CSMA/CA + ACK protocol | Siemens IEEE802.11 specs

CSMA/CA + ACK protocol

DIFS

Src

Dest

Other

Data

SIFS

Ack	Contention Window
Ack

DIFS

Next MPDU

Defer Access

Backoff after Defer

νDefer access based on Carrier Sense.

–CCA from PHY and Virtual Carrier Sense state.

νDirect access when medium is sensed free longer then DIFS, otherwise defer and backoff.

νReceiver of directed frames to return an ACK immediately when CRC correct.

–When no ACK received then retransmit frame after a random backoff (up to maximum limit).

WLAN-IEEE802.11 Tutorial (Maximilian Riegel), 021018-wlan-tutorial.ppt	Page 33	© Siemens, 2002

Image 33

Siemens IEEE802.11 manual CSMA/CA + ACK protocol, Defer access based on Carrier Sense

Contents

Maximilian Riegel Prolog Ubiquitous Wlan Prolog Wlan has taken off Outline Part Wireless Internet system architecture Peer Generic Internet network architecture Html Layering means encapsulation IEEE802.11 seamless integration into the Internet $*6 IP based network architecture Wireless LAN IEEE802.11 basic architecture What is unique about wireless? Part 2 IEEE802.11 Overview With 11 Mbps using existing MAC layer Wireless IEEE802.11 Standard IEEE802.11 Configurations IndependentInfrastructure IEEE802.11 Architecture Overview Isochronous Service MAC MIB IEEE802.11 Protocol Architecture WIG Wireless LAN Standardization Part 3 Physical layer GHz Direct Sequence Spread Spectrum IEEE802.11 GHz & 5 GHz Physical Layers Frequency Hopping Spread Spectrum RF Energy is Spread by XOR of Data with PRN Sequence Direct Sequence Spread Spectrum Dsss Transmit Spectrum and Channels Specifications IEEE802.11a 5GHz PHY LayerKey milestones CCK-OFDM proposal for up to 54 Mbit/s from Intersil IEEE802.11g Further Speed Extension for the 2.4GHz Band Spectrum Designation in the 5 GHz range IEEE802.11h Spectrum Transmit Power Management TPC Transmission Power ControlDFS Dynamic Frequency Selection Issues of 5 GHz systems … when will 5 GHz WLANs come? PHY Terminology Physical Layer Convergence Protocol Plcp Part 4 Medium Access Control Robust for interference Basic Access Protocol Features Backoff-Window CSMA/CA Explained CSMA/CA + ACK protocol Defer access based on Carrier Sense Station Distributed Coordination Function DCF STA a STA B Hidden Node Provisions Edcf Enhanced Distributed Coordination Function IEEE802.11e MAC Enhancements for Quality of Service Edcf Point Coordination Function PCF HCF Hybrid coordination function IEEE802.11e MAC Enhancements for Quality of Service HCF MAC Header format differs per Type Frame Formats Address Field Description Summary MAC Protocol Features Part 5 MAC layer management Infrastructure Beacon Generation Timestamp contains timer value at transmit time Timing Synchronization Function TSF All stations maintain a local timerTiming Synchronization Function TSF Timing conveyed by periodic Beacon transmissions Scanning required for many functions ScanningMAC uses a common mechanism for all PHY Passive Scanning Active Scanning Example Initial connection to an Access Point Mobile devices are battery powered Power Management Considerations Power Management Approach Power Management Procedure If TIM indicates frame bufferedBroadcast frames are also buffered in AP Beacon MAC Management Frames Part 6 Wlan Mobility Independent networking IEEE802.11 Ad Hoc Mode IEEE802.11 Infrastructure Mode If AP accepts Reassociation Request Mobility inside a Wlan ‘hotspot’ By link layer functions Iapp defines procedures for IEEE802.11f Inter-Access Point Protocol Iapp Part 7 Wlan security IEEE802.11 Privacy and Access Control Provides for an authentication mechanismShared key authentication is based on WEP privacy mechanism WEP bit in Frame Control Field indicates WEP used WEP privacy mechanism Station Shared key authentication Shortcomings of plain WEP security Security association management IEEE802.11i Robust Security Network RSN Even IEEE802.11i may not be sufficient for public hot-spots Last word about Wlan security Summary MAC Functionality Part 8 Public hotspot operation 〈 Rqrwwrxfk Fxvwrphuhtxlsphqw Serving customers in public hot spots One solution for every place hotspot Cost issues Becoming a Wlan operator is easyLegal aspects in Germany How does your favorite storefront look like? Using a web page for initial user interaction Auth How does it work Web based access controlPage Real-time accounting based on service, duration and volume Functions of an integrated access gateway User management Dhcp server for assigning IP addresses to Wlan clients Functions of an integrated access gateway Network servicesPolicy engine IP router with NAT engine Part 9 Wlan Umts Interworking GSM/GPRS/UMTS Wlan IEEE802.11 Umts and Wireless LAN are different Wlan Umts Interworking Ancient approach ‘tight coupling’ Wlan as just another radio access technology of Umts Tight coupling Wlan as an extension of a mobile network Orjdulwkplfvfdoh Wlan is much cheaper than 2G/3G Conclusions for Mobile Network Operators Revenues without competing against aggressive Wlan operators Wlan Umts Interworking Now widely accepted ‘loose coupling’ Each hotspot is SS7 endpoint Wlan loosely coupled to a Mobile Network Web based authentication and mobile network security Standards for WLAN/UMTS interworking End Thank you for your attention Questions and comments?Literature