lecture 13
DESCRIPTION
mobile communicationTRANSCRIPT
1
Wireless LANCharacteristics IEEE 802.11
PHY MAC Roaming .11a, b, g, h, i …
HIPERLAN Bluetooth / IEEE
802.15.x IEEE
802.16/.20/.21/.22 RFID Comparison
2
Wireless LAN Components
The WLAN has the following configuration:
Access Point : Connects to the wired network from a fixed location via an
Ethernet cable Receives, transmits information from mobile devices such as
laptops, PDAs etc and the wired infrastructure network. A single access point can function anywhere in the range of 30
metres to several hundred feet.
WLAN Adapters: The mobile devices communicate with the operating system via
the WLAN Adapters(radio Network Interface Cards NIC), ISA or PCA adapters for desk top computers.
3
Software and HW Access Point
HW Access Point
4
SW Access Point - Advantages
does not limit the type or number of network interfaces you use.
allows considerable flexibility in providing access to different network types, such as different types of Ethernet, Wireless and Token Ring networks.
5
Range of Access Point
Typical indoor ranges are 150-300 feet, but can be shorter if the building construction interferes with radio transmissions. Longer ranges are possible, but performance will degrade with distance.
Outdoor ranges are quoted up to 1000 feet, but again this depends upon the environment.
There are ways to extend the basic operating range of Wireless communications, by using more than a single access point or using a wireless relay /extension point
6
No. of users on an Access Point
This depends upon the manufacturer. Some hardware access points have a recommended limit of 10, with other more expensive access points supporting up to 100 wireless connections. Using more computers than recommended will cause performance and reliability to suffer.
Software access points may also impose user limitations, but this depends upon the specific software, and the host computer's ability to process the required information.
7
Multiple Access Points
multiple access points can be connected to a wired LAN, or sometimes even to a second wireless LAN if the access point supports this.
In most cases, separate access points are interconnected via a wired LAN, providing wireless connectivity in specific areas such as offices or classrooms, but connected to a main wired LAN for access to network resources, such as file servers.
8
Extension Point
9
Roaming
10
Roaming
A wireless computer can "roam" from one access point to another, with the software and hardware maintaining a steady network connection by monitoring the signal strength from in-range access points and locking on to the one with the best quality. Usually this is completely transparent to the user; they are not aware that a different access point is being used from area to area. Some access point configurations require security authentication when swapping access points, usually in the form of a password dialog box.
Access points are required to have overlapping wireless areas to achieve this
*** NOT ALL ACCESS POINTS SUPPORT ROAMING
11
LAN to LAN Wireless Communication
Each Access Point acts as a Router or Bridge to connect its own LAN to the wireless network
12
Mobile Communication Technology according to IEEE
Local wireless networksWLAN 802.11
802.11a
802.11b802.11i/e/…/w
802.11g
WiFi802.11h
802.15.4
802.15.1 802.15.2
Bluetooth
802.15.4a/bZigBee
802.15.3
802.20 (Mobile Broadband Wireless Access)
+ Mobility
WiMAX
802.15.3a/b
802.15.5
13
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 like, 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
many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE 802.11)
products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e.g., IMT-2000
14
Design goals for wireless LANs
global, seamless operation low power for battery use no special permissions or 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 (no one should be able to read my data), privacy (no
one should be able to collect user profiles), safety (low radiation)
transparency concerning applications and higher layer protocols, but also location awareness if necessary
15
Comparison: infrared vs. radio transmission
Infrared uses IR diodes, diffuse light, multiple reflections (walls,
furniture etc.). Photo diodes act as receivers. Advantages
simple, cheap, available in many mobile devices no licenses needed simple shielding possible
Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light low bandwidth
Example IrDA (Infrared Data Association) interface available
everywhere
16
Paper : An Adhoc Network system based on Infra Red Communication
The network should solve the following problems:•Route maintenance How to maintain routes between the mobile hosts.• Host enumeration How to count up (and identify) the partici- pants(mobile hosts) of the network.
17
Comparison: infrared vs. radio transmission
Radio typically using the license free ISM band at 2.4 GHz
Advantages experience from wireless WAN and mobile phones can be
used coverage of larger areas possible (radio can penetrate walls,
furniture etc.) Disadvantages
very limited license free frequency bands shielding more difficult, interference with other electrical
devices Example
Many different products
18
Comparison: infrastructure vs. ad-hoc networks
Infrastructure network
ad-hoc network
APAP
AP
wired network
AP: Access Point
19
Comparison: infrastructure vs. ad-hoc networks
A very good coordination is required between the medium access of wireless nodes and access points. Else, collisions can occur (infrastructure networks)
If the access points control the medium access of individual terminals, collisions can be largely minimized.
20
Complexity with Adhoc Networks
Each node has to implement : Medium Access Mechanisms to handle hidden and exposed problems Priority Mechanisms
Greatest Advantage : Flexibility in installation and configuration
21
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 frequencyAccess 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
22
802.11 - Architecture of an infrastructure network
ESS has its own ESSID ESSID distinguishes different networks If a node wants to participate in the network, it has to
know the ESSID for WLAN communication. The access points get connected to the network via
a portal. IEEE 802.11f specifies the inter-Access Point
communication protocols.
23
802.11 - Architecture of an ad-hoc network
Direct communication within a limited range Station (STA):
terminal with access mechanisms to the wireless medium
Independent Basic Service Set (IBSS):group of stations using the same radio frequency802.11 LAN
IBSS2
802.11 LAN
IBSS1
STA1
STA4
STA5
STA2
STA3
24
IEEE standard 802.11
mobile terminal
access point
fixedterminal
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
infrastructurenetwork
LLC LLC
25
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
MIB : Management Information Base
26
802.11 – Physical Layer
Physical Layer
PLCP : Physical Layer Convergence Protocol
-Provides carrier sense signal
-Provides a common physical service access point(PSAP) independent of transmission technology
-PMD (Physical Medium Dependent Sublayer)
- Modulation
-Encoding-Decoding
27
802.11 Management Layer
MAC Management Provides: Association and re-associaltion of a station to an access point. Roaming (handover) between access points. Authentication Encryption Synchronization Power Management
28
802.11 - Physical layer (classical)
3 versions: 2 radio (typ. 2.4 GHz), 1 IR data rates 1 or 2 Mbit/s
FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s min. 2.5 frequency hops/s (USA), two-level GFSK modulation
DSSS (Direct Sequence Spread Spectrum) 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
29
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 rates of payload (1 or 2 Mbit/s) 0000 : 1 Mbps.
0010 : 1.5 Mbps (500 kbps granularity) HEC (Header Error Check)
CRC with x16+x12+x5+1
30
DSSS PHY packet format
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
synchronization SFD signal service HEC payload
PLCP preamble PLCP header
128 16 8 8 16 variable bits
length
16
31
PHY : DSSS
Sync SFD signal service length HEC Payload
128 16 8 8 16 16 Variable
Uses 11-chip Barker Code : +1 -1 +1 +1 -1 +1 +1 +1 -1 -1 -1
Start Frame Delimiter(SFD) : 1111001110100000
Signal : Data Rate of Payload 0x0A : 1Mbps; 0x14 : 2Mbps
Service : Future use; 0x00 : indicates IEEE 802.11 compliant
Length(16 bits) : Length of payload
HEC; Header error check : CRC 16 polynomial for signal, service and length fields
Payload
32
802.11 - MAC layer I – DFWMAC (Distributed Foundation Wireless Medium Access)
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){Distributed Coordination Function}
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 (Point Coordination Function-optional) access point polls terminals according to a list
33
Medium Access
Medium Busy Contention next frame
Time
DIFS
Direct Access if “Medium is Free” >= DIFS
DIFS
PIFS
SIFS
Short Interframe Spacing (SIFS) : Highest priority, acks, polling responses
PCF Inter-frame spacing (PIFS): medium priority, time bounded service
DCF Inter-frame Spacing (DIFS) : Asynchronous data service within a contention period – lowest priority
34
Medium Access
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, Distributed Coordination Function IFS)
lowest priority, for asynchronous data service
t
medium busySIFS
PIFS
DIFSDIFS
next framecontention
direct access if medium is free DIFS
35
Basic DFWMAC-DFC using CSMA/CA
Medium Busy Next Frame
DIFS
Slot time
Contention window
• A mobile device waits for DIFS and if the medium is free after DIFS, it accesses the medium. So, the medium is busy.
• Once the above device releases the resources in the medium, it waits for DIFS. The contention period starts. A few devices start their random back off timer and the countdown of the timers start.
• whichever device that completes the timer back off time first will get access to the medium.
•As soon as the device senses that the medium is busy, it loses the chance for this cycle and has to try after DIFS duration.
• Now, the backoff time is initialized for the rest of the devices and they start all over again after DIFS.
DIFS
36
Basic DFWMAC-DFC using CSMA/CA
ISSUE WITH THE ABOVE SCHEME:
A node will not have a priority once it has lost the chance. Irrespective of the amount of wait in the last cycle, it has to start all over again.
37
Medium Access Priorities
Short Interframe Spacing (SIFS) : Highest priority, acks, polling responses
PCF Inter-frame spacing (PIFS): medium priority, time bounded service
DCF Inter-frame Spacing (DIFS) : Asynchronous data service within a contention period – lowest priority
38
802.11 - competing stations - simple version(for broadcast)
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
39
802.11 - competing stations - simple version
St-3 has the first request and sends the packet. St-3 senses the medium, waits for DIFS and accesses the medium.
Stns 1,2 & 5 have to wait for at least DIFS after Stn-3 stops sending the data.
All three stations now start off a back off timer and start counting down their back off timers.
Back off time = Elapsed back off time + residual back off time.
@@ It is to be noted that if the residual time of device-1 is more than that of device-2, it means that device-1 had waited for a lesser time as compared to device-2 and so, device-2 gets a priority to access the medium.
40
802.11 - competing stations - simple version
Stn-2 gets an access since its backoff time is the least. The back off timers for Stns 1 & 5 stop and stns store
residual backoff times. Now Stn 4 wants to access the medium. In all, three stans
are trying to acess the medium. Since 4 & 5 have the same backoff time, they result in
collision. Transmitted Frames are destroyed
Stn-1 finally gets access to the medium.Stns 4 & 5 still have to contend in the next
cycle.
41
802.11 - competing stations - simple version
Problem with this scheme:
If the contention window is small, too many stns will contend and so, collisions will be substantial.
If the contention window is larger, there will be noticeable delays.
42
802.11 - CSMA/CA access method II (for unicast)
t
SIFS
DIFS
data
ACK
waiting time
otherstations
receiver
senderdata
DIFS
contention
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 (No ACK is sent) But sender has to wait for the medium access. No special privileges for retransmitted data. No. of retransmissions are limited.
43
802.11 – DFWMAC Hidden Terminal Avoidance using RTS & CTS)
t
SIFS
DIFS
data
ACK
defer access
otherstations
receiver
senderdata
DIFS
contention
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
Sending unicast packets station can send RTS with reservation parameter after waiting for DIFS (RTS specifies receiver’s Id,
amount of time needed for transmission of data and also time for ACK ) acknowledgement via CTS after SIFS by receiver (if ready to receive) (all stns receive this) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS. Other stns have to set ‘Net
allocation Vector(NAV)’ (contained in CTS) that specifies how long they need to wait before trying again for transmission.
44
Fragmentation
If frames of larger sizes are transported, any bit error will corrupt the entire frame and so, frame errors increase.
Hence, it is advantageous to consider shorter frame lengths so as to minimize frame errors.
45
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
46
Fragmentation
While transmitting frag1, one more duration is also transmitted corresponding to the duration of the following fragment and the acknowledgement.
Thus the medium is reserved for the following fragment(frag-2) Other nodes which receive this will adjust their NAV If there is no network change (static network), the set of nodes receiving
this duration is the same as that indicated in the original RTS control packet.
Because of mobility, this is not the case in most situations.
The receiver will receive frag1 and send an ACK1 that contains duration of the net fragment transmission.
The other set of nodes will adjust their NAV Thus, the current fragments would contain info about the following ones.
47
DFWMAC-PCF I
PIFS
stations‘NAV
wirelessstations
point coordinator
D1
U1
SIFS
NAV
SIFSD2
U2
SIFS
SIFS
SuperFramet0
medium busy
t1
48
DFWMAC-PCF I (Point Coordination Function -Polling)
To achieve a time bounded service, the PCF is used on top of the DCF. It requires an access point. The access point splits time into super frame periods.
Super Frame : contention-free period + contention period.(i) In the scheme above, contention-free period should ideally start at to. But the
medium is busy till t1.(ii) Actually, PCF has to wait for PIFS before accessing the medium. As PIFS is
less than DIFS, no other stn can send the data earlier than PCF.(iii) PCF sends data to stn-1(polling starts)(iv) Stn-1 responds after SIFS.(v) After waiting for SIFS, the PCF sends data to Stn-2. Stn-2 answers with U2.(vi) Polling continues for D3 but D3 has no data.(vii) Finally, after polling all stns, the PCF can send a ‘End Marker (CFend)
indicating that the contention period can start all over.(viii) Using PCF automatically sets NAV and prevents other stns from sending data.
49
DFWMAC-PCF I (Point Coordination Function -Polling)
The Process of polling with PCF is exactly like TDMA where all users get a fair and equal chance to send data.
If a node does not send data, it is an overhead.
50
DFWMAC-PCF II
tstations‘NAV
wirelessstations
point coordinator
D3
NAV
PIFSD4
U4
SIFS
SIFSCFend
contentionperiod
contention free period
t2 t3 t4
51
802.11 - Frame format
FrameControl
Duration/ID
Address1
Address2
Address3
SequenceControl
Address4
Data CRC
2 2 6 6 6 62 40-2312bytes
Protocolversion
Type SubtypeToDS
MoreFrag
RetryPowerMgmt
MoreData
WEP
2 2 4 1
FromDS
1
Order
bits 1 1 1 1 1 1
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
52
802.11 - Frame format
Frame control : 2 bytes Protocol ver : 2 bits, starts from zero
Type : 2 bits (00: Management, 01: control, 10: Data, 11: Reserved) Sub types: Ex., management functions 0000 : Association request 1000 : Beacon 1011 : RTS 1100 : CTS
Duration ID : 2 Bytes period for which the medium is occupied(used for setting NAV)
Addresses 1 to 4:contain MAC addresses (to address later)
53
802.11 - Frame format
Sequence Control : 2 Bytes Used to filter duplicates
Data : 0 to 2312 bytesChecksum(CRC) : 32 bits To protect from frame errorsTo DS/From DSShows How MAC frames are being
transmitted.
To DS From DS Address1 Address 2 Address 3 Address 4
0 0 Distribution address (DA)
Source Address (SA)
Basic Service Set Id (BSSID)
-
0 1 DA BSSID SA -
1 0 BSSID SA DA -
1 1 Receiver Address(RA)
Transmitter Address(TA)
DA -
54
802.11 - Frame format
Address 1: Physical receiver of the frame
Address 2: Physical transmitter of a frame
Addresses 3 & 4 : Logical assignment of frames.
More Fragments : Set to 1 if more fragments are to follow.
RETRY If the current frame is a duplicate because of retransmissions, this
field is set to 1.
55
802.11 - Frame format
Power Management 0 : Stn is in stand-by mode
1 : stn stays active
More Data : To indicate to the receiver that more data is to follow. To indicate to the stns that are in sleep mode that more data is to follow. To indicate to an access point that more polling is required.
Wired Equivalent Privacy (WEP) :
To indicate that standard 802.11 security algorithm is used.
Order :
If set to 1, frames have to be taken strictly in the same order that they are received.
56
MAC address format
scenario to DS fromDS
address 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
57
Special Frames: ACK, RTS, CTS
Acknowledgement
Request To Send
Clear To Send
FrameControl
DurationReceiverAddress
TransmitterAddress
CRC
2 2 6 6 4bytes
FrameControl
DurationReceiverAddress
CRC
2 2 6 4bytes
FrameControl
DurationReceiverAddress
CRC
2 2 6 4bytes
ACK
RTS
CTS
58
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 All parameters concerning the present state of the wireless stn and
access point are stored in MIB. These can be accessed via a protocol line SNMP.
59
Synchronization using a Beacon (infrastructure)
beacon interval
tmedium
accesspoint
busy
B
busy busy busy
B B B
value of the timestamp B beacon frame
60
Synchronization using a Beacon (infrastructure)
802.11 specifies a Timing and Synchronization Function (TSF). It is needed for PCF and for power management.
Needed for Synchronization of hopping sequence for all nodes etc.
Within a BSS, timing is conveyed by (quasi) periodic transmission of timing frame called “Beacon (contains time stamp and other management functions”.
Nodes need to hear the beacons and adjust the timing. But nodes need not adjust to every beacon.
If the medium is busy, access point may not be able to send the beacons on certain occasions. Beacon intervals are not shifted if a beacon is missed.
61
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
62
Synchronization using a Beacon (ad-hoc)
No Access Point Each node maintains a sync timer and starts sending to the rest
of the nodes. It uses a standard back-off algorithm and and only one beacon
wins. All other stns adjust their internal clock as per the received
beacon. They suppress their beacons for this cycle. If there is a collision, the beacon is lost. In this situation, beacon
intervals can be slightly shifted and the following cycles synchronizes all stns.
63
Power management
Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF)
For sender, it is not an issue as the transmitter knows when it is ready for sending frames. Transmitter has to buffer the frame to make sure that it will transmit when the receiver is ready to
receive. stations wake up at the same time periodically and listen to the transmitter. Waking up at the right time needs the TSF. Along with beacon, a Traffic Indication Map(TIM- containing the list of stns for which buffering has been done in the AP.) is also sent.
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?)
64
Power saving with wake-up patterns (infrastructure).........Only One Station Shown...........
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
65
Power saving with wake-up patterns (infrastructure)
All stations wake up prior to TIM/DTIM.
CASE WITH TIM : Access Point buffers frames when the receiver is in the sleep mode. With every beacon, a Traffic Information Map (TIM-containing the list of
stations for which uni-cast buffers are stored in AP) is sent. AP sends a broadcast frame and the receiver stays awake to receive it. Receiver then sleeps and wakes up just before the next TIM. TIM is delayed since the medium is busy. So, the receiver stays awake. AP has nothing to send and so, the receiver goes to sleep. In the next TIM interval, the AP indicates that the stn is the destination
for a buffered frame. Stn answers with a PS poll and stays awake to receive data. In the next DTIM interval, the AP has more broadcast data to send.
This is deferred since medium is busy.
66
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
ATIMwindow beacon interval
a acknowledge ATIM d acknowledge data
67
Power saving with wake-up patterns (ad-hoc)
Each participating station has to buffer the data since access points do not exist.
In the period that all stations are awake, all the participating station send a list of buffered frames and the stations that are targeted to receive these. These are sent through “Adhoc Traffic Information Map(ATIM)”
All stations stay awake during this ATIM period and listen to the ATIM.
In the example, ATIM of station-1 contains the address of station-2. Stn-2 acknowledges the ATIM, waits for the data and later
acknowledges the data. With more stations wanting to send their frames, collisions can be
substantial. Access delay is not easy to predict and so, QoS can’t be guaranteed.