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Cyber Book
Wireless LAN
What’s Wireless LAN?
Communication system established
through the use of RF technology
Can function either as an extension to an
existing LAN or an alternative for a wired
LAN
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General Characteristics
The need for WLAN
resolving the problems of Hard-wired LAN
providing user mobility and flexibility
enabling to extend the range of an existing
wired LAN
being cheaper to install than traditional wired
LAN
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Types of WLANs
Radio WLAN
DSSS
uses signalling schemes which are based on some form of
coding and which use a much wider bandwidth over the
information bandwidth
FHSS
uses multiple frequency channel randomly (in a predefined
pattern)
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Types of WLANs
Infrared WLAN
use part of the electromagnetic spectrum just
below visible light as a media
based on on-off pulse modulation and
detection of the optical signal
techniques
Directed (line of sight)
Diffuse (reflective)
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Infrastructure vs. ad-hoc networks
 Infrastructure networks
 Communications take place between the wireless
nodes and AP, not directly between nodes.
 can use different access schemes
 collision
 wireless nodes and AP are not coordinated
 collision free
 AP controls medium access (QoS support)
 Ad-hoc networks
 medium access: collision
 wireless node may transfer data (router)
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WLAN Protocol Topologies
Independent Topology
supports peer-to-peer connectivity
implemented quickly and easily
no need for special tools or skills, no network
administration
in business meeting or in the setting up of
temporary workgroups
limited coverage area
=> all stations must be within the airwave distance of other
stations with which they need to communicate
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Ad-hoc networks
Independent Topology
Ad hoc network
BSS
BSS
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Cyber Book of Internet Communication
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WLAN Protocol Topologies
Infrastructure topology using a AP
transition from the wireless media to the
wired media via an AP
Access Point
have network interface card connected to the wired LAN
functions as a radio relay to the mobile stations
involves the extension of an existing LAN
through the use of a wireless AP
all mobile stations must communicate with
the AP
within a predetermined distance,
approximately 50~300m
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Infrastructure Configuration
Infrastructure topology using a AP
wired network
AP
AP: Access Point
BSS
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WLAN Protocol Topologies
Infrastructure topology using multiple AP
can extend wireless LAN access capabilities
over a relatively large area,
such as college campus, industrial complex
enables mobile stations to move within a
larger coverage area
provide effectively notebooks and PDAs with a
roaming capability
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Cyber Book of Internet Communication
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Infrastructure Configuration
Infrastructure topology using multiple AP
Infrastructure network
BSS
AP
AP
AP: Access Point
wired network
AP
BSS
BSS
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802.11 Protocol Entities
LLC
MAC
MAC
Sublayer
MAC Layer
Management
Station
Management
PLCP Sublayer
PHY
PHY Layer
Management
PMD Sublayer
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IEEE 802 Network
802.2 LLC
802
802.1
802.3
MAC
802.5
MAC
802.3
PHY
802.5
PHY
802.11 MAC
OUTLINE
&
MANAGEMENT
ARCHITECTURE
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Cyber Book of Internet Communication
802.11
FHSS
PHY
802.11
DSSS
PHY
802.11a 802.11b
OFDM HR/DSSS
PHY
PHY
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IEEE 802.11 Standard
 Standard process
 802.11 MAC(1997)
 802.11a – PHY, 54Mbps, 5Ghz
 802.11b – PHY, 11Mbps, 2.4Ghz
 802.11g – PHY, 54Mbps, 2.4Ghz(Super set of 802.11b PHY)
 802.11e for QoS support
 802.11f for IAPP(Inter-Access Point Protocol)
 802.11h for spectrum and transmit power management
 802.11i for security enhancement
 802.11k for radio resource management enhancement
 802.11n for higher throughput
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802.11 - Architecture of an infrastruct
ure
802.11 LAN
BSS1
802.x LAN
STA1
Portal
AP
Distribution System
AP
ESS
Extended Service Set
STA2
BSS2
STA3
802.11 LAN
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802.11 - Architecture of an infrastruct
ure
 Station (STA)
 terminal with access mechanisms to the wireless mediu
m and radio contact to the access point
 Basic Service Set (BSS)
 group of stations using the same radio frequency
 Access Point
 station integrated into the wireless LAN and the distrib
ution system
 Portal
 bridge to other (wired) networks
 Distribution System
 interconnection network to form one logical network (E
ES: Extended Service Set) based
on several BSS
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802.11 - Architecture of an ad-hoc ne
twork
Direct communication
within a limited range
802.11 LAN
STA1
STA3
IBSS
STA2
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Cyber Book of Internet Communication
Station (STA):
terminal with access mec
hanisms to the wireless
medium
Independent Basic Servic
e Set (IBSS):
group of stations using t
he same radio frequency
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IEEE standard 802.11
mobile terminal
infrastructure
network
access point
Application
Application
TCP
TCP
IP
IP
LLC
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Fixed terminal
LLC
LLC
802.3 MAC
802.11 MAC
802.3 MAC
802.3 MAC
802.3 PHY
802.11 PHY
802.3 PHY
802.3 PHY
Cyber Book of Internet Communication
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802.11 - MAC layer - 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)
Contention-Free
General
PCF
DCF
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Cyber Book of Internet Communication
< MAC Extent >
2017-05-25
802.11 - MAC layer - DFWMAC
Distributed Foundation Wireless MAC
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
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DCF (Distributed Coordination Function)







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Distributed
Contention-based channel access
Based CSMA/CA
Asynchronous data transport
IBSS, Infrastructure BSS
FIFO transmission queue
Backoff
Cyber Book of Internet Communication
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802.11 - MAC layer
 Priorities
 Defined through different inter frame spaces
 No guaranteed, hard priorities
 SIFS (Short Inter Frame Spacing)
 highest priority, for ACK, CTS, polling response
 10usec (DSSS)
 PIFS (PCF IFS)
 medium priority, for time-bounded service using PCF
 SIFS + 1time slot (20 usec)
 DIFS (DCF, Distributed Coordination Function IFS)
 lowest priority, for asynchronous data service
 SIFS + 2 time slots
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Cyber Book of Internet Communication
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802.11 - CSMA/CA access method
DIFS
DIFS
Medium busy
direct access if
medium is free  DIFS
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contention window
(randomized back-off
mechanism)
Next frame
t
slot time
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CSMA/CA Explained
Free access when
medium is free
longer than DIFS
DIFS
PIFS
DIFS
Contention Window
SIFS
Busy Medium
Backoff-Window
Next Window
Slot time
Defer Access
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Cyber Book of Internet Communication
Select Slot and Decrement Backoff as long
as medium is idle
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CSMA/CA Explained
 Reduce collision probability where mostly
needed
 Stations are waiting for medium to become free
 Select Random Backoff after a Defer, resolving
contention to avoid collisions
 Efficient Backoff algorithm stable at high loads
 Exponential Backoff window increases for
retransmissions
 Backoff timer elapses only when medium is idle
 Implement different fixed priority levels
 To allow immediate responses and PCF coexistence
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CSMA/CA + ACK protocol
DIFS
Sender
DATA
SIFS
Receiver
ACK
DIFS
Next MPDU
Other
stations
t
contention
Defer Access
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Cyber Book of Internet Communication
Backoff after Defer
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CSMA/CA + ACK protocol
Defer access based on Carrier Sense
CCA from PHY and Virtual Carrier Sense state
Direct access when medium is sensed
free longer then DIFS, otherwise defer
and backoff
Receiver of directed frames to return an
ACK immediately when CRC correct
When no ACK received then retransmit frame
after a random backoff (up to maximum limit)
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Contention Window
CW(Contention Window)
 Contention window or backoff window
 Station selects random slot (probability same) [0,CW],
(Cwmin = 7, Cwmax=255)
DIFS
PIFS
DIFS
Busy medium
Contention window
SIFS
Backoff window
Next frame
Slot time
Defer access
Select slot and decrement backoff as long as
medium is idle
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802.11 - CSMA/CA access method (ACK)
Sending unicast packets
Station has to wait for DIFS before sending
data
Receivers acknowledge at once (after waiti
ng for SIFS) if the packet was received corr
ectly (CRC)
Automatic retransmission of data packets in
case of transmission errors
But the sender has to wait again and compete for the
access right.
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RTS / CTS Concept
C
A
B
D
A
RTS
DATA
(1) RTS
(2) CTS
B
(3) DATA
C
(4) ACK
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Cyber Book of Internet Communication
D
CTS
ACK
NAV (RTS overhearing)
NAV (CTS overhearing)
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802.11 - DFWMAC
Sending unicast packets
 Station can send RTS with reservation param
eter (duration) after waiting for DIFS (reserva
tion determines amount of time the data pack
et needs the medium)
 Acknowledgement via CTS after SIFS by recei
ver (if ready to receive)
 Sender can now send data at once, acknowle
dgement via ACK
 Other stations store medium reservations dist
ributed via RTS and CTS
 NAV (Net Allocation Vector)
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Hidden Node Problem
Using an RTS-CTS handshake to resolve hidden node problems
A
B
C
1. A transmits request (RTS) to B
2. B replies that the channel is Clear (CTS).
Both A & C overhear the broadcast
(1) RTS
(2) CTS
(3) DATA
3. A sends its data to B. C is blocked from
transmitting.
4. B acknowledges the data transfer
(4) ACK
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Exposed Node Problem
1. B is transmitting to A
2. C overhears this, and is blocked
A
B
C
D
3. P wants to transmit to Q,
but is being blocked by B
(1) RTS
B
(2) CTS
A
(3) DATA
C
(4) ACK
CTS_overhear
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others
DATA
RTS
CTS
ACK
NAV (RTS overhearing)
NAV (CTS overhearing)
RTS_overhear
Cyber Book of Internet Communication
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802.11 - DFWMAC
DIFS
sender
RTS
DATA
SIFS
receiver
other
stations
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Cyber Book of Internet Communication
CTS
SIFS
SIFS
ACK
NAV(RTS)
NAV(CTS)
t
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Fragmentation
RTS
Sender
frag1
frag2
DIFS
CTS
Receiver
SIFS
ACK1
SIFS
SIFS
ACK2
SIFS
SIFS
NAV (RTS)
NAV (CTS)
Other
stations
NAV (frag1)
NAV (ACK1)
DIFS
data
t
contention
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Optional PCF
Contention
Free Service
Contention
Service
PCF
Optional
MAC
DCF
(CSMA/CA)
PHY
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Optional PCF
Contention Free Service uses Point
Coordination Function (PCF) on a DCF
Foundation
PCF can provide lower transfer delay
variations to support Time Bounded Services
Async Data, Voice or mixed implementations
possible
Point Coordinator resides in AP
Coexistence between Contention and
optional Contention Free does not burden
the implementation
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PCF(Point Coordination Function)
Point Coordination Function
centrally controlled multiple access
mechanism
uses a poll and response protocol to eliminate
the possibility of contention for the medium
point coordinator (PC) is always located in an
AP
supports time bounded services
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Contention Free operation
PCF Defers for Busy Medium
CFP repetition interval
Contention Free
Period
PCF
(Optional)
CFP repetition interval
Contention Period
DCF
Busy
medium
CF-Burst
Variable Length
”Reset NAV”
PCF
Defers
NAV
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Contention Free operation
Alternating Contention Free and
Contention operation under PCF control
NAV prevents Contention traffic until
reset by the last PCF transfer
So variable length Contention Free period per
interval
Both PCF and DCF defer to each other
causing PCF Burst start variations
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PCF Burst
CFP repetition interval
Contention Free Burst
PIFS
Busy
medium
Contention Period
D2
D1
U1
SIFS
D3
U2
D4
No Up
U4
CF_End
Dx = AP-Frame
Ux = Station-Frame
Reset NAV
NAV
Min Contention Period
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Cyber Book of Internet Communication
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PCF Burst
 CF-Burst by Polling bit in CF-Down frame.
 Immediate response by Station on a CF_Poll.
 Stations to maintain NAV to protect CF-traffic.
 Responses can be variable length.
 “Reset NAV” by last (CF_End) frame from AP.
 "ACK Previous Frame" bit in Header.
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IEEE 802.11 Standardization
IEEE802.11 MAC & Other Activities
Ad-Hoc Publicity
Ad-Hoc Regulatory
MAC Layer
802.11 MAC
802.11
MAC
802.11e / TGe
MAC Enhancements -QoS
802.11f / TGf
Inter-Access Point Protocol
802.11i / TGi
Enhanced Security Mechanisms
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IEEE 802.11 Standardization
 IEEE802.11 current status
TG
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Substances
Remarks
802.11
Develop three PHY's and MAC Standards capable
of providing the transmission rate of 1,2Mbps,
using IR, 2.4 GHz FHSS and DSSS
Completed
(1997.6)
802.11a
Develop a PHY to operate in the newly allocated
UNII band (5GHz)
Completed
(1999.9)
802.11b
Develop a standard for a higher rate (11Mbps)
PHY in the 2.4GHz band
Completed
(1999.9)
802.11b
cor1
Corrigendum to the MIB (802.11b)
Completed
802.11d
Extensions to Operate in Additional Regulatory
Domains
802.11e
MAC Enhancements for Quality of Service
Cyber Book of Internet Communication
Ongoing
Ongoing
(D1.4)
2017-05-25
IEEE 802.11 Standardization
 IEEE802.11 current status
TG
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Substances
Remarks
Ongoing
(D2.0)
802.11f
Recommended Practice for Inter Access Point
Protocol
802.11g
Standard for Higher Rate (20+ Mbps)
Extensions in the 2.4GHz Band
802.11h
Spectrum Managed 802.11a
Ongoing
(D1.0)
802.11i
MAC Enhancements for Enhanced Security
Ongoing
(D1.0)
WNGSC
Synthesize industry inputs for the Next
Generation WLAN and recommends a global
standard initiative
Cyber Book of Internet Communication
Ongoing
(CCK-OFDM)
Ongoing 1st
draft
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IEEE 802.11 WG & Activities
IEEE802.11e/TG e
enhance the 802.11 MAC to improve and
manage Quality of Service
provide classes of service, and enhanced
security and authentication mechanisms
consider efficiency enhancements in the areas
of the DCF and PCF
applications
 transport of voice, audio, video
 video conferencing, media stream distribution, enhanced security
applications
 mobile and nomadic access applications
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IEEE 802.11 WG & Activities
IEEE802.11f/TG f
develop recommended practices for an InterAccess Point Protocol (IAPP) which provides
the necessary capabilities to achieve multivendor Access Point interoperability across a
Distribution System supporting IEEE P802.11
Wireless LAN Links
This IAPP will be developed for the following
environment(s)
 A DS consisting of IEEE 802 LAN supporting an IETF IP
environment
 Others as deemed appropriate
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IEEE 802.11 WG & Activities
IEEE802.11g/TG g
to develop a higher rate PHY extension to
802.11b
 date rates more than 20Mbps
 backwards compatible and interoperable with existing 2.4GHz
IEEE 802.11b 11Mbps standard
 uses existing MAC layer
Extended rate OFDM : mandatory mode
 using 802.11a and 802.11b
 the capability to detect either 802.11a or 802.11b preambles
automatically
Optional modes
 CCK-OFDM
 PBCC-22, PBCC-33
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Cyber Book of Internet Communication
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IEEE 802.11 WG & Activities
IEEE802.11h/TG h
enhance the current 802.11 MAC and 802.11a
PHY in 5GHz license exempt bands, enabling
regulatory acceptance of 802.11 5GHz
products.
provide improvements in channel energy
measurement and reporting, channel
coverage to improve spectrum and transmit
power management
provide Dynamic Channel Selection and
Transmit Power Control mechanisms
Dynamic Channel Selection
Transmit Power Control
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Cyber Book of Internet Communication
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IEEE 802.11 WG & Activities
IEEE802.11i/TG i
enhance the 802.11 MAC to enhance security
and authentication mechanisms
 Address security mechanisms beyond RC4 WEP 40-bit encryption,
more robust against eavesdropping
 Focus on key distribution and authentication between Stations and
Stations to APs
 Scaleable security framework to serve: home networks, enterprise,
Ad-Hoc, Public Environments
 Data traffic: Voice, Data, Multi-media
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IEEE 802.11 WG & Activities
WNG SC
changed from 5GSG which has investigated
the globalization and harmonization of the
5GHz band jointly with ETSI-BRAN, and MMAC
PPDU frame format
synthesize industry inputs for the Next
Generation WLAN and recommend a global
standard initiative
will deliver recommended requirements and
standards plan
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Cyber Book of Internet Communication
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IEEE 802.11 WG & Activities
Roadmap
1st Generation
2nd Generation
3rd Generation
4th Generation
Analog Cellular
IMT-2000
Cellular
PCS - 2.5G
WCDMA/CDMA2000
Mobile 384kbps
Stationary : 2Mbps
2.4GHz 2Mbps,
11Mbps
IEEE802.11x
1Mbps
Wired LAN
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• Outdoor
5.15-5.35GHz
25-50Mbps
Digital Cellular
825-890MHz
Voice only
HiperLAN2
IEEE802.11a
IEEE802.11
IEEE802.11b
Wireless LAN Wireless LAN
Wireless
Wireless
Multimedia
Multimedia
Internet
Internet Appliance
Appliance
IEEE802.11a+
5G NG WLAN
• Indoor
60-65GHz
155-622Mbps
• 5GHz/24Mbps
• 5GHz/54Mbps
•
Cyber Book of Internet Communication
2017-05-25