: 咨询热线
 考试咨询  培训咨询  MSN咨询




>>>>>进入技术文摘 列表


点击进入:技术文摘  专栏  添加时间: 2010-5-17 10:13:57  阅读数: 8066

802.11n Frequently Asked Questions

While 802.11n technology brings enormous benefits in terms of increased coverage, throughput and capacity, it also requires careful planning and optimization for organizations to fully realize its benefits.  This comprehensive FAQ attempts to demystify 802.11n and offers guidance on important issues facing organizations with regards to 802.11n adoption.


Q: What is 802.11n?
IEEE 802.11n is the next generation wireless LAN standard. The excitement about 802.11n is driven by the promise of two to four times longer range and ten-fold increase in data transmission rate as compared to its predecessors 802.11a, b and g.

Q. Is 802.11n already an IEEE standard?
Yes, 802.11n was ratified by IEEE as a standard in September 2009.

Q. How does 802.11n achieve the multi-hundred megabits throughput and longer coverage range?
IEEE 802.11n brings many new features to deliver the performance gains. It uses the multiple-input-multiple-output (MIMO) technology that enables spatial diversity and spatial multiplexing for respectively increasing the range and data transmission rate. In addition, 802.11n allows use of wider 40 MHz channels to double the bandwidth as compared to the legacy 20 MHz operation. 802.11n uses frame aggregation and block acknowledgements for improving the throughput efficiency.

Q. My legacy (802.11a/b/g) AP already has two antennas. Isn’t it capable of MIMO?
MIMO is not just about having multiple antennas but about having multiple transmit/receive RF chains. Though a legacy (802.11a/b/g) AP has two antennas, it hosts only one RF chain that selects one of the antennas to transmit and receive a signal. This is commonly known as “switched diversity” or "selection combining." 802.11n devices instead use the MIMO capability to simultaneously process signals over multiple antennas.

Q. Which frequency bands can 802.11n operate in?
802.11n can operate in the 2.4 GHz and 5 GHz ISM bands that are respectively used by 802.11b/g and 802.11a.

Q. Can I use my legacy (802.11a/b/g) clients with an 802.11n AP?
Yes, you can use legacy clients with 802.11n APs and vice versa. 802.11n standard supports a non-high-throughput legacy mode and defines a high-throughput mixed mode operation that is backward compatible with legacy 802.11a/b/g protocols.

Q. If legacy clients associate with 802.11n AP, will they get better range?
An AP-client association is half duplex, which means gains in both directions (client to AP and AP to client) are needed to sustain a connection over a longer distance. The asymmetry in transmission power common in wireless LANs (clients usually transmit at lower power than AP) can actually allow legacy clients to achieve higher data rates over a longer range when associated with an 802.11n AP — the receive spatial diversity will improve the signal-to-noise ratio (SNR) from the client to the 802.11n AP, and higher transmission power of the AP will allow it to reach the client. In absence of this asymmetry, an 802.11n AP will need to additionally implement transmit beamforming to reach legacy clients over a longer distance.

Q. Is it true that an 802.11n AP consumes more power than a legacy (802.11a/b/g) AP?
Yes, with multiple transmit and receive RF chains, a full-featured 802.11n AP usually consumes lot more power than a legacy 802.11 AP. The actual power consumption depends on the implementation and may vary across vendors.

Q. Can I power an 802.11n AP using the existing 802.3af Power over Ethernet (PoE) standard?
With dual radios and 3x3 MIMO per radio, the typical power consumption (~18 W) of a full-capacity 802.11n AP is more than the 12.95 W the 802.3af PoE standard can handle. The 802.3at standard when available will be able to support the higher power required by 802.11n APs. However some vendors are offering proprietary solutions that support enough power for running an 802.11n AP.

Q. Should I deploy 802.11n APs?
If you are planning to deploy a fresh WLAN, then 802.11n offers the opportunity to plan for the future. If you already have a large, fully functional 802.11a or 802.11g WLAN, and the answer to these four questions is YES, you are ready for an 802.11n upgrade:
(1) Will you also upgrade most of your legacy Wi-Fi clients to 802.11n in the near future?
(2) Are you looking for more capacity or coverage?
(3) Are you planning on deploying bandwidth intensive applications (e.g., streaming audio/video, imaging applications)?
(4) Are you ready to invest into solutions capable of securing and troubleshooting your 802.11n WLAN?

Q. Most vendors recommend simply swapping each of my existing legacy APs with an 802.11n AP. Is this a good 802.11n migration strategy?
Deploying 802.11n requires careful planning. Swapping each legacy AP with an 802.11n AP may not be necessary or in fact may even be detrimental due to increased contention or interference among your APs, thanks to the better coverage range of 802.11n APs. To get improvement in capacity, you should also upgrade your clients to 802.11n.

Q. How should I plan my 802.11n WLAN?
Use a predictive planning tool that—based on your capacity or coverage requirements, and the chosen 802.11n AP model and its configuration—estimates the number of APs required, their placement, and their channel assignment. If you have a legacy 802.11b/g or 802.11a WLAN, then the planning tool should allow you to run “what-if” scenarios and design a migration plan to gradually upgrade your legacy WLAN to 802.11n or even plan for coexistence of legacy and 802.11n APs. If you expect co-existence of legacy and 802.11n clients, you should consider using dual radio APs to separately serve legacy and 802.11n clients for better performance.

Q. If I deploy 802.11n, do I also need to upgrade my Ethernet LAN to Gigabit Ethernet?
An 802.11n AP serving only 802.11n clients can deliver over 150 Mbps throughput. In this case, the 10/100 Ethernet LAN is the bottleneck and you should consider upgrading it to Gigabit Ethernet. Using 5 GHz 802.11n WLAN is another alternative to design a wireless backhaul.

Back to top

Performance Issues

Q. Can 802.11n achieve the multi-hundred megabit data rates and two to four times longer range than legacy 802.11 at the same time?
The multiple transmit/receive RF chains in MIMO can be leveraged either for achieving better range by processing multiple copies of the same signal over multiple RF chains (spatial diversity), or for achieving higher data rates by pushing multiple unique data streams over multiple RF chains (spatial multiplexing). In short, one is usually traded off for the other. For instance, in a 2x2 system, if two unique data streams are transmitted then the receiver can use MIMO to decode the two streams (no diversity gain). If only one stream is transmitted then transmitter could use transmit beamforming and the receiver could use MIMO to combine the copies of the signal from both chains and improve the range (no multiplexing gain).

Q. Can 802.11n APs deliver the promised multi-hundred megabits throughput with a mix of 802.11n and legacy (802.11a/b/g) clients?
Presence of legacy clients will limit the performance improvement for 802.11n WLANs. Legacy clients will continue to operate at legacy speeds occupying the medium for longer intervals per frame transmission. Further, legacy clients associating with an 802.11n AP will trigger the HT-protection mode in this AP leading to additional overhead. Both these factors will reduce the net throughput of the 802.11n devices associated with the same AP.

Q. Will 802.11n interfere with my legacy 802.11 network?
802.11n can operate in 2.4 and 5 GHz frequency bands and hence can interfere with legacy 802.11 networks—802.11b/g in 2.4 GHz and 802.11a in 5 GHz.

Q. Is it a good idea to use 802.11n’s 40 MHz operation in 2.4 GHz band?
Due to limited bandwidth in 2.4 GHz ISM band, only three non-overlapping 20 MHz channels (1, 6, and 11) are feasible in most regulatory domains. This allows deployment of multiple APs and facilitates frequency reuse. With 40 MHz channel width, only one AP can be deployed. Further, interference/contention with neighboring WLANs is more likely due to increased coverage. Hence, 40 MHz operation in the 2.4 GHz band should be avoided.

Q. 802.11n is often touted as the brass ring for VoIP and video applications. Will simply replacing our existing APs with 802.11n APs improve the performance of VoIP and video applications?
VoIP and video applications are certainly enhanced by the benefits of high speeds of 802.11n but only if the devices (e.g., WiFi phones) running these applications are 802.11n capable. Using legacy Wi-Fi devices with 802.11n APs will not provide much advantage as applications will continue to run on legacy data rates.

Back to top

Security Concerns

Q. Does 802.11n impact existing security vulnerabilities?
The longer range and higher speeds of 802.11n amplify existing security vulnerabilities. Signal from authorized devices will spill over longer distances from your premises and signal from unauthorized devices from farther will spill into your premises. The existence of 802.11n clients using 802.11n data rates in a/b/g environment poses a security threat by creating somewhat of a blind spot, allowing hackers to evade existing monitoring systems.

Q. Does 802.11n introduce new security vulnerabilities?
802.11n gives rise to new DoS attacks that exploit the Block Ack (BA) feature. An attacker can launch a DoS attack by inserting fake, out-of-order ADDBA packets and cause the receiver to drop valid packets that it perceives as out of order. An attacker can also tear down sessions with a fake DELBA packet.

Q. Will my current wireless intrusion prevention system (WIPS) detect 802.11n rogue devices and unauthorized client associations?
So long as the 802.11n rogue devices operate in legacy or high-throughput mixed mode, 802.11a/b/g WIPS can detect their existence. However, if these rogue devices operate using 802.11n data rates, they can be detected and prevented using only 802.11n-ready WIPS solution.

Back to top


Q. What is MIMO?
MIMO stands for multiple inputs and multiple outputs, which means a device with multiple transmitters emitting signals and a device with multiple receivers receiving the signals. A MIMO system is often represented as NxM where N is the number of inputs and M is the number of outputs.

Q. What is spatial diversity?
Spatial diversity is a MIMO feature that can be achieved either at the transmitter (transmit diversity) or at the receiver (receive diversity) or both. It involves use of multiple antennas with enough separation in distance so that the receiver can receive multiple independently fading signal paths. Spatial diversity improves the signal-to-noise ratio (SNR) at the receiver over what would be obtained without diversity (single receive antenna). The improvement in SNR is often termed as “diversity gain.” In theory, the maximum achievable diversity gain is the product of the number of transmit and receive antennas.

Q. What is maximal ratio combining (MRC)?
Maximal ratio combining (MRC) is a way to exploit spatial diversity at the receiver. In MRC, a weighted sum of signals received on all antennas results in the combined signal being stronger than the signal received with maximum power. The improved signal-to-noise ratio leads to better reception range.

Q. What is beamforming?
Simply put, beamforming is to achieve MIMO transmitter spatial diversity. Beamforming is the process in which the transmitter codes or assigns weights to signals before transmission to maximize the signal-to-noise ratio at the receiver. The weights depend on the transmitter’s estimate of the channel to the receiver. The information to estimate the channel can be obtained through implicit of explicit feedback. Beamforming allows coherent combining of the multiple independently fading signal paths at the receiver. Transmit beamforming is an optional feature in the 802.11n standard.

Q. What is space-time block coding (STBC)?
STBC is a pre-transmission encoding done by a MIMO transmitter that allows it to improve the signal-to-noise ratio even at a single RF receiver (non-MIMO). STBC is an optional feature in the 802.11n standard. Alamouti coding is an example of STBC that can work with at least a 2x1 system or any multiple of that. The beauty of Alamouti coding lies in the fact that it enables STBC without knowledge about the channel to the receiver.

Q. What is spatial multiplexing?
Spatial multiplexing is a MIMO feature where multiple unique data streams are multiplexed over different transmit/receive RF chains in parallel. In other words, more bits are pushed per unit time from a transmitter to receiver increasing the link capacity or data rate. This gain in capacity is often termed as “multiplexing gain.” In theory, the maximum achievable multiplexing gain is equal to the minimum of the number of inputs (transmitter chains) and outputs (receiver chains).

Q. What is channel bonding in 802.11n?
802.11n can operate on two “bonded” 20 MHz channels to give an effective bandwidth of 40 MHz. The higher bandwidth can more than double the data rates as compared to 20 MHz operation.

Q. How does frame aggregation in 802.11n work?
To improve the protocol efficiency and achieve higher throughput, 802.11n allows devices to aggregate and transmit multiple frames together. This is termed as “frame aggregation.”

Q. What is a block acknowledgement (Block Ack)?
Instead of sending an acknowledgement for each frame, 802.11n allows a receiver to cumulatively acknowledge multiple frames with a single Block Ack. Block Ack was first proposed in the IEEE 802.11e standard and has been enhanced in 802.11n.

Q. What is an A-MSDU?
Aggregation of MAC service data units (MSDUs) or frames at the MAC layer gives an aggregated MSDU (A-MSDU).

Q. What is an A-MPDU?
Aggregation of MAC protocol data units (MPDUs) or frames at the physical layer gives an aggregated MPDU (A-MPDU).

Q. What is the Greenfield mode in 802.11n?
The Greenfield mode is an optional high-throughput mode in the 802.11n standard, which is not backward compatible with legacy (802.11a/b/g) protocols and is expected to provide maximum performance benefits of 802.11n.

Q. What does MCS stand for?
MCS stands for Modulation and Coding Scheme. The 802.11n standard defines a total of 77 MCS. Each MCS is a combination of a certain modulation (e.g., BPSK, QPSK, 64-QAM), coding rate (e.g., 1/2, 3/4), guard interval (800 or 400 ns), and number of spatial streams. Support for MCS 0-15 is mandatory for 802.11n APs and support for MCS 0-7 is mandatory for 802.11n clients.

地址:广东省深圳市罗湖区宝安南路2014号振业大厦A座15A-B(地王大厦旁) Email:kevinzhu#cntimes.biz(将#改为@)
电话:0755-25022500 25025151 传真:0755-25022400  ICP备案:粤ICP备14017173号