192620010 Mobile & Wireless Networking - Universiteit Twenteheijenkgj/mwn/slides/Lecture-7.pdf ·...
Transcript of 192620010 Mobile & Wireless Networking - Universiteit Twenteheijenkgj/mwn/slides/Lecture-7.pdf ·...
Mobile and Wireless Networking 2013 / 2014
192620010 Mobile & Wireless Networking
Lecture 7: Wireless LAN
[Schiller, Section 7.3]
[Reader, Part 6] [Optional: "IEEE 802.11n Development: History, Process, and Technology",
Perahia, IEEE Communications Magazine, July 2008]
Geert Heijenk
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Outline of Lecture 9
q Wireless LAN q General Characteristics / IEEE 802.11 standard q IEEE 802.11 Physical Layers q Medium Access Control
l CSMA/CA l CSMA with RTS/CTS l MAC Quality of Service Enhancements
q MAC Frame Format q MAC Management q IEEE 802.11: Important Developments
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Design goals for wireless LANs
q global, seamless operation q low power for battery use q no special permissions or licenses needed to use the LAN q robust transmission technology q simplified spontaneous cooperation at meetings q easy to use for everyone, simple management q protection of investment in wired networks q security (no one should be able to read my data), privacy (no
one should be able to collect user profiles), safety (low radiation) q transparency concerning applications and higher layer
protocols, but also location awareness if necessary
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Comparison: infrastructure vs. ad-hoc networks
infrastructure network
ad-hoc network
AP AP
AP
wired network
AP: Access Point
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IEEE 802.11 - Architecture of an infrastructure network
Station (STA) q terminal with access mechanisms
to the wireless medium and radio contact to the access point
Basic Service Set (BSS) q group of stations using the same
radio frequency Access Point
q station integrated into the wireless LAN and the distribution system
Portal q bridge to other (wired) networks
Distribution System q interconnection network to form
one logical network (ESS: Extended Service Set) based on several BSS
Distribution System
Portal
802.x LAN
Access Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access Point
STA1
STA2 STA3
ESS
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IEEE 802.11 - Architecture of an ad-hoc network
Direct communication within a limited range
q Station (STA): terminal with access mechanisms to the wireless medium
q Independent Basic Service Set (IBSS): group of stations using the same radio frequency
Exception: q IEEE 802.11p for Vehicular
Networks: communication without setting up a basic service set.
802.11 LAN
IBSS2
802.11 LAN
IBSS1 STA1
STA4
STA5
STA2
STA3
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IEEE standard 802.11
mobile terminal
access point
fixed terminal
application
TCP
802.11 PHY
802.11 MAC
IP
802.3 MAC
802.3 PHY
application
TCP
802.3 PHY
802.3 MAC
IP
802.11 MAC
802.11 PHY
LLC
infrastructure network
LLC LLC
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IEEE 802.11 - Layers and functions
PLCP Physical Layer Convergence Protocol q clear channel assessment
signal (carrier sense) PMD Physical Medium Dependent
q modulation, coding PHY Management
q channel selection, MIB Station Management
q coordination of all management functions
PMD
PLCP
MAC
LLC
MAC Management
PHY Management
MAC q access mechanisms,
fragmentation, encryption MAC Management
q synchronization, roaming, authentication, MIB, power management
PH
Y D
LC
Sta
tion
Man
agem
ent
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The Physical Layer q IEEE 802.11
l Direct Sequence Spread Spectrum (DSSS) PHY – 2.4 GHz : RF : 1 – 2 Mbps
l Frequency Hopping Spread Spectrum (FHSS) PHY – 2.4 GHz : RF : 1- 2 Mbps
l Infrared (IR) PHY – Indoor : IR : 1 and 2 Mbps
q IEEE 802.11b l High Rate DSSS PHY
– 2.4 GHz : 5.5 Mbps – 11 Mbps q IEEE 802.11a
l OFDM PHY – 5 GHz : 6-54 Mbps
q IEEE 802.11g l OFDM PHY
– 2.4 GHz : 6-54 Mbps q IEEE 802.11n
l OFDM PHY + MIMO (up to 4x4) + 40 MHz channel (instead of 20 MHz) – Extension of a/g: 6.5 – 600 Mbps
q IEEE 802.11ac l OFDM PHY + MIMO (up to 8x8) + Multi-User MIMO (up to 4 simultaneous users)
+ 80 MHz + 256 QAM – extension of n in 5GHz: up to 4 x 1.69 Gbit/s
q IEEE 802.11ad l WiGig in 60 GHz, upto 7 Gbps
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IEEE 802.11a
Data rate q 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR q User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) q 6, 12, 24 Mbit/s mandatory
Transmission range
q 100m outdoor, 10m indoor l E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40
m, 12 up to 60 m
Frequency q Free 5 GHz ISM-band
Special Advantages/Disadvantages
q Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band
q Disadvantage: stronger shading due to higher frequency
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IEEE 802.11g
Frequency q Free 2.4 GHz ISM-band q But shared with many legacy WLANs and other systems, e.g. Bluetooth
Backward compatible with IEEE 802.11b Data rate (as 802.11a)
q 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR q User throughput significantly lower in presence of 802.11b stations
Transmission range
q Somewhat higher than 802.11a
Special Advantages/Disadvantages
q Advantage: backward compatibility, better propagation conditions q Disadvantage: crowded band, lower speed due to backward compatibility
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2400 [MHz]
2412 2483.5 2442 2472
channel 1 channel 7 channel 13
Europe (ETSI)
US (FCC)/Canada (IC)
2400 [MHz]
2412 2483.5 2437 2462
channel 1 channel 6 channel 11
22 MHz
22 MHz
802.11 b/g Channel selection (non-overlapping)
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rate service payload
variable bits
6 Mbit/s
PLCP preamble signal data
symbols 12 1 variable
reserved length tail parity tail pad
6 16 6 1 12 1 4 variable
6, 9, 12, 18, 24, 36, 48, 54 Mbit/s
PLCP header
IEEE 802.11a/g – PHY frame format
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802.11 - MAC layer I
Access methods q DCF CSMA/CA (mandatory)
l collision avoidance via randomized „back-off“ mechanism l minimum distance between consecutive packets l ACK packet for acknowledgements (not for broadcast)
q DCF w/ RTS/CTS (optional) l reduces hidden terminal problem
q PCF (optional) l access point polls terminals according to a list
q HCF EDCA (optional) l CSMA/CA with priority levels
q HCF CCA (optional) l Improved polling
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t
medium busy SIFS PIFS DIFS DIFS
next frame contention
direct access if medium is free ≥ DIFS
802.11 - MAC layer II
Priorities q defined through different inter frame spaces q no guaranteed, hard priorities q SIFS (Short Inter Frame Spacing)
l highest priority, for ACK, CTS, polling response q PIFS (PCF IFS)
l medium priority, for time-bounded service using PCF q DIFS (DCF, Distributed Coordination Function IFS)
l lowest priority, for asynchronous data service
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t
medium busy
DIFS DIFS
next frame
contention window (randomized back-off mechanism)
slot time direct access if medium is free ≥ DIFS
802.11 - CSMA/CA access method (DCF) I
q station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment)
q if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type)
q if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time)
q if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)
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t
busy
boe
station1
station2
station3
station4
station5
packet arrival at MAC
DIFS boe
boe
boe
busy
elapsed backoff time
bor residual backoff time
busy medium not idle (frame, ack etc.)
bor
bor DIFS
boe
boe
boe bor DIFS
busy
busy
DIFS boe busy
boe
boe
bor
bor
802.11 - competing stations - simple version
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802.11 - Binary Exponential Backoff
q Stations choose their backoff time randomly from contention window
q Ideal contention window size is trade-of between acceptable load and experienced delay
q Initial contention window size (CWmin) is 7 slots (backoff time between 0 and 7)
q After collision (no ack), contention window is “doubled” until CWmax = 255 is reached: 7 -> 15 -> 31 -> 63 -> 127 -> 255
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t
SIFS
DIFS
data
ACK
waiting time
other stations
receiver
sender data
DIFS
contention
802.11 - CSMA/CA access method (DCF) II
q Sending unicast packets q station has to wait for DIFS before sending data q receivers acknowledge at once (after waiting for SIFS) if the packet
was received correctly (CRC) q automatic retransmission of data packets in case of transmission
errors
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t
SIFS
DIFS
data
ACK
defer access
other stations
receiver
sender data
DIFS
contention
RTS
CTS SIFS SIFS
NAV (RTS) NAV (CTS)
802.11 – CSMA/CA with RTS/CTS
q Sending unicast packets q station can send RTS with reservation parameter after waiting for DIFS
(reservation determines amount of time the data packet needs the medium) q acknowledgement via CTS after SIFS by receiver (if ready to receive) q sender can now send data at once, acknowledgement via ACK q other stations store medium reservations distributed via RTS and CTS
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t
SIFS
DIFS
data
ACK1
other stations
receiver
sender frag1
DIFS
contention
RTS
CTS SIFS SIFS
NAV (RTS) NAV (CTS)
NAV (frag1) NAV (ACK1)
SIFS ACK2
frag2
SIFS
Fragmentation
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802.11e - MAC services (QoS support)
• Distributed Coordination Function • Point Coordination Function (not implemented) • + Hybrid Coordination Function (HCF)
• backward compatible with DCF
HCF: • HCF Controlled Channel Access (HCCA) • Enhanced Distributed Channel Access (EDCA)
• Differentiation priority levels • 4 access categories with separate queues in each STA
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EDCA
Multiple Access Categories with their own transmission queue Differentiation by means of: • Arbitray Inter Frame Spacing (AIFS) • Initial Contention Window (CWmin) • Maximum Contention Window (CWmax) • Transmission Opportunity Limit (TXOP Limit)
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DCF Summary
frame at the beginning of the immediately following slot. Otherwise, the station defers its frame transmissionaccording to the binary exponential backo! algorithm: The station waits until the medium has become idle foran interval of DIFS and then sets its backo! timer. The backo! timer gives the additional time it should waitbefore the next transmission attempt. It is set as the product of aStotTime and a pseudorandom integer that isdrawn from a uniform distribution over the interval [0,CW]. The contention window CW is set to its minimumvalue CWmin (CWmin = 31) for the first time a frame is backed o!. Every time the frame incurs a collision, CWis increased to (CW + 1) · 2 ! 1, with the restriction that CW cannot exceed its maximum possible valueCWmax (CWmax = 1023). The backo! timer is decreased by one time slot for every consecutive idle slot afterthe medium has been sensed idle for an interval of DIFS, and is suspended whenever the medium becomesbusy. The backo! process resumes when the wireless medium has been detected to be idle for an intervalof DIFS. When the backo! timer reaches zero, the station transmits the frame immediately.
If the destination station receives the frame correctly, it sends a positive acknowledgment (ACK) frameafter a time interval of Short InterFrame Space (SIFS). Note that SIFS is shorter than DIFS. When the sourcestation does not receive an ACK within an ACKTimeout interval, it assumes the frame has encountered a col-lision, updates the contention window CW according to the binary exponential backo! algorithm describedabove, and sets its backo! timer according to a newly selected random value within [0,CW]. A maximumof 7 retransmissions (4 retransmissions) for short frames (long frames) are allowed before the frame isdropped. The basic access procedure is depicted in Fig. 1(a). To further decrease the overhead incurred inframe collision and hidden terminal e!ects, Request-To-Send (RTS) and Clear-To-Send (CTS) frames mayalso be exchanged before the data transmission.
It is worth noting that collision avoidance is achieved by a virtual carrier sense mechanism. Whether thechannel is idle or busy is not solely determined by the physical carrier sense result, but also by the value ofthe NAV timer maintained by the MAC. A duration value is included in each frame that is transmitted bya station and indicates how long the transmission will last, including any subsequent acknowledgments andfragments. Each station in the vicinity of the transmitting station receives the frame and uses the durationvalue to update its NAV. Therefore the NAV value indicates how long another station has access to the wire-less medium no matter what is the real activity of that station.
2.2. IEEE 802.11E enhanced distributed channel access
As mentioned in Section 1, IEEE 802.11e defines a new HCF coordination function that includes both thecontention-based channel access method, EDCA, and the controlled channel access method, HCCA. AsEDCA is essentially an extension of the legacy DCF mechanism for which several analytical models existand can be leveraged, we focus on EDCA in this paper.
In EDCA, several parameters that control how and when a node gains access to the medium (e.g., the con-tention window, the inter-frame space, and the transmission opportunity) di!er among di!erent priority levels,so as to favor/disfavor data transmission from high-priority/low-priority flows. A total of eight user prioritylevels are available, and mapped to four access categories (ACs). Each AC corresponds to one of the fourtransmit queues that implement the EDCA contention algorithm with di!erent parameters. These parameters
DIFS
DIFS
PIFS
SIFSBusy Medium Backoff-Window Next Frame
Slot Time
Select Slot and Decrement Backoff as longas medium is idle
Defer Access
Contention Window
Immediate access whenmedium is free >=DIFS
DIFS/AIFS
Busy Medium Backoff Slots Next Frame
Slot Time
Select Slot and Decrement Backoff as longas medium is idle
Defer Access
SIFS
PIFS
DIFS
AIFS[i]
AIFS[j]
Contention Window
Immediate access whenMedium is free >= DIFS/AIFS[i]
Fig. 1. Access methods in IEEE 802.11 DCF and IEEE 802.11e EDCA: (a) basic access in DCF and (b) basic access in EDCA.
1958 Y. Ge et al. / Computer Networks 51 (2007) 1955–1980
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EDCA Operation (AIFS)
frame at the beginning of the immediately following slot. Otherwise, the station defers its frame transmissionaccording to the binary exponential backo! algorithm: The station waits until the medium has become idle foran interval of DIFS and then sets its backo! timer. The backo! timer gives the additional time it should waitbefore the next transmission attempt. It is set as the product of aStotTime and a pseudorandom integer that isdrawn from a uniform distribution over the interval [0,CW]. The contention window CW is set to its minimumvalue CWmin (CWmin = 31) for the first time a frame is backed o!. Every time the frame incurs a collision, CWis increased to (CW + 1) · 2 ! 1, with the restriction that CW cannot exceed its maximum possible valueCWmax (CWmax = 1023). The backo! timer is decreased by one time slot for every consecutive idle slot afterthe medium has been sensed idle for an interval of DIFS, and is suspended whenever the medium becomesbusy. The backo! process resumes when the wireless medium has been detected to be idle for an intervalof DIFS. When the backo! timer reaches zero, the station transmits the frame immediately.
If the destination station receives the frame correctly, it sends a positive acknowledgment (ACK) frameafter a time interval of Short InterFrame Space (SIFS). Note that SIFS is shorter than DIFS. When the sourcestation does not receive an ACK within an ACKTimeout interval, it assumes the frame has encountered a col-lision, updates the contention window CW according to the binary exponential backo! algorithm describedabove, and sets its backo! timer according to a newly selected random value within [0,CW]. A maximumof 7 retransmissions (4 retransmissions) for short frames (long frames) are allowed before the frame isdropped. The basic access procedure is depicted in Fig. 1(a). To further decrease the overhead incurred inframe collision and hidden terminal e!ects, Request-To-Send (RTS) and Clear-To-Send (CTS) frames mayalso be exchanged before the data transmission.
It is worth noting that collision avoidance is achieved by a virtual carrier sense mechanism. Whether thechannel is idle or busy is not solely determined by the physical carrier sense result, but also by the value ofthe NAV timer maintained by the MAC. A duration value is included in each frame that is transmitted bya station and indicates how long the transmission will last, including any subsequent acknowledgments andfragments. Each station in the vicinity of the transmitting station receives the frame and uses the durationvalue to update its NAV. Therefore the NAV value indicates how long another station has access to the wire-less medium no matter what is the real activity of that station.
2.2. IEEE 802.11E enhanced distributed channel access
As mentioned in Section 1, IEEE 802.11e defines a new HCF coordination function that includes both thecontention-based channel access method, EDCA, and the controlled channel access method, HCCA. AsEDCA is essentially an extension of the legacy DCF mechanism for which several analytical models existand can be leveraged, we focus on EDCA in this paper.
In EDCA, several parameters that control how and when a node gains access to the medium (e.g., the con-tention window, the inter-frame space, and the transmission opportunity) di!er among di!erent priority levels,so as to favor/disfavor data transmission from high-priority/low-priority flows. A total of eight user prioritylevels are available, and mapped to four access categories (ACs). Each AC corresponds to one of the fourtransmit queues that implement the EDCA contention algorithm with di!erent parameters. These parameters
DIFS
DIFS
PIFS
SIFSBusy Medium Backoff-Window Next Frame
Slot Time
Select Slot and Decrement Backoff as longas medium is idle
Defer Access
Contention Window
Immediate access whenmedium is free >=DIFS
DIFS/AIFS
Busy Medium Backoff Slots Next Frame
Slot Time
Select Slot and Decrement Backoff as longas medium is idle
Defer Access
SIFS
PIFS
DIFS
AIFS[i]
AIFS[j]
Contention Window
Immediate access whenMedium is free >= DIFS/AIFS[i]
Fig. 1. Access methods in IEEE 802.11 DCF and IEEE 802.11e EDCA: (a) basic access in DCF and (b) basic access in EDCA.
1958 Y. Ge et al. / Computer Networks 51 (2007) 1955–1980
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Frame Control
Duration/ ID
Address 1
Address 2
Address 3
Sequence Control
Address 4 Data CRC
2 2 6 6 6 6 2 4 0-2312 bytes
Protocol version Type Subtype To
DS More Frag Retry Power
Mgmt More Data WEP
2 2 4 1 From DS
1
Order
bits 1 1 1 1 1 1
802.11 – MAC Frame format
q Types q control frames, management frames, data frames
q Duration /ID q for NAV during RTS/CTS and fragmentation, and CFP
q Sequence control q important against duplicated frames due to lost ACKs
q Addresses q receiver, transmitter (physical), BSS identifier, sender (logical)
q Miscellaneous q checksum, frame control, data
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MAC address format
scenario to DS fromDS
address 1 address 2 address 3 address 4
ad-hoc network 0 0 DA SA BSSID -infrastructurenetwork, from AP
0 1 DA BSSID SA -
infrastructurenetwork, to AP
1 0 BSSID SA DA -
infrastructurenetwork, within DS
1 1 RA TA DA SA
DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address
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Special Frames: ACK, RTS, CTS
Acknowledgement Request To Send Clear To Send
Frame Control Duration Receiver
Address Transmitter
Address CRC
2 2 6 6 4 bytes
Frame Control Duration Receiver
Address CRC
2 2 6 4 bytes
Frame Control Duration Receiver
Address CRC
2 2 6 4 bytes
ACK
RTS
CTS
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802.11 - MAC management
Synchronization q try to find a LAN, try to stay within a LAN q timer etc.
Power management q sleep-mode without missing a message q periodic sleep, frame buffering, traffic measurements
Association/Reassociation q integration into a LAN q roaming, i.e. change networks by changing access points q scanning, i.e. active search for a network
MIB - Management Information Base q managing, read, write
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beacon interval
t medium
access point
busy
B
busy busy busy
B B B
value of the timestamp B beacon frame
Synchronization using a Beacon (infrastructure)
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t medium
station1
busy
B1
beacon interval
busy busy busy
B1
value of the timestamp B beacon frame
station2 B2 B2
random delay
Synchronization using a Beacon (ad-hoc)
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Power management
Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF)
q stations wake up at the same time Infrastructure
q Traffic Indication Map (TIM) l list of unicast receivers transmitted by AP
q Delivery Traffic Indication Map (DTIM) l list of broadcast/multicast receivers transmitted by AP
Ad-hoc q Ad-hoc Traffic Indication Map (ATIM)
l announcement of receivers by stations buffering frames l more complicated - no central AP l collision of ATIMs possible (scalability?)
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TIM interval
t
medium
access point
busy
D
busy busy busy
T T D
T TIM D DTIM
DTIM interval
B B
B broadcast/multicast
station
awake
p PS poll
p
d
d
d data transmission to/from the station
Power saving with wake-up patterns (infrastructure)
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awake
A transmit ATIM D transmit data t
station1 B1 B1
B beacon frame
station2 B2 B2
random delay
A
a
D
d
ATIM window beacon interval
a acknowledge ATIM d acknowledge data
Power saving with wake-up patterns (ad-hoc)
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802.11 - Roaming
No or bad connection? Then perform: Scanning
q scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer
Reassociation Request q station sends a request to one or several AP(s)
Reassociation Response q success: AP has answered, station can now participate q failure: continue scanning
AP accepts Reassociation Request q signal the new station to the distribution system q the distribution system updates its data base (i.e., location
information) q typically, the distribution system now informs the old AP so it can
release resources
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IEEE 802.11 – standards and amendments
802.11e: MAC Enhancements – QoS 802.11h: Spectrum Managed 802.11a (DCS, TPC) – 802.11i: Enhanced Security Mechanisms IEEE 802.11-2007: new release of the standard that includes a, b, d, e, g, h, i & j. 802.11n: > 100 Mbps (MIMO) 802.11p: inter-vehicle communications 802.11s: Mesh Networking IEEE 802.11-2012: new release of the standard that includes k – z. 802.11ac: 1 Gbps in 5 GHz band (> bw, > mimo-channels, 256 QAM) 802.11ad: 7 Gbps in 60 GHz band (a.o. beamforming)