1 CSE401n:Computer Networks Lecture 16 Wireless Link & LANs WS: ch-14 KR: 5.7

2 IEEE Wireless LANs r b m operate at 2.4 GHz, 11 Mbps m widely deployed r a m 5-6 GHz range m up to 54 Mbps r g m 2.4 GHz m up to 54 Mbps r All have base-station and ad-hoc network versions r All use CSMA/CA for multiple access

3 IEEE Wireless LAN r IEEE standard: m unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz and b a

4 Frequency Band

5

6 IEEE Wireless LAN r Basic Service Set (BSS) (a.k.a. “cell”) contains: m wireless station (WS) m access point (AP): base station r BSS’s combined to form distribution system (DS) r Two operation modes: m Infrastructure mode everything through AP m Peer-to-peer mode called ad hoc network

7 IEEE Architecture r Distribution system (DS) r Access point (AP) r Basic service set (BSS) m Stations competing for access to shared wireless medium m Isolated or connected to backbone DS through AP r Extended service set (ESS) m Two or more basic service sets interconnected by DS

8 Ad Hoc Network approach r No AP r No connection to the outside world r Wireless hosts communicate with each other r Applications: m “laptop” meeting in conference room, car

9 Distribution of Messages Within a DS r Distribution service m Used to exchange MAC frames from station in one BSS to station in another BSS r Integration service m Transfer of data between station on IEEE LAN and station on integrated IEEE 802.x LAN

10 Transition Types Based On Mobility r No transition m Stationary or moves only within BSS r BSS transition m Station moving from one BSS to another BSS in same ESS r ESS transition m Station moving from BSS in one ESS to BSS within another ESS

11 Association-Related Services r Association m Establishes initial association between station and AP r Reassociation m Enables transfer of association from one AP to another, allowing station to move from one BSS to another r Disassociation m Association termination notice from station or AP

12 Access and Privacy Services r Authentication m Establishes identity of stations to each other r Deathentication m Invoked when existing authentication is terminated r Privacy m Prevents message contents from being read by unintended recipient

– Traffic and Access Methods r Two types of traffic services m Asynchronous Data Service (mandatory) exchange of data packets based on “best-effort” m Time-Bounded Service (optional) r Two types of coordination function m DCF (Distributed Coordination Function) m PCF (Point Coordination Function) r Access methods (why not CSMA/CD?) m DCF-CSMA/CA (collision avoidance) collision avoidance via randomized back-off mechanism (if sense busy, random backoff even if no collision) ACK packet for acknowledgements (not for broadcasts) m DCF-RTS/CTS avoids hidden terminal problem m PCF access point polls

14 IEEE Medium Access Control r MAC layer covers three functional areas: m Reliable data delivery m Access control m Security

15 Reliable Data Delivery r More efficient to deal with errors at the MAC level than higher layer (such as TCP) r Frame exchange protocol m Source station transmits data m Destination responds with acknowledgment (ACK) m If source doesn’t receive ACK, it retransmits frame r Four frame exchange m Source issues request to send (RTS) m Destination responds with clear to send (CTS) m Source transmits data m Destination responds with ACK

16 The Hidden Terminal Problem BAC r A is sending to B, but C cannot receive from A m Friis Law (power decay proportional to distance square) r Therefore C sends to B, without detecting the transmission from A to B r In summary, A is “hidden” for C r Implication: How to do carrier sense and collision detection?

17 The Exposed Terminal Problem BACD r B is sending to A, C intends to send to D r C senses an “in-use” medium, thus C waits r But A is outside the radio range of C, therefore waiting is not necessary r In summary, C is “exposed” to B r Implication: false carrier sense

18 Basic Solution: RTS-CTS r Short signaling packets m RTS (request to send) and CTS (clear to send) m Contain sender address, receiver address, transmission duration r Example: solve the hidden terminal problem BAC RTSCTS D

19 Access Control

20 Medium Access Control Logic

21 Interframe Space (IFS) Values r Short IFS (SIFS) m Shortest IFS m Used for immediate response actions r Point coordination function IFS (PIFS) m Midlength IFS m Used by centralized controller in PCF scheme when using polls r Distributed coordination function IFS (DIFS) m Longest IFS m Used as minimum delay of asynchronous frames contending for access

22 IFS Usage r SIFS m Acknowledgment (ACK) m Clear to send (CTS) m Poll response r PIFS m Used by centralized controller in issuing polls m Takes precedence over normal contention traffic r DIFS m Used for all ordinary asynchronous traffic

– Inter Frame Spacing r Defined different inter frame spacing r SIFS (Short Inter Frame Spacing) m highest priority, for ACK, CTS, polling response r PIFS (PCF IFS) m medium priority, for time-bounded service using PCF r DIFS (DCF, Distributed Coordination Function IFS) m lowest priority, for asynchronous data service direct access if medium is free  DIFS t medium busy SIFS PIFS DIFS next framecontention

– RTS/CTS + ACK r Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) after waiting for DIFS r Receiver acknowledges via CTS after SIFS (if ready to receive) m CTS reserves channel for sender, notifying possibly hidden stations r Sender can now send data at once, acknowledgement via ACK r Other stations store NAV distributed via RTS and CTS t SIFS DIFS data ACK defer access other stations receiver sender data DIFS new contention RTS CTS SIFS NAV (RTS) NAV (CTS)

Collision Avoidance: RTS-CTS + ACK DIFS: Distributed Inter-Frame Spacing SIFS: Short Inter-Frame Spacing

: PCF for Polling t NAV polled wireless stations point coordinator NAV PIFS D U SIFS D contention period contention free period medium busy D: downstream poll, or data from point coordinator U: data from polled wireless station

27 MAC Frame Format

28 MAC Frame Fields r Frame Control – frame type, control information r Duration/connection ID – channel allocation time r Addresses – context dependant, types include source and destination r Sequence control – numbering and reassembly r Frame body – MSDU or fragment of MSDU r Frame check sequence – 32-bit CRC

29 Frame Control Fields r Protocol version – version r Type – control, management, or data r Subtype – identifies function of frame r To DS – 1 if destined for DS r From DS – 1 if leaving DS r More fragments – 1 if fragments follow r Retry – 1 if retransmission of previous frame

30 Frame Control Fields r Power management – 1 if transmitting station is in sleep mode r More data – Indicates that station has more data to send r WEP – 1 if wired equivalent protocol is implemented r Order – 1 if any data frame is sent using the Strictly Ordered service

31 Control Frame Subtypes r Power save – poll (PS-Poll) r Request to send (RTS) r Clear to send (CTS) r Acknowledgment r Contention-free (CF)-end r CF-end + CF-ack

32 Data Frame Subtypes r Data-carrying frames m Data m Data + CF-Ack m Data + CF-Poll m Data + CF-Ack + CF-Poll r Other subtypes (don’t carry user data) m Null Function m CF-Ack m CF-Poll m CF-Ack + CF-Poll

33 Management Frame Subtypes r Association request r Association response r Reassociation request r Reassociation response r Probe request r Probe response r Beacon

34 Management Frame Subtypes r Announcement traffic indication message r Dissociation r Authentication r Deauthentication

35 Wired Equivalent Privacy

36 Authentication r Open system authentication m Exchange of identities, no security benefits r Shared Key authentication m Shared Key assures authentication

37 IEEE a802.11b Standard approved Sep July 1997 Available bandwidth 300 MHZ83.5 MHZ Unlicensed freq. of operation G G G No. of non- overlapping Ch. 433 Rate per channel 6,9,12,18,24,36, 48,54 Mbps 1, 2, 5.5, 11 Mbps 1,2 Mbps Range225 feet ?? ModulationOFDMDSSSDSSS, FHSS DSSS: direct sequence spread spectrum FHSS: frequency hopping spread spectrum OFDM: orthogonal frequency division multiplexing