lecture8-mac protocols.pdf
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-
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
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