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Transcript
Elements of a wireless network network infrastructure wireless hosts laptop, PDA, IP phone run applications may be stationary (non-mobile) or mobile wireless does not always mean mobility Csci 183/183W/232: Wireless LAN 1 Elements of a wireless network network infrastructure base station typically connected to wired network relay - responsible for sending packets between wired network and wireless host(s) in its “area” e.g., cell towers for cellular networks, 802.11 access points Csci 183/183W/232: Wireless LAN 2 Elements of a wireless network network infrastructure wireless link typically used to connect mobile(s) to base station also used as backbone link (WiMAX) multiple access protocol coordinates link access various data rates, transmission distance Csci 183/183W/232: Wireless LAN 3 Elements of a wireless network network infrastructure infrastructure mode base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network Csci 183/183W/232: Wireless LAN 4 Elements of a wireless network Ad hoc mode no base stations nodes can only transmit to other nodes within link coverage nodes organize themselves into a network: route among themselves Csci 183/183W/232: Wireless LAN 5 Wireless Link Characteristics Differences from wired link …. decreased signal strength: radio signal attenuates as it propagates through media (path loss) interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well multipath propagation: radio signal reflects off objects, ground, arriving at destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult” Csci 183/183W/232: Wireless LAN 6 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B A C’s signal strength A’s signal strength B Hidden terminal problem C space B, A hear each other Signal fading: A, C can not hear each other B, C hear each other B, C hear each other B, A hear each other means A, C unaware of their interference at B A, C can not hear each other interferring at B Csci 183/183W/232: Wireless LAN 7 Special challenge for Wireless MAC Collision detection is difficult • Transmission power is much higher than receiving power Hidden station problem • A station is not able to detect a potential competitor for the medium because the competitor is too far away • Example: A is transmitting to B. C cannot hear the transmission, thus falsely concludes that it can transmit. Exposed station problem • A station hears the on-going transmission, and falsely assume it cannot transmit • Example: B tis transmitting to A. C hears the transmission, thus falsely concludes that it cannot transmit o D. Csci 183/183W/232: Wireless LAN 8 IEEE 802.11 Wireless LAN 802.11b 2.4-2.485 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer • all hosts use same chipping code (insensitive to multipath fading) widely deployed, using base stations 802.11a 5.1-5.8 GHz range up to 54 Mbps OFDM 802.11g 2.4-2.485 GHz range up to 54 Mbps DSSS All use CSMA/CA for multiple access All have base-station and ad-hoc network versions Csci 183/183W/232: Wireless LAN 9 Spread Spectrum Idea spread signal over wider frequency band than required originally deigned to thwart jamming Frequency Hopping transmit over random sequence of frequencies sender and receiver share… • pseudorandom number generator • seed 802.11 uses 79 x 1MHz-wide frequency bands Csci 183/183W/232: Wireless LAN 10 Spread Spectrum (cont) Direct Sequence for each bit, send XOR of that bit and n random bits random sequence known to both sender and receiver called n-bit chipping code 802.11 defines an 11-bit chipping code 1 0 Data stream: 1010 1 0 Random sequence: 0100101101011001 1 0 XOR of the two: 1011101110101001 Csci 183/183W/232: Wireless LAN 11 OFDM A large number of closely-spaced orthogonalsub-carriers are used to carry data. The data are divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. Csci 183/183W/232: Wireless LAN 12 Base Station Approach Wireless host communicates with a base station base station = access point (AP) Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless hosts access point (AP): base station BSS’s combined via a distribution system (DS) The DS runs at layer 2 of the ISO architecture! Csci 183/183W/232: Wireless LAN 13 Ad Hoc Network Approach No access point (i.e., base station) “peer-to-peer” mode wireless hosts communicate with each other to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z Applications: “laptop” meeting in conference room, car interconnection of “personal” devices battlefield IETF MANET (Mobile Ad hoc Networks) working group Csci 183/183W/232: Wireless LAN 14 802.11: Channels, association 802.11b/g: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! host: must associate with an AP scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address selects AP to associate with may perform authentication will typically run DHCP to get IP address in AP’s subnet Csci 183/183W/232: Wireless LAN 15 Definitions Definitions MAC-level Acknowledgement • Indicate the received frame is correct • The source should wait ACKTimeout amount of time for ACK Inter-frame Space (IFS) • • • • SIFS: short IFS PIFS: PCF IFS DIFS: DCF IFS DIFS>PIFS>SIFS Csci 183/183W/232: Wireless LAN 16 IEEE 802.11: multiple access avoid collisions: 2+ nodes transmitting at same time 802.11: CSMA - sense before transmitting don’t collide with ongoing transmission by other node 802.11: no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can’t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) A C A B B C C’s signal strength A’s signal strength space Csci 183/183W/232: Wireless LAN 17 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff timer; timer counts down while channel idle; transmits when timer expires; if no ACK, increase random backoff interval, repeat 2 sender receiver DIFS 802.11 receiver - if frame received OK data SIFS ACK return ACK after SIFS (ACK needed due to hidden terminal problem) Csci 183/183W/232: Wireless LAN 18 Avoiding collisions – Option I idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they’re short) BS broadcasts clear-to-send CTS in response to RTS RTS heard by all nodes sender transmits data frame other stations defer transmissions Avoid data frame collisions completely using small reservation packets! Csci 183/183W/232: Wireless LAN 19 Collision Avoidance: RTS-CTS exchange A AP B reservation collision DATA (A) defer time Csci 183/183W/232: Wireless LAN 20 IEEE 802.11 MAC Layer – Option II Protocol Architecture Distributed Coordination Function (DCF) Point Coordination Function (PCF) Csci 183/183W/232: Wireless LAN 21 802.11 MAC Layer Carrier Sensing Carrier sense at two levels Physical carrier sense: done by physical layer Virtual carrier sense at MAC layer using Network Allocation Vector (NAV) set while RTS/CTS/Data/Ack are overheard: partially solves problem of Hidden and Exposed terminal • The Duration field reserves the media! Reduces collision by deferring transmission if any of the carrier sense mechanisms sense the channel busy Csci 183/183W/232: Wireless LAN 22 DCF Basic Access Basic Access When a STA has data to send, it senses medium The STA may transmit a MAC Protocol Data Unit (MPDA) when medium idle time is greater or equal to DIFS If medium is busy, wait for a random backoff time Csci 183/183W/232: Wireless LAN 23 DCF Backoff Procedure Backoff procedure is invoked for a STA to transfer a frame but the medium is busy Set Backoff Timer to be random backoff time Backoff Timer start decreasing after an idle time of DIFS following the medium busyness Backoff Timer is suspended when medium is busy, and won’t resume until the medium is idle for DIFS A frame may be transmitted immediately when Backoff Timer is 0 Csci 183/183W/232: Wireless LAN 24 DCF Recovery procedures Collision may happen during contention When collision happens, retransmission STAs maintain a station short retry counter (SSRC) and long retry counter (SLRC) for each MSDU and MMPDU • SSRC increases by one for each failed RTS or MPDU whose length is <= dot11RTSThreshold • SLRC increases by one for each failed MPDU whose length is > dot11RTSThreshold • Both counter is reset upon a successful MPDU • Retry is aborted when SSRC>=dot11ShortRetryLimt =7 or SLRC>=aLongRetryLimit Csci 183/183W/232: Wireless LAN 25 DCF Random backoff time=random()xaSlotTime aSlotTime: the value of the correspondingly named PHY characteristic (20s for DSSS) Random(): a random integer uniformly distributed over [0, CW] CW (contention window) Increases exponentially after each retry fails (so does average backoff time. Why to do this?) Keep constant after reaching the maximum Reset after a successful transmission Csci 183/183W/232: Wireless LAN 26 DCF RTS/CTS Scheme RTS/CTS Scheme Four way handshake: RTS-CTS-DATA-ACK NAV (Network Allocation Vector) An indicator, maintained at each STA, for the period that transmission will not be initiated Setting and resetting NAV according to “Duration” in MAC header when receiving a valid frame Csci 183/183W/232: Wireless LAN 27 DCF -- Fragmentation Control of the channel Once the STA has contented for the channel, it shall continue to send fragments until • All fragments of a MSDU or MMPDU have been sent • An ACK is not received • STA is restricted from sending additional fragments by PHY layer Duration field RTS/CTS: time till the end of ACK0 Fragments/ACK: time till the end of the ACK for the next fragment • Last fragment/ACK: length of ACK/0 Csci 183/183W/232: Wireless LAN 28 DCF Directed (Unicast) MPDU STA uses RTS/CTS for directed MPDU only when the length of a MPDU >= dot11RTSThreshold • Always use RTS/CTS: set dot11RTSThreshold=0 • Don’t use RTS/CTS: set dot11RTSThreshold=maximum MPDU length=2304 octets Broadcast and multicast Regardless of the length of frame, no RTS/CTS No ACK No MAC layer recovery Csci 183/183W/232: Wireless LAN 29 PCF Point Coordinator PCF provides contention free frame transfer PC resides in AP; It is an option of AP to become PC Fundamental Access PC senses the medium When medium is idle for PIFS, PC transmit a Beacon frame After beacon, PC shall wait for SIFS, and then transmit • Data frame • CF-Poll (contention free poll) frame • Data + CF-Poll frame Csci 183/183W/232: Wireless LAN 30 PCF Polling list STA indicates its CF-Pollability via Association and Reassociation PC shall send a CF-Poll to at least one STA when there are entries in the polling list NAV Each STA set the NAV to CFPMaxDuration PC shall transmit a CF-End frame at the end of CFP Csci 183/183W/232: Wireless LAN 31 PCF Only available for infrastructured architecture, why? PCF is on top of DCF Super frame contains a contention-free period and a contention period Question: how time-bounded service is provided? Csci 183/183W/232: Wireless LAN 32 802.11 frame: addressing 2 2 6 6 6 frame address address address duration control 1 2 3 Address 1: MAC address of wireless host or AP to receive this frame 2 6 seq address 4 control 0 - 2312 4 payload CRC Address 4: for Intra DS communication Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame Csci 183/183W/232: Wireless LAN 33 802.11 frame: more frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) 2 2 6 6 6 frame address address address duration control 1 2 3 2 Protocol version 2 4 1 Type Subtype To AP 6 2 1 seq address 4 control 1 From More AP frag 1 Retry 0 - 2312 4 payload CRC 1 1 Power More mgt data 1 1 WEP Rsvd Frame subtype (RTS, CTS, ACK, Beacon, etc.) frame type (management, control, data) Csci 183/183W/232: Wireless LAN 34 Interpretation of the MAC Addrs To AP From AP Addr 1 Addr 2 Addr 3 Addr 4 0 0 DA SA 0 1 DA BSSID SA 1 0 BSSID SA DA - 1 1 RA DA SA TA BSSID - Csci 183/183W/232: Wireless LAN - 35 802.11 frame: addressing R1 router H1 Internet AP R1 MAC addr H1 MAC addr dest. address source address 802.3 frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame Csci 183/183W/232: Wireless LAN 36 Supporting Mobility Case 1: ad hoc net working Case 2: access points (AP) tethered each mobile node associates with an AP Distribution system AP-1 AP-3 F AP-2 A B G H C E D Csci 183/183W/232: Wireless LAN 37 Mobility (cont) Scanning (selecting an AP) node sends Probe frame all AP’s w/in reach reply with ProbeResponse frame node selects one AP; sends it AssociateRequest frame AP replies with AssociationResponse frame new AP informs old AP via tethered network When active: when join or move passive: AP periodically sends Beacon frame Csci 183/183W/232: Wireless LAN 38