MobileComputingWLAN MobileComputingWLAN.pptx

Mobile Computing
Wireless LAN (802.11)
Mayank Pandey, MNNIT, Allahabad, India

Introduction: WLAN
• A wireless LAN (WLAN) is a :
– flexible data communication system implemented as
• an alternative for a wired LAN within a building or campus.
• WLANs :
– combine data connectivity with user mobility.
– a network client can migrate between different physical
locations within the LAN environment without losing
connectivity.
• Flexible:
– reconfigure /add more nodes to the network
• without much planning effort and cost of re-cabling.
07/11/2025 Mayank Pandey, MNNIT, Allahabad, India 2

WLANs: Topologies
• Two types of network topologies:
– ad hoc and infrastructure.
• An adhoc topology supports p2p connectivity :
– mobile nodes communicate directly with each
other using wireless adapters.
• ideal for meetings or setting up of temporary
workgroups.
– However, it has disadvantage of limited coverage area
07/11/2025 3

WLAN: Ad-hoc

Infrastructure wireless LANs
• Use an access point that controls :
– transmission and assigns priority levels
– monitors network load and
– allows mobile stations to roam from cell to cell.
• Also:
– to handle traffic from a mobile station to another
mobile station
• within the coverage area or to the wired backbone.
07/11/2025 5

WLAN: Infrastructure

7
IEEE 802.11
• Wireless LAN standard defined in the
unlicensed spectrum (2.4 GHz and 5 GHz U-
NII bands)

8
802.11 (contd.)
• Standards covers the MAC sublayer and PHY
layers
• Three different physical layers in the 2.4 GHz
band
– FHSS, DSSS and IR
• OFDM based PHY layer in the 5 GHz band

9
802.11 architecture
• The basic service set (BSS) is the basic
building block of an IEEE 802.11 LAN
ad-hoc network BSS2
BSS1

10
802.11 architecture (contd.)
• 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
• IBSS is also called the ad hoc mode or DCF
mode in 802.11

11
802.11 - ad-hoc network
• Direct communication within
a limited range
– Station (STA):
terminal with access
mechanisms to the wireless
medium
– Basic Service Set (BSS):
group of stations using the
same radio frequency
802.11 LAN
BSS2
802.11 LAN
BSS1
STA1
STA4
STA5
STA2
STA3
Source: Schiller

12
802.11 - infrastructure
Source: Schiller
Distribution System
Portal
802.x LAN
Access
Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access
Point
STA1
STA2 STA3
ESS

13
Infrastructure 802.11 components
• Station (STA): terminal with access mechanisms to the
wireless medium 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 distribution system
• Portal: bridge to other (wired) networks
• Distribution System: interconnection network to form
one logical network (ESS: Extended Service Set) based
on several BSS

14
Distribution System (DS)
• 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

15
DS (contd.)
• 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)

16
802.11- in the TCP/IP stack
mobile terminal
access point
server
fixed terminal
application
TCP
802.11 PHY
802.11 MAC
IP
802.3 MAC
802.3 PHY
application
TCP
802.3 PHY
802.3 MAC
IP
802.11 MAC
802.11 PHY
LLC
infrastructure network
LLC LLC

Mayank Pandey, MNNIT, Allahabad, India 17
802.11 - Layers and functions
• PLCP Physical Layer Convergence
Protocol
– clear channel assessment
signal (carrier sense)
• PMD Physical Medium Dependent
– modulation, coding
• PHY Management
– channel selection, MIB
• Station Management
– coordination of all
management functions
PMD
PLCP
MAC
LLC
MAC Management
PHY Management
• MAC
– access mechanisms,
fragmentation, encryption
• MAC Management
– synchronization, roaming,
MIB, power management
PHY
DLC
Station
Management

18
802.11 - Physical layer
• 3 versions: 2 radio (typically 2.4 GHz), 1 IR
– data rates 1, 2, 5.5, or 11 Mbit/s
• Infrared
– 850-950 nm, diffuse light, typ. 10 m range
– carrier detection, energy detection,
synchronization
• FHSS (Frequency Hopping Spread Spectrum)
– spreading, despreading, signal strength
– typically 1 Mbit/s (mandatory), 2Mbits/s (optional)
– min. 2.5 frequency hops/s (USA), two-level GFSK
(Gaussian FSK) modulation

19
802.11 DSSS
• DSSS (Direct Sequence Spread Spectrum)
– DBPSK modulation for 1 Mbit/s (Differential Binary
Phase Shift Keying),
– DQPSK (differential quadrature PSK) for 2 Mbit/s, CCK
(complementary code keying) for 5.5 and 11 Mbits/s
– preamble and header of a frame is always transmitted
with 1 Mbit/s
– chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1,
-1 (Barker code) (11 chip)
– max. radiated power 1 W (USA), 100 mW (EU)
– min. 1mW

20
Spread-spectrum communications

21
DSSS Barker Code modulation

22
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)

23
802.11 access methods
– DCF CSMA/CA (mandatory)
– DCF with RTS/CTS (optional)
• avoids hidden terminal problem
– PCF (optional)
• access point polls terminals according to a list

24
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

25
802.11 virtual carrier sensing
• 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)

26
802.11 – Reliability: ACKs
– 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
A B C
RTS
CTS CTS
DATA
D
RTS
ACK

27
802.11 - CSMA/CA
– station ready 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)
t
medium busy
DIFS
DIFS
next frame
contention window
(randomized back-off
mechanism)
slot time
direct access if
medium is free  DIFS

28
802.11 – CSMA/CA
– if the medium is busy, the station has to wait for a
free IFS, then the station must additionally wait a
random back-off time (collision avoidance,
multiple of slot-time)
– if another station occupies the medium during the
back-off time of the station, the back-off timer
stops (fairness)

29
802.11 –CSMA/CA example
t
busy
boe
station1
station2
station3
station4
station5
packet arrival at MAC
DIFS
boe
boe
boe
busy
elapsed backoff time
bor residual backoff time
busy medium not idle (frame, ack etc.)
bor
bor
DIFS
boe
boe
boe bor
DIFS
busy
busy
DIFS
boe busy
boe
boe
bor
bor

30
802.11 - Collision Avoidance
• Collision avoidance: Once channel becomes
idle, the node waits for a randomly chosen
duration before attempting to transmit
– When transmitting a packet, choose a backoff
interval in the range [0,cw]; cw is contention
window
– Count down the backoff interval when medium is
idle
– Count-down is suspended if medium becomes busy
– When backoff interval reaches 0, transmit RTS

31
DCF Example
data
wait
B1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervals
at nodes 1 and 2
cw = 31
B2 = 10

32
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

33
Congestion control (contd.)
• 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)
– Upon successful completion data transfer, restore
cw to CWmin

34
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)

35
802.11 – Priorities (contd.)
• 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 (DIFS): DIFSTime = SIFSTime + 2xSlotTime

36
802.11 - CSMA/CA II
• station has to wait for DIFS before sending 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
t
SIFS
DIFS
data
ACK
waiting time
other
stations
receiver
sender
data
DIFS
contention

37
802.11 –RTS/CTS
t
SIFS
DIFS
data
ACK
defer access
other
stations
receiver
sender
data
DIFS
contention
RTS
CTS
SIFS
SIFS
NAV (RTS)
NAV (CTS)

38
802.11 –RTS/CTS
• station can send RTS with reservation parameter after waiting for
DIFS (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 (NAV) distributed via
RTS and CTS

Fragmentation
t
SIFS
DIFS
data
ACK1
other
stations
receiver
sender
frag1
DIFS
contention
RTS
CTS
SIFS SIFS
NAV (RTS)
NAV (CTS)
NAV (frag1)
NAV (ACK1)
SIFS
ACK2
frag2
SIFS

802.11 - PCF
PIFS
stations‘
NAV
wireless
stations
point
coordinator
D1
U1
SIFS
NAV
SIFS
D2
U2
SIFS
SIFS
SuperFrame
t0
medium busy
t1
t0 = time when the superframe should have started
t1 = time when it actually started due to contention in the prev period

802.11 - PCF
t
stations‘
NAV
wireless
stations
point
coordinator
D3
NAV
PIFS
D4
U4
SIFS
SIFS
CFend
contention
period
contention free period
t2 t3 t4
t2 = time when CFP actually finished
t3 = initial planned CFP (but PCF finished polling earlier than expected)

42
CFP
• The length of CFP is controlled by PC
– CFPMaxDuration field is used for this
• When CFP is more than beacon interval
– CFP_Dur_Remaining is included in beacons
– CFP_Dur_Remaining is set to 0 for beacons in CP

43
Polling Mechanisms
• With DCF, there is no mechanism to
guarantee minimum delay for time-bound
services
• PCF wastes bandwidth (control overhead)
when network load is light, but delays are
bounded
• Implicit signaling mechanism for STAs to
indicate when they have data to send
improves performance

44
Coexistence of PCF and DCF
• PC controls frame transfers during a
Contention Free Period (CFP).
– CF-Poll control frame is used by the PC to invite a
station to send data
– CF-End is used to signal the end of the CFP
• CFPs are generated at the CFP repetition rate
and each CFP begins with a beacon frame

45
PCF and DCF (contd.)
• The CFP alternates with a CP, when DCF
controls frame transfers
– The CP must be large enough to send at least one
maximum-sized MPDU including RTS/CTS/ACK
• Superframe: One CFP + One CP. It repeats
according to the CFP repetition rate and each
CFP begins with a beacon frame

46
802.11 - Frame format
bytes
Frame
Control
Duration
ID
Address
1
Address
2
Address
3
Sequence
Control
Address
4
Data CRC
2 2 6 6 6 6
2 4
0-2312
version, type, fragmentation, security, ...

47
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

Frame Control Field

49
Types of Frames
• Control Frames
– RTS/CTS/ACK
– CF-Poll/CF-End
• Data Frames
• Management Frames
– Beacons
– Probe Request
– Probe Response
– Association Request
– Association Response
– Dis/Reassociation
– Authentication
– Deauthentication
– ATIM (Announcement TIM)

50
MAC address format
scenario to DS from
DS
address 1 address 2 address 3 address 4
ad-hoc network 0 0 DA SA BSSID -
infrastructure
network, from AP
0 1 DA BSSID SA -
infrastructure
network, to AP
1 0 BSSID SA DA -
infrastructure
network, within DS
1 1 RA TA DA SA
DS: Distribution System
AP: Access Point
DA: Destination Address
SA: Source Address
BSSID: Basic Service Set Identifier
RA: Receiver Address
TA: Transmitter Address

51
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
• Association/Reassociation
– scanning, i.e. active search for a network
– roaming, i.e. change networks by changing APs
• MIB - Management Information Base
– managing, read, write

52
Synchronization using Beacon (infrastructure)
• Synchronized clocks are needed for PCF, Power
saving and for frequency hopping
• Within a BSS timing is conveyed by a periodic beacon
• STAs use the timestamp in beacon to adjust its
internal local clock
• AP always tries to send beacon at scheduled period
(even if the prev beacon was delayed)

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

54
Synchronization using Beacon (ad-hoc)
• Synchronization in ad hoc mode is more difficult,
since there is no AP for beacon transmission
• Each STA maintains its synchronization timer and
starts transmission of a beacon periodically
• Standard random back off is applied to beacon
frames so that only one STA wins transmitting beacon

Synchronization using Beacon (ad-hoc)
t
medium
station1
busy
B1
beacon interval
busy busy busy
B1
value of the timestamp B beacon frame
station2
B2 B2
random delay

56
Power saving in 802.11
• Basic idea is to switch off transceiver when
there is no communication
• Easy for sender since they know when to send
data
• Receivers should wakeup periodically to check
if it has to receive anything

57
Power saving :wake-up patterns (infra)
• All stations (one station shown) wake up prior
to TIM or DTIM
– With every beacon the AP sends TIM (Traffic
Indication Map)
• TIM contains a list of stations for which unicast data
frames are waiting
• DTIM (Delivery TIM) is for sending broadcast frames
– PS (Power Saving) poll is sent by STA in response
to TIM destined to the STA

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

59
Power saving:wakeup patterns (ad-hoc)
• PS in ad hoc mode is more complex (no central AP)
• All stations announce a list of buffered frames during
a period when all of them are awake
– Destinations are announced using ATIM (Adhoc TIM)

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

61
802.11 - Roaming
• Scanning
– scan the environment, i.e.,
– passive scanning
• listen into the medium for beacon signals (to detect other network)
– active scanning
• send probes into the medium on each channel and wait for an
answer
• Station then selects the best AP (e.g. based on signal strength)
– sends association Request to the AP
• association Response
– success: AP has answered, station is now associated with the new AP
– failure: continue scanning

62
Roaming (contd.)
• AP accepts Association Request
– signal 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

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

64
IEEE 802.11 Summary
• Infrastructure (PCF) and adhoc (DCF) modes
• Signaling packets for collision avoidance
– Medium is reserved for the duration of the
transmission
– Beacons in PCF
– RTS-CTS in DCF
• Acknowledgements for reliability
• Binary exponential backoff for congestion
control
• Power save mode for energy conservation

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