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1. DISCLAIMER STATEMENT
In preparation of these slides, materials have been
taken from different online sources in the shape of
books, websites, research papers and presentations
etc. However, the author does not have any intention
to take any benefit of these in her/his own name. This
lecture (audio, video, slides etc) is prepared and
delivered only for educational purposes and is not
intended to infringe upon the copyrighted material.
Sources have been acknowledged where applicable.
The views expressed are presenter’s alone and do not
necessarily represent actual author(s) or the
institution.
1

2. Mobile & WirelessNetworks

3. Typical LayeringArchitecture and
Wireless Networks
(Data LinkLayer)

4. Layering in WirelessNetworks
ApplicationLayer
Transport Layer
Network Layer
Data linklayer
Physical Layer
 Modulation and Demodulation
 Analog Modulations: AM, FM,PM
 Digital Modulations: ASK,FSK,PSK
 Spread Spectrum
 FHSS
 DSSS

5. Data LinkLayer
(MACLayer)

6. Data LinkLayer Issues in WirelessNetworks
 Framesizeand overhead
 Frame error
 Error recovery(ARQ)
 Forward error correction
 Multiple access (Wireless ChannelAccessMechanism)
Application Layer
TransportLayer
Network Layer
Data LinkLayer
Physical Layer

7. MACLayer Issues and Wireless Networks
Application Layer
TransportLayer
Network Layer
Data LinkLayer
Physical Layer
 Multiple UserPerspective
 MACLayerIssuesin Wireless Networks
Hidden/Exposed Station Problem
Multiple AccessSchemes
Logical LinkControl
Medium AccessControl

8. Wired/Ethernet MAC – CSMA/CD
Carrier sense
 Nodes can distinguish between an idle and busy
link
Multiple access
 Nodes send and receive frames over a shared
link
Collision detection
 A node listens as it transmits to detect collision

9. CSMA/CD MAC Algorithm
 If line is idle (no carriersensed)
 Sendimmediately
 Must wait between back-to-backframes
 If line is busy (carriersensed)
 Wait until the line becomes idle (continuously
sensemedium) and then transmit immediately
 If collisiondetected
 Stop sending data
 Try again later

10. MACLayer Issues in
Wireless Networks

11. Reason for not using CSMA/CD in Wireless LANs
 Wireless LANscannot implement CSMA/CDfor three
reasons
 For Collision Detection, Voltagelevelcheckinginwireless
networksisexpensiveoperation.
 Collision may not be detected because of Hiddenstation
problem
 Thedistance between stations can be great. Signalfading
could prevent astation at one end from hearing acollisionat
the other

12. Multiple User Perspective/MAC LayerIssuesin
Wireless Networks
 How to share the broadcast wirelesschannel
efficiently among multiple users?
How to separate transmissions from differentusers?
How to avoid interference andcollisions?
How to achieve flexible, efficient and fairshare of
bandwidth?
 How to addressHidden/ExposedstationProblems?

13. Hidden/Exposed StationProblem
 Hidden nodes in a wireless network refer to nodes
that are out of range of other nodes or a collection
of nodes
 In wireless networks, the exposed nodeproblem
occurs when anode is prevented from sending
packets to other nodes due to aneighboring
transmitter

14. Hidden StationProblem
 NodeAcancommunicate with Band Cboth
 Band C cannot hear each other
 When B transmits to A, Ccannot detect the transmission
using the carrier sensemechanism
 If C also transmits, collision willoccur at nodeA

15. Exposed StationProblem
 Nodes Band D are out of range of eachother
 NodesAand Care in range of eachother
 If Ais transmitting to B,Node Cis prevented from transmitting to D
 Cconcludes after carrier sense that it will interfere with the transmission
by its neighborA
 However, Dcould still receive the transmission of C withoutinterference
becauseitis out of range of theongoingcommunicationbetweenAandB
Node Cprevented
to Transmit toD

16. Multiple UserPerspective
Classification of MAC LayerSchemes

17. Contention FreeMAC – Polling
 Works with topologies in which one device isdesignated as a
primary station and other devices are secondary devices and
all data exchangesmust be made through primarydestination
 Theselect (SEL) function is used whenever the primarydevice
hassomething to send. Primary must alert the secondary to
upcoming transmission.
 Poll function is used by primary device to ask for
transmission from the secondarydevices.
 When primary is ready to receive data, it must ask(poll) each
station if it has something to send?

18. Contention FreeMAC – Polling
A B A B

19. Contention FreeMAC – Polling
 Polling Limitations
 Polling overhead
 Latency
 Single point of failure(Master)

20. FDMA – Frequency Division Multiple Access
 In an area, multiple users or wireless subscribersare
served by asingle BS
 In FDMA,frequency band divided into sub-bands, called
channels
 FDMA isused in all 1Gcellular systems
User 1
User 2
User n
…
Time
Frequency
Frequency1
User1
User2
…
Usern
Frequency2
…
Frequencyn
Mobile Stations BaseStation

21. TDMA – TimeDivision Multiple Access
Time
1
Time
2 …
…
User1
User2
…
User n
Time
n
Mobile Stations Base Station
 In TDMA, one channel used by several users
 Time slots
 Round-robin
 TDMAused in most 2Gcellular systems

22. CDMA – Code Division Multiple Access
 CDMAused in some 2Gand most 3Gcellular systems
 The frequency of the transmitted signal is varied according to a defined pattern (code), so it
can be interpreted only by a receiver
User
2
User
1
User
n
Frequency
.. .
Time
Code

23. Multiple UserPerspective
Classification of MAC LayerSchemes

24. Random Access -Aloha
 Earliest RandomAccessMethod for Wireless LANs.
 Eachstation sendsaframe whenever it hasframe to send
 However, there is one channel to share, collisioncan
occur between frames from differentstations

25. Random Access – Slotted Aloha
 Slotted ALOHAdivides time into discrete intervalsand each
interval corresponds to aframe ofdata.
 It requires users to agree on slot boundaries.
 It does not allow a systemto transmit any time. Instead the
systemhasto wait for the beginning of the next slot
 Packetscompletely overlap or no overlapping. No
partial overlapping

26. ALOHA vs SlottedALOHA

27. Multiple UserPerspective
Classification of MAC LayerSchemes

28. CSMA – Persistence Methods
 What should astation do if the channel isbusy?
 What should astation do if the channel isidle?
 1-Persistence
 Non-Persistence
 p-Persistence

29. CSMA – Persistence Methods
 1-persistent CSMA Protocol:
 Step 1: If the medium is idle, transmit immediately
 Step 2:If the medium is busy, continue to listen until medium
becomes idle, and then transmitimmediately
 There will always be acollision if two nodes want to retransmit
(usually you stop transmission attempts after few tries)
 Theprotocol is called 1-persistent because the stationtransmits
with aprobability of 1 when it finds the channel idle
 Non-persistent CSMA Protocol:
 Step 1: If the medium is idle, transmit immediately
 Step 2: If the medium is busy, wait arandom amount of time
and repeat Step1
 Random backoff reduces probability of collisions
 Waste idle time if the backoff time is too long

30. CSMA – Persistence Methods
 p-persistent CSMA Protocol:
 It applies toslotted channels
 Step 1: If the medium is idle, check probability outcome if
p<=1 station can transmit. If >1 then wait atime slot
 Step 2: If the medium is busy, continue to listen until medium
becomes idle, then go to Step1
 Step 3: If transmission is delayed by one timeslot, continue
with Step1

31. Multiple UserPerspective
Classification of MAC LayerSchemes

32. Contention-Based MAC Protocols
with ReservationMechanisms
 Motivation
 Tosolve the hidden and exposed-terminal problems inwireless
 usethe request-to-send/clear-to-send (RTS/CTS)control
packets to preventcollisions
 Dynamic Reservation
 Thedynamic reservation approach involves setting upsome
sort of areservation prior todata transmission
 sender-initiatedprotocol:anode that wants to send datatakes
the initiative of setting up this reservation
 receiver-initiated protocol: the receiving node polls apotential
transmitting node for data

33. Multiple Access Collision Avoidance(MACA)
 MACA (Multiple Access Collision Avoidance)
 When node Awants to send apacket to node B,node Afirst
sendsaRequest-to-Send (RTS)to B
 Onreceiving RTS,node Bresponds by sending Clear-to-Send
(CTS),provided node Bis able to receive the packet
 When anode (such asC)overhears aCTS,it keeps quiet for
the duration of thetransfer
 Transfer duration is included in CTS

34. Working ofMACA
 If Calso transmits RTS,collision atB
A B C
RTS

35. Working ofMACA
 Cknows the expected DATAlength fromCTS
A B C
CTS
Defers untilDATA
completion
CTS

36. Working ofMACA
 Avoids the hidden terminalproblem
A B C
DA
T
A
RemainsSilent

37. Working ofMACA
 B want to communicate withA
A B C D
RTS RTS

38. Working ofMACA
 Cdoes not hear aCTS
A B C D
CTS

39. Working ofMACA
 Is Creally free to send toD?
A C D
RTS
B
DA
T
A DA
T
A

40. Working ofMACA
 In MACA, Cincreases its backoff counter in caseof
any collision
A B C D
DA
T
A CTS
DA
T
A

41. Limitations ofMACA
 RTS-CTSapproach does not always solve the hidden-terminal
problem completely and collision can occur when different
nodes send the RTSand CTSpackets

42. Working ofMACAW (Multiple Access with
Collision Avoidance for Wireless)
heard fromA. Hence my
transmission isdeferred
D
B A F
E
G
1.RTS
3.CTS
4.I heard CTSfrom B hencemy
transmission isdeferred till data
is to bereceived
4.Iheard CTSfrom B
hence my transmission
is deferred till data is to
be received
3.CTS
5.DS
2.(From both Eand F)RTSand RTS/CTS is a
success. Hence Idefer
my transmissiontill
till CTSshould be received dataends
7.Data
9.Ican not listen any
thing anymore
6.DS tells me that
what is datalength
10. Fis not replying.
Congestion may be?
I Backoff
11.ACK
10.Cis not replying.Congestion
may be? Ibackoff
C
1.RTS
12.Now I can replyto D
for old RTS withRRTS
11.ACK
7.Data
9.ReceivesRTS

43. Disadvantages ofMACAW
 MACAWdoes not solve the exposed terminalproblem
 Assume that node Ghas data to send to node Fin example explained
previously. It initiates the process by sending an RTSsignal to node F.
Node Fis in the transmission range of node Aand cannot hear the RTS
from node G, since it is exposed to co-channel interference. Node G
assumes that its RTSwas not successful because of collision and hence
backsoff before it triesagain.

44. IEEE 802.11Standard Wireless MAC
(CSMA/CA)

45. IEEE 802.11Standard – Network Configuration
 Wireless networking mainly can be viewed asadistributed
function among afew stations in aBasicServiceArea (BSA).
 Thegroup in aBSAis called aBasicServiceSet(BSS).Stations in a
BSSare connected via acommon central point
 More than one BSScould be grouped through aDistribution
System(DS)to form an extendedserviceset (ESS).
 E
S
Sis defined only for the infrastructure topology. For ad hocor
independenttopology, the BSSis called Independent BSSorIBSS.

46. Distribution System
Portal
802.xLAN
Access
Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access
Point
ST
A1
ST
A2 ST
A3
ESS
IEEE 802.11standard – Network Configuration

47. IEEE 802.11Standard – Modes of Operation
 Infrastructure-based
 Themain most mature technology for WLANs
 Most commonly used to construct Wi-Fihotspots
 Costly for dynamicenvironments
 Infrastructureless
 Also calledAd Hoc mode
 Stations form an Independent BasicService Set(IBSS)
 Any stations within the sametransmission range cancommunicate

48. IEEE 802.11Standard – Basic Services
 Thebasic services supported are the mandatory
data serviceand an optional real-time service
 Real-time service is supported only in infrastructure-
based networks where APscontrol accessto shared
medium

49. Distributed Foundation Wireless MAC (DFWMAC)
 Theprimary accessmethodof IEEE802.11 is by meansof a
distributed coordination function(DCF),also known as
DFWMAC
 DCFis basedon CSMA/CA
 Thesecondaryaccessmethodis point coordinationfunction
(PCF)implemented to provide real-timeservices
 When the PCFis in operation, APcontrols medium accessand
avoids simultaneous transmissions by thenodes

50. Components of IEEE 802.11MAC
1.Useof RTS/CTS to avoid hidden stationproblem
(Optional)
2.ChannelSensing:Physical carrier sense,and Virtual
carrier sense
3.Backoffintervals used to reduce collisionprobability
4.UseInterframe Spacing(IFS)to prioritize accessibility
5.ACK to achievereliability

51. 1.Use of RTS/CTS – Avoid Hidden Node Problem
 Idea: allow senderto “reserve”channelrather than random
access of dataframes:avoid collisions
 Sender first transmits small request-to-send (RTS)packetsto
receiver usingCSMA
 RTSmay still collide with each other (but they’re short)
 Receiver broadcasts clear-to-send CTSin response to RTS
 CTSheard by all nodes
 sender transmits dataframe
 other stations defertransmissions
Avoiddata frame collisionscompletely
usingsmallreservation packets!

52. AP
A B
Time
DATA(A)
reservation collision
defer
1.Use of RTS/CTS – Avoid Hidden Node Problem

53. 2.Channel Sensing
 Channel sensing is carried out
 by either sensing the presence of acarrier signal in the wireless
medium
 or by checking the value of aparameter,called Network
AllocationVector (NAV).
 TheMAClayer gets help from the physical layer electronicsto
check carrier signal presence. ThePHYfunction to sense
channel is called ClearChannelAssessment(CCA).
 NAVresults in what is termed virtual sensing, asit does not
involve aphysical signal-detection mechanism.

54. Virtual ChannelSensing
 In virtual sensing,NAVvalueischeckedthat wassetbythe
sending station ondetecting shortpacketexchange(called
handshake) between the intendedtransmitter andreceiver.
 Thetransmitter neighbors set the NAVparameter value on
detecting aRequest to Send(RTS)type of MACpacket. RTSis
sent by the transmitter to inform the intended receiver
about the reservation request and to askitspermission.
 Onreceiving RTS,the receiver responds with Clearto Send
(CTS),indicating its permission, aswell asinforming its
neighbors of the transmission. Ondetecting CTS,the stationsin
the neighborhood of the receiver set their NAVvalue.

55. Virtual CarrierSensing
C F
A E
D
RTS
B
NAV =10
RTS=Request-to-Send
Pretending a circularrange
NAV =NetworkAllocationVector,
remaining duration to keepquiet

56. Virtual CarrierSensing
C F
A B E
D
CTS
NAV =8
CTS=Clear-to-Send

57. Virtual CarrierSensing
DATApacket follows CTS.
C F
A B E
D
DA
T
A

58. Virtual CarrierSensing
Successful data receptionacknowledged
usingACK.
C F
A B E
ACK
D

59. Virtual CarrierSensing
C F
A B E
D
ACK
Reservedarea

60. Virtual CarrierSensing
when carrier sensed
 Nodes stay silent
(physical/virtual).
 NAV is updated
RTS/CTS/DATA/ACK
based
packets,
on overheard
each of which
specified duration of apendingtransmission

61. 3.Random Backoff
 In the caseof astation finding achannel busy, it hasto
wait for some time before it can restart sensing.This
time is called backofftime.
 Thebackoff time is selected randomly in IEEE802.11.
Thisensures that two stations should not start andfinish
to sensethe channel togetheragain
 Thetime is generated from arandom numbergenerator
every time astation hasto backoff.

62. 4.Inter-Frame Spacing (IFS)
 DCFincludes delays (Inter-frame spacing) that act asapriority
schemes
 Inter-frame spacingrefersto the time-interval between the
transmissionof two successiveframesbyanystation
 Four types of IFS
 SIFS(Short Inter-FrameSpacing)
 PIFS(PCFInter-FrameSpacing)
 DIFS(DCFInter-Frame Spacing)
 EIFS(Extended Inter-FrameSpacing)

63. Inter-Frame Spacing
 SIFS
highest priority, for ACK,CTS,pollingresponse
 PIFS
medium priority, for time-bounded service usingPCF
 DIFS
Lower priority than PIFS,for RTS
 EIFS
 Longest IFSand denotes the least priority to accessthe
medium. Usedfor resending an RTS packet whenever
physical layer detects incorrect MACframereception

64. Data Transmission in802.11
 Asender transmits an RTSframe to the receiver. set the NAVtimer
by the other stations.
 The receiver responds with a CTS frame containing a shorter
duration field. Setthe NAVtimer by the other stations.
 After this exchange all clients in the BSSthen contend based upon
the contention window after the DIFS.

65. 5.Uses ACK to achievereliability
 Wireless links are prone to errors.
 Mechanisms are needed to reduce packet loss rate
 When node Breceives adata packet from nodeA, node Bsends
anAcknowledgement (Ack). Thisapproach adopted in many
protocols
 If nodeAfails to receive anAck, it will retransmit the packet
A B C

66. THANK YOU