chuong 3_tsl
TRANSCRIPT
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CHÖÔNG 3
CÔ SÔÛ GIAO THÖÙC
(PROTOCOL BASICS)
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CÔ SÔÛ GIAO THÖÙC
3.1 Toång quan
3.2 Kieåm soaùt loãi
3.3 Idle RQ (stop and wait)
3.4 Continuous RQ (RQ lieân tuïc)
3.5 Ñieàu khieån luoàng (flow control)
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3.2 Kieåm soaùt loãi
Kieåm soaùt loãi baèng tay
Kieåm tra doäi laïi (echo checking)
Kieåm soaùt loãi töï ñoäng (ARQ)
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Kieåm soaùt loãi töï ñoäng (ARQ)
ARQ:
Hình 3.1
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Kieåm soaùt loãi töï ñoäng (ARQ)
Hình 3.2
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RQ raûnh
(Idle RQ)
ARQ
RQ lieân tuïc
(Continuous
RQ)
Truyeàn laïi ngaàm
ñònh
(implicit)
Yeâu caàu roõ
(explicit)
Truyeàn laïi coù choïn
löïa (seclective
repeat)
Truyeàn laïi moät nhoùm
(go-back-N)
Kieåm soaùt loãi töï ñoäng
• Hình 3.3: Phaân loaïi caùc kieåu RQ
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P S
I – frame
ACK – frame hoaëc NAK - frame
+ Primary – P (phía sô caáp) hoaëc Tx (Transmitter)
+ Secondary – S (phía thöù caáp) hoaëc Rx (Receiver)
+ I – frame (Information frame): I(N)
+ ACK – frame (Acknowledge frame): ACK(N)
+ NAK – frame (Negative Acknowledge frame): NAK(N)
• Hình 3.4: Chieàu truyeàn tin cuûa P vaø S
Kieåm soaùt loãi töï ñoäng
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Söû duïng trong kieåu truyeàn döõ lieäu ñònh
höôùng kyù töï.
Hoaït ñoäng theo cheá ñoä baùn song coâng .
P
S
One
frame
ACK
Another
fame
ACK time
Another
frame
3.3.1 Ñaëc ñieåm
Hình 3.5: Hoïat ñoäng cuûa giao thöùc Idle RQ
3.3 Idle RQ
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CRC Information
packet
Header
Information frame
Control frame: ACK
CRC Header
Packet Error-free
packet
Information frame
Control frame
Transmitter
(Tx)
Receiver
(Rx)
Stop-and-Wait ARQ
Truyền khung Information, đợi tín hiệu ACK
Timer được khởi
động tại mỗi thời
điểm bắt đầu
truyền frame
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Tröôøng hôïp truyeàn ñuùng
Hình 3.6a: Tröôøng hôïp truyeàn ñuùng
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I(N)
I(N)
I(N+1)
I(N+1)
I(N+2)
Time
I(N+2)
I(N+3)
Timer stopped
Timer started Timer started
Timer stopped
Timer started
Primary (P)
Secondary (S)
Timer stopped
tT tTtT
Tröôøng hôïp truyeàn ñuùng
Hình 3.6b: Tröôøng hôïp truyeàn ñuùng
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I(N)
I(N)
I(N+1)
I(N+1)
Time
Timer stopped
Timer started Timer started
Timer stopped
Primary (P)
Tp
Tix
Tip
Tp
Tax
Tap
Tp
= Treã lan truyeàn frame töø P->S
Tix
= Thôøi gian truyeàn frame töø P->S
Tip
= Thôøi gian xöû lyù frame taïi S
Tp= Treã lan truyeàn ACK töø S->P
Tax
= Thôøi gian truyeàn ACK töø S->P
Tap
= Thôøi gian xöû lyù ACK taïi P
Secondary (S)
• Hình 3. 7 : Caùc khoaûng thôøi gian xöû lyù trong quaù trình truyeàn daãn
tT tT
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Tröôøng hôïp coù loãi xaûy ra
Hình 3.8a: Khung thoâng tin (Information) bò loãi
A
B
One
frame
Another
frame
ACK
The
frame
ACK
time
Time-out
Another
frame
A
B
One
frame
Another
frame
ACK
the
frame
ACK time
Time-out
Another
frame
ACK
Hình 3.8b: Khung ñieàu khieån (control) bò loãi
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Coù 2 loaïi Idle RQ:
+ Truyeàn laïi ngaàm ñònh (Implicit retransmission)
+Truyeàn laïi töôøng minh (Explicit retransmission)
Tröôøng hôïp coù loãi xaûy ra
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• a/ Tröôøng hôïp truyeàn laïi ngaàm ñònh
• b./ Tröôøng hôïp truyeàn laïi töôøng minh
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3.3.2 Idle RQ ngaàm ñònh
• Hình 3.9a: Tröôøng hôïp khung I(N+1) bò loãi
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I(N)
I(N)
I(N)
I(N)
I(N+1)
Time
Timer expires/Restarted
Timer started
Timer stopped
Primary (P)
Secondary (S)
3.3.2 Idle RQ ngaàm ñònh
• Hình 3.9b: Tröôøng hôïp khung I(N) bò loãi
OT
tT
tT
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3.3.2 Idle RQ ngaàm ñònh
• Hình 3.10a: Tröôøng hôïp ACK(N+1) bò loãi
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I(N)
I(N)
I(N)
I(N)
I(N+1)
Time
Timer expires/Restarted
Timer started
Timer stopped
Primary (P)
Secondary (S)
Duplicate detected/discarded
3.3.2 Idle RQ ngaàm ñònh
• Hình 3.10a: Tröôøng hôïp ACK(N) bò loãi
tTOT
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3.3.3 Idle RQ töôøng minh
• Hình 3.11a: Tröôøng hôïp khung I(N+1) bò loãi
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I(N)
I(N)
I(N)
I(N)
I(N+1)
Time
Timer expires/Restarted
Timer started
Timer stopped
Primary (P)
Secondary (S)
Duplicate detected/discarded
3.3.3 Idle RQ töôøng minh
• Hình 3.12b: Tröôøng hôïp ACK(N) bò loãi
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I(N)
I(N)
I(N+1)
I(N+1)
Time
Timer stopped
Timer started Timer started
Timer stopped
Primary (P)
Tp
Tix
Tip
Tp
Tax
Tap
Tp
= Treã lan truyeàn frame töø P->S
Tix
= Thôøi gian truyeàn frame töø P->S
Tip
= Thôøi gian xöû lyù frame taïi S
Tp= Treã lan truyeàn CK töø S->P
Tax
= Thôøi gian truyeàn ACK töø S->P
Tap
= Thôøi gian xöû lyù ACK taïi P
Secondary (S)
• Hình 3.7: caùc khoaûng thôøi gian xöû lyù trong quaù trình truyeàn daãn
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3.3.5 Hieäu suaát söû duïng lieân keát
Tröôøng hôïp truyeàn daãn laø lyù töôûng (töùc khoâng coù loãi xaûy ra):
Goïi Tt laø thôøi gian toång ñeå hoaøn thaønh vieäc truyeàn moät I – frame vaø
xöû lyù xong ACK – frame.
Khi ñoù:
Thoâng thöôøng, Tap
, Tip
vaø Tax
raát nhoû so vôùi Tp vaø T
ix.
Do ñoù:
Hieäu suaát söû duïng ñöôøng truyeàn ñöôïc ñònh nghóa laø:
Ñaët:
2.t ix p ap ip axT T T T T T
2.t ix pT T T
2.(1 2. )
ix ix ix
pt ix pix
ix
T T TU
TT T TT
T1
1 2.
p
ix
Ta U
T a
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3.3.5 Hieäu suaát söû duïng lieân keát
Nhaän xeùt:
-Vôùi tuyeán thoâng tin coù toác ñoä truyeàn tin thaáp vaø khoaûng
caùch gaàn thì Tix
raát lôùn, Tp
nhoû a raát nhoû hieäu suaát
lieân keát cao ( 100%).
-Ngöôïc laïi, vôùi tuyeán thoâng tin coù toác ñoä truyeàn tin cao
vaø khoaûng caùch xa thì Tix
raát nhoû, Tp
raát lôùn a raát lôùn
hieäu suaát lieân keát thaáp.
Vì vaäy, giao thöùc Idle RQ phuø hôïp trong tröôøng hôïp
tuyeán lieân keát coù khoaûng caùch ngaén vaø toác ñoä truyeàn
thaáp.
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3.3.5 Hieäu suaát söû duïng lieân keát
Tröôøng hôïp truyeàn daãn laø khoâng lyù töôûng (töùc laø coù loãi xaûy ra), thì caùc
khung bò loãi phaûi thöïc hieän vieäc truyeàn laïi. Giaû söû, moãi khung ñöôïc
truyeàn laïi trung bình laø Nr laàn. Khi ñoù:
Hieäu suaát söû duïng lieân keát:
Goïi P laø xaùc suaát 1 bit bò loãi. Khi ñoù, xaùc suaát 1 bit ñuùng laø 1 – P.
Neáu goïi Pf laø xaùc suaát moät khung thu ñöôïc bò loãi thì: P
f = 1 - (1 – P)
Ni
Xaùc suaát moät khung ñuùng laø:1 – Pf = (1 – P)
Ni. Do vaäy:
Hieäu suaát lieân keát:
( 2. ). 2. .t ix p r r ix p rT T T N N T T N
1 1
. 2. . .(1 2. ).(1 2. )
ix ix
pt r ix r p rr
ix
T TU
TT N T N T N aN
T
1
1r
f
NP
1
1 2.
fPU
a
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3.4 Continuous RQ
3.4.1 Ñaëc ñieåm
3.4.2 Truyeàn laïi coù löïa choïn
3.4.3 Truyeàn laïi moät nhoùm (Go-back_N)
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3.4.1 Ñaëc ñieåm
P truyeàn caùc I- frame tôùi S moät caùch lieân tuïc maø
khoâng döøng laïi ñeå chôø ACK – frame truyeàn veà töø S.
P giöõ laïi moät baûn sao cuûa moãi I- frame trong danh
saùch truyeàn laïi cho tôùi khi P nhaän ñöôïc ACK –
frame töông öùng cuûa khung ñoù truyeàn veà töø S.
S traû veà ACK – frame cho moãi I – frame nhaän ñuùng.
S thu giöõ moät danh saùch thu, chöùa ñöïng ñònh danh
cuûa I – frame cuoái cuøng nhaän ñuùng.
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• Tuaàn töï khung cuûa Continuous RQ
Hình 3.14a: Hoïat ñoäng cuûa giao thöùc Continuous RQ
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I(N) I(N+2)
N
Thôøi gian
Primary (P)
Secondary (S) I(N+3) I(N+4) I(N+1)
I(N) I(N+2) I(N+3) I(N+4) I(N+1)
N N N+1 N+2 N+3
N+1 N+1
N+2
N+2
N+3
N+3
N+4
N+4
N+3 N+4 N+5 N+2 N+1 N V(S)
Noäi dung cuûa
danh saùch
truyeàn laïi
N N+1 N+2 N+3 N+4
N+3 N+4 N+2 N+1 N
V(R)
Noäi dung cuûa
danh saùch thu
V(S)= Bieán tuaàn töï truyeàn
V(R)= Bieán tuaàn töï nhaän
• Hình 3.14b: Tuaàn töï khung cuûa Continuous RQ
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Trong tröôøng hôïp coù loãi xaûy ra trong quaù trình
truyeàn daãn, coù 2 caùch truyeàn laïi:
+ S phaùt hieän vaø yeâu caàu P truyeàn laïi chæ nhöõng
khung bò loãi (Selective – Repeat)
+ S phaùt hieän vaø yeâu caàu P truyeàn laïi nhöõng khung
chöa ñöôïc traû lôøi ACK, nghóa laø taát caû caùc
khung keå töø khung cuoái cuøng nhaän ñuùng. Kieåu
truyeàn naøy ñöôïc goïi laø truyeàn laëp laïi N (Go –
back - N)
Tröôøng hôïp coù loãi xaûy ra
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Hình 3.15: Hoïat ñoäng cuûa giao thöùc Selective – Repeat (explicit)
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Hình 3.16: Hoïat ñoäng cuûa giao thöùc Go – back - N
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• Hình 3.17a: Truyeàn laïi coù löïa choïn – I(N+2) bò loãi
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I(N) I(N+2)
N
Thôøi gian
Primary (P)
Secondary (S) I(N+3) I(N+4)
I(N) I(N+2) I(N+3) I(N+4) I(N+1)
N N N+1 N+1 N+3
N+1 N+1
N+2
N+2
N+3
N+2
N+3
N+4
N+3 N+4 N+5 N+2 N+1 N V(S)
Noäi dung cuûa
danh saùch
truyeàn laïi
N N+2 N+2
N+4
N+1 N+1 N+1 N+1 N V(R)
Noäi dung
cuûa danh
saùch thu
I(N+1) I(N+5)
I(N+1) I(N+5)
N+3
N+4
N+1
N+4
N+1
N+5
N+2 N+5
N+3
N+4
N+1
N+2
N+3
P vaøo traïng thaùi truyeàn laïi P ra khoûi traïng thaùi truyeàn laïi
N+1 N+5
• Hình 3.17b: truyeàn laïi coù löïa choïn – khoâng töôøng minh – I (N+1) bò loãi
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I(N) I(N+2)
N
Thôøi gian
Primary (P)
Secondary (S) I(N+3) I(N)
I(N) I(N+2) I(N+3) I(N) I(N+1)
N N N N+2
N+3
N+1 N+1
N+2
N+1
N+2
N+3
N+3
N+3 N+4 N+4 N+2 N+1 N V(S)
Noäi dung cuûa danh
saùch truyeàn laïi
N N+2
N+3 N+4 N+2 N+1 N
V(R)
Noäi dung cuûa
danh saùch thu
N+3
P vaøo traïng thaùi truyeàn laïi P ra khoûi traïng thaùi truyeàn laïi
N+4
I(N+1)
N+1
N
N
•Hình 3.18: truyeàn laïi coù löïa choïn – khoâng töôøng minh – ACK – frame bò loãi
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I(N) I(N+2)
N
Thôøi gian
Primary (P)
Secondary (S) I(N+3) I(N+4)
I(N) I(N+2) I(N+3) I(N+4) I(N+1)
N N N+1 N+1 N+2
N+1 N+1
N+2
N+2
N+3
N+2
N+3
N+3
N+3 N+4 N+5 N+2 N+1 N V(S)
N N+2 N+2
N+4
N+1 N+1 N+1 N+1 N V(R)
I(N+1) I(N+5)
I(N+1) I(N+5)
N+3
N+4
N+4
N+2
N+3
N+5
N+2 N+5
N+3
N+4
N+1
N+2
N+3
P vaøo traïng thaùi truyeàn laïi P ra khoûi traïng thaùi truyeàn laïi
N+1 N+5
N+2
N+3
N+5
N+1
N+4
N+1
N+5
N+4
N+1
N+5
N+6
N+6
S vaøo traïng thaùi truyeàn laïi S ra khoûi traïng thaùi truyeàn laïi
Noäi dung cuûa
danh saùch
truyeàn laïi
Noäi dung cuûa
danh saùch thu
• Hình 3.19: truyeàn laïi coù löïa choïn – töôøng minh – I(N+1) bò loãi
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I(N) I(N+2)
N
Thôøi gian
Primary (P)
Secondary (S) I(N+3) I(N+4)
I(N) I(N+2) I(N+3) I(N+4) I(N+1)
N N N+1 N+1 N+1
N+1 N+1
N+2
N+2
N+3
N+2
N+3
N+2
N+3 N+4 N+5 N+2 N+1 N V(S)
Noäi dung cuûa danh
saùch truyeàn laïi
N N+2 N+2
N+4
N+1 N+1 N+1 N+1 N V(R)
Noäi dung cuûa
danh saùch thu
I(N+5) I(N+6)
I(N+5) I(N+6)
N+3
N+4
N+3
N+4
N+5
N+6
N+2
N+3
N+4
N+5
N+2
N+3
N+1 N+5
N+6
N+4
N+5
•Hình 3.20: truyeàn laïi coù löïa choïn –töôøng minh – NAK(N+1) bò loãi
![Page 38: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/38.jpg)
GO-BACK-N: khung I(N+2) bò loãi
•Hình 3.21a: Go-back-N: I(N+2) bò loãi
![Page 39: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/39.jpg)
Hình 3.21b: Go-back-N: I(N+2) bò maát
![Page 40: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/40.jpg)
I(N) I(N+2)
N
Thôøi gian
Primary (P)
Secondary (S) I(N+3) I(N+4)
I(N) I(N+2) I(N+3) I(N+4) I(N+1)
N N N+1 N+1 N+1
N+1 N+1
N+2
N+2
N+3
N+2
N+3
N+2
N+3 N+4 N+5 N+2 N+1 N V(S)
N N+2 N+3
N+1 N+1 N+1 N+1 N V(R)
I(N+1)
I(N+1) I(N+2)
N+4
N+3
N+1 N+4
P vaøo traïng thaùi truyeàn laïi
P ra khoûi traïng thaùi
truyeàn laïi
N+1 N+2
N+4
Frame bò loaïi
Noäi dung
cuûa danh
saùch
truyeàn laïi
Noäi dung cuûa
danh saùch thu
I(N+4)
•Hình 3.21c: Go-back-N: I (N+1) bò maát
![Page 41: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/41.jpg)
I(N) I(N+2)
N
Thôøi gian
Primary (P)
Secondary (S) I(N+3) I(N+4)
I(N) I(N+2) I(N+3) I(N+4) I(N+1)
N N
N+1 N+1
N+3
N+1 N+1
N+2 N+2
N+3
N+2
N+3
N+4
N+3 N+4 N+5 N+2 N+1 N
V(S)
Noäi dung cuûa danh
saùch truyeàn laïi
V(R)
Noäi dung cuûa
danh saùch thu
N+4
N+6
N N+2 N+3
N+3 N+4 N+2 N+1 N
N+4
N+5
N+1
I(N+1)
N N
•Hình 3.22: Go-back-N: ACK – frame bò loãi
![Page 42: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/42.jpg)
3.5 Ñieàu khieån luoàng
3.5.1 Kieåu ñieàu khieån X – ON/ X – OFF
3.5.2 Phöông phaùp cöûa soå tröôït (Sliding
window)
3.5.3 Hieäu suaát söû duïng lieân keát
![Page 43: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/43.jpg)
3.5.1 Kieåu ñieàu khieån X – ON/ X – OFF
![Page 44: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/44.jpg)
I(N) I(N+2) I(N+3) I(N+4) I(N+5) ---- I(N+1) I(N+6)
Caùc frame ñang
ñôïïi baùo nhaän
Döøng luoàng
Caùc frame ñang
ñôïïi truyeàn
Caùc frame ñaõ
ñöôïc baùo nhaän
Bieân döôùi cuûa cöûa
soå (LWE)
Cöûa soå truyeàn, K =3
Bieân treân cuûa cöûa soå
(UWE)
Thöù töï truyeàn
•Hình 3.24a: ví duï veà cöûa soå tröôït
3.5.2 Phöông phaùp cöûa soå tröôït
![Page 45: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/45.jpg)
Giao thöùc Cöûa soå truyeàn Cöûa soå thu
Idle RQ
Selective Repeat
Go-back-N
1
K
K
1
K
1
•Hình 3.24b: caùc giôùi haïn cöûa soå truyeàn vaø nhaän
3.5.2 Phöông phaùp cöûa soå tröôït
![Page 46: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/46.jpg)
Nguyên lý đánh số
• P đơn giản đánh số tuần tự khung sau bằng số khung trước đó cộng thêm 1, do đó tầm đánh số là vô cùng.
• Thực tế, việc đánh số còn phụ thuộc vào lược đồ điều khiển truyền lại và kích thước của cửa sổ gởi và nhận.
Ví dụ: Với Idle RQ thì cửa sổ gởi và cửa sổ nhận đều bằng 1. Như vậy, chỉ cần 2 số là đủ để S xác định là khung là mới hoặc khung đã nhận tốt nhưng được truyền lại. Thông thường hai số được sử dụng là 0 và 1, P đánh số tuần tự tăng theo modulo-2
Với lược đồ Go back - N có một cửa sổ gởi là K thì việc đánh
số ít nhất đến K + 1.
Với lược đồ Selective Repeat, cửa sổ gởi và cửa sổ nhận đều là K thì việc đánh số ít nhất đến 2K + 1.
![Page 47: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/47.jpg)
Stop-and-Wait ARQ -Normal operation-
Expect to receive
frame 0
Expect to receive
frame 1
Sent frame 0 recently
and expect to receive
acknowledgment 1
Frame 0 received and expect
to receive frame 1 next time Received ACK 1, set
s=1, and sent frame 1
![Page 48: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/48.jpg)
Stop-and-Wait ARQ - Lost or damaged frame-
Damaged frames will be
treated as lost frames
![Page 49: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/49.jpg)
Stop-and-Wait ARQ -Lost ACK-
Numbering frames prevents the
retaining of duplicate frames
![Page 50: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/50.jpg)
Stop-and-Wait ARQ -Delayed ACK-
Numbered acknowledgments are needed
if an acknowledgment is delayed and the
next frame is lost
![Page 51: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/51.jpg)
Stop-and-Wait ARQ – Piggybacking
Piggybacking: a method to combine a data frame with an acknowledgment
Bidirectional transmission
![Page 52: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/52.jpg)
Nguyên lý đánh số (tiếp theo)
• Với lược đồ Go - back - n ARQ:
– Kích thước cửa sổ gởi phải nhỏ hơn 2m
– Kích thước cửa sổ thu luôn luôn bằng 1
![Page 53: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/53.jpg)
Cửa sổ trượt phía gởi (Sender Sliding Window)
Sliding sau khi nhận được ACK
Sequence numbers: 0, 1, …, 7
![Page 54: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/54.jpg)
Cửa sổ trượt phía thu (Receiver Sliding Window)
Window size = 1
![Page 55: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/55.jpg)
Các biến điều khiển
SF: first frame
SL: last frame
S: frame sent
R: frame to receive
![Page 56: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/56.jpg)
Go-Back-N ARQ, Chế độ họat động bình thường
![Page 57: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/57.jpg)
=> Inefficient
Timer for each data frame
Go back to the first frame and resend every data frames => Inefficient in such case
Go-Back-N ARQ, frame bị lỗi
![Page 58: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/58.jpg)
Go-Back-N ARQ: sender window size
= 4 (m=2) = 3 (m=2)
![Page 59: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/59.jpg)
Nguyên lý đánh số (tiếp theo)
• Với lược đồ Selective repeat ARQ:
– Cửa sổ gởi và cửa sổ thu phải có cùng kích thước và bằng 2m-1
![Page 60: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/60.jpg)
Same size windows
Selective Repeat ARQ, sender and receiver window
![Page 61: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/61.jpg)
Accepted => efficient
If it receives ACK 0, then sends Frame 0, not Frame 3 again. No retransmission if possible => efficient
Selective Repeat ARQ, lost frame
![Page 62: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/62.jpg)
Selective Repeat ARQ, sender window size
![Page 63: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/63.jpg)
Ví duï ñieàu khieån truyeàn söû duïng Sliding Window
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0
1
2
Sender Receiver
A3
3
4 5
6
A4
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
![Page 64: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/64.jpg)
3.5.3 Hieäu suaát söû duïng lieân keát:
- Moät lieân keát coù kích thöôùc cöûa soå K. Khi ñoù, neáu trong
quaù trình truyeàn khoâng xaûy ra loãi thì hieäu suaát lieân
keát laø:
1 neáu 1+ 2.a
U = Kneáu < 1+ 2.a
1+ 2.a
K
K
- Neáu coù loãi xaûy ra trong quaù trình truyeàn daãn:
Goïi Pf laø xaùc suaát moät khung thu ñöôïc bò loãi. Khi ñoù soá
laàn truyeàn laïi trung bình cuûa moãi khung laø:
r
f
1N =
1-P
![Page 65: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/65.jpg)
3.5.3 Hieäu suaát söû duïng lieân keát:
- Vôùi kieåu truyeàn laïi coù löïa choïn thì:
- Vôùi kieåu truyeàn Go – back – N, hieäu suaát lieân keát seõ
giaûm vì khi coù moät khung bò loãi thì seõ coù nhöõng khung
khoâng bò loãi phaûi truyeàn laïi
r
r
1neáu 1+ 2.a
N
U =
Kneáu < 1+ 2.a
(1+ 2.a).N
K
K
f
f
f
f
1- Pneáu 1+ 2.a
1+ P (K -1)
U =
K.(1- P )neáu < 1+ 2.a
(1+ 2.a)[1+ P (K -1)]
K
K
![Page 66: Chuong 3_TSL](https://reader034.vdocuments.mx/reader034/viewer/2022042607/55376e7255034686768b4d9f/html5/thumbnails/66.jpg)