CSE401n:Computer Networks Lecture 16 Wireless Link & LANs WS: ch-14 KR: 5.7 IEEE 802.11 Wireless LANs 802.11b operate at 2.4 GHz, 11 Mbps widely.
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Transcript CSE401n:Computer Networks Lecture 16 Wireless Link & LANs WS: ch-14 KR: 5.7 IEEE 802.11 Wireless LANs 802.11b operate at 2.4 GHz, 11 Mbps widely.
CSE401n:Computer Networks
Lecture 16
Wireless Link & LANs
WS: ch-14
KR: 5.7
1
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
2
IEEE 802.11 Wireless LAN
IEEE 802.11 standard:
unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz
and 802.11b
802.11a
3
Frequency Band
4
5
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
6
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
8
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
10
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
12
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
13
IEEE 802.11 Medium Access
Control
MAC layer covers three functional areas:
Reliable data delivery
Access control
Security
14
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
A
B
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?
16
The Exposed Terminal Problem
A
B
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
17
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
A
RTS
CTS
B
CTS
C
D
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Access Control
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Medium Access Control Logic
20
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
DIFS
DIFS
medium busy
PIFS
SIFS
contention
next frame
t
direct access if
medium is free DIFS
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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
DIFS
sender
data
RTS
SIFS
receiver
other
stations
CTS SIFS
SIFS
NAV (RTS)
NAV (CTS)
defer access
ACK
DIFS
new contention
data
t
24
802.11 Collision Avoidance: RTS-CTS + ACK
DIFS: Distributed Inter-Frame Spacing
SIFS: Short Inter-Frame Spacing
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802.11: PCF for Polling
PIFS
point
coordinator
D
D
SIFS
U
polled
wireless
stations
NAV
SIFS
NAV
medium
busy
contention free period
contention
period
t
D: downstream poll, or data from point coordinator
U: data from polled wireless station
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MAC Frame Format
27
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
28
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
36
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. 5.15-5.35G
of operation
5.725-5.825G
2.4-2.4835G
2.4-2.4835G
No. of nonoverlapping 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, FHSS
DSSS: direct sequence spread spectrum
FHSS: frequency hopping spread spectrum
OFDM: orthogonal frequency division multiplexing
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