Siemens IEEE802.11 manual CSMA/CA Explained, Backoff-Window

Page 32

CSMA/CA Explained

IFS: Inter Frame Space

Free access when medium	DIFS
is free longer than DIFS

Contention Window

DIFS	PIFS
DIFS
	SIFS	Next Frame
Busy Medium	Backoff-Window	Next Frame
	Slot time
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νReduce collision probability where mostly needed.

–Stations are waiting for medium to become free.

–Select Random Backoff after a Defer, resolving contention to avoid collisions.

νEfficient Backoff algorithm stable at high loads.

–Exponential Backoff window increases for retransmissions.

–Backoff timer elapses only when medium is idle.

νImplement different fixed priority levels

WLAN-IEEE802.11 Tutorial (Maximilian Riegel), 021018-wlan-tutorial.ppt	Page 32	© Siemens, 2002

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Contents Maximilian Riegel Prolog Ubiquitous Wlan Prolog Wlan has taken off Outline Part Wireless Internet system architecture Generic Internet network architecture PeerLayering means encapsulation HtmlIEEE802.11 seamless integration into the Internet IP based network architecture $*6Wireless LAN IEEE802.11 basic architecture What is unique about wireless? Part 2 IEEE802.11 Overview Wireless IEEE802.11 Standard With 11 Mbps using existing MAC layerInfrastructure IEEE802.11 ConfigurationsIndependent Isochronous Service IEEE802.11 Architecture OverviewIEEE802.11 Protocol Architecture MAC MIBWireless LAN Standardization WIGPart 3 Physical layer IEEE802.11 GHz & 5 GHz Physical Layers GHz Direct Sequence Spread SpectrumFrequency Hopping Spread Spectrum Direct Sequence Spread Spectrum RF Energy is Spread by XOR of Data with PRN SequenceDsss Transmit Spectrum and Channels Key milestones SpecificationsIEEE802.11a 5GHz PHY Layer IEEE802.11g Further Speed Extension for the 2.4GHz Band CCK-OFDM proposal for up to 54 Mbit/s from IntersilSpectrum Designation in the 5 GHz range DFS Dynamic Frequency Selection IEEE802.11h Spectrum Transmit Power ManagementTPC Transmission Power Control … when will 5 GHz WLANs come? Issues of 5 GHz systemsPHY Terminology Physical Layer Convergence Protocol Plcp Part 4 Medium Access Control Basic Access Protocol Features Robust for interferenceCSMA/CA Explained Backoff-WindowDefer access based on Carrier Sense CSMA/CA + ACK protocolDistributed Coordination Function DCF StationHidden Node Provisions STA a STA BIEEE802.11e MAC Enhancements for Quality of Service Edcf Edcf Enhanced Distributed Coordination FunctionPoint Coordination Function PCF IEEE802.11e MAC Enhancements for Quality of Service HCF HCF Hybrid coordination functionFrame Formats MAC Header format differs per TypeAddress Field Description Summary MAC Protocol Features Part 5 MAC layer management Timestamp contains timer value at transmit time Infrastructure Beacon GenerationAll stations maintain a local timer Timing Synchronization Function TSFTiming Synchronization Function TSF Timing conveyed by periodic Beacon transmissionsScanning Scanning required for many functionsMAC uses a common mechanism for all PHY Passive ScanningInitial connection to an Access Point Active Scanning ExamplePower Management Considerations Mobile devices are battery poweredPower Management Approach Broadcast frames are also buffered in AP Power Management ProcedureIf TIM indicates frame buffered MAC Management Frames BeaconPart 6 Wlan Mobility IEEE802.11 Ad Hoc Mode Independent networkingIEEE802.11 Infrastructure Mode Mobility inside a Wlan ‘hotspot’ By link layer functions If AP accepts Reassociation RequestIEEE802.11f Inter-Access Point Protocol Iapp Iapp defines procedures forPart 7 Wlan security Shared key authentication is based on WEP privacy mechanism IEEE802.11 Privacy and Access ControlProvides for an authentication mechanism WEP privacy mechanism WEP bit in Frame Control Field indicates WEP usedShared key authentication StationShortcomings of plain WEP security IEEE802.11i Robust Security Network RSN Security association managementLast word about Wlan security Even IEEE802.11i may not be sufficient for public hot-spotsSummary MAC Functionality Part 8 Public hotspot operation Serving customers in public hot spots 〈 Rqrwwrxfk FxvwrphuhtxlsphqwOne solution for every place hotspot Legal aspects in Germany Cost issuesBecoming a Wlan operator is easy How does your favorite storefront look like? Using a web page for initial user interaction How does it work Web based access control AuthPage Functions of an integrated access gateway User management Real-time accounting based on service, duration and volumeFunctions of an integrated access gateway Network services Dhcp server for assigning IP addresses to Wlan clientsPolicy engine IP router with NAT enginePart 9 Wlan Umts Interworking Umts and Wireless LAN are different GSM/GPRS/UMTS Wlan IEEE802.11Wlan as just another radio access technology of Umts Wlan Umts Interworking Ancient approach ‘tight coupling’Wlan as an extension of a mobile network Tight couplingWlan is much cheaper than 2G/3G OrjdulwkplfvfdohConclusions for Mobile Network Operators Wlan Umts Interworking Now widely accepted ‘loose coupling’ Revenues without competing against aggressive Wlan operatorsWlan loosely coupled to a Mobile Network Each hotspot is SS7 endpointWeb based authentication and mobile network security Standards for WLAN/UMTS interworking Literature EndThank you for your attention Questions and comments?