Transcript IEEE 802.11 based WLANs

IEEE 802.11 Wireless Local
Area Networks (WLANs)
802.11 WLANs - Outline
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801.11 bands and layers
Link layer
Media access layer
 frames and headers
 CSMA/CA
Physical layer
 frames
 modulation
 Frequency hopping
 Direct sequence
 Infrared
Security
Implementation
2
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How to share a medium?
Medium sharing techniques
Static
channelization
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FDMA,TDMA, CDMA
Uses partition
medium
Dedicated allocation
to users
Examples:
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Satellite
transmission
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Cellular
Telephone
Dynamic medium
access control
Scheduling
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Medium sharing
required for multiple
users to access the
channel
Communications by
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unicasting
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multicasting
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broadcasting
Random access
(contention)
Polling (take turns):
Token ring (=round
robin)
Reservation systems:
Request for slot in
transmission schedule
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Loose coordination
Send, wait, retry if
necessary
Aloha
CSMA/CD (Ethernet)
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MAC Techniques - overview
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Contention
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Medium is free for all
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A node senses the free medium and occupies it as long as data packet
requires it
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Example: Ethernet (CSMA/CD), IEEE 802.3
Token ring
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gives everybody a turn
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reservation time depends on token holding time (set by network
operator)
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for heavy loaded networks
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Example: Token Ring/IEEE 802.5, Token Bus/IEEE 802.4, FDDI
Reservation (long term)
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link reservation for multiple packets
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Example: schedule a time slot: GSM using TDMA
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How ring-network works
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A node functions as a repeater
A
only destination copies
frame to it,
C
A
all other nodes
have to discarded
B transmits frame
the frame
addressed to A
Unidirectional link
A
B
C
C ignores frame
A
C
A
A copies frame
B
A
A
B
C
A
B
C absorbs
returning frame
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Token ring
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A ring consists of a single or dual (FDDI) cable in the shape of a
loop. Ring reservation supervised by rotating token.
Each station is only connected to each of its two nearest
neighbors. Data in the form of packets passes around the ring
from one station to another in uni-directional way.
Advantages :
 (1) Access method supports heavy load without
degradation of performance because the medium is not
shared.
 (2) Several packets can simultaneous circulate between
different pairs of stations.
Disadvantages:
 (1) Complex management
 (2) Re-initialization of the ring whenever a failure occurs
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How bus-network works
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In a bus network, one node’s transmission traverses the entire
network and is received and examined by every node. The access
method can be :
 (1) Contention scheme : multiple nodes attempt to access
bus; only one node succeed at a time (e.g. CSMA/CD in
Ethernet)
 (2) Round robin scheme : a token is passed between nodes;
node holding the token can use the bus (e.g.Token bus)
Advantages:
 (1) Simple access method
C
D
A
B
 (2) Easy to add or remove
D
stations
term
term
Disadvantages:
 (1) Poor efficiency with high
network load
 (2) Relatively insecure, due to
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the shared medium
term: terminator impedance
Selecting a Medium Access Control
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Applications:
 What type of traffic?
 Voice streams? Steady traffic, low delay/jitter
 Data? Short messages? Web page downloads?
 Enterprise or consumer market? Reliability, cost
Scale:
 How much traffic can be carried?
 How many users can be supported?
Examples:
 Design MAC to provide wireless DSL-equivalent access for rural
communities
 Design MAC to provide Wireless-LAN-equivalent access to mobile
users (user in car travelling at 130 km/hr)
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Wireless LAN standard requirements
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Wireless channel is unreliable
 error control
 security/secrecy
Stations movable and may be operated while moved
 addressing and association procedures
 interconnections (roaming)
Wireless channel is also the reason why access method for
802.11 is called as CSMA/CA and not CSMA/CD (Collisions
can not be avoided)
CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance
CSMA/CD: Carrier Sense Multiple Access/Collision Detection
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IEEE 802.11 defines physical (PHY), logical link (LLC) and media
access control (MAC) layers for wireless local area networks
802.11 networks can work as
Network
 basic service set (BSS)
LLC
MAC
 extended service set (ESS)
FHSS DSSS IR PHY
BSS can also be used in ad-hoc
networking
802.xx
IEEE 802.11 Architecture
DS,
ESS
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
PHY: Physical Layer
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
SS: Spread spectrum
IR: Infrared light
BSS: Basic Service Set
ESS: Extended Service Set
AP: Access Point
DS: Distribution System
ad-hoc network
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BSS and ESS
Defined
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Basic Service Set (BSS)
Basic (independent)
service set (BSS)
Extended service set (ESS)
Group of stations that coordinate their access using a given instance of
MAC
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Located in a Basic Service Area (BSA)
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Stations in BSS can communicate with each other
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Distinct collocated BSS’s can coexist
Extended Service Set (ESS)
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Multiple BSSs interconnected by Distribution System (DS)
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Each BSS is like a cell and stations in BSS communicate with an Access
Point (AP). Gateway operation:
 Portals attached to DS provide access to Internet
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BSSs may overlap, be physically disjoint, or they may be collocated (one
BSS may use several antennas)
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11
IEEE 802-series of LAN standards
802 standards free to
download from
http://standards.ieee.org
/getieee802/portfolio.html
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hub
stations
hub
stations
hub
stations
hub
router
server
Demand priority: A round-robin (see token rings-later) arbitration
method to provide LAN access based on message priority level
DQDB: Distributed queue dual buss, see PSTN lecture 2
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The IEEE 802.11 and
supporting LAN Standards
IEEE 802.2
Logical Link Control (LLC)
OSI Layer 2
(data link)
MAC
IEEE 802.3 IEEE 802.4 IEEE 802.5
IEEE 802.11
Carrier
Token
Token
Wireless
Sense
Bus
Ring
a b g
bus

star
PHY
OSI Layer 1
(physical)
ring
See also IEEE LAN/MAN Standards Committee Web site
http://www.ieee802.org/
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802.11 WLAN technologies
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IEEE 802.11 standards and rates
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IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band ) [FH, DS]
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IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi [QPSK]
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IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz
band) [OFDM]
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IEEE 802.11g (2001 ... 2003) up to 54 Mbps (2.4 GHz) backward
compatible to 802.11b [OFDM]
IEEE 802.11 networks work on license free Industrial, Science,
Medicine (ISM) bands:
26 MHz
902
EIRP power
in Finland
928
83.5 MHz
2400
2484
100 mW
200 MHz
5150
5350
255 MHz
5470
200 mW
indoors only
5725 f/MHz
1W
EIRP: Effective Isotropically Radiated Power - radiated power measured immediately after antenna
Equipment technical requirements for radio frequency usage defined in ETS 300 328
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Other WLAN technologies
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High performance LAN or HiperLAN (ETSI-BRAN EN 300
652) in the 5 GHz ISM
 version 1 up to 24 Mbps
 version 2 up to 54 Mbps
HiperLAN provides also QoS for data, video, voice and
images
Bluetooth
 range up to 100 meters only (cable replacement tech.)
 Bluetooth Special Interest Group (SIG)
 Operates at max of 740 kbps at 2.4 GHz ISM band
 Applies fast frequency hopping 1600 hops/second
 Can have serious interference with 802.11 2.4 GHz
range network
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IEEE 802.11 Mobility
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Standard defines the following mobility types:
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No-transition: no movement or moving within a local BSS
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BSS-transition: station movies from one BSS in one ESS to another
BSS within the same ESS
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ESS-transition: station moves from a BSS in one ESS to a BSS in a
different ESS (continuos roaming not supported)
Especially: 802.11 don’t support roaming with GSM!
- Address to destination
mapping
- seamless integration
of multiple BSS
ESS 1
ESS 2
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802.11 Logical architecture
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802.xx
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LLC provides addressing and data link control
MAC provides
Network
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access to wireless medium
LLC
 CSMA/CA
MAC
 Priority based access (802.12)
FHSS DSSS IR PHY
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joining the network
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authentication & privacy
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Services
 Station service: Authentication, privacy, MSDU* delivery
 Distributed system: Association**, participates to data distribution
Three physical layers (PHY)
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FHSS: Frequency Hopping Spread
Spectrum (SS)
CSMA/CA: Carrier Sense Multiple Access
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DSSS: Direct Sequence SS
with Collision Avoidance
LLC: Logical Link Control Layer
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IR: Infrared transmission
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*MSDU: MAC service data unit
** with an access point in ESS or BSS
MAC: Medium Access Control Layer
PHY: Physical Layer
FH: Frequency hopping
DS: Direct sequence
IR: Infrared light
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802.11 DSSS
DSSS-transmitter
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Supports 1 and 2 Mbps data transport, uses BPSK and QPSK modulation
Uses 11 chips Barker code for spreading - 10.4 dB processing gain
Defines 14 overlapping channels, each having 22 MHz channel
bandwidth, from 2.401 to 2.483 GHz
Power limits 1000mW in US, 100mW in EU, 200mW in Japan
Immune to narrow-band interference, cheaper hardware
PPDU:Baseband Data Frame Unit, BPSK: Binary Phase Shift Keying, QPSK: Quadrature PSK
DSSS: Direct Sequence Spread Spectrum, PN:Pseudo Noise
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802.11 FHSS
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Supports 1 and 2 Mbps data transport and applies two level - GFSK
modulation* (Gaussian Frequency Shift Keying)
79 channels from 2.402 to 2.480 GHz ( in U.S. and most of EU
countries) with 1 MHz channel space
78 hopping sequences with minimum 6 MHz hopping space, each
sequence uses every 79 frequency elements once
Minimum hopping rate
2.5 hops/second
Tolerance to multi-path,
narrow band interference,
security
Low speed, small range
due to FCC TX power
regulation (10mW)
* f  f c  f , f nom  160 kHz
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26 MHz
802.11a
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902
928
83.5 MHz
2400
2484
200 MHz
5150
5350
255 MHz
5470
5725 f/MHz
Operates at 5 GHz band
Supports multi-rate 6 Mbps, 9 Mbps,… up to 54 Mbps
Uses Orthogonal Frequency Division Multiplexing (OFDM) with 52
subcarriers, 4 us symbols (0.8 us guard interval)
Applies inverse discrete Fourier transform (IFFT) to combine multicarrier signals to single time domain symbol
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IEEE 802.11a rates and modulation
formats
Data Rate
Coded bits per
Code bits per
Data bits per
sub-carrier
OFDM symbol
OFDM symbol
1/2
1
48
24
BPSK
3/4
1
48
36
12
QPSK
1/2
2
96
48
18
QPSK
3/4
2
96
72
24
16QAM
1/2
4
192
96
36
16QAM
3/4
4
192
144
48
64QAM
2/3
6
288
192
54
64QAM
3/4
6
288
216
Modulation
Coding Rate
6
BPSK
9
(Mbps)
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IEEE 802.3 Media
Access Control (MAC)
Carrier-sense multiple access protocol
with collision avoidance (CSMA/CA)
DIFS: Distributed Inter-Frame Spacing
SIFS: Short Inter-Frame Spacing
ack: Acknowledgement
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Other ARQ - Techniques
forward
channel
ARQ-system:
TX-buffer
RX-buffer
return
channel
ack. of erroneous frame
correct pre-error frames
correct post-error frames
‘corrected’ frame
acknowledgment
ack. of error received
n-1 frames send due
to RX-TX propagation
delay
TX-buffer
TX-buffer
re-send only the erroneous frame
RX-buffer (after reordering)
n frames to be re-send
Selective repeat
RX-buffer
Go-back-n
- also some correct frames re-send
- small receiver buffer size enough
- no reordering in RX
- reordering required in TX
-- large
buffer
required
in in
TXTX
a large
buffer
required
Stop-and-wait
- for each packet wait for ack.
- if negative ack received, re-send packet
- inefficient if long propagation delays
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MAC frame
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NOTE: This frame structure is common for all data send by a 802.11 station
control info (WEP, data type as management, control, data ...)
next frame duration
frame ordering
info for RX
-Basic service identification*
-source/destination address
-transmitting station
-receiving station
*BSSID: a six-byte address typical for a particular access point
(network administrator sets)
CRC: Cyclic Redundancy Check
WEP: Wired Equivalent Privacy
frame specific,
variable length
frame check
sequence
(CRC)
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Logical Link Control Layer (LLC)
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Specified by ISO/IEC 8802-2 (ANSI/IEEE 802.2)
purpose: exchange data between users across LAN using 802-based
MAC controlled link
provides addressing and data link control (routing)
independent of topology, medium, and chosen MAC access method
Data to higher level protocols
Info: carries user data
Supervisory: carries
flow/error control
Unnumbered: carries protocol
control data
Source
SAP
LLC’s functionalities
LLC’s protocol data unit (PDU)
SAP: service access point
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Logical Link Control Layer Services
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A Unacknowledged connectionless service
 no error or flow control - no ack-signal usage
 unicast (individual), multicast, broadcast addressing
 higher levels take care or reliability - thus fast for
instance for TCP
B Connection oriented service
 supports unicast only
 error and flow control for lost/damaged data packets
by cyclic redundancy check (CRC)
C Acknowledged connectionless service
 ack-signal used
 error and flow control by stop-and-wait ARQ
 faster setup than for B
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A TCP/IP packet in 802.11
TPC/IP sends data packet
Control
header
LLC constructs PDU by
adding a control header
SAP (service access point)
MAC frame with
new control fields
Traffic to the
target BSS / ESS
*BDU: protocol data unit
MAC lines up packets using carrier
sense multiple access (802.3 CSMA)
PHY layer transmits packet
using a modulation method
(DSSS, OFDM, IR, FHSS)
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Authentication and privacy
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Goal: to prevent unauthorized access & eavesdropping
Realized by authentication service prior access
Open system authentication
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station wanting to authenticate sends authentication
management frame - receiving station sends back frame for
successful authentication
Shared key authentication (included in WEP*)
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Secret, shared key received by all stations by a separate, 802.11
independent channel
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Stations authenticate by a shared knowledge of the key properties
WEP’s privacy (blocking out eavesdropping) is based on ciphering:
*WEP: Wired Equivalent Privacy 28
Vertical
handovers
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Vertical handovers deal with communication between different systems
targeting to seamless communications:
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802 and global roaming (GSM, UMTS)
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802 and Internet distribution (ADSL)
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802 and home networking (Bluetooth)
Users can route traffic to different interfaces based on their price, security,
bandwidth, etc. requirements
Current (Cellular) operators are in a (very) good position to extend their
service offerings beyond a single wireless network (subscriptions incl.
several access methods, e.g. 2.5/3G, WLAN, xDSL, etc.).
For more information search Google with ‘6WINIT’
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Simultaneous
Multiple Access
(“SIMA”)
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SIMA=Ability to use simultaneously multiple network interfaces:
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Route different traffic flows through different interfaces in parallel
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Access and network selection could be based on user
profiles/policies
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Dynamic operation (possibly hidden from the end-user)
Scope of mobility (Homogeneous vs. Heterogeneous environments):
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micro-mobility, intra-domain mobility (Horizontal handovers)
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macro-mobility, inter-domain/global mobility (Vertical handovers)
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WLAN Network Planning
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Network planning target
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Maximize system performance with limited resource
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Including
 coverage
 throughput
 capacity
 interference
 roaming
 security, etc.
Planning process
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Requirements for project management personnel
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Site investigation
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Computer-aided planning practice
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Testing and verifying planning
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Base station location planning tools
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NPS/indoor (Nokia Network, Finland)
 Indoor radio planning designed originally for GSM/DCS
 Support three models
 One slop model
 Multi-wall model
 Enhanced Multi-wall model
 System parameters can be adjusted
and optimized by field measurement
 Graphical planning of interface
and coverage view
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Field measurements
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Measurement targets: power levels - throughput - error rate
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Laptop or PDA
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Utility come with radio card HW (i.e. Lucent
client manager)
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Supports channel scan, station search
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Indicate signal level, SNR, transport rate
Advanced tools: detailed protocol data flows
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Special designed for field measurement
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Support PHY and MAC protocol analysis
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Integrated with network planning tools
Examples
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Procycle™ from Softbit, Oulu, Finland
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SitePlaner™ from WirelessValley, American
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Capacity planning
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802.11b has usually about 6.5 Mbps rate throughput due to
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CSMA/CA MAC protocol
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PHY and MAC management overhead
More user connected, less capacity offered
Example of supported users in different application cases:
Environment
Traffic content
Corporation
Web, Email, File
Wireless LAN
transfer
Branch Office
All application via
Network
WLAN
Public Access
Web, Email, VPN
Traffic Load
Nu mber of simu ltaneous users
802.11b
802.11b
802.11
11Mbps
5.5Mbps
2Mbps
150 kbits/user
40
20
9
300 kbits/user
20
10
4
100 kbits/user
60
30
12
tunneling
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Frequency
planning
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83.5 MHz
200 MHz
255 MHz
802.11b
902
928
2400
2484
5150
5350
5470
5725 f/MHz
Interference from other WLAN systems or cells
IEEE 802.11 operates at uncontrolled ISM band
14 channels of 802.11b are overlapping, only 3 channels
are disjointed. For example Ch1, 6, 11
Throughput decreases with less channel spacing
A example of frequency allocation in multi-cell network
6
5
4
Mbit/s
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26 MHz
11Mb if/frag 512
2Mb if/frag 512
2Mb if/frag 2346
3
2
1
0
Offset
25MHz
Offset
20MHz
Offset
15MHz
Offset
10MHz
Offset
5MHz
Offset
0MHz
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Interference from microwave ovens
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Microwave oven magnetrons have central frequency at
2450~2458 MHz
Burst structure of radiated radio signal, one burst will affect
several 802.11 symbols
18 dBm level measured from 3 meter away from oven
-> masks all WLAN signals!
Solutions
 Use unaffected channels
 Keep certain distance
 Use RF absorber near
microwave oven
100 mW
902
928
26 MHz
2400
2484
83.5 MHz
indoors only
200 mW
5150
5350
200 MHz
1W
5470
5725 f/MHz
255 MHz
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Interference from Bluetooth
The received signal level from two systems are comparable at
mobile side

In co-existing environment, the probability of frequency collision
for one 802.11 frame vary from 48% ~62%

Deterioration level is relevant to many factors
 relative signal levels
 802.11 frame length
 activity in Bluetooth
channel
Solution

Co-existing protocol
IEEE 802.15 (not ready)

Limit the usage of BT
in 802.11 network

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WLAN benefits

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Mobility
 increases working efficiency and productivity
 extends the On-line period
Installation on difficult-to-wire areas
 inside buildings
 road crossings
Increased reliability
 Note: Pay attention to security!
Reduced installation time
 cabling time and convenient to users and difficult-towire cases
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WLAN benefits (cont.)


Broadband
 11 Mbps for 802.11b
 54 Mbps for 802.11a/g (GSM:9.6Kbps,
HCSCD:~40Kbps, GPRS:~160Kbps, 2.5G: ~340 kbps,
WCDMA:up to 2Mbps)
Long-term cost savings
 O & M cheaper that for wired nets
 Comes from easy maintenance, cabling cost, working
efficiency and accuracy
 Network can be established in a new location just by
moving the PCs!
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WLAN technology challenges

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Date Speed
 IEEE 802.11b supports rates up to 11 MBps (in practice 6
Mb/s), sometimes this is not enough - far lower than 100
Mbps fast Ethernet. (… 802.11a reaches up to 54 Mb/s …)
Interference
 Works in ISM band, shares the same frequency with
microwave oven, Bluetooth, and others
Security
 Current WEP algorithm is weak - usually not ON!
Roaming
 No industry standard is available and propriety solution are
not interoperable - especially with GSM
Inter-operability
 Only few basic functionality are interoperable, other vendor’s
features can’t be used in a mixed network
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WLAN implementation challenges

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Lack of wireless networking experience for most IT
engineer
No well-recognized operation process on network
implementation
Selecting access points with ‘Best Guess’ method
Unaware of interference from/to other networks
Weak security policy
As a result, your WLAN may have
 Poor performance (coverage, throughput, capacity,
security)
 Unstable service
 Customer dissatisfaction
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References
[1] Jim Geier: Wireless LANs, SAMS publishing
[2] A. Leon-Garcia, I. Widjaja: Communication Networks (2th
ed.), Instructor’s Slide Set, McGraw-Hill
[3] Kurose, Ross: Computer Networking (2th ed.), Pearson
Education
[4] IEEE 802 Standard
[5] Raimo Vuopionperä: Background of the VHO project,
TEKES-seminar (Vertical handover goes alive! )
31.10.2003
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