1

8

ng

2

ng

n ch

ch

n

c cn

ng Internet

i.

n

ng

ng LAN

n KTQS 3

i 5

ng

p

nh (Scheduling)

4

6

p

5

p

i M n

c

m

: o?

n KTQS 6

1 2

3

4

5 M

chung

ng

n KTQS 7

ng

nh p

ng

nh

p

MAC

schemes

Polling

Token ring

WLANs

Aloha

Ethernet

FDMA TDMA

CDMA

nh

m

ng

c (steady stream)

n KTQS 8

tinh

n KTQS 9

tinh

uplink fin downlink fout

o

n KTQS 10

uplink f1 ; downlink f2

uplink f3 ; downlink f4

ng

packets.

i MAC:

nh (scheduling)

n KTQS 11

nh: Polling

n KTQS 12

Inbound line

Outbound line Host

computer

Stations

1 2 3 M

Poll 1

Data from 1

Poll 2

Data from 2

Data to M

nh: Trao Token

n KTQS 13

Ring networks

token

ng

token Data to M

n KTQS 14

Multitapped Bus

ng

Crash!!

p

Wireless LAN

n KTQS 15

- - m

c (base station)

n MACs

ng

ng?

c (steady)

p?

ng?

ng?

?

n KTQS 16

t MACs

i nhau

ng

m

-

n

i

m

m, c i

n KTQS 17

m

n KTQS 18

ng chung

m

t = 0

ch d meters tprop = d /v seconds

A B

A B m

t = tprop

Case 1

m

t = tprop

A B

A B m t = 2 tprop

Case 2

m

ng 2tprop ng tprop c t

R bit/s

bit trong 1 frame L bits/frame

max1 1

2 1 2 / 1 2prop prop

LL t R t R L a

/propta

L R

n KTQS 19

-

a ng

n 1 frame

t

1 bits/second

/ 2 1 2eff prop

LR R

L R t a

nh

n KTQS 20

c CSMA-CD (Ethernet):

ng Token-ring

a a khung

m:

CSMA1

1 6.44a

token1

1 a

2tram1

1 2a Khi a1, 100%

Khi a 1, m

- nh

a n i Ethernet frame max: 1500 bytes = 12.000 bits

max segment: 65.000bytes=520.000bit

-

n KTQS 21

ch 10 Mbps 100 Mbps 1 Gbps ng

1 m 3.33 x 10-02 3.33 x 10-01 3.33 x 100 ng Desk area network

100 m 3.33 x 1001 3.33 x 1002 3.33 x 1003 (LAN)

10 km 3.33 x 1002 3.33 x 1003 3.33 x 1004 (MAN)

1000 km 3.33 x 1004 3.33 x 1005 3.33 x 1006 ng

(WAN)

100000 km 3.33 x 1006 3.33 x 1007 3.33 x 1008 u (GAN)

t MAC

n Khung (Frame Transfer Delay): T n

ch

ng (Throughput)

frames/sec hay bits/sec

n: R bits/sec

frame: L bits/frame

n 1 frame: X= L/R seconds/frame

nh: frames/second

t): R/L fr/sec

i: = X = L/ R

n KTQS 22

i

n KTQS 23 i

n

E[T]/X

max 1

1

E[T] =

nh

X =

nh

n

frame

100%

i a= tpropR / L

n KTQS 24

Tra

nsfe

r D

ela

y

Load

E[T]/X

max 1

1

max

a a

a > a

Khi a

p, max 1, khi a

m

i khung

6

25

ALOHA

c Hawaii

t: just do it! t

t

u

u

n KTQS 26

ALOHA

nh

c (ACK) sau 2tprop

Sau 2tpropB)

ng

m i m

n KTQS 27

t t0 t0-X t0+X t0+X+2tprop

t0+X+2tprop + B

i gian

m Time-out

i gian B

u i

nh ALOHA

t n khung X=L/R nh

Sng X

G

Psuccess

successS GP

n KTQS 28

X X

frame

transmission

Prior interval

m

ng 2X

ch Abramson

n

a Abramson:

G t k i

gian t

i t i

n KTQS 29

( )[ ( ) ]

!

kttP A t k e

k

ch Abramson

Do t= 2X, = G/X

( 2 )

2

( 2 )[ arrivals in 2X]

!

(2 )!

GX

kGXX

kG

XP k e

k

Ge

k

02 2

[0 arrivals in 2X]

(2 )0!

success

G G

P P

Ge e

n KTQS 30

a ALOHA

2 [frame/X s]GsuccessS GP Ge

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0

0.00

7812

5

0.01

5625

0.03

125

0.06

25

0.12

50.

25 0.5 1 2 4

G

S

n KTQS 31

ng max Smax= 1/ 2e (18.4%) [frame/X s]

c:

, S

n, S 0

(2e)-1 = 0.184

a ALOHA

t Snowball effect

a ALOHA

i B

nh

2ALOHA[ ] / 1 1 (1 / ) [packet2 ]

GE T X a e a B X

/prop propt ta

L R X

n KTQS 32

propX t

2 propt X B

2ALOHA[ ] 1 ( 2 ) [sec]

Gprop propE T X t e X t B

i

i - a

t ALOHA

n

n

ng Slotted ALOHA

n KTQS 33

i gian

c khe X

a khung

a khe i

c khe

n KTQS 34

t (k+1)X kX t0 +X+2tprop+ B

Vulnerable

period

Time-out

i gian backoff B

t0 +X+2tprop

m

a S-ALOHA

n KTQS 35

a S-ALOHA

a S-ALOHA