but the extension modes offered devices, weak signals at range, and so forth. What ittle gain for their additional complexity and have makes the raw speed of 802.1 lac so valuable are the not made it to products. Indeed, only the most basic extensions that help to deliver reliable throughput under mode, 2x1, has been certified by the Wi-Fi Alliance realistic conditions With this experience, 802 1 l ac defines only the core 2×1,4×2,6×3,and8x4 STBC modes, but again only 2x 1 is expected to make it to products: if you had an 2.3.1 Technology overview AP with four antennas, why would you be satisfied By design, 802 1 l ac is intended to operate only in the With 4 x2 STBC When you could-and should-be 5-GHz band, as shown in Table 3. This avoids much of using beamforming? the interference at 2. 4 GHz, including bluetooth headsets and microwave ovens, and provides a strong incent What 802 1 lac also gets right is to define a single for users to upgrade their mobile devices(and hotspot way of performing channel sounding for beamforming APs) to dual-band capability so that the 5-GHz band is SO-called explicit compressed feedback. Although more universally usable. This choice also streamlines the optional, if an implementer wants to offer the benefits of IEEE process by avoiding the possibility of contention standards-based beamforming, there is no choice but to between 802.11 and 802.15 proponents. And there is select that single mechanism, which can then be tested barely 80 MHZ of bandwidth at 2. 4 GHz anyway for interoperability As we've already seen, 802.11 introduces higher-order Because of the wider channel bandwidths of 802 1 lac modulation, up to 256QAM; additional channel bonding is much more likely that an 80-MHZ AP will overlap up to 80 or 1 60 MHZ; and more spatial streams, up to with another 20-or 40-MHz AP -and similarly an 80-or eight. There is an alternative way to send a 160-MHz 160-MHz AP -or even several of them, all potentially on signal, known as“80+80”MHz, discussed later different channels. To enable reliable operation amid this (see Section 2.3.6) complexity, 802 11 ac mandates extensions to the RTS/ CTS mechanism, stronger Clear-Channel Assessment 802.11ac continues some of the more valuable features (CCA)requirements, and new primary channel selection of 802. 11n, including the option of a short guard interval rules. See Section 2.3.4 ( for a 10 percent bump in speed and an incrementally better rate at range using the advanced low-density 802.1 1ac also introduces a valuable new technology parity check(LDPC) forward error-correcting codes called multiuser MIMO. This is challenging to get righ These LDPC codes are designed to be an evolutionary so it is deferred until the second wave of 802.11ac extension of the 802.11n LDPC codes, so implementers products and will likely be optional. More on this later in can readily extend their current hardware designs Section 2.3.9 c2018 Cisco andyor its affiliates. All rights reserved White paper Cisco public Table 3. Primary ingredients of 802.lac Parameter 802.lac Draft 3.0 802.11ac(subset of 802.11ac complete Wave 1 Wi-Fi Alliance ratified amendment) amendment certification Potential Wave 2 Wi-Fi Alliance certification Spectrum 5 GHz(varied support by regulatory domain; nearly 600 <6 GHz, excluding 2.4 GHz MHZ in the United States Bandwidth Mandatory: 20, 40, and 80 Mandatory: 20, 40, and 80 MHZ tional: 160 and 80+80 MHZ Modulation Mandatory: BPSK, QPSK, 1 6QAM, 64QAM Optional: 256QAM Number of spatial Mandatory: 2(nonmobile Mandatory: 2(nonmobile Mandatory: 1 streams APs*), 1(others) APs*), 1(others) Optional: 2 to 8 Optional: up to 3 spati tional: up to 4 spatial streams streams Forward error correction Mandatory: BCC Optional: LDPC STBC Optional: 2x1 AP to client Optional:2×1,4×2,6×3, 8×4 Short guard interval Optional Sounding (a single Optional interoperable protocol CTS in response to Mandatory RTS With bandwidth indication RTS With bandwidth Optional indication Aggregation Mandatory: TX and RX of Mandatory: TX and Rx of A-MDPU, A-MDPU of A-MPDU A-MPDU A-MSDU Optional: RX A-MPDU of Optional: RX A-MPDU of A-MSDU A-MSDU MU-MIMO Optional Additional requirement introduced by the Wi-Fi Alliance c2018 Cisco andyor its affiliates. All rights reserved White paper Cisco public 2.3.2 Differences between 802.lac and 802,11n transmissions, but experience has shown that A-MDPU 802 1 ac has avoided the battles of 802.11n and solves much the same problem even more efficiently instead has focused on extending the tremendous 802.1 lac devices operating in 802 1 lac mode are not advances made in 802 1 1n to deliver the next generation permitted to transmit RiFs (as of Draft 3.0) of speed and robustness 802.1 1n features that are not updated for 802.1 1ac(or For instance, 802.1 1n pioneered aggregation through are explicitly forbidden for 802 11 ac devices operating the selective use of a-mPdu. a-msdu, and a-mPdu of 802.11ac mode) include all the 802 1 1n sounding A MSDU(see the Appendix). 802 1 lac actually requires options, including extension LTFS, the calibration every 802 1 1 ac transmission to be sent as an A-MPDU procedure, antenna selection, PCo, L-SIG TXOP aggregate. This is due in part to the intrinsic efficiency protection, unequal modulation, 4x3 and 3x2 STBC of A-MPdu, as well as to some other factors modes, MCS32, and dual Cts protection. Don't worry if (see Section 2.3.5) you don t know these terms; you'll almost certainly never need to understand them In a further example, 802 1 lac extends the 802.1In channel access mechanism: virtual carrier sense and 2.3.3 Standards-based beamforming backoff occur on a single 20-MHz primary channel; CCA is then used for the remaining 20-MHz subchannels Any device(with multiple antennas) can beamform to immediately before transmitting on them any other device at any time. What 802.1 1 ac adds is the opportunity for the receiver to help the beamforming Given the power of A-MPDU and the 802.11n channel transmitter to do a better job of beamforming. T his access mechanism, 802 1 lac actually didn't need to is called"sounding, and it enables the beamformer innovate much in the mac. indeed, extensions to the to precisely steer its transmitted energy toward the RTS/CTS mechanism are the only new mandatory receiver 802 1 l ac defines a single, though optiona MAC feature protocol for one 802 1 1ac device to sound other 802.1 1ac devices. The protocol selected closely follows 802.1 In does include many options with reduced value. the 802.1 1n explicit compressed feedback protocol and 802.1 1 ac takes a very pragmatic approach to them as follows If a useless option is used and affects a third-party device, 802 1 lac typically forbids an 802.1 1ac device A device, typically an AP, sends a"Very High Throughput (operating in 802 1 1ac mode)from using the option. If a(VHT) Null Data Packet(NDP)Announcement"frame useless"option has not been used in 802 1 1n products its only purpose is to contain the address of the AP and or affects only the devices that activate the option, the of the target recipients. The VHT NDP Announcement feature is not updated for 802 1 lac but is instead frame is immediately followed by a VhT Null Data “ eft to die.” Packet"(VHT NDP)intended for those target recipients Each intended recipient measures the rf channel from For instance, there is no 802 1 1 ac version of the the aP to itself using the preamble of the Vht NDP and 802.1 1n greenfield"preamble format. 802 1 lac defines empresses the channel. The first intended recipient only one preamble format, which, to legacy 802. 11a/n responds with the compressed channel information in devices, will look safely like an 802 1 1a preamble followed by a payload with a bad CRC. This means that a VhT Compressed Beamforming frame immediately and other recipients respond when they are polled by legacy devices won't try to transmit over the top of the the ap the vht ndP announcement frame, the vht 802.1 1ac transmission, nor will they attempt to send a NDP and the Vht compressed beamforming frame are bad payload up the stack all similar to features in 802.1 1n. However because of 802 1 1n introduced Reduced Interframe spacing some subtle differences, the 802 1 1ac sounding is not (RIFS), which reduces overheads between consecutive backward compatible with 802.1 1n devices c2018 Cisco andyor its affiliates. All rights reserved

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