Microsoft PowerPoint - W.lan.3.ppt

Size: px

Start display at page:

Download "Microsoft PowerPoint - W.lan.3.ppt"

柿札庄
4 years ago
Views:

1 Wireless LAN: MAC Module W.lan.3 Shanghai Jiaotong University Shanghai, China University of New Mexico Albuquerque, NM, USA 1

2 W.lan.3-2 Wireless LANs: MAC MAC MAC services MAC: DCF, CSMA/CA MAC: DCF, RTS/CTS MAC: PCF MAC frames & addresses MAC management End of module W.lan.3 PANs & Bluetooth: module W.lan.4 End of modules W.lan

3 IEEE protocol stack W.lan.3-3 App Transport IEEE Network Link PHY LLC MAC Layer PLCP (Physical Layer Convergence Procedure) PMD (Physical Medium Dependent) MAC management PHY management Station management

4 802.11: MAC W.lan.3-4 MAC Access mechanisms Fragmentation Encryption MAC management Synchronization Roaming MIB Power management Link MAC Layer MAC management

5 W.lan.3-5 MAC services Wireless transmissions are inherently broadcast Contention/collision only can be reduced, but inevitable Basic service: data transmission Mandatory: asynchronous data service, uni or multi-cast DCF (Distributed Coordination Function) exchange of data packets based on best-effort Optional: real-time service, infrastructure-based only PCF (Point Coordination Function) Primary algorithm defined DFWMAC (Distributed Foundation Wireless Medium Access Control)

6 W.lan MAC: DCF & PCF Access mechanism DCF CSMA/CA (mandatory) collision avoidance via randomized back-off mechanism minimum distance between consecutive packets ACK packet for acknowledgements (for unicast) DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC avoids hidden terminal problem PCF (optional) Access point polls terminals according to a list

7 W.lan MAC: Priority Priorities are implemented by IFS (Inter-Frame Spacing) IFS = time interval between transmission of two successive frames. Slot time, DSSS 20 μs, FHSS 50 μs, function of various delay SIFS (Short Inter Frame Spacing, DSSS 10 μs, FHSS 28 μs) highest priority, for ACK, CTS, polling response PIFS (PCF IFS, PIFS = SIFS + 1 slot time medium priority, for time-bounded service using PCF DIFS (DCF IFS, DIFS = SIFS + 2 slot time) lowest priority, for asynchronous data service DIFS DIFS medium busy PIFS SIFS contention next frame direct access if medium is free DIFS t

8 W.lan MAC: parameters Parameter (FHSS) (DSSS) (IR) b a CWmin CWmax 1 K T slot 50 μs 20 μs 8 μs 20 μs 9 μs SIFS 28 μs 10 μs 10 μs 10 μs 16 μs PIFS SIFS + T slot DIFS SIFS + T slot +T slot

9 W.lan.3-9 Carrier sensing & collision detection 802.3, cable as a medium The receiver reads back the peak voltage on the cable Compare it against a threshold Sensing idle or detect collision , radio as a medium Rely on PHY layer s CCA (Clear Channel Assessment) CCA is based on sensing of air interface To sense the detected bits in the air Slow, relatively reliable To check the received signal strength (RSS) against a threshold Possible false alarm due to level of interference Collision detection is not always possible

10 W.lan.3-10 Wireless LANs: MAC MAC MAC services MAC: DCF, CSMA/CA MAC: DCF, RTS/CTS MAC: PCF MAC frames & addresses MAC management End of module W.lan.3 PANs & Bluetooth: module W.lan.4 End of modules W.lan

11 W.lan DCF: CSMA/CA Sender/receiver protocol, overall sender has to wait for DIFS before sending data receivers after receiving data if the packet was received correctly (CRC) acknowledge at once after waiting for SIFS Else, waiting sender s waiting timer off automatic retransmission of data packets if no ACK sender receiver other stations DIFS data SIFS ACK DIFS data waiting time contention t

12 W.lan DCF: CSMA/CA DIFS DIFS contention window (randomized back-off mechanism) medium busy direct access if medium is free DIFS slot time next frame t Init: station ready to send starts sensing the medium w/ CCA Light loaded: if the medium is free for the duration of an IFS, start sending, short access delay If the medium is busy, the station has to wait for DIFS, then enter a contention phase Back-off: wait T w, collision avoidance T w is randomly chosen from a contention window CW (>CW min, <CW max ) T w is multiple of slot-time

13 W.lan DCF: CSMA/CA DIFS DIFS contention window (randomized back-off mechanism) medium busy direct access if medium is free DIFS slot time next frame t During back-off duration, continue to check CCA If detect a busy channel, (transmission is going) the back-off timer stops: freeze resume remainder of the timer after channel becomes idle for DIFS Upon back-off timer reaches zero start sending Must execute the contention phase at least once between every successive transmission

14 DCF CSMA/CA example W.lan.3-14 DIFS DIFS boe bo r DIFS bo e bo r DIFS boe busy station 1 bo e busy station 2 busy station 3 bo e busy bo e bo r station 4 bo e bo r bo e busy bo e bo r station 5 t busy medium not idle (frame, ack etc.) bo e elapsed backoff time packet arrival at MAC bo r residual backoff time

15 W.lan.3-15 Wireless LANs: MAC MAC MAC services MAC: DCF, CSMA/CA MAC: DCF, RTS/CTS MAC: PCF MAC frames & addresses MAC management End of module W.lan.3 PANs & Bluetooth: module W.lan.4 End of modules W.lan

16 W.lan.3-16 Hidden terminal problem One can receive from two others, which cannot hear each other Station A can exchange frames with B, but not C Station C can exchange frames with B, but not A Both A and C want to communicate with B Two frames collide with each other at B A and C are said to be hidden nodes with respect to each other Solution: RTS/CTS extension: small reservation packets RTS Request To Send CTS Clear To Send

17 DCF: RTS/CTS W.lan.3-17 Sender/receiver protocol, overall Sender can send RTS with NAV after waiting for DIFS NAV (Network Allocation Vector): reservation determines amount of time needs the medium Receiver acknowledges via CTS after SIFS if ready to receive Sender can now send data at once, acknowledgement via ACK Other stations (hidden nodes) store NAV distributed via RTS & CTS sender receiver DIFS RTS SIFS CTS SIFS data SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS contention data t

18 DCF: RTS/CTS W.lan.3-18 Overhead involved in RTS/CTS Pro: avoid collisions Con: overhead When to use RTS? Define RTS threshold ϕ If frame size > ϕ Activate RTS 4-way handshaking RTS/CTS/Data/ACK If frame size < ϕ Disable RTS 2-way handshaking Data/ACK

19 W.lan DCF: RTS/CTS example

20 W.lan.3-20 Wireless LANs: MAC MAC MAC services MAC: DCF, CSMA/CA MAC: DCF, RTS/CTS MAC: PCF MAC frames & addresses MAC management End of module W.lan.3 PANs & Bluetooth: module W.lan.4 End of modules W.lan

21 W.lan PCF: polling PCF (Point Coordination Function) Applicable only in real-time service, infrastructure-based Access is centrally controlled by AP, point coordinator AP perform poll & response protocol Build a centralized contention resolution scheme AP polls the nodes in BSS Time partitioned into super frame periods CFP (Contention Free Period), run PCF CP (Contention Period), run DCF Both DCF & PCF operate simultaneously, PCF has a higher priority due to PIFS < DIFS Once AP gains access, send beacon to start CFP with NAV Possible to define super frame for CP is zero, all CFP

22 PCF: polling W.lan.3-22 Once in CFP PC waits for PIFS, send data D1 to STA1, PC transmits packets to STA and polls STA that requested contention-free service All spacing uses PIFS or SIFS rather than DIFS to remain in CFP D: downstream poll or data from PC U: data from STA to PC t 0 t 1 SuperFrame medium busy point coordinator PIFS D 1 SIFS SIFS D 2 SIFS SIFS wireless stations U 1 U 2 stations NAV NAV

23 W.lan.3-23 Wireless LANs: MAC MAC MAC services MAC: DCF, CSMA/CA MAC: DCF, RTS/CTS MAC: PCF MAC frames & addresses MAC management End of module W.lan.3 PANs & Bluetooth: module W.lan.4 End of modules W.lan

24 W.lan MAC: frame format Types control frames, management frames, data frames Sequence numbers important against duplicated frames due to lost ACKs Addresses, four 48-bit MAC addresses receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous sending time, checksum, frame control, data bytes Duration/ Address Address Address Sequence ID Control Frame Control Address Data 4 bits Protocol version 1 To Type Subtype DS From DS More Frag RetryPower Mgmt More Data WEP Order CRC

25 MAC: frame format W.lan.3-25 Frame control 2-bit protocol version 2-bit type 00: management; 01: control; 10: data 4-bit subtype 10/0000: data, 00/0000: association request 00/1000: beacon; 01/1011: RTS; 01/1100: CTS; etc 1-bit more fragment Having another fragment to follow 1-bit retry As a retransmission 1-bit power management 1: STA goes into power-save mode 0: STA stays active 1-bit more data 1-bit WEP 1-bit order: 1: frames must be processed in strict order

26 MAC: address format W.lan.3-26 Frame control 1-bit to DS & 1-bit from DS DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address scenario to DS from DS addr1 addr2 addr3 adddr4 ad-hoc network 0 0 DA SA BSSID - infrastructure 0 1 DA BSSID SA - network, from AP infrastructure 1 0 BSSID SA DA - network, to AP infrastructure network, within DS 1 1 RA TA DA SA

27 MAC: address format W.lan.3-27 Frame control IBSS (ad-hoc network) 00: DA, SA, BSSID BSS (infrastructure network) 01: from AP 10: to AP 11: STA1 to STA2 via AP scenario to DS from DS addr1 addr2 addr3 adddr4 ad-hoc network 0 0 DA SA BSSID - infrastructure 0 1 DA BSSID SA - network, from AP infrastructure 1 0 BSSID SA DA - network, to AP infrastructure network, within DS 1 1 RA TA DA SA

28 MAC: special frames W.lan.3-28 ACK bytes Frame Duration Receiver CRC Control Address RTS CTS bytes Frame Duration Receiver Transmitter CRC Control Address Address bytes Frame Duration Receiver CRC Control Address

29 W.lan.3-29 Wireless LANs: MAC MAC MAC services MAC: DCF, CSMA/CA MAC: DCF, RTS/CTS MAC: PCF MAC frames & addresses MAC management End of module W.lan.3 PANs & Bluetooth: module W.lan.4 End of modules W.lan

30 W.lan MAC management Synchronization try to find a LAN, stay within a LAN timer etc. Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements Association/Reassociation integration into a LAN roaming, i.e. change networks by changing access points scanning, i.e. active search for a network MIB - Management Information Base managing, read, write

31 W.lan Synchronization Hardware & software support in each node Internal clock TSF (Timing Synchronization Function) Use of synchronization Power management Coordination of PCF Frequency hopping Timer of IFS (SIFS, PIFS, DIFS, etc) Synchronization mechanism Beacon = timestamp + other management info BBS (infrastructure-based), Beacon sent by AP IBBS (ad-hoc-based), Beacon sent by a random STA

32 W.lan.3-32 Sync using a beacon (infrastructure) beacon interval access point medium B busy B B B busy busy busy t value of the timestamp B beacon frame AP transmit the quasi-periodic beacon signal All STAs adjust their local timer by beacon s timestamp IF medium is busy, beacon will be delayed within its interval, but not accumulative Beacon s timestamp always reflects the real transmit time, not the scheduled time Beacon interval =??? around 100ms

33 W.lan.3-33 Sync using a beacon (ad-hoc) beacon interval station 1 B 1 B 1 station 2 B 2 B 2 medium busy busy busy busy t value of the timestamp B beacon frame random delay Every node maintains its own synchronization timer At the beacon interval, starts transmission of beacon frame Since the standard random back-off algorithm will be applied, only one STA wins All other STAs receive the beacon and suppress their beacons for this interval

34 W.lan : Power management Idea: switch the transceiver off if not needed States of a station: sleep and awake Stations wake up at the same time by TSF Announce going to sleep by mark the bit in frame control Sender or AP need to buffer frames if receiver is not awake BBS/Infrastructure Traffic Indication Map (TIM) list of unicast receivers transmitted by AP Delivery Traffic Indication Map (DTIM) list of broadcast/multicast receivers transmitted by AP IBBS/Ad-hoc Ad-hoc Traffic Indication Map (ATIM) announcement of receivers by stations buffering frames more complicated - no central AP collision of ATIMs possible (scalability?)

35 Power saving with wake-up patterns W.lan.3-35 Infrastructure example TIM interval DTIM interval access point medium D B busy T T d D busy busy busy B station T TIM D DTIM p awake d t B broadcast/multicast p PS poll Assumption: beacon interval = TIM interval DTIM internal = multiple of TIM interval d data transmission to/from the station

36 Power saving with wake-up patterns W.lan.3-36 Ad-hoc ATIM window beacon interval B station 1 A D B 1 1 station 2 B 2 B 2 a d B beacon frame random delay A transmit ATIM D transmit data t awake a acknowledge ATIM d acknowledge data Announce an ATIM window (< beacon interval) to stay awake Send ATIM to its receiver if buffered any Listen for any incoming ATIM, if not addressed, sleep again

37 W.lan : Roaming Roaming moving between APs within a single ESS Poor connection from AP 1? Scanning for another AP Passive: listen into the medium for beacon signals Active: send probes into the medium and wait for an answer Pick the best AP from scanning Sends a Reassociation request Wait for Reassociation response success: AP has answered, station can now participate failure: continue scanning AP 2 accepts Reassociation Request Signal the new station to the DS, which updates its data base (i.e., location information) Joining AP 2, leaving AP 1, via IAPP (Inter-AP Protocol), non standard

38 802.11: Message types W.lan.3-38 Management Type (2-bit) 00 Subtype (4-bit) 0000/ / / /1100 Description Association request/response Rassociation request/response Probe request/response Beacon Announcement of TIM Dissociation Authentication/deauthentication

39 W.lan : Message types Control Type (2-bit) 01 Subtype (4-bit) / Power-save poll, request the sender to transmit the buffered frame while this STA was in power-save mode RTS /CTS ACK CF-End (Contention Free), the last message in PCF CF-End & CF-ACK Description

40 W.lan : Message types Data Type (2-bit) 10 Subtype (4-bit) / / /1100 Description Data Data + CF-Ack Data + CF-Poll No data CF_ACK CF_Poll CF_Ack, C+CF-Poll

41 W.lan.3-41 LLC (Link layer control, not ) Framing, link access: encapsulate datagram into frame, adding header, trailer physical addresses used in frame header to identify source, dest example: MAC address Flow control: pacing between adjacent sending and receiving nodes Error detection and correction Reliable delivery between adjacent nodes seldom used on low bit error link (fiber, some twisted pair) On the top of MAC

42 W.lan.3-42 Wireless LANs: MAC MAC MAC services MAC: DCF, CSMA/CA MAC: DCF, RTS/CTS MAC: PCF MAC frames & addresses MAC management End of module W.lan.3 PANs & Bluetooth: module W.lan.4 End of modules W.lan

解除身份验证机密性 Wep 等一些加密机制 MSDU 传递 (MAC Service Data Unit) 负责将数据传送给实际的接收端传输功率控制 (Transmit Power Control 简称 TPC) 欧洲标准

解除身份验证机密性 Wep 等一些加密机制 MSDU 传递 (MAC Service Data Unit) 负责将数据传送给实际的接收端传输功率控制 (Transmit Power Control 简称 TPC) 欧洲标准一网络服务 802.11 总共提供 9 种服务 : 分布式 (distribution) 接入点收到帧, 就会使用分布式服务将真传送至目的地整合 (integration) 该服务由分布式系统提供, 它让分布式系统得以链接至非 IEEE802.11 网络关联 (association)