1. Introduction to 802.11 Wireless LANs
WLAN History WLAN History Early Wireless LAN proprietary products WaveLAN (AT&T) the ancestor of 802.11 HomeRF (Proxim) Support for Voice, unlike 802.11 45% of the home network in 2000; 30% in 2001,?% today Abandoned by major chip makers (e.g. Intel: dismissed in april 2001) IEEE 802.11 Committee formed in 1990 Charter: specification of MAC and PHY for WLAN First standard: june 1997 1 and 2 Mbps operation Reference standard: september 1999 Multiple Physical Layers 2.4GHz Industrial, Scientific & Medical shared unlicensed band» Legacy; 802.11b/g 5 GHz ISM (802.11a) 1999: Wireless Ethernet Compatibility Alliance (WECA) certification Later on named Wi-Fi Boosted 802.11 deployment!!
Why so much talking about of 802.11 today? 802.11: no more just a WLAN Hot-spots Where the user goes, the network is available: home, school, office, hotel, university, airport, convention center Freedom to roam with seamless connectivity in every domain, with single client device May compete (complement) with 3G for Wireless Internet access Which of these two is the proper (closer) picture of Wireless Internet and Mobile Computing? Which technology is most suited?
Some citations we look at [802.11 technology] as becoming a requirement. Some carriers see it as a threat while other says, it could have something for me Bruce Dale, vice president of UMTS/W-CDMA products at Lucent (July 2002): Qualcomm: so sure of the coexistence of these technologies that it is intending to include 802.11b capability in its CDMA chipset during late 2003.
GPRS, 3G UMTS < 400 Kb/s Kms Mobile Broadband Internet IEEE 802.11 (b) > 10 Mb/s 100 m Bluetooth < 800 Kb/s 10 m The global picture Wide Area Local Area Personal Area 802.11/UMTS switching BT/802.11 switching PAN: collection of secure connections between devices in a very local area WAN: everywhere outside of the hotspots, where wireless Internet connection are provided LAN: collection of secure hot spot connections, providing broadband access to the Internet
WLAN Market - Products year 2002: overall spending for Wi-Fi (802.11b) products has increased of 38% up to 2.3 billion dollars [source: Gartner Dataquest] (15.4 million Wi-Fi cards, and 4.4 million Access Points have been sold). Projections: 5 billion dollars by 2006 [source: Gartner] end of 2003, 50% of the medium-large companies are expected to own a WLAN infrastructure (source: Gartner).
WLAN Enterprise Market Shares Vendor NIC Units (k) 2001 Share 2001 Vendor AP Units (k) Share 2001 Agere 392 14,9% Cisco 63,2 12,3% Cisco 272 10,3% Linksys 61,8 12,1% Avaya 199 7,5% Agere 36,3 7,1% Buffalo Technology (Melco) 124 4,7% Buffalo Technology (Melco) 32,4 6,3% Enterasys 106 4,0% D-Link 28,0 5,5% 3Com 95 3,6% Apple 25,2 4,9% Intermec 90 3,4% Symbol 23,2 4,5% Proxim 89 3,4% SOHOware 20,4 4,0% Linksys 83 3,1% Enterasys 20,3 4,0% Compaq 80 3,0% 3Com 19,5 3,8% Apple 75 2,8% Avaya 13,5 2,6% Dell 73 2,8% Compaq 13,0 2,5% Intel 55 2,1% Proxim 10,7 2,1% Siemens 48 1,8% Intel 10,4 2,0% Symbol 46 1,7% Intermec 10,4 2,0% SOHOware 46 1,7% IBM 9,6 1,9% IBM 46 1,7% SMC 9,4 1,8% Nokia 44 1,7% Toshiba 9,0 1,8% HP 41 1,6% Nokia 7,9 1,5% SMC 36 1,4% Netgear 7,3 1,4% Toshiba 34 1,3% HP 7,2 1,4% D-Link 32 1,2% Dell 7,0 1,4% Other 506 20,1% Fujitsu 6,2 1,2% Nortel 5,0 1,0% Other 55,0 10,8% Total 2.637 100% Total 511,8 100% Source: Cahners In-Stat Group Sep 2001
WLAN Market - HotSpots 4.500 4.000 3.500 3.000 2.500 2.000 1.500 U.S. Commercial Hotspots 2001-2002: exceeded expectations by 14% 2001 2002 Forecasted Actual 50000 40000 30000 20000 10000 0 U.S. Hotspots growth (2002-2006) 2002 2004 2006 Unique U.S. Hotspots 2003: 125.000 regular hotspot US users End 2006: 9 Million regular hotspot US users End 2006: 1 Billion dollars revenue predicted from HotSpot operation
PHY-related task groups 802.11a: PHY for 5 GHz published in 1999 Products available since early 2002 802.11b: higher rate PHY for 2.4 GHz Published in 1999 Products available since 1999 Interoperability tested (wifi) 802.11g: OFDM for 2.4 GHz Published in june 2003 Products available, though no extensive interoperability testing yet 802.11n:??? (Higher data rate) Launched in september 2003 Minimum goal: 108 Mbps (but higher numbers considered) Support for space division multiple access and smart antennas?
PHY rates at a glance Standard Transfer Method Frequency Band Data Rates Mbps 802.11 legacy FHSS, DSSS, IR 2.4 GHz, IR 1, 2 802.11b DSSS, HR-DSSS 2.4 GHz 1, 2, 5.5, 11 "802.11b+" non-standard DSSS, HR-DSSS, (PBCC) 2.4 GHz 1, 2, 5.5, 11, 22, 33, 44 802.11a OFDM 5.2, 5.5 GHz 6, 9, 12, 18, 24, 36, 48, 54 802.11g DSSS, HR-DSSS, OFDM 2.4 GHz 1, 2, 5.5, 11; 6, 9, 12, 18, 24, 36, 48, 54
PHY rates at a glance
802.11a Data rates
PHY distance/rate tradeoffs (open office) Distance (m) 140.0 120.0 100.0 80.0 60.0 40.0 5 GHz OFDM (.11a) 2.4 GHz OFDM (.11g) 2.4 GHz (.11b) 20.0 0.0 1Mbps 5.5Mbps 6Mbps 11Mbps 12Mbps 24Mbps 36Mbps 54Mbps
Coverage performance Cisco Aironet 350 Access Point 11 Mb/s DSS da ~30 a ~45 metri Configurable TX power: 50, 30, 20, 5, 1 mw (100 mw outside Europe) 5.5 Mb/s DSS da ~45 a ~76 metri 2 Mb/s DSS da ~76 a ~107 metri Greater TX power, faster battery consumptions!
The 2.4 (wifi) GHz spectrum 2.4 GHz bandwidth: 2.400-2.483,5 Europe (including Italy): 13 channels (each 5 MHz) - 2.412 2.417-2.422 2.427 2.472 Channel spectrum: Non Overlapping Channels 2412 2437 2462
The 5.0 (OFDM) GHz spectrum US regulation: 3 x 100 MHz channels 5.150-5.250 (40 mw) 5.250-5-350 (200 mw) 5.725-5.825 (800 mw) Operating 20 MHz channels 5.180, 5.200, 5.220, 5.240 5.260, 5.280, 5.300, 5.320 5.745, 5.765, 5.785, 5.805 20 MHz / 64 subcarriers = 0.3125 MHz per subcarriers (52 used)
Task groups e/f/i 802.11e: QoS extensions (packet prioritization) Currently in draft version 7.0; final version: end-year? Technical details later 802.11f: Interoperation between APs Working Practice Standard: june 2003 Inter-Access Point Protocol for roaming between APs of different vendors 802.11i: improved security mechanisms Wi-Fi Protected Access Strongly pushed by the Wi-Fi Alliance 802.11i components: Advanced Encryption Standard algorithm Extensible Authentication Protocol authentication schemes Temporal Key Integrity Protocol
Spectrum-related related task groups 802.11d: global harmonization working group not really a standard Different countries have different usable parts of the 2.4- and 5-GHz bands; Goal: create standards that satisfy different regulations, and will be approvable in as many different countries as possible. 802.11h: adds features to 802.11a 5GHz To deal with Hiperlan-a ETSI European standard To avoid interference with radar and satellite european services Features: Power management Dynamic channel/frequency selection 802.11j: equivalent to 802.11h, but for Japan available spectrum 4.9- to 5.0-GHz
Less-known task groups 802.11c: how to implement media-access-control bridging completed in 1997 useful only to people who design wireless LAN hardware 802.11k: measurement report Recently activated Goal: standardize the way 802.11a, b, and g networks report measurements of radio and network conditions to other parts of the network stack and new applications. 802.11m: collection of maintenance releases for the whole 802.11 WG used for internal IEEE housekeeping
2. Wireless LAN Networks and related Addressing
Basic Service Set (BSS) group of stations that can communicate with each other Infrastructure BSS or, simply, BSS Stations connected through AP Independent BSS or IBSS Stations connected in ad-hoc mode Network infrastructure AP
Frame Forwarding in a BSS Network infrastructure AP BSS: AP = relay function No direct communication allowed! IBSS: direct communication between all pairs of STAs
Why AP = relay function? Management: Mobile stations do NOT neet to maintain neighbohr relationship with other MS in the area But only need to make sure they remain properly associated to the AP Power Saving: APs may assist MS in their power saving functions by buffering frames dedicated to a (sleeping) MS when it is in PS mode Obvious disadvantage: use channel bandwidth twice
Extended Service Set BSS1 AP1 BSS2 BSS3 BSS4 AP2 AP3 AP4 ESS: created by merging different BSS through a network infrastructure (possibly overlapping BSS to offer a continuous coverage area) Stations within ESS MAY communicate each other via Layer 2 procedures APs acting as bridges MUST be on a same LAN or switched LAN or VLAN (no routers in between)
The concept of Distribution System Distribution system (physical connectivity + logical service support) AP1 AP2 AP3 MSs in a same ESS need to 1) communicate each other 2) move through the ESS Ethernet backbone: Distribution system medium (but DS is more than just a medium!!) DS role: - track where an MS is registrered within an ESS area - deliver frame to MS
Association and DS AP1 AP2 AP3 IAPP IAPP Association DS implementation: - an AP must inform other APs of associated MSs MAC addresses - proprietary implementation no interoperability (must use APs from same vendor) - standardized protocol on the way (?): IAPP (802.11f) - 802.11f Working Practice Standard: june 2003
Wireless Distribution System AP1 AP2 AP3 DS medium: - not necessarily an ethernet backbone! - could be the 802.11 technology itself Resulting AP = wireless bridge
Addresses At least three addresses Receiving station Transmitting station BSS address To make sure a frame is valid within the considered BSS For filtering purpose (filter frame within a BSS)
BSSID Address of a BSS Infrastructure mode: AP MAC address Ad-hoc mode: Random value» With universal/local bit set to 1 Generated by STA initiating the IBSS 802 IEEE 48 bit addresses 1 bit = individual/group 1 bit = universal/local 46 bit address
Addressing in an IBSS SA DA Frame Control Duration / ID Address 1 DA Address 2 SA Address 3 BSSID Sequence Control Address 4 --- Data FCS SA = Source Address DA = Destination Address
Addressing in a BSS? AP SA X DA
Addressing in a BSS! AP Distribution system SA DA Frame must carry following info: 1) Destined to DA 2) But through the AP What is the most general addressing structure?
Addressing in a BSS (to AP) AP BSSID Distribution system Address 2 = wireless Tx Address 1 = wireless Rx Address 3 = dest SA DA Frame Control Duration / ID Address 1 BSSID Address 2 SA Address 3 DA Sequence Control Address 4 --- Data FCS Protocol version Type 2 2 Sub Type To DS 1 0 From DS More Frag Retry Pwr MNG More Data WEP Order 4 1 1 1 1 1 1 1 1
Addressing in an ESS Distribution System AP AP BSSID DA DA SA Idea: DS will be able to forward frame to dest (either if fixed or wireless MAC) Frame Control Duration / ID Address 1 BSSID Address 2 SA Address 3 DA Sequence Control Address 4 --- Data FCS Protocol version Type 2 2 Sub Type To DS 1 0 From DS More Frag Retry Pwr MNG More Data WEP Order 4 1 1 1 1 1 1 1 1
Addressing in a BSS (from AP) AP BSSID Distribution system Address 2 = wireless Tx Address 1 = wireless Rx Address 3 = src SA DA Frame Control Duration / ID Address 1 DA Address 2 BSSID Address 3 SA Sequence Control Address 4 --- Data FCS Protocol version Type 2 2 Sub Type To DS 0 1 From DS More Frag Retry Pwr MNG More Data WEP Order 4 1 1 1 1 1 1 1 1
From AP: do we really need 3 addresses? Distribution system AP BSSID SA DA DA correctly receives frame, and send 802.11 ACK to BSSID (wireless transmitted) DA correctly receives frame, and send higher level ACK to SA (actual transmitter)
Addressing within a WDS Wireless Distribution System AP TA AP RA SA Address 4: initially forgotten DA Frame Control Duration / ID Address 1 RA Address 2 TA Address 3 DA Sequence Control Address 4 SA Data FCS Protocol version Type 2 2 Sub Type To DS 1 1 From DS More Frag Retry Pwr MNG More Data WEP Order 4 1 1 1 1 1 1 1 1
Addressing: summary Receiver Transmitter Function To DS From DS Address 1 Address 2 Address 3 Address 4 IBSS 0 0 RA = DA SA BSSID N/A From AP 0 1 RA = DA BSSID SA N/A To AP 1 0 RA = BSSID SA DA N/A Wireless DS 1 1 RA TA DA SA BSS Identifier (BSSID) unique identifier for a particular BSS. In an infrastructure BSSID it is the MAC address of the AP. In IBSS, it is random and locally administered by the starting station. (uniqueness) Transmitter Address (TA) MAC address of the station that transmit the frame to the wireless medium. Always an individual address. Receiver Address (RA) to which the frame is sent over wireless medium. Individual or Group. Source Address (SA) MAC address of the station who originated the frame. Always individual address. Maynot match TA because of the indirection performed by DS of an IEEE 802.11 WLAN. SA field is considered by higher layers. Destination Address (DA) Final destination. Individual or Group. Maynotmatch RA because of the indirection.
3. 802.11 MAC: CSMA/CA Distributed Coordination Function
Wireless Ethernet 802.3 (Ethernet) CSMA/CD Carrier Sense Multiple Access Collision Detect 802.11(wireless LAN) CSMA/CA (Distributed Coordination Function) Carrier Sense Multiple Access Collision Avoidance A B C Both A and C sense the channel idle at the same time they send at the same time. Collision can be detected at sender in Ethernet. Why this is not possible in 802.11? 1. Either TX or RX (no simultaneous RX/TX) 2. Large amount of power difference in Tx and Rx (even if simultaneous txrx, no possibility in rx while tx-ing) 3. Wireless medium = additional problems vs broadcast cable!!
Hidden Terminal Problem Large difference in signal strength; physical channel impairments (shadowing) It may result that two stations in the same area cannot communicate Hidden terminals A and C cannot hear each other A transmits to B C wants to send to B; C cannot receive A;C senses idle medium (Carrier Sense fails) Collision occurs at B. A cannot detect the collision (Collision Detection fails). A is hidden to C. A B C
802.11 MAC approach Still based on Carrier Sense: Listen before talking But collisions can only be inferred afterwards, at the receiver Receivers see corrupted data through a CRC error Transmitters fail to get a response Two-way handshaking mechanism to infer collisions DATA-ACK packets TX packet ACK RX
Channel Access details A station can transmit only if it senses the channel IDLE for a DIFS time DIFS = Distributed Inter Frame Space Packet arrival TX DIFS DATA RX SIFS ACK What about a station arriving in this frame time? Key idea: DATA and ACK separated by a different Inter Frame Space SIFS = Short Inter Frame Space Second station cannot hear a whole DIFS, as SIFS<DIFS
DIFS & SIFS in wi-fi DIFS = 50? s SIFS = 10? s
STA1 DIFS Why backoff? DATA SIFS ACK STA2 DIFS STA3 Collision! RULE: when the channel is initially sensed BUSY, station defers transmission; But when itis sensed IDLE for a DIFS, defer transmission of a further random time (BACKOFF TIME)
STA2 STA3 DIFS Slotted Backoff w=7 Extract random number in range (0, W-1) Decrement every slot-time? w=5 Note: slot times are not physically delimited on the channel! Rather, they are logically identified by every STA Slot-time values: 20?s for DSSS (wi-fi) Accounts for: 1) RX_TX turnaround time 2) busy detect time 3) propagation delay
Backoff freezing When STA is in backoff stage: It freezes the backoff counter as long as the channel is sensed BUSY It restarts decrementing the backoff as the channel is sensed IDLE for a DIFS period STATION 1 DIFS DATA SIFS ACK DIFS STATION 2 DIFS SIFS ACK 6 5 BUSY medium Frozen slot-time 4 DIFS 3 2 1
Backoff rules First backoff value: Extract a uniform random number in range (0,CW min ) If unsuccessful TX: Extract a uniform random number in range (0,2 (CW min +1)-1) If unsuccessful TX: Extract a uniform random number in range (0,2 2 (CW min +1)-1) Etc up to 2 m (CW min +1)-1 Exponential Backoff! CWmin = 31 CWmax = 1023 (m=5)
Throughput vs CWmin P=1000 bytes
RTS/CTS Request-To-Send / Clear-To-Send 4-way handshaking Versus 2-way handshaking of basic access mechanism Introduced for two reasons Combat hidden terminal Improve throughput performance with long packets
RTS/CTS and hidden terminals Packet arrival TX DIFS RTS DATA RX SIFS CTS SIFS SIFS ACK others TX RTS CTS data RX CTS hidden (Update NAV) NAV (RTS) NAV (CTS) RTS/CTS: carry the amount of time the channel will be BUSY. Other stations may update a Network Allocation Vector, and defer TX even if they sense the channel idle (Virtual Carrier Sensing)
RTS/CTS and performance RTS/CTS cons: RTS/CTS pros: larger overhead reduced collision duration
RTS/CTS throughput RTS/CTS convenient with long packets and large number of terminals (collision!);
RTS/CTS more robust to number of users and CWmin settings
Relation between IFS Immediate access when medium is idle >= DIFS DIFS Busy Medium SIFS DIFS PIFS Contention Window Backoff Window Next Frame Slot Time Parameters 802.11b PHY Defer Access PIFS used by Point Coordination Function - Time-bounded services - Polling scheme PCF Never deployed SIFS (? sec) 10 DIFS (?sec) 50 Select Slot and decrement backoff as long as medium stays idle Slot Time (? sec) 20 CWmin 31 CWmax 1023
EIFS Source station Destination station Data ACK DIFS Back-off Other stations receiving Data frame correctly SIFS NAV Back-off Back-off Other stations receiving Data frame incorectly EIFS
Time in microseconds. Update the NAV time in the neighborhood Data Frame formats PHY IEEE 802.11 Data 0-2312 FCS Frame Control Duration / ID Address 1 Address 2 Address 3 Sequence Control Address 4 2 2 6 6 6 2 6 0-2312 4 Data Frame check sequence Protocol version Type Sub Type info 2 2 12 Fragment number Sequence number 4 12 Sub Type To DS From DS More Frag Retry Pwr MNG More Data WEP Order 4 1 1 1 1 1 1 1 1
Why NAV (i.e. protect ACK)? TX C RX Station C receives frame from station TX Station C IS NOT in reach from station RX But sets NAV and protects RX ACK
Why EIFS (i.e. protect ACK)? TX C RX Station C DOES NOT receive frame from station TX but still receives enough signal to get a PHY.RXEND.indication error Station C IS NOT in reach from station RX But sets EIFS (!!) and protects RX ACK
4. DCF Overhead
Frame formats DATA FRAME (28 bytes excluded address 4) Frame Control Duration / ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data FCS RTS (20 bytes) Frame Control Duration RA TA FCS CTS / ACK (14 bytes) Frame Control Duration RA FCS
S? station DCF overhead Frame_ Tx Epayload [ ] ET [ ]? DIFS? CW /2 min T? T? SIFS? T Frame_ Tx MPDU ACK T? T? SIFS? T? SIFS? T? SIFS? T Frame_ Tx RTS CTS MPDU ACK T T T T MPDU ACK RTS CTS?? T? T? T T PLCP PLCP PLCP PLCP? 8? (28? L) /? 8? 14 / R? 8? 20/ R? 8? 14 / R ACK _ Tx RTS _ Tx CTS _ Tx R MPDU _ Tx
DCF overhead (802.11b) RTS/CTS Basic RTS/CTS Basic 0 2000 4000 6000 8000 Transmssion Time (usec) DIFS Ave Backoff RTS+SIFS CTS+SIFS Payload+SIFS ACK
DCF overhead (802.11b) Normalized Throughput 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 BAS-2Mbps RTS-2Mbps BAS-11Mbps RTS-11Mbps 100 300 500 700 900 1100 1300 1500 1700 1900 2100 2300 Payload Size (Bytes)
5. Point Coordination Function
PCF vs DCF PCF DCF PHY PCF deployed on TOP of DCF Backward compatibility
PCF Token-based access mechanism Polling Channel arbitration enforced by a point Coordinator (PC) Typically the AP, but not necessarily Contention-free access No collision on channel PCF deployment: minimal!! Optional part of the 802.11 specification As such, almost never deployed But HCCA (PCF extension in 802.11e) is getting considerable attention
PCF frame transfer SIFS Polling strategy: very elementary!! - send polling command to stations with increasing Association ID value - (regardless whether they might have or not data to transmit)