MBN Lec5 Part1-Wlan

EEE613 (MobiNets)
Mobile Broadband Networks
Shahzad A. Malik, Ph.D.
smalik@comsats.edu.pk
Lecture 5
Wireless LANs: 802.11
Outline
 Overview of wireless networks
– Single-hop wireless: Cellular, Wireless LANs (WLANs)
– multiple wireless hops – Mobile ad hoc networks (MANETS)
 Challenges of wireless communications
 IEEE 802.11 WLANs
– spread spectrum and physical layer specification
– MAC functional specification: DCF mode, PCF mode
 QoS in Wireless LANs
– IEEE 802.11e
 Security in Wireless LANs
– IEEE 802.11i
 Roaming and Handover
Shahzad Malik Mobile Broadband Networks - WLANs 5-1-3
References
 http://standards.ieee.org/getieee802/802.11.html IEEE
Computer Society 1999, Wireless LAN MAC and PHY
layer specification
 J. Schiller, “Mobile Communications”, Addison Wesley,
1999. – several figures
 Short tutorials on 802.11 and spread spectrum by
J.Zyren, A.Petrick, C.Andren http://www.intersil.com

Overview of
wireless networks

Wireless networks
 Access computing/communication services, on the move
 Cellular Networks
– traditional base station infrastructure systems
 Wireless LANs
– infrastructure as well as ad-hoc networks possible
– very flexible within the reception area
– low bandwidth compared to wired networks (1-10 Mbit/s)
 Multihop Ad hoc Networks

– useful when infrastructure not available, impractical, or
expensive
– military applications, rescue, home networking

Cellular Wireless
 Single hop wireless connectivity to the wired
world
– Space divided into cells, and hosts assigned to a cell
– A base station is responsible for communicating with
hosts/nodes in its cell
– Mobile hosts can change cells while communicating
– Hand-off occurs when a mobile host starts
communicating via a new base station

Evolution of cellular networks
 First-generation: Analog cellular systems (450-900 MHz)
– Frequency shift keying; FDMA for spectrum sharing
– NMT (Europe), AMPS (US)
 Second-generation: Digital cellular systems (900, 1800
MHz)
– TDMA/CDMA for spectrum sharing; Circuit switching
– GSM (Europe), IS-136 (US), PDC (Japan)
– <9.6kbps data rates
 2.5G: Packet switching extensions
– Digital: GSM to GPRS; Analog: AMPS to CDPD
– <115kbps data rates
 3G: Full-fledged data services
– High speed, data and Internet services
– IMT-2000, UMTS
– <2Mbps data rates
Wireless LANs
 Infrared (IrDA) or radio links (Wavelan)
 Advantages
– very flexible within the reception area
– Ad-hoc networks possible
– (almost) no wiring difficulties
 Disadvantages
– low bandwidth compared to wired networks
– many proprietary solutions
• Bluetooth, HiperLAN and IEEE 802.11

Wireless LANs vs. Wired LANs
 Destination address does not equal destination
location
 The media impact the design
– wireless LANs intended to cover reasonable geographic
distances must be built from basic coverage blocks
 Impact of handling mobile (and portable) stations

– Propagation effects
– Mobility management
– Power management

Infrastructure vs. Ad hoc WLANs
infrastructure
network
AP: Access Point
AP
AP wired network
AP
ad-hoc network

Multi-Hop Wireless
 May need to traverse multiple links to reach
destination
 Mobility causes route changes

Mobile Ad Hoc Networks (MANET)
 Do not need backbone infrastructure support
 Host movement frequent
 Topology change frequent
B
A B A
 Multi-hop wireless links

 Data must be routed via intermediate nodes

Applications of MANETS
 Military - soldiers at Kargil, tanks, planes
 Disaster Management – Orissa, Gujarat
 Emergency operations – search-and-rescue, police
and firefighters
 Sensor networks
 Taxicabs and other closed communities
 airports, sports stadiums etc. where two or more
people meet and want to exchange documents
 Presently MANET applications use 802.11 hardware
 Personal area networks - Bluetooth

Wireless Technology Landscape
72 Mbps
Turbo .11a
54 Mbps 802.11{a,b}
5-11 Mbps 802.11b .11 p-to-p link
Bluetooth 802.11
1-2 Mbps
µwave p-to-p links
3G
384 Kbps WCDMA, CDMA2000
2G
56 Kbps IS-95, GSM, CDMA
Indoor Outdoor Mid range Long range Long distance

outdoor outdoor com.
10 – 30m 50 – 200m 200m – 4Km 5Km – 20Km 20m – 50Km

802.11 Market Evolution
802.11
Industry Campus Public hotspots Broadband access

Enterprise
Verticals Networking Mobile Operators to home
Warehouses
Factory floors
Medical
Remote data Freedom from

entry; business Mobile user wires for laptop Revenue generation Untested
process population users; opportunity; proposition;
efficiency without any productivity low cost alternative attempts are on-
improvement office space enhancement to GPRS going

Challenges of
Wireless Communications
Wireless Media
 Physical layers used in wireless networks
– have neither absolute nor readily observable
boundaries outside which stations are unable to
receive frames
– are unprotected from outside signals
– communicate over a medium significantly less
reliable than the cable of a wired network
– have dynamic topologies
– lack full connectivity and therefore the assumption
normally made that every station can hear every
other station in a LAN is invalid (i.e., STAs may be
“hidden” from each other)
– have time varying and asymmetric propagation
properties

Limitations of the mobile
environment
 Limitations of the Wireless Network
 limited communication bandwidth
 frequent disconnections
 heterogeneity of fragmented networks
 Limitations Imposed by Mobility

 route breakages
 lack of mobility awareness by system/applications
 Limitations of the Mobile Device

 short battery lifetime
 limited capacities

Wireless v/s Wired networks
 Regulations of frequencies
– Limited availability, coordination is required
– useful frequencies are almost all occupied
 Bandwidth and delays
– Low transmission rates
• few Kbps to some Mbps.
– Higher delays
• several hundred milliseconds
– Higher loss rates
• susceptible to interference, e.g., engines, lightning
 Always shared medium
– Lower security, simpler active attacking
– radio interface accessible for everyone
– Fake base stations can attract calls from mobile phones
– secure access mechanisms important
Difference Between Wired and
Wireless
Ethernet LAN Wireless LAN
B
A B C
A C
 If both A and C sense the channel to be idle at the

same time, they send at the same time.
 Collision can be detected at sender in Ethernet.
 Half-duplex radios in wireless cannot detect collision at
sender.

Hidden Terminal Problem
A B C
– A and C cannot hear each other.

– A sends to B, C cannot receive A.
– C wants to send to B, C senses a “free” medium
(CS fails)
– Collision occurs at B.
– A cannot receive the collision (CD fails).
– A is “hidden” for C.

Exposed Terminal Problem
A B
D
C
– A starts sending to B.
– C senses carrier, finds medium in use and has to
wait for A->B to end.
– D is outside the range of A, therefore waiting is
not necessary.
– A and C are “exposed” terminals
Effect of mobility on protocol stack
 Application
– new applications and adaptations
 Transport
– congestion and flow control
 Network
– addressing and routing
 Link
– media access and handoff
 Physical
– transmission errors and interference

802.11-based Wireless LANs
Architecture and Physical Layer
IEEE 802.11
 Wireless LAN standard defined in the unlicensed
spectrum (2.4 GHz and 5 GHz U-NII bands)
 33cm 12cm 5cm
26 MHz 83.5 MHz 200 MHz
902 MHz 2.4 GHz 5.15 GHz

928 MHz 2.4835 GHz 5.35 GHz
 Standards covers the MAC sublayer and PHY layers

 Three different physical layers in the 2.4 GHz band
– FHSS, DSSS and IR
 OFDM based Phys layer in the 5 GHz band (802.11a)

802.11- in the TCP/IP stack
fixed terminal
mobile terminal
server
infrastructure network
access point
application application
TCP TCP
IP IP
LLC LLC LLC
802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC
802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

802.11 - Layers and functions
 MAC
– access mechanisms,
fragmentation, encryption
 MAC Management  PLCP Physical Layer Convergence
Protocol
– synchronization, roaming,
MIB, power management – clear channel assessment
signal (carrier sense)
 PMD Physical Medium Dependent
– modulation, coding
 PHY Management
– channel selection, MIB
Station Management
LLC  Station Management
DLC
MAC MAC Management – coordination of all

management functions
PLCP
PHY
PHY Management
PMD

Components of IEEE 802.11
architecture
 The basic service set (BSS) is the basic building block
of an IEEE 802.11 LAN
 The ovals can be thought of as the coverage area
within which member stations can directly communicate
 The Independent BSS (IBSS) is the simplest LAN. It
may consist of as few as two stations
ad-hoc network BSS1 BSS2

802.11 - ad-hoc network
802.11 LAN  Direct communication
within a limited range
STA1 – Station (STA):
BSS1 STA3 terminal with access
mechanisms to the
wireless medium
STA2 – Basic Service Set (BSS):
group of stations using
the same radio frequency
BSS2
STA5
STA4 802.11 LAN

802.11 - infrastructure network
Station (STA)
802.11 LAN – terminal with access
802.x LAN mechanisms to the wireless
medium and radio contact to the
access point
STA1 Basic Service Set (BSS)
BSS1
Portal – group of stations using the
Access
same radio frequency
Point
Access Point
Distribution System
– station integrated into the
Access wireless LAN and the
ESS Point distribution system
Portal
BSS2
– bridge to other (wired)
networks
Distribution System
– interconnection network to
STA2 802.11 LAN STA3
form one logical network (EES:
Extended Service Set) based on
several BSS
Distribution System (DS) concepts
 The Distribution system interconnects multiple
BSSs
 802.11 standard logically separates the wireless
medium from the distribution system – it does not
preclude, nor demand, that the multiple media be
same or different
 An Access Point (AP) is a STA that provides
access to the DS by providing DS services in
addition to acting as a STA.
 Data moves between BSS and the DS via an AP
 The DS and BSSs allow 802.11 to create a wireless
network of arbitrary size and complexity called
the Extended Service Set network (ESS)

Extended Service Set network

802.11 - Physical layer
 3 versions of spread spectrum: 2 radio (typ. 2.4 GHz), 1 IR
– data rates 1 or 2 Mbps
 FHSS (Frequency Hopping Spread Spectrum)
– spreading, despreading, signal strength, typically 1 Mbps
– min. 2.5 frequency hops/s (USA), two-level GFSK modulation
 DSSS (Direct Sequence Spread Spectrum)
– DBPSK modulation for 1 Mbps (Differential Binary Phase Shift
Keying), DQPSK for 2 Mbps (Differential Quadrature PSK)
– preamble and header of a frame is always transmitted with 1 Mbps,
rest of transmission 1 or 2 Mbps
– chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)
– max. radiated power 1 W (USA), 100 mW (EU), min. 1mW
 Infrared
– 850-950 nm, diffuse light, typ. 10 m range
– carrier detection, energy detection, synchronization
Spread-spectrum communications

DSSS Barker Code modulation

DSSS properties

Hardware
 Original WaveLAN card (NCR)
– 914 MHz Radio Frequency
– Transmit power 281.8 mW
– Transmission Range ~250 m (outdoors) at 2Mbps
 WaveLAN II (Lucent)
– 2.4 GHz radio frequency range
– Transmit Power 30mW
– Transmission range 376 m (outdoors) at 2 Mbps (60m
indoors)
– Receive Threshold = - 81dBm
– Carrier Sense Threshold = -111dBm
 Many others….Agere, Cisco,………

MAC functional spec - DCF
802.11 - MAC layer
 Traffic services
– Asynchronous Data Service (mandatory) – DCF
– Time-Bounded Service (optional) - PCF
 Access methods
– DCF CSMA/CA (mandatory)
• collision avoidance via randomized back-off mechanism
• ACK packet for acknowledgements (not for broadcasts)
– DCF w/ RTS/CTS (optional)
• avoids hidden/exposed terminal problem, provides reliability
– PCF (optional)
• access point polls terminals according to a list

802.11 - Coordination Functions

802.11 - CSMA/CA
contention window
DIFS DIFS (randomized back-off
mechanism)
medium busy next frame
direct access if t
medium is free  DIFS slot time
– station which has data to send starts sensing the medium

(Carrier Sense based on CCA, Clear Channel Assessment)
– if the medium is free for the duration of an Inter-Frame
Space (IFS), the station can start sending (IFS depends on
service type)
– if the medium is busy, the station has to wait for a free IFS
plus an additional random back-off time (multiple of slot-time)
– if another station occupies the medium during the back-off
time of the station, the back-off timer stops (fairness)

802.11 DCF – basic access
 If medium is free for DIFS time, station sends data
 receivers acknowledge at once (after waiting for SIFS) if
the packet was received correctly (CRC)
 automatic retransmission of data packets in case of
transmission errors
DIFS
data
sender
SIFS
ACK
receiver
DIFS
other data
stations t
waiting time contention

802.11 –RTS/CTS
 If medium is free for DIFS, station can send RTS with reservation
parameter (reservation determines amount of time the data packet needs
the medium)
 acknowledgement via CTS after SIFS by receiver (if ready to receive)
 sender can now send data at once, acknowledgement via ACK
 other stations store medium reservations distributed via RTS and CTS
DIFS
RTS data
sender
SIFS SIFS
CTS SIFS ACK
receiver
NAV (RTS) DIFS

other NAV (CTS) data
stations t
defer access contention

802.11 - Carrier Sensing
 In IEEE 802.11, carrier sensing is performed
– at the air interface (physical carrier sensing), and
– at the MAC layer (virtual carrier sensing)
 Physical carrier sensing
– detects presence of other users by analyzing all
detected packets
– Detects activity in the channel via relative signal
strength from other sources
 Virtual carrier sensing is done by sending MPDU duration
information in the header of RTS/CTS and data frames
 Channel is busy if either mechanisms indicate it to be
 Duration field indicates the amount of time (in
microseconds) required to complete frame transmission
 Stations in the BSS use the information in the duration field
to adjust their network allocation vector (NAV)

802.11 - Collision Avoidance
 If medium is not free during DIFS time..
 Go into Collision Avoidance: Once channel becomes
idle, wait for DIFS time plus a randomly chosen
backoff time before attempting to transmit
 For DCF the backoff is chosen as follows:
– When first transmitting a packet, choose a backoff
interval in the range [0,cw]; cw is contention window,
nominally 31
– Count down the backoff interval when medium is idle
– Count-down is suspended if medium becomes busy
– When backoff interval reaches 0, transmit RTS
– If collision, then double the cw up to a maximum of 1024
 Time spent counting down backoff intervals is part
of MAC overhead

Example - backoff
B1 = 25 B1 = 5
wait data
data wait
B2 = 20 B2 = 15 B2 = 10
B1 and B2 are backoff intervals

cw = 31 at nodes 1 and 2

Backoff - more complex example
DIFS DIFS DIFS DIFS
boe bor boe bor boe busy
station1
boe busy
station2
busy
station3
boe busy boe bor

station4
boe bor boe busy boe bor

station5
t
busy medium not idle (frame, ack etc.) boe elapsed backoff time
packet arrival at MAC bor residual backoff time

Source: Schiller
802.11 - Priorities
 defined through different inter frame spaces –
mandatory idle time intervals between the transmission of
frames
 SIFS (Short Inter Frame Spacing)
– highest priority, for ACK, CTS, polling response
– SIFSTime and SlotTime are fixed per PHY layer (10 s and
20 s respectively in DSSS)
 PIFS (PCF IFS)
– medium priority, for time-bounded service using PCF
– PIFSTime = SIFSTime + SlotTime
 DIFS (DCF IFS)
– lowest priority, for asynchronous data service
– DCF-IFS: DIFSTime = SIFSTime + 2xSlotTime

Solution to Hidden/Exposed Terminals
 A first sends a Request-to-Send (RTS) to B

 On receiving RTS, B responds Clear-to-Send (CTS)
 Hidden node C overhears CTS and keeps quiet
– Transfer duration is included in both RTS and CTS
 Exposed node overhears a RTS but not the CTS
– D’s transmission cannot interfere at B
RTS RTS
D A B C
CTS CTS
DATA
802.11 - Reliability
 Use acknowledgements
– When B receives DATA from A, B sends an ACK
– If A fails to receive an ACK, A retransmits the DATA
– Both C and D remain quiet until ACK (to prevent collision
of ACK)
– Expected duration of transmission+ACK is included in
RTS/CTS packets
RTS RTS
D A B C
CTS CTS
DATA
ACK

802.11 - Congestion Control
 Contention window (cw) in DCF: Congestion
control achieved by dynamically choosing
cw
 large cw leads to larger backoff intervals
 small cw leads to larger number of
collisions
 Binary Exponential Backoff in DCF:

– When a node fails to receive CTS in response to
its RTS, it increases the contention window
• cw is doubled (up to a bound cwmax =1023)
– Upon successful completion data transfer,
restore cw to cwmin=31
Fragmentation
DIFS
RTS frag1 frag2
sender
SIFS SIFS SIFS
CTS SIFS ACK1 SIFS ACK2
receiver
NAV (RTS)
NAV (CTS)
NAV (frag1) DIFS
other NAV (ACK1) data
stations t
contention

802.11 - MAC management
 Synchronization
– try to find a LAN, try to stay within a LAN
– timer etc.
 Power management
– sleep-mode without missing a message
– periodic sleep, frame buffering, traffic measurements
 Association/Reassociation
– integration into a LAN
– roaming, i.e. change networks by changing access points
– scanning, i.e. active search for a network
 MIB - Management Information Base
– managing, read, write

802.11 - Synchronization
 All STAs within a BSS are synchronized to a common

clock
– Infrastructure mode: AP is the timing master
• periodically transmits Beacon frames containing Timing
Synchronization function (TSF)
• Receiving stations accepts the timestamp value in TSF
– Ad hoc mode: TSF implements a distributed algorithm
• Each station adopts the timing received from any beacon that
has TSF value later than its own TSF timer
 This mechanism keeps the synchronization of the
TSF timers in a BSS to within 4 s plus the
maximum propagation delay of the PHY layer

Synchronization using a Beacon
(infrastructure mode)
beacon interval
B B B B
access
point
busy busy busy busy
medium
t
value of the timestamp B beacon frame

Source: Schiller
Synchronization using a Beacon
(ad-hoc mode)
beacon interval
B1 B1
station1
B2 B2
station2
busy busy busy busy

medium
t
value of the timestamp B beacon frame random delay

802.11 - Power management
 Idea: switch the transceiver off if not needed
 States of a station: sleep and awake
 Timing Synchronization Function (TSF)
– stations wake up at the same time
 Infrastructure
– Traffic Indication Map (TIM)
• list of unicast receivers transmitted by AP
– Delivery Traffic Indication Map (DTIM)
• list of broadcast/multicast receivers transmitted by AP
 Ad-hoc
– Ad-hoc Traffic Indication Map (ATIM)
• announcement of receivers by stations buffering frames
• more complicated - no central AP
• collision of ATIMs possible (scalability?)

802.11 - Energy Conservation
 Power Saving in infrastructure mode
– Nodes can go into sleep or standby mode
– An Access Point periodically transmits a beacon
indicating which nodes have packets waiting for
them
– Each power saving (PS) node wakes up periodically
to receive the beacon
– If a node has a packet waiting, then it sends a PS-
Poll
• After waiting for a backoff interval in [0,CWmin]
– Access Point sends the data in response to PS-poll

Power saving with wake-up patterns
(infrastructure)
TIM interval DTIM interval
D B T T d D B
access
point
busy busy busy busy
medium
p d
station
t
T TIM D DTIM awake
B broadcast/multicast p PS poll d data transmission

to/from the station

Power saving with wake-up patterns
(ad-hoc)
ATIM
window beacon interval
B1 A D B1
station1
B2 B2 a d
station2
t
B beacon frame random delay A transmit ATIM D transmit data
awake a acknowledge ATIM d acknowledge data

802.11 - Frame format
 Types
– control frames, management frames, data frames
 Sequence numbers
– important against duplicated frames due to lost ACKs
 Addresses
– receiver, transmitter (physical), BSS identifier, sender (logical)
 Miscellaneous
– sending time, checksum, frame control, data
bytes 2 2 6 6 6 2 6 0-2312 4
Frame Duration Address Address Address Sequence Address
Data CRC
Control ID 1 2 3 Control 4
version, type, fragmentation, security, ...

Types of Frames
 Control Frames
– RTS/CTS/ACK
– CF-Poll/CF-End
 Management Frames
– Beacons
– Probe Request/Response
– Association Request/Response
– Dissociation/Reassociation
– Authentication/Deauthentication
– ATIM
 Data Frames

802.11 - Roaming
 Bad connection in Infrastructure mode? Perform:
 scanning of environment
– listen into the medium for beacon signals or send probes into
the medium and wait for an answer
 send Reassociation Request
– station sends a request to a new AP(s)
 receive Reassociation Response
– success: AP has answered, station can now participate
– failure: continue scanning
 AP accepts Reassociation Request and
– signals the new station to the distribution system
– the distribution system updates its data base (i.e., location
information)
– typically, the distribution system now informs the old AP so it
can release resources

Point Coordination Function (PCF)
802.11 - Point Coordination Function

Coexistence of PCF and DCF
 A Point Coordinator (PC) resides in the Access Point
and controls frame transfers during a Contention Free
Period (CFP)
 A CF-Poll frame is used by the PC to invite a station to
send data. Stations are polled from a list maintained by
the PC
 The CFP alternates with a Contention Period (CP) in
which data transfers happen as per the rules of DCF
 This CP must be large enough to send at least one
maximum-sized packet including RTS/CTS/ACK
 CFPs are generated at the CFP repetition rate
 The PC sends Beacons at regular intervals and at the
start of each CFP
 The CF-End frame signals the end of the CFP

CFP structure and Timing

802.11 - PCF I
t0 t1
SuperFrame
medium busy PIFS SIFS SIFS

D1 D2
point
coordinator SIFS SIFS
U1 U2
wireless
stations
stations‘ NAV
NAV

802.11 - PCF II
t2 t3 t4
PIFS SIFS
D3 D4 CFend
point
coordinator SIFS
U4
wireless
stations
stations‘ NAV
NAV contention free period contention t
period

Throughput – DCF vs. PCF
 Overheads to throughput and delay in DCF mode come
from losses due to collisions and backoff
 These increase when number of nodes in the network
increases
 RTS/CTS frames cost bandwidth but large data packets
(>RTS threshold) suffer fewer collisions
 RTC/CTS threshold must depend on number of nodes
 Overhead in PCF modes comes from wasted polls
 Polling mechanisms have large influence on throughput
 Throughput in PCF mode shows up to 20% variation with
other configuration parameters – CFP repetition rate
 Saturation throughput of DCF less than PCF in all studies
presented here (‘heavy load’ conditions)

Shahzad Malik Mobile Broadband Networks - WLANs ICCC 2002
5-1-72
IEEE 802.11 Summary
 Infrastructure and ad hoc modes using DCF
 Carrier Sense Multiple Access
 Binary exponential backoff for collision avoidance
and congestion control
 Acknowledgements for reliability
 Power save mode for energy conservation
 Time-bound service using PCF
 Signaling packets for avoiding Exposed/Hidden
terminal problems, and for reservation
– Medium is reserved for the duration of the transmission
– RTS-CTS in DCF
– Polls in PCF

802.11 current status
802.11i LLC
security
WEP MAC
802.11f MAC Mgmt
Inter Access Point Protocol
802.11e MIB
QoS enhancements
PHY
DSSS FH IR
OFDM
802.11b
5,11 Mbps
802.11a
6,9,12,18,24
802.11g 36,48,54 Mbps
20+ Mbps

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EEE613 (MobiNets)

Mobile Broadband Networks

Shahzad A. Malik, Ph.D.

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-4

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-5

 Multihop Ad hoc Networks

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-6

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-7

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-9

 Impact of handling mobile (and portable) stations

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-10

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-11

 Mobility causes route changes

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-12

 Multi-hop wireless links

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-13

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-14

Indoor Outdoor Mid range Long range Long distance

10 – 30m 50 – 200m 200m – 4Km 5Km – 20Km 20m – 50Km

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-15

Industry Campus Public hotspots Broadband access

Remote data Freedom from

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-16

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-18

 Limitations Imposed by Mobility

 Limitations of the Mobile Device

Shahzad Malik Mobile Broadband Networks - WLANs 5-1-19

 If both A and C sense the channel to be idle at the

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– A and C cannot hear each other.

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 33cm 12cm 5cm

26 MHz 83.5 MHz 200 MHz

902 MHz 2.4 GHz 5.15 GHz

 Standards covers the MAC sublayer and PHY layers

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MAC MAC Management – coordination of all

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ad-hoc network BSS1 BSS2

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STA4 802.11 LAN

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medium busy next frame

– station which has data to send starts sensing the medium

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NAV (RTS) DIFS

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