Download MAC layer

Introduction to MAC and
IEEE 802.11 (WiFi)
1
Outline
• Introduction to MAC Layer
• Introduction to IEEE 802.11:
Schiller Sec 7.3.1, 7.3.2
• 802.11 Physical layer:
Schiller Sec 7.3.3
• 802.11 MAC layer:
Schiller Sec 7.3.4
• 802.11 Management:
Schiller Sec 7.3.5
2
Link Layer Services
• Framing, link access:
– encapsulate datagram into frame, adding header,
trailer
– implement channel access if shared medium (e.g.,
Ethernet)
– ‘physical addresses’ used in frame headers to
identify source, dest
• different from IP address!
• coordinate access to a shared medium
• reliable delivery between two physically
connected devices
• flow control
• error detection/correction
3
Link Layer: setting the context
4
Multiple Access Protocols
• Determine how stations share channel
– single shared communication channel
– two or more simultaneous transmissions by nodes:
interference
• only one node can send successfully at a time
• What to look for in MAC protocols
– Synchronous vs. asynchoronous
– Centralized vs. decentralized
– Performance
• Efficiency
• fairness
5
MAC Protocols: a taxonomy
• Channel Partitioning
– divide channel into smaller “pieces” (time slots,
frequency, code)
– allocate piece to node for exclusive use
– Examples
• TDMA: partition time slots
• FDMA: partition frequency
• CDMA: partition code
• Random Access
– allow collisions
– “recover” from collisions
6
Random Access protocols
• When a node has packet to send
– transmit at full channel data rate R.
– no a priori coordination among nodes
• two or more transmitting nodes -> “collision”
• random access MAC protocol specifies:
– how to detect collisions
– how to recover from collisions (e.g., via delayed
retransmissions)
• Examples of random access MAC protocols:
– Pure ALOHA
– Slotted ALOHA
– CSMA and CSMA/CD
7
Pure ALOHA
• Transmit whenever a message is
ready
• Retransmit when there is a collision
8
Slotted Aloha
• time is divided into equal size slots (= pkt
trans. time)
• node with new arriving pkt: transmit at
beginning of next slot
• if collision: retransmit pkt in future slots
with probability p, until successful.
Success (S), Collision (C), Empty (E) slots
9
Problems with Pure/Slotted ALOHA
• Pure ALOHA
– Transmit whenever a message is ready
– Retransmit when there is a collision
• Slotted ALOHA
–
–
–
–
Time is divided into equal time slots
Transmit only at the beginning of a time slot
Avoid partial collisions
Increase delay, and require synchronization
Problem: do not listen to the channel.
10
CSMA: Carrier Sense Multiple Access
CSMA: listen before transmit:
• If channel sensed idle: transmit entire pkt
• If channel sensed busy, defer transmission
– Persistent CSMA: retry immediately with
probability p when channel becomes idle
(may cause instability)
– Non-persistent CSMA: retry after random
interval
11
CSMA collisions
spatial layout of nodes along Ethernet
collisions can occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
note:
role of distance and
propagation delay in
determining collision prob.
12
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
– collisions detected within short time
– colliding transmissions aborted, reducing channel
wastage
– persistent or non-persistent retransmission
• collision detection:
– easy in wired LANs: measure signal strengths,
compare transmitted, received signals
13
CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA
– collisions detected within short time
– colliding transmissions aborted, reducing channel
wastage
– persistent or non-persistent retransmission
• collision detection:
– easy in wired LANs: measure signal strengths,
compare transmitted, received signals
– difficult in wireless LANs:
• receiver shut off while transmitting
• receiver’s channel condition is different from that of the
14
sender
Outline
• Introduction to MAC
• Introduction to IEEE 802.11:
Schiller Sec 7.3.1, 7.3.2
• 802.11 Physical layer:
Schiller Sec 7.3.3
• 802.11 MAC layer:
Schiller Sec 7.3.4
• 802.11 Management:
Schiller Sec 7.3.5
15
Characteristics of wireless LANs
• Advantages
–
–
–
–
very flexible within the reception area
Ad-hoc networks without previous planning possible
(almost) no wiring difficulties (e.g. historic buildings, firewalls)
more robust against disasters
• e.g., earthquakes, fire - or users pulling a plug...
• Disadvantages
– typically very low bandwidth compared to wired networks
(1-10 Mbit/s) due to shared medium
– Less reliable
16
Design Goals for Wireless LANs
–
–
–
–
–
–
–
–
–
–
global, seamless operation
low power for battery use
no special licenses needed to use the LAN
robust transmission technology
simplified spontaneous cooperation at meetings
easy to use for everyone, simple management
protection of investment in wired networks
Security, privacy, safety
Transparent to applications and higher layer protocols
Location aware if necessary
17
Infrastructure vs. ad-hoc networks
infrastructure
network
AP
AP
wired network
AP: Access Point
AP
ad-hoc network
18
802.11: Infrastructure
802.11 LAN
STA1
802.x LAN
•Station (STA)
– terminal with access mechanisms
to the wireless medium and radio
contact to the access point
BSS1
Portal
Access
Point
Distribution System
– station integrated into the
wireless LAN and the
distribution system
•Basic Service Set (BSS)
Access
Point
ESS
•Access Point
– group of stations using the same
AP
•Portal
BSS2
– bridge to other (wired) networks
•Distribution System
STA2
802.11 LAN
STA3
– interconnection network to form
one logical network (EES:
Extended Service Set) based
on several BSS
19
802.11: Ad hoc mode
• Direct communication
within a limited range
802.11 LAN
STA1
STA3
IBSS1
STA2
IBSS2
STA5
STA4
– Station (STA):
terminal with access
mechanisms to the
wireless medium
– Independent Basic
Service Set (IBSS):
group of stations using
the same network
802.11 LAN
20
IEEE standard 802.11
fixed
terminal
mobile terminal
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
21
802.11 - Layers and functions
• PLCP Physical Layer Convergence
– access mechanisms,
fragmentation, error
control, encryption
• MAC Management
PHY
DLC
– synchronization,
roaming, MIB, power
management
LLC
MAC
MAC Management
PLCP
PHY Management
PMD
Protocol
– CCA, clear channel
assessment signal (carrier
sense)
• PMD Physical Medium Dependent
– modulation, coding
• PHY Management
– channel selection, MIB
• Station Management
Station Management
• MAC
– coordination of all
management functions
22
Outline
• Introduction to MAC
• Introduction to IEEE 802.11:
Schiller Sec 7.3.1, 7.3.2
• 802.11 Physical layer:
Schiller Sec 7.3.3
• 802.11 MAC layer:
Schiller Sec 7.3.4
• 802.11 Management:
Schiller Sec 7.3.5
23
WLAN: IEEE 802.11b
• Data rate
• Connection set-up time
– 1, 2, 5.5, 11 Mbit/s, depending
– Connectionless/always on
on SNR
• Quality of Service
– User data rate max. approx. 6
– Best effor: no delay bounds can
Mbit/s
• Transmission range
– 300m outdoor, 30m indoor
– Max. data rate ~10m indoor
• Frequency
– 2.4-5 GHz unlicensed ISMband
• Security
– Limited: Wired Equivalent
Privacy (WEP insecure ),
Service set identifier
( SSID)
• Availability
– Many products and vendors
be given for transmission
– Manageability
– Limited (no automated key
distribution, sym. Encryption)
• Pros
– Many installed systems and
vendors
– Available worldwide
– Free ISM-band
• Cons
– Heavy interference on ISMband
– No service guarantees
– Relatively low data rate
24
• Data rate
WLAN: IEEE 802.11a
– 6, 9, 12, 18, 24, 36, 48, 54
Mbit/s, depending on SNR
– User throughput (1500 byte
packets): 5.3 (6), 18 (24), 24
(36), 32 (54)
– 6, 12, 24 Mbit/s mandatory
• Transmission range
– 100m outdoor, 10m indoor
• E.g., 54 Mbit/s up to 5 m, 48 up
to 12 m, 36 up to 25 m, 24 up to
30m, 18 up to 40 m, 12 up to 60
m
• Frequency
– Free 5.15-5.25 (50mW), 5.255.35 (250mW), 5.725-5.825
GHz (1 W) ISM-band
• Security
– Limited, WEP insecure, SSID
• Availability
– Some products, some vendors
• Connection set-up time
– Connectionless/always on
• Quality of Service
– Best effort, no guarantees
(same as all 802.11 products)
• Manageability
– Limited (no automated key
distribution, sym. Encryption)
• Pros
–
–
–
–
–
Fits into 802.x standards
Free ISM-band
Available, simple system
Uses less crowded 5 GHz band
Higher data rates
• Cons
– Shorter range
25
Outline
• Introduction to MAC
• Introduction to IEEE 802.11:
Schiller Sec 7.3.1, 7.3.2
• 802.11 Physical layer:
Schiller Sec 7.3.3
• 802.11 MAC layer:
Schiller Sec 7.3.4
• 802.11 Management:
Schiller Sec 7.3.5
26
802.11: MAC layer I – DFWMAC
Distributed Foundation Wireless Media Access Control
• Traffic services
– Asynchronous Data Service (mandatory):
• exchange of data packets based on “best-effort”
• support of broadcast and multicast
– Time-Bounded Service (optional): DFWMAC- PCF
(optional)
• implemented using PCF (Point Coordination Function)
• Broadcast, multicast, and unicast
• Uses ACK and retransmission to achieve reliability for unicast
frames
• No ACK/retransmission for broadcast or multicast frames
27
802.11 MAC Layer II:
Asynchronous Data Service
• 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)
– collision avoidance via randomized “back-off“
& RTS/CTS mechanisms
– avoids hidden terminal problem
28
802.11 - MAC layer III
• Priorities
– defined through different inter frame spaces
– no guaranteed, hard priorities
– 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 IFS)
• lowest priority, for asynchronous data service
DIFS
DIFS
medium busy
CCA detects
whether medium is
free
PIFS
SIFS
direct access if
medium is free  DIFS
contention
next frame
Several nodes try to
access medium
t
29
Values of SIFS, PIFS, and DIFS
• Depend on PHY
• Defined in relation to a slot time
– Slot time: derived from medium propagation
delay, transmitter delay, and other PHY
dependent parameters
• 20 μs for DSSS
• 50 μs for FHSS
• SIFS: 10 μs (DSSS); 28 μs (FHSS)
• PIFS: SIFS plus one slot time
• DIFS: SIFS plus two slot times
30
IEEE 802.11 DCF
• DCF is CSMA/CA protocol
– Why not CSMA/CD?
• DCF suitable for multi-hop ad hoc networking
• Optionally uses RTS-CTS exchange to avoid
hidden terminal problem
– Any node overhearing a CTS cannot transmit for the
duration of the transfer
• Uses ACK to provide reliability
31
CSMA/CA: I
• CSMA/CA:
– Wireless MAC protocols often use collision
avoidance techniques, in conjunction with a
(physical or virtual) carrier sense mechanism
• Collision avoidance
– Using random backoff only or with RTS/CTS
• Nodes hearing RTS or CTS stay silent for the
duration of the corresponding transmission.
• Once channel becomes idle, the node waits for a
randomly chosen duration before attempting to
transmit.
32
CSMA/CA II: Carrier sense
• Physical carrier sense
– A Station CCA (Clear Channel Assessment) function is
used to indicate if there is traffic on the medium.
• Virtual carrier sense using Network Allocation
Vector (NAV)
– NAV is an indicator, maintained by each station, of
time periods when transmission will not be initiated
even though the station’s CCA (Clear Channel
Assessment) function does not indicate traffic on the
medium.
– A node receiving RTS sets its net allocation vector
(NAV) in accordance with the “duration field” of the
RTS frame which will cover time for CTS/DATA/ACK
frames
– NAV then specifies the earliest point at which the
station can try to access the medium again
33
Hidden Terminal Problem
A
B
C
• B can communicate with both A and C
• A and C cannot hear each other
• Problem
– When A transmits to B, C cannot detect the
transmission using the carrier sense mechanism
– If C transmits, collision will occur at node B
• Solution
– Hidden sender C needs to defer
34
Solution for
Hidden Terminal Problem: MACA
A
B
C
• When A wants to send a packet to B, A first sends a
Request-to-Send (RTS) to B
• On receiving RTS, B responds by sending Clear-to-Send
(CTS), provided that A is able to receive the packet
• When C overhears a CTS, it keeps quiet for the duration
of the transfer
– Transfer duration is included in both RTS and CTS
35
Reliability
• Wireless links are prone to errors. High
packet loss rate detrimental to transportlayer performance.
• Mechanisms needed to reduce packet loss
rate experienced by upper layers
36
A Simple Solution to
Improve Reliability
• When B receives a data packet from A, B
sends an Acknowledgement (ACK) to A.
• If node A fails to receive an ACK, it will
retransmit the packet
A
B
C
37
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
C
D
E
F
Pretending a circular range
38
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
C
D
E
F
NAV = 10
NAV = remaining duration to keep quiet
39
IEEE 802.11
CTS = Clear-to-Send
CTS
A
B
C
D
E
F
40
IEEE 802.11
CTS = Clear-to-Send
CTS
A
B
C
D
E
F
NAV = 8
41
IEEE 802.11
•DATA packet follows CTS. Successful data reception
acknowledged using ACK.
DATA
A
B
C
D
E
F
42
IEEE 802.11
ACK
A
B
C
D
E
F
43
IEEE 802.11
Reserved area
ACK
A
B
C
D
E
F
44
IEEE 802.11
Carrier sense
range
Interference
“range”
DATA
A
B
C
D
E
F
Transmit “range”
45
CSMA: Review
CSMA: listen before transmit:
1. If channel sensed idle: transmit entire pkt
2. If channel sensed busy, defer transmission
a. Persistent CSMA: retry immediately with
probability p when channel becomes idle
(may cause instability)
b. Non-persistent CSMA: after random
Backoff interval, transmit pkt otherwise
repeat 2b
46
Non-persistent CSMA:
Backoff Interval
• Backoff intervals used to reduce collision
probability
• 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
• Transmit when backoff interval reaches 0
47
DCF Example
busy
B1 = 25
B1 = 5
wait
data
data
B2 = 20
busy
cw = 31
wait
B2 = 15
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
A backoff interval is chosen in the range [0, 31]
48
Backoff Interval (continued)
• The time spent counting down backoff
intervals is a part of MAC overhead
• Important to choose CW appropriately
– large CW  large overhead
– small CW  may lead to many collisions (when
two nodes count down to 0 simultaneously)
49
Backoff Interval (Cont.)
• Since the number of nodes attempting to
transmit simultaneously may change with time,
some mechanism to manage contention is needed
• IEEE 802.11 DCF: contention window CW is
chosen dynamically depending on collision
occurrence
50
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 an upper bound)
– More collisions  longer waiting time to
reduce collision
• When a node successfully completes a
data transfer, it restores CW to CWmin
51
MILD Algorithm in MACAW,
Multiple Access with Collision Avoidance for Wireless
• MACAW
– used in Ad-hoc network
– foundation of other MAC protocols used in Wireless
Sensor Networks (WSN)
• uses exponential increase linear decrease to
update CW
– When a node successfully completes a transfer,
reduces CW by 1
– In 802.11 CW is restored to CWmin
– In 802.11, CW reduces much faster than it increases
• MACAW can avoid wild oscillations of CW when
many nodes contend for the channel
52
802.11 Overhead
Random
backoff
RTS/CTS
Data Transmission/ACK
Overhead because of:
• Backoff and (optional) RTS/CTS
handshake before transmission of data
packet
• 802.11 has room for improvement
53
802.11 – DFWMAC (review)
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 NAV as distributed via RTS and
CTS
DIFS
sender
RTS
data
SIFS
receiver
other
stations
CTS SIFS
SIFS
NAV (RTS)
NAV (CTS)
defer access
Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/
ACK
DIFS
data
t
contention
MC SS05
7.54
Fragmentation
Use shorter frames to decrease error probability of frames.
Bit error rate is the same, but now only short frames are destroyed and, the
frame error rate decreases.
DIFS
sender
RTS
frag1
SIFS
receiver
CTS SIFS
frag2
SIFS
ACK1 SIFS
SIFS
ACK2
NAV (RTS)
NAV (CTS)
other
stations
NAV (frag1)
NAV (ACK1)
DIFS
contention
data
t
Sender sends frag1, after waiting only for SIFS; Another duration value included in frag1, This
reserves medium for duration of transmitting frag2 & ACK2( also ACK1). Several nodes may receive
this reservation and adjust their NAV.
Receiver of frag1 answers after SIFS with ACK1 including reservation for next transmission.
A set of nodes may receive this reservation and adjust their NAV
frag2 is last fragment; Sender does not reserve the medium; Receiver acknowledges frag2, not
reserving the medium. After ACK2, all nodes compete for medium after having waited for DIFS
55
DFWMAC-PCF
• DFWMAC-DCF & DFWMAC-DCF w/
RTS/CTS
– cannot guarantee a maximum access delay or
minimum transmission bandwidth
• DFWMAC-PCF
– provides time-bounded service
– an access point controls medium access and
polls nodes
– point coordinator in access point splits the
access time into super frame periods
56
DFWMAC-PCF I
t0 t1
medium busy PIFS
point
coordinator
wireless
stations
stations‘
NAV
SuperFrame
SIFS
D1
SIFS
SIFS
D2
SIFS
U1
U2
NAV
57
DFWMAC-PCF II
t2
point
coordinator
wireless
stations
stations‘
NAV
D3
PIFS
SIFS
D4
t3
t4
CFend
SIFS
U4
NAV
contention free period
contention
period
If only PCF is used and polling is distributed evenly,
the bandwidth is also distributed evenly among all
polled nodes.
t
58
802.11: MAC Frames Addressing
infrastructure
network
BSS1
AP
AP
MAC frames
can be
transmitted
between:
AP: Access Point
wired network
BSS2
mobile
stations
AP
mobile
stations and
an access
point
BSS3
ad-hoc networks
access points
over a DS
IBSS2
IBSS1
59
• Types
802.11 - Frame format
– control frames, management frames, data frames
• Sequence numbers
– important against duplicated frames due to lost ACKs
• Addresses
– Sender, receiver, BSS identifier
• Miscellaneous
– duration time, checksum, frame control, data
bytes
2
2
6
6
6
2
6
Frame Duration/ Address Address Address Sequence Address
Control
ID
1
2
3
Control
4
bits
2
2
4
1
1
1
1
1
1
1
0-2312
4
Data
CRC
1
Protocol
To From More
Power More
Type Subtype
Retry
WEP Order
version
DS DS Frag
Mgmt Data
60
Duration/ID
• If the field value is less than 32,768
(1000 000 0000 0000)2
– the duration field contains the value indicating
the period of time in which the medium is
occupied (in μs). This field is used for setting
the NAV for the virtual reservation
mechanism using RTS/CTS and during
fragmentation.
– Certain values above 32,768 are reserved for
identifiers.
61
Type & Subtype Fields
• Type
–
–
–
–
00: management frame
01: control frame
10: data frame
11 reserved
• Subtype
– Examples
• for 00 frames : 0000 association request, 1000
beacon
• for 01 frames: 1011 RTS, 1100 CTS
• for 10 frames: 0000 User data
62
MAC address format
scenario
ad-hoc network
infrastructure
network, from AP
infrastructure
network, to AP
infrastructure
network, within DS
to DS from
DS
0
0
0
1
address 1 address 2 address 3 address 4
DA
DA
SA
BSSID
BSSID
SA
-
1
0
BSSID
SA
DA
-
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
63
Special Frames: ACK, RTS, CTS
• Acknowledgement
• Request To Send
RTS
bytes
2
2
6
Frame
Receiver
Duration
Control
Address
ACK
CRC
bytes
2
2
6
6
Frame
Receiver Transmitter
Duration
Control
Address Address
bytes
• Clear To Send
4
CTS
2
2
6
Frame
Receiver
Duration
Control
Address
4
CRC
4
CRC
64
Outline
• Introduction to MAC
• Introduction to IEEE 802.11:
Schiller Sec 7.3.1, 7.3.2
• 802.11 Physical layer:
Schiller Sec 7.3.3
• 802.11 MAC layer:
Schiller Sec 7.3.4
• 802.11 Management:
Schiller Sec 7.3.5
65
802.11 - MAC management
• Roaming
– Functions for joining a network (association), changing
access points, scanning (i.e. active search) for access points.
• Synchronization
– Functions to support finding a wireless LAN,
synchronization of internal clocks, generation of beacon
signals
• timing
• Power management
– sleep-mode without missing a message
– periodic sleep, frame buffering, traffic measurements
• MIB - Management Information Base
– Stores parameters representing current state of wireless
stations and access point
– Accessed via standardized protocols (e.g. SNMP)
66
Association and Reassociation
• Integration into a LAN
• Scanning: find a network to connect
• Roaming: change networks by changing access points
67
Scanning
• Goal: Find a network to connect
• Passive scanning
– Does not require transmission by station
– Move to each channel, and listen for Beacon frames
• Active scanning
– Requires transmission by station
– Move to each channel, and send Probe Request frames
to solicit Probe Responses from a network
68
Association in 802.11
1: Association request
2: Association response
3: Data traffic
Client
AP
69
Reassociation in 802.11
1: Reassociation request
3: Reassociation response
5: Send buffered frames
Client
6: Data traffic
New AP
2: verify
previous
association
Old AP
4: send
buffered
frames 70
802.11 - Roaming
• No or bad connection? Then perform:
• Scanning
– scan the environment, i.e., listen into the medium for beacon
signals or send probes 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
71
Synchronization using a Beacon
(infrastructure)
beacon interval
access
point
medium
B
B
busy
busy
B
busy
B
busy
t
value of the timestamp
Synchronization needed for:
1. power management,
2. coordination of PCF
3. synchronization of hopping sequence
B
beacon frame
beacon contains
timestamp & management
information used for power
management and roaming
Timestamp is used by nodes to
adjust local clocks
72
Synchronization using a Beacon
(ad-hoc)
beacon interval
station1
B1
B1
B2
station2
medium
busy
busy
B2
busy
busy
t
value of the timestamp
B
beacon frame
random delay
73
Power Management : Basic Idea
• Idea: switch the transceiver off if not needed
• Two states for a station: sleep and awake
– If a sender intends to communicate with a sleeping
station, it buffers data
– Sleeping station wakes up periodically and stay awake
for a certain time
– During this time, all senders announce the destinations
of their buffered data frames
– If a station detects that it is a destination of a
buffered packet it has to stay awake until the
transmission takes place.
• Waking up at the right moment requires the
timing synchronization function (TSF).
– stations wake up at the same time
74
Power management (continued)
• Infrastructure
– AP buffers frames destined for stations in
power-save mode
– With every beacon, a traffic indication map
(TIM) is transmitted by AP
• TIM contains a list of stations for which unicast
frames are buffered
– Delivery Traffic Indication Map (DTIM)
• list of broadcast/multicast receivers; transmitted
by AP
75
Power management (continued)
• 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?)
76
Power saving with wake-up
patterns (infrastructure)
TIM interval
access
point
DTIM interval
D B
T
busy
medium
busy
T
d
D B
busy
busy
p
station
d
t
T
TIM
D
B
broadcast/multicast
DTIM
awake
p PS poll
d data transmission
to/from the station
AP transmits a beacon frame each beacon interval which is the same as the TIM interval
AP maintains a delivery traffic indication map (DTIM) interval for broadcast/multicast frames
77
DTIM interval is always a multiple of TIM interval
Power saving with wake-up
patterns (ad-hoc)
ATIM
window
station1
beacon interval
B1
station2
A
B2
B2
D
a
B1
d
t
B
beacon frame
awake
random delay
a acknowledge ATIM
A transmit ATIM
D transmit data
d acknowledge data
All stations announce a list of buffered frames during a period when they are all awake
Destinations announced using ATIMs
78
Announcement period is called the ATIM window
IEEE 802.11 further developments
•
802.11i: Enhanced Security Mechanisms
•
802.11j: Extensions for operations in Japan
•
802.11k: Methods for channel measurements
•
•
802.11m: Updates of the 802.11 standards
802.11n: Higher data rates above 100Mbit/s
•
802.11p: Inter car communications
– Enhance the current 802.11 MAC to provide improvements in security.
– TKIP enhances the insecure WEP, but remains compatible to older WEP systems
– AES provides a secure encryption method and is based on new hardware
– Changes of 802.11a for operation at 5GHz in Japan using only half the channel
width at larger range
– Devices and access points should be able to estimate channel quality in order to
be able to choose a better access point of channel
– Changes of PHY and MAC with the goal of 100Mbit/s at MAC SAP
– MIMO antennas (Multiple Input Multiple Output), up to 600Mbit/s are currently
feasible
– However, still a large overhead due to protocol headers and inefficient
mechanisms
– Communication between cars/road side and cars/cars
– Planned for relative speeds of min. 200km/h and ranges over 1000m
– Usage of 5.850-5.925GHz band in North America
79
IEEE 802.11 further developments
•
802.11c: Bridge Support
•
802.11d: Regulatory Domain Update
•
802.11e: MAC Enhancements – QoS
•
802.11f: Inter-Access Point Protocol
•
802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM
•
802.11h: Spectrum Managed 802.11a
– Definition of MAC procedures to support bridges as extension to 802.1D
– Support of additional regulations related to channel selection, hopping sequences
– Enhance the current 802.11 MAC to expand support for applications with Quality
of Service requirements, and in the capabilities and efficiency of the protocol
– Definition of a data flow (“connection”) with parameters like rate, burst, period…
– Additional energy saving mechanisms and more efficient retransmission
– Establish an Inter-Access Point Protocol for data exchange via the distribution
system
– Currently unclear to which extend manufacturers will follow this suggestion
– Successful successor of 802.11b, performance loss during mixed operation with
11b
– Extension for operation of 802.11a in Europe by mechanisms like channel
measurement for dynamic channel selection (DFS, Dynamic Frequency Selection)
and power control (TPC, Transmit Power Control)
80
IEEE 802.11 further developments
•
802.11r: Faster Handover between BSS
–
–
–
Secure, fast handover of a station from one AP to another within an ESS
Current mechanisms (even newer standards like 802.11i) plus incompatible devices from
different vendors are massive problems for the use of, e.g., VoIP in WLANs
Handover should be feasible within 50ms in order to support multimedia applications
efficiently
•
802.11s: Mesh Networking
•
802.11t: Performance evaluation of 802.11 networks
•
•
802.11u: Interworking with additional external networks
802.11v: Network management
•
802.11w: Securing of network control
•
•
Note: Not all “standards” will end in products, many ideas get stuck at working group
Info: www.ieee802.org/11/, 802wirelessworld.com, standards.ieee.org/getieee802/ 81
–
–
Design of a self-configuring Wireless Distribution System (WDS) based on 802.11
Support of point-to-point and broadcast communication across several hops
–
Standardization of performance measurement schemes
–
–
Extensions of current management functions, channel measurements
Definition of a unified interface
–
Classical standards like 802.11, but also 802.11i protect only data frames, not the control
frames. Thus, this standard should extend 802.11i in a way that, e.g., no control frames can
be forged.