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CSE401n:Computer Networks

Lecture 16Wireless Link & LANs

WS: ch-14KR: 5.7

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IEEE 802.11 Wireless LANs 802.11b

operate at 2.4 GHz, 11 Mbps widely deployed

802.11a 5-6 GHz range up to 54 Mbps

802.11g 2.4 GHz up to 54 Mbps

All have base-station and ad-hoc network versions

All use CSMA/CA for multiple access

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IEEE 802.11 Wireless LAN IEEE 802.11 standard:

unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz

and 802.11b 802.11a

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Frequency Band

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IEEE 802.11 Wireless LAN

Basic Service Set (BSS) (a.k.a. “cell”) contains: wireless station (WS) access point (AP): base

station BSS’s combined to form

distribution system (DS) Two operation modes:

Infrastructure mode• everything through AP

Peer-to-peer mode• called ad hoc network

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IEEE 802.11 Architecture

Distribution system (DS) Access point (AP) Basic service set (BSS)

Stations competing for access to shared wireless medium Isolated or connected to backbone DS through AP

Extended service set (ESS) Two or more basic service sets interconnected by DS

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Ad Hoc Network approach No AP No connection to the outside world Wireless hosts communicate with each

other Applications:

“laptop” meeting in conference room, car

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Distribution of Messages Within a DS Distribution service

Used to exchange MAC frames from station in one BSS to station in another BSS

Integration service Transfer of data between station on IEEE 802.11 LAN and

station on integrated IEEE 802.x LAN

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Transition Types Based On Mobility

No transition Stationary or moves only within BSS

BSS transition Station moving from one BSS to another BSS in same ESS

ESS transition Station moving from BSS in one ESS to BSS within

another ESS

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Association-Related Services

Association Establishes initial association between station and AP

Reassociation Enables transfer of association from one AP to another, allowing

station to move from one BSS to another

Disassociation Association termination notice from station or AP

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Access and Privacy Services

Authentication Establishes identity of stations to each other

Deathentication Invoked when existing authentication is terminated

Privacy Prevents message contents from being read by unintended

recipient

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802.11 – Traffic and Access Methods Two types of traffic services

Asynchronous Data Service (mandatory)• exchange of data packets based on “best-effort”

Time-Bounded Service (optional) Two types of coordination function

DCF (Distributed Coordination Function) PCF (Point Coordination Function)

Access methods (why not CSMA/CD?) 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) DCF-RTS/CTS

• avoids hidden terminal problem PCF

• access point polls 7.12.1

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IEEE 802.11 Medium Access Control MAC layer covers three functional areas:

Reliable data delivery Access control Security

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Reliable Data Delivery

More efficient to deal with errors at the MAC level than higher layer (such as TCP)

Frame exchange protocol Source station transmits data Destination responds with acknowledgment (ACK) If source doesn’t receive ACK, it retransmits frame

Four frame exchange Source issues request to send (RTS) Destination responds with clear to send (CTS) Source transmits data Destination responds with ACK

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The Hidden Terminal Problem

BA C

A is sending to B, but C cannot receive from A Friis Law (power decay proportional to distance square)

Therefore C sends to B, without detecting the transmission from A to B

In summary, A is “hidden” for C Implication: How to do carrier sense and collision

detection?

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The Exposed Terminal Problem

BA C D

B is sending to A, C intends to send to D C senses an “in-use” medium, thus C waits But A is outside the radio range of C,

therefore waiting is not necessary In summary, C is “exposed” to B Implication: false carrier sense

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Basic Solution: RTS-CTS

Short signaling packets RTS (request to send) and CTS (clear to send) Contain sender address, receiver address,

transmission duration Example: solve the hidden terminal

problem

BA CRTS

CTSCTS D

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Access Control

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Medium Access Control Logic

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Interframe Space (IFS) Values

Short IFS (SIFS) Shortest IFS Used for immediate response actions

Point coordination function IFS (PIFS) Midlength IFS Used by centralized controller in PCF scheme when using polls

Distributed coordination function IFS (DIFS) Longest IFS Used as minimum delay of asynchronous frames contending for

access

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IFS Usage

SIFS Acknowledgment (ACK) Clear to send (CTS) Poll response

PIFS Used by centralized controller in issuing polls Takes precedence over normal contention traffic

DIFS Used for all ordinary asynchronous traffic

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802.11 – Inter Frame Spacing Defined different inter frame spacing 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

direct access if medium is free DIFS

t

medium busy SIFSPIFS

DIFSDIFS

next framecontention

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802.11 – RTS/CTS + ACK 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

Receiver acknowledges via CTS after SIFS (if ready to receive) CTS reserves channel for sender, notifying possibly hidden stations

Sender can now send data at once, acknowledgement via ACK Other stations store NAV distributed via RTS and CTS

t

SIFS

DIFS

data

ACK

defer access

otherstations

receiver

senderdata

DIFS

new contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

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802.11 Collision Avoidance: RTS-CTS + ACK

DIFS: Distributed Inter-Frame SpacingSIFS: Short Inter-Frame Spacing

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802.11: PCF for Polling

tNAV

polledwirelessstations

point coordinator

NAV

PIFSD

USIFS

SIFSD

contentionperiod

contention free periodmediumbusy

D: downstream poll, or data from point coordinatorU: data from polled wireless station

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MAC Frame Format

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MAC Frame Fields

Frame Control – frame type, control information Duration/connection ID – channel allocation time Addresses – context dependant, types include source and

destination Sequence control – numbering and reassembly Frame body – MSDU or fragment of MSDU Frame check sequence – 32-bit CRC

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Frame Control Fields

Protocol version – 802.11 version Type – control, management, or data Subtype – identifies function of frame To DS – 1 if destined for DS From DS – 1 if leaving DS More fragments – 1 if fragments follow Retry – 1 if retransmission of previous frame

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Frame Control Fields

Power management – 1 if transmitting station is in sleep mode

More data – Indicates that station has more data to send WEP – 1 if wired equivalent protocol is implemented Order – 1 if any data frame is sent using the Strictly Ordered

service

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Control Frame Subtypes

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

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Data Frame Subtypes

Data-carrying frames Data Data + CF-Ack Data + CF-Poll Data + CF-Ack + CF-Poll

Other subtypes (don’t carry user data) Null Function CF-Ack CF-Poll CF-Ack + CF-Poll

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Management Frame Subtypes

Association request Association response Reassociation request Reassociation response Probe request Probe response Beacon

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Management Frame Subtypes

Announcement traffic indication message Dissociation Authentication Deauthentication

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Wired Equivalent Privacy

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Authentication

Open system authentication Exchange of identities, no security benefits

Shared Key authentication Shared Key assures authentication

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IEEE 802.11

802.11a 802.11b 802.11

Standard approved

Sep. 1999 Sep. 1999 July 1997

Available bandwidth

300 MHZ 83.5 MHZ 83.5 MHZ

Unlicensed freq. of operation

5.15-5.35G 5.725-5.825G

2.4-2.4835G 2.4-2.4835G

No. of non-overlapping Ch.

4 3 3

Rate per channel

6,9,12,18,24,36,48,54 Mbps

1, 2, 5.5, 11 Mbps

1,2 Mbps

Range 225 feet 225 feet ??

Modulation OFDM DSSS DSSS, FHSSDSSS: direct sequence spread spectrum

FHSS: frequency hopping spread spectrum

OFDM: orthogonal frequency division multiplexing