Wireless LAN IEEE WLAN Standard. Muhammad Jaseemuddin Ryerson University. Application. Presentation. Session. Transport.

Transcription

1 Wireless LAN Muhammad Jaseemuddin Ryerson University IEEE WLAN Standard Application Presentation Session Transport Network Data Link LLC MAC LLC Layer - IEEE MAC Layer - IEEE Carrier Sense Multiple Access (CSMA) Virtual Collision Detection (VCD) Asynchronous Data Service Time-bounded Service Error Correction, Access Control Encryption, Roaming, Power Saving PHY Layer - IEEE Radio 900MHz, 2.4GHz & 5.8GHz Frequency Hopping Spread Spectrum Direct Sequence Spread Spectrum 1, 2, 5.5 & 11Mbps Data Rates m Transmission Range Physical

2 Ad-Hoc Network STA1 STA2 STA3 Basic Service Set (BSS) - BSSID Infrastructure Network Distribution System STA1 BSS1 STA2 STA BSS2 Cellular Structure Cells operating in different frequency channel Roaming across BSS through Distribution System

3 Radio Frequency Spectrum The Industrial Scientific and Medical (ISM) Bands in N. America 900MHz 928MHz GHz GHz 5.725GHz 5.850GHz 900MHz 2.4GHz 5.8GHz FHSS IEEE PHY divides ISM band into a series of 1-MHz channels Approximately 99% of the radio energy is confined to the channel Channel sequence starts from GHZ with a step of 1 goes up to for total 95 channels In NA 78 channels are permitted from channel 2 (2.402) to channel 79 (2.479) An FH pattern Fxconsists of a permutation of all 79 channels, given as Fx = {fx(1), fx(2),, fx(79)} Where Fx is the FH pattern fx(i) is the channel number for the ith frequency in the xth FH pattern fx(i) = [b(i) + x] mod(79) + 2 The sequences are designed to ensure some minimum distances between the frequencies of the contiguous hops 6-MHZ gap in NA

4 FHSS PHY The FH patterns are divided into three sets The sets are defined to avoid prolonged collision periods between different sequences in a set Each set contains 26 patterns for NA S1: x = {0,3,6,9,12,15,18,21,, 72, 75} S2: x = {1,4,7,10,13,16,19,22,, 73,76} S3: x = {2,5,8,11,14,17,20,23,, 74,77} 2.5 hops per second yields maximum dwell time to be 390 TUs ~ 0.4 second Beacon Frame contains time stamp and FH Parameter Set element Hop Set # Hop pattern # Hop index All STAs in the IBSS synchronized their clock with the TBTT using beacon advertised timestamp They all tuned to the same FH pattern advertised in the beacon Hop occurs when the timestamp modulo dwell time becomes zero DSSS IEEE b PHY It requires more power to achieve the same throughput than the FH SS It is readily adaptable to much higher data rate adopted 11-bit Barker code { } It is tolerant to multipath delay spread Each data bit is encoded using the entire Barker word as the chipping sequence The DS PHY has 14 channels each 5MHz wide Channel 1: GHz, channel 2: GHz and so on up to channel 14: GHz In NA 11 channels are allowed: 1-11 ( GHz) Within a channel most of the energy is spread across a 22-MHz band To prevent interference caused by networks operating in adjacent channels IBSSes are required to operate on center frequencies that are 22-MHz apart With 5-MHz channel spacing it means the adjacent IBSSes must operate with 5 channels apart Typically they are configured at channels 1,6 and 11 Only three IBSSes can be adjacent

5 DS Channels (22 MHz Wide) Channel Center NA-ANZ ETSI Japan Frequency x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Source: CISCO WLAN Adapters Software Guide DS PHY was announced in 1997 allowed 1 or 2 bits encoding per Barker word resulting into 1-2 Mbps The system is capable of processing 1 million chips per second b as announced in 1999 allowing 5 or 8 bits encoding per Barker word resulting into 5.5 to 11 Mbps bandwidth Using different encoding technique

6 Medium Access Control Why not CSMA/CA? Collision detection is difficult in radio environment Stations may interfere from other LANs (BSS) Hidden node problem Distributed Co-ordinated Function (DCF) For asynchronous data service CSMA/CA Virtual Collision Detection (VCD) Point Co-ordinated Function (PCF) For time-bounded data service Access Point (AP) serves as the co-ordinator Carrier Sensing Two carrier sensing mechanisms are defined A physical carrier sensing Depending upon the PHY layer, it senses the availability of the carrier frequency A virtual carrier sensing This is a logical carrier sensing at the MAC layer Every packet (with some exceptions) announces the duration for which the current transmission will hold the channel it is called Network Allocation Vector (NAV) All stations monitoring the channel read the MAC header, which contains the NAV. They all backoff for NAV microseconds before starting the contention for the next transmission

7 Virtual Carrier Sensing Sender Unicast Frame NAV ACK Data Access to medium is deferred Every unicast frame contains NAV value, which indicates the time in microseconds this transaction will take including the time for ACK All other monitoring stations will next sense the medium after NAV and the subsequent Basic Transmission Algorithm NAV=0? Sense the medium (perform physical channel assessment) Medium Idle? Random Backoff Time Transmit Frame Collision?

8 Medium Acces and IFS PIFS medium busy contention next frame PIFS Slot time Short Inter-Frame Spacing PCF Inter-Frame Spacing = + slot time DCF Inter-Frame Spacing = + 2*slot time For DSS = 10 µs Slot time = 20 µs acwmin = 31 acwmax = 1023 Exponential Back-off random back-off time within a contention window [0, CW] contention window size increases with retransmission back-off time = random() * slot time random() = a pseuodo random integer in [0,CW] acwmin <= CW <= acwmax, CW starts with acwmin and increases by every retransmission upto acwmax, and is reset after successful transmission time DSS Contention Window Initial Transmission medium busy CW = 31 slots 1st Retransmission medium busy CW = 63 slots time 2nd Retransmission medium busy CW = 127 slots time 3rd Retransmission medium busy CW = 255 slots time 4th Retransmission medium busy CW = 511 slots time 5th Retransmission medium busy CW = 1023 slots time 6th Retransmission CW = 1023 slots time

9 Transmission Mode (CSMA/CA) Station 1 bo e bo r bo e bo r bo e busy Station 2 bo e busy Station 3 busy Station 4 bo e busy bo e bo r Station 5 bo e bo r bo e busy bo e bo r Source: Mobile Communications - Jochen Schiller The Hidden Node Problem The Hidden Node problem occurs when two clients exist that can both connect to an AP but cannot see each other This can cause as much as 40% data loss through collisions and re-transmissions. Using VCD (the RTS/CTS mechanism) avoids these problems. Collision STA1 Maximum Range Access Point Maximum Range STA2

10 Transmission Mode (VCD) Virtual Collision Detection With the RTS threshold set (valid range Bytes, 128 Bytes recommended), this becomes CSMA/CA with VCD. Sender Receiver Other STA RTS Data CTS NAV (RTS) NAV (CTS) ACK Data When Mobile Units hear a CTS that is not for them, they back off for the duration specified Point Co-ordination Function (PCF) CFP Repetition Interval CFP Repetition Interval CFP CP CFP CP B PCF DCF B PCF DCF NAV NAV Co-existence of PCF and DCF Beacon marks the beginning of Contention Free Period (CFP) it contains the CFP maximum duration, which is used by other stations to set their NAV the CFP max duration must be at least equivalent to the transmission time a frame of maximum size

11 Frames Exchanged during PCF Data Vanilla data transmission CF-Ack Acknowledging the data transmitted in the previous frame CF-Poll Polling a station to transmit the data frame Data + CF-Ack Data is destined to any station and CF-Ack is to acknowledge the data received in the previous frame Data + CF-Poll Data is destined to the same station that is polled CF-Ack + CF-Poll CF-Ack is to acknowledge the data received in the previous frame and CF-Poll is to poll the next station in the poling list Data + CF-Ack + CF-Poll Data and CF-Ack are for the same station, and CF-Ack is to acknowledge the data received in the previous frame CF-End Marks the end of contention period CF-End + CF-Ack CF-End also contains acknowledgment for potentially the last data received Any management frame PCF An Example PIFS AP B P1 D2 CF end Station CFA2 NAV CFP Max Duration Contention Free Period (CFP) Released by AP

12 PCF Foreshortening CP Begins Expected CFP Start PIFS CFP CP Frame ACK Frame B ACK CFP Foreshortening Actual CFP Start CFP Max Duration CFP End Since the next time when a station is expected to be polled for data transfer may vary from its intended time, a hard bound on the data delivery time cannot be guaranteed near isochronous service PCF More Operations AP Data + CFP1 STA1 Data + CFA1 PCF continues STA1 STA2 Stations Data + CFA1 Data + CFA2 AP Data + CFA1 + CFP2 PCF continues

13 MAC Frame Frame Control Duration ID Address 1 Address 2 Address 3 Seq. Ctrl Address 4 Data CRC MAC Frame Control Prot. Version Type Subtype To DS From DS More Frag. Retry Power Mgmt More Data WEP Order Type Sub-type 0 0 Management Frame 0 1 Control Frame 1 0 Data Frame 1 1 Reserved Management Control Data Association Req, Resp; Reassociation Req, Resp; Disassociation Probe Req, Resp; Beacon; ATIM; Authentication; Deauthentication PS Poll; RTS; CTS; ACK; CF End; CF End + CF ACK Data; Data + CF Ack; Data + CF Poll; Data + CF ACK + CF Poll CF ACK; CF Poll; CF ACK + CF Poll

14 Address Assignment to DS from DS Address Address Address Address Comments DA SA BSSID - Ad hoc 0 1 DA BSSID SA - From AP 1 0 BSSID SA DA - To AP 1 1 RA TA DA SA Within DS Address 1 Receiver The node that receives the frame over the air and is responsible for acknowledging the reception Address 2 Transmitter The node the transmits the frame over the air and is responsible for retransmission in case of no acknowledgment Address 3 and 4 take different values depending upon the mode of operation BSSID BSSID uniquely identifies a BSS In infrastructure mode BSSID is the MAC address of the wireless interface of the AP that is creating the BSS In case of ad-hoc mode BSSID is a 48-bit number in the MAC address format, which is composed of 46-bit randomly generated number and local/universal bit is set to 1 and the group bit is set to 0 Address Assignment - Scenarios STA AP STA AP To Distribution System A1(RA)=BSSID (AP s MAC) A2(TA)=SA=STA s MAC A3(DA)=FN s MAC FN From Distribution System A1(RA)=DA=STA s MAC A2(TA)=BSSID (AP s MAC) A3(SA)=FN s MAC FN Within Wireless Distribution System (A to B) A1(RA)=AP2 s MAC A2(TA)=AP1 s MAC A3(DA)=B s MAC A4(SA)=A s MAC DS1 A AP1 AP2 DS2 B

15 Fragmentation Sender Frag 1 Frag 2 Frag 3 Receiver ACK1 ACK2 ACK3 Other STA NAV= F2+2*ACK+3* NAV= F2+ACK+2* NAV= F3+2*ACK+3* NAV= F2+ACK+2* To deal with interference Interference is often in the form of short bursts Breaking large frames into fragments (smaller frames) increase the percentage of reception of undamaged frames Fragmentation Every fragment is acknowledged individually Retransmission of fragments (small frames) are less expensive Fragmentation Threshold Any frame larger than the threshold undergoes fragmentation It is a configurable parameter All but non-final ACK contains NAV value Final ACK contains NAV value 0 Fragmentation with RTS/CTS Sender Receiver RTS CTS Frag 1 ACK1 Frag 2 ACK2 Other STA NAV((RTS)= CTS+F1+ACK+3* NAV (CTS)= F1+ACK+2* NAV= F2+2*ACK+3* NAV= F2+ACK+2* Fragmentation with RTS/CTS Often fragmentation is combined with RTS/CTS RTS/CTS provides exclusive access to the medium

16 Power Management 1 Power Management Frame (Any frame with PM bit on) 2 Beacon (TIM ) 3 PS Poll Frame AP BSS1 STA Transceivers can be turned off to put the station in power saving mode to conserve the battery power Access points perform following power-management tasks it maintains the power management state of every station it buffers the frames for the station in sleeping mode it announces the buffer status of every station every TIM interval powering up the receiver at the sleeping station to receive the buffer status consumes far less power than if the station periodically polls for the buffer status Station wakes up every listen interval listen interval is its contract with the AP that is negotiated at the association time Broadcast/Multicast packets are transmitted every DTIM interval Power Management More Data Station sens PS-Poll frame for every frame buffered at the AP AP sets more data bit in the frame header if more frames are waiting in the buffer for transmission Station sends acknowledgment for every frame An unacknowledged frame is retransmitted AP if not received ACK readvertises the frame in the next TIM Station can turn off PM bit anytime indicating to the AP of switchintg its mode from PS to normal operation STA PS-Poll Frame, More Data ACK PS-Poll Frame, More Data ACK PS-Poll Frame ACK AP

17 Power Management - Scenario Beacon Interval Frames for 1 Frames for 1 and 2 Frames for 2 Frames for 1 and 2 No Frames No Frames AP T T T T T T STA 1 STA 2 Listen interval of station 1 is 2 it wakes up every second beacon interval Listen interval of station 2 is 3 During the fourth beacon interval both stations 1 and 2 contend for the medium Station 1 wins and retrieves the frame from the AP Station 2 may next gain access to the medium if no other station contends for that Otherwise if it loses the access to another station then it will remain awake for the subsequent beacon intervals until it retrieves its frame from the AP, after that it resumes its normal power saving mode of operation Roaming Registry Distribution System AP1 AP2 BSS1 BSS2 STA STA ESS - SSID A station can attach to a single AP at any time Handoff detection active scanning - probe request + response passive scanning - beacon Mobile initiated handover mobile sends association/reassociation request The AP responds with Association/Reassociation Response it returns Association ID (AID) unique for each registered mobile

18 Registration Service Maintain a table of mappings: <BSSID, IP Address, UDP Port #> Perform add in response to add request refresh in response to refresh request refresh time is 5 minutes remove in response to deregistration if the entry is not refreshed (within 15 minutes) Supply mappings in response to query Handover Registry Distribution System BSS1 4 0 BSS2 Mobile Unit Mobile Unit 0 Reassociation Frame 1 AP lookup 2 Lookup response 3 Move Notify 4 Move Response

19 IAPP Packet Format General Packet Format IAPP Version Command Data 1 byte 1 byte 0-n bytes Move-notify Packet Format Add. Length Pad MAC Address Seq. # Move-response Packet Format Add. Length Pad MAC Address Length CB Context Blob