S72 1130 Telecommunication Systems Wireless Local Area Networks

S-72. 1130 Telecommunication Systems Wireless Local Area Networks

Outline n n LAN basics n Structure/properties of LANs WLANs n Link layer services n Media access layer n frames and headers n CSMA/CA n Physical layer n frames n modulation n Frequency hopping n Direct sequence n Infrared Installation Security 2

S-72. 1130 Telecommunication Systems LAN Basics

What is a LAN? Local area means: n Freedom from regulatory constraints at ISM Band (Industrial, Science and Medical) n Short distance (~1 km) between computers n Low cost n High-speed (10 Mb/s. . 10 Gb/s); support for TCP or UDP type of communications n Flexible error control: in MAC and in upper levels n Computers move, machines have unique MAC address n MAC protocol takes care of optimizing throughput for the expected services n Physical level takes care of physical transmission of packets over a medium 4

Typical Wired LAN n n n Transmission Medium Network Interface Card (NIC) Unique MAC “physical” address Serial format 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 5

IEEE 802 -series of LAN Standards 802 standards free to download from http: //standards. ieee. org /getieee 802 n hub stations Wi. MAX hub router server Demand priority: A round-robin (see token rings-later) arbitration method to provide LAN access based on message priority level DQDB: Distributed queue dual buss, see PSTN lecture 6

Example: How Ring Networks Work n n n A node functions as a repeater A only destination copies frame to it, C A all other nodes have to discarded B transmits frame the frame addressed to A Unidirectional link A Signal propagates encoded by line codes A C Example: 802. 5 Problem in reliability if A copies frame a station fails A B C B A C ignores frame A B C absorbs returning frame 7

Token Ring n n A ring consists of a single or dual (FDDI) cable in the shape of a loop. Ring reservation supervised by rotating token. Each station is only 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 : n (1) Access method supports heavy load without degradation of performance because the medium is not shared. n (2) Several packets can simultaneous circulate between different pairs of stations. Disadvantages: n (1) Complex management - especially for several rings n (2) Re-initialization of the ring whenever a failure occurs 8

Example: Bus Network n n n 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 : n (1) Contention scheme : multiple nodes attempt to access bus; only one node succeed at a time (e. g. CSMA/CD in Ethernet 802. 3) n (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: n (1) Simple access method C D A B n (2) Easy to add or remove stations D term Disadvantages: - Line coded, serial data n (1) Poor efficiency with high - twisted pair or coaxial cable network load n (2) Relatively insecure, due to 9 the shared medium term: terminator impedance

S-72. 1130 Telecommunication Systems IEEE 802 LAN Standard

The IEEE 802 LAN Standards (http: //www. ieee 802. 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 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. 3 u…) MAC OSI Layer 1 (physical) Ring (802. 5…) 11

IEEE 802 Layers Logical Link Control (LLC) Sublayer n Utilizes services of HDLC (High-level Data Link Control) n Therefore, LLC SAPs separate upper layer data exchanges => NIC IEEE 802. 3 applies different buffer segments Carrier for each SAP (port) Sense n LLC provides means to exchange Ethernet frames between LANs using different MACs Medium Access Control Sublayer n Coordinates access to medium n Connectionless/Connection oriented frame transfer service n Machines identified by MAC/physical address (in NIC) n Broadcasts frames with MAC addresses n Examples: CSMA/CD, CSMA/CA Physical § § IEEE 802. 2 Logical Link Control (LLC) LLC b: Wi-Fi IEEE 802. 4 IEEE 802. 5 IEEE 802. 11 Token Wireless Bus Ring MAC abg Physical layers PHY level Star, bus or ring topology Cabling and electrical interfaces Twisted pair, coaxial, fiber Line coding (wired LANs) or modulation (WLANs) (More of HDLC in supplementary…) 12

S-72. 1130 Telecommunication Systems IEEE 802 LAN Standard: Logical Link Layer (LLC)

Logical Link Control Layer (LLC) n n 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) 14

LLC Protocol Data Unit (PDU) 1 byte Destination SAP Address 1 to 2 bytes Source SAP Address Control Source SAP Address Destination SAP Address C/R I/G 1 Information (network layer packet) 7 bits 1 I/G = Individual or group address C/R = Command or response frame Examples of SAP Addresses: 06 IP packet E 0 Novell IPX FE OSI packet AA Sub Network Access protocol (SNAP) IP Packet LLC header IP Packet MAC header LLC header IP Packet 7 bits FCS Packet encapsulation into a MAC frame FCS: Frame Check Sequence 15

LLC Services n n A Unacknowledged connectionless service n no error or flow control - no ack-signal usage n unicast (individual), multicast, broadcast addressing n higher levels take care or reliability - thus fast for instance for TCP n Unnumbered frame mode of HDLC B Connection oriented service n supports unicast only n error and flow control for lost/damaged data packets by cyclic redundancy check (CRC) n Asynchronous balanced mode of HDLC: error control, sequencing, flow control n Phases: Connection setup, data exchange, and release C Acknowledged connectionless service Problem: A workstation has a single, physical MAC address, how to separate network or higher level service access? Ans: HDLC SAP addressing: n Can handle several logical connections, distinguished by their SAP (service access points, next slides). n n n ack-signal used error and flow control by stop-and-wait ARQ faster setup than for B 16

A TCP/IP Packet in 802. 11: 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 *BDU: protocol data unit MAC lines up packets using by using a MAC protocol PHY layer transmits packet using a modulation method (DSSS, OFDM, IR, FHSS) 17

Encapsulation … Reference: W. Stallings: Data and Computer Communications, 7 th ed 18

SAP Addressing IEE 802. 11 (CSMA/CA). . . IEE 802. 11 (CDMA). . . ATM. . . Reference: W. Stallings: Data and Computer Communications, 7 th ed 19

S-72. 1130 Telecommunication Systems IEEE 802 LAN Standard: Media Access Control (MAC) Layer

Media Access Control: Ways to Share a Medium n Medium sharing techniques Static channelization n n FDMA, TDMA, CDMA Uses partition medium Dedicated allocation to users Examples: n Satellite transmission n Cellular Telephone Dynamic medium access control Scheduling n n n Medium sharing required for multiple users to access the channel Communications by n unicasting n multicasting n broadcasting Random access (contention) Polling (take turns): Token ring 802. 5 Reservation systems: Request for slot in transmission schedule 802. 4 n n n Loose coordination Send, wait, retry if necessary Aloha CSMA/CD (Ethernet) CSMA/CA (802. 11 WLAN) 21

Selecting a Medium Access Control n n Environment: Wired / Wireless? Applications: n What type of traffic? n Voice streams? Steady traffic, low delay/jitter n Data? Short messages? Web page downloads? n Enterprise or consumer market? Reliability, cost Scale: n How much traffic can be carried? n How many users can be supported? Examples: n Design MAC to provide wireless DSL-equivalent access for rural communities n Design MAC to provide Wireless-LAN-equivalent access to mobile users (user in car travelling at 130 km/hr) 22

MAC Techniques in LANs n n Contention n Medium is free for all n A node senses the free medium and occupies it as long as data packet requires it n Example: Ethernet (IEEE 802. 3 CSMA/CD) Reservation (short term statistical access) n Gives everybody a turn n Reservation time depends on token holding time (set by network operator) n For heavy loaded networks n Example: Token Ring/IEEE 802. 5, Token Bus/IEEE 802. 4, FDDI Mixed n Flexible compromise: 802. 11 WLANs Reservation (long term) n Link reservation for multiple packets (whole session) n Example: scheduling a time slot: GSM using TDMA of FDMA (uplink/dowlink) 23

Example 802. 3: MAC of Ethernet (CSMA/CD) n 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 24

Throughput Performance of CSMA/CD r (Load) We can see that in Ethernet transfer delays grow very fast as the load approaches the maximum possible value for the given value of a (tprop: one-way delay, R: signaling rate, L: frame length) Reference: A. Leon-Garcia, I. Widjaja, Communication Networks, 2 nd ed 25

S-72. 1130 Telecommunication Systems IEEE 802. 11 Wireless Local Area Networks (WLANs)

Why WLANs? n n Mobility n Increases working efficiency and productivity n Roaming support: extended on-line times -> universal access & seamless services No new wiring and installation on difficult-to-wire areas n Offices, public places, and homes n Factories, vehicles, roads, and railroads Increased reliability - several networks & nodes secure links n However, AAA (Authentication, Authorization, Accounting) challenging Reduced installation time n No cabling time n Easy setup 27

WLAN Technology Challenges n n n High date rates n IEEE 802. 11 b supports rates up to 11 MBps (in practice 6 Mb/s), and 802. 11 g reaches up to 54 Mb/s, need to have the bandwidth Interference n Working in ISM band means sharing the frequency bands with microwave oven, and Bluetooth. Modulation and MAC design challenge Security n Original WEP (Wired Equivalent Privacy) algorithm is weak – often not set ON by users, more efficient algorithms developed later Roaming, especially with GSM and UMTS Inter-operability between different vendors n Only few basic functionality are interoperable, other vendor’s features can’t be used in a mixed network 28

Requirements for 802. 11 Wireless LAN Standard n n n Dynamic network management n Stations movable and may be operated while moved n addressing and association procedures n interconnections (roaming) License free operation Wireless channel is unreliable n error control n security/secrecy n Wireless channel is also the reason why access method for 802. 11 is CSMA/CA and not CSMA/CD n Difficult to detect collisions in wireless environment n External interference, especially at ISM n Hidden terminal problem CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance CSMA/CD: Carrier Sense Multiple Access/Collision Detection 29

802. 11 WLAN Architecture Overview n n n LLC provides addressing and data link control – common to all 802 LANs IEEE 802. 2 LLC Logical Link Control (LLC) 802. 11 MAC provides b: Wi-Fi n Access to wireless medium IEEE 802. 3 IEEE 802. 4 IEEE 802. 5 n CSMA/CA (DCF) IEEE 802. 11 Carrier MAC Token Wireless n Contention-free access (PCF) Sense Bus Ring abg n Joining the network (NAV, addressing) Ethernet n Services Physical layers: DSSS, FHSS, IR. . . PHY n Station service: Authentication, privacy, MSDU* delivery CSMA/CA: Carrier Sense Multiple Access n Distributed system: Association**, with Collision Avoidance participates to data distribution LLC: Logical Link Control Layer MAC: Medium Access Control Layer Three physical layers (PHY) SS: Spread Spectrum n FHSS: Frequency Hopping Spread FHSS: Frequency hopping SS DSSS: Direct sequence SS Spectrum (SS) IR: Infrared light n DSSS: Direct Sequence SS NAV: Network Allocation Vector SAP: Service Access Point n IR: Infrared transmission *MSDU: MAC service data unit ** with an access point in ESS or BSS DCF: Distributed Coordination Function PCF: Point Coordination Function 30

S-72. 1130 Telecommunication Systems IEEE 802. 11 Wireless Local Area Networks (WLANs): Service Sets

n n 802. 11 networks can work in n Basic service set (BSS) n Extended service set (ESS) BSS can also be used in ad-hoc networking Network LLC MAC FHSS DSSS IR 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) Portal: gateway access to other networks/Internet 32

Basic and Extended Service Sets n Basic Service Set (BSS) – tens of meters Operate in Basic Service Area (BSA) that is much like the are of cell in mobile communications n BSSs may geographically overlap, be physically disjoint, or they may be collocated (one BSS may use several antennas) n Ad-hoc or Infrastructure (nomadic) mode: Access coordinated by the given instance of MAC Extended Service Set (ESS) n Multiple BSSs interconnected by Distribution System (DS) n Each BSS is like a cell and stations in BSS communicate with an Access Point (AP). n Portals attached to DS provide gateways as access to Internet or other ESS n n 33

Distribution system (DS) services n n DS provides distribution services: n Transfer MAC SDUs between APs in ESS (I) n Transfer MSDUs between portals & BSSs in ESS (II) n Transfer MSDUs between stations in same BSS (III) n Multicast, broadcast, or stations’s preference ESS looks like a single BSS to LLC layer 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 MSDU: MAC Service Data Unit AP: Access Point Internet II III IIIb Distribution system Station B 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 34

IEEE 802. 11 Mobility n n Standard defines the following mobility types: n No-transition: no movement or moving within a local BSS n BSS-transition: station movies from one BSS in one ESS to another BSS within the same ESS n 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! - Address to destination mapping - seamless integration of multiple BSS ESS 2 ESS 1 35

S-72. 1130 Telecommunication Systems IEEE 802. 11 Wireless Local Area Networks (WLANs): Media Access Protocol

Hidden Terminal Problem (a) A C Data Frame A transmits data frame B (b) Data Frame B A n New MAC: CSMA with Collision Avoidance Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set C senses medium, station A is hidden from C Data Frame C C transmits data frame & collides with A at B RTS: Request to Send CTS: Clear to Send 37

CSMA with Collision Avoidance (a) B RTS C A requests to send (b) CTS B CTS A C B announces A ok to send (c) Data Frame A sends Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set B C remains quiet RTS: Request to Send CTS: Clear to Send 38

IEEE 802. 11 Coordination Functions Reference: W. Stallings: Data and Computer Communications, 7 th ed 39

Media Access Control in 802. 11 WLANs n n Distributed Wireless Foundation MAC (DWFMAC): n Distributed access control mechanism (CSMA/CA) n Optional centralized control on top (PCF) MAC flavours provided by coordination functions: n Distributed coordination function (DCF) - CSMA n Contention algorithm to provide access to all traffic n Asynchronous, best effort-type traffic n Application: bursty traffic, add-hoc networks n Point coordination function (PCF) – polling principle n Centralized MAC algorithm n Connection oriented n Contention free n Built on top of DCF n Application: timing sensitive, high-priority data 40

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, 7 th 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 41

Collisions, Losses & Errors n n Collision Avoidance n When station senses channel busy, it waits until channel becomes idle for DIFS period & then begins random backoff time (in units of idle slots) n Station transmits frame when backoff timer expires n If collision occurs, recompute backoff over interval Receiving stations of error-free frames send ACK n Sending station interprets non-arrival of ACK as loss n Executes backoff and then retransmits n Receiving stations use sequence numbers to identify duplicate frames 42

IEEE 802. 11 MAC Logic (DWFMAC) IFS: Inter Frame Space (= DIFS, SIFS, or PIFS) DWFMAC: Distributed Wireless Foundation MAC Reference: W. Stallings: Data and Computer Communications, 7 th ed 43

Carrier Sensing in 802. 11 MAC n Physical Carrier Sensing Analyze all detected frames n Monitor relative signal strength from other sources Virtual Carrier Sensing at MAC sublayer n Source stations informs other stations of transmission time (in msec) for an MPDU n Carried in Duration field of RTS & CTS n Stations adjust Network Allocation Vector (NAV) to indicate when channel will become idle Channel busy if either sensing is busy n n n Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set 44

Transmission of MPDU without RTS/CTS DIFS NAV: Network allocation vector DIFS: DCF Inter Frame Space (async) 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 Access Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set Wait for Reattempt Time 45

Transmission of MPDU with RTS/CTS DIFS RTS Data NAV: Network allocation vector DIFS: DCF Inter Frame Space (async) 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 Source SIFS CTS 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 46

PCF Frame Transfer Fixed super-frame interval TBTT Contention-free (CF) repetition interval SIFS B PIFS SIFS CF D 2+Ac k+Poll D 1 + Poll Contention period (DCF) End U 2+ ACK U 1+ ACK Reset NAV CF_Max_duration D 1, D 2 = frame sent by point coordinator U 1, U 2 = frame sent by polled station TBTT = target beacon transmission time B = beacon frame NAV: Network allocation vector DIFS: DCF Inter Frame Space (async) 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 47

Point Coordination Function n n PCF provides connection-oriented, contention-free service through polling Point coordinator (PC) in AP performs PCF Polling table up to implementor Contention free period (CFP) repetition interval n Determines frequency with which CFP occurs n Initiated by beacon frame transmitted by Point Coordinator (PC) in AP n 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 48

MAC Frame Types n n n Management frames n Station association & disassociation with AP (this establishes formally BSS) n Timing & synchronization n Authentication & deauthentication (option for identifying other stations) Control frames n Handshaking n ACKs during data transfer Data frames n Data transfer Reference: A. Leon-Garcia, I. Widjaja, Communication Networks , Instructor's Slide Set 49

MAC Frame n 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* -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) 50

S-72. 1130 Telecommunication Systems IEEE 802. 11 Wireless Local Area Networks (WLANs): Physical Level

802. 11 WLAN bands and technologies n n IEEE 802. 11 standards and rates n IEEE 802. 11 (1997) 1 Mbps and 2 Mbps (2. 4 GHz band ) [FH, DS] n IEEE 802. 11 b (1999) 11 Mbps (2. 4 GHz band) = Wi-Fi [QPSK] n IEEE 802. 11 a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz band) [OFDM] n IEEE 802. 11 g (2001. . . 2003) up to 54 Mbps (2. 4 GHz) backward compatible to 802. 11 b [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 m. W 200 MHz 5150 5350 255 MHz 5470 200 m. W indoors only 5725 f/MHz 1 W EIRP: Effective Isotropically Radiated Power - radiated power measured immediately after antenna Equipment technical requirements for radio frequency usage defined in ETS 300 328 52

Physical Level of 802. 11: DSSS-transmitter n n n 802. 11 supports 1 and 2 Mbps data transport, uses BPSK and QPSK modulation (802. 11 b, a, g apply higher rates) 802. 11 applies 11 chips Barker code for spreading - 10. 4 d. B processing gain Defines 14 overlapping channels, each having 22 MHz channel bandwidth, from 2. 401 to 2. 483 GHz Power limits 1000 m. W in US, 100 m. W in EU, 200 m. W 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 53

Physical Level of 802. 11: FHSS n n n 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 (10 m. W) 54

Example: PHY of 802. 11 a n n 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 55

References and Supplementary Material - A. Leon-Garcia, I. Widjaja: Communication Networks (2 th ed. ) - W. Stallings: Data and Computer Communications, 7 th ed - Kurose, Ross: Computer Networking (2 th ed. ) - Jim Geier: Wireless LANs, SAMS publishing - 802 Standards, IEEE Supplementary Material: n HDLC: A. Leon-Garcia, I. Widjaja: Communication Networks, 2 th ed. : pp. 333 -340 n WLANs: W. Stallings: Data and Computer Communications, 7 th ed, pp. 544 -568 56