the problems with microcell (1) how cochannel interference destroys microcell throughput
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The Problems With Microcell (1) How cochannel interference destroys microcell throughput. The Question. What is it about microcell WLAN’s that have made so many WLAN administrators and end-users unhappy and frustrated ?. What happens:. WLAN Operation. X. X. AP. - PowerPoint PPT PresentationTRANSCRIPT
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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