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Special Topics on Wireless Ad-hoc Networks Lecture 6: Wireless Local Area Networks (WLANs) University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Univ. of Tehran Computer Network 1 Covered topic How to build a small wireless local area network? Different current wireless technologies References Chapter 3 of the book “Design alternative for Wireless local area networks”, Many other sources for 802.11 Univ. of Tehran Computer Network 2 Outlines Some basic issues Wireless area network standards 802.11 standard 802.11 management Univ. of Tehran Computer Network 3 Ideal Wireless Area network? Wish List High speed (Efficiency) Low cost No use/minimal use of the mobile equipment battery Can work in the presence of other WLANs (Heterogeneity) Easy to install and use Etc Univ. of Tehran Computer Network 4 Wireless LAN Design Goals Wireless LAN Design Goals Portable product: Different countries have different regulations concerning RF band usage. Low power consumption License free operation Multiple networks should co-exist Univ. of Tehran Computer Network 5 Wireless LAN Design Alternatives Design Choices Physical Layer: diffused Infrared (IR) or Radio Frequency (RF)? Radio Technology: Direct-Sequence or FrequencyHopping? Which frequency range to use? Which MAC protocol to use. Peer-Peer architecture or Base-Station approach? Univ. of Tehran Computer Network 6 Physical Layer Alternatives IR Simple circuitry, cost-effective, no regulatory constraints, no Rayleigh fading (waves are small), also nice for micro-cellular networks... (multiple cells can exist in a room providing more bandwidth) RF more complicated circuitry, regulatory constraints (2.4 GHz Industrial Scientific Medical, ISM, bands) in the U.S. Univ. of Tehran Computer Network 7 Physical Layer Alternatives Cost IR <$10 Regulation None RF <$20 Interference No license on ISM bands Ambient Light Radiators coverage Performance Spot Moderate Multiple networks Univ. of Tehran Limited Computer Network Wide Area Depends on Bandwidth Possible 8 Spread spectrum technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code power interference spread signal power detection at receiver f spread interference f Side effects: signal coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping DSSS (Direct Sequence Spread Spectrum) XOR of the signal with pseudo-random number (chipping sequence) generate a signal with a wider range of frequency: spread spectrum tb user data 0 1 XOR tc chipping sequence 01101010110101 = resulting signal 01101011001010 tb: bit period tc: chip period FHSS (Frequency Hopping Spread Spectrum) • Discrete changes of carrier frequency – sequence of frequency changes determined via pseudo random number sequence • Two versions – Fast Hopping: several frequencies per user bit – Slow Hopping: several user bits per frequency • Advantages – frequency selective fading and interference limited to short period – simple implementation – uses only small portion of spectrum at any time FHSS: Example tb user data 0 1 f 0 1 1 t td f3 slow hopping (3 bits/hop) f2 f1 f t td f3 fast hopping (3 hops/bit) f2 f1 t tb: bit period td: dwell time Comparison between Slow Hopping and Fast Hopping Slow hopping Pros: cheaper Cons: less immune to narrowband interference Fast hopping Pros: more immune to narrowband interference Cons: tight synchronization increased complexity Radio Technology Spread Spectrum Technologies Frequency Hopping: The sender keeps changing the carrier wave frequency at which its sending its data. Receiver must be in synch with transmitter, and know the ordering of frequencies. Direct-Sequence: The receiver listens to a set of frequencies at the same time. The subset of frequencies that actually contain data from the sender is determined by spreading code, which both the sender and receiver must know. This subset of frequencies changes during transmission. Non-Spread Spectrum requires licensing Univ. of Tehran Computer Network 14 Frequency Hopping versus Direct Sequence DS advantages Lower cost FH advantages Higher capacity Interference avoidance capability: If some frequency has interference on it, simply don't hop there. Multiple networks can co-exist: Just use a different frequency hopping pattern. Univ. of Tehran Computer Network 15 Wireless Standards Univ. of Tehran Computer Network 16 Distance vs. Data Rate Univ. of Tehran Computer Network 17 Mobility vs. Data Rate Univ. of Tehran Computer Network 18 LAN Industry WANs are offered as service LANs are sold as “end products” Cost of infrastructure Coverage area You own, no service charge Analogy with PSTN/PBX WLAN vs. WAN Cellular Networks Data rate (2 Mbps vs. 54 Mbps) Frequency band regulation (Licensing) Method of data delivery (Service vs. own) Univ. of Tehran Computer Network 19 LAN standard Univ. of Tehran Computer Network 20 Early Experiences IBM Switzerland,Late 1970 HP Labs, Palo Alto, 1980 Factories and manufacturing floors Diffused IR technology Could not get 1 Mbps 100 Kbps DSSS around 900 Mhz CSMA as MAC Experimental licensing from FCC Frequency administration was problematic, thus abandoned Motorola, ~1985 1.73 GHz Abandoned after FCC difficulties Univ. of Tehran Computer Network 21 WiFi Almost all wireless LANs now are IEEE 802.11 based Competing technologies, e.g., HiperLAN can’t compete on volume and cost 802.11 is also known as WiFi = “Wireless Fidelity” Fidelity = Compatibility between wireless equipment from different manufacturers WiFi Alliance is a non-profit organization that does the compatibility testing (WiFi.org) Univ. of Tehran Computer Network 22 Architectures Distributed wireless Networks: also called Ad-hoc networks Centralized wireless Networks: also called last hop networks. They are extension to wired networks. Univ. of Tehran Computer Network 23 Centralized Wlan Ad Hoc Laptop Server Laptop DS Access Point Access Point Pager PDA Univ. of Tehran Laptop Computer Network Laptop 24 Base-Station Approach Advantages over Peer-Peer No hidden terminal: base station hears all mobile terminals, are relays their information to ever mobile terminal in cell. Higher transmission range Easy expansion Better approach to security Problem? Point of failure, Feasibility? Univ. of Tehran Computer Network 25 Access Point Functions Access point has three components Wireless LAN interface to communicate with nodes in its service area Wireline interface card to connect to the backbone network MAC layer bridge to filter traffic between sub-networks. This function is essential to use the radio links efficiently Univ. of Tehran Computer Network 26 Medium Access Control Wireless channel is a shared medium Need access control mechanism to avoid interference MAC protocol design has been an active area of research for many years. See Survey. Univ. of Tehran Computer Network 27 MAC: A Simple Classification Wireless MAC Centralized Distributed On Demand MACs, Polling Guaranteed or controlled access Random access Our focus SDMA, FDMA, TDMA, Polling Univ. of Tehran Computer Network 28 Wireless LAN Architecture, Cont… Logical Link Control Layer MAC Layer: Consist of two sub layer, physical Layer and physical convergence layer Physical convergence layer, shields LLC from the specifics of the physical medium. Together with LLC it constitutes equivalent of Link Layer of OSI Univ. of Tehran Computer Network 29 Power Management Battery life of mobile computers/PDAs are very short. Need to save The additional usage for wireless should be minimal Wireless stations have three states Sleep Awake Transmit Univ. of Tehran Computer Network 30 Power Management, Cont… AP knows the power management of each node AP buffers packets to the sleeping nodes AP send Traffic Delivery Information Message (TDIM) that contains the list of nodes that will receive data in that frame, how much data and when? The node is awake only when it is sending data, receiving data or listening to TDIM. Univ. of Tehran Computer Network 31 802.11 Features Power management: NICs to switch to lower-power standby modes periodically when not transmitting, reducing the drain on the battery. Put to sleep, etc. Bandwidth: To compress data Security: Addressing: destination address does not always correspond to location. Univ. of Tehran Computer Network 32 IEEE 802.11 Topology Independent basic service set (IBSS) networks (Ad-hoc) Basic service set (BSS), associated node with an AP Extended service set (ESS) BSS networks Distribution system (DS) as an element that interconnects BSSs within the ESS via APs. Univ. of Tehran Computer Network 33 Starting an IBSS One station is configured to be “initiating station,” and is given a service set ID (SSID); Starter sends beacons; Other stations in the IBSS will search the medium for a service set with SSID that matches their desired SSID and act on the beacons and obtain the information needed to communicate; There can be more stations configured as “starter.” Univ. of Tehran Computer Network 34 ESS topology connectivity between multiple BSSs, They use a common DS Univ. of Tehran Computer Network 35 Starting an ESS The infrastructure network is identified by its extended service set ID (ESSID); All APs will have been set according to this ESSID; On power up, stations will issue probe requests and will locate the AP that they will associate with. Univ. of Tehran Computer Network 36 802.11 Logical Architecture •PLCP: Physical Layer Convergence Procedure •PMD: Physical Medium Dependent •MAC provides asynchronous, connectionless service •Single MAC and one of multiple PHYs like DSSS, OFDM, IR and FHSS. Univ. of Tehran Computer Network 37 802.11 MAC Frame Format Bytes 32 Preamble 34~2346 6 MPDU PLCP header MAC Header Frame Duration Addr 1 Addr 2 Addr 3 Sequence Address 4 User Control Control Data Bytes 2 2 6 6 2 6 6 CRC 4 Encrypted to WEP Bits 2 2 Protocol Version 4 1 1 1 Type Sub type To From DS DS Univ. of Tehran Last Retry Power Fragment Mgt Computer Network EP RSVD 38 802.11 MAC Frame Format Address Fields contains Source address Destination address AP address Transmitting station address DS = Distribution System User Data, up to 2304 bytes long Univ. of Tehran Computer Network 39 Special Frames: ACK, RTS, CTS bytes Acknowledgement 2 2 6 Frame Receiver Duration Control Address ACK 4 CRC bytes Request To SendRTS 2 2 6 6 Frame Receiver Transmitter Duration Control Address Address bytes Clear To Send CTS 2 2 6 Frame Receiver Duration Control Address 4 CRC 4 CRC IEEE 802.11 LLC Layer Provides three type of service for exchanging data between (mobile) devices connected to the same LAN Acknowledged connectionless Un-acknowledged connectionless, useful for broadcasting or multicasting. Connection oriented Higher layers expect error free transmission Univ. of Tehran Computer Network 41 IEEE 802.11 LLC Layer, Cont.. Destination Source SAP SAP Control Data Each SAP (Service Access Point) address is 7 bits. One bit is added to it to indicate whether it is order or response. Control has three values Information, carry user data Supervisory, for error control and flow control Unnumbered, other type of control packet Univ. of Tehran Computer Network 42 IEEE 802.11 LLC <-> MAC Primitives Four types of primitives are exchanged between LLC and MAC Layer Request: order to perform a function Confirm: response to Request Indication: inform an event Response: inform completion of process began by Indication Univ. of Tehran Computer Network 43 Reception of packets AP Buffer traffic to sleeping nodes Sleeping nodes wake up to listen to TIM (Traffic Indication Map) in the Beacon AP send a DTIM (Delivery TIM) followed by the data for that station. Beacon contains, time stamp, beacon interval, DTIM period, DTIM count, sync info, TIM broadcast indicator Univ. of Tehran Computer Network 44 Frame type and subtypes Three type of frames Management Control Asynchronous data Each type has subtypes Control RTS CTS ACK Univ. of Tehran Computer Network 45 Frame type and subtypes, Cont.. Management Association request/ response Re-association request/ response: transfer from AP to another. Probe request/ response privacy request/ response: encrypting content Authentication: to establish identity Beacon (Time stamp, beacon interval, channels sync info, etc.) Univ. of Tehran Computer Network 46 Frame type and subtypes, Cont.. Management… TIM (Traffic Indication Map) indicates traffic to a dozing node dissociation Univ. of Tehran Computer Network 47 802.11 Management Operations Scanning Association/Reassociation Time synchronization Power management Univ. of Tehran Computer Network 48 Scanning in 802.11 Goal: find networks in the area Passive scanning Not require transmission Move to each channel, and listen for Beacon frames Active scanning Require transmission Move to each channel, and send Probe Request frames to solicit Probe Responses from a network Univ. of Tehran Computer Network 49 Association in 802.11 1: Association request 2: Association response 3: Data traffic AP Client Univ. of Tehran Computer Network 50 Reassociation in 802.11 1: Reassociation request 3: Reassociation response 5: Send buffered frames Client 6: Data traffic New AP 2: verify previous association Old AP Univ. of Tehran Computer Network 4: send buffered 51 frames Time Synchronization in 802.11 Timing synchronization function (TSF) AP controls timing in infrastructure networks All stations maintain a local timer TSF keeps timer from all stations in sync Periodic Beacons convey timing Beacons are sent at well known intervals Timestamp from Beacons used to calibrate local clocks Local TSF timer mitigates loss of Beacons Univ. of Tehran Computer Network 52 Power Management in 802.11 A station is in one of the three states Transmitter on Receiver on Both transmitter and receiver off (dozing) AP buffers packets for dozing stations AP announces which stations have frames buffered in its Beacon frames Dozing stations wake up to listen to the beacons If there is data buffered for it, it sends a poll frame to get the buffered data Univ. of Tehran Computer Network 53 Authentication Three levels of authentication Open: AP does not challenge the identity of the node. Password: upon association, the AP demands a password from the node. Public Key: Each node has a public key. Upon association, the AP sends an encrypted message using the nodes public key. The node needs to respond correctly using it private key. Univ. of Tehran Computer Network 54 Inter Frame Spacing SIFS = Short inter frame space = dependent on PHY PIFS = point coordination function (PCF) inter frame space = SIFS + slot time DIFS = distributed coordination function (DCF) inter frame space = PIFS + slot time The back-off timer is expressed in terms of number of time slots. Univ. of Tehran Computer Network 55 802.11 Frame Priorities Busy DIFS PIFS SIFS content window Frame transmission Time Short interframe space (SIFS) PCF interframe space (PIFS) For highest priority frames (e.g., RTS/CTS, ACK) Used by PCF during contention free operation DCF interframe space (DIFS) Minimum medium idle time for contention-based services Univ. of Tehran Computer Network 56 SIFS/DIFS SIFS makes RTS/CTS/Data/ACK atomic Example: Slot Time = 1, CW = 5, DIFS=3, PIFS=2, SIFS=1, Univ. of Tehran Computer Network 57 Priorities in 802.11 CTS and ACK have priority over RTS After channel becomes idle If a node wants to send CTS/ACK, it transmits SIFS duration after channel goes idle If a node wants to send RTS, it waits for DIFS > SIFS Univ. of Tehran Computer Network 58 SIFS and DIFS DATA1 ACK1 SIFS DIFS Univ. of Tehran backoff RTS SIFS Computer Network 59 Energy Conservation Since many mobile hosts are operated by batteries, MAC protocols which conserve energy are of interest Two approaches to reduce energy consumption Power save: Turn off wireless interface when desirable Power control: Reduce transmit power Univ. of Tehran Computer Network 60 Power Control with 802.11 Transmit RTS/CTS/DATA/ACK at least power level needed to communicate with the receiver A B C D A/B do not receive RTS/CTS from C/D. Also do not sense D’s data transmission B’s transmission to A at high power interferes with reception of ACK at C Univ. of Tehran Computer Network 61 A Plausible Solution RTS/CTS at highest power, and DATA/ACK at smallest necessary power level Data sensed A B C D Data Interference range RTS Ack A cannot sense C’s data transmission, and may transmit DATA to some other host This DATA will interfere at C This situation unlikely if DATA transmitted at highest power level Interference range Network Univ. of Tehran range ~ sensingComputer 62 Transmitting RTS at the highest power level also reduces spatial reuse Nodes receiving RTS/CTS have to defer transmissions Univ. of Tehran Computer Network 63 Bridge Functions Speed conversion between different devices, results in buffering. Frame format adaptation between different incompatible LANs Adding or deleting fields in the frame to convert between different LAN standards Univ. of Tehran Computer Network 64 02.11 Activities IEEE 802.11c: Bridge Operation (Completed. Added to IEEE 802.1D) 802.11d: Global Harmonization (PHYs for other countries. Published as IEEE Std 802.11d-2001) 802.11e: Quality of Service. IEEE Std 802.11e-2005 802.11f: Inter-Access Point Protocol (Published as IEEE Std Std 802.11F-2003) 802.11h: Dynamic Frequency Selection and transmit power control to satisfy 5GHz band operation in Europe. Published as IEEE Std 802.11h-2003 802.11i: MAC Enhancements for Enhanced Security. Published as IEEE Std 802.11i-2004 802.11j: 4.9-5 GHz operation in Japan. IEEE Std 802.11j-2004 802.11k: Radio Resource Measurement interface to higher layers. Active. Univ. of Tehran Computer Network 65 02.11 Activities IEEE 802.11m: Maintenance. Correct editorial and technical issues in 802.11a/b/d/g/h. Active. 802.11n: Enhancements for higher throughput (100+ Mbps). Active. 802.11p: Inter-vehicle and vehicle-road side communication at 5.8GHz. Active. 802.11r: Fast Roaming. Started July 2003. Active. 802.11s: ESS Mesh Networks. Active. 802.11T: Wireless Performance Metrics. Active. 802.11u: Inter-working with External Networks. Active. 802.11v: Wireless Network Management enhancements for interface to upper layers. Extension to 80211.k. Active. Study Group ADS: Management frame security. Active Standing Committee Wireless Next Generation WNG: Globalization jointly w ETSI-BRAN and MMAC. Active. Univ. of Tehran Computer Network 66 802.11n Trend: HDTV and flat screens are taking off Media Center Extenders from Linksys and other vendors Application: HDTV and streaming video (over longer distances than permitted by 802.15.3 WPANs) 11n = Next Generation of 802.11 At least 100 Mbps at MAC user layer ⇒ 200+ Mbps at PHY ⇒ 4x to 5x faster than 11a/g (802.11a/g have 54 Mbps over the air and 25 Mbps to user) Pre-11n products already available Task Group n (TGn) setup: Sept 2003 Expected Completion: March 2007 v. of Tehran Computer Network 67 802.11n Uses multiple input multiple output antenna (MIMO) Data rate and range are enhanced by using spatial multiplexing (N antenna pairs) plus antenna diversity occupies one WLAN channel, and in compliance with 802.11 Backwards compatible with 802.11 a,b,g One access point supports both standard WLAN and MIMO devices v. of Tehran Computer Network 68 HIPERLAN 1995 ETSI technical group RES 10 (Radio Equipment and Systems) developed HIPERLAN/1 wireless LAN standards using 5 channels in 5.15-5.3 GHz frequency range Technical group BRAN (Broadband Radio Access Network) is standardizing HIPERLAN/2 for wireless ATM ETSI URL for Hiperlan information http://www.etsi.org/frameset/home.htm? /technicalactiv/Hiperlan/hiperlan2.htm Univ. of Tehran Computer Network 69 HIPERLAN Characteristics HIPERLANs with same radio frequencies might overlap Stations have unique node identifiers (NID) Stations belonging to same HIPERLAN share a common HIPERLAN identifier (HID) Stations of different HIPERLANs using same frequencies cause interference and reduce data transmission capacity of each HIPERLAN Packets with different HIDs are rejected to avoid confusion of data Univ. of Tehran Computer Network 70 HIPERLAN Protocol Layers Data link layer = logical link control (LLC) sub layer + MAC sub layer + channel access control (CAC) sub layer network data link physical Univ. of Tehran LLC MAC CAC Computer Network 71 HIPERLAN Protocol Layers, Cont.. MAC sub layer: Keeps track of HIPERLAN addresses (HID + NID) in overlapping HIPERLANs Provides lookup service between network names and HIDs Converts IEEE-style MAC addresses to HIPERLAN addresses Provides encryption of data for security Univ. of Tehran Computer Network 72 HIPERLAN Protocol Layers, Cont.. MAC sub layer: Provides “multi hop routing” – certain stations can perform store-andforwarding of frames Recognizes user priority indication (for time-sensitive frames) Univ. of Tehran Computer Network 73 HIPERLAN Protocol Layers, Cont.. CAC sub layer: Non-preemptive priority multiple access (NPMA) gives high priority traffic preference over low priority Stations gain access to channel through channel access cycles consisting of 3 phases: Univ. of Tehran Computer Network 74 HIPERLAN CAC Protocol CAC sub layer: Prioritization Contention Phase Phase Cycle Transmission Phase 1 2 3 4 Data ACK AP 1 2 3 4 5 Univ. of Tehran Computer Network Time 75 HIPERLAN Protocol Layers, Cont… CAC is designed to give each station (of same priority) equal chance to access the channel First stations with highest priority data are chosen. The rest will back off until all higher priority data is transmitted. Stations with the same priority level data, compete according to a given rule to choose “survivors” Survivors wait a random number of time slots and then listen to see if the channel is idle Univ. of Tehran Computer Network 76 HIPERLAN Protocol Layers, Cont… If the channel is idle then it starts transmitting. Those who could not transmit wait until next period Univ. of Tehran Computer Network 77 HIPERLAN/2 To support QoS, Handoff and integrate WLAN with next generation Cellular sys. Supporting IP& ATM at 54Mbps Use TDMA as MAC DLC (Data Link Control) layer constitutes a logical link Between AP and MT to ensure a connection oriented Communication. Univ. of Tehran Computer Network 78 Related Standards Activities IEEE 802.11 Hiperlan/2 http://grouper.ieee.org/groups/802/11/ http://www.etsi.org/technicalactiv/hiperlan2.htm IETF manet (Mobile Ad-hoc Networks) working group http://www.ietf.org/html.charters/manet-charter.html Univ. of Tehran Computer Network 79