Overview of the IEEE 802.11 (Physical & MAC Layer) for WLAN

1. WLAN – IEEE 802.11

2. IEEE 802.11


802.11 standard only covers the physical layer PHY
and medium access layer MAC.
PHY divided into two sublayers :−
−
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Physical layer convergence protocol (PLCP)
Physical medium dependent (PMD)
PLCP – it provides a carrier sense signal called
clear channel assessment (CCA), and provides a
common PHY service access point (SAP).
PMD – handles modulation and encoding
/decoding of signals.

3. Cont..

MAC – support the association and re-association
of a station to an access point and roaming between
different access point ,control
authentication,encryption, synchronization and
power management.

4. PHY Layer

802.11 supports three versions of PHY layer
−
FHSS – Frequency hopping spread spectrum
−
DSSS – Direct sequence spread spectrum
−
Infra red
[Spread Spectrum-techniques involve spreading the bandwidth
needed to transmit data.]

5. FHSS Frame Structure
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Synchronization – 80 bit SYN (010101....), used to
SYN potential receiver.
Start frame delimiter (SFD) – indicate the start of
frame.
PLCP_PDU length word (PLW) – length of payload
PLCP Signalling field (PSF) – indicate data rate of
payload 0000 – 1Mbits/sec, granularity 500 Kbits/sec.
Header error check (HEC) – 16 bit checksum

6. DSSS Frame Structure

Self Study

7. MAC Layer

Three basic access mechanisms have been
defined for IEEE 802.11:(based on
CSMA/CA,RTS/CTS and polling) :−
First two method summarized as distributed
coordination function (DCF).

−
offers ASYN service and not require any access point
to control.
Third, point coordination function (PCF).

offers both ASYN and time-bounded service but need
an access point to control medium access.

8. Medium Access and Inter-frame Spacing

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Short inter-frame spacing (SIFS) – shortest waiting
for medium access (highest priority) ex. Control msg.
PCF inter-frame spacing (PIFS) – used for time
bounded services.
DCF inter-frame spacing (DCF) – longest waiting
time and has the lowest priority for medium access.
[Contention – duration in which several nodes try to access
the medium]

9. Access Mechanism


DFWMAC-DCF using access mechanism
based on CSMA/CA.
Based on random access scheme with
carrier sense and collision avoidance
through random backoff.

10. Basic DFWMAC-DCF example

11. DFWMAC-DCF with RTS/CTS


In this,after waiting for DIFS + backoff time , the sender can issue a request
to send(RTS).
RTS will have same priority , it includes the recevier of the data transmission
and whole duration (data frame + ACK).

Other nodes receive this RTS will set its NAV(net allocation vector).

Receiver will answer RTS with CTS message after waiting for SIFS.

This CTS will also include duration of transmission

12. DFWMAC-PCF with polling


PCF- Point Coordination Function
It requires an access point that controls
medium access and polls the single nodes.

13. MAC frames

Frame control – serve
several purposes.





Duration/ID – indicating the
period of time in which the
medium is occupied, used to
set the NAV.
Address 1 to 4 – four
address fields contain
standard IEEE 802 MAC
addresses (48 bits each),
meaning of each address is
depend upon the DS bits.
Sequence control – seq.
no used to filter duplicates.

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Protocol version – current protocol
version.
Type – function of frame : management
(00), control (01), or data (10) and value
(11) is reserved.
Subtype – subtype for management
frame: (0000) for association req. ,
(1000) beacon , RTS (1011), CTS
(1100).
More fragments – 1 more frag.
Present.
Retry – if retransmission set to 1.
Power Management – set to 1 then
power saver mode.
More data – indicate more data is
present between access point and
station nodes.

14. Interpretation of MAC addresses

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Ad-hoc network – exchange of MAC frame between two
wireless nodes without a distribution system.
Infrastructure network(from AP) – frame physically
originates from an access point.
Infrastructure network (to AP) – station sends a packet to
another station via the access point.
Infrastructure network (within DS) – packet transmitted
between two access points.
BSSID – basic service set ID , DA – destination address , SA – source
address,RA – receiver address , TA – transmitter address.

15. MAC Management
Perform following tasks:



SYN – finding a wireless LAN, SYN of internal clock,
generation of becon signals.
Power Management – to control, transmitter activity
for power management (periodic sleep,buffering,
without missing a frame).
Roaming – joining a network,changing of access
points,scanning for access points.
Management information base(MIB) – parameters
representing the current state, can be accessed via
standardized protocol : simple network management
protocol (SNMP).

16. SYN

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To SYN clocks of all nodes , timing
synchronization function (TSF).
Within a BSS, timing is conveyed by the
periodic transmissions of a beacon frame.
Beacon contains a timestamp and other
management information used for power
management and roaming.
Transmission of a beacon frame is not always
periodic as it deferred if the medium is busy.
In infra-based network , AP performs SYN by
transmitting the periodic beacon signal.

17. SYN example
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
Beacon intervals are not shifted if one beacon is
delayed.
Timestamp of beacon always reflects the real
transmit time, not the scheduled time.

18. Power Management



Wireless devices are battery powered , i.e power
saving mechanisms are crucial.
Basic idea of power management is to switch off the
transceiver whenever it is not needed.
PM is simple for sending device, but PM for receiver
can't known is advance.

Receiver “walk up” the transceiver periodically.

Power saving includes two states for a station:
Sleep and awake

Buffering of data in sender.

19. Power Management (Cont.)


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
Sender communicate with power saving station then
it has to buffer data.
If a station detects that it is a destination of a
buffered packet it has to stay awake until the
transmission takes place.
Walk up at right moment require the timing
synchronization function(TSF).
Power management in infra-based network is
simpler.
Beacon sent by the access point, a traffic indicating
map(TIM) is transmitted.

20. Power Management example
DTIM – delivery traffic indication map

21. Power Management in Ad-hoc
Self Study

22. Roaming

Moving between AP is called roaming.
Steps are as follows :
Check current link quality and scanning for another AP.

Scanning involves the active search for another BSS.
–
Passive scanning means listening into the medium to find
other n/w. i.e receiving beacon msg.
–
Active scanning means also sending a probe
–
and , beacon and prob responses used to join the new BSS

Select the best AP and send association request.

New BSS response the association request

23. IEEE 802.11b & 802.11a
Self Study

24. HIPERLAN


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HIPERLAN – High Performance Local Area
Network
ETSI (European Telecommunication Standard
Institute) standardized HIPERLAN
HIPERLAN is a WLAN allowing for node
mobility and supporting ad-hoc and
infrastructure-based topologies.

25. Bluetooth

In 1998 five companies
(Ericsson,Intel,IBM,Nokia,Toshiba) founded
the Bluetooth consortium with the goal of
developing a single-chip,low-cost,radio-based
wireless network technology.

26. Bluetooth Architecture


Networking
Protocol Stack

27. Bluetooth Architecture (Networking)

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It operate on 79 channels in the 2.4 GHz
band with 1 MHz carrier spacing.
Each device perform frequency hopping
with 1600 hops/s in a random fashion.
Piconet – it is a collection of Bluetooth device
which are SYN to the same hopping
sequence.
Piconet is a collection of devices with different
roles.

28. Cont.
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One device in piconet is master (M)
All other device connected to the
master are act as a slaves(S).
Two additional devices : parket
devices (P) and stand-by (SB)
P – known but they do not have
connection.
SB – do not participated in the
piconet.
M determine the hopping pattern in
the piconet
Slaves have to SYN to this pattern.
Each piconet has one master and
exctly 7 simultaneous slaves.

29. Formation of Piconet

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As all devices use a same hopping
sequence they must be SYN.
Step 1 Master sending its clock
and device ID .
Step 2 hopping pattern is
determined by the device ID , a 48bit worldwide unique identifier.
Step 3 slaves adjust their internal
clock according to the master and
participated in the piconet.
All active devices are assigned a
3-bit active member address
(AMA).
All parket devices use an 8-bit
parket member address(PMA).

30. Formation of Scatternet



Groups of piconet called
scatternet.
Many piconet with
overlapping coverage can
form scatternet.
If a device wants to
participate in more then
one piconet, then it has to
SYN to the hopping seq. of
the piconet it wants to take
part in.

31. Bluetooth (Protocol Stack)

Protocol stack can be divided into two parts



Core specification
Profile specification
Core specification comprise following elements:
Radio

Baseband
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Link Manager Protocol

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Logical Link Control and adaptation
protocol (L2CAP)
Service discovery protocol

32. Bluetooth:Protocol
Stack:Core:Radio
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It specifies the air interface i.e frequencies,
modulation and transmit power.
Bluetooth uses the license-free frequency
band at 2.4 GHz.
A frequency hopping/time-division duplex
scheme is used for transmission, with a fast
hopping rate of 1600 hops/sec.
A time interval between two hops is 625
mico sec.
Each slot uses different frequency

33. Bluetooth:Protocol
Stack:Core:Baseband

It manages frequncy hopping ,medium access and also defines the
packet format.

It defines 1-slot, 3-slot and 5 slot for higher data rates.

No frequency hopping is performed within packets.

In above example the master or one out of seven slaves may transmit data
in an alternative fashion.

34. Bluetooth:Protocol
Stack:Core:Baseband (Cont..)

Components of a bluetooth packet at baseband layer, packet
contains following three fields :


Access code– used for timing SYN and piconet
identification.
Packet header – used for addess, packet type , flow
and error control and checksum.The three bit active
member address (AMA) represent the active address
of the slave.
Payload – up to 343 bytes payload can be
transferred.

35. Bluetooth:Protocol
Stack:Core:Physical Link

It offers two different types of links:
Synchronous connection-oriented link(SCO)


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For this master reserve two consecutive slots
(forward and return slots) at fixed intervals
Master can support upto three simultaneous
SCO links to the same slave or to different
slaves.
Asynchronous connectionless link(ACL)


Data applications , point-to-multipoint
transfer scenarios
Only one ACL links can exist between a master
and slave.

36. Bluetooth:Example data
transmission
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Master always uses the even number of frequency slots ,
odds slots are for the slaves
Every sixth slot is used for an SCO link
ACL link use single or multiple slots providing asymmetric
bandwith

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