The Problems With Microcell (1)The Problems With Microcell (1)How cochannel interference destroys microcell throughputHow cochannel interference destroys microcell throughput

What is it about microcell WLAN’s that have made so many WLAN administrators and end-

users unhappy and frustrated?

The Question

What happens:

WLAN Operation

AP

XX

Classic Cellular Operation

BTS

WLAN Operation

AP

XX

Microcell Architecture – 3 channels

AP AP

AP

2.4 GHz Band

Covering Floor 1 With WLAN

1

4

2 3

5 6

7 8 9

But We Have Only 3 Frequencies

1

6

6 11

11 1

11 1 6

1

6

6 11

11 1

1 611

Microcell Architecture: the ugly realityCells are actually a lot larger !!

Lower rate transmissions travel far from the AP.

Cochannel interference zones

A client transmitting in this zone quiets not only his AP, but also the neighboring AP.

System throughput is lowered drastically.

The Reason: RF Energy propagation

Distance

Client C

onnects @

54 Mbps

Client C

onnects @

6 Mbps

Client C

onnects @

1 Mbps

Radio Transmission Still Continues

Radio Coverage:This is the area a client can hear an Access Point and reply successfully – Typically 10 Metres radius from the AP at 54 Mbps

Range:The RF energy does not stop simply because the client and AP can no longer interpret the data, typical Range may be 2,000 meters

Client C

onnects @

300 M

bps

1

6

6 11

11 1

1 611

Microcell Architecture: the ugly realityCells are actually a lot larger !!

Lower rate transmissions travel far from the AP.

Cochannel interference zones

A client transmitting in this zone quiets not only his AP, but also the neighboring AP.

System throughput is lowered drastically.

So Instead of This….

1

4

2 3

5 6

7 8 9

We’re Back To This…

AP AP

AP

Or worse…

Conclusion

Actual microcell throughput is up to 70% lower than expected due to cochannel interference

And If You Try To Spread Out the Cells To Lower The Interference…

1

6

6 11

11 1

11 1 6

You get coverage holes

And lower air rates

1

6

6 11

11 1

11 1 6

Most of the coverage area now has the lower air rates

So most users get the lower rates, and lower throughput.

Making the situation even worse, the users inside the high rate areas need to wait for those outside to finish transmitting.Throughput is reduced even further.

And If You Try To Spread Out the Cells There Is A 2nd Impact:

1

6

6 11

11 1

11 1 6

Most of the coverage area now has the lower air rates

So most users get the lower rates, and lower throughput.

Making the situation even worse, the users inside the high rate areas need to wait for those outside to finish transmitting.Throughput is reduced even further.

1

6

6 11

11 1

1 611

Microcell Architecture: roaming hell

Mobile device must cross several cells as it moves across the floor.

Every time the unit changes cells, the call drops!

Disconnect From AP on Channel 6Request to join AP on Channel 1Authenticate with central RadiusConnect and start recovering data

Cochannel Interference and Cell Planning: Even Worse With 802.11n

802.11n RF patterns are spikey, less predictable

How the competition Tries To Fix The Inherent Problems of Microcell Architecture Its Band aid Time….

Bandaid #1 TPC Transmission power control: does not work so well, There is a fundamental hole in the solution: clients do not alter their

power!!

Bandaid #2: Dynamic Channel Assignment Disconnects any VoIP calls in progress Sometimes chooses wrong channel, increasing cochannel interference

Cisco RRM: 2.4 GHz case study

RF Experts went into an office and tested RRM. For some reason, instead of choosing channels 1,6,11, RRM chose channels 1,7,11 and also put two channel 7 cells next to each other. End result might have looked something like this:

1 7 7

11111

11

7 1

Some interference between lobes of 7 and 11

Cochannel interference between adjacent cells on same channel

Classic cochannel interference between nearby cells on same channel (unavoidable in microcell architecture)

Other Attempts To Fix The Inherent Problems of Microcell Architecture

Bandaid #3: Beamforming a/b/g only Independent tests showed no significant impact to throughput Clients can’t beamform, so when they transmit it’s omnidirectional

Bandaid #4: 802.11k Attempts to enhance ability of AP’s to hear each other Not very effective as number of AP’s and AP density increases:

algorithm does not scale well.

Bandaid #5: 802.11r Attempts to fix the inherent roaming problem of microcell architecture Not a very big success.

Bandaid #6 802.11e (WMM) Can cause dropped VoIP calls

Netronics: we don’t like band aid solutionsSo we changed the architecture to this:

11

11

11 11

11 11

11 11 11

Channel BlanketArchitecture

Can Group Cells As Close As Needed

11

11

11 11

11 11

11 11 11

1. Gapless Coverage

2. Higher throughput, since more users are in higher air rate areas (closer to AP’s)

3. Seamless roaming: no more handoffs!

Benefits:

Avoiding A Single Collision DomainStack the Channel BlanketsFor Bandwidth Multiplication

Biproduct: built-in quality of service (segregate traffic type per blanket)

Dividing A Single Collision DomainTrue reuse, for even more bandwidth

11

11

11 11

11 11

11 11 11

NetGlide switch can transmit to 3 clients on same channel simultaneously when those clients are out of range of each other

Bandwidth is multiplied even further

Built-in Uplink Diversity

11

11

11 11

11 11

11 11 11

Client signal is transmitted to switch by the AP’s that hear it.

Switch takes care of redundant packets

Uplink redundancy ideal for highly mobile, mission critical environments like logistics and healthcare

Uplink redundancy does not exist in microcell architectures. In microcell, only one AP can receive client’s transmissions.

Cisco AP’s: Need A Controller

Cisco AP’s (even though they are layer 3 devices) cannot function independently in an enterprise setting.

The Cisco AP’s do not have computing resources for filtering, policy enforcement, authentication, encryption, that enterprises must activate to be secure (ie. WPA2)

Inherent RF problems of microcell architecture require controller-based monitoring and control of RF environment

Requires communication between access points and Cisco wireless controller(s) + Cisco WCS (Wireless Control Management System) Provides access point device discovery, information exchange, and configuration Provides access point certification and software control Packet encapsulation (L2 mode) and tunneling (L3 mode)

Cisco Lightweight Access Point Protocol (LWAPP)

What it does? Reduces amount of processing within access points, freeing up their computing

resources to focus exclusively on wireless instead of filtering and policy enforcement

Enable centralized traffic handling, authentication, encryption, and policy enforcement for an entire WLAN system

Provide a generic encapsulation and transport mechanism for multivendor access point interoperability, using either a Layer 2 infrastructure or an IP-routed network

How? Requires communication between access points and Cisco wireless controller(s) +

Cisco WCS (Wireless Control Management System) Provides access point device discovery, information exchange, and configuration Provides access point certification and software control Packet encapsulation (L2 mode) and tunneling (L3 mode) Aironet 1250 can automatically detect best available controller

The LWAPP Problem: Heavy traffic between AP’s and controller is driven into the layer 3 cloud

Thank youThank you

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