Wireless LANs (802.11 Networks) lecture notes

Wireless LANs (IEEE802.1)
Lecture 5
G. Noubir
noubir@ccs.neu.edu
Textbook: chapters 13, 14
Slides partially from “Mobile Communications” by J. Schiller Chapter 7.

COM3525, W02, Wireless LANs – IEEE802.11
2
Outline
 Wireless LAN Technology
 Medium Access Control for Wireless
 IEEE802.11

3
Wireless LAN Applications
 LAN Extension
 Cross-building interconnect
 Nomadic Access
 Ad hoc networking

4
LAN Extension
 Wireless LAN linked into a wired LAN on
same premises
 Wired LAN

Backbone

Support servers and stationary workstations
 Wireless LAN

Stations in large open areas

Manufacturing plants, stock exchange trading floors, and
warehouses

6
Cross-Building Interconnect
 Connect LANs in nearby buildings
 Wired or wireless LANs
 Point-to-point wireless link is used
 Devices connected are typically bridges or
routers

7
Nomadic Access
 Wireless link between LAN hub and mobile
data terminal equipped with antenna
 Laptop computer or notepad computer
 Uses:
 Transfer data from portable computer to office
server
 Extended environment such as campus

8
Ad Hoc Networking
 Temporary peer-to-peer network set up to meet
immediate need
 Example:
 Group of employees with laptops convene for a
meeting; employees link computers in a temporary
network for duration of meeting

9
Wireless LAN Requirements
 Throughput
 Number of nodes
 Connection to backbone LAN
 Service area
 Battery power consumption
 Transmission robustness and security
 Collocated network operation
 License-free operation
 Handoff/roaming
 Dynamic configuration

10
Wireless LAN Categories
 Infrared (IR) LANs
 Spread spectrum LANs
 Narrowband microwave

11
Strengths of Infrared Over
Microwave Radio
 Spectrum for infrared virtually unlimited
 Possibility of high data rates
 Infrared spectrum unregulated
 Equipment inexpensive and simple
 Reflected by light-colored objects
 Ceiling reflection for entire room coverage
 Doesn’t penetrate walls
 More easily secured against eavesdropping
 Less interference between different rooms

12
Drawbacks of Infrared Medium
 Indoor environments experience infrared
background radiation
 Sunlight and indoor lighting
 Ambient radiation appears as noise in an infrared
receiver
 Transmitters of higher power required

Limited by concerns of eye safety and excessive power
consumption
 Limits range

13
IR Data Transmission
Techniques
 Directed Beam Infrared
 Ominidirectional
 Diffused

14
Directed Beam Infrared
 Used to create point-to-point links
 Range depends on emitted power and degree of
focusing
 Focused IR data link can have range of
kilometers
 Cross-building interconnect between bridges or
routers

15
Ominidirectional
 Single base station within line of sight of all
other stations on LAN
 Station typically mounted on ceiling
 Base station acts as a multiport repeater
 Ceiling transmitter broadcasts signal received by IR
transceivers
 IR transceivers transmit with directional beam
aimed at ceiling base unit

16
Diffused
 All IR transmitters focused and aimed at a point
on diffusely reflecting ceiling
 IR radiation strikes ceiling
 Reradiated omnidirectionally
 Picked up by all receivers

17
Spread Spectrum LAN
Configuration
 Multiple-cell arrangement
 Within a cell, either peer-to-peer or hub
 Peer-to-peer topology
 No hub
 Access controlled with MAC algorithm

CSMA
 Appropriate for ad hoc LANs

18
Spread Spectrum LAN
Configuration
 Hub topology
 Mounted on the ceiling and connected to backbone
 May control access
 May act as multiport repeater
 Automatic handoff of mobile stations
 Stations in cell either:

Transmit to / receive from hub only

Broadcast using omnidirectional antenna

19
Narrowband Microwave LANs
 Use of a microwave radio frequency band for
signal transmission
 Relatively narrow bandwidth
 Licensed
 Unlicensed

20
Licensed Narrowband RF
 Licensed within specific geographic areas to
avoid potential interference
 Motorola - 600 licenses in 18-GHz range
 Covers all metropolitan areas
 Can assure that independent LANs in nearby
locations don’t interfere
 Encrypted transmissions prevent eavesdropping

21
Unlicensed Narrowband RF
 RadioLAN introduced narrowband wireless
LAN in 1995
 Uses unlicensed ISM spectrum
 Used at low power (0.5 watts or less)
 Operates at 10 Mbps in the 5.8-GHz band
 Range = 50 m to 100 m

22
Motivation for Wireless MAC
 Can we apply media access methods from fixed networks?
 Example CSMA/CD
 Carrier Sense Multiple Access with Collision Detection
 send as soon as the medium is free, listen into the medium if a
collision occurs (original method in IEEE 802.3)
 Problems in wireless networks
 signal strength decreases proportional to the square of the distance
 the sender would apply CS and CD, but the collisions happen at the
receiver
 it might be the case that a sender cannot “hear” the collision, i.e., CD
does not work
 furthermore, CS might not work if, e.g., a terminal is “hidden”

23
 Hidden terminals
 A sends to B, C cannot receive A
 C wants to send to B, C senses a “free” medium (CS fails)
 collision at B, A cannot receive the collision (CD fails)
 A is “hidden” for C
 Exposed terminals
 B sends to A, C wants to send to another terminal (not A or B)
 C has to wait, CS signals a medium in use
 but A is outside the radio range of C, therefore waiting is not
necessary
 C is “exposed” to B
Motivation - hidden and
exposed terminals
B
A C

24
 Terminals A and B send, C receives
 signal strength decreases proportional to the square of the distance
 the signal of terminal B therefore drowns out A’s signal
 C cannot receive A
 If C for example was an arbiter for sending rights, terminal B
would drown out terminal A already on the physical layer
 Also severe problem for CDMA-networks - precise power control
needed!
Motivation - near and far
terminals
A B C

25
Access methods
SDMA/FDMA/TDMA
 SDMA (Space Division Multiple Access)

segment space into sectors, use directed antennas

cell structure
 FDMA (Frequency Division Multiple Access)

assign a certain frequency to a transmission channel between a sender
and a receiver

permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast
hopping (FHSS, Frequency Hopping Spread Spectrum)
 TDMA (Time Division Multiple Access)

assign the fixed sending frequency to a transmission channel between
a sender and a receiver for a certain amount of time
 The multiplexing schemes are now used to control medium access!

26
FDD/FDMA - general
scheme, example GSM
f
t
124
1
124
1
20 MHz
200 kHz
890.2 MHz
935.2 MHz
915 MHz
960 MHz

27
TDD/TDMA - general
scheme, example DECT
1 2 3 11 12 1 2 3 11 12
t
downlink uplink
417 µs

28
Frequency Division Multiple
Access
 Concept:
 assign different frequency bands to different users
 no sharing of a frequency band between two users
 user separation using band-pass filters
 continuous flow
 two-way: two frequency bands or Time Division Duplex (TDD)
 Advantages: simple receivers
 longer symbol duration: no-need for equalization

low inter-symbol interference

e.g., 50kb/s QPSK =>40s >> 1-10s delay spread
 Drawbacks:
 frequency guard bands, costly tight RF band-filters,
 long fading duration: need slow frequency hopping
 may need spatial diversity (multiple antennas/beam forming) Rx/Tx

29
Frequency Selection
 Frequency management:
 Fixed (cellular phones-base stations): reuse factor
 On demand (cellular phones-mobile terminals)
 Dynamic (cordless/WLAN): based on sensing interference levels
 Problems: congestion management, dynamic load, …
 Antenna implications:
 High antennas (e.g., 50m): higher coverage but higher
interference between base stations (need for synchronization)
 Low antennas: higher attenuation, lower coverage, better reuse
 Conclusion:
 Pure FDMA is only interesting for simple cordless systems (CT-2)

30
Time Division Multiple
Access
 Concept:
 use the same frequency over non-overlapping periods of time
 Advantages:
 simple filters (window)
 transmit and receive over the same frequency channel
 Drawbacks:
 users must be synchronized with BS (master clock over a BCH)
 guard times: common 30-50s, may be less in recent systems
 short symbol duration: need for equalization, training
sequences...

high inter-symbol interference

e.g., 50Kbps, QPSK, 8 users:
 5 s symbol duration
 delay spread: 1s (cordless), upto 20s for cellular

31
FDMA/TDMA
 First channel allocation:
 random access channel (RACH) to send short requests
 ALOHA type protocol over the RACH
 One can use both FDMA and TDMA
 examples: GSM system, D-AMPS
M9
M9
M5
M5 M6
M6
M1
M1 M2
M2 M3
M3 M4
M4
M9
M9
M5
M5 M6
M6
M1
M1 M2
M2 M3
M3 M4
M4
Time
Time
Frequency
Frequency
cycle
cycle

32
Access method CDMA
CDMA (Code Division Multiple Access)
 all terminals send on the same frequency probably at the same time and can use the
whole bandwidth of the transmission channel
 codes generate signals with “good-correlation” properties

signals from another user appear as “noise” (use spread spectrum technology)

signals are spread over a wideband using pseudo-noise sequences (e.g., each sender
has a unique random number, the sender XORs the signal with this random number)
 the receiver can “tune” into this signal if it knows the pseudo random number, tuning
is done via a correlation function
Disadvantages:
 higher complexity of a receiver (receiver cannot just listen into the medium and start
receiving if there is a signal)

all signals should have the same strength at a receiver (near-far effect)
Advantages:

all terminals can use the same frequency => no planning needed; macrodiversity

huge code space (e.g. 232
) compared to frequency space

33
 Mechanism
 random, distributed (no central arbiter), time-multiplex
 Slotted Aloha additionally uses time-slots, sending must always start at
slot boundaries
 Aloha
 Slotted Aloha
Aloha/slotted aloha
sender A
sender B
sender C
collision
sender A
sender B
sender C
collision
t
t

34
Carrier Sense Protocols
Use the fact that in some networks you can sense the
medium to check whether it is currently free
 1-persistent CSMA
 non-persistent CSMA
 p-persistent protocol
 CSMA with collision Detection (CSMA/CD): not applicable to
wireless systems
 1-persistent CSMA
 when a station has a packet:

it waits until the medium is free to transmit the packet

if a collision occurs, the station waits a random amount of time
 first transmission results in a collision if several stations are
waiting for the channel

35
Carrier Sense Protocols
(Cont’d)
 non-persistent CSMA
 when a station has a packet:

if the medium is free, transmit the packet

otherwise wait for a random period of time and repeat the algorithm
 higher delays, but better performance than pure ALOHA
 p-persistent protocol
 when a station has a packet wait until the medium is free:

transmit the packet with probability p

wait for next slot with probability 1-p
 better throughput than other schemes but higher delay
 CSMA with collision Detection (CSMA/CD)
 stations abort their transmission when they detect a collision
 e.g., Ethernet, IEEE802.3 but not applicable to wireless systems

36
DAMA - Demand Assigned
Multiple Access
 Channel efficiency only 18% for Aloha, 36% for Slotted
Aloha (assuming Poisson distribution for packet arrival
and packet length)
 Reservation can increase efficiency to 80%
 a sender reserves a future time-slot
 sending within this reserved time-slot is possible without
collision
 reservation also causes higher delays
 typical scheme for satellite links
 Examples for reservation algorithms:
 Explicit Reservation according to Roberts (Reservation-ALOHA)
 Implicit Reservation (PRMA)
 Reservation-TDMA

37
Access method DAMA:
Explicit Reservation
Explicit Reservation (Reservation Aloha):
 two modes:

ALOHA mode for reservation:
competition for small reservation slots, collisions possible

reserved mode for data transmission within successful reserved slots
(no collisions possible)
 it is important for all stations to keep the reservation list
consistent at any point in time and, therefore, all stations have
to synchronize from time to time
Aloha reserved Aloha reserved Aloha reserved Aloha
collision
t

38
Access method DAMA: PRMA
Implicit reservation (PRMA - Packet Reservation MA):
 a certain number of slots form a frame, frames are repeated
 stations compete for empty slots according to the slotted aloha principle
 once a station reserves a slot successfully, this slot is automatically
assigned to this station in all following frames as long as the station has
data to send
 competition for this slots starts again as soon as the slot was empty in
the last frame
frame1
frame2
frame3
frame4
frame5
1 2 3 4 5 6 7 8 time-slot
collision at
reservation
attempts
A C D A B A F
A C A B A
A B A F
A B A F D
A C E E B A F D
t
ACDABA-F
ACDABA-F
AC-ABAF-
A---BAFD
ACEEBAFD
reservation

39
Access method DAMA:
Reservation-TDMA
Reservation Time Division Multiple Access
 every frame consists of N mini-slots and x data-slots
 every station has its own mini-slot and can reserve up to k
data-slots using this mini-slot (i.e. x = N * k).
 other stations can send data in unused data-slots according
to a round-robin sending scheme (best-effort traffic)
N mini-slots N * k data-slots
reservations
for data-slots
other stations can use free data-slots
based on a round-robin scheme
e.g. N=6, k=2

40
MACA - collision avoidance
 MACA (Multiple Access with Collision Avoidance) uses
short signaling packets for collision avoidance
 RTS (request to send): a sender request the right to send from
a receiver with a short RTS packet before it sends a data packet
 CTS (clear to send): the receiver grants the right to send as
soon as it is ready to receive
 Signaling packets contain
 sender address
 receiver address
 packet size
 Variants of this method can be found in IEEE802.11 as
DFWMAC (Distributed Foundation Wireless MAC)

41
 MACA avoids the problem of hidden terminals
 A and C want to
send to B
 A sends RTS first
 C waits after receiving
CTS from B
 MACA avoids the problem of exposed terminals
 B wants to send to A, C
to another terminal
 now C does not have
to wait for it cannot
receive CTS from A
MACA examples
A B C
RTS
CTS
CTS
A B C
RTS
CTS
RTS

42
MACA variant: DFWMAC in
IEEE802.11
idle
wait for the
right to send
wait for ACK
sender receiver
packet ready to send; RTS
time-out;
RTS
CTS; data
ACK
RxBusy
idle
wait for
data
RTS; RxBusy
RTS;
CTS
data;
ACK
time-out 
data;
NAK
ACK: positive acknowledgement
NAK: negative acknowledgement
RxBusy: receiver busy
time-out 
NAK;
RTS

43
Polling mechanisms
 If one terminal can be heard by all others, this “central” terminal (a.k.a.
base station) can poll all other terminals according to a certain scheme
 now all schemes known from fixed networks can be used (typical mainframe
- terminal scenario)
 Example: Randomly Addressed Polling
 base station signals readiness to all mobile terminals
 terminals ready to send can now transmit a random number without
collision with the help of CDMA or FDMA (the random number can be seen
as dynamic address) or with collisions (over the Random Access CHannel)
 the base station now chooses one address for polling from the list of all
random numbers (collision if two terminals choose the same address)
 the base station acknowledges correct packets and continues polling the
next terminal
 this cycle starts again after polling all terminals of the list

44
ISMA (Inhibit Sense Multiple
Access)
 Current state of the medium is signaled via a “busy tone”

the base station signals on the downlink (base station to terminals)
if the medium is free or not

terminals must not send if the medium is busy
 terminals can access the medium as soon as the busy tone stops

the base station signals collisions and successful transmissions via
the busy tone and acknowledgements, respectively (media access
is not coordinated within this approach)
 mechanism used, e.g.,
for CDPD
(USA, integrated
into AMPS)

45
Comparison
SDMA/TDMA/FDMA/CDMA
Approach SDMA TDMA FDMA CDMA
Idea segment space into
cells/sectors
segment sending
time into disjoint
time-slots, demand
driven or fixed
patterns
segment the
frequency band into
disjoint sub-bands
spread the spectrum
using orthogonal codes
Terminals only one terminal can
be active in one
cell/one sector
all terminals are
active for short
periods of time on
the same frequency
every terminal has its
own frequency,
uninterrupted
all terminals can be active
at the same place at the
same moment,
uninterrupted
Signal
separation
cell structure, directed
antennas
synchronization in
the time domain
filtering in the
frequency domain
code plus special
receivers
Advantages very simple, increases
capacity per km²
established, fully
digital, flexible
simple, established,
robust
flexible, less frequency
planning needed, soft
handover
Dis-
advantages
inflexible, antennas
typically fixed
guard space
needed (multipath
propagation),
synchronization
difficult
inflexible,
frequencies are a
scarce resource
complex receivers, needs
more complicated power
control for senders
Comment only in combination
with TDMA, FDMA or
CDMA useful
standard in fixed
networks, together
with FDMA/SDMA
used in many
mobile networks
typically combined
with TDMA
(frequency hopping
patterns) and SDMA
(frequency reuse)
still faces some problems,
higher complexity,
lowered expectations; will
be integrated with
TDMA/FDMA

46
Throughputs of Some
Random Access Protocols
G: load (includes both successful transmissions and retransmissions)
S: successful transmission
a: ratio of propagation delay to the packet transmission delay
)
1
(
)
2
1
(
)
1
(
)
1
(
)
2
1
(
)]
2
/
1
(
1
[
a
G
aG
a
G
e
aG
e
a
G
e
aG
G
aG
G
G
S 














)
1
(
)
1
(
)
1
(
)
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(
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a
G
aG
a
G
aG
ae
e
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e
e
G
G
S 












aG
aG
e
a
Ge
S 




)
2
1
(
a
e
aGe
S aG
aG


 

1
Protocol Throughput
Pure-ALOHA S = Ge-2G
Slotted-ALOHA S = Ge-G
Non slotted 1-persistent
Slotted 1-persistent CSMA
Nonpersistent non slotted
CSMA
Nonpersistent slotted
CSMA

47
Comparison of MAC
Algorithms

48
IEEE802.11
infrastructure
network
ad-hoc network
AP
AP
AP
wired network
AP: Access Point

49
802.11 - Architecture of an
infrastructure network
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 (EES:
Extended Service Set) based
on several BSS
Distribution System
Portal
802.x LAN
Access
Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access
Point
STA1
STA2 STA3
ESS

50
802.11 - Architecture of an
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

51
IEEE standard 802.11
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

52
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

53

54
802.11 - Physical layer
 5 versions: 2 radio (typ. 2.4 GHz), 1 IR
 data rates 1 or 2 Mbit/s
 FHSS (Frequency Hopping Spread Spectrum) 2.4 GHz
 spreading, despreading, signal strength, typ. 1 Mbit/s
 min. 2.5 frequency hops/s (USA), two-level GFSK modulation
 DSSS (Direct Sequence Spread Spectrum) 2.4GHz
 DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying),
DQPSK for 2 Mbit/s (Differential Quadrature PSK)
 preamble and header of a frame is always transmitted with 1 Mbit/s,
rest of transmission 1 or 2 Mbit/s
 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

55
IEEE 802.11a and IEEE 802.11b
 IEEE 802.11a
 Makes use of 5-GHz band
 Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps
 Uses orthogonal frequency division multiplexing (OFDM)
 Subcarrier modulated using BPSK, QPSK, 16-QAM or 64-
QAM
 IEEE 802.11b
 Provides data rates of 5.5 and 11 Mbps
 Complementary code keying (CCK) modulation scheme

56
FHSS PHY packet format
synchronization SFD PLW PSF HEC payload
PLCP preamble PLCP header
80 16 12 4 16 variable bits
 Synchronization
 synch with 010101... pattern
 SFD (Start Frame Delimiter)
 0000110010111101 start pattern
 PLW (PLCP_PDU Length Word)
 length of payload incl. 32 bit CRC of payload, PLW < 4096
 PSF (PLCP Signaling Field)
 data rate of payload (1 or 2 Mbit/s)
 HEC (Header Error Check)
 CRC with x16
+x12
+x5
+1

57
DSSS PHY packet format
synchronization SFD signal service HEC payload
PLCP preamble PLCP header
128 16 8 8 16 variable bits
length
16
 Synchronization
 synch., gain setting, energy detection, frequency offset
compensation
 SFD (Start Frame Delimiter)
 1111001110100000
 Signal
 data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK)
 Service Length
 future use, 00: 802.11 compliant  length of the payload
 HEC (Header Error Check)
 protection of signal, service and length, x16
+x12
+x5
+1

58
802.11 - MAC layer I -
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)
 Access methods
 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

59
802.11 - MAC layer II
 Priorities
 defined through different inter frame spaces
 SIFS (Short Inter Frame Spacing)

highest priority, for ACK, CTS, polling response
 PIFS (PCF IFS)

medium priority, for time-bounded service using PCF
 DIFS (DCF, Distributed Coordination Function IFS)

lowest priority, for asynchronous data service
t
medium busy
SIFS
PIFS
DIFS
DIFS
next frame
contention
direct access if
medium is free  DIFS

60
t
medium busy
DIFS
DIFS
next frame
contention window
(randomized back-off
mechanism)
802.11 - CSMA/CA access
method I
 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)
 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)
slot time
direct access if
medium is free  DIFS

61
802.11 - competing stations -
simple version
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

62
802.11 - CSMA/CA access
method II
 Sending unicast packets
 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

63
802.11 - DFWMAC
 Sending unicast packets
 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 distributed via RTS and CTS
t
SIFS
DIFS
data
ACK
defer access
other
stations
receiver
sender
data
DIFS
contention
RTS
CTS
SIFS SIFS
NAV (RTS)
NAV (CTS)

64
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

65
DFWMAC-PCF I
PIFS
stations‘
NAV
wireless
stations
point
coordinator
D1
U1
SIFS
NAV
SIFS
D2
U2
SIFS
SIFS
SuperFrame
t0
medium busy
t1

66
DFWMAC-PCF II
t
stations‘
NAV
wireless
stations
point
coordinator
D3
NAV
PIFS
D4
U4
SIFS
SIFS
CFend
contention
period
contention free period
t2 t3 t4
7.20.1

67
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
Duration
ID
Address
1
Address
2
Address
3
Sequence
Control
Address
4
Data CRC
2 2 6 6 6 6
2 4
0-2312
bytes
Version, Type, Subtype, To DS, From DS, More Fragments, Retry,
Power Management, More Data, Wired Equivalent Privacy (WEP), and Order

68
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 (final recipient)
SA: Source Address (initiator)
BSSID: Basic Service Set Identifier
RA: Receiver Address (immediate recipient)
TA: Transmitter Address (immediate sender)

69
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

70
Synchronization using a
Beacon (infrastructure)
beacon interval
t
medium
access
point
busy
B
busy busy busy
B B B
value of the timestamp B beacon frame

71
Synchronization using a
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

72
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?)

73
Power saving with wake-up
patterns (infrastructure)
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

74
Power saving with 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

75
802.11 - Roaming
 No or bad connection? Then perform:
 Scanning
 scan the environment, i.e., listen into the medium for beacon signals
(passive) or send probes (active) into the medium and wait for an answer
 Reassociation Request
 station sends a request to one or several AP(s)
 Reassociation Response
 success: AP has answered, station can now participate
 failure: continue scanning
 AP accepts Reassociation 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

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