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Wireless Local Area Networks. The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level.

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Presentation on theme: "Wireless Local Area Networks. The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level."— Presentation transcript:

1 Wireless Local Area Networks

2 The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless local area networking. Companies jumped in, quickly developing incompatible wireless products in the 1990’s. Industry decided to entrust standardization to IEEE committee that dealt with wired LANS – namely, the IEEE 802 committee!!

3 Wireless MAC Protocol Outline: design challenges for wireless MAC –hidden/exposed stations –flexible control for QoS support two design paradigms –multiple access based –token based rationale for design choices

4 Wireless Networking Environment A simple model: –A single shared physical channel among users –Omni-directional antenna, limited transmission range –Same transmission rate for all users Channel characteristics(illustrated with examples) –wireless transmission is spatial and local –sender & receiver: different views of the world –relevant contention is at the receiver side –contention may induce collisions –contention/collision/congestion is location dependent –channel access is a collective behavior from the fairness perspective: the notion of “ local ” is misnomer Wireless MAC: how to address channel access in a wireless environment

5 Design Goals for Wireless MAC Requirements for a wireless MAC protocol: –robustness –efficiency –fairness –support for priority and QoS –support for multicast

6 Hidden Station Problem Hidden Stations: within the range of the intended receiver, but out of range of the transmitter –hidden sender C ABDC ABDC Problem: A transmits to B, if C transmits (to D), collision at B Solution: hidden sender C needs to defer (Question: who tells C, A or B?) Problem: A transmits to B, if D xmits to C, C cannot reply. D confuses (4 cases) Solution: D needs to be notified that its receiver C is hidden – hidden receiver C

7 Exposed Station Problem Exposed Stations: within the range of the intended sender, but out of range of the receiver –exposed sender B ABDC ABDC Problem: C transmits to D, if B transmits (to A), B cannot hear from A Solution: exposed sender B needs to defer Problem: C transmits to D, if A xmits to B, B cannot reply. A confuses (4 cases) Solution: A needs to be notified that its receiver B is exposed (how can B hears A?) – exposed receiver B

8 Summary of hidden and exposed station problem Receiver ’ s perception of a clean/collided packet is critical Hidden/exposed senders need to defer their transmissions Hidden/exposed receivers need to notify their senders about their status

9 MAC Protocol Resolve channel contention & access: Channel access arbitration –know who are there –allocate the channel among multiple senders & receivers who share the channel Collision avoidance –multiple access based –token based Collision resolution –backoff based

10 Solution Space for channel contention Multiple access approach –with carrier sensing carrier sensing: provides collision information at the sender, NOT the receiver FAMA, 802.11 –without carrier sensing MACA, MACAW –cons and pros: robust, solves hidden/exposed station problem, hard to provide QoS Token based approach –TDMA, DQRUMA –cons and pros: easy to provide QoS, less robust, hard to handle hidden/exposed stations

11 IEEE 802 Standards Working Groups The important ones are marked with *. The ones marked with  are hibernating. The one marked with † gave up.

12 IEEE Standards for Wireless Networks IEEE 802.11Wireless LANs IEEE 802.15Wireless Personal Area Networks (WPAN) IEEE 802.16Broadband Wireless Access (BBWA) IEEE 802.20Mobile Broadband Wireless Access (MBWA) IEEE 802.21Media Independent Handover (MIH) IEEE 802.22Wireless Regional Area Networks

13 IEEE 802.11 (WLAN) 802.11a5 GHz, up to 54 Mbps 802.11b2.4 GHz, up to 11 Mbps 802.11dEnables 802.11 to work in various countries where it can't today 802.11eQoS Enhancement 802.11fAdds Access Point Interoperability 802.11g2.4 GHz, up to 54 GHz, compatible with 802.11b 802.11hResolves interference issues 802.11iSecurity Enhancement 802.11jJapanese regulatory extensions 802.11kRadio resource measurement 802.11mEnhanced maintenance features, improvements, and amendments 802.11nNext generation of 802.11 with throughput in excess of 100Mbps 802.11rEnhancements for fast roaming of WLAN units 802.11sWireless mesh networks

14 Common Aliases of Wireless Standards 802.11b/gWi-Fi 802.15.1Bluetooth 802.15.3Ultra Wideband 802.15.4ZigBee 802.16WiMAX

15 Categories of Wireless Networks Base Station :: all communication through an access point {note hub topology}. Other nodes can be fixed or mobile. Infrastructure Wireless :: base station network is connected to the wired Internet. Ad hoc Wireless :: wireless nodes communicate directly with one another. MANETs (Mobile Ad Hoc Networks) :: ad hoc nodes are mobile.

16 Wireless LANs a) Wireless networking with a base station. (b) Ad hoc networking.

17 The 802.11 Protocol Stack Part of the 802.11 protocol stack.

18 IEEE standard: 802.11 mobile terminal access point server fixed terminal application TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY application TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC infrastructure network LLC

19 Wireless Physical Layer Physical layer conforms to OSI (five options) –1997: 802.11 infrared, FHSS, DHSS –1999: 802.11a OFDM and 802.11b HR-DSSS –2001: 802.11g OFDM 802.11 Infrared –Two capacities 1 Mbps or 2 Mbps. –Range is 10 to 20 meters and cannot penetrate walls. –Does not work outdoors. 802.11 FHSS (Frequence Hopping Spread Spectrum) –The main issue is multipath fading. –79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band. –Same pseudo-random number generator used by all stations. –Dwell time: min. time on channel before hopping (400msec).

20 Wireless Physical Layer 802.11 DSSS (Direct Sequence Spread Spectrum) –Spreads signal over entire spectrum using pseudo-random sequence (similar to CDMA). –Each bit transmitted using an 11 chips Barker sequence, PSK at 1Mbaud. –1 or 2 Mbps. 802.11a OFDM (Orthogonal Frequency Divisional Multiplexing) –Compatible with European HiperLan2. –54Mbps in wider 5.5 GHz band  transmission range is limited. –Uses 52 FDM channels (48 for data; 4 for synchronization). –Encoding is complex ( PSM up to 18 Mbps and QAM above this capacity). –E.g., at 54Mbps 216 data bits encoded into into 288-bit symbols. –More difficulty penetrating walls.

21 Wireless Physical Layer 802.11b HR-DSSS (High Rate Direct Sequence Spread Spectrum) –11a and 11b shows a split in the standards committee. –11b approved and hit the market before 11a. –Up to 11 Mbps in 2.4 GHz band using 11 million chips/sec. –Note in this bandwidth all these protocols have to deal with interference from microwave ovens, cordless phones and garage door openers. –Range is 7 times greater than 11a. –11b and 11a are incompatible!!

22 Wireless Physical Layer 802.11g OFDM(Orthogonal Frequency Division Multiplexing) –An attempt to combine the best of both 802.11a and 802.11b. –Supports bandwidths up to 54 Mbps. –Uses 2.4 GHz frequency for greater range. –Is backward compatible with 802.11b.

23 802.11 - MAC layer Access methods –MAC-DCF CSMA/CA (mandatory) collision avoidance via randomized “ back-off ” mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) –MAC-DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC avoids hidden terminal problem –MAC- PCF (optional) access point polls terminals according to a list

24 Distribute Coordination Function (DCF) Uses CSMA/ CA (CSMA with Collision Avoidance). –Uses both physical and virtual carrier sensing. –Two methods are supported: 1.based on MACAW(Multiple Access with Collision Avoidance for Wireless) with virtual carrier sensing. 2.1-persistent physical carrier sensing.

25 Virtual Channel Sensing in CSMA/CA “virtual” implies source station sets duration field in data frame or in Ready-to-Send (RTS) and Clear-to-Send (CTS ) frames. Stations then adjust their NAV (Network Allocation Vector) accordingly!

26 1-Persistent Physical Carrier Sensing Station senses the channel when it wants to send. If idle, station transmits. –Station does not sense channel while transmitting. If the channel is busy, station defers until idle and then transmits. Upon collision, wait a random time using binary exponential backoff.

27 802.11 - MAC layer (cont) Priorities –defined through different inter frame spaces –no guaranteed, hard priorities –SIFS (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response –PIFS (PCF IFS) medium priority, for time-bounded service using PCF –DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service t medium busy SIFS PIFS DIFS next framecontention Access (after CWmin) if medium is free  DIFS

28 t medium busy DIFS next frame contention window (randomized back-off mechanism) 802.11 - CSMA/CA basic access method –station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) –if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending after CWmin (IFS depends on packet type) –if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) –if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) slot time direct access if medium is free  DIFS

29 802.11 - CSMA/CA (cont) Sending unicast packets –station has to wait for DIFS (and CWmin) before sending data –receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) –automatic retransmission of data packets in case of transmission errors t SIFS DIFS data ACK waiting time other stations receiver sender data DIFS contention

30 IEEE 802.11 MAC Protocol CSMA Version of the Protocol: sense channel idle for DISF sec (Distributed Inter Frame Space) transmit frame (no Collision Detection) receiver returns ACK after SIFS (Short Inter Frame Space) if channel sensed busy => binary backoff NAV: Network Allocation Vector (min time of deferral)

31 802.11 - CSMA/CA with RTS/CTS Sending unicast packets –station can send RTS with reservation parameter after waiting for DIFS (reservation declares amount of time the data packet needs the medium) –acknowledgement via CTS after SIFS by receiver (if ready to receive) –sender can now send data at once, acknowledgement via ACK –other stations store medium reservations distributed via RTS and CTS t SIFS DIFS data ACK defer access other stations receiver sender data DIFS contention RTS CTS SIFS NAV (RTS) NAV (CTS)

32 Collision Avoidance RTS freezes stations near the transmitter CTS “ freezes ” stations within range of receiver (but possibly hidden from transmitter); this prevents collisions by hidden station during data transfer RTS and CTS are very short: collisions during data phase are thus very unlikely (similar effect as Collision Detection) Note: IEEE 802.11 allows CSMA, CSMA/CA and “ polling ” from AP

33 Fragmentation in 802.11 High wireless error rates  long packets have less probability of being successfully transmitted. Solution: MAC layer fragmentation with stop-and- wait protocol on the fragments.

34 Point Coordinated Function (PCF) PCF uses a base station to poll other stations to see if they have frames to send. No collisions occur. Base station sends beacon frame periodically. Base station can tell another station to sleep to save on batteries and base stations holds frames for sleeping station.

35 MAC-PCF (Point Coordination Function) like polling PIFS stations‘ NAV wireless stations point coordinator D1D1 U1U1 SIFS NAV SIFS D2D2 U2U2 SuperFrame t0t0 medium busy t1t1

36 MAC-PCF (cont) t stations‘ NAV wireless stations point coordinator D3D3 NAV PIFS D4D4 U4U4 SIFS CF end contention period contention free period t2t2 t3t3 t4t4

37 DCF and PCF Co-Existence Distributed and centralized control can co-exist using InterFrame Spacing. SIFS (Short IFS) :: is the time waited between packets in an ongoing dialog (RTS,CTS,data, ACK, next frame) PIFS (PCF IFS) :: when no SIFS response, base station can issue beacon or poll. DIFS (DCF IFS) :: when no PIFS, any station can attempt to acquire the channel. EIFS (Extended IFS) :: lowest priority interval used to report bad or unknown frame.

38 Interframe Spacing in 802.11.

39 CSMA/CA Protocol: congestion control

40 Congestion Avoidance: IEEE 802.1 DCF Before transmitting a packet, randomly choose a backoff interval in the range [0,cw] –cw is the contention window “ Count down ” the backoff interval when medium is idle –Count-down is suspended if medium becomes busy When backoff interval reaches 0, transmit packet (or RTS)

41 DCF Example data wait B1 = 5 B2 = 15 B1 = 25 B2 = 20 data wait B1 and B2 are backoff intervals at nodes 1 and 2 Let cw = 31 B2 = 10

42 Congestion Avoidance The time spent counting down backoff intervals contributes to MAC overhead Choosing a large cw leads to large backoff intervals and can result in larger overhead Choosing a small cw leads to a larger number of collisions (more likely that two nodes count down to 0 simultaneously)

43 Congestion Control Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage congestion is needed IEEE 802.11 DCF: Congestion control achieved by dynamically adjusting the contention window cw

44 Binary Exponential Backoff in DCF When a node fails to receive CTS in response to its RTS, it increases the contention window –cw is doubled (up to an upper bound – typically 5 times) When a node successfully completes a data transfer, it restores cw to CW min

45 MILD Algorithm in MACAW [Bharghavan94Sigcomm] When a node fails to receive CTS in response to its RTS, it multiplies cw by 1.5 –Less aggressive than 802.11, which multiplies by 2 When a node successfully completes a transfer, it reduces cw by 1 –More conservative than 802.11, where cw is restored to Cwmin –802.11 reduces cw much faster than it increases it –MACAW: cw reduction slower than the increase Exponential Increase Linear Decrease MACAW can avoid wild oscillations of cw when congestion is high

46 CSMA/CA Protocol: fairness

47 Fairness Issue Many definitions of fairness plausible Simplest definition: All nodes should receive equal bandwidth AB CD Two flows

48 Fairness Issue Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide Nodes A and B then choose from range [0,63] –Node A chooses 4 slots and B choose 60 slots –After A transmits a packet, it next chooses from range [0,31] –It is possible that A may transmit several packets before B transmits its first packet AB CD Two flows

49 Fairness Issue Observation: unfairness occurs when one node has backed off much more than some other node AB CD Two flows

50 MACAW Solution for Fairness When a node transmits a packet, it appends its current cw value to the packet All nodes hearing that cw value use it for their future transmission attempts The effect is to reset all competing nodes to the same ground rule

51 Weighted Fair Queueing Assign a weight to each node Goal: bandwidth used by each node should be proportional to the weight assigned to the node

52 Distributed Fair Scheduling (DFS) [Vaidya00Mobicom] A fully distributed algorithm for achieving weighted fair queueing Key idea: if sender A has weight =1 and sender B has weight = 2, they split the bandwidth 1 to 2 Choose backoff intervals proportional to (packet size / weight) DFS attempts to mimic the centralized Self-Clocked Fair Queueing algorithm [Golestani] Works well on a LAN

53 Distributed Fair Scheduling (DFS) data wait B1 = 15 B2 = 5 B1 = 15 (DFS actually picks a random value with mean 15) B2 = 5 (DFS picks a value with mean 5) Weight of node 1 = 1 Weight of node 2 = 3 Assume equal packet size B1 = 10 B2 = 5 data wait B1 = 5 B2 = 5 Collision !

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