wireless ethernet wireless lans: design goals · † fhss (frequency hopping spread spectrum) –...
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Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 1Chapter 3.2: WLAN
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� IEEE 802.11 (data rate of 2 MBit/s), standardised in 1997
� IEEE 802.11a with 54 MBit/s, use of a 5 GHz frequency band� IEEE 802.11b with 11 MBit/s in a 2.4 GHz frequency range
� IEEE 802.11g: enhancement of 802.11b with up to 54 MBit/s� IEEE 802.11n: data rates up to several hundreds of MBit/s (not finished)
� …
802.11• 1 or 2 MBit/s• 2.4 GHz• FHSS, DSSS
802.11a
• 54 MBit/s• 5 GHz• OFDM
802.11b
• 11 MBit/s• 2.4 GHz• DSSS
802.11g• 54 MBit/s• 2.4 GHz• OFDM, DSSS
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Page 2Chapter 3.2: WLAN
Wireless LANs: Design Goals
• Global, seamless operation
• Low power for battery use • No special permissions or licenses needed to use the LAN
• Robust transmission technology• Simplified spontaneous cooperation at meetings • Easy to use for everyone, simple management
• Protection of investment in wired networks • Security (no one should be able to read my data), privacy (no one should be able
to collect user profiles), safety (low radiation)
• Transparency concerning applications and higher layer protocols, but also location awareness if necessary
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Page 3Chapter 3.2: WLAN
Structure of a WLAN
1. Infrastructure network
• Access Points (APs) are attached to an existing fixed network (Ethernet, Satellites, …)
• Each AP manages all communication in its reception range
• APs using the same frequency range must have enough distance to avoid disturbances
• Control functionality (medium access, mobility management, authentication, …) are realized within the infrastructure, wireless devices only need a minimum of functionality
2. 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
• Higher complexity needed within the stations (control functionality)
Fixed networkL 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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Page 4Chapter 3.2: WLAN
Infrastructure Network
Distribution System
Portal
802.x LAN
AccessPoint
802.11 LAN
BSS2
802.11 LAN
BSS1
AccessPoint
STA1
STA2STA3
ESS
• 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
• PortalGateway to another fixed network
• Distribution systemConnection of different AP areas to one logical network (EES: Extended service set). Simplest principle: switch
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Page 5Chapter 3.2: WLAN
Ad-hoc Network
802.11 LAN
IBSS2
802.11 LAN
IBSS1
STA1
STA4
STA5
STA2
STA3
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 frequenciesNo designated stations for the forwarding of data, routing,… …
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802.11 Protocols
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 the existence of the wireless network (except capacity, longer access times)
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IEEE 802.11 Variants
Improved measurement/evaluation/management of radio parameters (e.g. signal strength), e.g. for enabling location based services
802.11k
Japanese variant of 802.11a for the frequency range of 4,9 GHz - 5 GHz802.11j
Authentication/encryption for 802.11a/b/g, e.g. WPA802.11i
54 MBit/s WLAN in the 5 GHz band with dynamic adaptation of channel and frequency choice as well as automatic adaptation of transmission power (enhancement of IEEE 802.11a for Europe)
802.11h
54 MBit/s WLAN in the 2,4 GHz band 802.11g
Inter Access Point Protocol (IAPP), allows communication between Access Points of different vendors, e.g. for exchanging roaming information
802.11f
QoS und streaming enhancement for 802.11a/g/h 802.11e
"World Mode", Adaptation to regional regulations (e.g. used frequency ranges)802.11d
Wireless Bridging between Access Points802.11c
11 MBit/s WLAN in the 2,4 GHz band 802.11b
54 MBit/s WLAN in the 5 GHz band 802.11a
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IEEE 802.11 Variants
Support of Virtual WLANs802.11q
3650-3700 MHz Operation in the U.S. 802.11y
Protection of Management Frames 802.11w
Wireless network management802.11v
Interworking with non-802 networks (for example, cellular) 802.11u
Wireless Performance Prediction (WPP) - test methods and metrics802.11t
ESS Mesh Networking 802.11s
Fast roaming between APs to avoid gaps in Voice over WLAN audio802.11r
WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars)
802.11p
Enhancement for a future, faster WLAN with data rate of 100 - 600 MBit/s802.11n
Summary of earlier enhancements, correction of errors in former specifications (maintenance)
802.11m
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802.11 – Physical Layer
Variants for transmission: 2 using radio (in the 2.4 GHz band), 1 using infrared
• FHSS (Frequency Hopping Spread Spectrum)
– 79 different channels with 1 MHz bandwidth each– Hopping between 2 channels for 1 MBit/s, between 4 channels for 2 MBit/s
– Min. 2.5 hops/sec– GFSK modulation
– Max. transmission power: 1 W (USA)/100 mW (EU), min. 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)
– Chipping sequence: (+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1), a Barker-Code
– Max. transmission power: 1 W (USA)/100 mW (EU), min. 1 mW
• Infrared
– 850-950nm, diffuse light, typically 10 m range
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IEEE 802.11b
• Data rate– 1, 2, 5.5, 11 MBit/s, depending
on SNR – User throughput max. approx.
6 MBit/s• Transmission range
– 100m outdoor, 30m indoor (directed links: several km)
– Max. data rate ~ 10m (indoor)• Frequency range
– Unlicensed 2.4 GHz ISM band• Security
– SSID, WPA2
• Connection setup time– Connectionless, „always on“
• QoS– Best effort, no guarantees (some
defined in “bad” way, later on much better standardized in 802.11e)
• Manageability– Limited (no automatic key distribution,
symmetrical encryption)
• Special advantages/disadvantages– Advantages: free ISM band, many
vendors, simple system
– Disadvantage: heavy interferences on the ISM band, no QoS, relatively low data rates
• Usage– Preferred version in Europe
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Channels in IEEE 802.11b
2400[MHz]
2412 2483.52437 2462
Channel 1 Channel 6 Channel 11
22 MHz
• Two APs using the same frequency would have interferences in the overlapping area – thus: divide the whole frequency range in channels
• Each channel in IEEE 802.11b has a bandwidth of 22 MHz• 13 channels in Germany (2412, 2417, 2422, …, 2472 MHz), 11 in USA/Canada
• Channels overlap! Non-overlapping choice of channels:
• Ideal case: only use channels 1, 6 und 11:
116
1
611
1
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Channels in IEEE 802.11b
Available in the ISM band (most of Europe): 2400 – 2483,5 MHz
MHz2400 2410 2420 2430 2440 2450 2460 2470 2480
Channel 12401 2412 2423
Channel 12401 2412 2423
Carrier frequencyChannel 6
2426 2437 2448Channel 11
2451 2462 2473
Channel 22406 2417 2428
Channel 72431 2442 2453
Channel 122456 2467 2478
Channel 32411 2422 2433
Channel 82436 2447 2458
Channel 132461 2472 2483
Channel 42416 2427 2438
Channel 92441 2452 2463
Channel 142473 2484 2495
Channel 52421 2432 2443
Channel 102446 2457 2468
Japan ( 1 – 14)
USA/Canada: channel 1 - 11
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Dynamic Rate Shifting
Bits/SymbolUsed Symbol RateModulationCode lengthData Rate
811 Mbit/s
41,375 MS/s
Modified DSSS/QPSK
8 (CCK)5,5 Mbit/s
2DSSS/QPSK2 Mbit/s
11 MS/s
DSSS/PSK11 (barker code)
1 Mbit/s
Adjustment of the data rate to the transmission quality:
CCK: Complementary Code Keying
• Use of an 8-chip spreading sequence where each chip is modulated with QPSK• QPSK has 4 states, chipping sequence has length 8 → 48 resulting states
• Select 64 (for 11 Mbit/s) resp. 4 (for 5,5 Mbit/s) of the states which have as good cross correlation characteristics as possible (i.e. are as different as possible)
• That means: make use of 4 resp. 16 code words which can be transferred instead of only 1 as with the barker code (i.e. skip some robustness)
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Channels
The whole 2.4GHz ISM band is divided into 11 resp. 13 overlapping channels. On each channel, DSSS is used for signal spreading:
→One sub-band has a bandwidth of 22 MHz. The sent data are spread to those bandwidth to avoid environmental disturbances
→ The chips of the barker code resp. CCK are sent in sequence – this increases the number of symbols per second compared with “pure” sending of the data, thus a larger bandwidth is needed
→Purpose: even if the frequency range is disturbed partly, enough of the signal power reaches the receiver on the rest of the channel; if a non-spread transmission would take place, the whole data would be lost in case of narrowband interference
→ If CCK is used, we use “several codes” instead of the same chipping sequence everytime - the transmission becomes more susceptible for disturbances thanwith use of the barker code, if we have a distortion (maybe caused by an overlapping channel)!
Channel n
22 MHz
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Range of IEEE 802.11b
10 30 60 100 m0
2
4
6
8
10
Data rate
Mbit/s
Distance
802.11
802.11b
Due to “abused” spreading in case of CCK, 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 802.11a
• Data rates
– 6, 9, 12, 18, 24, 36, 48, 54 MBit/s, depending on SNR
– User Throughput: max. 32 MBit/s
– 6, 12, 24 MBit/s mandatory• Transmission range
– 100m outdoor, 10m indoor (e.g. 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30 m, 18 up to 40 m, 12 up to 60 m)
• Frequency range– Free 5.15-5.35 + 5.725-5.825
GHz ISM band
• Security– SSID, WPA2
• Connection setup time
– Connectionless, „always on“• QoS
– Best effort, no guarantees (same as for 802.11b)
• Manageability
– Limited (same as for 802.11b)• Special advantages/disadvantages
– Advantages: uses less crowded free ISM band, available worldwide, simple system, many vendors
– Disadvantages: strong shading due to high frequencies, no QoS
• Usage– Preferred version in USA
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Channels in IEEE 802.11a
5150 [MHz]5180 53505200
36 44
16,6 MHz
center frequency = 5000 + 5·channel-no. [MHz]
channel-no.40 48 52 56 60 64
149 153 157 161
5220 5240 5260 5280 5300 5320
5725 [MHz]5745 58255765
16,6 MHz
channel-no.
5785 5805
Channels are also overlapping, as in 802.11b:
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subcarriernumber
Modulation in 802.11a: OFDM
• OFDM with 52 subcarriers (64 in total, 6 as guard space on each side)
• Subcarriers overlap with 312,5 kHz spacing, but orthogonality of chosen frequencies allows for clear separation
• 48 data subchannels + 4 subchannels for phase reference (pilot)
• Pilots are used by the receiver to deal with multipath propagation: phase references for the whole band are sent here, the receiver can interpolate phase shifts for the data carriers
1 7 21 26-26 -21 -7 -1
channel center frequency
312,5 kHzphase reference (pilot)
And: IEEE 802.11g simply is introducing OFDM on the existing 802.11b system, i.e. replacing of DSSS by OFDM for higher data rates (while keeping the ability to switch to DSSS for interworking with 802.11b)
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Medium Access Control
We can assign one channel with an AP – but then we have to coordinate all mobile stations in their communication with the AP. Chosen for IEEE 802.11a/b/g/…:
„Wireless Ethernet“ – MAC protocol is oriented at CSMA/CD• Hidden Station Problem
• Exposed Station Problem
Solution of the problems, especially Hidden Station
CSMA/CA – CSMA with Collision Avoidance
Types of traffic
• Asynchronous data service (standard)� Exchange of data by „best effort“� Support of broadcast and multicast
• Time-bound services (optional)� Implementation of some degree of QoS
� Only for infrastructure networks
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802.11 – MAC Layer: DFWMAC
Access strategies
• DFWMAC-DCF CSMA/CA (standard)� DFWMAC: Distributed Foundation Wireless MAC
� DCF: Distributed Coordination Function� collision avoidance by random access with backoff mechanism� Minimum time between two frames
� ACKs for acknowledging correct receipt (not for broadcast)
• DFWMAC-DCF with RTS/CTS (optional)
� Avoidance of Hidden Stations� MACA variant (Multiple Access with Collision Avoidance)
• DFWMAC-PCF (optional)� PCF: Point Coordination Function
� Collision-free, centralized Polling strategy where the AP has a list of all connected stations
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802.11 – MAC Layer
Priorities for medium access
• defined through different timing intervals• no guaranteed priorities
• SIFS (Short Inter Frame Spacing) – 10µs– highest priority, used for ACK, CTS, polling response
• PIFS (PCF IFS) – 30µs– medium priority, for time-bounded services using PCF
• DIFS (DCF IFS) – 50µs– lowest priority, für asynchronous data service
t
Medium busy SIFSPIFS
DIFSDIFS
next framecontention
direct access, if time the medium is free ≥ DIFS
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t
Medium busy SIFSPIFS
DIFSDIFS
next frame
contention window(randomized backoffmechanism)
802.11 - CSMA/CA Method
time slot (20 µs)waiting time
• Mandatory for all implementations• Before sending, a station performs carrier sense
• 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
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Example - Backoff
data
wait
B1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervalsat nodes 1 and 2
B2 = 10
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Competing Stations
boe
boe
boe
t
busy
Station1
Station2
Station3
Station4
Station5
DIFSboe
boe
boe
busy
bor
bor
DIFS
boe
boe
boe bor
DIFS
busy
busy
DIFSboe busy
boe
boe
bor
bor
boe
Sending request
elapsed backoff time
bor remaining backoff time
busy Medium busy (Frame, ACK, etc.)
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 Method
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 time
otherstations
receiver
senderData
DIFS
contention
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Competing Stations (with ACK)
t
busy
boij
Station1
Station2
Station3
Station4
Station5
Sending request
SIFSbo11
bo21
bo51
busy
jth backoff time of station ibusy Medium occupied (Frame, ACK, etc.)
DIFS
bo41
bo51
bo11
DIFS
busy
busy
DIFSbo11 busy
bo42
bo52
The size of the competition window (Contention Window, CW) affects the efficiency. Therefore (similar to Ethernet) it starts with CW = 8 and is doubled with each collision up to CWmax = 256
ACK
DIFS
ACK Acknowledgement
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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 time
otherstations
receiver
sender
contention
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
contention
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
NAV (frag1)NAV (ACK1)
SIFSACK2
frag2
SIFS
otherstations
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
contentioncontention-free period
t2 t3 t4
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What is implemented?
Any vendor has to implement the standard CSMA/CA variant, the other two are optional
• RTS/CTS very often is implemented by AP manufacturers, but: disabled!
• Usual method:� A frame size threshold is defined, and only frames longer than the threshold
are sent with RTS/CTS (to avoid overhead for small frames)
� The threshold value in basic configuration is sent to maximum allowed frame length…
� Changing the threshold value allows you to enable the RTS/CTS
� Only possibility to really avoid collisions• PCF mechanism usually is not implemented
� Not needed in many cases, and not possible in ad-hoc networks� Would allow for real-time data transmission, but is not good in it, thus it
doesn’t became prominent – instead, a QoS enhancement for real-time transmission was defined (IEEE 802.11e)
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Frame Format
• Types
� Control frames, administrative frames, data frames• Sequence numbers
� For detecting duplicated frames due to lost ACKs
• Addresses� Receiver, transmitter (physical), sender (logical), BSS identifier
• Misc� Duration of transmission, data
FrameControl
Duration/ID
Address1
Address2
Address3
SequenceControl
Address4
Data CRC
2 2 6 6 6 62 40-2312bytes
Protocolversion
Type SubtypeToDS
MoreFrag
RetryPowerMgmt
MoreData
WEP
2 2 4 1
FromDS
1
Order
bits 1 1 1 1 1 1
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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
Sequence Control• Recognition of duplicated frames by sequence numbers
CRC• Checksum for detecting transmission errors
Addresses• Each field contains a 48-Bit MAC address. MAC frames can be transferred
between two stations, between station and AP or between two APs within 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
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MAC Address Format
DS: Distribution SystemAP: Access PointDA: Destination AddressSA: Source AddressBSSID: Basic Service Set IdentifierRA: Receiver AddressTA: Transmitter Address
Scenario to DS from DS address 1 address 2 address 3 address 4 ad-hoc network 0 0 DA SA BSSID - infrastructure network, from AP
0 1 DA BSSID SA -
infrastructure network, to AP
1 0 BSSID SA DA -
infrastructure network, within DS
1 1 RA TA DA SA
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Special Frames
FrameControl
DurationReceiverAddress
TransmitterAddress
CRC
2 2 6 6 4bytes
FrameControl
DurationReceiverAddress
CRC
2 2 6 4bytes
FrameControl
DurationReceiverAddress
CRC
2 2 6 4bytes
Acknowledgement, ACK
Request to Send, RTS
Clear to Send, CTS
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FHSS Frame Format (PHY)
Synchronization SFD PLW PSF HEC Payload
Preamble Header
80 16 12 4 16 variable Bits
• Synchronization
– Synchronization of receivers by the pattern 010101... • SFD (Start Frame Delimiter)
– 0000110010111101 to announce start of frame• PLW (PLCP_PDU Length Word)
– Length of payload including the 32 Bit CRC (at the end of the payload). Allowed values are between 0 and 4095
• PSF (PLCP Signaling Field)
– Data rate of payload (1 or 2 Mbit/s)• HEC (Header Error Check)
– CRC with x16+x12+x5+1
transmission with 1 Mbit/s
transmission with1 or 2 Mbit/s
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DSSS Frame Format (PHY)
Synchronization SFD Signal Service HEC Payload
Preamble Header
128 16 8 8 16 variable Bits
Length
16
• Synchronization– Synchronization, gain setting, energy detection, frequency offset compensation
• SFD (Start Frame Delimiter)– 1111001110100000 as start pattern
• Signal– Data rate of payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)
• Service– Reserved for future use, standard: 00 for 802.11 frames
• Length (length of payload) and HEC (CRC) as for FHSS
transmission with 1 Mbit/s
transmission with1 or 2 Mbit/s
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IEEE 802.11b – Frame Format (PHY)
synchronization SFD signal service HEC payload
Preamble Header
128 16 8 8 16 variable Bits
length
16
192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s
short synch. SFD signal service HEC Payload
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, optional:
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IEEE 802.11a – Frame Format (PHY)
rate service payload
variable Bits
6 Mbit/s
Preamble, SFD Signal Data
Symbols12 1 variable
reserved length tailparity tail pad
616611214 variable
6, 9, 12, 18, 24, 36, 48, 54 Mbit/s
PLCP-Header
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Kommunikation und verteilte Systeme
Page 40Chapter 3.2: WLAN
802.11 - MAC Management
• Synchronization
� Find a LAN, try to remain in the LAN� Synchronization of internal clocks (e.g. FHSS, PCF, power saving
mechanisms)
� 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
• MIB - Management Information Base
� Managing, read and write of management attributed and state variables inside APs, the distribution system, etc
Lehrstuhl für Informatik 4
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Page 41Chapter 3.2: WLAN
tMedium
AP
busy
B
busy busy busy
B B B
value of the timestamp B beacon frame
Synchronization using a Beacon
• 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
Interval of the periodic radio
signal (beacon): 20ms - 1s
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 42Chapter 3.2: WLAN
Synchronization using a Beacon (Ad-hoc)
tMedium
Station1
busy
B1
beacon interval
busy busy busy
B1
value of the timestamp B beacon frame
Station2B2 B2
random backoff
• 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.
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 43Chapter 3.2: WLAN
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
� Similar to the infrastructure mod, an aA-hoc Traffic Indication Map (ATIM) is defined
� Stations, which have data to send, announce the receivers of stored packages
� More complex, no central AP: all stations have to temporarily store frames� Collisions of ATIMs possible (scalability?)
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 44Chapter 3.2: WLAN
Power Management with Wake-up Patterns (Infrastructure)
TIM interval
t
Medium
AP
busy
D
busy busy busy
T T D
T TIM D DTIM
DTIM interval
BB
B Broadcast/Multicast
Station
awake
p PS Poll
p
d
d
d Data transmissionfrom/to the station
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 45Chapter 3.2: WLAN
Power Management with Wake-up Patterns (Ad-hoc)
awake
A ATIM transmission D data transmission
t
Station1B1 B1
B beacon frame
Station2B2 B2
random backoff
A
a
D
d
ATIMwindow beacon interval
a ACK for ATIM d ACK for data
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 46Chapter 3.2: WLAN
802.11 - Roaming
Bad or even no connection?
• Scanning– Scanning of environment (listen for beacons of APs or send a probe and
wait for a response)
• Reassociation Request– Station requests joining the network to AP(s)
• Reassociation Response– If an AP responds, the station takes part in the network
– Otherwise, go on scanning
• AP accepts Reassociation Request
– Announce new station to the Distribution System– Distribution System updates its databases (location information)
– The old AP is informed by the Distribution System
Lehrstuhl für Informatik 4
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Page 47Chapter 3.2: WLAN
Quality of Service – IEEE 802.11e
The PCF variant of CSMA/CA should allow some quality in data transmission:• By polling at certain times, allow for deterministic delay of information
• Also, guarantee a certain data rate to each participant• But…frames in polling can be between 0 and 2304 bytes… and the data rate on
physical layer can change due to channel conditions…
→ no way to calculate transmission time of a frame in advance, thus the above quality cannot be given
Solution: define additional CSMA/CA variants which can give priority to real-time data (defined in IEEE 802.11e)
• Only an add-on the IEEE 802.11a/b/g, not a stand-alone WLAN standard
• Definition of� Extended Distributed Channel Access (EDCA) as better version of DCF using
several classes of access priority by refining the inter-frame gaps and introducing so-called Transmission Opportunities (TXOP)
� Hybrid Coordination Function Controlled Channel Access (HCCA) as better version of PCF also using TXOP
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Kommunikation und verteilte Systeme
Page 48Chapter 3.2: WLAN
Extended Distributed Channel Access
The scheme from before (all stations use the DIFS time interval) is refined:
• Assign different priorities to different data streams (traffic classes, TC)• As before, priority is given by waiting times: the Arbitration Inter-Frame space (AIFS)
t
busy SIFS
PIFS
DIFS =AIFS[TC7]
RTS
contention window
AIFS[TC6]
AIFS[TC0]
• Classify all data streams in traffic classes regarding their QoS• 8 priority classes, TC 7 has highest priority
• Give longer waiting times to lower priority – thus higher priority streams can start sending earlier
• Fairness is given – even high priority senders can draw a large backoff number
Best EffortBackgroundBackgroundVideo Probe
VideoVideoVoiceVoice
01122233
01234567
PurposeAccess Category (AC)TC
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Page 49Chapter 3.2: WLAN
EDCF Implementation
With EDCF, each station has to handle up to 8 queues performing the same access procedure as “plain” DCF with backoff counter (BC) and contention window (CW):
One more enhancement: each class also a TXOP is assigned, which is a maximum sending duration – after getting medium access, for time of TXOP several frames can be sent (Contention Free Burst)
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Kommunikation und verteilte Systeme
Page 50Chapter 3.2: WLAN
HCCA
As in PCF, HCCA is a combination of a contention-free period and a contention period
• In the contention-free period the AP polls the stations
�Difference to PCF: stations can place reservations for the polling phase�The AP polls stations by granting a TXOP oriented at reservation wishes and
current traffic load
• In the contention period, EDCF is used
Question: why giving QoS? Why not overprovisioning, i.e. only increase the data rate?
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 51Chapter 3.2: WLAN
Faster!
Not an end with 802.11a/g – go on with 802.11n• up to 600 MBit/s!
• over 70 – 250m!
How to achieve such a data rate while keeping compatibility to 802.11a/b/g?
• Applied to 2.4 as well as 5 GHz ISM band to only have a single variant for the future
• Modify OFDM with increasing symbol rate and slightly enlarge the bandwidth:→ increase data rate from 54 MBit/s to 65 MBit/s
• Optional: Greenfield mode, i.e. skip support for 802.11a/b/g (an increasing number of legacy devices reduces the average throughput in the whole network)
• Optional: increase a channel’s bandwidth to 40 MHz (dynamic adaptation to other WLANs in the environment necessary!)
• Use MIMO – multiple input multiple output
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Kommunikation und verteilte Systeme
Page 52Chapter 3.2: WLAN
MIMO
MIMO means: use several antennas in parallel to send data to one receiver• Apply Space Division Multiplexing (SDM) – i.e. split the data stream into multiple
parts (called spatial stream) and transmit each part with a separate antenna (for up to 4 antennas)
• Necessary: power control – only use MIMO if necessary, otherwise lots of power is consumed
• Apply beam-forming to focus the sender’s antennas to the receiver’s antennas
• By antenna diversity, a receiver can find out the angle of incidence of certain spatial streams and thus distinguish between several streams
• Optional: apply diversity on improving signal strength, i.e. improve signal by receiving the same stream with several antennas and combine the outputs (for up to 4 antennas, but only if the number of receiver antennas is larger than the number of spatial streams)
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 53Chapter 3.2: WLAN
802.11n – MAC Layer
Many improvements on PHY layer, only a few on the MAC layer:• Introduce Reduced Inter-Frame Space (RIFS) to shorten the waiting time after
detecting the medium to be idle• Use frame aggregation, i.e. pack together several frames of one station and
remove redundant header information
Availability of 802.11n?• Draft version 2 finished this year• Lot of products of several vendors (compliance to a non-finished standard?)
• Potential problems with a patent?• Planned release date – varies between September 2008 and March 2009…
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Kommunikation und verteilte Systeme
Page 54Chapter 3.2: WLAN
802.11s – WLAN Mesh Networking
Other WLAN variant: mesh networks• Classical WLAN: wired
infrastructure between APs
• Sometimes called “Wireless Paradox”
Let APs interconnect in wireless manner, also using WLAN (lower costs, simple installation, resilient, …)
Figures from: IEEE 802.11s tutorial
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Kommunikation und verteilte Systeme
Page 55Chapter 3.2: WLAN
Mesh Topology
Figures from: IEEE 802.11s tutorial
Mesh PointSpecial component, establishes peer links with neighbors
Mesh APAs mesh point, but additionally implements AP functionallity
Mesh PortalAs mesh point, but additionally connects to some other network
Changes in the 802.11 standard regarding:• Addresses
• MAC scheme (oriented at 802.11e)• Synchronization / power modes
• Security
• And: routing (layer 3!)
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 56Chapter 3.2: WLAN
Secure or not Secure…
Within a WLAN „data are flying free through the air“.
Within WLAN everybody in transmission range can share your Access Point.Thus: security!
WEP: Wired Equivalent Privacy• Authentication at the Access Point, encryption of data before transmission• Connection is only possible if knowing the WEP key
• But: no key management, short keys• Thus: WPA/WPA2 (Wi-Fi Protected Access) today give much better security
... but many users are overtaxed with configuring an Access Point – even if today a good user guide to install security functions is implemented on APs, there is a lot of open networks...
Registration of allowed MAC addresses
• But: MAC addresses can be faked, large effort for large networks
Hiding of SSID
• Broadcast of SSID in beacons can be switched of, thus only someone knowing the SSID can join the network (but: intuitive names? Default names?)
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 57Chapter 3.2: WLAN
Wardriving
New kind of sports: search for open WLANs.Just take:
• A notebook with WLAN card and a connector for a GPS device• A software for detcting Access Points,
e.g. Network Stumbler
• A GPS receiver• Time for driving around
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 58Chapter 3.2: WLAN
Warchalking
What can be found at walls after a wardiver has passed...
Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Page 59Chapter 3.2: WLAN
• Bluetooth may act like a rogue member of a 802.11 network– does not know anything about gaps, IFS etc.
• IEEE 802.15-2 discusses these problems– Proposal: Adaptive Frequency Hopping (only co-existence, no collaboration)
• Real effects? Many different opinions, tests, formulae, …
– Results from complete breakdown to almost no effect– Bluetooth (FHSS) seems to be more robust than 802.11b (DSSS)
– Maybe Bluetooth adaptive frequency hopping has better effect
802.11 vs. 802.15/Bluetooth
t
f [MHz]
2402
2480 802.11b 3 channles(separated by installation)
AC
K
DIF
S
DIF
S
SIF
S
1000 byte
SIF
S
DIF
S
500 byte
AC
K
DIF
S
500 byte
SIF
SA
CK
DIF
S
500 byte
DIF
S 100byte S
IFS
AC
K
DIF
S 100byte S
IFS
AC
K
DIF
S 100byte S
IFS
AC
K
DIF
S 100byte S
IFS
AC
K
DIF
S 100byte S
IFS
AC
K
802.15 79 channels(separated by hopping pattern)