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Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 1Chapter 2.8: WLAN
Design Aspects for Wireless Networks (WLAN)
• World-wide standardization
• Low power consumption to enable battery operation for mobile devices• Usage is possible without special permission and/or licenses
• Robust transmission technology• Simplification of (spontaneous) cooperation at meetings
• Simple use and administration• Retain of former investments in the fixed network area
• Security regarding the tapping of confidential data and also regarding emissions
• Transparency regarding applications and protocols of higher layers
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Page 2Chapter 2.8: WLAN
General Structure of Radio Networks
1. Networks with fixed infrastructure• Infrastructure means: stationary network e.g.
Ethernet or satellite routes
• Central Access Point (AP), wireless devices communicate only with the AP
• Control functionalities (media access, mobility management, authentication,…) are realized in the infrastructure
• Complexity lies in the infrastructure components, wireless devices only need to realize a minimum of functionality
2. Ad-hoc networks• No infrastructure – the wireless devices
are communicating directly
• Higher complexity of the devices, since every device has to implement all access and control mechanisms
InfrastructureL a p to pAP
APAPL a p to pL a p to p L a p to p L a p to p
LaptopLaptop
Laptop
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Wireless Ethernet
• Wireless equivalent to Ethernet: “Wireless LAN” (WLAN)• Exclusively data-oriented, wide-band Internet access solution
• Standardized by the IEEE as IEEE 802.11� 1997: IEEE 802.11 (capacity of maximally 2 MBit/s)
� IEEE 802.11a with 54 MBit/s, use of a (more susceptible for disturbances) frequency band
� 1999: IEEE 802.11b (data rate of 11 MBit/s with a utilizable data rate of of up to 6-7 MBit/s)
� IEEE 802.11g: enhancement of 802.11b with up to 54 MBit/s� …
802.11• 1 or 2 MBit/s• 2.4 GHz• FHSS, DSSS
802.11a
• 54 MBit/s• 5 GHz• FHSS, DSSS
802.11b
• 11 MBit/s• 2.4 GHz• only DSSS
802.11g• 54 MBit/s• 2.4 GHz• only DSSS
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WLAN: IEEE 802.11b
• Data rates– 1, 2, 5.5, 11 Mbit/s, depending on SNR (signal-to-noise ratio)– Utilizable data rate max. approx. 7 Mbit/s
• Range– 300m outside, 30m in buildings– Maximum data rate only can be used up to ~10m (in buildings)
• Frequency range– License-free 2,4 GHz ISM band (2,4 - 2,4835 GHz)
• Transmitting power– Maximally 100 mWatt
• Advantages:– Many installed systems, world-wide availability, free ISM-band, many
companies, integrated into laptops, simple system• Disadvantages:
– Strong disturbances on the ISM band (Bluetooth, microwave ovens,microwave lighting, analogue television, monitoring systems, license-free urban networks), no service quality, relatively low data rates
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Page 5Chapter 2.8: WLAN
Structure of a WLAN
Integration into an existing fixed network:
Fixed networkLaptopAP
APAP
LaptopLaptop Laptop Laptop
• Access Points (APs) are attached to an existing fixed network
• Each AP manages all communication in its reception range
• APs using the same channel must have enough distance to avoid disturbances
LaptopLaptop
Laptop
Forming an Ad-hoc network:• If no AP is available, stations also can
build up an own LAN• The transmission now takes place directly
between the stations
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APs in Aachen - MoPS
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Architecture: Infrastructure Network
• Station (STA)
Computer with access mechanism to the wireless medium and by this radio connection to the AP
• Access Point (AP)
Station which is integrated both in the radio and the wired network (distribution system)
• Basic Service Set (BSS)
Group of stations incl. the AP within an AP transmission range
• Portal
Gateway to another fixed network• Distribution system
Connection of different AP areas to one logical network (EES: Extended service set). Simplest principle: switch
Distribution system
Portal
802.x LAN
AccessPoint
802.11 LAN
BSS2
802.11 LAN
BSS1
AccessPoint
STA1
STA2STA3
ESS
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Architecture: Ad-hoc Network
• Direct communication within limited range
– Station (STA)Computer with access mechanism tothe wireless medium
– Independent Basic Service Set (IBSS)Group of stations which use the same carrier frequency within a transmission range
• Different IBSS are possible by spatial separation or by using different carrier frequencies
• No designated stations for the forwarding of data, routing,…
802.11 LAN
IBSS2
802.11 LAN
IBSS1
STA1
STA4
STA5
STA2
STA3
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Protocol Architecture
Medium Access Control• Access mechanism, fragmenting, encryption
• MAC management: synchronization, roaming between APs, power management
Physical layer• Channel selection, modulation, coding
Applications should not be aware of theexistence of the wireless network (except capacity, longer access times)
802.11 MAC
802.11 PHY
IP
TCP
Application
802.3 MAC
802.3 PHY
IP
TCP
Application
802.11 MAC
802.11 PHY
802.3 MAC
802.3 PHY
Fixed Terminal
AccessPoint
Mobile Terminal
Infrastructure Network
Server
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802.11 - Physical Layer
Variants for transmission: 2 radio (in 2,4 the GHz band), 1 infrared
• FHSS (Frequency Hopping Spread Spectrum)
– 2 frequencies at 1 Mbit/s, 4 frequencies at 2 Mbits/s– Frequency band divided into 79 different channels of 1 MHz bandwidth
– min. 2.5 frequency changes per second– GFSK modulation
– Max. transmission power: 1 W (USA)/100 mW (EU), minimum: 1 mW
• DSSS (Direct Sequence Spread Spectrum)
– DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK)
– Chip sequence: (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1), a barker code
– Maximum transmitting power: 1 W (USA)/100 mW (EU), minimum: 1 mW
• Infrared
– 850-950nm, diffuse light, typically 10m range
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FHSS PHY Frame Format
Synchronization SFD PLW PSF HEC Utilizable data
Preamble Header
80 16 12 4 16 variable Bits
• Synchronization
– Synchronization of the receiver by sequence 010101…• SFD (Start Frame Delimiter)
– 0000110010111101 as starting pattern• PLW (PLCP_PDU Length Word)
– Length of the utilizable data in bytes inclusive 32-bit CRC (at the end of the utilizable data). Permitted values are between 0 and 4095
• PSF (PLCP Signaling Field)
– Data rate of the utilizable data (1 or 2 Mbit/s)• HEC (Header Error Check)
– CRC with x16+x12+x5+1
Transmission always with 1 Mbit/s
Transmission alternatively with 1 or 2 Mbit/s
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DSSS PHY Frame Format
Synchronization SFD Signal Service HEC Pay load
Preamble Header
128 16 8 8 16 variable Bits
Length
16
• Synchronization– Snychronization, power management, signal detection, frequency adjustment
• SFD (Start Frame Delimiter)– 1111001110100000 as starting pattern
• Signal– Data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)
• Service
– Reserved for later use, standard: 00 for 802.11 frames
• Length (length of the utilizable data) and HEC (CRC) as for FHSS
Transmission always with 1 Mbit/s
Transmission alternatively with 1 or 2 Mbit/s
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802.11b - Physical Layer
Achieved bits/symbol
Used Symbol Rate
ModulationCode lengthData Rate
81,375 MS/sQPSK8 (CCK)11 Mbit/s
41,375 MS/sQPSK8 (CCK)5,5 Mbit/s
21 MS/sQPSK11 (barker code)2 Mbit/s
11 MS/sPSK11 (barker code)1 Mbit/s
Dynamic Rate Shifting: Adjustment of the data rate to the transmission quality:
• Only DSSS
• CCK: Complementary Code KeyingUse of an 8-Chips spreading sequence: select 64 (11 Mbit/s) resp. 4 (5,5 Mbit/s) of the 48 possible states, which have as good cross correlation characteristics as possible. I.e.: use spreading for the transmission of several bits at the same time
Thus the transmission becomes clearly more susceptible for disturbances than for 1 resp. 2 Mbit/s
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Range of IEEE 802.11
10 30 60 100 m0
2
4
6
8
10
Data rate
Mbit/s
Distance
802.11
802.11b
Due to missing spreading, the higher data transmission rates are more susceptible for disturbances. Thus, a smaller range results:
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Range of 802.11b
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IEEE 802.11b – PHY Frame Formats
synchronization SFD signal service HEC Utilizable data
Preamble Header
128 16 8 8 16 variable Bits
length
16
192 µs with 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s
short synch. SFD signal service HEC Utilizable data
Preamble(1 Mbit/s, DBPSK)
Header(2 Mbit/s, DQPSK)
56 16 8 8 16 variable Bits
length
16
96 µs 2, 5.5 or 11 Mbit/s
Long frame format:
Short frame format, optionally:
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Communication Channels in IEEE 802.11
The whole 2.4GHz ISM band is divided into several channels. To avoid interference, distances must be left between the channels. To avoid collisions, when configuring an access point a channel is assigned to it.
• FHSS: The band is divided into 79 sub-bands, the channel number determines the hop sequence
• DSSS: The band is divided into 11 resp. 13 sub-bands, each of these forms an own channel. Signal spreading is performed in the sub-bands:
→One sub-band has a bandwidth of 22 MHz. The sent data are spread to those bandwidth to avoid environmental disturbances:
Channel n
22 MHz
Purpose: even if the frequency range is disturbed partly, enough of the signal power reaches the receiver. If only on one frequency transmission would take place, the whole data would be lost.
Signal is spread to 11 frequencies (when using a barker code)
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Channel Selection with IEEE 802.11b
2400[MHz]
2412 2483.52442 2472
Channel 1 Channel 7 Channel 13
22 MHz
• Channels are 22 MHz wide• Channels are overlapping
• 13 channels in Germany (2412, 2417, 2422,… 2467, 2472 MHz), 11 in the USA/Canada
• Non overlapping channel selection:
• Ideally: assign only e.g. channels 1, 6 and 11: 116
1
611
1
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Medium Access Control
Procedure for Ethernet: CSMA/CD• Send as soon as the medium is free, listen whether a collision took place
Problems in wireless networks• Signal strength decreases squarely with the distance• CS/CD are used by the sender, but collisions happen at the receiver
• CS can deliver wrong results, e.g. if a terminal is “hidden” (Hidden station problem)• Therefore, a collision possibly is not detectable by the sender, i.e. CD fails
BA C
• A sends to B, C does not receive signal from A
• C wants to send to B, medium is unused for C (CS failed)
• Collision at B, A does not detect it (CD failed)
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Further Problems with CSMA/CD
Exposed station
• B sends to A, C wants to send to D• C must wait, because CS signals an
“occupied” medium
• Since A however is outside the range of C, C is blocked unnecessarily
BA C D
Terminals A and B send, C has to receive
• the signal strength weakens squarely with the distance, therefore the signal of B “drowns” the signal of A
• C cannot hear A
• Accurate efficiency control is necessary!A B C
Solution for the problems, especially hidden station:
CSMA/CA – CSMA with Collision Avoidance
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Collision avoidance
Collision detection does not work. Therefore: Collision Avoidance
Multiple Access with Collision Avoidance (MACA)• Three-way handshake minimizes number of hidden terminals
• Signaling frames contain sender and receiver address as well as frame size• Sender sends a short Request to Send (RTS) frame• Receiver answers with Clear to Send (CTS) frame
• Sender sends data
Multiple Access with Collision Avoidance by Invitation (MACA-BI)• Sender needs “invitation” before transmitting data to the receiver• RTS is omitted, Ready to Receive (RTR) instead of CTS
• Less complex than MACA, since fewer signaling frames are sent• But: the receiver must be able to estimate the traffic volume of the sender
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Avoidance of the hidden station problem:• A and C want to send to B
• A first sends RTS• C waits since it hears the CTS of B
RTS/CTS Handshake
A B C
RTS
CTSCTS
A B C
RTS
CTS
RTS
Avoidance of the exposed station problem:• B wants to send to A,
C wants to send to D
• C does not wait unnecessarily, because it does not receivethe CTS of A
D
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Further Procedures for Collision Avoidance
Collision avoidance through out-of-band signaling• Use of an additional channel for signaling
Busy Tone Multiple Access (BTMA)• Everyone who hears a continuous transmission on the data channel, sends “Busy
Tone” on another transmission channel (control channel)
• All devices in the range of 2 hops of an active station wait• No hidden stations, but many exposed stations
Receiver Initiated Busy Tone Multiple Access (RI-BTMA)• Only the receiver sends “Busy Tone”
• Hardly exposed stations, but the Busy Tone can be sent only if the receiver decoded the transmission wish
Wireless Collision Detect (WCD) Protocol• Combines BTMA and RI-BTMA: Two kinds of “Busy Tones”• First like BTMA: Stations send Busy Tone “collision detect”
• Receiver then sends “feedback tone” as soon as he detects transmission wish
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802.11 - MAC Layer: DFWMAC• Traffic types
– Asynchronous data service (standard)• Exchange of packets on “best effort” basis
• Support of broadcast and multicast– Time-limited service (optional)
• implemented via PCF (Point Coordination Function)
• Access methods– DFWMAC-DCF CSMA/CA (standard, DCF: Distributed Coordination Function)
• DFWMAC: Distributed Foundation Wireless MAC• Collision avoidance by random access with “backoff” mechanism
• Short time interval introduced between successive frames• Receipt acknowledgment by ACK (not with broadcast)
– DFWMAC-DCF with RTS/CTS (optional)• Avoidance of the hidden station problem
– DFWMAC-PCF (optional)• Collision-free, centralized polling with a list of stations in the AP
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802.11 - MAC LayerPriorities for media accesses
• Access times are controlled by introducing three waiting periods
• No guaranteed priorities
• SIFS (Short Inter-Frame Spacing) – 10µs– highest priority, for ACK, CTS, answer to polls
• PIFS (PCF Inter-Frame Spacing) – 30µs– medium priority, for time-limited services using PCF
• DIFS (DCF Inter-Frame Spacing) – 50µs– lowest priority, for asynchronous data services
t
Medium occupied SIFSPIFS
DIFSDIFS
next frameCompetition
direct access, if the channel was unused for a time ≥ DIFS
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t
Medium occupied SIFSPIFS
DIFSDIFS
next frame
Competition window(random backoffmechanism)
802.11 - CSMA/CA Procedure
• All implementations have to support the procedure• A station, which wants to send, listens to the medium• If the medium is free for at least the duration of a DIFS, the station may send
• If the medium is occupied, when becoming free the station waits for one DIFS and then randomly chooses a backoff time (collision avoidance, in multiples of a slot time). The station continues to listen to the medium
• If the medium is occupied by another station during the backoff time, the backofftimer stops. In the next try, no new backoff time is chosen randomly, but the old timer is gone on with.
• Also usable for broadcast
Time slot (20 µs)Waiting period
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boebor
boebor
boebor
Stations in Competition
t
busy
boe
Station1
Station2
Station3
Station4
Station5
Sending request
DIFSboe
boe
boe
busy
applied backoff time
bor remaining backoff time
busy Medium occupied (Frame, ACK, etc.)
DIFS
boe
boe
boe
DIFS
busy
busy
DIFSboe busy
boe
boe
bor
bor
The size of the competition window (Contention Window, CW) affects the efficiency. Therefore (similar to Ethernet) it starts with CW = 7 and is doubled with each collision up to CWmax = 255
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802.11 - CSMA/CA Procedures
Unicast transmission: the receipt is additionally confirmed, since collisions possibly are not detected by the transmitter
• Data can be sent after waiting for DIFS
• Receivers answer immediately (after SIFS, without additional backoff time), if the frame arrived correctly (CRC)
• In case of an error the frame is repeated automatically. No special treatment of a transmission repetition, same access mechanism as before
t
SIFS
DIFS
Data
ACK
Waiting period
FurtherStations
Receiver
SenderData
DIFS
Competition
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802.11 – DFWMAC with RTS/CTS
Optional extension for the avoidance of the hidden station problem:
• RTS with holding time as parameter can be sent after waiting for DIFS (plus backoff time)
• Confirmation of the receiver by CTS after SIFS (also containing holding time)
• Immediate sending of the data is possible, confirmation by ACK• Other stations store the holding time, which were sent in the RTS and CTS, in their
NAV (Network Allocation Vector)
• Collisions are only possible with RTS/CTS messages, but substantial overhead through RTS/CTS messages
tWaiting period
FurtherStations
Receiver
Sender
Competition
SIFS
DIFS
Data
ACK
Data
DIFS
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
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802.11 – DFWMAC with RTS/CTS
t
SIFS
DIFS
Data
ACK1
frag1
DIFS
Competition
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
NAV (frag1)NAV (ACK1)
SIFSACK2
frag2
SIFS
FurtherStations
Receiver
Sender
• Fragmenting data can decrease the damage caused by transfer errors • Special mechanism: adapt size of the fragments to current error rate of the
medium
• First: normal reservation with RTS/CTS• Fragments and ACKs (except the last for each case) contain reservation durations
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DFWMAC-PCF
PIFSD1
U1
SIFS
NAV
SIFSD2
U2
SIFS
SIFS
Super-framet0 t1
• PCF for guarantees concerning bandwidth and access delay
• AP controls medium access and cyclic queries all stations (Polling)• Super-frames with competition-free period and competition period (like before)• If the medium gets free (t1) after the begin of the super-frame (t0), the coordinator
cyclic asks all stations x (Dx) for sending needs. If necessary, they answer with Ux(the data to be sent)
• If the phase is ended earlier than planned (t2 instead of t3), more time remains for the competition phase (end is announced by a control frame CFend)
t
D3
PIFSD4
U4
SIFS
SIFSCFend
Competitioncompetition-free period
t2 t3 t4
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802.11 - Frame Format
Frame Control• Protocol version, frame type (administration, control, data), fragmenting,
encryption information, meaning of the following address fields
Duration ID• Sent along with RTC, CTS for setting the NAV
Addresses• In each case contains 48-Bit MAC addresses. MAC frames can be transferred
between two stations, between station and AP or between two APs by the distribution system. In the field Frame Control, two bits are determining the current meaning of the addresses. Addresses can be: Final destination, source address, BSS Identifier, intermediate sender address, intermediate receiver address
Sequence Control
• Recognition of duplicated frames
FrameControl
Duration/ID
Address1
Address2
Address3
SequenceControl
Address4
Data CRC
2 2 6 6 6 62 40-2312Byte
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802.11 - MAC Management
• Synchronization– Find a LAN, try to remain in the LAN
– Synchronization of internal clocks (e.g. FHSS, PCF, power savingmechanisms)
– Timer etc.
• Power management– Sleep mode without missing a message
– Periodic sleeping, frame buffering, traffic monitoring
• Association/Re-association
– Integration into a LAN– Roaming, i.e. moving between networks from one Access Point to another– Scanning, i.e. active search for a network
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Interval of the periodic radio
signal (beacon): 20ms - 1s
tMedium
AccessPoint
busy
B
busy busy busy
B B B
Value of the time stamp B Beacon frame
Synchronization by “Beacons”
• Beacon frame contains time stamps and administrative information for power saving mechanisms and roaming
• Varying times between beacon frames, since the medium can be occupied
• In infrastructure networks: AP takes over the sending of the beacons
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Synchronization by “Beacons” (ad-hoc)
tMedium
Station1
busy
B1
Beacon interval
busy busy busy
B1
Value of the time stamp B beacon frame
Station2B2 B2
random delay
• All stations try to send a Beacon frame in fixed intervals
• Standard access procedure with backoff
• One station wins and sends a beacon frame at first. All other stations synchronize to this frame.
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Power Management
• Idea: Switch off the sending/receiving device when not needed• Timing Synchronization Function
– Regular activation of all stations. Transmissions for sleeping stations are buffered; when waking up, the stations receive the transmission
• Infrastructure:– AP can store all pending frameworks for sleeping stations– With each beacon frame, a Traffic Indication Map (TIM) is sent along which
indicates, for which stations frames are buffered.– Additionally: List for broadcast/multicast receivers (Delivery Traffic Indication
Map, DTIM)
• Ad-hoc– Ad-hoc Traffic Indication Map (ATIM)
• The storing stations announce the receivers of stored packages
• More complex, since no central AP: all stations have to temporarily store frames
• Collisions of ATIMs possible (scalability?)
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Energy Saving with Activity Patterns (Infrastructure)
TIM interval
t
Medium
AccessPoint
busy
D
busy busy busy
T T D
T TIM D DTIM
DTIM interval
BB
B broadcast/multicast
Station
active
p poll confirmation
p
D
D
D data transmissionfrom/to the station
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Energy Saving with Activity Patterns (Ad-hoc)
awake
A ATIM transmission D Data transmission
t
Station1B1 B1
B Beacon frame
Station2B2 B2
random delay
A
a
D
d
ATIMwindow Beacon interval
a confirmation of ATIM d Confirmation of the data
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802.11 - Roaming
No or only bad connection?
• Scanning
– Scanning of the environment (listen to medium for beacons of APs or send a probe into the medium and wait for an answer)
• Re-association Request– Station sends inquiry at AP(s)
• Re-association Response– If success, i.e. an AP answered, the station now enters the network
– Further scanning, if no response is received
• AP accepts Re-association Request
– Indicate the distribution system about the new station– Distribution system updates location database (i.e. who is where)– Normally, the AP which was responsible for the station before, is
informed by the distribution system