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

2

12

3

4

5M

Shared MultipleAccess Medium

Multiple Access Communications

3

Medium Sharing Techniques

Static Channelization

Dynamic Medium Access Control

Scheduling Random Access

Approaches to Sharing

4

Satellite Channel = fin

= fout

Satellite Communications

5

Multidrop telephone lines

Inbound line

Outbound line

Multidrop Telephone line

6

Ring networks

Multitapped Bus

Ring Networks and Buses

7

Wireless LAN

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

9

Tra

nsfe

r D

elay

Load

E[T]/E[X]

max 1

1

Typical Frame Transfer Delay versus Load

10

Tra

nsfe

r D

elay

Load

E[T]/E[X]

max 1

1

max

aa

a > a

BW-Delay Product and Performance

11

(a)

RAM

RAMROM

Ethernet Processor

(b)

Typical LAN

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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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PHY

MAC

PHY

MAC

PHY

MAC

Unreliable Datagram Service

MAC Sublayer

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PHY

MAC

PHY

MAC

PHY

MAC

Reliable Packet Service

LLCLLC LLC

A C

A C

LLC Layer

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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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LLC Header

IP

Data

MAC Header

FCS

LLC PDU

IP Packet

LLC PDU and MAC Frame

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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

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t(k+1)XkX t0 +X+2tpropt0 +X+2tprop

Vulnerableperiod

Time-out Backoffperiod

Retransmission if necessary

Slotted ALOHA

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A

Station A begins transmission at t=0

A

Station A captureschannelat t=tprop

CSMA Random Access Scheme

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sensing

P-Persistent CSMA 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

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0

0.1

0.2

0.3

0.4

0.5

0.6

0.02

0.03

0.06

0.13

0.25

0.5 1 2 4 8 16

32

64

1-PersistentCSMA

0.53

0.45

0.16

S

G

0.01

0.1

1

Throughput of 1-persistent CSMA