Module 3 –Wireless Technology

Ref :- Mobile Communication by JOCHEN SCHILLER

Forming Wireless Networks

• Two major transmission methods are used.• Infra Red and Radio waves.• Infra red devices standardized by Infrared data

association IrDA• Radio transmission works in License free band

of 2.4Ghz.• IEEE 802.11 (Wifi) ,Bluetooth etc works in this

spectrum.

InfraRed

• Makes use of Infrared lights and sensors.• Use Line of Sight transmission.• Was cheaper and easy to build.• Signals could not penetrate thick walls and

obstacles.• Speed varied from 11 kbits/sec to 4 Mbits/Sec.• Occasional interference issues with other light

forms.

Radio waves

• Does not rely on Line of sight.• Can penetrate most common obstacles.• Offered speed from 54Mbits/Sec.• Easy interference with other radio transmission schemes.

• Infrared devices faded off the scene owing to serious performance limitations.

Infrastructure V/S Adhoc

• Wireless connections can be made in two ways.

• One by using special devices to handle various functionalities.

• Other by which devices add among themselves.

Infrastructure based

• Special devices known as access points are present.

• Access points perform complex tasks like access control, flow regulation etc.

• Communication happens in between nodes and Access Points.

• One end of Access points are physically connected to a Wired Network.

Contd..

• The computing devices may not include complex functionalities.

• But Infrastructure devices will fail to deliver if Infrastructure is damaged.

AdHoc Networks

• Each node communicate with each other.• Nodes themselves have to take care of

complex functionalities like access control.• Devices need to be in the radio range of one

another.

Things to Think

• Is Data visible to every other node ??• What will happen if two nodes transmit at the

same time?• Does access points have physical address?

IEEE 802.11 Wireless LAN

• Standardized by IEEE in 1999.• Variations are prescribed mostly based on the

modulation used.

IEEE 802.11 System architecture

• Supports Adhoc and infrastructure mode.• Involves access points and stations.• Stations and Access points in the same Radio

area forms Basic Service Set.• The overall network now is called Extended

service set.

Protocol architecture• IEEE 802.11 standard covers protocols for Physical

layer and Medium Access Layer.• Physical layer is subdivided into Physical Layer

Convergence Protocol [PLCP]• Physical medium dependant (PMD) sub layer.• PLCP is the access point and PMD deals with

modulation, encoding/Decoding..

PHY layers

• Frequency Hopping Spread Spectrum(FHSS)• Usage of different frequencies in a range.• May use z7 + z4 + 1 pattern to scramble payload.

• Synchronization :- 80 bits of 0101….• Start frame delimiter :- 0000110010111101 • PLCP PDU Length word PLW:- Length of

payload including CRC.• PLCP Signaling field :- 4 bits indicate speed • 0000 indicates 1 Mbits/s ,1111 indicate 8.5

Mbits/s• Header error check (HEC)

Direct Sequence Spread Spectrum

• In DSSS we use 11 chip barker sequence to alter the signal form.

• For 1 Mbps it uses differential binary phase shift keying

• For 2 Mbps it uses differential quadrature phase shift keying.

• PLCP part of the frame is always transmitted at 1 Mbps

• Signal ->0X0A indicates 1 Mbps ,0X14 2 Mbps• SFD -> 1111001110100000..• Service -> Not used.• Rest is ‘Classical’

802.11 MAC Layer

• Medium Access control layer has give access to the media.

• It also has to handle roaming and energy conservation.

• Methods are defined to avoid nodes competing with each other for the media.

Classical Medium access Problems in IEEE 802.11

• Hidden Node problem

• Exposed node problem

IEEE 802.11 MAC

• MAC defines two basic functions .• Distributed Coordination Function (DCF).• Point Coordination Function (PCF).• Has also defined 3 time type of time slots.• Short Interframe spacing(SIFS)- Shortest time frame

for ACK’s short messages etc.• PCF interframe spacing (PCF) – Medium time frame,

> SIFS• DCF interframe spacing (DISF)- Longest waiting time.

DCF• DCF is defined in two ways.DCF with CSMA/CA and DCF with RTS

and CTS.• DCF with CSMA/CA• CSMA/CA also uses exponential back off time.• Rules.• 1.Sense if the medium is free• 2.If free wait for DIFS time before transmitting.• 2.1 If again busy then go for exponential waiting • Else Transmit.• 2.3 If waiting time expires • 2.3.1 If medium free transmit else select a timer and wait.

• In step 2.3.1 it may be noted that another timer is selected second time so as to avoid another large waiting time.

• In other words nodes which were waiting need to be given a priority.

• DCF with RTS and CTS• Every node which wants to send a data has to

inform the receiver using Ready to Send message.• RTS include an estimate of time duration required

and other parameters.• The receiver if ready will send back Clear to

Send ,again including the time frame required .• This RTS and CTS will enable the neighboring

nodes not to interfere.• They set their Net Allocation Vector (NAV)

accordingly.

Point coordination Function

• PCF uses the access point as the main party.• The access points polls each station only after

waiting for PIFS time.• If the stations wants to send anything they

wait SIFS time and sends.

• Remember that every transmission happens between AP and nodes.

MAC Header.

• More fragments – Set if original data was fragmented and more to follow.

• Retry -> Set to 1 if it is a retry of old attempt.• Power mgmt -> set to 1 if its going to power saving

mode.• More data-> if the sender wants to send more

data.• WEP -> Set if WEP is used.• Order-> set to 1 if receiving order is important.

• To DS and From DS -> Set if the data has to be send to the distributed system or if its coming from DS.

• If both are 0 -> Adr1 is destn and Adr 2 source.• If both are 1 -> Adr1 is final destn and Adr2 is

immediate sender, Adr3 -> intermediate destn,Adr4 -> original source.

Other MAC functions

• Synchornization -> Synchronization of clocks • Power management • Roaming facilities

• All the informations of an AP or a node is saved in a Management Information Base(MIB).

Synchronization

• Special frames known as Beacon frames are used.• Beacon frames are send out periodically after

specific time.• In infrastructure based wifi, Beacons are send out

by the AP.• In Adhoc each node maintains its own timing

details and will transmit it periodically.• If collision backoff algorithm is used.

Power management

• Nodes and AP’s can be in awake state or sleep state.

• Needs to provide buffering so that data is not lost.

• All the stations are synchronised to wake up at the same time.

• In infrastructure based -> AP buffers all the frames

• With beacon signals ,Traffic indication map(TIM) is send out to all.

• TIM contains list of all the destinations .• Those will stay awake • Another map is the Delivery TIM for broadcast

/Multicast frames.• DTIM takes double time of TIM

• In the case of Adhoc wifi, buffering is done at individual nodes.

• Since all the nodes wake up at the same time Adhoc TIM is send by each node .

• If collision happens ,backoff random time.

Roaming

• It’s the switching of weak signal BSS to high signal BSS.

• Has to be performed seamlessly.• Mainly applicable in infrastructure based.• After identifying the best AP,association request

has to be sent.• New AP responds with Association response.• Now the AP will inform the DS about the change.

Bluetooth

• Uses 2.4 GHz spectrum.• Operates in 79 channels separated by 1 Mhz.• Devices use frequency hopping.• Bluetooth devices form a small network

known as PICONET.• One node will be master followed by 7 slaves

at any time (Maximum 7).

• Some other devices can be designated as Parked (Max 200)

• They do not participate in any transfer but may be included.

• All other devices will be in Standby Mode.• All slaves are given 3 bit Active member

address while parked devices are given Parked Member address

• The master and slaves synchronize their clocks.

• They also follow a same Hopping Sequence.• The master and slaves are having similar

capabilities.• One who establishes the piconet becomes the

master.• Piconets can join to form scatternet.• But the hopping sequence and other

parameters need to be synchronized.

Yet another Protocol Stack !!!

Radio Band Layer

• Uses frequency hopping.• Each frequency used for 625microsecond.• Three power classes defined.• Class 1-> Min 1 mW ,100 mW (milli)• Class 2-> Max 2.5 mW,Normal 1 mW,min

0.25mW• Class 3-> Max 1 mW.

Base band layer

• Specifies frequency hopping nature.• Also specifies various formats of data for

various needs• Specifies 1-slot,3-slot,5-slot frequency model.

Baseband packet structure

• The Sync field is derived from the device address.• AM addr -> Slave or master address.• Master to slave data-> Address = receiver addr• Slave to Master -> Sender address

• Type-> whether data, control ,synchronous, asynchronous etc.

• Flow->In asynchronous mode if flow=0 stop transmission. Start only after getting another flow=1.

• Used only to get ACK’s• Single bit Sequence and ACK number.• Header error check