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1
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
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
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?
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
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
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
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
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.
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.
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!
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
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