12 multiple access
Transcript of 12 multiple access
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OutlineOutline Multiple access mechanismsMultiple access mechanisms Random accessRandom access Controlled accessControlled access ChannelizationChannelization
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Sublayers of Data Link Sublayers of Data Link LayerLayer
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Multiple Access Multiple Access MechanismsMechanisms
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Random AccessRandom Access
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Random AccessRandom Access Also called Also called contention-basedcontention-based access access No station is assigned to control No station is assigned to control
anotheranother
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ALOHA NetworkALOHA Network
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Frames in Pure ALOHAFrames in Pure ALOHA
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ALOHA ProtocolALOHA Protocol
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ExampleExample Calculate possible values of Calculate possible values of TTBB, when , when
stations on an ALOHA network are a stations on an ALOHA network are a maximum of 600 km apartmaximum of 600 km apart
TTpp = (600 × 10 = (600 × 1033) / (3 × 10) / (3 × 1088) = 2 ms) = 2 ms
When When KK=1, =1, TTBB {0ms,2ms} {0ms,2ms} When When KK=2, =2, TTBB {0ms,2ms,4ms,6ms} {0ms,2ms,4ms,6ms} ::
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ALOHA: Vulnerable TimeALOHA: Vulnerable Time
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Slotted ALOHASlotted ALOHA
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Slotted ALOHA: Vulnerable Slotted ALOHA: Vulnerable TimeTime
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CSMACSMA CCarrier arrier SSense ense MMultiple ultiple AAccessccess
““Sense Before Transmit"Sense Before Transmit" Reduce the possibility of collisionReduce the possibility of collision
But cannot completely eliminate itBut cannot completely eliminate it
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Collision in CSMACollision in CSMA
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CSMA: Vulnerable TimeCSMA: Vulnerable Time
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Persistence MethodsPersistence Methods What a station does when channel is idle or What a station does when channel is idle or
busybusy
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Persistence MethodsPersistence Methods
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CSMA/CDCSMA/CD CCarrier arrier SSense ense MMultiple ultiple AAccess with ccess with
CCollision ollision DDetectionetection Station monitors channel when Station monitors channel when
sending a framesending a frame
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Energy LevelsEnergy Levels
ZERO, NORMAL AND ABNORMAL
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CSMA/CD: Minimum Frame CSMA/CD: Minimum Frame SizeSize Each frame must be large enough for a Each frame must be large enough for a
sender to detect a collisionsender to detect a collision Worst case scenario:Worst case scenario:
"A" is transmitting"A" is transmitting "D" starts transmitting just before A's signal "D" starts transmitting just before A's signal
arrivesarrivesA B C D
Long enough tohear colliding signalfrom D
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ExampleExample A CSMA/CD network has a bandwidth A CSMA/CD network has a bandwidth
of 10 Mbps. If the maximum of 10 Mbps. If the maximum propagation time is 25.6 μs, what is propagation time is 25.6 μs, what is the minimum size of the frame?the minimum size of the frame?
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CSMA/CD: Flow DiagramCSMA/CD: Flow Diagram
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CSMA/CACSMA/CA CCarrier arrier SSense ense MMultiple ultiple AAccess with ccess with
CCollision ollision AAvoidancevoidance Used in a network where collision Used in a network where collision
cannot be detectedcannot be detected E.g., wireless LANE.g., wireless LAN
IFS – Interframe Space
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CSMA/CA: Flow DiagramCSMA/CA: Flow Diagram
contention window size is 2K-
1
After each slot:- If idle, continue counting- If busy, stop counting