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1
ELEN602 Lecture 10
• Review of last lecture
– M/G/1 and M/D/1 Results
• Multi-access Networks
• Medium Access Control
– ALOHA
– Slotted ALOHA
3
Medium Sharing Techniques
Static Channelization
Dynamic Medium Access Control
Scheduling Random Access
Approaches to Sharing
8
A transmits at t = 0
Distance d meterstprop = d / seconds
A B
B transmits before t = tprop and detectscollision shortlythereafter
A B
A BA detectscollision at t = 2 tprop
Impact of Bandwidth-Delay Product
12
Data LinkLayer
802.3CSMA-CD
802.5Token Ring
802.2 Logical Link Control
PhysicalLayer
MAC
LLC
802.11Wireless
LAN
Network Layer Network Layer
PhysicalLayer
OSIIEEE 802
Various Physical Layers
OtherLANs
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DestinationSAP Address
Source SAP Address Information
1 byte 1
Control
1 or 2
Destination SAP Address Source SAP Address
I/G
7 bits1
C/R
7 bits1
I/G = Individual or group address C/R = Command or response frame
LLC PDU Structure
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tt0t0-X t0+X t0+X+2tprop
t0+X+2tprop
Vulnerableperiod
Time-out Backoffperiod
Retransmission if necessary
First transmission Retransmission
ALOHA Random Access Scheme
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0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.01563
0.03125
0.0625
0.125
0.25 0.5 1 2 4 8
Ge-G
Ge-2G
G
S0.184
0.368
Throughput for ALOHA and Slotted ALOHA
19
t(k+1)XkX t0 +X+2tpropt0 +X+2tprop
Vulnerableperiod
Time-out Backoffperiod
Retransmission if necessary
Slotted ALOHA
20
A
Station A begins transmission at t=0
A
Station A captureschannelat t=tprop
CSMA Random Access Scheme
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0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.02
0.03
0.06
0.13
0.25
0.5 1 2 4 8 16
32
64
Non-PersistentCSMA
0.81
0.51
0.14
S
G
0.01
0.1
1
Throughput of CSMA