Upload
bheemsain
View
553
Download
4
Tags:
Embed Size (px)
Citation preview
11
OutlineOutline Multiple access mechanismsMultiple access mechanisms Random accessRandom access Controlled accessControlled access ChannelizationChannelization
22
Sublayers of Data Link Sublayers of Data Link LayerLayer
33
Multiple Access Multiple Access MechanismsMechanisms
Random AccessRandom Access
55
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
66
ALOHA NetworkALOHA Network
77
Frames in Pure ALOHAFrames in Pure ALOHA
88
ALOHA ProtocolALOHA Protocol
99
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} ::
1010
ALOHA: Vulnerable TimeALOHA: Vulnerable Time
1111
Slotted ALOHASlotted ALOHA
1212
Slotted ALOHA: Vulnerable Slotted ALOHA: Vulnerable TimeTime
1313
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
1414
Collision in CSMACollision in CSMA
1515
CSMA: Vulnerable TimeCSMA: Vulnerable Time
1616
Persistence MethodsPersistence Methods What a station does when channel is idle or What a station does when channel is idle or
busybusy
1717
Persistence MethodsPersistence Methods
1818
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
1919
Energy LevelsEnergy Levels
ZERO, NORMAL AND ABNORMAL
2020
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
2121
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?
2222
CSMA/CD: Flow DiagramCSMA/CD: Flow Diagram
2323
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
2424
CSMA/CA: Flow DiagramCSMA/CA: Flow Diagram
contention window size is 2K-
1
After each slot:- If idle, continue counting- If busy, stop counting