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IT 6011 IT 601: Mobile Computing Wireless LANs (most of the slides are borrowed from Prof. Sridhar...
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Transcript of IT 6011 IT 601: Mobile Computing Wireless LANs (most of the slides are borrowed from Prof. Sridhar...
IT 601 1
IT 601: Mobile Computing
Wireless LANs(most of the slides are borrowed from
Prof. Sridhar Iyer)
IT 601 2
Wireless LANs: Characteristics
• Advantages– Flexible deployment; Minimal wiring problems – More robust against disasters – Historic buildings, conferences, …
• Disadvantages– Low bandwidth compared to wired networks– Need to follow wireless spectrum regulations
IT 601 3
Infrastructure and Adhoc Networksinfrastructure network
ad-hoc network
APAP
AP
wired network
AP: Access Point
Source: Schiller
IT 601 4
Wireless LANs are different…
• 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 (portable) stations– Propagation effects – Mobility management– power management
IT 601 5
Difference Between Wired and Wireless
• 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.
A B CA
B
C
Ethernet LAN Wireless LAN
IT 601 6
Wireless PHY
– Medium has neither absolute nor readily observable boundaries outside which stations are unable to receive frames
– Are unprotected from outside signals and are significantly less reliable than wired PHYs
– Have time varying and asymmetric propagation properties
– Lack full connectivity • the assumption that every station (STA)
can hear every other STA in invalid
IT 601 7
Wireless MAC: Motivation• 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)
IT 601 8
Wireless MAC
– signal strength decreases inversely proportional to the square of the distance
– sender would apply CS and CD, but the collisions happen at the receiver
– sender may not “hear” the collision, i.e., CD does not work
– CS might not work, e.g. if a terminal is “hidden”
IT 601 9
– 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.
Hidden Terminal Problem
BA C
IT 601 10
Exposed Terminal Problem
– 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 BCD
IT 601 11
Solution for Hidden 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
A B CRTS
CTS CTSDATA
DRTS
IT 601 12
IEEE 802.11
• Wireless LAN standard defined in the unlicensed spectrum (2.4 GHz and 5 GHz U-NII bands)
IT 601 13
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
IT 601 14
802.11 architecture• The basic service set (BSS) is the basic building
block of an IEEE 802.11 LAN
ad-hoc network BSS2BSS1
IT 601 15
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
IT 601 16
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
IT 601 17
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
IT 601 18
PCF 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
IT 601 19
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
IT 601 20
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)
IT 601 21
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
IT 601 22
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
PH
YD
LC
Sta
tion
Man
agem
ent
7.8.1
IT 601 23
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, synchonization
• 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
IT 601 24
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
IT 601 25
Spread-spectrum communications
Source: Intersil
IT 601 26
DSSS Barker Code modulation
Source: Intersil
IT 601 27
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)
IT 601 28
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
IT 601 29
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
IT 601 30
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)
IT 601 31
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 CRTS
CTS CTSDATA
DRTS
ACK
IT 601 32
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
DIFSDIFS
next frame
contention window(randomized back-offmechanism)
slot timedirect access if medium is free DIFS
IT 601 33
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)
IT 601 34
802.11 –CSMA/CA example
t
busy
boe
station1
station2
station3
station4
station5
packet arrival at MAC
DIFSboe
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
DIFSboe busy
boe
boe
bor
bor
IT 601 35
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
IT 601 36
DCF Example
data
waitB1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervalsat nodes 1 and 2cw = 31
B2 = 10
IT 601 37
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
IT 601 38
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
IT 601 39
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)
IT 601 40
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
IT 601 41
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
otherstations
receiver
sender data
DIFS
contention
IT 601 42
802.11 –RTS/CTS
t
SIFS
DIFS
data
ACK
defer access
otherstations
receiver
sender data
DIFS
contention
RTS
CTS
SIFS SIFS
NAV (RTS)
NAV (CTS)
IT 601 43
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
IT 601 44
Fragmentation
t
SIFS
DIFS
data
ACK1
otherstations
receiver
senderfrag1
DIFS
contention
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
NAV (frag1)NAV (ACK1)
SIFSACK2
frag2
SIFS
IT 601 45
802.11 - Point Coordination Function
IT 601 46
802.11 - PCF I
PIFS
stations‘NAV
wirelessstations
point coordinator
D1
U1
SIFS
NAV
SIFSD2
U2
SIFS
SIFS
SuperFramet0
medium busy
t1
t0 = time when the superframe should have startedt1 = time when it actually started due to contention in the prev period
IT 601 47
802.11 - PCF II
tstations‘NAV
wirelessstations
point coordinator
D3
NAV
PIFSD4
U4
SIFS
SIFSCFend
contentionperiod
contention free period
t2 t3 t4
t2 = time when CFP actually finishedt3 = initial planned CFP (but PCF finished polling earlier than expected)
IT 601 48
CFP structure and Timing
CFP is greater than beacon intervalDTIM – Delivery Traffic Indication Message
Source: 802.11 spec
IT 601 49
CFP
• Then 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
IT 601 50
PCF- Data transmission
Source: 802.11 standard
IT 601 51
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
IT 601 52
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
IT 601 53
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
IT 601 54
802.11 - Frame format
bytes
FrameControl
DurationID
Address1
Address2
Address3
SequenceControl
Address4 Data CRC
2 2 6 6 6 62 40-2312
version, type, fragmentation, security, ...
IT 601 55
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
IT 601 56
Frame Control Field
IT 601 57
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)
IT 601 58
MAC address formatscenario to DS from
DSaddress 1 address 2 address 3 address 4
ad-hoc network 0 0 DA SA BSSID -infrastructurenetwork, from AP
0 1 DA BSSID SA -
infrastructurenetwork, to AP
1 0 BSSID SA DA -
infrastructurenetwork, within DS
1 1 RA TA DA SA
DS: Distribution SystemAP: Access PointDA: Destination AddressSA: Source AddressBSSID: Basic Service Set IdentifierRA: Receiver AddressTA: Transmitter Address
IT 601 59
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
IT 601 60
Synchronization using a 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)
IT 601 61
Synchronization using a Beacon (infrastructure)
beacon interval
tmedium
accesspoint
busy
B
busy busy busy
B B B
value of the timestamp B beacon frame
IT 601 62
Synchronization using a 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
IT 601 63
Synchronization using a Beacon (ad-hoc)
tmedium
station1
busy
B1
beacon interval
busy busy busy
B1
value of the timestamp B beacon frame
station2
B2 B2
random delay
IT 601 64
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
IT 601 65
Power saving with wake-up patterns (infrastructure)
• 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 (Deliverry TIM) is for sending
broadcast frames– PS (Power Saving) poll is sent by STA in response to
TIM destined to the STA
IT 601 66
Power saving with wake-up patterns (infrastructure)
TIM interval
t
medium
accesspoint
busy
D
busy busy busy
T T D
T TIM D DTIM
DTIM interval
BB
B broadcast/multicast
station
awake
p PS poll
p
d
d
d data transmissionto/from the station
IT 601 67
Power saving with wake-up patterns (ad-hoc)
• PS in ad hoc mode is more complex (no centralized 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)
IT 601 68
Power saving with wake-up patterns (ad-hoc)
awake
A transmit ATIM D transmit datat
station1
B1 B1
B beacon frame
station2
B2 B2
random delay
A
a
D
d
ATIMwindow beacon interval
a acknowledge ATIM d acknowledge data
IT 601 69
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
IT 601 70
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
IT 601 71
Hardware• Original WaveLAN card (NCR)
– 914 MHz Radio Frequency– Transmit power 281.8 mW– Transmission Range ~250 m (outdoors) at 2Mbps– SNRT 10 dB
• 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
IT 601 72
802.11 status
MAC
MIB
DSSS FH IRPHY
WEP
LLC
MAC Mgmt
802.11b5,11 Mbps
802.11g20+ Mbps
802.11a6,9,12,18,24
36,48,54 Mbps
OFDM
802.11isecurity
802.11fInter Access Point Protocol
802.11eQoS enhancements
IT 601 73
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
IT 601 74
HIPERLAN• Wireless LAN ratified by ETSI• HIPERLAN1
– First of the series of spec– Supports five different priorities– Data rate of 23.5 Mbps– Forwards packets using several relays
• Extends communication beyond the radio range– Power conservation by specific sleep and wakeup pattern– MSDU lifetime can be set to have time bound services– MAC layer uses residual lifetime and user priority to choose
the next MSDU to be transmitted
IT 601 75
HIPERLAN2• Operates at 5 GHz• Data rates up to 54 Mbps• OFDM in the physical layer and a dynamic TDMA/TDD based MAC• QoS support
– Each connection has its QoS parameters (delay, jitter, bit error)• Connection oriented
– Negotiation of QoS parameter during connection establishment• Dynamic frequency selection
– Best frequency chosen based on interference level and usage of radio channels
• Power save– Mobile devices can negotiate certain sleep and wakeup pattern for power
save• Access Points can have multiple transceivers• APs can have sectorized antenna
IT 601 76
HIPERLAN2• Two modes of operation• Centralized Mode (CM)
– Like the infrastructure mode in 802.11, APs are connected to a core network and Mobile Stations (MS) are associated with APs.
• Direct Mode (DM)– This is the optional ad hoc mode of HiperLAN2– Data is directly exchanged between MS
• But the network is still controlled• Done via an AP that has the central controller (CC)
functionality or via an MS that has CC functionality• This ensures QoS support in ad hoc mode also
IT 601 77
HIPERLAN2
AP
MS MS
datacontrolcontrol
Centralized mode
AP/CC
MS MSdata
control
Direct mode
Different modes of operation of HiperLAN2
IT 601 78
Bluetooth
• Design goal was to set up short range ad hoc network (called piconets)
• 79 channels in the 2.4 GHz band with 1 MHz carrier spacing
• Devices perform frequency hopping at 1600 hops/s• Maximum data rate of 1Mbps• Range of about 10m
IT 601 79
Bluetooth
M
S
S
S
SB
P
P SBM – MasterS - SlaveSB – StandbyP - Park
Bluetooth Piconet
IT 601 80
Bluetooth• Piconet
– A collection of bluetooth devices which are synchronized to the same hopping sequence
– One of the devices is the master, all others are slaves– Master determines the hopping pattern in the piconet and the
slaves have to synchronize to this pattern– Each piconet has a unique hopping pattern– Parked devices
• Cannot participate in the piconet, but are known and can be activated within few msec
• Devices in standby do not participate in piconet.
IT 601 81
Bluetooth
• Piconet– Active members assigned a 3-bit active member
address (AMA)• Upto 8 devices can be active in a piconet
– Parked devices use 8-bit parked member address (PMA).
– Standby devices do not need address
IT 601 82
Bluetooth• Scatternet
– Only having 1 piconet within 80 MHz in total is not very efficient– Many piconets with overlapping coverage can exist simulatenously
• A device may participate in two different piconets– Bluetooth uses FH-CDMA for separation of piconet– A slave first syncs to one piconet and communicates, then leaves that piconet
and enters the other piconet (of scatternet) by syncing to its FH sequence.– A master cannot be shared between two piconets of a scatternet– Master can leave one piconet and enter the other as a slave
• All traffic in the former piconet is suspended until the master returns