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