Transcript IEEE 802.11 based WLANs

S-72.1130 Telecommunication
Systems
Wireless Local Area Networks
Outline




LAN basics

Structure/properties of LANs
WLANs

Link layer services

Media access layer
 frames and headers
 CSMA/CA

Physical layer
 frames
 modulation
 Frequency hopping
 Direct sequence
 Infrared
Installation
Security
2
S-72.1130 Telecommunication
Systems
LAN Basics
What is a LAN?
Local area means:

Freedom from regulatory constraints at ISM Band (Industrial,
Science and Medical)

Short distance (~1km) between computers

Low cost

High-speed (10 Mb/s.. 10 Gb/s); support for TCP or UDP type
of communications

Flexible error control: in MAC and in upper levels

Computers move, machines have unique MAC address

MAC protocol takes care of optimizing throughput for the
expected services

Physical level takes care of physical transmission of packets over
a medium
4
Typical Wired LAN



Transmission Medium
Network Interface Card
(NIC)
Unique MAC “physical”
address
Serial format
Ethernet
Processor
RAM
ROM
RAM
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
NIC implements MAC protocol &
physical port. Parallel interface to PC 5
IEEE 802-series of LAN Standards
802 standards free to
download from
http://standards.ieee.org
/getieee802

hub
stations
hub
stations
hub
stations
WiMAX
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
6
Example: How Ring Networks Work






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
Signal propagates
encoded by line codes
A
C
Example: 802.5
Problem in reliability if
A copies frame
a station fails
A
B
C
B
A
C ignores frame
A
B
C
A
B
C absorbs
returning frame
7
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 - especially for several rings
 (2) Re-initialization of the ring whenever a failure occurs
8
Example: Bus Network



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 802.3)
 (2) Round robin scheme : a token is passed between nodes;
node holding the token can use the bus (e.g.Token bus 802.4)
Advantages:
 (1) Simple access method
C
D
A
B
 (2) Easy to add or remove
stations
D
term
term
Disadvantages:
- Line coded, serial data
 (1) Poor efficiency with high
- twisted pair or coaxial cable
network load
 (2) Relatively insecure, due to
9
the shared medium
term: terminator impedance
S-72.1130 Telecommunication
Systems
IEEE 802 LAN Standard
The IEEE 802 LAN Standards
(http://www.ieee802.org/)
OSI Layer 3
Network
IEEE 802.2
Logical Link Control (LLC)
LLC
OSI Layer 2
(data link)
b: Wi-Fi
IEEE 802.3 IEEE 802.4 IEEE 802.5
IEEE 802.11
Carrier
Token
Token
Wireless
Sense
Bus
Ring
Ethernet
a b g
Physical Layers
- options: twisted pair, coaxial, optical, radio paths;
(not for all MACs above!)
Bus (802.3…)
Star (802.3u…)
MAC
OSI Layer 1
(physical)
Ring (802.5…)
11
IEEE 802 Layers
Logical Link Control (LLC) Sublayer

Utilizes services of HDLC (Highlevel Data Link Control)

Therefore, LLC SAPs separate
upper layer data exchanges =>
NIC applies different buffer
segments for each SAP (port)

LLC provides means to exchange
frames between LANs using
different MACs
IEEE 802.2
Logical Link Control (LLC)
LLC
b: Wi-Fi
IEEE 802.3 IEEE 802.4 IEEE 802.5
IEEE 802.11
Carrier
Token
Token
Wireless
Sense
Bus
Ring
abg
Ethernet
Medium Access Control Sublayer

Coordinates access to medium

Connectionless/Connection oriented
frame transfer service

Machines identified by
MAC/physical address (in NIC)

Broadcasts frames with MAC
addresses

Examples: CSMA/CD, CSMA/CA
Physical layers
Physical




MAC
PHY
level
Star, bus or ring topology
Cabling and electrical interfaces
Twisted pair, coaxial, fiber
Line coding (wired LANs) or
modulation (WLANs)
(More of HDLC in supplementary…)
12
S-72.1130 Telecommunication
Systems
IEEE 802 LAN Standard:
Logical Link Layer (LLC)
Logical Link Control Layer (LLC)

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Specified by ISO/IEC 8802-2 (ANSI/IEEE 802.2)
Objective: 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 Protocol Data Unit (PDU)
(SAP: Service Access Point)
14
LLC Protocol Data Unit (PDU)
1 byte
1 byte
Destination
SAP Address
1 to 2 bytes
Source
SAP Address
Control
Source SAP Address
Destination SAP Address
C/R
I/G
1
Information
(network layer packet)
7 bits
1
I/G = Individual or group address
C/R = Command or response frame
Examples of SAP Addresses:
06 IP packet
E0 Novell IPX
FE OSI packet
AA Sub Network Access protocol (SNAP)
IP Packet
LLC header
IP Packet
MAC header LLC header
IP Packet
7 bits
FCS
Packet encapsulation into a MAC frame
FCS: Frame Check Sequence
15
LLC 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

Unnumbered frame mode of HDLC
B Connection oriented service

supports unicast only

error and flow control for lost/damaged data packets by cyclic
redundancy check (CRC)

Asynchronous balanced mode of HDLC: error control, sequencing,
flow control

Phases: Connection setup, data exchange, and release
C Acknowledged connectionless service
Problem: A workstation has a single, physical MAC address, how to separate
network or higher level service access? Ans: HDLC SAP addressing:

Can handle several logical connections, distinguished by their SAP (service
access points, next slides).


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ack-signal used
error and flow control by stop-and-wait ARQ
faster setup than for B
16
A TCP/IP Packet in 802.11:
Encapsulation
Control
header
TCP makes logical connection
to deliver the packet
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
by using a MAC protocol
PHY layer transmits packet
using a modulation method
(DSSS, OFDM, IR, FHSS)
17
Encapsulation …
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
18
SAP Addressing
IEE802.11 (CSMA/CA)...
IEE802.11 (CDMA)...
ATM...
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
19
S-72.1130 Telecommunication
Systems
IEEE 802 LAN Standard:
Media Access Control (MAC) Layer
Media Access Control:
Ways to Share a Medium
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Medium sharing techniques
Static
channelization
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FDMA,TDMA, CDMA
Uses partition
medium
Dedicated allocation
to users
Examples:

Satellite
transmission

Cellular
Telephone
Dynamic medium
access control
Scheduling
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Medium sharing
required for multiple
users to access the
channel
Communications by

unicasting

multicasting

broadcasting
Random access
(contention)
Polling (take turns):
Token ring 802.5
Reservation systems:
Request for slot in
transmission schedule
802.4
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Loose coordination
Send, wait, retry if
necessary
Aloha
CSMA/CD (Ethernet)
CSMA/CA (802.11
WLAN)
21
Selecting a Medium Access Control

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Environment: Wired / Wireless?
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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MAC Techniques in LANs

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Contention
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Medium is free for all

A node senses the free medium and occupies it as long as data packet
requires it

Example: Ethernet (IEEE 802.3 CSMA/CD)
Reservation (short term statistical access)

Gives everybody a turn

Reservation time depends on token holding time (set by network
operator)

For heavy loaded networks

Example: Token Ring/IEEE 802.5, Token Bus/IEEE 802.4, FDDI
Mixed

Flexible compromise: 802.11 WLANs
Reservation (long term)

Link reservation for multiple packets (whole session)

Example: scheduling a time slot: GSM using TDMA of FDMA
(uplink/dowlink)
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Example 802.3: MAC of Ethernet (CSMA/CD)

CSMA/CD:
1. If the medium is idle, transmit; otherwise, go to step 2
2. If the medium is busy, continue listening (CS: carrier
sensing) until the channel is idle, then transmit
immediately
3. If a collision is detected (CD) during transmission,
transmit brief jamming signal to assure all stations
know about collision and then cease transmission
4. After transmitting the jamming signal, wait a random
time (back-off time), then attempt to transmit again
24
Throughput Performance of CSMA/CD
a  t prop R / L

   L / R
a : normalized
delay-bandwidth product
 : normalized load
 : aggregated rate [frames/second]
 (Load)
We can see that in Ethernet transfer delays grow very fast as the
load approaches the maximum possible value for the given value of a
(tprop: one-way delay, R: signaling rate, L: frame length)
Reference: A. Leon-Garcia, I. Widjaja,
Communication Networks, 2nd ed
25
S-72.1130 Telecommunication
Systems
IEEE 802.11 Wireless Local
Area Networks (WLANs)
Why WLANs?

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Mobility
 Increases working efficiency and productivity
 Roaming support: extended on-line times
-> universal access & seamless services
No new wiring and installation on difficult-to-wire areas
 Offices, public places, and homes
 Factories, vehicles, roads, and railroads
Increased reliability - several networks & nodes secure links
 However, AAA (Authentication, Authorization, Accounting)
challenging
Reduced installation time
 No cabling time
 Easy setup
27
WLAN Technology Challenges
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High date rates

IEEE 802.11b supports rates up to 11 MBps (in practice 6 Mb/s),
and 802.11g reaches up to 54 Mb/s, need to have the bandwidth
Interference

Working in ISM band means sharing the frequency bands with
microwave oven, and Bluetooth. Modulation and MAC design
challenge
Security

Original WEP (Wired Equivalent Privacy) algorithm is weak – often
not set ON by users, more efficient algorithms developed later
Roaming, especially with GSM and UMTS
Inter-operability between different vendors

Only few basic functionality are interoperable, other vendor’s
features can’t be used in a mixed network
28
Requirements for
802.11 Wireless LAN Standard


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Dynamic network management
 Stations movable and may be operated while moved
 addressing and association procedures
 interconnections (roaming)
License free operation
Wireless channel is unreliable
 error control
 security/secrecy
 Wireless channel is also the reason why access method
for 802.11 is CSMA/CA and not CSMA/CD
 Difficult to detect collisions in wireless environment
 External interference, especially at ISM
 Hidden terminal problem
CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance
CSMA/CD: Carrier Sense Multiple Access/Collision Detection
29
802.11 WLAN Architecture Overview



LLC provides addressing and data link control
– common to all 802 LANs
IEEE 802.2
LLC
Logical Link Control (LLC)
802.11 MAC provides
b: Wi-Fi

Access to wireless medium
IEEE 802.3 IEEE 802.4 IEEE 802.5
 CSMA/CA (DCF)
IEEE 802.11
Carrier
MAC
Token
Token
Wireless
 Contention-free access (PCF)
Sense
Bus
Ring
abg

Joining the network (NAV, addressing) Ethernet

Services
Physical layers: DSSS, FHSS, IR ...
PHY
 Station service: Authentication,
privacy, MSDU* delivery
CSMA/CA: Carrier Sense Multiple Access
 Distributed system: Association**,
with Collision Avoidance
participates to data distribution
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
Three physical layers (PHY)
SS: Spread Spectrum

FHSS: Frequency Hopping Spread
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
Spectrum (SS)
IR: Infrared light

DSSS: Direct Sequence SS
NAV: Network Allocation Vector
SAP: Service Access Point

IR: Infrared transmission
*MSDU: MAC service data unit
** with an access point in ESS or BSS
DCF: Distributed Coordination Function
PCF: Point Coordination Function
30
S-72.1130 Telecommunication
Systems
IEEE 802.11 Wireless Local
Area Networks (WLANs): Service Sets


802.11 networks can work in
 Basic service set (BSS)
 Extended service set (ESS)
BSS can also be used in ad-hoc
networking
Network
LLC
MAC
FHSS DSSS IR
Propagation
boundary
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
PHY
802.xx
IEEE 802.11 Architecture
Internet
Distribution
system
Station B
Station A
BSS 1
Basic (independent)
service set (BSS)
Access Point
BSS 2
Extended service set (ESS)
Portal: gateway access to other networks/Internet
32
Basic and Extended Service Sets

Basic Service Set (BSS) – tens of meters
Operate in Basic Service Area (BSA) that is much like the
are of cell in mobile communications
 BSSs may geographically overlap, be physically disjoint, or
they may be collocated (one BSS may use several
antennas)
 Ad-hoc or Infrastructure (nomadic) mode: Access
coordinated by the given instance of MAC
Extended Service Set (ESS)
 Multiple BSSs interconnected by Distribution System (DS)
 Each BSS is like a cell and stations in BSS communicate
with an Access Point (AP).
 Portals attached to DS provide gateways as access to
Internet or other ESS


33
Distribution system (DS) services


DS provides distribution services:

Transfer MAC SDUs between APs in ESS (I)

Transfer MSDUs between portals & BSSs in ESS (II)

Transfer MSDUs between stations in same BSS (III)

Multicast, broadcast, or stations’s preference
ESS looks like a single BSS to LLC layer
Propagation
boundary
Internet
II
III
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
MSDU: MAC Service Data Unit
AP: Access Point
III
Distribution
system
Station B
IIIb
Station A
BSS 1
Basic (independent)
service set (BSS)
Access Point
I
BSS 2
Extended service set (ESS)
Portal: gateway access to other networks/Internet
34
IEEE 802.11 Mobility


Standard defines the following mobility types:

No-transition: no movement or moving within a local BSS

BSS-transition: station movies from one BSS in one ESS to another
BSS within the same ESS

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
35
S-72.1130 Telecommunication
Systems
IEEE 802.11 Wireless Local
Area Networks (WLANs): Media
Access Protocol
Hidden Terminal Problem
(a)
C
A
Data Frame
A transmits data frame
B
(b)
Data Frame
B
A

New MAC: CSMA with Collision Avoidance
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
C senses medium,
station A is hidden from C
Data Frame
C
C transmits data frame
& collides with A at B
RTS: Request to Send
CTS: Clear to Send
37
CSMA with Collision Avoidance
(a)
B
RTS
C
A requests to send
(b)
CTS
B
CTS
A
C
B announces A ok to send
(c)
Data Frame
A sends
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
B
C remains quiet
RTS: Request to Send
CTS: Clear to Send
38
IEEE 802.11 Coordination Functions
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
39
Media Access Control in 802.11 WLANs


Distributed Wireless Foundation MAC (DWFMAC):
 Distributed access control mechanism (CSMA/CA)
 Optional centralized control on top (PCF)
MAC flavours provided by coordination functions:
 Distributed coordination function (DCF) - CSMA
 Contention algorithm to provide access to all traffic
 Asynchronous, best effort-type traffic
 Application: bursty traffic, add-hoc networks
 Point coordination function (PCF) – polling principle
 Centralized MAC algorithm
 Connection oriented
 Contention free
 Built on top of DCF
 Application: timing sensitive, high-priority data
40
IEEE 802.11 MAC (DWFMAC):
Timing in Basic Access
duration depends
on MAC load type
duration depends
on network condition
MAC frame: Control,
management , data + headers
(size depends on frame load and type)
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
PCF: Point Coordination Function (asynchronous, connectionless access)
DCF: Distributed Coordination Function (connection oriented access)
DIFS: DCF Inter Frame Space (minimum delay for asynchronous frame
access)
PIFS: PCF Inter Frame Space (minimum poll timing interval)
SIFS: Short IFS (minimum timing for high priority frame access as ACK, CTS,
MSDU…)
MSDU: MAC Service Data Unit
41
Collisions, Losses & Errors


Collision Avoidance
 When station senses channel busy, it waits until channel
becomes idle for DIFS period & then begins random backoff
time (in units of idle slots)
 Station transmits frame when backoff timer expires
 If collision occurs, recompute backoff over interval
Receiving stations of error-free frames send ACK
 Sending station interprets non-arrival of ACK as loss
 Executes backoff and then retransmits
 Receiving stations use sequence numbers to identify
duplicate frames
42
IEEE 802.11
MAC Logic
(DWFMAC)
IFS: Inter Frame Space (= DIFS, SIFS, or
PIFS)
DWFMAC: Distributed Wireless Foundation
MAC
Reference: W. Stallings: Data
and Computer Communications,
7th ed
43
Carrier Sensing in 802.11 MAC

Physical Carrier Sensing
Analyze all detected frames
 Monitor relative signal strength from other sources
Virtual Carrier Sensing at MAC sublayer
 Source stations informs other stations of transmission
time (in msec) for an MPDU
 Carried in Duration field of RTS & CTS
 Stations adjust Network Allocation Vector (NAV) to
indicate when channel will become idle
Channel busy if either sensing is busy



Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
44
Transmission of MPDU without RTS/CTS
DIFS
NAV: Network allocation vector
DIFS: DCF Inter Frame Space (async)
SIFS: SIFS: Short IFS (ack, CTS…)
RTS: Request to send
CTS: Clear to send
MPDU: MAC Protocol Data Unit
DCF: Distributed Coordination Function
PCF: Point Coordination Function
Data
Source
SIFS
ACK
Destination
DIFS
Other
NAV
Defer Access
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
Wait for
Reattempt Time
45
Transmission of MPDU
with RTS/CTS
NAV: Network allocation vector
DIFS: DCF Inter Frame Space (async)
SIFS: SIFS: Short IFS (ack, CTS…)
RTS: Request to send
CTS: Clear to send
MPDU: MAC Protocol Data Unit
DCF: Distributed Coordination Function
PCF: Point Coordination Function
DIFS
RTS
Data
Source
SIFS
CTS
SIFS
SIFS
Ack
Destination
DIFS
NAV (RTS)
Other
NAV (CTS)
NAV (Data)
Reference: A. Leon-Garcia, I. Widjaja,
Communication Networks , Instructor's Slide Set
Defer access
RTS: Request to Send
CTS: Clear to Send
46
PCF Frame Transfer
Fixed super-frame interval
TBTT
Contention-free (CF) repetition interval
SIFS
B
PIFS
SIFS
SIFS
SIFS SIFS
D2+Ac
k+Poll
D1 +
Poll
CF
Contention period (DCF)
End
U2+
ACK
U1+
ACK
Reset NAV
NAV
CF_Max_duration
D1, D2 = frame sent by point coordinator
U1, U2 = frame sent by polled station
TBTT = target beacon transmission time
B = beacon frame
NAV: Network allocation vector
DIFS: DCF Inter Frame Space (async)
SIFS: SIFS: Short IFS (ack, CTS…)
RTS: Request to send
CTS: Clear to send
MPDU: MAC Protocol Data Unit
DCF: Distributed Coordination Function
PCF: Point Coordination Function
47
Point Coordination Function





PCF provides connection-oriented, contention-free service
through polling
Point coordinator (PC) in AP performs PCF
Polling table up to implementor
Contention free period (CFP) repetition interval
 Determines frequency with which CFP occurs
 Initiated by beacon frame transmitted by Point
Coordinator (PC) in AP
 During CFP stations may only transmit to respond to a
poll from PC or to send ACK
All stations adjust Network Allocation Vector (NAV) to
indicate when channel will becomes idle
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
48
MAC Frame Types



Management frames
 Station association & disassociation with AP (this
establishes formally BSS)
 Timing & synchronization
 Authentication & deauthentication (option for
identifying other stations)
Control frames
 Handshaking
 ACKs during data transfer
Data frames
 Data transfer
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
49
MAC Frame

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)
50
S-72.1130 Telecommunication
Systems
IEEE 802.11 Wireless Local
Area Networks (WLANs):
Physical Level
802.11 WLAN bands and technologies


IEEE 802.11 standards and rates

IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band ) [FH, DS]

IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi [QPSK]

IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz
band) [OFDM]

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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Physical Level
of 802.11: DSSS
DSSS-transmitter





802.11 supports 1 and 2 Mbps data transport, uses BPSK and QPSK modulation
(802.11b,a,g apply higher rates)
802.11 applies 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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Physical Level of 802.11: FHSS






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
54
26 MHz
902
Example: PHY of 802.11a




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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References and Supplementary Material
- A. Leon-Garcia, I. Widjaja: Communication Networks (2th
ed.)
- W. Stallings: Data and Computer Communications, 7th ed
- Kurose, Ross: Computer Networking (2th ed.)
- Jim Geier: Wireless LANs, SAMS publishing
- 802 Standards, IEEE
Supplementary Material:
 HDLC: A. Leon-Garcia, I. Widjaja: Communication
Networks, 2th ed.: pp. 333-340
 WLANs: W. Stallings: Data and Computer
Communications, 7th ed, pp. 544-568
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