Transcript 2005-guest-leture

Introduction to Wireless LAN
Background
 Media access principle
 Architecture

MAC control
 MAC management
 PHY layer

802.11 Wireless LAN:
Background (15 min)
Wireless LANs


Wireless networks today
Features and benefits

Mobility
 Flexibility
 Scalability

Wireless LANs: ready now and ready for the future

Standards
 Security
 Service
 Roaming
Wireless LAN
Basic Service Set (BSS): a single cell controlled by a Base Station (also called Access
Point or AP)
Distribution System: the interconnection network of base stations
Extended Service Set (ESS): the whole interconnected Wireless LAN (seen as a single
802 network), including the different cells, their respective Access Points, and the
Distribution System
Radio Frequency In Wireless Networks



Radio spectrum
Narrowband interference
Spread spectrum




FHSS – Frequency Hopping Spread Spectrum
DSSS – Direct Sequence Spread Spectrum
Multi-path interference
IEEE 802.11 series standard





802.11: 2M
802.11b: 1M, 2M, 5.4M, 11M
802.11a: 6M, 9M, 12M, 18M, 24M, 36M, 48M, 54M
802.11g: compatible with 802.11b and 802.11a
Other standards:

802.11e: provides Quality of Service (QoS)
 802.11h: supplementary to comply with European regulations
 802.11i: improved WLAN security
Multipath Effect
Multipath radio effect. Transmitter signals are reflected or diffracted
by structures, changing the signals’ timing, strength, and quality.
IEEE 802.11b Standard
802.11b allows unconnected client devices to communicate with an Ethernet
network through an RF (Radio Frequency) transmitter that is physically
connected to the wired network.
Deploying Wireless LAN



Ad-hoc network
Association and roaming
Deploying access point and wireless LANs




Deploying access point
Load balancing
Channel selection for neighboring wireless LANs
Multiple channel rate in a wireless LAN
US (FCC)/Canada (IC)
channel 1
2400
2412
channel 6
channel 11
2437
2462
22 MHz
2483.5
[MHz]
802.11 Wireless LAN:
Media Access Principle (20 min)
Media Access
Can we apply media access methods from fixed networks?
Example CSMA/CD


Carrier Sense Multiple Access with Collision Detection
send as soon as the medium is free, listen into the medium if a collision
occurs (original method in IEEE 802.3)
Problems in wireless networks

signal strength decreases proportional to the square of the distance
 the sender would apply CS and CD, but the collisions happen at the
receiver
 it might be the case that a sender cannot “hear” the collision, i.e., CD does
not work
 furthermore, CS might not work if, e.g., a terminal is “hidden”
Motivation - hidden and exposed terminals
Hidden terminals

A sends to B, C cannot receive A
 C wants to send to B, C senses a “free” medium (CS fails)
 collision at B, A cannot receive the collision (CD fails)
 A is “hidden” for C
Exposed terminals

A
B
C
B sends to A, C wants to send to another terminal (not A or B)
 C has to wait, CS signals a medium in use
 but A is outside the radio range of C, therefore waiting is not
necessary
 C is “exposed” to B
MACA - collision avoidance
MACA (Multiple Access with Collision Avoidance) uses short signaling
packets for collision avoidance

RTS (request to send): a sender request the right to send from a receiver
with a short RTS packet before it sends a data packet
 CTS (clear to send): the receiver grants the right to send as soon as it is
ready to receive
Signaling packets contain

sender address
 receiver address
 packet size
Variants of this method can be found in IEEE802.11 as DFWMAC
(Distributed Foundation Wireless MAC)
MACA examples
MACA avoids the problem of hidden terminals

A and C want to
send to B
 A sends RTS first
 C waits after receiving
CTS from B
RTS
CTS
A
CTS
B
C
MACA avoids the problem of exposed terminals

B wants to send to A, C
to another terminal
 now C does not have
to wait for it cannot
receive CTS from A
RTS
RTS
CTS
A
B
C
Polling mechanisms
If one terminal can be heard by all others, this “central” terminal
(a.k.a. base station) can poll all other terminals according to a
certain scheme

now all schemes known from fixed networks can be used (typical
mainframe - terminal scenario)
Example: Randomly Addressed Polling





base station signals readiness to all mobile terminals
terminals ready to send can now transmit a random number without
collision with the help of CDMA or FDMA (the random number can be
seen as dynamic address)
the base station now chooses one address for polling from the list of
all random numbers (collision if two terminals choose the same
address)
the base station acknowledges correct packets and continues polling
the next terminal
this cycle starts again after polling all terminals of the list
802.11 Wireless LAN:
Architecture (15 min)
Comparison: infrastructure vs. ad-hoc networks
infrastructure
network
AP
AP
ad-hoc network
wired network
AP: Access Point
AP
802.11 - Architecture of an infrastructure network
Station (STA)
802.11 LAN
STA1
802.x LAN

Basic Service Set (BSS)
BSS1
Portal
Access
Point
Access
Point
ESS

group of stations using the same
radio frequency
Access Point
Distribution System

station integrated into the wireless
LAN and the distribution system
Portal

BSS2
bridge to other (wired) networks
Distribution System

STA2
terminal with access mechanisms
to the wireless medium and radio
contact to the access point
802.11 LAN
STA3
interconnection network to form
one logical network (EES:
Extended Service Set) based
on several BSS
802.11 - Architecture of an ad-hoc network
Direct communication within a limited
range
802.11 LAN

Station (STA):
terminal with access mechanisms to
the wireless medium
 Independent Basic Service Set
(IBSS):
group of stations using the same
radio frequency
STA1
STA3
IBSS1
STA2
IBSS2
STA5
STA4
802.11 LAN
IEEE standard 802.11
fixed
terminal
mobile terminal
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
802.11 - Layers and functions
MAC

PLCP Physical Layer Convergence Protocol
MAC Management


access mechanisms, fragmentation,
encryption
clear channel assessment signal
(carrier sense)
PMD Physical Medium Dependent
synchronization, roaming, MIB,
power management

modulation, coding
PHY Management

channel selection, MIB
Station Management
LLC
MAC
MAC Management
PLCP
PHY Management
PMD
Station Management
PHY
DLC

coordination of all management
functions
802.11 Wireless LANs:
MAC Control (25 min)
802.11 - MAC layer I - DFWMAC
Traffic services

Asynchronous Data Service (mandatory)
exchange of data packets based on “best-effort”
 support of broadcast and multicast


Time-Bounded Service (optional)

implemented using PCF (Point Coordination Function)
Access methods

DFWMAC-DCF CSMA/CA (mandatory)
collision avoidance via randomized „back-off“ mechanism
 minimum distance between consecutive packets
 ACK packet for acknowledgements (not for broadcasts)


DFWMAC-DCF w/ RTS/CTS (optional)

Distributed Foundation Wireless MAC
 avoids hidden terminal problem

DFWMAC- PCF (optional)

access point polls terminals according to a list
802.11 - MAC layer II
Priorities

defined through different inter frame spaces
 no guaranteed, hard priorities
 SIFS (Short Inter Frame Spacing)


PIFS (PCF IFS)


highest priority, for ACK, CTS, polling response
medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS)

lowest priority, for asynchronous data service
DIFS
DIFS
medium busy
PIFS
SIFS
direct access if
medium is free  DIFS
contention
next frame
t
802.11 - CSMA/CA access method I
DIFS
DIFS
medium busy
direct access if
medium is free  DIFS

contention window
(randomized back-off
mechanism)
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 (collision
avoidance, multiple of slot-time)
 if another station occupies the medium during the back-off time of
the station, the back-off timer stops (fairness)
802.11 - competing stations - simple version
DIFS
DIFS
station1
station2
DIFS
boe
bor
boe
busy
DIFS
boe bor
boe
busy
boe busy
boe bor
boe
boe
busy
station3
station4
boe bor
station5
busy
bor
t
busy
medium not idle (frame, ack etc.)
boe elapsed backoff time
packet arrival at MAC
bor residual backoff time
802.11 - CSMA/CA access method II
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
receiver
ACK
DIFS
other
stations
waiting time
data
t
contention
802.11 - DFWMAC
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
other
stations
CTS SIFS
SIFS
NAV (RTS)
NAV (CTS)
defer access
ACK
DIFS
data
t
contention
Fragmentation
DIFS
sender
RTS
frag1
SIFS
receiver
CTS SIFS
frag2
SIFS
ACK1 SIFS
SIFS
ACK2
NAV (RTS)
NAV (CTS)
other
stations
NAV (frag1)
NAV (ACK1)
DIFS
contention
data
t
DFWMAC-PCF I
t0 t1
medium busy PIFS
point
coordinator
wireless
stations
stations‘
NAV
SuperFrame
SIFS
D1
SIFS
SIFS
D2
SIFS
U1
U2
NAV
DFWMAC-PCF II
t2
point
coordinator
wireless
stations
stations‘
NAV
D3
PIFS
SIFS
D4
t3
t4
CFend
SIFS
U4
NAV
contention free period
contention
period
t
802.11 - Frame format
Types

control frames, management frames, data frames
Sequence numbers

important against duplicated frames due to lost ACKs
Addresses

receiver, transmitter (physical), BSS identifier, sender (logical)
Miscellaneous

sending time, checksum, frame control, 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
MAC address format
scenario
ad-hoc network
infrastructure
network, from AP
infrastructure
network, to AP
infrastructure
network, within DS
to DS from
DS
0
0
0
1
address 1 address 2 address 3 address 4
DA
DA
SA
BSSID
BSSID
SA
-
1
0
BSSID
SA
DA
-
1
1
RA
TA
DA
SA
DS: Distribution System
AP: Access Point
DA: Destination Address
SA: Source Address
BSSID: Basic Service Set Identifier
RA: Receiver Address
TA: Transmitter Address
Special Frames: ACK, RTS, CTS
Acknowledgement
bytes
ACK
2
2
6
Frame
Receiver
Duration
Control
Address
4
CRC
Request To Send
bytes
RTS
2
2
6
6
Frame
Receiver Transmitter
Duration
Control
Address Address
Clear To Send
bytes
CTS
2
2
6
Frame
Receiver
Duration
Control
Address
4
CRC
4
CRC
802.11 Wireless LANs:
MAC Management
802.11 - MAC management
Synchronization

try to find a LAN, try to stay within a LAN
 timer 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
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
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
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

Traffic Indication Map (TIM)


list of unicast receivers transmitted by AP
Delivery Traffic Indication Map (DTIM)

list of broadcast/multicast receivers transmitted by AP
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?)
Power saving with wake-up patterns (infrastructure)
TIM interval
access
point
DTIM interval
D B
T
busy
medium
busy
T
d
D B
busy
busy
p
station
d
t
T
TIM
D
B
broadcast/multicast
DTIM
awake
p PS poll
d data transmission
to/from the station
Power saving with wake-up patterns (ad-hoc)
ATIM
window
station1
beacon interval
B1
station2
A
B2
B2
D
a
B1
d
t
B
beacon frame
awake
random delay
a acknowledge ATIM
A transmit ATIM
D transmit data
d acknowledge data
Power Mode of Wireless NIC
Transmit mode

Used during data transmission (sending)
 Power consumption: high (e.g., 450mA)
Receive mode

Default mode for both data receiving and listening
 Power consumption: medium (e.g., 270mA)
Sleep mode

Power consumption: low (e.g., 15mA)
Power Saving Mechanism
Frame types

Data Frames: used for data transmission
 Control Frames: used to control access to the medium
 Management Frames: such as Beacon Frame (for synchronization)
The Access Point

maintains a continually updated record of the stations currently working in
Power Saving mode
 buffers the packets addressed to these stations
 periodically transmits information (as part of its Beacon Frames) about
which Power Saving Stations have frames buffered at the AP
The Power Saving Station


wake up periodically (100ms) in order to receive the Beacon Frame
if there are frames stored at the AP waiting for delivery, the station stays
awake and sends a Polling message to the AP to get these frames
 otherwise goes back sleep
Power Consumed during PS Mode
Power consumed by Orinoco Gold
NIC during Power Save Mode
Power consumed by Cisco AIR-PCM350
NIC during Power Save Mode
Ecycle (n,t) = 0.060nt + 3300, 0 =< n =< 65535
Ecycle (n,t) = 0.060nt + 3300, 0 =< n =< 65535
Problems
Energy consumption

Wireless networking card consumes a great amount of energy in mobile
devices
 Over 50% total energy of handheld PC
 Up to 10% total energy of laptop PC
Networking performance


Highly depends on the signal strength and transmit power
signal attenuation: distance, obstacles, and environment (humidity,
temperature, etc)
 transmit power levels: 1mW, 5mW, 20mW, 30mW, 50mW, and 100mW for
Cisco Aironet 350 NIC
Latency


Data transmission time: effective bandwidth, loss rate
Mode transition time: power saving mode – normal mode
 AP handoff time
802.11 - Roaming
No or bad connection? Then perform:
Scanning

scan the environment, i.e., listen into the medium for beacon signals or
send probes into the medium and wait for an answer
Reassociation Request

station sends a request to one or several AP(s)
Reassociation Response

success: AP has answered, station can now participate
 failure: continue scanning
AP accepts Reassociation Request

signal the new station to the distribution system
 the distribution system updates its data base (i.e., location information)
 typically, the distribution system now informs the old AP so it can release
resources
802.11 Wireless LANs:
PHY Layer
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, synchonization
FHSS PHY packet format
Synchronization

synch with 010101... pattern
SFD (Start Frame Delimiter)

0000110010111101 start pattern
PLW (PLCP_PDU Length Word)

length of payload incl. 32 bit CRC of payload, PLW < 4096
PSF (PLCP Signaling Field)

data of payload (1 or 2 Mbit/s)
HEC (Header Error Check)

CRC with x16+x12+x5+1
80
synchronization
16
12
4
16
variable
SFD
PLW
PSF
HEC
payload
PLCP preamble
PLCP header
bits
DSSS PHY packet format
Synchronization

synch., gain setting, energy detection, frequency offset compensation
SFD (Start Frame Delimiter)

1111001110100000
Signal

data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)
Service

Length
future use, 00: 802.11 compliant

length of the payload
HEC (Header Error Check)

protection of signal, service and length, x16+x12+x5+1
128
synchronization
16
SFD
PLCP preamble
8
8
16
16
signal service length HEC
PLCP header
variable
payload
bits
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

100€ adapter, 250€ base station,
dropping
Availability

Many products, many vendors
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)
Special 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
IEEE 802.11b – PHY frame formats
Long PLCP PPDU format
128
16
synchronization
SFD
8
8
16
16
signal service length HEC
PLCP preamble
bits
variable
payload
PLCP header
192 µs at 1 Mbit/s DBPSK
1, 2, 5.5 or 11 Mbit/s
Short PLCP PPDU format (optional)
56
short synch.
16
SFD
8
8
16
16
signal service length HEC
PLCP preamble
(1 Mbit/s, DBPSK)
variable
payload
PLCP header
(2 Mbit/s, DQPSK)
96 µs
2, 5.5 or 11 Mbit/s
bits
Channel selection (non-overlapping)
Europe (ETSI)
channel 1
2400
2412
channel 7
channel 13
2442
2472
22 MHz
2483.5
[MHz]
US (FCC)/Canada (IC)
channel 1
2400
2412
channel 6
channel 11
2437
2462
22 MHz
2483.5
[MHz]
WLAN: IEEE 802.11a
Data rate



Connection set-up time
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


Limited, WEP insecure, SSID

280€ adapter, 500€ base station
Cost
Availability
Some products, some vendors
Typ. best effort, no guarantees (same as
all 802.11 products)
Manageability

Limited (no automated key distribution,
sym. Encryption)
Special Advantages/Disadvantages



Connectionless/always on
Quality of Service
Free 5.15-5.25, 5.25-5.35, 5.725-5.825
GHz ISM-band
Security


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 QoS
IEEE 802.11a – PHY frame format
4
1
12
1
rate reserved length parity
6
16
tail service
variable
6
variable
payload
tail
pad
bits
PLCP header
PLCP preamble
12
signal
1
6 Mbit/s
data
variable
6, 9, 12, 18, 24, 36, 48, 54 Mbit/s
symbols
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
5725 5745 5765 5785 5805 5825 [MHz]
16.6 MHz
center frequency =
5000 + 5*channel number [MHz]
OFDM in IEEE 802.11a (and HiperLAN2)
OFDM with 52 used subcarriers (64 in total)
 48 data + 4 pilot
 (plus 12 virtual subcarriers)
 312.5 kHz spacing
312.5 kHz
pilot
-26 -21
-7 -1 1
7
channel center frequency
21 26
subcarrier
number
WLAN: IEEE 802.11 – future developments (08/2002)
802.11d: Regulatory Domain Update – completed
802.11e: MAC Enhancements – QoS – ongoing

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 – ongoing

Establish an Inter-Access Point Protocol for data exchange via the
distribution system.
802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM – ongoing
802.11h: Spectrum Managed 802.11a (DCS, TPC) – ongoing
802.11i: Enhanced Security Mechanisms – ongoing
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Enhance the current 802.11 MAC to provide improvements in security.
Study Groups
5 GHz (harmonization ETSI/IEEE) – closed
 Radio Resource Measurements – started
 High Throughput – started
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