© 2009

802.11n – The Good, the Bad and the Ugly

Phil Belanger

2© 2009

What is 802.11n? A new IEEE 802.11 standard for high throughput wireless LANs.

The biggest change in wireless LANs since the first Wi-Fi.

The only 802.11 standard to change both the MAC and PHY.• 11n changes everything - modulation, framing, antennas...

The official IEEE standard has not yet been published.• the official standard is now expected in 2010

• huge changes and a bureaucratic process led to a very long development

• the technical debate has been over for years• draft 11n chipsets are widely available

Wi-Fi Alliance is certifying interoperability for 802.11n Draft 2• interoperability has been good• this effectively creates a pre-standard version that is standard• the formal standard will include backwards compatibility with Draft 2

3© 2009

What is 802.11n? Features MIMO - Multiple Input/Multiple Output

• multiple radios and multiple antennas for receive and transmit• signal processing that combines the inputs from multiple antennas to improve signal

• multipath reflections previously cause errors, now 802.11n MIMO uses multipath as an advantage• Draft 2.0 enterprise products are typically 3 receive and 3 transmit

Spatial multiplexing• supports multiple simultaneous streams of data on same channel• Draft 2.0 equipment must support at least 2 streams

Improved modulation• better OFDM achieves 65 Mbps in a single stream 20 MHz channel

Channel bonding• two 20 MHz channels can be combined to form a single 40 MHz channel that delivers twice the data

rate

Frame Aggregation, Block ACK, Short Guard Interval• more efficient protocol which provides higher effective throughput

More spectrum available• 802.11n supports both bands - 2.4 GHz and 5 GHz• 802.11h opens up more channels in 5 GHz band through DFS

4© 2009

The Promise of 802.11n

Speed - High Throughput• Current Draft 2 products can achieve 300 Mbps raw data rate

• 160 Mbps of actual throughput• Under ideal conditions, when configured properly• All of the enhancements must be enabled, only 11n products on channel• 40 MHz channel, 2 streams

• Six to eight times improvement in performance

Improved Range and Coverage• much better range for most clients• twice the range?• What is coverage?

Perfect backwards compatibility• can support legacy 802.11 products on the same system• 11n clients can operate in legacy mode on older networks

5© 2009

Reality of 802.11n Consumer success

Works great• Great speed and great coverage• Perfect backwards compatibility

• 802.11n clients work with legacy infrastructure and perform as well as the best 802.11g or 802.11a client

• 802.11n APs also support legacy 802.11g or 802.11a clients

Built into many devices• Mature chipsets, second or third generation 802.11n• Still priced higher, but prices of 11n declining at the device level

Just starting in the enterprise...• 802.11n Draft 2 products are stable• There is no reason to wait from an interoperability point of view

Single 11n AP and a few clients in the home works great! Can 802.11n scale up to multiple AP systems in the enterprise?

• Yes - but it takes careful planning.

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Enterprise WLAN 11n Challenges Wireless Network Challenges

• Full speed requires channel bonding which reduces available channels• Lack of 40 MHz channels in 2.4 GHz band, few 40 MHz channels in 5 GHz• MIMO makes channel planning impractical

• Unpredictable coverage increases interference and creates coverage gaps• More range forces larger spacing between same-channel APs

• which consumes more channels or leaves coverage gaps• legacy planning tools do not work

Wired Network Challenges• Each AP needs at least a gigabit Ethernet uplink• Dramatic bandwidth improvements on the wireless network may require wired

network upgrades• 802.3af PoE power budget may be insufficient for some multi-radio 11n APs

Wireless Client Challenges• Legacy 802.11b/g clients are not going to disappear• There will be a variety of 802.11n clients with different capabilities too…

7© 2009

802.11 Speeds and Feeds...

Mode Operating Band

Channel Width

Raw Data Rate

Useful Throughput

Non-Overlapping Channels

802.11b 2.4 GHz 20 MHz 11 Mbps 5.5 Mbps 3

802.11g 2.4 GHz 20 MHz 54 Mbps 20 Mbps 3

802.11a 5 GHz 20 MHz 54 Mbps 24 Mbps 24 with DFS

Mode Operating Band

Channel Width

Raw Data Rate

Useful Throughput

Non-Overlapping Channels

802.11n 2.4 GHz 40 MHz 300 Mbps 160 Mbps 1

802.11n 2.4 GHz 20 MHz 150 Mbps 80 Mbps 3

802.11n 5 GHz 40 MHz 300 Mbps 160 Mbps 3 without DFS11 with DFS

802.11n 5 GHz 20 MHz 150 Mbps 80 Mbps 24 with DFS

8© 2009

The Effect of Channel Bonding and Backward Compatibility

Today’s 3 non-overlapping channels collapse down to 1 x 40 MHz and 1 x 20 MHz channel in the 2.4 GHz band

160 Mbps‘n’ devices only 80 Mbps

n & g devices in mixed mode

38 Mbps? 19 Mbps?

2402 2407 2412 2417 2422 2427 2432 2437 2442 2447 2452 2457 2462 2467 2472 2484

Ch 1 Ch 6 Ch 11

Ch 2 Ch 7 Ch 12

Ch 3 Ch 8 Ch 13

Ch 4 Ch 9 Ch 14

Ch 5 Ch 10

MHz

802.11g (Japan)

Today

802.11g/n40MHz 20MHz

9© 2009

Unpredictable Coverage The techniques that bring greater range and better coverage to

11n also make coverage unpredictable.• Very sensitive to the environment since MIMO leverages RF reflections• Different depending on client capabilities

802.11 b/g/a802.11 n

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Impractical Cell Planning Cell planning attempts to insure continuous coverage but minimize overlap

to reduce co-channel interference between APs.

With each AP having erratic coverage, hard to avoid coverage gaps.

11n has great range, but interference range is greater too!• 11n will generate more co-channel interference that 802.11a/b/g

How to minimize coverage gaps while avoiding co-channel interference?

802.11 b/g/a 802.11 n

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Variety of Clients

Legacy clients• 802.11b/g and 802.11a clients• Mixed mode protocol slows down 802.11n clients dramatically

New 802.11n clients with different capabilities• High power 3 x 3 MIMO supporting 2 streams with beamforming• Low power, 2x2 MIMO, single stream• Very low power, no MIMO, single stream

The AP coverage pattern will be different for each client type• Makes the AP cell planning even more difficult

12© 2009

Wired Network Challenges

The increased bandwidth of 802.11n APs may move the network bottleneck.• In legacy WLAN systems bottleneck was always in the wireless access edge• 802.11n may create enough bandwidth to move the bottleneck back to the wired

network

A dual band 802.11n AP could generate 320 Mbps of traffic load for the wired Ethernet.• 10 indepndent APs is 3.2 Gigabits!• Gigabit Ethernet backbones may no longer be sufficient.• A big centralized WLAN controller may not be the best architecture

• creates too much traffic back to the network core• routing at network edge is more efficient• Upgrade to 10 gigabit Ethernet core?

New wired and wireless architectures may be required to get the most out of 802.11n in the enterprise.

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Enterprise 802.11n Design Considerations

Applications and Devices• What applications will be supported?• What device types will be supported?• What percentage of the users will use legacy devices?

Network Architecture• Does the wired network have enough capacity?• Where shall the wireless network connect?

Wireless Network Architecture• Which bands will be used? Shift 802.11n users to 5 GHz?• Centralized WLAN controller or switch?• Routing at the edge? Fat APs?

Implementation Approach• Gradual introduction or re-start?• Wireless layout redesign needed?• Impact to current cabling?• Required powering scheme?

14© 2009

Summary

802.11n is a significant upgrade to the 802.11 standard.

It is great technology that delivers excellent performance.• higher data rates and better throughout• better range and more robust coverage• perfect backwards compatibility with legacy Wi-Fi equipment

Getting all of the 802.11 benefits in multiple AP enterprise deployments can be challenging.• careful planning is required• new tools and new network upgrades may be needed• new network architectures may be required to deliver the full benefit• wireless and wired

© 2009

Thank You

16© 2009

Extricom 802.11n Channel Blankets

Practical deployment of 802.11n.• Multiple APs on same channel form a contiguous blanket of coverage

• Overlapping AP coverage is good in the Channel Blanket, more diversity

• Simpler planning, Channel Blanket works well with unpredictable coverage of 802.11n APs

Full-performance for both 11n and 11b/g devices in the 2.4 GHz band.• 11n and 11b/g in separate blankets allows 11n clients to run full speed in “greenfield mode”

• 1 40 MHz channel for 11n and 1 20 MHz channel for 802.11g

Channel Blanket provides system level diversity for 802.11n.• Excellent complement to device level diversity of 802.11n MIMO system

Smooth migration from legacy 802.11 a/b/g to full performance 802.11n

2.4 GHz ‘n’

2.4 GHz ‘b/g’

5 GHz ’n’

5 GHz ‘a’

A separate Channel Blanket for each

client type

Optimized for IEEE 802.11n