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Roaming Between AccessPoints

Roaming Between AccessPoints. Multi-channel roaming Associating multiple AccessPoints within the same wireless network. Multi-channel Roaming. Multiple AccessPoints can be in the same wireless network as long as they do not overlap frequencies (Channels)

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Roaming Between AccessPoints

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  1. Roaming Between AccessPoints • Multi-channel roaming • Associating multiple AccessPoints within the same wireless network

  2. 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

  3. Multiple Cells on One Wireless Network Wired LAN (Ethernet) Channel 11 Channel 6 Channel 1

  4. 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!!

  5. 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

  6. 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 Out Of Range Threshold Cell Search Threshold Cell Search (CS) zone Out of Range Not Searching (NS) zone CQ Lowest Quality Highest Quality

  7. 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 Out Of Range Threshold Cell Search Threshold Cell Search (CS) zone Out of Range Not Searching (NS) zone CQ Lowest Quality Highest Quality

  8. 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!! Out Of Range Threshold Cell Search Threshold Cell Search (CS) zone Out of Range Not Searching (NS) zone CQ Lowest Quality Highest Quality

  9. 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

  10. 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

  11. 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

  12. Station A Roaming • As Station A is associated with AccessPoint 1 and decides to move away from away from AccessPoint 1. Wired LAN (Ethernet) AccessPoint 2 AccessPoint 1 Channel 1 Channel 5

  13. Station A 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) Wired LAN (Ethernet) AccessPoint 2 AccessPoint 1 Channel 1 Channel 5

  14. Station A Roaming • Station A now sees AccessPoint 2 offers the best CQ • Station A now creates an association withAccessPoint 2 Wired LAN (Ethernet) AccessPoint 2 AccessPoint 1 Channel 1 Channel 5

  15. Cell Search Threshold exceeded Sweep for new AccessPoint New Access Point found NO denied NO Security check (if required) Same network name (SSID)? YES Associated with new AccessPoint granted YES Determining AccessPoint to Associate with

  16. 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

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