cse401n: computer networks
DESCRIPTION
CSE401n: Computer Networks. Lecture 16 Wireless Link & LANs WS: ch-14 KR: 5.7. 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 - PowerPoint PPT PresentationTRANSCRIPT
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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