Wireless Ethernet Wireless LANs: Design Goals

Transcrição

Lehrstuhl für Informatik 4
Kommunikation und verteilte Systeme
Wireless Ethernet
Wireless LANs: Design Goals
• 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)
…
•
•
•
•
•
•
•
•
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
802.11a
802.11
• 1 or 2 MBit/s
• 2.4 GHz
• FHSS, DSSS
• 54 MBit/s
• 5 GHz
• OFDM
802.11b
802.11g
• 11 MBit/s
• 2.4 GHz
• DSSS
• 54 MBit/s
• 2.4 GHz
• OFDM, DSSS
Page 1
Chapter 3.2: WLAN
Page 2
Chapter 3.2: WLAN
Structure of a WLAN
Infrastructure Network
1. Infrastructure network
• Access Points (APs) are attached to an existing
AP
fixed network (Ethernet, Satellites, …)
Fixed network
• Each AP manages all communication
AP
AP
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
802.11 LAN
802.x LAN
L a p to p
L a p to p
L a p to p
L a p to p
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)
STA1
BSS1
L a p to p
Distribution System
Chapter 3.2: WLAN
BSS2
• Portal
Gateway to another fixed network
Laptop
STA2
802.11 LAN
Page 3
Chapter 3.2: WLAN
• 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
Access
Point
ESS
Laptop
Laptop
Portal
Access
Point
• Station (STA)
Computer with access mechanism to
the wireless medium and by this radio
connection to the AP
STA3
• Distribution system
Connection of different AP areas to
one logical network (EES: Extended
service set). Simplest principle: switch
Page 4
Ad-hoc Network
802.11 Protocols
802.11 LAN
Direct communication within limited range
STA1
STA3
IBSS1
STA2
IBSS2
STA5
STA4
• Station (STA)
Computer with access mechanism to
the 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
Chapter 3.2: WLAN
Page 5
Applications should not
be aware of the
existence of the wireless
network (except
capacity, longer access
times)
Medium Access Control
• Access mechanism, fragmenting, encryption
• MAC management: synchronization, roaming between APs, power management
Physical layer
• Channel selection, modulation, coding
Chapter 3.2: WLAN
IEEE 802.11 Variants
IEEE 802.11 Variants
802.11a
54 MBit/s WLAN in the 5 GHz band
802.11b
11 MBit/s WLAN in the 2,4 GHz band
802.11c
Wireless Bridging between Access Points
802.11d
"World Mode", Adaptation to regional regulations (e.g. used frequency ranges)
802.11e
QoS und streaming enhancement for 802.11a/g/h
802.11f
Inter Access Point Protocol (IAPP), allows communication between Access Points of
different vendors, e.g. for exchanging roaming information
802.11g
54 MBit/s WLAN in the 2,4 GHz band
802.11h
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)
802.11m
Summary of earlier enhancements, correction of errors in former specifications
(maintenance)
802.11n
Enhancement for a future, faster WLAN with data rate of 100 - 600 MBit/s
802.11p
WAVE - Wireless Access for the Vehicular Environment (such as ambulances and
passenger cars)
802.11q
Support of Virtual WLANs
802.11r
Fast roaming between APs to avoid gaps in Voice over WLAN audio
802.11s
ESS Mesh Networking
802.11t
Wireless Performance Prediction (WPP) - test methods and metrics
802.11u
Interworking with non-802 networks (for example, cellular)
802.11i
Authentication/encryption for 802.11a/b/g, e.g. WPA
802.11v
Wireless network management
802.11j
Japanese variant of 802.11a for the frequency range of 4,9 GHz - 5 GHz
802.11w
Protection of Management Frames
802.11k
Improved measurement/evaluation/management of radio parameters (e.g. signal
strength), e.g. for enabling location based services
802.11y
3650-3700 MHz Operation in the U.S.
Chapter 3.2: WLAN
Page 6
Page 7
Chapter 3.2: WLAN
Page 8
802.11 – Physical Layer
IEEE 802.11b
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
Page 9
Chapter 3.2: WLAN
• 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
Page 10
Chapter 3.2: WLAN
Channels in IEEE 802.11b
Channels in IEEE 802.11b
• 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:
Available in the ISM band (most of Europe): 2400 – 2483,5 MHz
Channel 1
Channel 6
Channel 1
2401 2412 2423
USA/Canada: channel 1 - 11
Carrier frequency
Channel 1
2401 2412 2423
Channel 6
2426 2437 2448
Channel 2
2406 2417 2428
Channel 11
Channel 11
2451 2462 2473
Channel 7
2431 2442 2453
Channel 3
2411 2422 2433
Channel 12
2456 2467 2478
Channel 8
2436 2447 2458
Channel 13
2461 2472 2483
[MHz]
2400
2412
2437
22 MHz
2462
Channel 4
2416 2427 2438
2483.5
11
Channel 5
2421 2432 2443
6
• Ideal case: only use channels 1, 6 und 11:
1
Channel 9
2441 2452 2463
Channel 10
2446 2457 2468
Channel 14
2473 2484 2495
11
6
Chapter 3.2: WLAN
Japan ( 1 – 14)
1
Page 11
2400
2410
Chapter 3.2: WLAN
2420
2430
2440
2450
2460
2470
2480
MHz
Page 12
Dynamic Rate Shifting
Channels
Adjustment of the data rate to the transmission quality:
The whole 2.4GHz ISM band is divided into 11 resp. 13 overlapping channels. On
each channel, DSSS is used for signal spreading:
Data Rate
Code length
1 Mbit/s
Modulation
Used Symbol Rate Bits/Symbol
DSSS/PSK
11 (barker code)
2 Mbit/s
1
1 MS/s
DSSS/QPSK
5,5 Mbit/s
8 (CCK)
11 Mbit/s
Modified
DSSS/QPSK
2
1,375 MS/s
4
8
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)
Page 13
Chapter 3.2: WLAN
→ 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
Channel n
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
22 MHz
narrowband interference
→ If CCK is used, we use “several codes” instead of the same chipping sequence
everytime - the transmission becomes more susceptible for disturbances than
with use of the barker code, if we have a distortion (maybe caused by an
overlapping channel)!
Chapter 3.2: WLAN
Page 14
Range of IEEE 802.11b
Range of 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:
Data rate
10
Mbit/s
8
6
802.11b
4
2
802.11
0
10
Chapter 3.2: WLAN
30
60
100 m
Distance
Page 15
Chapter 3.2: WLAN
Page 16
IEEE 802.11a
Channels in 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
Page 17
Chapter 3.2: WLAN
312,5 kHz
phase 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)
-7 -1 1
7
channel center frequency
5150
40
44
48
52
56
60
channel-no.
64
5180 5200 5220 5240 5260 5280 5300 5320
5350 [MHz]
16,6 MHz
149
153
157
161
channel-no.
center frequency =
5000 + 5·channel-no. [MHz]
5725 5745 5765 5785 5805 5825 [MHz]
16,6 MHz
Chapter 3.2: WLAN
Page 18
Medium Access Control
• 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
-26 -21
36
Modulation in 802.11a: OFDM
Chapter 3.2: WLAN
Channels are also overlapping, as in 802.11b:
21 26
subcarrier
number
Page 19
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
Chapter 3.2: WLAN
Page 20
802.11 – MAC Layer: DFWMAC
802.11 – MAC Layer
Priorities for medium access
• defined through different timing intervals
• no guaranteed priorities
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)
• 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
• DFWMAC-DCF with RTS/CTS (optional)
Avoidance of Hidden Stations
MACA variant (Multiple Access with Collision Avoidance)
• DIFS (DCF IFS) – 50µs
– lowest priority, für asynchronous data service
DIFS
• DFWMAC-PCF (optional)
PCF: Point Coordination Function
Collision-free, centralized Polling strategy where the AP has a list of
all connected stations
Medium busy
Page 21
Chapter 3.2: WLAN
Medium busy
next frame
t
direct access, if
time the medium is free ≥ DIFS
Page 22
Example - Backoff
DIFS
PIFS
SIFS
contention window
(randomized backoff
mechanism)
B1 = 25
B1 = 5
wait
next frame
data
t
waiting time
time slot (20 µs)
•
•
•
•
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 backoff
timer stops. In the next try, no new backoff time is chosen randomly, but the old
timer is gone on with.
• Also usable for broadcast
Chapter 3.2: WLAN
contention
802.11 - CSMA/CA Method
DIFS
Chapter 3.2: WLAN
DIFS
PIFS
SIFS
Page 23
data
B2 = 20
wait
B2 = 15
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
Chapter 3.2: WLAN
Page 24
Competing Stations
DIFS
802.11 - CSMA/CA Method
DIFS
DIFS
boe bobo
e r
Station1
boe
Station2
DIFS
bobo
e bo
e r
boe
boe busy
boe bor
boe
boe
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
busy
busy
busy
Station3
Station4
bobo
e bo
e r
Station5
busy
DIFS
bor
Data
sender
t
SIFS
busy
Medium busy (Frame, ACK, etc.)
boe elapsed backoff time
Sending request
bor remaining backoff time
receiver
DIFS
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 CW max = 255
Page 25
Chapter 3.2: WLAN
SIFS
DIFS
Station1
DIFS
bo11
bo11
bo41 busy
bo42
bo51 busy
bo52
busy
bo21 busy
Station2
busy
Station3
ACK
Station4
bo51
Station5
Medium occupied (Frame, ACK, etc.)
Sending request
Page 26
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
DIFS
RTS
data
SIFS
boij jth backoff time of station i
CTS SIFS
SIFS
ACK
receiver
contention
ACK Acknowledgement
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 CW max = 256
Chapter 3.2: WLAN
t
contention
Chapter 3.2: WLAN
sender
t
busy
waiting time
802.11 – DFWMAC with RTS/CTS
DIFS
bo11
Data
other
stations
Competing Stations (with ACK)
DIFS
ACK
Page 27
other
stations
Chapter 3.2: WLAN
NAV (RTS)
NAV (CTS)
waiting time
DIFS
data
t
Page 28
802.11 – DFWMAC with RTS/CTS
DFWMAC-PCF
• 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
DIFS
RTS
frag1
sender
SIFS
CTS SIFS
SIFS
ACK1 SIFS
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)
t0 t1
frag2
receiver
•
•
•
•
SIFS
ACK2
PIFS
NAV (RTS)
NAV (CTS)
other
stations
SIFS
D1
DIFS
SIFS
D2
SIFS
NAV (frag1)
NAV (ACK1)
Chapter 3.2: WLAN
PIFS
U1
SIFS
D4
t4
CFend
SIFS
U2
U4
data
Page 29
NAV
contention-free period
Chapter 3.2: WLAN
contention
Page 30
What is implemented?
Frame Format
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)
Chapter 3.2: WLAN
D3
SIFS
t
contention
t2 t3
super-frame
Page 31
• 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
bytes
2
2
6
6
6
2
6
Frame Duration/ Address Address Address Sequence Address
Control
ID
1
2
3
Control
4
bits
2
2
4
1
1
1
1
1
1
1
0-2312
4
Data
CRC
1
Protocol
To From More
Power More
Type Subtype
Retry
WEP Order
version
DS DS Frag
Mgmt Data
Chapter 3.2: WLAN
Page 32
t
Frame Format
MAC Address 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
Page 33
Chapter 3.2: WLAN
2
2
6
Frame
Receiver
Duration
Control
Address
4
CRC
bytes
2
2
6
6
Frame
Receiver Transmitter
Duration
Control
Address Address
bytes
Clear to Send, CTS
Chapter 3.2: WLAN
address 3
BSSID
SA
address 4
-
SA
DA
-
TA
DA
SA
Distribution System
Access Point
Destination Address
Source Address
Basic Service Set Identifier
Receiver Address
Transmitter Address
Page 34
Chapter 3.2: WLAN
FHSS Frame Format (PHY)
bytes
Request to Send, RTS
DS:
AP:
DA:
SA:
BSSID:
RA:
TA:
address 2
SA
BSSID
Special Frames
Acknowledgement, ACK
Scenario
to DS from DS address 1
ad-hoc network
0
0
DA
infrastructure
0
1
DA
network, from AP
infrastructure
1
0
BSSID
network, to AP
infrastructure
1
1
RA
network, within DS
2
2
6
Frame
Receiver
Duration
Control
Address
4
CRC
• 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
4
80
Synchronization
CRC
Page 35
Chapter 3.2: WLAN
Preamble
transmission
with 1 Mbit/s
16
12
4
16
variable
SFD
PLW
PSF
HEC
Payload
Header
transmission with
1 or 2 Mbit/s
Bits
Page 36
DSSS Frame Format (PHY)
IEEE 802.11b – Frame Format (PHY)
• 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
Long frame format:
128
16
Synchronization
8
8
16
16
SFD Signal Service Length HEC
Preamble
transmission
with 1 Mbit/s
variable
Bits
6
16
tail service
12
Signal
1
6 Mbit/s
Chapter 3.2: WLAN
payload
1, 2, 5.5 or 11 Mbit/s
short synch.
16
SFD
8
8
16
16
variable
signal service length HEC
Bits
Payload
Header
(2 Mbit/s, DQPSK)
2, 5.5 or 11 Mbit/s
Chapter 3.2: WLAN
Page 38
802.11 - MAC Management
variable
6
variable
payload
tail
pad
Bits
• 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
PLCP-Header
Preamble, SFD
Bits
variable
IEEE 802.11a – Frame Format (PHY)
1
16
Short frame format, optional:
96 µs
Page 37
12
16
192 µs at 1 Mbit/s DBPSK
transmission with
1 or 2 Mbit/s
1
8
Header
Preamble
(1 Mbit/s, DBPSK)
Chapter 3.2: WLAN
4
8
signal service length HEC
Payload
Header
rate reserved length parity
SFD
Preamble
56
128
16
synchronization
Data
variable
Symbols
• 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
6, 9, 12, 18, 24, 36, 48, 54 Mbit/s
Page 39
Chapter 3.2: WLAN
Page 40
Synchronization using a Beacon
Synchronization using a Beacon (Ad-hoc)
• Beacon frame contains time stamps and administrative information for power
saving mechanisms and roaming
• All stations try to send a Beacon frame in fixed intervals
• Varying times between beacon frames, since the medium can be occupied
• One station wins and sends a beacon frame at first. All other stations synchronize
to this frame.
• In infrastructure networks: AP takes over the sending of the beacons
beacon interval
Interval of the
periodic radio
signal (beacon):
20ms - 1s
B
B
busy
B1
Station1
B
busy
busy
B2
busy
Medium
busy
value of the timestamp
Chapter 3.2: WLAN
busy
busy
value of the timestamp
beacon frame
Page 41
busy
B
random backoff
beacon frame
Page 42
Chapter 3.2: WLAN
Power Management with Wake-up Patterns
(Infrastructure)
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?)
Chapter 3.2: WLAN
B2
t
t
B
B1
Station2
B
AP
Medium
• Standard access procedure with backoff
Page 43
TIM interval
AP
DTIM interval
D B
T
busy
Medium
busy
T
d
D B
busy
busy
p
Station
d
t
Chapter 3.2: WLAN
T
TIM
D
B
Broadcast/Multicast
DTIM
awake
p PS Poll
d Data transmission
from/to the station
Page 44
Power Management with Wake-up Patterns
(Ad-hoc)
ATIM
window
Station1
Bad or even no connection?
B1
beacon frame
awake
• Scanning
– Scanning of environment (listen for beacons of APs or send a probe and
wait for a response)
beacon interval
A
B2
Station2
B
802.11 - Roaming
random backoff
a ACK for ATIM
B2
D
a
B1
• Reassociation Request
– Station requests joining the network to AP(s)
d
t
A ATIM transmission D data transmission
d ACK for data
Chapter 3.2: WLAN
Page 45
• 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
Page 46
Chapter 3.2: WLAN
Quality of Service – IEEE 802.11e
Extended Distributed Channel Access
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
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)
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
Chapter 3.2: WLAN
Page 47
AIFS[TC0]
AIFS[TC6]
DIFS =
AIFS[TC7]
PIFS
busy
SIFS
RTS
contention window
t
TC
Access Category (AC)
Purpose
0
1
2
3
4
5
6
7
0
1
1
2
2
2
3
3
Best Effort
Background
Background
Video Probe
Video
Video
Voice
Voice
• 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
•Chapter
Fairness
is given – even high priority senders can draw a large backoff number
Page 48
3.2: WLAN
EDCF Implementation
HCCA
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):
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?
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)
Chapter 3.2: WLAN
Page 49
Chapter 3.2: WLAN
Faster!
MIMO
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
Chapter 3.2: WLAN
Page 50
Page 51
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)
Chapter 3.2: WLAN
Page 52
802.11n – MAC Layer
802.11s – WLAN Mesh Networking
Other WLAN variant: mesh networks
• Classical WLAN: wired
infrastructure between APs
• Sometimes called “Wireless
Paradox”
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
Figures from: IEEE 802.11s tutorial
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…
Let APs interconnect in
wireless manner, also using
WLAN (lower costs, simple
installation, resilient, …)
Page 53
Chapter 3.2: WLAN
Page 54
Mesh Topology
Secure or not Secure…
Mesh Point
Special component,
establishes peer
links with neighbors
Mesh Portal
As mesh point, but
additionally connects
to some other
network
Figures from: IEEE 802.11s tutorial
Mesh AP
As mesh point, but
additionally implements
AP functionallity
Changes in the 802.11 standard regarding:
• Addresses
• Security
• MAC scheme (oriented at 802.11e)
• And: routing (layer 3!)
• Synchronization / power modes
Chapter 3.2: WLAN
Chapter 3.2: WLAN
Page 55
Within a WLAN „data are flying free through the air“.
Within WLAN everybody in transmission range can share your Access Point.
Thus: security!
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?)
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...
Page 56
Chapter
3.2: WLAN
Wardriving
Warchalking
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
What can be found at walls after a wardiver has
passed...
• A GPS receiver
• Time for driving around
Page 57
Chapter 3.2: WLAN
(separated by
installation)
100
byte
802.15
79 channels
SIFS
ACK
DIFS
500 byte
SIFS
ACK
100
byte
802.11b
3 channles
DIFS
DIFS
SIFS
ACK
SIFS
ACK
DIFS
100
byte
DIFS
DIFS
100
byte
500 byte
SIFS
ACK
SIFS
ACK
100
byte
DIFS
SIFS
ACK
1000 byte
500 byte
DIFS
DIFS
DIFS
f [MHz]
2480
SIFS
ACK
802.11 vs. 802.15/Bluetooth
(separated by
hopping pattern)
2402
t
• 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
Chapter 3.2: WLAN
Page 59
Chapter 3.2: WLAN
Page 58

Wireless Ethernet Wireless LANs: Design Goals

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