Mobile Communications I Prof. Dr.-Ing. Rolf Kraemer chair owner telefon: 0335 5625 342 fax: 0335 5625 671 e-mail: kraemer [ at ] ihp-microelectronics.com web:
Mobile Communications Chapter 4: Wireless LANs Characteristics IEEE 802.11 PHY MAC Roaming.11a, b, g, n, e, i, k, s Bluetooth
Characteristics of Wireless LANs Advantages Very flexible within the reception area Ad-hoc networks without previous planning possible (almost) no wiring difficulties (e.g. historic buildings, firewalls) More robust against disasters like, e.g., earthquakes, fire - or users pulling a plug... WiFi alliance helped the acceptance to take place (profiling the standard) 802.11 a/b/g/n is now worldwide accepted Today a very active community extends the 802.11 standards into several further direction (e.g. 802.11ad, 802.11ac Gigabit WLAN) Disadvantages Many proprietary solutions, especially for higher bit-rates, standards take their time; its not only the standard but both the standard and the profile that is necessary for interoperability Products have to follow many national restrictions if working wireless, it takes a very long time to establish global solutions like, e.g., IMT-2000 From the early stage of idea to the introduction typically 10-15 years are needed Chapter 4 Page 179
Design Goals for Wireless LANs Global, seamless operation Frequency regulation Low power for battery use Design of HW/SW, special protocol options Sleep Modes; Dynamic Power Adaptation No special permissions or licenses needed to use the LAN ISM bands (industry, scientific and medical) Robust transmission technology Signal processing techniques; access mechanisms; e.g OFDM and CSMA/CA Simplified spontaneous cooperation at meetings Ad-Hoc Mode ; Meshed network Easy to use for everyone, simple management Protection of investment in wired networks Security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation), reliability, QoS Additional standards and regulations (e.g. 802.11i; 802.e, 802.h) Transparency concerning applications and higher layer protocols, but also location awareness if necessary IEEE802.2; IP protocols Triangulation, Fingerprints etc. Chapter 4 Page 180
Early system considerations: Infrared vs. Radio Transmission Infrared Uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) Advantages Simple, cheap, available in many mobile devices No licenses needed Simple shielding possible Very fast (up to 1Gb/s) Disadvantages Interference by sunlight, heat sources etc. Many things shield or absorb IR light Example IrDA (Infrared Data Association) interface available everywhere Radio Typically using the license free ISM band at 2.4 GHz (5.2 GHz, 17 GHz, 60 GHz) Advantages Experience from wireless WAN and mobile phones can be used Coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages Very limited license free frequency bands Shielding more difficult, interference with other electrical devices Example WiFi-WLAN, WaveLAN, HIPERLAN, Bluetooth Chapter 4 Page 181
Comparison: Infrastructure vs. Ad-hoc Networks infrastructure network AP AP wired network AP: Access Point AP ad-hoc network Chapter 4 Page 182
IEEE 802.11 - Architecture of an Infrastructure Network STA 1 ESS 802.11 LAN BSS 1 Access Point BSS 2 Portal Distribution System Access Point 802.x LAN STA 2 802.11 LAN STA 3 Station (STA) Terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) Group of stations using the same radio frequency to communicate with each other or the access point Access Point Station integrated into the wireless LAN and the distribution system Portal Bridge to other (wired) networks Distribution System Interconnection network to form one logical network Extended Service Set (ESS) Based on several BSS Chapter 4 Page 183
802.11 - Architecture of an Ad-hoc Network 802.11 LAN STA 1 IBSS 1 STA 2 STA 3 Direct communication within a limited range Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS): group of stations using the same radio frequency IBSS 2 STA 5 STA 4 802.11 LAN Chapter 4 Page 184
Cube Model of Layers and Planes Example: B-ISDN 3 dimensional reference model three vertical planes (columns) user plane control plane management plane three hierarchical layers physical layer ATM layer ATM adaptation layer Out-of-Band-Signaling: user data is transmitted separately from control information layers control plane higher layers ATM adaptation layer ATM layer user plane higher layers physical layer management plane N-layer management system management planes Data plane Chapter 4 Page 185
IEEE Standard 802.11 mobile terminal fixed terminal application TCP IP LLC access point LLC infrastructure network application TCP IP LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY Chapter 4 Page 186
Using the layers for functional extension: OMEGA OMEGA was an EU-project within the 7th framework program The consortium was led by France Telecom In total the project had 18 partners The project budget was 20 Mio (total funding was 12 Mio ) Chapter 4 Page 187
User paradigms Devices Services and Applications Today s situation Broadband access Chapter 4 Page 188 3rd OMEGA Open Event 2011, 24 February 2011, Rennes, France ICT-213311 OMEGA
Network topology Technology-dependent view LD LD LD OD WIFI OD UWB OG OD OD LEGEND UWB link PLC link WIFI link PLC OD Heterogeneous meshed network topology OD: OMEDA device OG: OMEGA Gateway LD: Legacy device Chapter 4 Page 189
Network topology Technology-independent view OD OD OG OD OD Omega network LEGEND Omega link A Omega link B OD OD: OMEDA device OG: OMEGA Gateway LD: Legacy device Chapter 4 Page 190
Possible options for convergence ISO OSI reference model Option A: Convergence mechanisms at the network layer (or above) Option B: Convergence below the network layer and above the technologydependent MAC layers Option C: Common and generic MAC layer for all technologies J.-P. Javaudin, M. Bellec, P. Jaffré, A. Foglar, O. Hoffmann, O. Isson, Inter-MAC concept for Gigabit Home Networks, 20th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC'09), Tokyo, 13. Sept. 2009 Chapter 4 Page 191
Invention of a new protocol layer: OSI 2.5 In Option C we can hide all technology and low layer protocol specifics within a new domain: the OMEGA network Routing, Error handling, QoS-issues, Handover (vertical and horizontal) are performed within the OMEGA network. A client has only one IP-Address and one logical service delivery point All physical details are hidden and mostly automatically addressed by the OMEGA engines The interface to the higher layer is identical to IEEE802.2 such that no changes to current network services is needed Legacy devices can be connected and legacy packet are encapsulated into OMEGA frames For a legacy device the OMEGA network looks like a distributed switch Chapter 4 Page 192
Inter-MAC reference implementation Middleware Shared library UPnP QoS applications Linux-based reference implementation Based on concept, architecture and algorithms developed in Omega WP5 Proof-of-concept Portable to different platforms due to hardware abstraction layers Generic IPC QoS Engine Control Plane Linux User Space Linux Kernel Space Data Plane Path Selection Engine Linux Network Interface Local Link Table Management Information Base Local Network Traffic (Layer 3) Monitoring Engine Date Plane Adaptor Generic IPC Control Plane Interface Inter-MAC Adaptor... Generic Netlink Messages LinkDown M. Brzozowski, S. Nowak, F.-M. Schaefer, R. Jennen, Andi Palo Inter-MAC - From Vision To Demonstration, Enabling Heterogeneous Meshed Home Area Networks, accepted for publication at 14th ITG Conference on Electronic Media Technology, Dortmund (Germany), March 2011 Chapter 4 Page 193 Linux Network Stack Hardware Driver 1 Date Plane Core Linux Network Stack Hardware Driver 2...... MAC + PHY 1 MAC + PHY 2 MAC + PHY n Forwarding Table Linux Network Stack Hardware Driver n Interface Table... Probe Frame Generation...
InterMAC Demonstrator (Rennes February 2011) Chapter 4 Page 194
PHY DLC Station Management 802.11 - Layers and Functions MAC Access mechanisms, fragmentation, encryption MAC Management Synchronization, roaming, MIB, power management LLC PLCP Physical Layer Convergence Protocol Clear channel assessment signal (carrier sense) PMD Physical Medium Dependent Modulation, coding PHY Management Channel selection, MIB Station Management Coordination of all management functions MAC PLCP PMD MAC Management PHY Management Data n-layer MM Sys-MM Chapter 4 Page 195
WLAN: IEEE 802.11 (legacy) Data rate 1, 2 Mbit/s, depending on SNR 20 MHz bandwidth Transmission range 20 m Frequency Free 2.4 GHz ISM-band Security Limited, WEP insecure, SSID Availability First WLANs in 1997 Last version of 802.11 (legacy) in 1999 Obsolete today Special Three physical layer technologies Diffuse infrared at 1 Mbit/s FHSS at 1 or 2 Mbit/s DSSS at 1 or 2 Mbit/s (rapidly replaced by 802.11b) Chapter 4 Page 196
802.11 - Physical Layer 3 versions: 2 radio (typ. 2.4 GHz), 1 IR Very first Version in 1997 Data rates 1 or 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) Spreading, de-spreading, signal strength, typ. 1 Mbit/s Min. 2.5 frequency hops/s (USA), two-level GFSK modulation DSSS (Direct Sequence Spread Spectrum) DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK) Preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s Chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (11 bit Barker code) Max. radiated power 1 W = 30 dbm (USA), 100 mw = 20 dbm (EU), min. 1 mw=0 dbm Infrared 850-950 nm, diffuse light, typ. 10 m range Carrier detection, energy detection, synchronization Chapter 4 Page 197
FHSS PHY Packet Format Synchronization Synch with 010101... pattern SFD (Start Frame Delimiter) 0000110010111101 start pattern PLW (PLCP_PDU Length Word) Length of payload incl. 32 bit CRC of payload, PLW < 4096 PSF (PLCP Signaling Field) Data of payload (1 or 2 Mbit/s) HEC (Header Error Check) CRC with x 16 + x 12 + x 5 + 1 80 16 12 4 16 variable bits synchronization SFD PLW PSF HEC payload PLCP preamble PLCP header Chapter 4 Page 198
DSSS PHY Packet Format Synchronization Bit synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) 1111001110100000 Signal Data rate of the payload (e.g. 0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service Future use, 00: 802.11 compliant Length length of the payload HEC (Header Error Check; CRC) Protection of signal, service and length, x 16 + x 12 + x 5 + 1 128 16 8 8 16 16 variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header Chapter 4 Page 199
802.11 - MAC Layer I Distributed Foundation Wireless MAC (DFWMAC) Traffic services Asynchronous Data Service (mandatory) Implemented using the DCF (Distributed Coordination Function) Exchange of data packets based on best-effort Support of broadcast and multicast Time-Bounded Service (optional) Implemented using PCF (Point Coordination Function) Access methods DFWMAC DCF CSMA/CA (mandatory) Collision avoidance via randomized back-off mechanism Minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts, multicasts) DFWMAC-DCF w/ RTS/CTS (optional) Avoids (limits) hidden terminal problem DFWMAC- PCF (optional) Access point polls terminals according to a list Chapter 4 Page 200
802.11 - MAC Layer II Priorities Defined through different inter frame spaces No guaranteed, hard priorities The shorter the minimal wait-time the higher the priority SIFS (Short Inter Frame Spacing) Highest priority, for ACK, CTS, polling response PIFS (PCF IFS) Medium priority, for time-bounded service using PCF DIFS (DCF IFS) Lowest priority, for asynchronous data service SIFS, PIFS and DIFS depend on the detailed physical layer used DIFS DIFS medium busy PIFS SIFS contention next frame direct access if medium is free DIFS Chapter 4 Page 201 t
802.11 - CSMA/CA Access Method I DIFS DIFS contention window (randomized back-off mechanism) medium busy direct access if medium is free DIFS slot time next frame t Station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) If the medium is free for the duration of (at least) one Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) If the medium is busy, the station has to wait for at least one free IFS, then the station has additionally to wait for a random back-off time (collision avoidance, multiple of slot-time) If another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) Chapter 4 Page 202
Determining Backoff-Time in variing load situations The number of backoff-slots is set to 8 by default If the number of slots is too low (high load situation) the probability that two or more station choose the same backoff number the collision causes a non successful transmission the number of slots is doubled up to 128 slots If the number of slots is too high (low load situation) The probability that stations choose a high number becomes high The waiting period until transmission starts is waisted time and resource The number of slots is halved to reduce possible waist Chapter 4 Page 203
IEEE 802.11 IFSs PHY SlotTime SIFS RIFS DIFS PIFS CWmin CWmax Preamble FHSS 50µs 28µs - 128µs 78µs 15 1023 96µs IR 8µs 10µs - 26µs 18µs 63 1023 16µs (1Mbit/s) 20µs (2Mbit/s) DSSS 2,4GHz 20µs 10µs - 50µs 30µs 31 1023 144µs DSSS HR 20µs 10µs - 50µs 30µs 31 1023 144µs OFDM 20MHz 9µs 16µs - 34µs 25µs 15 1023 16µs 10MHz 13µs 32µs - 58µs 45µs 15 1023 32µs 5MHz 21µs 64µs - 106µs 85µs 15 1023 64µs ERP Long: 20µs 50µs 30µs 10µs - Short: 9µs 28µs 19µs OFDM HT Long: 20µs 50µs 30µs 2,4GHz Short: 9µs 10µs 2µs 28µs 19µs 5GHz 9µs 16µs 34µs 25µs 31 1023 20µs 15 1023 16µs SlotTime and SIFS specified per PHY DIFS = SIFS + 2*SlotTime PIFS = SIFS + SlotTime Chapter 4 Page 204
Accessing the Medium CSMA/CD station A CRS defer station B CRS station C CRS defer CRS collision Adapters that can detect collisions (e.g. Ethernet adapters) Carrier Sensing: listen to the media to determine if it is free Initiate transmission as soon as carrier drops When collision is detected station defers When defer timer expires: repeat carrier sensing and start transmission Chapter 4 Page 205
Accessing the Medium CSMA/CA station A station B CRS defer station C CRS defer defer CRS CRS Wireless LAN adapters cannot detect collisions: Carrier Sensing - listen to the media to determine if it is free Collision Avoidance - minimize chance for collision by starting (random) back-off timer, when medium is sensed free, and prior to transmission Chapter 4 Page 206
802.11 - Competing Stations - Simplified Version (without Ack) station 1 DIFS DIFS bo e bo r DIFS bo e bo r DIFS bo e busy station 2 bo e busy station 3 busy station 4 bo e busy bo e bo r station 5 bo e bo r bo e busy bo e bo r busy medium not idle (frame, ack etc.) bo e elapsed backoff time t packet arrival at MAC bo r residual backoff time Chapter 4 Page 207
802.11 - CSMA/CA Access Method II Sending unicast packets Station has to wait for DIFS before sending data Receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) Automatic retransmission of data packets in case of transmission errors sender DIFS data receiver SIFS ACK other stations waiting time DIFS contention data t Chapter 4 Page 208
802.11 - DFWMAC Sending unicast packets Station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) Acknowledgement via CTS after SIFS by receiver (if ready to receive) Sender can now send data at once, acknowledgement via ACK Other stations store medium reservations distributed via RTS and CTS sender receiver DIFS RTS SIFS CTS SIFS data SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS contention data t Chapter 4 Page 209
Fragmentation sender receiver DIFS RTS SIFS CTS SIFS frag 1 SIFS ACK SIFS 1 frag 2 SIFS ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t Chapter 4 Page 210
DFWMAC-PCF t 0 t 1 SuperFrame medium busy point coordinator PIFS D 1 SIFS SIFS D 2 SIFS SIFS wireless stations U 1 U 2 stations NAV NAV t 2 t 3 t 4 point coordinator wireless stations stations NAV D 3 PIFS D 4 SIFS U 4 NAV contention free period Chapter 4 Page 211 SIFS CFend contention period t
PCF versus Infrastructure The PCF and DCF mode address the PHY-Layer operation of the WLAN Infrastructure- and Ad-Hoc-Mode address the network topology It is possible to run PCF in an Ad-Hoc network to achieve quality of service operation It is possible (and that is the most usual case) to run DCF operation in Infrastructure Mode Chapter 4 Page 212
802.11 MAC-Frame Format Types Control frames, management frames, data frames Sequence numbers Important against duplicated frames due to lost ACKs Addresses Receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous bytes Sending time, checksum, frame control, data 2 2 6 6 6 6 Duration/ Address Address Address Sequence ID 1 2 3 Control Frame Control 2 0-2312 4 Address Data 4 bits 2 2 4 1 1 1 1 1 1 1 1 Protocol version Type Subtype To DS From DS More Frag Retry Power Mgmt More Data WEP Order CRC Chapter 4 Page 213
MAC Address Format scenario to DS from address 1 address 2 address 3 address 4 DS ad-hoc network 0 0 DA SA BSSID - infrastructure 0 1 DA BSSID SA - network, from AP infrastructure 1 0 BSSID SA DA - network, to AP infrastructure network, within DS 1 1 RA TA DA SA DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address Chapter 4 Page 214
Ad-Hoc Network Communication 802.11 LAN STA 1 STA 3 IBSS 1 STA 2 to DS from DS Adr. 1 Adr. 2 Adr. 3 Adr. 4 0 0 STA1 BSS1 STA3 - Chapter 4 Page 215
Infrastructure Communication I 802.11 LAN 802.x LAN STA 1 BSS 1 BS1 Portal Distribution System ESS BS2 BSS 2 STA 2 STA 3 To BS From BS Adr. 1 Adr. 2 Adr. 3 Adr. 4 1 0 BSS1 STA1 STA2-1 1 BSS2 BSS1 STA2 STA1 0 1 STA2 BSS2 STA1 - Chapter 4 Page 216
Control Frames: ACK, RTS, CTS Acknowledgement bytes 2 2 6 4 Frame ACK Duration Receiver CRC Control Address Request To Send bytes 2 2 6 6 4 Frame Duration Receiver Transmitter RTS CRC Control Address Address Clear To Send bytes 2 2 6 4 Frame CTS Duration Receiver CRC Control Address Chapter 4 Page 217
802.11 - MAC Management Synchronization Try to find a WLAN, try to stay within a WLAN Timer etc. Power management Sleep-mode without missing a message Periodic sleep, frame buffering, traffic measurements Association/Re-association Integration into a WLAN Roaming, i.e. change networks by changing access points Scanning, i.e. active search for a network MIB - Management Information Base Managing, read, write Chapter 4 Page 218
Synchronization Using a Beacon (Infrastructure) beacon interval access point medium B B B B busy busy busy busy value of the timestamp B beacon frame t Chapter 4 Page 219
Synchronization Using a Beacon (Adhoc) beacon interval: 20 ms 1s station 1 B 1 B 1 station 2 B 2 B 2 medium busy busy busy busy t value of the timestamp B beacon frame random delay Chapter 4 Page 220
Power Management Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF) All stations wake up at the same time Infrastructure Traffic Indication Map (TIM) List of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM) List of broadcast/multicast receivers transmitted by AP Ad-hoc Ad-hoc Traffic Indication Map (ATIM) Announcement of receivers by stations buffering frames More complicated - no central AP Collision of ATIMs possible (scalability?) Chapter 4 Page 221
Power Saving with Wake-up Patterns (Infrastructure) TIM interval DTIM interval Access point D B T T d D B Medium (other traffic) busy busy busy busy Station T TIM D DTIM p awake d t B broadcast/multicast p PS poll d data transmission to/from the station Chapter 4 Page 222
Power Saving with Wake-up Patterns (Ad-hoc) ATIM window beacon interval station 1 B 1 A D B 1 station 2 B 2 B 2 a d B beacon frame random delay A transmit ATIM D transmit data t awake a acknowledge ATIM d acknowledge data Chapter 4 Page 223
Scanning Scanning required for many functions finding and joining a network finding a new AP while roaming (better hand-over) initializing an Independent BSS (ad hoc) network 802.11 MAC uses a common mechanism for all PHY single or multi channel passive or active scanning Passive Scanning Find networks simply by listening for Beacons Active Scanning On each channel Send a Probe, Wait for a Probe Response Beacon or Probe Response contains information necessary to join new network Chapter 4 Page 224
Active Scanning Example Initial connection to an Access Point Reassociation follows a similar process Steps to Association: Station sends Probe APs send Probe Response Station selects best AP Station sends Association Request to selected AP AP sends Association Response Chapter 4 Page 225
802.11 - Roaming No or bad connection? Then perform: Scanning Scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer Reassociation Request Station sends a request to one or several AP(s) Reassociation Response Success: AP has answered, station can now participate Failure: continue scanning AP accepts Reassociation Request Signal the new station to the distribution system The distribution system updates its data base (i.e., location information) Typically, the distribution system now informs the old AP so it can release resources Chapter 4 Page 226
Wireless LAN Infrastructure Network AP 2 Station 1 Station 2 Station 5 Station 6 AP 1 AP 3 Station 3 Station 4 Station 7 Each Station is Associated with a particular AP Stations 1, 2, and 3 are associated with Access Point 1 Stations 4 and 5 are associated with Access Point 2 Stations 6 and 7 are associated with Access Point 3 Chapter 4 Page 227
Roaming (Hand-Over) AP 2 Station 1 Station 2 Station 5 Station 6 AP 1 Station 1 AP 3 Station 7 Station 3 Station 4 Station 1 Mobile station 1 may move beyond the coverage area of their Access Point 1 but within range of another Access Points (2, 3) Reassociation allows station to continue operation Chapter 4 Page 228
802.11 Overview Chapter 4 Page 229
WLAN: IEEE 802.11 enhancements Standard Year Description Standard Year Description 802.11c 2001 Wireless Bridging 802.11T - Wireless Performance Prediction (WPP) 802.11d 2001 Roaming 802.11u 2011 Interworking to non-802 networks 802.11e 2004 Quality of Service 802.11v 2011 Network management 802.11F 2003 Handover 802.11w 2009 Security for management frames 802.11h 2006 DFS (Dynamic Frequency Selection) and TPC (Transmission Power Control) 802.11y 2008 3.6 GHz for USA 802.11i 2004 Security 802.11z 2010 Direct Link Setup (Ad-hoc) 802.11j 2004 5 GHz for Japan 802.11aa MAC Enhancements for Robust Audio Video Streaming 802.11k 2007 Radio resource measurement 802.11ac MIMO + QAM (5GHz) 802.11m 2006 Maintenance 802.11ad Beamforming + QAM (60GHz) 802.11n 2009 MIMO 802.11ae Prioritization of Management Frames (QoS) 802.11p 2010 Car-to-Car communication 802.11af TV Whitespace (Sub-1GHz) 802.11r 2010 Fast Handover (VoIP) 802.11ah Ultra-low-power version of Wi-Fi with immediate benefits for the so-called "Internet of things (Sub-1GHz) 802.11s 2011 Meshed networks 802.11ai Fast Initial Link Setup Chapter 4 Page 230
WLAN: IEEE 802.11e Scope Extension of standard MAC layer functionality in order to provide QoS Necessary for delay sensitive applications (e.g. A/V Streaming) Dates and Facts Published in 2005 Now part of IEEE 802.11-2007 Complies to WMM certificate of WiFi alliance Legacy 802.11 MAC DCF mandatory (RTS/CTS optional) PCF optional (better than DCF regarding QoS but still no guarantees) Best effort design 802.11e MAC EDCF/EDCA and HCF/HCCA Prioritization in EDCF Required for Prioritized QoS Services Required for Contention- Free Services for non-qos STA, optional otherwise MAC Extend Chapter 4 Page 231 Point Coordination Function (PCF) Hybrid Coordination Function (HCF) HCF Contention Access (EDCA) HCF Controlled Access (HCCA) Distributed Coordination Function (DCF) Required for Parameterized QoS Services Used for Contention Services, basis for PCF and HCF IEEE 802.11-2007 MAC Layer (Src: 802.11-2007, p. 251) Key Concept TXOP (Transmission opportunity) EDCA TXOP contention-based channel access TXOP HCCA TXOP HCF controlled channel access TXOP Hybrid Coordinator (HC) Centralized; higher prioritized access than EDCF
IEEE 802.11e EDCA I Prioritization through 4 Access Categories (ACs) Each MSDU is mapped to one AC Each AC has its own queue and its own parameter set to contend for TXOP Parameter Set is given by AP (or default if not) and consists of: Individual AIFS (Arbitrary IFS): AIFS DIFS Individual CW (Contention Window) to generate random backoff counter Priority UP AC Description Lowest 1 AC_BK Background 2 AC_BK Background 0 AC_BE Best Effort 3 AC_BE Best Effort 4 AC_VI Video 5 AC_VI Video 6 AC_VO Voice Highest 7 AC_VO Voice (MSDU, UP) Mapping to Access Category Transmit queues for ACs Per-queue EDCA functions with Internal collision resolution Concept of 802.11e MAC layer to provide prioritization: Different queues for each priority (Src: 802.11-2007, p. 287) Chapter 4 Page 232
IEEE 802.11e EDCA II Internal collision: backoff counters of two or more ACs get 0 simultaneously Solution: higher prioritized AC gets TXOP (TXOP = time interval to send data) AC(s) that lost contention react like collision would be external (on wireless medium) External collision: backoff counters of two or more stations get 0 simultaneously Solution: CW is increased next backoff counter will be higher 802.11e EDCA: At least a station has to wait for AIFS=DIFS before it is allowed to count down its backoff. (Src: 802.11-2007, p. 258) Chapter 4 Page 233
IEEE 802.11e HCCA I HCF (Hybrid coordination function) Only usable in infrastructure QoS network configurations To be used during both the contention period (CP) and the contention free period (CFP) Uses a QoS-aware point coordinator ( hybrid coordinator ) by default collocated with the enhanced access point (QAP) uses the point coordinator's higher priority (PIFS < DIFS) to allocate transmission opportunities (TXOPs) to stations (AIFS DIFS) Meets predefined service rate, delay and/or jitter requirements of particular traffic flows Caused long delays in standardization process due to its complexity Chapter 4 Page 234
IEEE 802.11e Conclusions Legacy 802.11 MAC vs. 802.11e MAC Multiplexing of CPs and CFPs is flexible in 802.11e whereas it was fixed in legacy standard In 802.11e HC sends periodically beacon frames like in 802.11 legacy standard stations still have to register by HC in CP in order to get part of polling list that is one reason why duration of CFP is bounded in 802.11e, too In 802.11e prioritized (EDCA) and parameterized (HCCA) QoS is provided Remaining Problem: Additional effort is necessary if several BSS/QBSS in overlapping physical space Chapter 4 Page 236