Transcript busy - TU Ilmenau

IEEE 802.11
Integrated Communication Systems Group
Ilmenau University of Technology
Integrated Communication Systems Group
Characteristics of Wireless LANs
•
Advantages
–
–
–
–
•
very flexible alternative to wired LANs
(almost) no wiring difficulties (e.g. historic buildings, firewalls)
ad-hoc networks without previous planning possible
more robust against disasters, e.g. earthquakes, fire, or users pulling a
plug ...
Disadvantages
–
–
–
–
–
–
lower bandwidth compared to wired networks
possible interference may reduce bandwidth
no guaranteed service due to license-free spectrum
need to consider security issues
proprietary solutions, especially for higher bit-rates
wireless products have to follow many national restrictions
=> long time to establish global standards like, e.g. UMTS
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Integrated Communication Systems Group
Design goals for Wireless LANs
• 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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Integrated Communication Systems Group
Comparison: Infrastructure vs. Ad-hoc Networks
infrastructure
network
AP
AP
wired network
AP: Access Point
AP
ad-hoc network
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Integrated Communication Systems Group
802.11: Architecture of an Infrastructure Network
• Station (STA)
802.11 LAN
802.x LAN
STA1
– terminal with access mechanisms
to the wireless medium and radio
contact to the access point
• Basic Service Set (BSS)
BSS1
Portal
Access
Point
• Access Point
Distribution System
– station integrated into the wireless
LAN and the distribution system
Access
Point
ESS
– group of stations using the same
radio frequency
• Portal
– bridge to other (wired) networks
BSS2
• Distribution System
STA2
STA3
802.11 LAN
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– interconnection network to form
one logical network (ESS:
Extended Service Set) based
on several BSS
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Integrated Communication Systems Group
802.11: Architecture of an Ad-hoc Network
802.11 LAN
• Direct communication within a limited
range
STA1
– Station (STA):
terminal with access mechanisms to
the wireless medium
– Independent Basic Service Set (IBSS):
group of stations using the same radio
frequency
STA3
IBSS1
STA2
IBSS2
STA5
STA4
802.11 LAN
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Integrated Communication Systems Group
IEEE Standard 802.11
fixed terminal
mobile terminal
server
infrastructure network
access point
application
application
TCP
TCP
IP
IP
LLC
LLC
LLC
802.11 MAC
802.11 MAC
802.3 MAC
802.3 MAC
802.11 PHY
802.11 PHY
802.3 PHY
802.3 PHY
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Integrated Communication Systems Group
802.11 – Layers and Functions
• MAC
• PLCP (Physical Layer Convergence
– access mechanisms,
fragmentation, encryption
• MAC Management
– synchronization, roaming, MIB,
power management
Protocol)
– clear channel assessment signal
(carrier sense)
• PMD (Physical Medium Dependent)
– modulation, coding
• PHY Management
– channel selection, MIB
DLC
LLC
MAC
MAC Management
PHY
PLCP
PHY Management
PMD
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Station Management
• Station Management
– coordination of all management
functions
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Integrated Communication Systems Group
802.11 – Physical Layer
•
3 versions: 2 radio (typ. 2.4 GHz), 1 IR
– data rates 1 or 2 Mbit/s
•
FHSS (Frequency Hopping Spread Spectrum)
– spreading, despreading, signal strength, typ. 1 Mbit/s
– min. 2.5 frequency hops/s (USA), two-level GFSK modulation
•
DSSS (Direct Sequence Spread Spectrum)
– DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying),
DQPSK for 2 Mbit/s (Differential Quadrature PSK)
– preamble and header of a frame is always transmitted with 1 Mbit/s, rest
of transmission 1 or 2 Mbit/s
– chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)
– max. radiated power 1 W (USA), 100 mW (EU), min. 1mW
•
Infrared
– 850-950 nm, diffuse light, typ. 10 m range
– carrier detection, energy detection, synchronization
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Integrated Communication Systems Group
802.11 MAC Layer – DFWMAC
•
Traffic services
– Asynchronous Data Service (mandatory)
• exchange of data packets based on "best-effort"
• support of broadcast and multicast
• implemented using DCF (Distributed Coordination Function)
– Time-Bounded Service (optional)
• implemented using PCF (Point Coordination Function)
•
Access methods
– DFWMAC-DCF CSMA/CA (mandatory)
•
•
•
•
Distributed Foundation Wireless MAC
collision avoidance via randomized "back-off" mechanism
minimum distance between consecutive packets
ACK packet for acknowledgements (not for broadcasts)
– DFWMAC-DCF with RTS/CTS Extension (optional)
• avoids hidden terminal problem
– DFWMAC-PCF (optional)
• access point polls terminals according to a list
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Integrated Communication Systems Group
802.11 MAC
• Priorities
– defined through different Inter-Frame Spaces (IFSs)
– no guaranteed or hard priorities
– 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 IFS)
• lowest priority, for asynchronous data service
DIFS
DIFS
PIFS
medium busy
SIFS
contention
next frame
t
direct access if
medium is free  DIFS
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Integrated Communication Systems Group
802.11 – CSMA/CA Access Method
DIFS
DIFS
contention window
(randomized back-off
mechanism)
PIFS
medium busy
SIFS
direct access if
medium is free  DIFS
•
•
•
next frame
t
slot time
Station ready to send starts sensing the medium (Carrier Sense based on
CCA, Clear Channel Assessment)
If the medium is free for the duration of an Inter-Frame Space (IFS), the
station can start sending (IFS depends on service type)
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
Random() = Pseudorandom integer drawn from a uniform distribution over the interval
[0, CW]
CW = an integer between CWmin and CWmax
•
•
If another station occupies the medium during the back-off time of the station,
the back-off timer stops (fairness)
If another collision happens, the stations double CW
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Integrated Communication Systems Group
802.11 – Competing Stations – Simple Version
DIFS
DIFS
DIFS
boe
station1
bor
DIFS
boe bor
boe
busy
boe busy
boe bor
boe busy
boe bor
boe busy
station2
busy
station3
station4
boe bor
station5
t
busy
medium not idle (frame, ack etc.)
boe
elapsed backoff time
packet arrival at MAC
bor
residual backoff time
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Integrated Communication Systems Group
802.11 – CSMA/CA Access Method
• Sending unicast packets
– station has to wait for DIFS before sending data
– receivers acknowledge at once (after waiting for SIFS) if the packet
was received correctly (CRC)
– automatic retransmission of data packets in case of transmission
errors
DIFS
sender
data
SIFS
ACK
receiver
DIFS
data
other
stations
t
waiting time
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contention
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Integrated Communication Systems Group
802.11 – DCF with RTS/CTS Extension
•
Sending unicast packets
– station can send RTS with reservation parameter after waiting for DIFS
(reservation determines amount of time the data packet needs the
medium)
– acknowledgement via CTS after SIFS by receiver (if ready to receive)
– sender can now send data at once, acknowledgement via ACK
– other stations store medium reservations distributed via RTS and CTS
DIFS
sender
RTS
data
SIFS
receiver
SIFS
SIFS
CTS
other
stations
ACK
NAV (RTS)
NAV (CTS)
DIFS
data
t
defer access
contention
NAV: network allocation vector (implicit in RTS and CTS)
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Integrated Communication Systems Group
Fragmentation
DIFS
sender
RTS
frag1
SIFS
receiver
SIFS
frag2
SIFS
CTS
SIFS
SIFS
ACK1
ACK2
NAV (RTS)
NAV (CTS)
other
stations
NAV (frag1)
NAV (ACK1)
DIFS
data
t
contention
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Integrated Communication Systems Group
802.11 – PCF (Polling)
SuperFrame defines time span for polling of (all) wireless stations
by AP (including time to reply)
t0
t1
SuperFrame
medium busy
point
coordinator
PIFS
SIFS
D1
SIFS
D2
SIFS
wireless
stations
stations‘
NAV
SIFS
U1
U2
NAV
D1: polling of wireless station 1
U1: station 1 responds to polling by sending its data
D2: polling of wireless station 2
U2: station 2 responds to polling by sending its data
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Integrated Communication Systems Group
802.11 – PCF (Polling)
t2
PIFS
point
coordinator
D3
t4
SIFS
D4
CFend
SIFS
U4
wireless
stations
stations‘
NAV
t3
NAV
contention free period
contention
period
t
D3: polling of wireless station 3
U3: no data -> no response by station 3 within SIFS
D4 (after PIFS): polling of wireless station 4
U4: station 4 responds to polling by sending its data
Discussion:
 unpredictable beacon delays
 unknown transmission duration of polled
stations
=> no QoS guarantees
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Integrated Communication Systems Group
802.11 – MAC Management
•
Synchronization
– try to find a LAN, try to stay within a LAN
– synchronization of internal clocks to coordinate access (e.g. SIFS, PIFS,
etc.), send beacons, etc.
•
Power management
– sleep-mode without missing a message
– periodic sleep, frame buffering, traffic measurements
•
Association/Reassociation
– integration into a LAN
– roaming, i.e. change networks by changing access points
– scanning, i.e. active search for a network
•
MIB - Management Information Base
– managing, read, write
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Integrated Communication Systems Group
Synchronization Using a Beacon (Infrastructure)
beacon interval
access
point
medium
B
B
busy
busy
B
busy
B
busy
t
value of the timestamp
B
beacon frame
• Beacon sent only by access point
• Beacon contains a timestamp and other management information used for
power management and roaming
• Beacon may be delayed due to busy medium; beacon interval is not influenced
by this!
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Integrated Communication Systems Group
Synchronization Using a Beacon (Ad-hoc)
beacon interval
station1
B1
B1
B2
station2
medium
busy
busy
B2
busy
busy
t
value of the timestamp
B
beacon frame
random delay
Beacon may be sent by any station (sending of beacon employs a random
delay to avoid collisions)
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Integrated Communication Systems Group
Power Management
•
•
•
Idea: switch the transceiver off if not needed
States of a station: sleep and awake
Timing Synchronization Function (TSF)
– stations wake up at the same time
•
Infrastructure
– AP stores frames intended for sleeping stations
– AP transmits indication about stored frames in periodic beacons
(Indication Maps) sent during awake interval
• Traffic Indication Map (TIM): List of unicast receivers
• Delivery Traffic Indication Map (DTIM): List of broadcast/multicast
receivers
•
Ad-hoc
– Ad-hoc Traffic Indication Map (ATIM)
• announcement of receivers by stations buffering frames
• more complicated - no central AP
• collision of ATIMs possible (scalability?)
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Integrated Communication Systems Group
Power Saving with Wake-up Patterns
(Infrastructure)
 beacon indicates station that data are
available
 station replies with PS (power save) poll
and continues listening to the medium
 AP transmits data
 station acknowledges the data
TIM interval
access
point
DTIM interval
D B
T
T
busy busy
medium
d
D B
busy
busy
p
station
d
t
T
TIM
D
B
broadcast/multicast
DTIM
awake
p
PS poll
d
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data transmission
to/from the station
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Integrated Communication Systems Group
Power Saving with Wake-up Patterns (Ad-hoc)
ATIM
window
beacon interval
B1
station1
A
B2
station2
medium
busy
busy
B2
D
a
B1
d
busy
t
B
beacon frame
awake
a
random delay
A
transmit ATIM
acknowledge ATIM
d
acknowledge data
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D
transmit data
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Integrated Communication Systems Group
WLAN: IEEE 802.11b
•Data rate
– 1, 2, 5.5, 11 Mbit/s, depending
on SNR
– User data rate max. approx. 6
Mbit/s
•Transmission range
– 300m outdoor, 30m indoor
– Max. data rate ~10m indoor
•Frequency
– Free 2.4 GHz ISM-band
•Security
– Limited, WEP insecure, SSID
•Cost
– 25€ adapter, 100€ base station
•Availability
– Many products, many vendors
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•Connection set-up time
– Connectionless/always on
•Quality of Service
– Typ. best effort, no guarantees
(unless polling is used, limited
support in products)
•Manageability
– Limited (no automated key
distribution, sym. encryption)
•Advantages/Disadvantages
– Advantage: many installed
systems, lot of experience,
available worldwide, free ISM
band, many vendors, integrated
in laptops, simple system
– Disadvantage: heavy
interference on ISM band, no
service guarantees, slow relative
speed only
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Integrated Communication Systems Group
Channel Selection (Non-overlapping)
Europe (ETSI)
channel 1
2400
channel 7
2412
2442
channel 13
2472
22 MHz
2483.5
[MHz]
US (FCC)/Canada (IC)
channel 1
2400
2412
channel 6
2437
22 MHz
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channel 11
2462
2483.5
[MHz]
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Integrated Communication Systems Group
WLAN: IEEE 802.11g
•Data rate
– 6, 9, 12, 18, 24, 36, 48, 54
Mbit/s, depending on SNR
– User throughput (1500 byte
packets): 5.3 (6), 18 (24), 24
(36), 32 (54)
– 6, 12, 24 Mbit/s mandatory
•Transmission range
– 150m outdoor, 20m indoor
54 Mbit/s up to 6 m
•Frequency
– 2.412~2.472GHz (Europe ETSI)
– 2.457~2.462GHz (Spain)
– 2.457~2.472GHz (France)
•Security
– Limited, WEP insecure, SSID
•Cost
– 50€ adapter, 200€ base station
•Availability
– Some products, some vendors
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•Connection set-up time
– Connectionless/always on
•Quality of Service
– Typ. best effort, no guarantees
(same as all 802.11 products)
•Manageability
– Limited (no automated key
distribution, sym. encryption)
•Advantages/Disadvantages
– Advantage: free ISM band,
compatible with 802.11b
standard
– Disadvantage: heavy
interference on ISM band, no
service guarantees
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Integrated Communication Systems Group
WLAN: IEEE 802.11a
•Data rate
– 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s,
depending on SNR
– User throughput (1500 byte packets):
5.3 (6), 18 (24), 24 (36), 32 (54)
– 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 30m, 18
up to 40 m, 12 up to 60 m
•Frequency
– Free 5.15-5.25, 5.25-5.35, 5.7255.825 GHz ISM-band
•Security
– Limited, WEP insecure, SSID
•Cost
– 100€ adapter, 200€ base station
•Availability
– Some products, some vendors
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•Connection set-up time
– Connectionless/always on
•Quality of Service
– Typ. best effort, no guarantees (same
as all 802.11 products)
•Manageability
– Limited (no automated key
distribution, sym. encryption)
•Advantages/Disadvantages
– Advantage: fits into 802.x standards,
free ISM band, available, simple
system, uses less crowded 5 GHz
band
– Disadvantage: stronger shading due
to higher frequency, no service
guarantees
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Integrated Communication Systems Group
WLAN: IEEE 802.11h – Regulatory Details
• Spectrum Managed 802.11a
– power control
– dyn. channel/frequency selection
• 4 frequency bands:
 5.150 - 5.250 GHz
–
–
–
–
–
–
4 usable channels (100 MHz)
indoor only
max. 30mW EIRP (.11a)
TPC (Transmit Power Control) max.
60mW EIRP
combined TPC and DCS/DFS
(Dynamic Channel/Frequency
Selection) max. 200mW EIRP
Turbo Mode: combination of two
carriers to reach 108 Mbps
 5.250 - 5.350 GHz
–
–
4 usable channels
TPC, DCS/DFS mandatory
 5.470 – 5.725 GHz
–
–
–
–
indoor and outdoor
max. 1W EIRP
disallowed in US
not supported by all chipsets
 5.725 bis 5.825 GHz
–
disallowed in Germany
EIRP (Equivalent Isotropic Radiated Power)
Usage in Germany according to the German Federal Regulation (Vorschrift der
Regulierungsbehörde für das Telekommunikations- und Postwesen, RegTP, 35/2002)
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Integrated Communication Systems Group
Operating Channels for 802.11a / US U-NII
36
5150
40
44
48
52
56
60
64
5180 5200 5220 5240 5260 5280 5300 5320
channel
5350 [MHz]
16.6 MHz
149
153
157
161
channel
center frequency =
5000 + 5*channel number [MHz]
5725 5745 5765 5785 5805 5825 [MHz]
16.6 MHz
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Integrated Communication Systems Group
WLAN: IEEE 802.11 – Extensions and
developments (11/2011)
•
•
802.11d: Regulatory Domain Update – completed
802.11e: MAC Enhancements – QoS – completed
– Enhance the current 802.11 MAC to expand support for applications with Quality of
Service requirements, and in the capabilities and efficiency of the protocol
•
802.11f: Inter-Access Point Protocol (IAPP) – withdrawn 2006
– Establish an Inter-Access Point Protocol for data exchange via the distribution
system
•
•
802.11h: Spectrum Managed 802.11a (DCS, TPC) – completed
802.11i: Enhanced Security Mechanisms – completed
– Enhance the current 802.11 MAC to provide improvements in security
•
•
802.11j: MAC and PHY Specifications for Operation in 4.9-5 GHz Band in
Japan – completed
802.11n: Throughput enhancement to 108-320 Mbps – completed
•
Study Groups
– 5 GHz (harmonization ETSI/IEEE) – closed
– Radio Resource Measurements – started
– High Throughput – started
•
See http://standards.ieee.org/getieee802/802.11.html for an update
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Integrated Communication Systems Group
WLAN: IEEE 802.11 – Details
 802.11d aims to produce versions of 802.11b that work at other frequencies, making it suitable
for parts of the world where the 2.4GHz band isn't available. Most countries have now released
this band, thanks to an ITU recommendation and extensive lobbying by equipment
manufacturers. The only holdout is Spain, which may follow soon.
 802.11e add QoS capabilities to 802.11 networks. It replaces the Ethernet-like MAC layer with
a coordinated Time Division Multiple Access (TDMA) scheme, and adds extra error-correction
to important traffic. The technology is similar to Whitecap, a proprietary protocol developed by
Sharewave and used in Cisco's 802.11a prototypes. A standard was supposed to be finalized
by the end of 2001, but has run into delays thanks to arguments over how many classes of
service should be provided and exactly how they should be implemented.
 802.11f tries to improve the handover mechanism in 802.11 so that users can maintain a
connection while roaming between two different switched segments (radio channels), or
between access points attached to two different networks. This is vital if wireless LANs are to
offer the same mobility that cell phone users take for granted.
 802.11h attempts to add better control over transmission power and radio channel selection to
802.11a. Along with 802.11e, this could make the standard acceptable to European regulators.
 802.11i deals with 802.11's most obvious weakness: security. Rather than WEP, this is an
entirely new standard based on the Advanced Encryption Standard (AES), the U.S.
government's "official" encryption algorithm.
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Integrated Communication Systems Group
WLAN: IEEE 802.11 – More Details
Within the IEEE 802.11 Working Group,[6] the following IEEE Standards Association Standard and Amendments exist:
•
IEEE 802.11-1997: The WLAN standard was originally 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and infrared (IR) standard (1997), all the
others listed below are Amendments to this standard, except for Recommended Practices 802.11F and 802.11T.
•
IEEE 802.11a: 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)
•
IEEE 802.11b: Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)
•
IEEE 802.11c: Bridge operation procedures; included in the IEEE 802.1D standard (2001)
•
IEEE 802.11d: International (country-to-country) roaming extensions (2001)
•
IEEE 802.11e: Enhancements: QoS, including packet bursting (2005)
•
IEEE 802.11F: Inter-Access Point Protocol (2003) Withdrawn February 2006
•
IEEE 802.11g: 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)
•
IEEE 802.11h: Spectrum Managed 802.11a (5 GHz) for European compatibility (2004)
•
IEEE 802.11i: Enhanced security (2004)
•
IEEE 802.11j: Extensions for Japan (2004)
•
IEEE 802.11-2007: A new release of the standard that includes amendments a, b, d, e, g, h, i and j. (July 2007)
•
IEEE 802.11k: Radio resource measurement enhancements (2008)
•
IEEE 802.11n: Higher throughput improvements using MIMO (multiple input, multiple output antennas) (September 2009)
•
IEEE 802.11p: WAVE—Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) (July 2010)
•
IEEE 802.11r: Fast BSS transition (FT) (2008)
•
IEEE 802.11s: Mesh Networking, Extended Service Set (ESS) (July 2011)
•
IEEE 802.11T: Wireless Performance Prediction (WPP)—test methods and metrics Recommendation cancelled
•
IEEE 802.11u: Improvements related to HotSpots and 3rd party authorization of clients, e.g. cellular network offload (February 2011)
•
IEEE 802.11v: Wireless network management (February 2011)
•
IEEE 802.11w: Protected Management Frames (September 2009)
•
IEEE 802.11y: 3650–3700 MHz Operation in the U.S. (2008)
•
IEEE 802.11z: Extensions to Direct Link Setup (DLS) (September 2010)
•
IEEE 802.11-2012: A new release of the standard that includes amendments k, n, p, r, s, u, v, w, y and z (March 2012)
•
IEEE 802.11aa: Robust streaming of Audio Video Transport Streams (June 2012)
•
IEEE 802.11ad: Very High Throughput 60 GHz (December 2012) - see WiGig
•
IEEE 802.11ae: Prioritization of Management Frames (March 2012)
In process
•
IEEE 802.11ac: Very High Throughput <6 GHz;[28] potential improvements over 802.11n: better modulation scheme (expected ~10%
throughput increase), wider channels (estimate in future time 80 to 160 MHz), multi user MIMO;[29] (~ February 2014)
•
IEEE 802.11af: TV Whitespace (~ June 2014)
•
IEEE 802.11ah: Sub 1 GHz sensor network, smart metering. (~ January 2016)
•
IEEE 802.11ai: Fast Initial Link Setup (~ February 2015)
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Integrated Communication Systems Group
Reference
Books on 802.11:
 Franz-Joachim Kauffels: Wireless LANs: Drahtlose Netze planen und verwirklichen, der Standard
IEEE 802.11 im Detail, WLAN-Design und Sicherheitsrichtlinien. 1. Aufl., mitp-Verl., Bonn, 2002

Frank Ohrtman: WiFi-Handbook – Building 802.11b wireless networks. McGraw-Hill, 2003

Jochen Schiller: Mobile Communications (German and English), Kap 7.3, Addison-Wesley, 2002
Details on 802.11e:




Anders Lindgren, Andreas Almquist, Olov Schelén. Quality of service schemes for IEEE 802.11
wireless LANs: an evaluation. Mobile Networks and Applications, Volume 8 Issue 3, June 2003
Daqing Gu; Jinyun Zhang. QoS enhancement in IEEE 802.11 wireless local area networks.
Communications Magazine, IEEE , Volume: 41 Issue: 6, June 2003
Qiu Qiang; Jacob, L., Radhakrishna Pillai, R., Prabhakaran, B.. MAC protocol enhancements for QoS
guarantee and fairness over the IEEE 802.11 wireless LANs. 11th Intl. Conf. on Computer
Communications and Networks, Oct. 2002
Mangold S, Choi S, May P, Klein O, Hiertz G, and Stibor L. IEEE 802.11e wireless LAN for quality of
service. Proc. Of European Wireless (EW2002), Feb. 2002
Web Links:
•
The IEEE 802.11 Wireless LAN Standards http://standards.ieee.org/getieee802/802.11.html
•
Introduction to the IEEE 802.11 Wireless LAN Standard http://www.wlana.org/learn/80211.htm
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