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Wireless LAN (WLAN): infrastructure mode
Access Point
F1
F1
F1
F1F1
F1
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
WLAN: Infrastructure Network
−→ shared uplink & downlink channel F1
−→ single baseband channel
• basic service set (BSS)
• base station: access point (AP)
• mobile stations must communicate through AP
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WLAN: ad hoc mode
F1
F1
F1
F1F1
F1
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
MobileMobile
F1
F1
F1
F1
F1
WLAN: Ad Hoc Network
−→ homogeneous: no base station
−→ everyone is the same
−→ share forwarding responsibility
• independent basic service set (IBSS)
• mobile stations communicate peer-to-peer
→ also called peer-to-peer mode
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WLAN: internetworking
Access Point
F1
F1
F1
F1F1
F1
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Access Point
F1
F1
F1
F1F1
F1
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Access Point
F1
F1
F1
F1F1
F1
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
Mobile
WLAN: Extended Service Set
Distribution System
−→ internetworking between BSS’s through APs
−→ mobility and handoff
• extended service set (ESS)
• APs are connected by distribution system (DS)
→ typically: Ethernet switch
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• How do APs and Ethernet switches know where to
forward frames?
→ spanning tree
→ IEEE 802.1 (Perlman’s algorithm)
→ learning bridge: source address discovery
→ if unclear: broadcast
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Additional headache: mobility
−→ how to perform handoff
−→ mobility management at MAC vs. IP
Mobility between BSSes in an ESS
• association
→ registration process
→ AP sends out periodic beacon
→ mobile station (MS) associates with one AP
• disassociation
→ upon permanent departure: notification
• reassociation
→ movement of MS from one AP to another
→ inform new AP of old AP
→ forwarding of buffered frames
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IEEE 802.11b/g WLAN spectrum 2.4–2.4835 GHz:
−→ 11 channels (U.S.)
−→ 2.412 GHz, 2.417 GHz, . . ., 2.462 GHz
−→ unlicensed ISM (Industrial, Scientific, Medical) band
−→ global: 2.4–2.4835 GHz
−→ up to 14 channels
IEEE 802.11a: 5.15–5.35 GHz and 5.725–5.825 GHz
−→ UNNI (unlicensed National Information Infrastructure)
−→ non-global
IEEE 802.11n: both 2.4 and 5 GHz
−→ 2.4 GHz: backward compatible
−→ also uses multiple antennae
−→ called MIMO (multiple input multiple output)
−→ e.g., Apple’s 802.11n has 3 antennae
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Non-interference specification for 802.11b:
• each channel has 22 MHz bandwidth
• require 25 MHz channel separation
−→ thus, only 3 concurrent channels possible
−→ e.g., channels 1, 6 and 11
−→ 3-coloring. . .
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Examples:
Purdue Univ.: IEEE 802.11b (11 Mbps) WLAN network
−→ PAL (Purdue Air Link)
−→ partial mobility: MAC roaming (within ESS)
−→ no mobile IP
−→ football scores at Ross-Ade through PDAs
Dartmouth College: IEEE 802.11b WLAN (500+ APs)
−→ full VoIP
−→ free long distance
Seattle, SF, San Diego, Boston, etc.: WiFi communities
−→ “free” Internet access
−→ city: lamp-post; called mesh networks
−→ private: roof-top
−→ cable & DSL companies don’t like it
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IEEE 802.11 MAC
−→ CSMA/CA with exponential backoff
−→ almost like CSMA/CD
−→ drop CD
−→ explicit positive ACK frame
−→ added optional feature: CA (collision avoidance)
Two modes for MAC operation:
• Distributed coordination function (DCF)
→ multiple access (default mode)
• Point coordination function (PCF)
→ polling-based priority
. . . neither PCF nor CA used in practice
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Timeline without collision:
DATA BOBO DIFSDIFS
SIFS
2−way handshake
. . .
. . .
Time
TimeSender
ACK
Receiver
• SIFS (short interframe space): 10 µs
• Slot Time: 20 µs
• DIFS (distributed interframe space): 50 µs
→ DIFS = SIFS + 2 × slot time
• BO: variable back-off (within one CW)
→ CWmin: 31; CWmax: 1023
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Time snapshot with Mira-come-lately:
−→ Sue sends to Arnold
Arnold
DATA
ACK
Time
Time
SIFS
Time
Sue
Mira
Mira wants to send a frame
DIFS BO
DIFS
Slot Time
. . .
BO
WAITCS = BUSY . . .
Contention Window CW
DATA
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Time snapshot with collision (Sue & Mira):
Mira
Sue
Time
SIFS
Time
Time
Arnold
Sue wants to send a frame
Mira wants to send a frame
DATA
SIFS
BACKOFF
BACKOFF
Slot Time
WINNER
NOACK
ACKNO
BO
BODIFS
DIFS
DATA . . .
. . .DATA
COLLISION
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MAC throughput and collision (simulation):
0
1
2
3
4
5
6
3.5 4 4.5 5 5.5 6 6.5
Tho
ughp
ut (
Mb/
s)
Offered Load (Mb/s)
node 2node 5
node 10node 20node 30node 50
node 100
0
10
20
30
40
50
60
70
3.5 4 4.5 5 5.5 6 6.5
Col
lisio
n P
roba
bilit
y (%
)
Offered Load (Mb/s)
node 2node 5
node 10node 20node 30node 50
node 100
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MAC throughput (experiment):
−→ HP iPAQ pocket PC running Linux
0
1
2
3
4
5
6
4 4.5 5 5.5 6 6.5 7 7.5 8
Thr
ough
put (
Mb/
s)
Offered Load (Mb/s)
Node 2Node 5
Node 10Node 12Node 16
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Additional issues with CSMA in wireless media:
Hidden station problem:
MobileMobile Mobile
A B C
Hidden Station Problem
(1) (2)
(3)
(1) A transmits to B
(2) C does not sense A; transmits to B
(3) interference occurs at B: i.e., collision
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But: collision does not always mean junk
−→ i.e., transmission failure
For example:
if A’s frame has stronger signal strength than C’s frame,
B may still be able to successfully decode A’s frame
but not C’s frame
−→ called capture effect
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Biggest problem: starvation problem
−→ related to hidden station problem
−→ A cannot hear C, C cannot hear A
−→ B can hear both A and C
−→ by CS, B gets little chance to speak
−→ hence “sandwiched” B may starve
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Consider 4 APs, each with 10 laptops (i.e., 4 adjacent
BSS):
AP 1 AP 2 AP 3 AP 4
hearing (i.e., CS) range
0
1e+006
2e+006
3e+006
4e+006
5e+006
Througput (bps)
AP0AP1AP2AP3
−→ middle two APs get half the throughput
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400-AP (20 × 20) mesh network:
−→ campus-scale wireless network
−→ could be city block
−→ large residential community
0 1 2 3 4 5 6
0 5 10 15 20 0
5
10
15
20
−→ black squares: regions of near-starvation
−→ note alternating checker pattern
−→ solution?
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Hidden station problem: CA (congestion avoidance)
−→ RTS/CTS reservation handshake
• Before data transmit, perform RTS/CTS handshake
• RTS: request to send
• CTS: clear to send
SIFSSIFSSIFS
CTS
RTS
Sender
Receiver
reservation handshake
20B
14B 14B
29B − 2347B
same as before
. . .
. . .
Time
Time
DIFS DIFSBO
ACK
DATA
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Hidden station problem: RTS/CTS handshake “clears”
hidden area
MobileMobile Mobile
A B CRTS
CTS CTS
RTS/CTS Handshake
"clears the area"
RTS/CTS perform only if data > RTS threshold
−→ why not for small data?
. . . feature available but not used