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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

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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.

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Software and HW Access Point

HW Access Point

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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.

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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

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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.

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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.

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Extension Point

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Roaming

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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

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LAN to LAN Wireless Communication

Each Access Point acts as a Router or Bridge to connect its own LAN to the wireless network

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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

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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

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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

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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

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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.

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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

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Comparison: infrastructure vs. ad-hoc networks

Infrastructure network

ad-hoc network

APAP

AP

wired network

AP: Access Point

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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.

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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.

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DFWMAC-PCF I

PIFS

stations‘NAV

wirelessstations

point coordinator

D1

U1

SIFS

NAV

SIFSD2

U2

SIFS

SIFS

SuperFramet0

medium busy

t1

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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.

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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.

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DFWMAC-PCF II

tstations‘NAV

wirelessstations

point coordinator

D3

NAV

PIFSD4

U4

SIFS

SIFSCFend

contentionperiod

contention free period

t2 t3 t4

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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

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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)

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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 -

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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.

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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.

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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

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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

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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.

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Synchronization using a Beacon (infrastructure)

beacon interval

tmedium

accesspoint

busy

B

busy busy busy

B B B

value of the timestamp B beacon frame

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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.

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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

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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.

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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?)

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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.

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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

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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.