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
 Direct sequence
 Frequency hopping
 Infrared
Installation
Security
2
LAN Basics
LAN
Basics
WLAN
Basics
802
LANs
802.11
Service Sets
802.11
LLC
802.11
MAC
802.11
PHY
Typical Wired LAN

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
Transmission Medium
Network Interface Card
(NIC)
Unique MAC “physical”
address
Serial format in 10BASE5
~ 10 Mb/s – baseband - 500 m
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 4
Example: How Ring Networks Work
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A node functions as a repeater
A
Only destination station copies
the frame,
C
B
A
all other nodes
discard
B transmits frame
the frame
addressed to A
Unidirectional link
A
Signal propagates
encoded by line codes
A
C
B
Example: 802.5
Reliability:
A copies the frame
link failure (FDDI applies
at the reception
double ring)
A
C
B
A
C ignores the frame
A
C
A
B
B absorbs the
returning frame
5
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 the rotating token.
Each station is physically 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 statistical
multiplexing degradation of performance because the
medium is shared for pair-wise stations
 (2) In practice 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
6
Example: Bus Network
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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 succeeds 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
D
stations
term
term
Disadvantages:
- Line coded, serial data

(1) Poor efficiency with high
- Twisted pair or coaxial cable
network load in contention
schemes

(2) Security taken care by upper
7
network levels
term: terminator impedance
Wireless Local Area networks
(WLANs) - basics
LAN
Basics
WLAN
Basics
802
LANs
802.11
Service Sets
802.11
LLC
802.11
MAC
802.11
PHY
Wireless LANs (WLANs) - features
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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
No new wiring and installation on difficult-to-wire areas
 Offices, public places, and homes
 Factories, vehicles, roads, and railroads
Mobility
 Increases working efficiency and productivity
 Roaming support: extended on-line times
-> universal access & seamless services
Reduced installation time
 No cabling time
 Easy setup
Standard enables interoperability between different vendors
 Roaming with GSM and UMTS is a research issue
9
WLAN Technology Challenges
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Flexible error control: in physical, MAC and/or in upper levels
Physical level takes care of physical transmission of packets over
a medium (modulation, line coding, channel coding)
 Interference & noise
 Working in ISM band means sharing the frequency bands
with microwave oven, cordless telephones, Bluetooth etc.
-> Modulation and MAC design challenge:
 Pros: Freedom from spectral regulatory constraints
at ISM Band (Industrial, Science and Medical)
 Multi-path propagation
 Remedies: channel coding / rake-reception
Dynamic network management
 Stations movable and may be operated while moved
 addressing and association procedures
 interconnections (roaming)
10
Challenges …
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MAC protocol takes care of optimizing throughput for the
expected services
 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
-> Hidden terminal problem (see PSTN lecture)
Security
 Traditional WEP (Wired Equivalent Privacy) now replaced by
WPA (Wi-Fi Protected Access) and 802.11i (WPA2)
 AAA (Authentication, Authorization, Accounting) can be
taken care by a dedicated server as RADIUS (Remote
Authentication Dial In User Service )
CSMA/CA: Carrier Sense Multiple Access/Collision Avoidance
CSMA/CD: Carrier Sense Multiple Access/Collision Detection
11
IEEE 802 LAN Standards
LAN
Basics
WLAN
Basics
802
LANs
802.11
Service Sets
802.11
LLC
802.11
MAC
802.11
PHY
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 (token rings)
method to provide LAN access based on message priority level
DQDB: Distributed queue dual buss, see PSTN lecture
13
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…)
14
IEEE 802.11 Wireless Local
Area Networks (WLANs):
Service Sets
LAN
Basics
WLAN
Basics
802
LANs
802.11
Service Sets
802.11
LLC
802.11
MAC
802.11
PHY
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
802.11 networks can work in
 Basic service set (BSS)
 Extended service set (ESS)
BSS can also be used in ad-hoc
Network
LLC
MAC
FHSS DSSS IR
networking
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)
(infrastructure-mode)
Portal: gateway access to other networks/Internet
16
Basic and Extended Service Sets
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Basic Service Set (BSS) – indoor radius of tens of meters with
a single AP
 Operates in Basic Service Area (BSA) that is much like the
area of a cell in cellular 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 MAC protocols
Extended Service Set (ESS)
 Multiple BSSs interconnected by a Distribution System (DS)
 Each BSS is like a cell and stations in BSS communicate via
an Access Point (AP) with the DS
 Portals attached to DS provide gateways as access to
Internet or other ESS
17
Distribution system (DS) services
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DS provides distribution services:
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Transfer MAC SDUs between APs in ESS (I)
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Transfer MSDUs between portals & BSSs in ESS (II)
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Transfer MSDUs between stations in same BSS (III)
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Multicast, broadcast, or stations’s preference
ESS looks like a single BSS to LLC layer
Propagation
boundary
Internet
II
III
SDU: Service Data Unit
(inter-layer data)
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
MSDU: MAC Service Data Unit
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
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
18
IEEE 802.11 Mobility (b/g)
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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!
For fast, seamless
roaming
802.11r
- Address to destination
mapping
- seamless integration
of multiple BSS
ESS 1
ESS 2
19
IEEE 802 LAN Standard:
Logical Link Layer (LLC)
LAN
Basics
WLAN
Basics
802
LANs
802.11
Service Sets
802.11
LLC
802.11
MAC
802.11
PHY
802.11 WLAN Architecture
Logical Link Control (LLC)

LLC provides addressing and data link
control
– common to all 802 LANs
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Utilizes services of HDLC
(High-level Data Link Control)
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Therefore, LLC SAPs separate
upper layer data exchanges =>
NIC applies different buffer
segments
for each SAP (port)
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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
Ethernet
MAC
abg
Physical layer: DSSS, FHSS, IR
CSMA/CA: Carrier Sense Multiple Access
with Collision Avoidance
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
SS: Spread Spectrum
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
IR: Infrared light
NAV: Network Allocation Vector
SAP: Service Access Point
DCF: Distributed Coordination Function
PCF: Point Coordination Function
NIC: Network Interface Card
PHY
21
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)
22
SAP Addressing
IEE802.11 (CSMA/CA)...
IEE802.11 (CDMA)...
ATM...
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
23
A TCP/IP Packet
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
*Protocol data unit
MAC lines up packets
by using a MAC protocol
PHY layer transmits packet
using a modulation method
(DSSS, OFDM, IR, FHSS)
24
Encapsulation …
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
25
LLC Services
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A Unacknowledged connectionless service
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B Connection oriented service
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Point-to-point, multicast (assigned users), broadcast (group of
users) addressing
no error or flow control - no ack-signal
higher levels take care or reliability - thus fast
Often referred as ‘Unnumbered frame mode of HDLC*’
connection phases: Connection setup, data exchange, and release
supports unicast only
error/flow control (cyclic redundancy check (CRC)), sequencing
‘Asynchronous mode of HDLC’
C Acknowledged connectionless service
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Can handle several logical connections, distinguished by their SAPs
ack-signal used
error and flow control by stop-and-wait ARQ
26
faster setup than for B
*High-Level Data Link Control
IEEE 802.11 Wireless Local
Area Networks (WLANs):
Media Access Protocol
LAN
Basics
WLAN
Basics
802
LANs
802.11
Service Sets
802.11
LLC
802.11
MAC
802.11
PHY
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 a car travelling at 130 km/h)
28
MAC techniques - examples
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Contention
 Medium is free for all, packet collisions do happen
 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
Reservation (long term)
 Link reservation for multiple packets (whole session)
 Example: scheduling a time slot: GSM using TDMA. FDMA
applied for uplink/dowlink separation.
Hybrid… (example: contention+reservation)
 Flexible compromise: 802.11 WLANs
29
Media Access Control (MAC):
Ways to Share a Medium
Medium sharing techniques
Static
channelization
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FDMA,TDMA, CDMA
Uses partition
medium
Dedicated allocation
to users
Examples:

Satellite
transmission

Cellular
Telephone
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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)
30
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
*Carrier sense multiple access/collision detection
31
Throughput Performance of CSMA/CD
a  t prop R / L

r   L / R
a : normalized
delay-bandwidth product
r : normalized load
 : aggregated rate [frames/second]
r (Load) ~ throughput
We can see that in Ethernet transfer delays grow very fast as the load
increases for the given value of delay-bw product a.
Note: Large value of parameter a scales results for propagation delay and/or
signaling rate – if their product becomes larger, throughput (in terms of
transfer delay) gets smaller.
tprop: one-way delay, R: signaling rate,
L: frame length
Reference: A. Leon-Garcia, I. Widjaja,
Communication Networks, 2nd ed
32
802.11 WLAN Architecture
Medium Access Control (MAC) - Summary

802.11 MAC

Services
 Station service:
Authentication,
privacy, MSDU* delivery
 Distributed system:
Association**,
participates to data
distribution

Transmits frames based on MAC
addresses (in NIC)
 Connectionless/Connection
oriented frame transfer
service

Coordinates access to medium
 Joining the network (NAV,
addressing)
 MAC scheme CSMA/CA:
 Contention-free
access (PCF)
 Contention access (DCF)
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
Ethernet
* MSDU: MAC service data unit
** with an access point in extended or basic service set (ESS,BSS)
MAC
abg
Physical layer: DSSS, FHSS, IR
PHY
CSMA/CA: Carrier Sense Multiple Access
with Collision Avoidance
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
SS: Spread Spectrum
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
IR: Infrared light
NAV: Network Allocation Vector
SAP: Service Access Point
DCF: Distributed Coordination Function
PCF: Point Coordination Function
NIC: Network Interface Card
33
IEEE 802.11 Coordination Functions
Reference: W.
Stallings: Data
and Computer
Communications,
7th ed
34
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/CA
 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
(rarely applied in practical devices)
 Centralized MAC algorithm
 Connection oriented
 Contention free
 Built on top of DCF
 Application: timing sensitive, high-priority data
35
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
36
IEEE 802.11
MAC Logic
(DWFMAC)
IFS: Inter Frame Space (= DIFS, SIFS, or
PIFS)
DWFMAC: Distributed Wireless Foundation
duration depends
on MAC load type
MAC
Reference: W. Stallings: Data
and Computer Communications,
7th ed
37
DWFMAC summarized
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
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
38
Carrier Sensing in 802.11 MAC - Summary

Physical Carrier Sensing
Analyze all detected frames for errors
 Monitor relative signal strength from other sources
Virtual Carrier Sensing at MAC sublayer (avoids hiddenterminal problem)
 Source stations inform other stations of transmission
time (in msec) for an MPDU (MAC Protocol Data Unit)
 Carried in Duration field of RTS (Request to send) &
CTS (Clear to send)
 Stations adjust their Network Allocation Vector (NAV)
to indicate when the channel will become idle
Channel busy if either sensing is busy



Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
39
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 (postpone) access
for other stations
Reference: A. Leon-Garcia, I. Widjaja, Communication
Networks , Instructor's Slide Set
Wait for
Reattempt Time
40
Transmission of MPDU
with RTS/CTS (DCF)
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
Hidden terminal solution
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
41
PCF Frame Transfer
TBTT Fixed super-frame interval
Contention-free repetition interval (PCF)
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 (initiation)
NAV: Network allocation vector
PIFS: PCF Inter Frame Space
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
42
Point Coordination Function
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




PCF provides connection-oriented, contention-free service
through polling
Point coordinator (PC) in AP performs PCF
Polling table up to implementer
Contention free period (CFP) repetition interval
 Determines frequency with which contention free
period 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
43
MAC Frame Types



Management frames
 Station association & disassociation with AP (this
establishes formally BSS)
 Timing & synchronization
 Authentication & de-authentication (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
44
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 BSSID*
-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)
45
IEEE 802.11 Wireless Local
Area Networks (WLANs):
Physical Level
LAN
Basics
WLAN
Basics
802
LANs
802.11
Service Sets
802.11
LLC
802.11
MAC
802.11
PHY
802.11 WLAN bands and
technologies - summary


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
47
802.11-wireless LANs, Dec. ’07
Ref: http://en.wikipedia.org/wiki/802.11n
48
802.11 WLAN Architecture
Physical Level (PHY)
802 Physical level specifies








Star, bus or ring
topology
Cabling and electrical
interfaces: Twisted pair,
coaxial, fiber…
Line coding (wired
LANs) or
modulation (WLANs)
Three physical layers for
802.11
FHSS: Frequency Hopping
Spread Spectrum (SS)
DSSS: Direct Sequence SS
IR: Infrared transmission
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
MAC
abg
Ethernet
Physical layers
CSMA/CA: Carrier Sense Multiple Access
with Collision Avoidance
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
SS: Spread Spectrum
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
IR: Infrared light
NAV: Network Allocation Vector
SAP: Service Access Point
DCF: Distributed Coordination Function
PCF: Point Coordination Function
NIC: Network Interface Card
PHY
49
Physical Level
of 802.11: DSSS
DSSS-transmitter





802.11 supports 1 and 2 Mbps data transmission, 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
50
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
51
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
52
Review questions
LAN
Basics
WLAN
Basics
802
LANs
802.11
Service Sets
802.11
LLC
802.11
MAC
802.11
PHY
Review questions








Logical link control (LLC) services in 802.11
What is the role of Distributed Coordination Function
(DCF) and Point Coordination Function (PFC) in 802.11
MAC?
Describe 802.3 MAC Scheme
What is the basic difference between CSMA/CD and
CSMA/CA? Which one is applied in 802.11 and why?
Discuss factors than should be considered while choosing
a medium access technique
Carrier sensing in 802.11 MAC
Mobility support in 802.11b/g
MAC frame types
54
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 (distributed by Edita):
 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
55