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S-72.1130 Telecommunication Systems Wireless Local Area Networks Outline LAN basics Structure/properties of LANs WLANs Link layer services Media access layer frames and headers CSMA/CA Physical layer frames modulation Direct sequence Frequency hopping Infrared Installation Security 2 LAN Basics LAN Basics WLAN Basics 802 LANs 802.11 Service Sets 802.11 LLC 802.11 MAC 802.11 PHY Typical Wired LAN Transmission Medium Network Interface Card (NIC) Unique MAC “physical” address Serial format in 10BASE5 ~ 10 Mb/s – baseband - 500 m Ethernet Processor RAM ROM RAM Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set NIC implements MAC protocol & physical port. Parallel interface to PC 4 Example: How Ring Networks Work A node functions as a repeater A Only destination station copies the frame, C B A all other nodes discard B transmits frame the frame addressed to A Unidirectional link A Signal propagates encoded by line codes A C B Example: 802.5 Reliability: A copies the frame link failure (FDDI applies at the reception double ring) A C B A C ignores the frame A C A B B absorbs the returning frame 5 Token Ring A ring consists of a single or dual (FDDI) cable in the shape of a loop. Ring reservation supervised by the rotating token. Each station is physically connected to each of its two nearest neighbors. Data in the form of packets passes around the ring from one station to another in uni-directional way. Advantages : (1) Access method supports heavy load without statistical multiplexing degradation of performance because the medium is shared for pair-wise stations (2) In practice several packets can simultaneous circulate between different pairs of stations. Disadvantages: (1) Complex management - especially for several rings (2) Re-initialization of the ring whenever a failure occurs 6 Example: Bus Network In a bus network, one node’s transmission traverses the entire network and is received and examined by every node. The access method can be : (1) Contention scheme : multiple nodes attempt to access bus; only one node succeeds at a time (e.g. CSMA/CD in Ethernet 802.3) (2) Round robin scheme : a token is passed between nodes; node holding the token can use the bus (e.g.Token bus 802.4) Advantages: (1) Simple access method C D A B (2) Easy to add or remove D stations term term Disadvantages: - Line coded, serial data (1) Poor efficiency with high - Twisted pair or coaxial cable network load in contention schemes (2) Security taken care by upper 7 network levels term: terminator impedance Wireless Local Area networks (WLANs) - basics LAN Basics WLAN Basics 802 LANs 802.11 Service Sets 802.11 LLC 802.11 MAC 802.11 PHY Wireless LANs (WLANs) - features High date rates IEEE 802.11b supports rates up to 11 MBps (in practice 6 Mb/s), and 802.11g reaches up to 54 Mb/s, need to have the bandwidth No new wiring and installation on difficult-to-wire areas Offices, public places, and homes Factories, vehicles, roads, and railroads Mobility Increases working efficiency and productivity Roaming support: extended on-line times -> universal access & seamless services Reduced installation time No cabling time Easy setup Standard enables interoperability between different vendors Roaming with GSM and UMTS is a research issue 9 WLAN Technology Challenges Flexible error control: in physical, MAC and/or in upper levels Physical level takes care of physical transmission of packets over a medium (modulation, line coding, channel coding) Interference & noise Working in ISM band means sharing the frequency bands with microwave oven, cordless telephones, Bluetooth etc. -> Modulation and MAC design challenge: Pros: Freedom from spectral regulatory constraints at ISM Band (Industrial, Science and Medical) Multi-path propagation Remedies: channel coding / rake-reception Dynamic network management Stations movable and may be operated while moved addressing and association procedures interconnections (roaming) 10 Challenges … MAC protocol takes care of optimizing throughput for the expected services Wireless channel is also the reason why access method for 802.11 is CSMA/CA and not CSMA/CD Difficult to detect collisions in wireless environment -> Hidden terminal problem (see PSTN lecture) Security Traditional WEP (Wired Equivalent Privacy) now replaced by WPA (Wi-Fi Protected Access) and 802.11i (WPA2) AAA (Authentication, Authorization, Accounting) can be taken care by a dedicated server as RADIUS (Remote Authentication Dial In User Service ) CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance CSMA/CD: Carrier Sense Multiple Access/Collision Detection 11 IEEE 802 LAN Standards LAN Basics WLAN Basics 802 LANs 802.11 Service Sets 802.11 LLC 802.11 MAC 802.11 PHY IEEE 802-series of LAN Standards 802 standards free to download from http://standards.ieee.org /getieee802 hub stations hub stations hub stations WiMAX hub router server Demand priority: A round-robin (token rings) method to provide LAN access based on message priority level DQDB: Distributed queue dual buss, see PSTN lecture 13 The IEEE 802 LAN Standards (http://www.ieee802.org/) OSI Layer 3 Network IEEE 802.2 Logical Link Control (LLC) LLC OSI Layer 2 (data link) b: Wi-Fi IEEE 802.3 IEEE 802.4 IEEE 802.5 IEEE 802.11 Carrier Token Token Wireless Sense Bus Ring Ethernet a b g Physical Layers - options: twisted pair, coaxial, optical, radio paths; (not for all MACs above!) Bus (802.3…) Star (802.3u…) MAC OSI Layer 1 (physical) Ring (802.5…) 14 IEEE 802.11 Wireless Local Area Networks (WLANs): Service Sets LAN Basics WLAN Basics 802 LANs 802.11 Service Sets 802.11 LLC 802.11 MAC 802.11 PHY 802.11 networks can work in Basic service set (BSS) Extended service set (ESS) BSS can also be used in ad-hoc Network LLC MAC FHSS DSSS IR networking Propagation boundary LLC: Logical Link Control Layer MAC: Medium Access Control Layer PHY: Physical Layer FHSS: Frequency hopping SS DSSS: Direct sequence SS SS: Spread spectrum IR: Infrared light BSS: Basic Service Set ESS: Extended Service Set PHY 802.xx IEEE 802.11 Architecture Internet Distribution system Station B Station A BSS 1 Basic (independent) service set (BSS) Access Point BSS 2 Extended service set (ESS) (infrastructure-mode) Portal: gateway access to other networks/Internet 16 Basic and Extended Service Sets Basic Service Set (BSS) – indoor radius of tens of meters with a single AP Operates in Basic Service Area (BSA) that is much like the area of a cell in cellular mobile communications BSSs may geographically overlap, be physically disjoint, or they may be collocated (one BSS may use several antennas) Ad-hoc or Infrastructure (nomadic) mode: Access coordinated by the MAC protocols Extended Service Set (ESS) Multiple BSSs interconnected by a Distribution System (DS) Each BSS is like a cell and stations in BSS communicate via an Access Point (AP) with the DS Portals attached to DS provide gateways as access to Internet or other ESS 17 Distribution system (DS) services DS provides distribution services: Transfer MAC SDUs between APs in ESS (I) Transfer MSDUs between portals & BSSs in ESS (II) Transfer MSDUs between stations in same BSS (III) Multicast, broadcast, or stations’s preference ESS looks like a single BSS to LLC layer Propagation boundary Internet II III SDU: Service Data Unit (inter-layer data) LLC: Logical Link Control Layer MAC: Medium Access Control Layer MSDU: MAC Service Data Unit PHY: Physical Layer FHSS: Frequency hopping SS DSSS: Direct sequence SS SS: Spread spectrum IR: Infrared light BSS: Basic Service Set ESS: Extended Service Set AP: Access Point III Distribution system Station B IIIb Station A BSS 1 Basic (independent) service set (BSS) Access Point I BSS 2 Extended service set (ESS) Portal: gateway access to other networks/Internet 18 IEEE 802.11 Mobility (b/g) Standard defines the following mobility types: No-transition: no movement or moving within a local BSS BSS-transition: station movies from one BSS in one ESS to another BSS within the same ESS ESS-transition: station moves from a BSS in one ESS to a BSS in a different ESS (continuos roaming not supported) Especially: 802.11 don’t support roaming with GSM! For fast, seamless roaming 802.11r - Address to destination mapping - seamless integration of multiple BSS ESS 1 ESS 2 19 IEEE 802 LAN Standard: Logical Link Layer (LLC) LAN Basics WLAN Basics 802 LANs 802.11 Service Sets 802.11 LLC 802.11 MAC 802.11 PHY 802.11 WLAN Architecture Logical Link Control (LLC) LLC provides addressing and data link control – common to all 802 LANs Utilizes services of HDLC (High-level Data Link Control) Therefore, LLC SAPs separate upper layer data exchanges => NIC applies different buffer segments for each SAP (port) LLC provides means to exchange frames between LANs using different MACs IEEE 802.2 Logical Link Control (LLC) LLC b: Wi-Fi IEEE 802.3 IEEE 802.4 IEEE 802.5 IEEE 802.11 Carrier Token Token Wireless Sense Bus Ring Ethernet MAC abg Physical layer: DSSS, FHSS, IR CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance LLC: Logical Link Control Layer MAC: Medium Access Control Layer SS: Spread Spectrum FHSS: Frequency hopping SS DSSS: Direct sequence SS IR: Infrared light NAV: Network Allocation Vector SAP: Service Access Point DCF: Distributed Coordination Function PCF: Point Coordination Function NIC: Network Interface Card PHY 21 Logical Link Control Layer (LLC) Specified by ISO/IEC 8802-2 (ANSI/IEEE 802.2) Objective: exchange data between users across LAN using 802-based MAC controlled link Provides addressing and data link control (routing) Independent of topology, medium, and chosen MAC access method Data to higher level protocols Info: carries user data Supervisory: carries flow/error control Unnumbered: carries protocol control data Source SAP LLC’s Protocol Data Unit (PDU) (SAP: Service Access Point) 22 SAP Addressing IEE802.11 (CSMA/CA)... IEE802.11 (CDMA)... ATM... Reference: W. Stallings: Data and Computer Communications, 7th ed 23 A TCP/IP Packet Encapsulation Control header TCP makes logical connection to deliver the packet LLC constructs PDU* by adding a control header SAP (service access point) MAC frame with new control fields Traffic to the target BSS / ESS *Protocol data unit MAC lines up packets by using a MAC protocol PHY layer transmits packet using a modulation method (DSSS, OFDM, IR, FHSS) 24 Encapsulation … Reference: W. Stallings: Data and Computer Communications, 7th ed 25 LLC Services A Unacknowledged connectionless service B Connection oriented service Point-to-point, multicast (assigned users), broadcast (group of users) addressing no error or flow control - no ack-signal higher levels take care or reliability - thus fast Often referred as ‘Unnumbered frame mode of HDLC*’ connection phases: Connection setup, data exchange, and release supports unicast only error/flow control (cyclic redundancy check (CRC)), sequencing ‘Asynchronous mode of HDLC’ C Acknowledged connectionless service Can handle several logical connections, distinguished by their SAPs ack-signal used error and flow control by stop-and-wait ARQ 26 faster setup than for B *High-Level Data Link Control IEEE 802.11 Wireless Local Area Networks (WLANs): Media Access Protocol LAN Basics WLAN Basics 802 LANs 802.11 Service Sets 802.11 LLC 802.11 MAC 802.11 PHY Selecting a Medium Access Control Environment: Wired / Wireless? Applications: What type of traffic? Voice streams? Steady traffic, low delay/jitter Data? Short messages? Web page downloads? Enterprise or consumer market? Reliability, cost Scale: How much traffic can be carried? How many users can be supported? Examples: Design MAC to provide wireless DSL-equivalent access for rural communities Design MAC to provide Wireless-LAN-equivalent access to mobile users (user in a car travelling at 130 km/h) 28 MAC techniques - examples Contention Medium is free for all, packet collisions do happen A node senses the free medium and occupies it as long as data packet requires it Example: Ethernet (IEEE 802.3 CSMA/CD) Reservation (short term statistical access) Gives everybody a turn Reservation time depends on token holding time (set by network operator) For heavy loaded networks Example: Token Ring/IEEE 802.5, Token Bus/IEEE 802.4, FDDI Reservation (long term) Link reservation for multiple packets (whole session) Example: scheduling a time slot: GSM using TDMA. FDMA applied for uplink/dowlink separation. Hybrid… (example: contention+reservation) Flexible compromise: 802.11 WLANs 29 Media Access Control (MAC): Ways to Share a Medium Medium sharing techniques Static channelization FDMA,TDMA, CDMA Uses partition medium Dedicated allocation to users Examples: Satellite transmission Cellular Telephone Dynamic medium access control Scheduling Medium sharing required for multiple users to access the channel Communications by unicasting multicasting broadcasting Random access (contention) Polling (take turns): Token ring 802.5 Reservation systems: Request for slot in transmission schedule 802.4 Loose coordination Send, wait, retry if necessary Aloha CSMA/CD (Ethernet) CSMA/CA (802.11 WLAN) 30 Example 802.3: MAC of Ethernet (CSMA/CD*) CSMA/CD: 1. If the medium is idle, transmit; otherwise, go to step 2 2. If the medium is busy, continue listening (CS: carrier sensing) until the channel is idle, then transmit immediately 3. If a collision is detected (CD) during transmission, transmit brief jamming signal to assure all stations know about collision and then cease transmission 4. After transmitting the jamming signal, wait a random time (back-off time), then attempt to transmit again *Carrier sense multiple access/collision detection 31 Throughput Performance of CSMA/CD a t prop R / L r L / R a : normalized delay-bandwidth product r : normalized load : aggregated rate [frames/second] r (Load) ~ throughput We can see that in Ethernet transfer delays grow very fast as the load increases for the given value of delay-bw product a. Note: Large value of parameter a scales results for propagation delay and/or signaling rate – if their product becomes larger, throughput (in terms of transfer delay) gets smaller. tprop: one-way delay, R: signaling rate, L: frame length Reference: A. Leon-Garcia, I. Widjaja, Communication Networks, 2nd ed 32 802.11 WLAN Architecture Medium Access Control (MAC) - Summary 802.11 MAC Services Station service: Authentication, privacy, MSDU* delivery Distributed system: Association**, participates to data distribution Transmits frames based on MAC addresses (in NIC) Connectionless/Connection oriented frame transfer service Coordinates access to medium Joining the network (NAV, addressing) MAC scheme CSMA/CA: Contention-free access (PCF) Contention access (DCF) IEEE 802.2 Logical Link Control (LLC) LLC b: Wi-Fi IEEE 802.3 IEEE 802.4 IEEE 802.5 IEEE 802.11 Carrier Token Token Wireless Sense Bus Ring Ethernet * MSDU: MAC service data unit ** with an access point in extended or basic service set (ESS,BSS) MAC abg Physical layer: DSSS, FHSS, IR PHY CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance LLC: Logical Link Control Layer MAC: Medium Access Control Layer SS: Spread Spectrum FHSS: Frequency hopping SS DSSS: Direct sequence SS IR: Infrared light NAV: Network Allocation Vector SAP: Service Access Point DCF: Distributed Coordination Function PCF: Point Coordination Function NIC: Network Interface Card 33 IEEE 802.11 Coordination Functions Reference: W. Stallings: Data and Computer Communications, 7th ed 34 Media Access Control in 802.11 WLANs Distributed Wireless Foundation MAC (DWFMAC): Distributed access control mechanism (CSMA/CA) Optional centralized control on top (PCF) MAC flavours provided by coordination functions: Distributed coordination function (DCF) – CSMA/CA Contention algorithm to provide access to all traffic Asynchronous, best effort-type traffic Application: bursty traffic, add-hoc networks Point coordination function (PCF) – polling principle (rarely applied in practical devices) Centralized MAC algorithm Connection oriented Contention free Built on top of DCF Application: timing sensitive, high-priority data 35 IEEE 802.11 MAC (DWFMAC): Timing in Basic Access duration depends on MAC load type duration depends on network condition MAC frame: Control, management , data + headers (size depends on frame load and type) Reference: W. Stallings: Data and Computer Communications, 7th ed PCF: Point Coordination Function (asynchronous, connectionless access) DCF: Distributed Coordination Function (connection oriented access) DIFS: DCF Inter Frame Space (minimum delay for asynchronous frame access) PIFS: PCF Inter Frame Space (minimum poll timing interval) SIFS: Short IFS (minimum timing for high priority frame access as ACK, CTS, MSDU…) MSDU: MAC Service Data Unit 36 IEEE 802.11 MAC Logic (DWFMAC) IFS: Inter Frame Space (= DIFS, SIFS, or PIFS) DWFMAC: Distributed Wireless Foundation duration depends on MAC load type MAC Reference: W. Stallings: Data and Computer Communications, 7th ed 37 DWFMAC summarized Collision Avoidance When station senses channel busy, it waits until channel becomes idle for DIFS period & then begins random backoff time (in units of idle slots) Station transmits frame when backoff timer expires If collision occurs, recompute backoff over interval Receiving stations of error-free frames send ACK Sending station interprets non-arrival of ACK as loss Executes backoff and then retransmits Receiving stations use sequence numbers to identify duplicate frames 38 Carrier Sensing in 802.11 MAC - Summary Physical Carrier Sensing Analyze all detected frames for errors Monitor relative signal strength from other sources Virtual Carrier Sensing at MAC sublayer (avoids hiddenterminal problem) Source stations inform other stations of transmission time (in msec) for an MPDU (MAC Protocol Data Unit) Carried in Duration field of RTS (Request to send) & CTS (Clear to send) Stations adjust their Network Allocation Vector (NAV) to indicate when the channel will become idle Channel busy if either sensing is busy Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set 39 Transmission of MPDU without RTS/CTS DIFS NAV: Network allocation vector DIFS: DCF Inter Frame Space (async) SIFS: SIFS: Short IFS (ack, CTS…) RTS: Request to send CTS: Clear to send MPDU: MAC Protocol Data Unit DCF: Distributed Coordination Function PCF: Point Coordination Function Data Source SIFS ACK Destination DIFS Other NAV Defer (postpone) access for other stations Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set Wait for Reattempt Time 40 Transmission of MPDU with RTS/CTS (DCF) NAV: Network allocation vector DIFS: DCF Inter Frame Space (async) SIFS: SIFS: Short IFS (ack, CTS…) RTS: Request to send CTS: Clear to send MPDU: MAC Protocol Data Unit DCF: Distributed Coordination Function PCF: Point Coordination Function Hidden terminal solution DIFS RTS Data Source SIFS CTS SIFS SIFS Ack Destination DIFS NAV (RTS) Other NAV (CTS) NAV (Data) Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set Defer access RTS: Request to Send CTS: Clear to Send 41 PCF Frame Transfer TBTT Fixed super-frame interval Contention-free repetition interval (PCF) SIFS B PIFS SIFS SIFS SIFS SIFS D2+Ac k+Poll D1 + Poll CF Contention period (DCF) End U2+ ACK U1+ ACK Reset NAV NAV CF_Max_duration D1, D2 = frame sent by point coordinator U1, U2 = frame sent by polled station TBTT = target beacon transmission time B = beacon frame (initiation) NAV: Network allocation vector PIFS: PCF Inter Frame Space DIFS: DCF Inter Frame Space (async) SIFS: SIFS: Short IFS (ack, CTS…) RTS: Request to send CTS: Clear to send MPDU: MAC Protocol Data Unit DCF: Distributed Coordination Function PCF: Point Coordination Function 42 Point Coordination Function PCF provides connection-oriented, contention-free service through polling Point coordinator (PC) in AP performs PCF Polling table up to implementer Contention free period (CFP) repetition interval Determines frequency with which contention free period occurs Initiated by beacon frame transmitted by Point Coordinator (PC) in AP During CFP stations may only transmit to respond to a poll from PC or to send ACK All stations adjust Network Allocation Vector (NAV) to indicate when channel will becomes idle Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set 43 MAC Frame Types Management frames Station association & disassociation with AP (this establishes formally BSS) Timing & synchronization Authentication & de-authentication (option for identifying other stations) Control frames Handshaking ACKs during data transfer Data frames Data transfer Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set 44 MAC Frame NOTE: This frame structure is common for all data send by a 802.11 station control info (WEP, data type as management, control, data ...) next frame duration frame ordering info for RX -Basic service identification BSSID* -source/destination address -transmitting station -receiving station *BSSID: a six-byte address typical for a particular access point (network administrator sets) CRC: Cyclic Redundancy Check WEP: Wired Equivalent Privacy frame specific, variable length frame check sequence (CRC) 45 IEEE 802.11 Wireless Local Area Networks (WLANs): Physical Level LAN Basics WLAN Basics 802 LANs 802.11 Service Sets 802.11 LLC 802.11 MAC 802.11 PHY 802.11 WLAN bands and technologies - summary IEEE 802.11 standards and rates IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band ) [FH, DS] IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi [QPSK] IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz band) [OFDM] IEEE 802.11g (2001 ... 2003) up to 54 Mbps (2.4 GHz) backward compatible to 802.11b [OFDM] IEEE 802.11 networks work on license free Industrial, Science, Medicine (ISM) bands: 26 MHz 902 EIRP power in Finland 928 83.5 MHz 2400 2484 100 mW 200 MHz 5150 5350 255 MHz 5470 200 mW indoors only 5725 f/MHz 1W EIRP: Effective Isotropically Radiated Power - radiated power measured immediately after antenna Equipment technical requirements for radio frequency usage defined in ETS 300 328 47 802.11-wireless LANs, Dec. ’07 Ref: http://en.wikipedia.org/wiki/802.11n 48 802.11 WLAN Architecture Physical Level (PHY) 802 Physical level specifies Star, bus or ring topology Cabling and electrical interfaces: Twisted pair, coaxial, fiber… Line coding (wired LANs) or modulation (WLANs) Three physical layers for 802.11 FHSS: Frequency Hopping Spread Spectrum (SS) DSSS: Direct Sequence SS IR: Infrared transmission IEEE 802.2 Logical Link Control (LLC) LLC b: Wi-Fi IEEE 802.3 IEEE 802.4 IEEE 802.5 IEEE 802.11 Carrier Token Token Wireless Sense Bus Ring MAC abg Ethernet Physical layers CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance LLC: Logical Link Control Layer MAC: Medium Access Control Layer SS: Spread Spectrum FHSS: Frequency hopping SS DSSS: Direct sequence SS IR: Infrared light NAV: Network Allocation Vector SAP: Service Access Point DCF: Distributed Coordination Function PCF: Point Coordination Function NIC: Network Interface Card PHY 49 Physical Level of 802.11: DSSS DSSS-transmitter 802.11 supports 1 and 2 Mbps data transmission, uses BPSK and QPSK modulation (802.11b,a,g apply higher rates) 802.11 applies 11 chips Barker code for spreading - 10.4 dB processing gain Defines 14 overlapping channels, each having 22 MHz channel bandwidth, from 2.401 to 2.483 GHz Power limits 1000mW in US, 100mW in EU, 200mW in Japan Immune to narrow-band interference, cheaper hardware PPDU:Baseband Data Frame Unit, BPSK: Binary Phase Shift Keying, QPSK: Quadrature PSK DSSS: Direct Sequence Spread Spectrum, PN:Pseudo Noise 50 Physical Level of 802.11: FHSS Supports 1 and 2 Mbps data transport and applies two level - GFSK modulation* (Gaussian Frequency Shift Keying) 79 channels from 2.402 to 2.480 GHz ( in U.S. and most of EU countries) with 1 MHz channel space 78 hopping sequences with minimum 6 MHz hopping space, each sequence uses every 79 frequency elements once Minimum hopping rate 2.5 hops/second Tolerance to multi-path, narrow band interference, security Low speed, small range due to FCC TX power regulation (10mW) * f f c f , f nom 160 kHz 51 26 MHz 902 Example: PHY of 802.11a 928 83.5 MHz 2400 2484 200 MHz 5150 5350 255 MHz 5470 5725 f/MHz Operates at 5 GHz band Supports multi-rate 6 Mbps, 9 Mbps,… up to 54 Mbps Uses Orthogonal Frequency Division Multiplexing (OFDM) with 52 subcarriers, 4 us symbols (0.8 us guard interval) Applies inverse discrete Fourier transform (IFFT) to combine multicarrier signals to single time domain symbol 52 Review questions LAN Basics WLAN Basics 802 LANs 802.11 Service Sets 802.11 LLC 802.11 MAC 802.11 PHY Review questions Logical link control (LLC) services in 802.11 What is the role of Distributed Coordination Function (DCF) and Point Coordination Function (PFC) in 802.11 MAC? Describe 802.3 MAC Scheme What is the basic difference between CSMA/CD and CSMA/CA? Which one is applied in 802.11 and why? Discuss factors than should be considered while choosing a medium access technique Carrier sensing in 802.11 MAC Mobility support in 802.11b/g MAC frame types 54 References and Supplementary Material - A. Leon-Garcia, I. Widjaja: Communication Networks (2th ed.) - W. Stallings: Data and Computer Communications, 7th ed - Kurose, Ross: Computer Networking (2th ed.) - Jim Geier: Wireless LANs, SAMS publishing - 802 Standards, IEEE Supplementary Material (distributed by Edita): HDLC: A. Leon-Garcia, I. Widjaja: Communication Networks, 2th ed.: pp. 333-340 WLANs: W. Stallings: Data and Computer Communications, 7th ed, pp. 544-568 55