Chapter 5 IEEE WLANs

Transcription

1 Chapter 5 IEEE WLANs Nov WLAN architecture Two types of topologies: single-hop ad hoc network and infrastructure network An ad hoc network forms an independent basic service set (IBSS) and cannot communicate with the external world An Access Point (AP) in an infrastructure network acts as a hub and connects the basic service set (BSS) network to an extended service set (ESS) network The architectural component used to connect two BSSs is called a distribution system Services provided by DS: distribution, integration, association, reassociation, disassociation 2

2 Network topologies Infrastructure network ad hoc network 3 Protocol stack Data link layer Physical layer LLC MAC PLCP PMD MAC management PHY management MAC layer and PHY layer are specified in MAC is divided into MAC and MAC Management PHY consists of three sublayers: PLCP (PHY layer convergence protocol), PMD (PHY medium dependent) and PHY layer management 4

3 Sub-layer responsibilities Data link layer Physical layer LLC MAC PLCP PMD MAC management PHY management MAC: access mechanism, fragmentation, encryption MAC layer management: roaming in ESS, power management, asso- disasso- reasso- ciation 5 Sub-layer responsibilities Data link layer Physical layer LLC MAC PLCP PMD MAC management PHY management PLCP: carrier sensing assessment, forming packets for PHYs PMD: modulation and coding PHY layer management: channel tuning 6

4 IEEE PHY Layer Two sub-layers: Physical Layer Convergence Protocol (PLCP) adapts the MPDU to different PMD and produces Clear Channel Assessment (CCA) for carrier sensing Physical Medium Dependent (PMD) responsible for signaling with the medium Several options Frequency Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (DSSS) Diffused Infra Red (DFIR) OFDM: orthogonal frequency division multiplexing, 54Mb/s CCK: complementary code keying, 5.5Mb/s and 11Mb/s 7 Frequency Hopping Spread Spectrum (FHSS) FHSS PHY consists of two protocol functions: A physical layer convergence function, which adapts the capabilities of the physical medium dependent (PMD) system to the PHY service: PLCP A PMD system, whose function defines the characteristics of, and method of transmitting and receiving data through a wireless medium between two and more STAs (STA: STAtion) Three entities: PLCP sublayer: simplifies the provision of a PHY service interface to MAC services Physical layer management entity (PLME): performs management of local PHY functions PMD sublayer: provides a transmission interface 8

5 Frequency Hopping Spread Spectrum (FHSS) North America GHz GHz Japan GHz GHz 9 PLCP sublayer This sublayer provides a convergence procedure to map MPDUs into a frame format designed for FHSS radio transceivers The PLCP protocol data unit (PPDU) frame format provides for the asynchronous transfer of MAC sublayer MPDUs from any transmitting STA to all receiving STAs within the wireless LAN s BSS The PPDU consists of three parts: preamble, header and PSDU 10

6 PLCP The PLCP preamble consists of two separate parts: the preamble synchronization field and start frame delimiter (SFD), to allow the PHY circuitry to reach steady-state demodulation and synchronization of bit clock and frame start SYNC is an 80-bit field containing an alternating 1-0 pattern, transmitted starting with 0, used by the PHY sublayer to detect a potentially received signal, reach a steady state frequency offset correction and synchronization SFD: Used to define frame timing 11 PLCP PLCP Header field PSDU length word: 12 bits, specifies the number of octets contained in the PSDU PLCP signaling field (PSF): 3 bits, indicates the data rate Header error check (HEC) field: 16 bits, to check the integrity of the header 12

7 PLCP signaling field Bit Parameter name Parameter values Description 0 reserved Default=0 reserved 1:3 PLCP_BITRATE b1 b2 b3 =Data Rate =1.0 Mbit/s =1.5 Mbit/s =2.0 Mbit/s =2.5 Mbit/s =3.0 Mbit/s This field indicates the data rate of the whitened PSDU from 1 Mbit/s to 4.5 Mbit/s in 0.5 Mbit/s increments =3.5 Mbit/s =4.0 Mbit/s =4.5 Mbit/s 13 Operating frequency range 79 channels provided in US and Europe, the frequency for channel k is 2402+k MHz Hopping rate: 2.5 hops per second 14

8 Hop sequences The hopping sequence of an individual PMD entity is used to create a pseudorandom hopping pattern. Sets of hopping sequences are used to co-locate multiple PMD entities in the same area to enhance the overall efficiency An FH pattern, F x, consists of a permutation of all frequency channels. For a given number, x, the hopping sequence can be written as: F x ={f x (1), f x (2),.., f x (p)} where f x (i) is the channel number for i th frequency in x th hopping pattern P is the number of frequency channels in hopping pattern f x (i)=[b(i)+x) mod (79) Base-hopping sequence b(i) 16

9 Hopping pattern set The hopping pattern numbers x are divided into three sets to avoid prolonged collision periods between different hopping sequences in a set For 79 channel system: For 23 channel system: 17 Direct sequence spread spectrum (DSSS) PHY The DSSS PHY system operates in 2.4 GHz band Supports 1Mb/s and 2Mb/s data connections Chipping rate 11MHz with 11-chip PN code (Barker code) Modulation scheme: phase shift keying 1Mb/s, DBPSK; 2Mb/s, DQPSK Three functional entities: PLCP sublayer, PMD sublayer and physical layer management entity (PLME) 18

10 DSSS PLCP sublayer This sublayer provides a convergence procedure in which MPDUs are converted to and from PPDUs 19 PLCP frame format SYNC field: 128 bits. For the receiver to perform necessary operations for synchronization Start frame delimiter (SFD): 16 bits, to indicate the start of frame 20

11 PLCP frame format (cont d) PLCP signal field: 8 bits, to indicate to the PHY the modulation that shall be used for transmission of the MPDU X 0A : 1Mb/s, DBPSK X 14 : 2Mb/s, DQPSK PLCP service: 8 bits, reserved 21 PLCP frame format (cont d) Length: 16 bits, indicates the number of microseconds required for transmitting the MPDU CRC: 16 bits, for header protection 22

12 DSSS PMD sublayer 11-chip Barker sequence is used as the PN code sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 Two modulation formats and data rates are specified for DSSS PHY: base access rate and enhanced access rate The basic access rate is on 1 Mb/s DBPSK modulation Enhanced access rate is for 2Mb/s DQPSK 23 Operating frequency range The DSSS PHY shall operate in the frequency range of 2.4GHz to GHz Channel spacing: 5MHz 24

13 FHSS VS DSSS FHSS is basically a narrowband system that is easier to implement and consumes less power DSSS provides better coverage and a more robust received signal RAKE implementation of DSSS improves the performance 25 MAC services Asynchronous data services: this service provides peer LLC entities with the ability to exchange MAC service data units (MSDUs) Best effort, connectionless Broadcast and multicast transport Two service classes: strictly ordered service and reorder-able multicast service Security services: provided by the authentication service and the WEP mechanism Limited to station-to-station data exchange Confidentiality Authentication Access control in conjunction with layer management 26

14 Frame control MAC frame format Octets: Duration Address 1 Address 2 Address 3 /ID Sequence control Address 4 Frame body FCS MAC Header Each frame consists of three basic components: A MAC header, which comprises frame control, duration, address, and sequence control information A variable length frame body, which contains information specific to the frame type A frame check sequence (FCS), which contains an IEEE 32-bit cyclic redundancy code (CRC) 27 Frame control field B0 B1B2 B3B4 B7 B8 B9 B10 B11 B12 B13 B14 B15 Protocol Type Subtype To From More Retry Pwr More WEP Order Version DS DS Frag Mgt Data Protocol version field: 2 bits, current version: 0 Type and subtype: used to identify the function of the frame. There are three frame types: control, data and management. Each frame type has several subtypes To DS: set to 1 in data type frames destined for the DS From DS: set to 1 in data type frames exiting the DS More Fragments: set to 1 in all data or management type frames that have another fragment of the current MSDU 28

15 Frame control field (cont d) B0 B1B2 B3B4 B7 B8 B9 B10 B11 B12 B13 B14 B15 Protocol Type Subtype To From More Retry Pwr More WEP Order Version DS DS Frag Mgt Data Retry: set to 1 in any data or management type frame that is a retransmission of an earlier frame Power management: used to indicate the power management mode of a STA. A value of 1 indicates that the STA will be in power-save mode More data: used to indicate a STA in power-save mode that more MSDUs, or MMSDUs are buffered for that STA in AP 29 Frame control field (cont d) B0 B1B2 B3B4 B7 B8 B9 B10 B11 B12 B13 B14 B15 Protocol Type Subtype To From More Retry Pwr More WEP Order Version DS DS Frag Mgt Data WEP: set to 1 if the frame body field contains information that has been processed by the WEP algorithm Order: set to 1 in any data type frame that contains an MSDU which is transferred using StrictlyOrdered service class 30

16 Other frame fields Duration/ID field: contains a duration value defined for each frame type Address fields: to indicate the BSSID, source address, destination address, transmitting station address, and receiving station address Sequence control field: Sequence number: each MSDU or MMSDU transmitted by a STA is assigned a sequence number from 0 to 4095, incremented by 1 Fragment number: indicates the number of each fragment of an MSDU or MMSDU B0 B3 B4 B15 Fragment Number Sequence Number 31 Other frame fields Octets: Frame control Duration Address 1 Address 2 Address 3 /ID Sequence control Address 4 Frame body FCS MAC Header Duration/ID field: contains a duration value defined for each frame type Address fields: to indicate the BSSID, source address, destination address, transmitting station address, and receiving station address 32

17 Other frame fields (cont d) Octets: Frame control Duration Address 1 Address 2 Address 3 /ID Sequence control Address 4 Frame body FCS MAC Header Sequence control field: Sequence number: each MSDU or MMSDU transmitted by a STA is assigned a sequence number from 0 to 4095, incremented by 1 Fragment number: indicating the number of each fragment of an MSDU or MMSDU B0 B3 B4 B15 Fragment Number Sequence Number 33 Format for individual frame types Request to send (RTS) frame: Octets: Frame control Duration RA TA FCS Clear to send (CTS) frame: Octets: Frame control Duration RA FCS Acknowledgement (ACK) frame: Octets: Frame control Duration RA FCS 34

18 Data frame format Octets: Frame control Duration Address 1 Address 2 Address 3 /ID Sequence control Address 4 Frame body FCS MAC Header 35 MAC architecture Includes the distributed coordination function (DCF), and the point coordination function (PCF) Required for Contention Free Services MAC Extent Point Coordination Function (PCF) Distributed Coordination Function (DCF) Used for Contention Services and basis for PCF 36

19 Distributed coordination function (DCF) Carrier sense multiple access with collision avoidance (CSMA/CA) Implemented in all STAs For an STA to transmit, it shall sense the medium to determine if another STA is transmitting A gap of a minimum specified duration exist between contiguous frame sequences If the medium is busy, the STA shall select a random backoff delay A refinement uses short control frames (RTS and CTS) 37 Point coordination function (PCF) Only usable for infrastructure network configurations A point coordinator (PC), operating at the access point, determines which STA currently has the right to transmit Polling Virtual carrier sense mechanism: setting network allocation vector (NAV) Contention-free (CF) access 38

20 Fragmentation/defragmentation Fragmentation is used for increasing reliability Each fragment can be transmitted independently Defragmentation is performed at the receiver 39 DCF DCF allows for automatic medium sharing between compatible PHYs through the use of CSMA/CA and a random backoff time CSMA/CA is designed to reduce collisions Carrier sense can be performed both through physical and virtual mechanisms Virtual carrier-sense is achieved by distributing reservation information announcing the impending use of the medium Exchange of RTS/CTS Fast collision inference and a transmission path check Hidden terminal problem solved RTS/CTS cannot be used with broadcast and multicast addresses 40

21 Carrier-sense mechanism Two carrier-sense mechanisms adopted: virtual and physical Either one indicates the medium is busy, it should be considered busy A physical carrier-sense mechanism is provided by PHY Virtual carrier-sense mechanism is provided by network allocation vector (NAV) NAV maintains a prediction of future traffic on the medium based on duration information announced Carrier-sense mechanism combines the NAV state and the STA s transmitter status with physical carrier-sense to determine the medium state 41 Interframe space (IFS) Four IFSs: SIFS: short interframe space PIFS: PCF interframe space DIFS: DCF interframe space EIFS: extended interframe space 42

22 Interframe space (IFS) (cont d) SIFS: used for an ACK frame, a CTS frame, the second or subsequent MPDU of a fragment burst, and by an STA responding to any polling by the PCF SIFS is used when STAs have seized the medium and need to keep it for the duration of the frame exchange sequence to be performed Priority is given for completion of the frame exchange sequence in progress 43 Interframe space (IFS) (cont d) PIFS: used only by STAs under the PCF to gain access to the medium during CFP (Contention-Free Period) DIFS: used by STAs operating under the DCF to transmit data frames and management frames EIFS: used by the DCF whenever the PHY has indicated to the MAC that a frame transmission was begun that did not result in correct reception of a complete MAC frame with a correct FCS value 44

23 Random backoff time When an STA desiring to initiate transfer of data MPDU and/or management MMPDUs senses the medium busy, it shall defer until the medium is idle. Then after waiting for another period of time equal to DIFS or EIFS, the STA shall generate a random backoff period for an additional deferral before transmitting Backoff time = Random()*aSlotTime Random() [0, CW] where CW is the contention window. CWmin CW CWmax. Initial value of CW = CWmin CW should be reset to CWmin after every successful attempt to transmit an MSDU or MMPDU With an unsuccessful attempt, CW should be sequentially ascending integer powers of 2, minus 1 45 An example of exponential increase of CW 46

24 DCF basic access procedure If medium is idle, and is still idle after a DIFS, transmit If medium is busy, defer until this transmission is complete, wait for another DIFS or EIFS. Then delay a random backoff time, then transmit For FH PHY, if the dwell time is not enough for transmitting the MPDU and ACK (if required), the STA shall defer the transmission by selecting a random backoff time 47 Backoff procedure When an STA senses the medium busy, random backoff procedure is invoked An STA performing the backoff procedure should use the carriersense mechanism to determine whether there is activity during each backoff time slot. If no, decrement the backoff timer Otherwise, backoff procedure suspended 48

25 Setting and resetting NAV STAs receiving a valid frame shall update their NAVs with the information received in the Duration/ID field, but only when the new NAV value is greater than the current NAV value. 49 Control of the channel Fragment burst SIFS SIFS SIFS SIFS SIFS Source Destination Fragment 0 Fragment 1 Fragment 2 ACK 0 ACK 1 ACK 2 SIFS is used to provide an efficient MSDU delivery mechanism. Once the STA has contended for the channel, that STA shall continue to send fragments until all fragments have been sent An STA shall transmit after the SIFS only when: The STA has just received a fragment that requires ACK The source STA has received an ACK for the previous fragment and has more fragment(s) to send 50

26 RTS/CTS usage with fragmentation The RTS/CTS frames define the duration of the following frame and ACK The duration/id field in the data and ACK frames specifies the total duration of the next fragment and ACK Other NAV(RTS) NAV(CTS) NAV(Fragment 0) NAV(Fragment 1) NAV(ACK 0) NAV(ACK 1) SIFS SIFS SIFS SIFS SIFS SIFS SIFS RTS Source Frag. 0 Frag. 1 Frag. 2 Destination CTS ACK 0 ACK 1 ACK 2 51 PCF Contention-free frame transfer Only for infrastructure network CF-pollable STA can transmit MPDU after being polled CF-pollable STA should not retransmit until polled Non-CF-pollable STA only reply ACK after receiving MPDU Data frames sent: PC: data+cf-poll, data+cf-ack+cf-poll, CF-poll, CF- Ack+CF-Poll PC or CF-pollable STA: data, data+cf-ack, null function, CF-Ack 52

27 CFP structure and timing CFP alternates with CP CFP begins with a beacon that contains a DTIM element CFPs occur at a predefined repetition rate Length of the CFP is controlled by the PC PC ends a CFP by sending CF-End or CF-End+Ack Delay (due to busy medium) B CF Period CFP repetition interval PCF Variable length (per superframe) CP DCF Busy medium B Foreshortened CFP CF Period PCF Contention Period DCF NAV 53 PCF access procedure Based on polling PC maintains control for the entire CFP by waiting a shorter time between transmissions than the STAs using DCF NAV is set to prevent most contentions At the beginning of each CFP, the PC senses the medium When the medium is idle for one PIFS period, beacon is sent to announce the beginning of CFP After the initial beacon frame, the PC waits for at least one SIFS period, then transmits one frame No traffic and poll to send, a CF-End should be sent 54

28 NAV operation during the CFP Each STA, except the STA with PC, shall preset its NAV to the CFPMaxDuration value at each target beacon transmission time at which a CFP is scheduled to start Each non-pc STA shall update its NAV using the CFPDurRemaining value in each beacon frame the STA receives Prevents STAs from taking control of the medium during the CFP The PC shall transmit CF-End or CF-End+Ack at the end of each CFP. An STA receiving CF-End should reset its NAV CFP_Dur_Remaining Value in beacon CFP CP CFP Beacons DTIM DTIM DTIM 55 NAV PIFS PCF transfer procedure Frame transfers under PCF typically consist of frames alternately sent from and to the AP/PC In an STA having an FH PHY, channel control is lost at a dwell time boundary. It is required that the current MPDU transmission and the accompanying Ack be transmitted before the dwell time boundary Beacon D1+poll U1+ack Contention-free Repetition Interval Contention-free Period D2+ack +poll U2+ ack SIFS SIFS SIFS SIFS SIFS D3+ack +poll PIFS SIFS SIFS D4 +poll No response To CF-Poll U4+ ack CF-End Contention Period Reset NAV CF_Max_Duration 56