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1999 Cabletron Systems
1999 Cabletron Systems
1999 Cabletron Systems
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Roaming Between AccessPoints
Multi-channel roamingAssociating multiple AccessPoints within the same wireless network
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Multi-channel Roaming
• Multiple AccessPoints can be in the same wireless network as long as they do not overlap frequencies (Channels)
– Overlapping cells should be at least 5 channels apart to provide protection from interference.
• Advantage here is the ability to have a wireless network larger than a single cell
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Multiple Cells on One Wireless Network
Wired LAN (Ethernet)
Channel 1
Channel 11Channel 6
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Multi-channel roaming
• AccessPoints are set to a fixed frequency
• Stations switch frequency when roaming between AccessPoints to associate with a particular AccessPoint
• Stations “associate” dynamically to the AccessPoint which offers the best service
• This implies
– Easier configuration!!
– Faster installation!!
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Communications Quality (CQ)
• Parameter used assess link between Station and AccessPoint
• Determined by Signal to Noise Ratio (SNR) on path between station and current AccessPoint
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Using Communications Quality
• Station monitors the CQ to the AccessPoint
• When station moves away from AccessPoint the CQ decreases
– In the example below the station has a good strong CQ to the AccessPoint
Highest QualityLowest Quality
Cell Search Threshold Out Of Range Threshold
Not Searching (NS) zoneCell Search (CS) zoneOut of Range
CQ
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Using Communications Quality
• As the station moves away from the AccessPoint the CQ will decrease
• This is normal and there is nothing to be alarmed about
Highest QualityLowest Quality
Cell Search Threshold Out Of Range Threshold
Not Searching (NS) zoneCell Search (CS) zoneOut of Range
CQ
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Highest QualityLowest Quality
Cell Search Threshold Out Of Range Threshold
Not Searching (NS) zoneCell Search (CS) zoneOut of Range
CQ
Using Communications Quality
• If the CQ falls below a set threshold, the station will enter the Cell Search (CS) zone
• In the Cell Search Zone the station is actively looking for another AccessPoint that is better able to service that station
• At this point the station is said to be roaming!!
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In the Cell Search State
• In the cell search state, the station will enter a sweep mode
– Sweep mode is when a station will actively scan all channels to determine if there is an AccessPoint on the network that is better able to service the Station
– The station also blocks its own transmissions during sweep
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Determining Which AccessPoint to Associate With
• Once a station discovers an AccessPoint (on the network) that can better service it, it will make an association to that AccessPoint.
• To be considered for association, an AccessPoint must:
– Have a CQ that is better than any other applicable AccessPoint and
• Have a matching Network Name of that station or
• Have the Network Name set to ANY and not be operating in the “closed” mode
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Creating an Association
• Station sends an “Association Request”
• AccessPoint enters the station in its Association Table (and assigns an “Association Code” to it)
• AccessPoint then sends an “Association Response” to the station
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Roaming • As Station A is associated with AccessPoint 1 and decides to move
away from away from AccessPoint 1.
AccessPoint 1AccessPoint 2
Wired LAN (Ethernet)
Channel 5 Channel 1
Station A
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Roaming • At some point the Cell search Threshold will be passed.
• Station A maintains its association to AccessPoint 1 since no other AccessPoint offers a better CQ (following a sweep)
AccessPoint 1AccessPoint 2
Wired LAN (Ethernet)
Channel 5 Channel 1
Station A
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Roaming • Station A now sees AccessPoint 2 offers the best CQ
• Station A now creates an association with AccessPoint 2
AccessPoint 1AccessPoint 2
Wired LAN (Ethernet)
Channel 5 Channel 1
Station A
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Determining AccessPoint to Associate with
Cell Search Threshold exceeded
Sweep for new
AccessPoint
NewAcces
sPoint found
Same network
name (SSID)?
Security check
(if required)
Associated with new AccessPoint
YES
YES
NO NO
granted
denied
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Roaming Summary
• For a station to begin to seek out another AccessPoint, the CQ must fall below the set threshold
• Once in the sweep mode, only other AccessPoints with the same Network Name will be considered
• Once a better AccessPoint is found then an association will be made
