Migrating to 802.11n While Supporting Legacy 802.11b,g Applications (2006)

8/12/2019 Migrating to 802.11n While Supporting Legacy 802.11b,g Applications (2006)

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Wi-Fi Technologyote WF1047 October 2006

Migrating to 802.11n While Supporting Legacy802.11b/g Applications

Overview

The IEEE 802.11n standard was conceived with the goal of increasing wireless local areanetwork (WLAN) data throughput to a theoretical level of 540Mbps, or 10 times the speedof 802.11g. Built on OFDM technology used in the 802.11g standard, 802.11n achieves itshigher throughput, in part, by increasing channel spectrum from 20MHz to 40MHz.

In addition to increasing channel utilization through MAC aggregation techniques, 802.11nalso incorporates multiple antennas using a scheme called multiple-input multiple-outputantenna technology, or MIMO. MIMO provides spatial diversity and spatial multiplexing forincreased range and throughput, respectively. A summary of the IEEE 802.11 standardfeatures is shown in the table below.

IEEE 802.11 Channels and Features

Description 802.11a 802.11b 802.11g 802.11nData rate maximum (Mbps) 54 11 54 600

Modulation technique OFDM CCK or DSSS CCK or DSSSor OFDM

CCK or DSSSor OFDM

Frequency band 5GHz 2.4GHz 2.4GHz 2.4 or 5GHz

Spatial streams 1 1 1 1 - 4

Channel width 20MHz 20MHz 20MHz 20 or 40MHz

The 802.11n standardization process has not yet been completed, and 802.11n remains apre-standard as debate continues about final details of the standard. 802.11n consumestwo of the three non-overlapping 2.4GHz channels. In a pure 802.11n network this is not an

issue, however, it is a problem in a hybrid network. Chipset manufacturer Airgo Networks1has publicly stated that pre-standard 802.11n products would degrade or even disableexisting 802.11b and 802.11g networks, and the following graphic depicts the conflict.

1http://www.tgdaily.com/2006/03/13/802_11n_backwards_compatibility_issues


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Note WF10417 October 2006

802.11n Overlaps 802.11b Channels

Source: Hyperlink Technologies and Meru Networks

Additionally, 802.11n uses OFDM modulation, while 802.11b does not. This means that ifOFDM clients want to communicate in the presence of 802.11b clients, they must use theslower and less-efficient 802.11b protocol to protect any higher-rate OFDM transmissions.

Failure to do so will affect both 802.11b signaling and 802.11n throughput. Pre-standard802.11n-based devices now on the market, most of which are consumer products, havebeen confirmed to cause interference with 802.11b/g devices.

The 802.11n standard is expected to be ratified in 2007, and wide-scale enterpriseadoption of 802.11n should commence in 2008. Concerned about interference withexisting WLAN infrastructure and clients, IT managers are seeking assurances today thatWLANs they purchase now can coexist with 802.11n networks that are deployed in thefuture.

Micro Cell (Traditional Architecture) vs. Virtual Cell Architecture

Micro cell WLAN architecture, as used by virtually all WLAN suppliers, requires that radiosbe deployed in a checkerboard pattern of radio coverage. Adjacent radios operate ondifferent (though somewhat overlapping) channels, and at lower than maximum power, tominimize mutual interference and thereby enhance performance.


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Clients decide when to switch between channels based on changes in signal strength, i.e.,moving away from channel 1 and towards channel 6 causes the former to drop in strength

and the latter to rise. It is therefore important that adjacent channels overlap as little aspossible to allow the clients to switch efficiently. Additionally, since adjacent channelshave overlapping radio energy it is important to isolate adjacent channels as much aspossible to prevent signal contention. Lowering output power increases the isolation ofadjacent channels but lowers both signal-to-noise ratio and the effective operating range.

Micro cell architecture is very inefficient with respect to the consumption of wirelessbandwidth, since three or four radio channels are required to avoid the overlap of adjacentradios. In a 2.4GHz 802.11b/g deployment, key channels 1, 6, and 11 must all be dedicatedto the micro cell deployment, leaving no free channels available for other uses.Additionally, channel planning and maintenance programs must be used to properly locateradios to minimize co-channel interference while optimizing radio coverage. Channel

planning must be conducted at both the initial deployment and with every addition, move,or change to the system.

By contrast, Merus Virtual Cell architecture aligns clients and Access Points on a singleradio channel. All of the clients and Access Points can be considered, metaphorically, toreside on a single plane which is defined by the radio channel on which they reside. AccessPoints are coordinated by a Meru Controller, which determines when clients are handed-offby analyzing signal quality - in this architecture clients do not themselves decide when toswitch between Access Points.

Adjacent channel interference is not an issue with Virtual Cell architecture; Access Pointscan be operated at full output power and situated where needed for optimum, overlapping

coverage, including being located in close proximity to one another. High output powerand overlapping coverage together minimize the number of Access Points and ensure thatclients remain networked in the event of a failure of one radio. In the example below theVirtual Cell is operating on channel 1.


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By using only channel 1, the example above preserves channels 6 and 11 for future capacity

or application requirements, i.e., additional single channel Virtual Cells or an 802.11ndeployment. The example below depicts how additional channels can be layered to addcapacity. The three-layer network shown has 3x the bandwidthof a single layer network.

Advantage Meru

Since micro cell networks require three 2.4GHz channels, they have a critical drawbackwith respect to 802.11n coexistence. By increasing channel spectrum from 20MHz to40MHz, 802.11n consumes two of the three 2.4GHz channels that are required for a microcell deployment. The enhanced capability of an 802.11n network is therefore incompatible


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with a legacy 2.4GHz 802.11b/g network. Instead, all of the legacy clients must either bemoved to 802.11a (5GHz) or abandoned, since this is the only range within which a three-channel deployment can be implemented.

Since most existing clients operate on 802.11b/g, overlaying 802.11n on a micro cellnetwork will require a complete overhaul of the radios, channel planning, radio placement,wiring plant, and clients to support 802.11a. In some cases, such as telephone handsets,clients will have to be replaced altogether because 802.11a versions are simply notavailable. Furthermore, 802.11a has shorter range than 802.11b/g, so additional radios willbe required, exacerbating co-channel interference by increasing radio density. Suchwholesale changes will displace, rather than leverage, existing capital assets, andrepresent a disruptive, costly, and time-consuming forklift upgrade.

When 802.11n 5GHz networks become available, co-existence will again arise as an issue

and additional network changes may be required.

In contrast, Merus Virtual Cell architecture places all of the 802.11b/g clients on onechannel, leaving available extra 2.4GHz or 5GHz channels for an 802.11n overlay. Existingradios, clients, wiring plant, and capital assets can all be retained, and the overlay can beaccomplished with minimal disruption.


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For applications that need multiple 802.11 channels for different services and/or higherbandwidth, Merus multi-radio RS 4000 Radio Switch can significantly expand capacity

while supporting an 802.11n overlay. The RS 4000 include two 802.11a channels and two802.11b/g channels. In the example below two 802.11a channels can be used for existingdata and video clients, one 802.11b channel used for existing voice clients, a second802.11b channel used for Merus AirFirewall over-the-air security, and the 802.11nchannels used for new clients. The total capacity provided by this innovative solution is farbeyond the capabilities of any micro cell-based alternatives.

Summary

Depending on the application and available budget, IT managers may consider manydifferent 802.11n deployment options: use 802.11n only in high capacity areas such aslecture halls; deploy only in new network additions or structures; phased deployment, one

building at a time; deployment only in green field application; or some other alternative.Regardless of the preferred deployment option, Meru offers the only WLAN solution thatco-exists with 802.11n, and IT managers can field an enterprise-grade Meru network todaywith the assurance that it will support a wide variety of 802.11n overlays in the future.

Copyright 2006 Meru Networks, Inc. All rights reserved worldwide. Meru Networks is a registered trademark of MeruNetworks, Inc. in the U.S. and worldwide. All other trademarks mentioned in this document are the property of theirrespective owners. MeruWF104_06-10-17

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Migrating to 802.11n While Supporting Legacy 802.11b,g Applications (2006)