Luan Vandam Bao Chat Luong Phuc Vu Cho Truyen Video Streaming Trong Mang Khong d 9OPtA 20121031090903 18338

7/30/2019 Luan Vandam Bao Chat Luong Phuc Vu Cho Truyen Video Streaming Trong Mang Khong d 9OPtA 20121031090903

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I HC QUC GIA H NI

TRNG I HC CNG NGH

Nguyn nh Ha

M BO CHT LNG DCH V CHO VIC

TRUYN VIDEO STREAMING TRONG MNGKHNG DY 802.11

KHO LUN TT NGHIP I HC H CHNH QUY

Ngnh: Cng ngh thng tin

H NI - 2010


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I HC QUC GIA H NI

TRNG I HC CNG NGH

Nguyn nh Ha

M BO CHT LNG DCH V CHO VICTRUYN VIDEO STREAMING TRONG MNG

KHNG DY 802.11

KHO LUN TT NGHIP I HC H CHNH QUY

Ngnh: Cng ngh thng tin

Cn b hng dn: TS. Nguyn Hoi Sn

H NI - 2010


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Li cm nEm xin chn thnh cm n cc thy c trong trng i hc Cng Ngh - HQG

H Ni tn tnh gip truyn t kin thc cho em trong sut 4 nm hc qua

em c kin thc hon thnh kha lun ny

c bit, em xin gi li cm n su sc ti thy Nguyn Hoi Sn. Ngi

nhin tnh ng vin hng dn v gip em trong sut qu trnh thc hin kha

lun ny.

Em xin cm n s nhit tnh chia s kinh nghim, ng gp kin ca nhm

nghin cu do thy Nguyn Hoi Sn hng dn.

Xin gi li cm n tha thit nht ti b m v nhng ngi thn khc trong gia

nh ng vin gip con/chu rt nhiu trong sut qu trnh hc tp cng nh

hon thnh kha lun ny.

Mc d rt c gng hon thnh kha lun ny vi tt c n lc ca bn thn,

xong kha lun s kh trnh khi nhng thiu st, knh mong qu thy c tn tnh ch

bo cho em nhng sai lm cn khc phc ca mnh. Mt ln na em xin gi ti tt c

mi ngi li cm n chn thnh nht.

H ni, thng 05 nm 2010

Sinh vin : Nguyn nh Ha


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Tm tt

Ngy nay, vi s pht trin ca Internet cc dch v truyn thng a phng tin

ngy cng pht trin v ng vai tr quan trng trong nhiu mt ca i sng. Video

streaming l mt trong s nhng dch v nh th. Trong video streaming, d liu ctruyn trc tip t ngun pht v hin th trc tip ti bn nhn trong khi truyn m

khng cn phi i cho vic truyn d liu kt thc. Vi c im nh vy dch v

video streaming c nhng rng buc kh cao v bng thng cao, tr thp, t l mt

gi tin thp...

Video streaming ngy cng c s dung nhiu trong mng khng dy nh mng

LAN khng dy theo quy chun IEEE 802.11. Bng chng l s ra i v pht trin

ca cc ng dng nh mobile tv, video call Nhng ng dng ny v ang cnghin cu ci tin vi mc tiu cu ch yu l lm sao khc phc cc nhc im

c hu ca mng khng dy cng nhp ng tt cc rng buc ca video streaming.

Tuy nhin, vn m bo cht lng dch v ca cc dch v video streaming vn

cn c s h trca mng khng dy. IEEE 802.11e nhm p ng yu cu .

IEEE 802.11e l mt bin th ca chun IEEE 802.11. N nh ngha tp cc ci

tin vm bo cht lng dch v dnh cho mng WLAN thng qua cc sa i

tng MAC. Trong vic truyn video streaming chun IEEE 802.11e c nhiu ci

tin mang tnh tch cc. IEEE 802.11e cho php phn cp cc dch v bng cch gn

cho lung d liu ca chng tng ng vi cc u tin khc nhau. Cc gi tin ca

dch v c u tin cao hn c th dnh c chi truyn tt hn cc gi tin ca

dch v c u tin thp hn.

Tuy nhin vic m bo cht lng truyn video streaming trong mng khng

dy s dng chun IEEE 802.11e vn tn ti nhng im bt cp. V d, vic phn

tng trong video streaming s cho kt qu l mt video sc truyn di dng cc

lung khc nhau p ng nhng nhu cu v tnh trng mng khc nhau. Nhng viIEEE 802.11e cc lung ny sc xp vo cng mt u tin vic ny s lm cho

cht lng dch v km i khi tnh trng mng tc nghn.

Kha lun s trnh by gii php h trtt hn vic m bo cht lng ca

vic truyn video streaming trong mng khng dy. l gii php gn u tin

ng cho cc lung d liu tng ng vi cc tng ca video streaming. Vic truyn

video streaming c tht c s linh ng hn v cc lung d liu s c kh nng


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thay i c u tin trong qu trnh truyn. Gii php ny sm bo duy tr dch

v cng nhm bo cht lng dch v ca video streaming mt cch tt hn.


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Mc lc

Chng 1. Tng quan v video streaming .................................................................... 1

1.1. Gii thiu v video streaming ............................................................................... 1

1.1.1. Nn d liu......................................................................................................... 2

1.1.2. iu khin cht lng dch v tng ng dng................................................... 3

1.1.2.1. iu khin tc nghn ...................................................................................... 4

1.1.2.2. iu khin li.................................................................................................. 7

1.1.2.3. Che du li...................................................................................................... 9

1.1.3. Dch v phn phi phng tin truyn thng lin tc ....................................... 9

1.1.4. My ch (streaming server) ............................................................................. 12

1.1.5. Cc giao thc video streaming......................................................................... 12

1.1.6.

Cchng b video streaming...................................................................... 13

1.2. K thut phn tng trong video streaming .......................................................... 13

1.3. m bo cht lng dch v truyn video streaming. ........................................ 15

Chng 2. Chun IEEE 802.11e vm bo cht lng dch v trong mng khngdy ................................................................................................................... 17

2.1. Tng quan v IEEE 802.11 MAC ....................................................................... 17

2.1.1. Cc khong lin khung (Inter-frame space)..................................................... 17

2.1.2. Chc nng phi hp phn tn (Distributed Coordination Function - DCF) .... 19

2.1.2.1. Th tc quay lui ngu nhin (random backoff) ............................................ 19

2.1.2.2. Th tc xc nhn ACK.................................................................................. 20

2.1.3. Chc nng phi hp im (Point Coordination Function - PCF) .................... 21

2.1.3.1. Chu k khng tranh chp.............................................................................. 21


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2.1.3.2. Th tc truy cp PCF...................................................................................22

2.2. Chun IEEE 802.11e ........................................................................................... 23

2.2.1. Chc nng phi hp lai (Hybrid Coordination Function - HCF) .................... 23

2.2.1.1. EDCF (Enhanced Distributed Coordinated Function) ................................ 24

2.2.1.2. HCCA(HCF controlled channel access) ...................................................... 27

2.3. Truyn video streaming trong mng khng dy IEEE 802.11e .......................... 28

Chng 3. Gii php gn u tin ng cho cc lung video streaming trong mngIEEE 802.11e................................................................................................................. 30

3.1. Vn nghin cu............................................................................................... 30

3.2. Gii php gn u tin ng cho cc lung video streaming .......................... 37

3.2.1. Cc tham snh gi mi trng truyn. ........................................................ 38

3.2.2. M t gii php. ............................................................................................... 39

3.3. Phn tch nh gi ............................................................................................... 41

Chng 4. nh gi gii php u tin ng cho cc lung video streaming trongmng IEEE 802.11e....................................................................................................... 43

4.1. Mi trng m phng ......................................................................................... 43

4.1.1. Xy dng chng trnh m phng. .................................................................. 43

4.1.2. Kch bn m phng .......................................................................................... 45

4.1.2.1. Cc tham smi trng ............................................................................... 45

4.1.2.2. Thng sti cc trm ...................................................................................46

4.2. Kt qu m phng ............................................................................................... 47

4.2.1. nh gi v thng lng.................................................................................. 47

4.2.2. nh gi v tr ............................................................................................ 49

Chng 5. Kt lun ..................................................................................................... 52

5.1. Kt lun ...............................................................................................................52


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5.2. Hng pht trin tip theo...................................................................................52

Ti liu tham kho ......................................................................................................... 53


9/63

Mu

Trong nhng nm gn y vi s pht trin mnh ca cng ngh mng ni

chung v cng ngh mng khng dy ni ring. Ngy cng c nhiu ng dng ra i,

mt trong s l cc ng dng video streaming vi in hnh nh l video call,

mobile tv Nhng ng dng ny, ang ngy cng c pht trin v hon thin dn.

Tuy nhin, duy tr v m bo cht lng cho cc ng dng ny khng phi l vic

n gin. Cc ng dng video streaming lun lun gn lin vi nhng rng buc v

tr, thng lng cng nh t l mt gi tin nht nh. Mng khng dy li l mi

trng khng c n nh nh mng c dy. Trong mng khng dy vic kim sot

bng thng v tr cng nh t l mt gi tin l rt kh.

Mc d, vic h tr m bo cht lng dch v c h tr trong mng

khng dy vi vic p dng chun IEEE 802.11e, tuy nhin chun ny ch dng li

mc cung cp vic cho php cc ng dng c gn u tin khc nhau t chi

truyn d liu ca cc lung d liu tng ng vi dch v s khc nhau. Tuy nhin,

bn thn dch v video streaming li l vic truyn mt tp hp cc lung d liu

tng ng vi cc tng trong c ch phn tng ca n. Theo chun 802.11e th cc

lung ny vn s c cng u tin v s c chi truyn ngang nhau.

Mt khc, cc tng ca video streaming l khng tng ng nhau, mi tng li

c mc quan trng khc nhau. Nu m bo cht lng dch v cho video

streaming mt cch hiu qu th cc lung ny phi c phn bit vi nhau tng

MAC c cch phn phi chi truyn ph hp. Chun 802.11e khng lm c

vic ny.

Vy phi c mt gii php no c th h trvic m bo cht lng dch

v ph hp vi c ch phn tng ca video streaming sao cho hiu sut vic truyn

video streaming t hiu qu cao hn.

Kha lun s xut gii php gn u tin ng cho cc lung d liu tng

ng vi cc tng ca video streaming. Trong video streaming cc tng video c

phn cp quan trng theo th t gim dn nh sau : tng cbn, tng m rng 1,

tng mrng 2 Vi trng hp mng rnh ri, vic cc lung d liu ca cc tng

ny u nhn gi tru tin cao nh trong chun IEEE 802.11e m t l hp l,

tn dng bng thng ca mi trng truyn cng nh gim thiu tr ca gi tin.

Nhng, vi trng hp mng bn vic phn bu tin nh vy l khng hp l bi


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v n s lm hiu qu truyn ca tt c cc lung cng gim xung. Do ty theo

mc bn ca mi trng truyn, cc lung d liu sc gn li u tin mt

cch hp l, sao cho m bo rng vic truyn ca lung d liu quan trng phi c

hiu qu cao, v c gng truyn d liu ca lung c quan trng nh hn mt mc

c th.

Kha lun chia thnh nm chng.

Chng 1. Gii thiu tng quan v dch v video streaming, c ch phn tng

trong video streaming v m bo cht lng dch v truyn video streaming.

Chng 2. M t tng quan v 802.11 MAC v chun 802.11e ng thi gii

thiu v nhn xt v vic truyn video streaming trong 802.11e

Chng 3. i su phn tch v chng minh s cn thit ca vic gn u tinng cho cc lung video streaming. Sau s trnh by gii php a ra v nhn xt

v gii php

Chng 4. Tin hnh nh gi gii php gn u tin ng cho cc lung video

streaming vi chun ieee 802.11e bng cch xy dng th nghim m phng trn b

cng c m phng ns2.

Chng 5. K lun v nu hng nghin cu tip theo.


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1

Chng 1. Tng quan v video streaming

1.1. Gii thiu v video streaming

Video streaming l phng thc chuyn giao d liu video dng nn trn

Internet mt cch u n v lin tc, sao cho ngi dng khng cn i ti khi c

ton b tp tin mi c th xem c ni dung ca n m c th bt u hin th cc d

liu a phng tin khi m mt phn d liu c nhn. Ngi dng phi dng mt

chng trnh t bit n c th gii nn d liu v phn phi d liu hnh nh cho

mnh hnh cng nh m thanh ti loa. Video streaming c pht trin da trn hai

cng ngh ch cht l cc cng ngh m ha video v cc cng ngh tng kh nng

mrng vic phn phi video.

Hnh 1.1. Kin trc h thng video streaming

Ban u d liu th c nn li thng qua b nn d liu, v c lu tr ti

thit b lu tr bn trong server. Sau khi c yu cu t client th server s ly cc d

liu tng ng c nn li trong thit b lu tr, v chuyn giao ti cho b phn

iu khin cht lng cht lng dch v tng ng dng. B phn ny s ty vo yu

cu ng dng, tnh trng mng to cc dng lu lng hp l, sau sy cc

lung d liu xung cho tng giao vn (transport layer). Cc giao thc tng giao vn s

c trch nhim gi d liu thng qua Internet ti client. Khi cc gi tin c lu

Raw Video

Raw Audio

Internet

(Continuous mediadistribution services)

Streaming Server Client/Receiver

Storage Device

Compressed

Video

Compressed

Audio

Application-layer

QoS Control

Video

Com ression

Audio

Com ressionTransport

Protocol

Video

Decoder

Audio

Decoder

Application-layer

QoS Control

Transport

Protocol

Media

Syschronization


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2

chuyn trn mng, n c th b mt mt hoc l c tr ln khi mng b tc nghn.

h trcht lng video/audio truyn ti, cc dch v h trtruyn thng a phng

tin lin tc c trin khai (v d nh caching ). Khi cc gi tin n c client

thnh cng, n c truyn ln tng giao vn, ri tip l b phn iu khin cht

lng dch v tng ng dng. Sau chng c gii m cho ra cc d liu th ban

u, tip theo c th s dng c nhng d liu ny, cn phi s dng n b

ng b ha a phng tin ng b video v audio mt cch chun xc.

Vy tc l mi mt h thng video streaming cn thit phi c su thnh phn

tng ng vi su bc thao tc nh trn, by gita si c th tng thnh phn .

1.1.1. Nn dliu

Cc d liu trong streaming server lun lun phi c nn trc khi truyn gim thiu dung lng truyn trn mng. ng thi, khi d liu c nhn client

cng phi thc hin thao tc gii nn thu c d liu gc. C hai kiu nn d liu

cbn l phng php nn khng mrng (non-scalable compress), v phng php

nn mrng (scalable compress).

Phng php nn khng mrng :

Trong phng php nn khng m rng, d liu thu c sau khi nn l mt

chui bit. Chui bit ny c gi ti bn nhn, sau qu trnh gii nn c thc thi

trn chui bit v kt qu thu c l d liu tng ng vi d liu gc bn gi.

Raw

videoDCT Q VLC

Compressed

Bit-stream

Compressed

Bit-streamDCT Q VLC

Decoded

video

DCT: Discrete Cosine

Transform

Q: Quantization

VLC: Variable Length Coding

VLD: Variable Length Decoding

IQ: Inverse Quantization

IDCT: Inverse DCT

Hnh 1.2. (a) nn khng m rng ; (b) gii nn khng m rng


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3

Phng php nn mrng :

Trong phng php nn mrng gc c chia thnh cc lung con, trong c

mt lung c gi l lung cbn, khi thc hin qu trnh gii nn n khng ph

thuc vo bt k mt lung con no khc, v cung cp cht lng video dng th.

Cc lung con khc gi l lung m rng, vic gii nn chng phi ph thuc vo

lung cbn, v chng cung cp cht lng video tt hn. Lung ton phn (kt hp

ca tt c cc lung ) s cung cp cht lng video tt nht.

1.1.2. iu khin cht lng dch v tng ng dng

Phn ny m t cc k thut iu khin tc nghn v iu khin li (ch p dng

cc h thng u cui v khng c bt k yu cu no vi mng)

Raw

Video DCT Q VLC

Base Layer

Compressed Bit-stream

IQ

Q VLCEnhancement Layer


-

(a)

Base LayerCompressed Bit-stream

Enhancement Layer


VLD

VLD

IQ

IQ

IDCT

IDCT

Base LayerDecode Video

Enhancement

Layer Decode

Video

(b)

Hnh 1.3. (a) nn m rng ; (b) gii nn khng m


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4

1.1.2.1. iu khin tc nghnNi dung chnh ca iu khin tc nghn l lm gim tr v mt gi tin. C

ch ca n da trn iu khin tc truyn (rate control) v nh hnh t l (rate

shaping).

iu khin tc truyn l k thut gim thiu mt cch nh nht c th s

tc nghn, bng cch iu chnh tc truyn video da vo tnh trng hin thi ca

mng. C ba loi iu khin tc truyn l : cn c vo bn gi, cn c vo bn

nhn, v phng php kt hp.

a. Phng php cn c vo bn gi :

Trong phng php ny, bn gi chu trch nhim iu chnh tc truyn d

liu. Da vo thng tin t cc gi tin phn hi, bn gi c th xc nh c tnh trng

hin thi ca mng, t c thiu chnh tc truyn mt cch hp l. K thut

ny c th s dng trong c truyn unicast cng nh multicast.

Hnh 1.5. (a) Truyn video unicast ; (b) Truyn video multicast

Quality of

service

Congestion

control

Error

control

Hnh 1.4. m hnh iu khin cht lng dch v


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5

Vi truyn video unicast c hai phng thc thc thi vic iu chnh tc

truyn bn gi, l da trn csthm d v da trn mu.

Phng thc da trn csthm d (probe-base) : T tng chnh ca phng

thc ny chnh l vic duy tr t l mt gi di mt ngng nht nh. C hai cch thit lp tc truyn mt l tng theo cp s cng v gim theo cp s nhn, hai l

tng theo cp s nhn v gim theo cp s nhn.

Phng thc da trn mu (model-base) : Da vo m hnh ca giao thc TCP,

thng lng ca bn gi sc xc nh theo cng thc :

pRTT

MTU

=

22.1

Trong :

: thng lng.

MTU: kch thc gi tin c dng trong kt ni.

RTT: round-trip time ca mt kt ni.

p: t l mt gi tin ca kt ni.

V l do da trn m hnh ca giao thc TCP m phng thc ny cn c gi

l phng thc iu chnh tc truyn TCP-frendly.

Vi truyn video multicast, bn gi s s dng mt knh truyn n gi d

liu cho cc bn nhn, trong trng hp multicast th ch c phng thc da trn c

sthm d c s dng (iu chnh t l truyn sao cho lun nm di mt ngng

nht nh). Phng thc s dng knh chung ny rt hiu qu cho cc bn nhn s

dng chung 1 knh. Tuy nhin, phng php ny thiu tnh mm do bi v cc yu

cu t cc bn nhn l khc nhau, bng thng truy cp knh chung ca cc bn cng

khc nhau.

b. Phng php cn c vo bn nhn :

Bn nhn kim sot tc nhn d liu bng cch thm hoc bt cc knh.

Thng thng, phng php ny s dng trong vic truyn multicast. Tng t nh

trong phng php cn c vo bn gi, phng php cn c vo bn nhn cng s

dng 2 phng thc : phng thc da trn cs thm d v phng thc da trn

mu.


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6

Phng thc da trn csthm d bao gm hai phn : Khi khng pht hin ra

tc nghn, bn nhn siu tra mi trng bng cch thm mt knh truyn, nu nh

vn khng pht hin ra tc nghn th xem nh vic thm knh truyn l thnh cng.

Ngc li nu sau khi thm m pht hin tc nghn, th bn nhn phi hy ngay knh

truyn va thm vo. Khi pht hin ra mi trng ang tc nghn, bn gi tng xa

mt hoc vi knh truyn gim tc truyn ti.

Phng thc da trn mu vn s dng cng thc tnh thng lng nh

phng php cn c vo bn gi, khng c g khc bit

c. Phng php kt hp :

Phng php ny l s tng hp ca c hai phng php cn c vo bn gi v

cn c vo bn nhn. Tc l bn nhn vn iu chnh tc nhn bng cch thm btcc knh truyn, ng thi bn gi cng thit lp tc gi hp l da vo cc gi tin

phn hi.

nh hnh t l (rate shaping) :

Mc tiu ca nh hnh t l l cn i t l gia tc truyn lung d liu

trc khi nn v tc hin thi ca mng. Bi v, tc video lu my ch c

th khng ph hp vi bng thng hin thi ca mng. Bnh hnh t l chc s

dng trong trng hp, phng php iu khin tc nghn l phng php cn c vobn gi. C mt s phng php nh hnh t l nh sau :

Lc m (Codec filter) : S dng gii nn v nn video. Thng thng

phng thc ny dng chuyn m gia cc phng thc nn khc nhau. Vic gii

nn v nn li l khng hon ton (khng gin nn, nn li ton b video) m ch thao

tc da vo phng thc nn.

Lc b khung (Frame-dropping filter) : Phn bit cc loi khung v hy cc

khung da vo vic phn chia quan trng ca chng (v d th t h dn l B-frame, P-frame, I-frame). B lc b frame dng lm gim tc truyn video bng

cch t chi mt s khung v truyn cc khung cn li tc thp.

Lc b tng (Layer-dropping filter) : Phn bit cc tng v loi b cc tng ty

theo quan trng ca chng. Vic loi b cc tng theo th t t cc tng m rng

cao nht ti tng cs.

Lc theo tn s (Frequency filter) : thc hin cc thao tc tng nn d liu.

Cch ca lc theo tn s gm ba thnh phn :


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7

-Lc thp : loi b h s DCT nhng tn s cao

-Lc gim mu : thc hin cng vic ging nhphn lc thp, tuy nhin n ch

thao tc trn thng tin v mu sc ca video.

-Lc mu thnh khi trng : loi b tt c cc thng tin v mu sc ca video.Trong MPEG n chn gin l thay th mi khi cha thng tin mu sc bi khi

trng. Khng ging vi lc b khi, lc theo tng s lm gim bt bng thng m

khng tc ng ti tc truyn khi. Nhng ci gi phi tr l cht lng ca khi s

b gim i.

Lc lng t ha li (Re-quantization filter) : Thc thi cc thao tc tng nn d

liu. u tin, b lc trch cc h s DCT t cc dng video c nn thng qua

cc k thut tng t nh b lng t ha. Sau , b lc lng tng ha li cc h sDCT vi cc bc lng t ha ln hn, iu to nn hiu qu l gim bt tc .

1.1.2.2. iu khin liBao gm bn c ch chnh FEC (forward error correction), truyn li

(retransmission), m ha li n hi (Error-resilient encoding) v che du li (Error

concealment.

a. FEC :

Nguyn l ca FEC l thm thng tin vo thng ip gc c th xy dng li

thng ip trong trng hp c mt mt xy ra. Cc loi cbn ca thng tin thm vo

gm : M ha knh (channel coding), M ha ngun (source coding-based FEC) v

M ha kt hp ngun/knh (joint source/channel coding). Ta s tm hiu tng loi

thng tin thm ny.

M ha knh (channel coding) : Trong cc ng dng Internet, m ha knh

thng thng s dng vi thut ng m kha khi (block codes). Mi mt video ban

u chia ra lm cc on (segment). Mi mt on tng ng vi k gi tin. Sau

ng vi mi on, phng php m ha khi c p dng cho k gi tin to ra mt

khi n gi tin (n>k). chuyn i tuyt i mt on, mt ngi dng ch cn bt k

k gi tin trong s n gi tin k trn.

M ha ngun (source coding-based FEC - SFEC) : L pht kin gn y nht

trong s cc phng php FEC dnh cho vic truyn video trn Internet. Ging m ha

knh, m ha ngun cng thm thng tin d tha phc hi. V d, gi tin th n s


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8

cha nhm khi th n (group of blocks) v thng tin d tha ca nhm khi th n-1.

l mt phin bn nn ca nhm khi th n-1 vi lng t ln hn.

M ha kt hp ngun/knh : l mt cch tip cn ti u phn b t l gia

m ho ngun v m ho knh.b. Truyn li vi rng buc tr :

Khi bn nhn pht hin mt gi N, nu (Tc + RTT + Ds < Td(N)) bn nhn s

gi yu cu cho bn gi gi li gi N. y Tc l thi gian hin ti, RTT l round

trip time, Ds l mt thut ng yu, Ds c th bao gm dung sai ca sc lng RTT,

thi gian tr li ca bn gi, thi gian gii m, T(N) l thi gian m N phi hin th.

c. M ha li n hi :

Mc ch l gim thiu li ca video nn do vic mt gi tin. Chun chung

ca phng php m ha li n hi bao gm ng b li nhn, chia d liu, khi

phc d liu. Nhng, nhng cch thc ny li khng ph hp vi mi trng wireless

cng p dng c cho mi trng internet. Do vy, chng ta s khng xt cc cng

c n hi li. Thay vo chng ta s trnh by phng thc m ha a m t

(MDC), phng thc ny ph hp hn vi vic truyn video trn Internet.

Vi MDC, mt video gc sc m ha thnh nhiu lung theo cch sau : Mi

mt s m t cung cp cht lng c th chp nhn c, kt hp nhiu m t li vinhau th thu c cht lng tt hn. u im ca MCD l :

-C tnh khng c tt vi vic mt mt : Nu mt bn nhn no ch nhn

c mt m t, th n vn c th thu c video vi cht lng chp nhn c.

-C tnh tng cng cht lng : Nu mt bn nhn no nhn c nhiu m

t khc nhau ca video, th n c th xy dng c video c cht lng tt bng cch

kt hp cc m t li vi nhau (cng nhiu m t th cht lng cng tt).

Tuy nhin, nhng u im c ci gi ca n. to mt m t cung cp cc

cht lng chp nhn c, mi m t cn phi a ra cc thng tin cn thit v truyn

hnh ban u. Thm vo , mc d kt hp cc m t cung cp cht lng tt hn,

sng n ca vic kt hp cc m t c th lm cho cht lng nn sau ny gim i.


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1.1.2.3. Che du liM ha li n hi c thc thi bn gi bng cch tng cng kh nng c vi

vic mt mt cho video nn ngay c khi c th khng c s mt mt gi tin. Ngi ta

cn gi y l phng php phng b, mt cch tip cn khc gi l phng php

chng mt mt, c thc thi pha nhn khi m vic mt gi tin l thc s xy ra

phng php ny gi l che du li. V cbn, phng php ny che du s mt mt

d liu bng cch trnh din video sao cho khng lm cho ngi xem nhn ra.

C hai phng thc thc thi vic che du li, l ni suy theo khng gian v

ni suy theo thi gian. Trong phng thc ni suy khng gian, cc d liu ti nhng

im nh b mt c thc ni suy thng qua nhng hng xm ca n. Cn ni suy

thi gian th s dng cc d liu ca cc frame nhn c ngay trc n xy dng

li d liu ca frame b mt.

1.1.3. Dch v phn phi phng tin truyn thng lin tc

S h trt tng mng c th ci thin c tr cng nh s mt mt gi tin.

Vic streaming video cng nh audio c xp vo truyn thng lin tc bi v chng

bao gm mt hng i phn ngch truyn thng. Chng truyn cc thng tin sao cho

trnh by theo thi gian. Xy dng trn c s Internet (giao thc IP), dch v

phn phi phng tin truyn thng lin tc hng ti mc ch l cung cp cht

lng dch v, v t c hiu qu trong vic truyn video /audio trn cch c gng

ti a ca Internet. Dch v phn phi phng tin truyn thng lin tc bao gm 3 c

ch chnh l lc d liu tng mng, multicast trn tng ng dng v nhn bn ni

dung.

a. Lc dliu tng mng

Nh k thut iu khin tc nghn, lc d liu tng mng c gng t c

cht lng truyn thng ti a trong iu kin mng tc nghn. B lc my ch

truyn thng c thiu chnh t l truyn video streaming theo tnh trng ca mng.

Tuy nhin, my ch qu bn nn khng iu khin c. Do , cc nh cung cp dch

v c th cung cp cc b lc mc mng.


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Hnh 1.6. B lc c ci t tng mng

Hnh 1.6 trn m t mt v d v vic t b lc tng mng. Cc node c

nh nhn R thng bo cho bnh tuyn rng khng c thng tin vnh dng

video streaming v c th ngu nhin vt b gi tin. Cc Node c nhn Fillter nhn

yu cu t my khch v iu chnh lung gi bi my ch tng ng. Gii php ny

cho php cc nh cung cp dch v c th thay th b lc trn mi node bng b lc

mng. Hn na, c th cung cp nhiu b lc trn ng t mt my chn my

khch.

Hnh 1.7. M hnh h thng ca b lc tng mng.

m phng vn hnh ca b lc, m hnh h thng c ch ra trong Hnh 1.7.

M hnh bao gm my ch, my khch v t nht mt b lc v hai knh o gia

chng. Mt knh o dng iu khin, knh khc dng cho d liu. Cc knh tng

ng tn ti thnh cp vi cc b lc. Knh iu khin c hai hng v c th l cc kt

ni TCP. M hnh trn ch cho php my khch c th kt ni vi ch mt my (b lc

cui cng), v s hoc chuyn yu cu hoc thc hin lun yu cu trn chng. S vn

hnh ca b lc trn d liu phng bao gm: Nhn video streaming t my ch hoc

b lc trc, thc hin cc yu cu, truyn tip cc yu cu ti b lc trc ca n.

Thng thng, cc b lc loi frame c s dng trong lc tng mng. Bn

nhn c th thay i bng thng ca video streaming bng cch gi yu cu ti b lc

tng/gim t l loi b frame. quyt nh xem b lc nn tng hay gim bng


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thng, bn nhn lin tc o c t l mt gi tin p. Da vo t l mt gi tin, cch

iu khin t l c thc thc hin. Nu t l mt gi tin ln hn ngng , my

khch s yu cu b lc tng t l loi frame. Nu t l thp hn ngng khc(

< ) th bn nhn s yu cu b lc gim t l loi frame.

u im ca b lc loi gi tin s dng trn tng mng l:

-Nng cao cht lng truyn thng: V d, khi mt video streaming t kt ni

lung trn vi bng thng cn li nhiu hn ti kt ni lung di vi bng thng cn

li t hn, s dng b lc loi frame ti im kt ni (gia kt ni lung di v kt

ni lung trn) c th nng cao cht lng truyn thng. Bi v b lc hiu rng nh

dng ca video streaming v c th loi b cc gi tin theo cch tt nht m bo cht

lng lung thay vo ngt lung.

-Hiu qu bng thng: b lc c th gip lu tr ti nguyn ca mng bng vic

loi b cc frame ti tr.

b. Multicast tng ng dng

Cc thit k trc cho video streaming trn Internet thch hp vi cch truyn

thng unicast (im-im) nh email, trao i file v trnh duyt Web, nhng kh c

th h trhiu qu vi vic truyn ni dung ln.

Chnh v vy m cch truyn multicast dn thay th vi nhng u im vttri. N gim ti cho my ch do mi node mng c thng vai tr nhn d liu v

ngun tip tc truyn d liu sang cc node khc.

c. Nhn bn ni dung

Mt k thut quan trng cho nng cao mrng ca cc h thng truyn thng

l vic nhn bn ni dung. Nhn bn ni dung c hai loi: lu tr v phn chiu. C

hai loi u tm mt ni dung cc my khc gn v c u im:

-Gim s dng bng thng trong kt ni mng.

-Gim ti trn cc my ch truyn thng.

-Gim tr ca my khch.

-Tng sn c.


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1.1.4. My ch (streaming server)

My ch lung ng mt vai tr quan trng trong vic cung cp dch v video

streaming. a ra nhng dch v cht lng cao, my ch lung cn phi x l d

liu a phng tin vi s rng buc v thi gian. Ngoi ra, my ch lung cng cn

h tr cc hot ng iu khin VCR. Hn na, n cng cn cung cp chc nng

truyn thng kiu ng b.

lm c iu , thng thng, my ch lung gm cc thnh phn sau:

-Ccu truyn: Mt ccu truyn tham gia vo tng ng dng v cc giao thc

giao vn c thc hin trong my ch. Thng qua ccu truyn, cc my khch c

th giao tip vi mt my ch v nhn ni dung a phng tin mt cch lin tc v

ng b.

-Hiu hnh: Khc vi hiu hnh thng thng, hiu hnh cho my ch

lung cn phi c thi gian thc an ton cho cc ng dng lung.

-H lu tr: phi h trlu tr truyn thng lin tc v c th truy cp c.

1.1.5. Cc giao thc video streaming

Nhiu giao thc c thit k v chun ho cho giao tip gia my ch lung

v my khch. Theo tnh nng, cc giao thc ny lin quan trc tip n video

streaming Internet v c th chia thnh ba loi sau:

-Giao thc tng mng: Cung cp dch v mng cbn nhnh a ch mng.

Giao thc Internet (IP) phc v nh giao thc tng mng cho vic video streaming trn

Internet.

-Giao thc tng giao vn: Cung cp cc chc nng truyn ti gia cc im u

cui trong ng dng video streaming. Cc giao thc bao gim UDP, TCP, giao thc

giao vn thi gian thc (RTP), giao thc iu khin thi gian thc (RTCP). UDP v

TCP l hai giao thc tng giao vn thp nht trong khi RTP v RTCP l cc giao thc

tng giao vn cao hn.

-Giao thc iu khin phin: nh ngha thng ip v cchiu hin d

liu truyn thng trong mt phin. RTSP v giao thc thit lp phin (SIP session

initiation protocol) l cc giao thc tng giao vn.


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1.1.6. Cchng b video streaming

ng b video streaming chnh l vic bo tr mi quan h tm thi gia mt

lung d liu vi mt s lung truyn thng khc. C ba mc ca ng b gi l ng

b trong lung, ng b lung v ng bi tng.

-ng b lung trong lung: Tng thp nht ca truyn thng lin tc hoc d

liu thi gian thc l tng d liu truyn thng. n v ca tng truyn thng l cc

khi d liu logic nh frame truyn hnh/truyn thanh. ng bmc ny c gi l

ng b lung d; bo tr s lin tc ca cc n v d liu. Khng c ng b lung

d, trnh din ca lung c th b ngt bi vic dng v s sai khc.

-ng b lung: tng hai ca d liu thi gian thc l tng lung. n v ca

tng lung chnh l mt lung. ng btng ny chnh l ng b lung, c nhim

v bo tr mi quan h gia cc truyn thng lin tc. Khng c ng b lung, s

khng khp gia cc lung c th gy ra li. V d, ngi dng s thy kh chu khi

vic di chuyn mi ngi ni khng tng ng vi m thanh.

-ng bi tng: Tng cao nht ca ti liu a phng tin l tng i tng,

l s kt hp lung v d liu thi gian thc nh ch v hnh nh. ng btng

ny gi l ng bi tng. Mc ch ca n l bt u v kt thc ca trnh din

thi gian thc trong mt khong thi gian va phi, nu ti cc im nh ngha trc

ca i tng truyn thng c lp thi gian c tht c. Khng c sng b

ny, ngi xem ca vic trnh din slide c th kh chu v ting bnh cho mt slide th

slide khc hin th.

1.2. K thut phn tng trong video streaming

Vic m ha video theo cch thng thng phi xc nh cc tham s kch c

khung, tc khung, tc d liu v cht lng video. Mt vn ny sinh l cc

tham s ny cnh khi m ha v rt kh thay i. Gi s nh m ha chun vi tc

6Mb/s sau lu tr video nn server, khi n c yu cu streaming bn

nhn mong mung tc nhn d liu s l 6Mb/s nhng v mt l do no tc

m ng truyn cung cp ch ti a l 4Mb/s. Khi , server phi thc hin chuyn

m ph hp vi cc tham s hin ti, iu ny i hi s tnh ton rt ln. C mt

phng php c th gii quyt vn ny l to ra nhiu bn sao ca video trn server

phc v cho yu cu khc nhau t ngi dng. Tng t nh vy, khi ngi xem

yu cu cht lng video khc nhau. iu ny l rt quan trng khi thng mi ha

bi v mi lp ngi dng, mi lp khch hng li c mt u tin khc nhau. T


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, dn n vic cung cp cht lng cho cc lp khch hng l khc nhau. gii

quyt nhng vn ny m ha video mrng sinh ra, n c s dng u tin trong

MPEG-2 v pht trin hon thin dn.

M ha video mrng l mt k thut cho php nn video mt ln v cung cpvic gii nn video vi cc thng s nh tc d liu, tc khung, phn gii v

cht lng video khc nhau. Ngi ta cn gi y l phng php mrng ng bi

v cc tham s trn c th thay i bt k thi gian no trong qu trnh truyn. V d,

khi mng tc nghn th c th truyn vi tc thp hn, v khi mng rnh ri th c

th chuyn sang truyn vi tc cao hn. Di y l m hnh cu trc m ha

video mrng.

Hnh 1.8. Cu trc m ha video m rng

M hnh m ha video m rng d liu video c m ha vo mt tng cs

(base layer) v mt hoc mt vi tng mrng (enhancement layer). Tng csc

m ha vi tc thp gii m dng video, do vy, cht lng video c gii

nn ra ng vi n c cht lng thp nht. Vic gii m tng cs l c lp khng

ph thuc vo bt k mt d liu no khc. Trong khi tng mrng c m ha

vi tc cao hn, mc ch ca n l lm ci thin cht lng video tng cs, tuy

nhin, vic gii m cc tng mrng l ph thuc vo tng cbn. V d, video gc

m ha s dng ba tng : tng csvi tc l 64kb/s, tng m rng th nht vi

tc l 128kb/s v tng m rng th hai vi tc l 128kb/s. bn nhn c th

gii m tc 64kb/s (tc l ch s dng tng cs) th kt qu gii m cho ra video

c cht lng thp nht. Nu bn nhn gii m tc 192kb/s (s dng tng cs

v tng mrng th nht) th kt qu cho ra l video c cht lng trung bnh. V

nhn c video c cht lng cao nht th bn nhn phi gii m tc 320kb/s (s

dng tng csv hai tng mrng).


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Vi phng php phn tng ny, vic truyn video streaming khng cn ti cc

phng thc m ha li tng thch vi iu kin mi trng. Mt mt na, vic

phn tng lm cho vn cung cp dch v hng ngi dng trnn d dng. Vi

lp ngi dng m c nhu cu cng nhu tin cao hn, server s gi cho h

nhiu hn cc tng m rng t tng cht lng video u cui. Vi lp ngi

dng c nhu cu v u tin thp hn, server vn c th cung cp cho h mt dch

v video bnh thng nhng vi cht lng thp hn bng cch gim bt gi cc

lung d liu cc mrng.

1.3. m bo cht lng dch v truyn video streaming.

Dch v video streaming yu cu mt siu kin kin quyt sau :

Bandwidth : Mi mt ng dng video streaming u c mt yu cu v bngthng nh nht duy tr ng dng. Tuy nhin h vi csh tng Internet hin ti,

khng c bt k cch no p ng iu kin ny.

Delay : L khi nim tr ca cc gi tin hai u cui, hoc cng c th bao

gm c thi gian gii m v hin th video. Nu gi tin khng n c ng thi

im th video c th b dng gy cm gic kh chu cho ngi dng

Loss : Mt gi tin l vic khng th trnh khi trn mi trng Internet. Vic

mt gi tin c th gy ra hin tng mo m hay mt i s lin kt trong vic hin thvideo.

Trong m hnh h thng video streaming nh trnh by phn trc, hu nh

tt c cc thnh phn ca h thng u thc thi nhng thao tc k thut thu c

hiu qu cao trong vic duy tr bng thng, gim tr v khc phc mt gi tin. y

cng chnh l t tng ca vic m bo cht lng dch v trong video streaming.

Dch v video streaming c tn ti v hot ng mt cch cht lng hay khng tt c

ph thuc vo vic m bo cht lng dch v.Tuy nhin, vi nhng mi trng mng khc nhau trong nhng iu kin khc

nhau th cn c thm nhng cch b sung cho vic ny. V d, trong mng LAN th

vic m bo cht lng dch v cho video streaming c thn gin hn l trong

mng WLAN bi v bn cht mi trng truyn ca chng l khc nhau. Do , cc

mi trng khc nhau cng s c nhng phng thc khc nhau lm sao m bo

cht lng dch v tt nht h tng, nhm mc ch h tr tt nht cho cc dch v

s dng h tng .


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Chng tip theo ta si nghin cu chun IEEE 802.11e l chun vm bo

cht lng dch v trong mng WLAN, tm hiu xem phng php xy dng h

tng mng khng dy ra sao c th h trcho dch v video streaming trong iu kin

mi trng c nhiu tc nhn gy nh hng xu ti dch v ny.


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Chng 2. Chun IEEE 802.11e vm bo cht lngdch v trong mng khng dy

2.1. Tng quan v IEEE 802.11 MACCng ging nh nhng chun 802.x khc, IEEE 802.11 ch m t chun giao

thc tng vt l (PHY) v tng iu khin truy cp mi trng truyn (medium

access control - MAC).

Tng MAC thc thi kh nhiu nhim v nhng nhim v chnh v quan trng

nht l truy cp mi trng truyn. Chun IEEE 802.11 MAC bao gm hai cch truy

cp mi trng truyn : th nht l chc nng phi hp phn tn DCF v th hai l

chc nng phi hp im PCF. DCF s dng giao thc a truy cp cm nhn sngmang trnh tc nghn CSMA/CA (Carrier sense multiple access with conllision

avoidance). PCF s dng phng thc thm d vi s h tr ca b phi hp im

PC. C hai phng thc c th cng c s dng trong mt mng BSS v sc s

dng ln lt xen k nhau. Cch DCF l bt buc trong khi PCF l ty chn. V vic

trin khai PCF phc tp nn nhiu nh sn xut khng h trPCF.

2.1.1. Cc khong lin khung (Inter-frame space)

Khong lin khung l khong thi gian gia hai frame lin tip xut pht t mt

trm trong mng khng dy. C bn loi khong lin khung tng ng vi bn mc

u tin truy cp knh truyn khc nhau. Di y l danh sch bn loi IFS ni trn

sp xp theo th t t ngn n di.

-SIFS (short inter-frame space)

-PIFS (PCF inter-frame space)

-DIFS (DCF inter-frame space)

-EIFS (extended inter-frame space)


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Hnh 2.1. Cc khong lin khung IFS trong 802.11

a. Short Interframe Space (SIFS) :

L khong thi gian nh gia cc frame d liu v ACK ca n. Chng c s

dng cho cc trm c u tin cao nht yu cu truy cp knh truyn u tin. Gi tr

ny l cnh da vo tng vt l pha di. Thng thng cc d liu c truyn

sau khong lin khung ny l ACK (acknowledgement), RTS (Request to send) v

CTS (Clear to send) messages.

b. PCF inter-frame space (PCFS) :

PIFS c s dng bi AP hot ng trong PCF, khi bt u chu k khng tranh

chp CFP. Gi tr ca PIFS nh hn DIFS v ln hn SIFS (DIFS > PIFS > SIFS), do

AP c u tin cao hn cc trm khc do n ch phi chkhong PIFS ngn hn

DIFS.

Gi tr PIFS c tnh thng qua cng thc : PIFS = SIFS + Slot time [4]

c. DCF inter-frame space (DIFS) :

L khong lin khung s dng trong phng php truy cp knh truyn DCF.

Cc khung d liu phi i mt khong thi gian l DIFS mi c php truyn i.

Gi tr DIFS c tnh theo cng thc : DIFS = SIFS + 2 * Slot time [4]

d. Extended Interframe space (EIFS) :

Nu frame n k trc c li th trm phi i mt khong thi gian l EIFS

thay v DIFS trc khi truyn frame.

EIFS c tnh theo cng thc : EIFS = SIFS + DIFS + ACKTxTime [4]

ACKTxTime l thi gian cn thit truyn mt frame ACK


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2.1.2. Chc nng phi hp phn tn (Distributed Coordination Function - DCF)

L phng php cbn s dng trong IEEE 802.11 MAC. DCF cn c bit

n nh l cch cm nhn sng mang a truy cp mi trng truyn trnh tc nghn

(CSMA/CA), s dng th tc quay lui ngu nhin (backoff). ng thi, tt c cc gi

gi i u c xc nhn bi frame ACK v bn gi s gi li gi tin nu khng nhn

c frame ACK xc nhn t bn nhn.

Khi mt trm mun truyn d liu, n phi xc nh l knh truyn c ri hay

khng. Nu ri th n c php truyn ngay, cn nu mi trng truyn c xc

nh l bn th trm phi chcho n khi phin truyn ang tn ti kt thc, sau

trm phi i mt khong thi gian backoff ngu nhin mi c php truy cp knh

truyn.

2.1.2.1. Th tc quay lui ngu nhin (random backoff)

Hnh 2.2. Th tc quay lui ngu nhin trong DFC

Khi mt trm mun truyn d liu, n s kim tra xem trng thi knh truyn l

bn hay ri bng cc s dng cc cch cm nhn sng mang (CS mechanism). Nu

mi knh truyn l bn, trm phi i cho ti khi mi trng ri, sau trm si

mt khong thi gian chnh bng gi tr ca DIFS trong trng hp khung d liu truyn trc c xc nh l khng c li, hoc n phi i mt khong thi gian

l EIFS nu nh pht hin ra c li xy ra khi nhn khung d liu truyn trc .

Sau khong lin khung ny , cc trm s sinh ra mt khong thi gian ngu nhin v s

gim dn bin m backoff. Khi bin m backoff gim n 0, trm sc php

truyn. Nu knh truyn c xc nh l bn trong khong thi gian backoff th th

tc backoff s tm dng, bin m backoffc gi nguyn gi tr. Khi knh truyn

ri th th tc backoff sc php bt u li vi gi trang tm dng ln trc.


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Vic cc trm chn cc khong thi gian ngu nhin khc nhau lm gim thiu

kh nng xung t gia cc trm m chknh truyn cng mt thi im.

Gi tr backoffc xc nh theo cng thc :

Backoff Time = Random() x aSlotTime

Trong :

-aSlotTime : l gi tr tng ng vi c im ca tng vt l (PHY).

-Random() : l mt gi tr ngu nghin nm trong khong [0,CW], y CW l

ca s tranh chp, mt gi tr s thc nm trong khong gi tr CWMin v CWMax

(hai gi tr ny l c im ca tng vt l).

CW khi to t gi tr CWMin v sau mi ln khung truyn khng thnh cng th

gi tr ny li c tng gp i, v th tc backoff mi li c bt u, gi tr CW

c tng vi cp s nhn nh th sau mi ln truyn d liu khng thnh cng cho ti

khi n t ti ngng CWMax, th CW s gi nguyn gi tr (khng tng na cho

d c truyn khng thnh cng). CW sc thit lp li sau khi d liu c truyn

thnh cng.

2.1.2.2. Th tc xc nhn ACK

Hnh 2.3. Th tc xc nhn ACK

Khi mt trm nhn c mt khung v xc nh rng n khng b li. Trm s

gi li cho bn gi mt khung ACK nhm mc ch xc nhn rng n nhn c

khung d liu. Khung ACK ch s dng trong trng hp nhn c gi tin unicast


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cn nu gi tin nhn l gi tin multicast hoc broadcast th khung ACK khng c s

dng. Khung ACK ch cn phi i mt khong lin khung l SIFS v khng phi thc

hin thao tc backoff trc khi truyn i, iu ny lm cho n c iu kin c

truyn i trc nu nh mi trng c nhiu trm c yu cu truyn. Trm gi s ch

mt khong thi gian l ACKTimeout, sau khong thi gian ny nu n khng nhn

c khung ACK ng vi khung d liu m n gi i th n s gi li khung d liu

.

2.1.3. Chc nng phi hp im (Point Coordination Function - PCF)

Mt phng php truy cp khc trong IEEE 802.11 MAC na l phng php

chc nng phi hp im (PCF). im khc bit u tin ca phng php ny vi

phng php DCF l DCF c th s dng trong c mng ad-hoc cng nh h thng

mng khng dy c cu trc BSS. Nhng phng php PCF th ch s dng c

trong cc h thng BSS, bi v trong phng php ny yu cu s dng b phi hp

im (PC) nm trong AP xc nh trm no c yu cu truyn. B phi hp im

PC nm trong AP siu khin vic truyn d liu. B phi hp im ci t danh

sch thm d v tin hnh vic thm d cc trm trong mng.

2.1.3.1. Chu k khng tranh chp

Hnh 2.4. Chu k tranh chp v chu k khng tranh chp

Trong PCF ta a ra khi nim chu k khng tranh chp v chu k tranh chp.

Chu k tranh chp (Contention period) s dng DCF v chu k khng tranh chp

(Contention free period) s dng PCF. Hai chu k ny l lun phin thay th nhau.

Mi chu k tranh chp c bt u bng vic AP s gi mt khung beacon l mt

khung qun l c bit, trong khung ny c cha gi tr CFP-MaxDuration l khong

thi gian ti a ca chu k khng tranh chp. Bi v vic truyn khung beacon c th


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b tr do knh truyn bn (frame trong chu k DCF ang c gi), cho nn chu k

khng tranh chp c th b thu ngn li.

2.1.3.2. Th tc truy cp PCF

Hnh 2.5. Th tc DCF

Cc trm v PC khng s dng RTS/CTS trong chu k khng tranh chp. thi

im bt u mi CFP, PC s cm nhn knh truyn, khi knh truyn c xc nh l

ri AP s i mt khong PIFS, sau PC s truyn khung qun l Beacon cha

thng tin CFPMaxDuration. Cc trm khi nhn c khung Beacon siu chnh gi

tr NAV (network allocation vector) ca chng theo gi tr CFPMaxDuration. iu

ny ngn cn cc trm chim quyn iu khin knh truyn trong CFP. Vic thit lp

NAV cng lm gim nguy ctrm n.

Sau khi gi khung beacon, PC s cht nht mt khong thi gian SIFS trc khi

n truyn khung thm d hoc d liu n cc trm. PC s duy tr mt danh sch thm

d v s thm d ln lt cc trm trong danh sch .

PC s nhn vo danh sch thm d ca n, v nu n kim tra thy trong bm

c d liu cn truyn cho trm m n nh thm d, PC s gi cho trm khung:

Data + CF-Poll (khung thm d), trong trng hp PC c d liu cn gi cho trm,

ngc li nu khng c d liu th PC ch gi khung thm d CF-Poll. Trm c

thm d s tr li vi khung Data/ACK sau khong SIFS n PC hoc n cc trm

khc trong mng. Nu sau khong PIFS m khng thy tr li, PC s ginh quyn

iu khin v tin hnh thm d trm tip theo trong danh sch thm d. i vi trm

khng tr li th ln thm d tip theo, PC s gi li d liu.


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i vi cc trm c thm d k trn, nu c d liu trm s tr li vi khung

Data + CF-ACK cn nu khng c d liu gi th n s tr li vi khung CF-ACK.

Qu trnh thm d, gi d liu v tr li ny s tip tc cho n khi PC khng c

khung d liu no gi v khng c trm no thm d. Khi PC s gi mt

khung kt thc CF-End hoc CF-End +CF-ACK (nu cn xc nhn d liu) thng

bo kt thc chu k khng tranh chp CFP. Lc , tt c cc trm s xc lp li gi tr

NAV ca chng v chu k tranh chp CP s bt u.

2.2. Chun IEEE 802.11e

Chun IEEE 802.11e l mt bin th ca chun 802.11. N nh ngha mt tp

hp cc ci tin m bo cht lng cht lng dch v cho mng khng dy WLAN

thng qua cc sa i trong tng MAC. Chun ny c tm quan trng v cn thit chocc ng dng yu cu thi gian thc cng nh chm tr thp nh voice over IP,

streaming multimedia.

IEEE 802.11e m bo cht lng dch v cbn da trn phn bit lu lng,

bng vic b sung thm trng QoS trong cu trc khung d liu MAC.

Hnh 2.6. Frame IEEE 802.11e

Vic b sung thm trng QoS Control trong khung d liu gip cho IEEE

802.11e c th cung cp vic m bo cht lng dch v . IEEE 802.11e cung cp s

u tin thng qua phn bit nh danh ca cc lung v ch r u tin.

2.2.1. Chc nng phi hp lai (Hybrid Coordination Function - HCF)Trong IEEE 802.11e cch truy cp knh truyn c s dng l HCF (Hybrid

coordination function). Cch ny c gi l cch lai bi v n c c hai phng

thc truy cp knh truyn c tranh chp v khng tranh chp. HCF s dng phng

thc truy cp knh truyn da trn tranh chp l EDCF (Enhance distributed channel

access), n l mt s mrng ca phng thc DCF c p dng trong IEEE 802.11

MAC. Hot ng ng thi vi EDCF l cch truy cp knh truyn c iu khin


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da trn k thut thm d HCCA(HCF controlled channel access), cch ny c th

s dng trong c chu k tranh chp ln chu k khng tranh chp.

2.2.1.1. EDCF (Enhanced Distributed Coordinated Function)Trong chun IEEE 802.11e mi station s phn loi cc dch v thnh 4 nhm

s phn chia ny da trn quyn truy cp ng truyn. Tc l mi frame c cc

tng trn gi xung tng MAC sc phn vo mt AC (Access categories) tng

ng vi u tin ca frame theo bng di y.

Bng 2.1. u tin cc dch v

y u tin 0 (thp nht) nm gia u tin 2 v 3 l do tun th theo

chun IEEE 802.11d.

Vn gin s khc bit ca cc AC ny da trn cc tham s : AIFSD[AC],

CWMin[AC], CWMax[AC] tng ng vi cc tham s DIFS, CWMin, Cwmax trong

DCF. Cc tham s trn l khc nhau vi mi AC, t mc u tin truy cpng truyn ca cc AC cng s thay i theo gi tr ca chng.

Gi tr ca AIFSD[AC] c xc nh thng qua cng thc :

AIFSD[AC] = SIFS + AIFS[AC] * SlotTime

AIFS[AC] l mt s nguyn c gi tr ln hn 0. Tc l u tin ca mt AC

sc quy nh bi 3 tham s l : AIFS[AC], CWMin[AC], CWMax[AC]. y

cng l ba tham s chnh trong chun EDCF, chng c cung cp cho cc trm t AP


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thng qua khung beacon. AP c th tng tnh ton cc gi tr ny thng qua vic

xem xt iu kin ca mng.

Hnh 2.7. Bn Access Categories trong EDCF

Hnh trn m t bn hng i trong tng MAC th hin cho bn AC, cc khung

d liu t tng trn gi n ty thuc vo u tin ca n sc sp xp vo mt

hng i tng ng. Mi hng i ny c coi nh l mt trm o, tc l n s c gitr AIFS[AC], CWMin[AC], CWMin[AC] ring v khi mun truyn d liu chng

cng s thc thi cc th tc backoff da vo cc tham s ny, v nu nh c nhiu hn

mt AC chn cng gi tr backoff th s xy ra hin tng xung t v ngi ta gi

hin tng ny l hin tng xung t o.

Trong chun IEEE 802.11e ngi ta a ra mt khi nim mi l c hi

truyn (XTOP - Transmission Opportunity). XTOP l khong thi gian mt trm c

quyn c truyn, n c xc nh bi thi gian bt u v thi gian kt thc truyn

ca trm , chnh da vo iu ny m cc trm khc trong mng c th xc nh

c mt cch chnh xc thi gian m trm ny s dng knh truyn. Nu trm ang

c cp pht XTOP thc hin xong vic truyn, trm c th truyn tip cc khung

tip theo, nhng phi m bo rng vic truyn cc khung cng nh nhn ACK tng

ng ca cc khung ny l khng vt qu thi gian cn li.


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Qu trnh thc thi EDCF c m t nh sau :

Hnh 2.8. Thc thi EDCF

Cch thc thi EDCF cng ging nh thc thi DCF, ch c mt s khc bit l

mi trm by gis tng ng vi 4 trm o, v mi trm ny s t thc thi cc cch

backoff, tranh chp ng truyn mt cch c lp, tuy nhin khi chn gi tr random

tnh gi tr backoff th gi tr random ny s l mt s nguyn nm trong khong [1,

1+ CW[AC]] (DCF l khong [0,CWMin]).

Thm vo , mi khi mt khung no khng thnh cng th thay v tng gi tr

ca ca s tranh chp ln gp i th trong EDCF c thay i cht t. Nu nh gi trCW[AC] hin ti l nh hn CWMax[AC] th gi tr CW[AC] sc thay i theo

cng thc :

CW[AC](new) = (CW[AC](old) + PF[AC]) * 2 1

Trong PE[AC] l mt nhn t cnh, trong chun IEEE 802.11 th PE l c

nh v nhn gi tr l 2, tuy nhin trong 802.11e gi tr ny thng thng l khc nhau

mi AC, iu ny lm tng s khc nhau gia cc gi tr CW ca cc AC. Nhng,

nu gi tr CW[AC] t ngng CWMax[AC] th n vn gi nguyn gi tr khng thay i cho ti khi khung c truyn i thnh cng th mi gn li gi tr

CW[AC].

Trong trng hp xy ra va chm o (cc AC truyn khung ti cng mt thi

im) th chi truyn (XTOP) sc cp pht cho AC c u tin cao hn, v

cc AC cn li c xem nh gp trng hp truyn khung khng thnh cng v phi

chn li gi tr CW cng nh lp li th tc backoff.


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2.2.1.2. HCCA(HCF controlled channel access)HCCA lm vic hu nh ging vi chc nng iu phi im (Point

Coordination Function). Tuy nhin, tri ngc vi PCF, y khong cch gia hai

khung cnh bo c chia thnh hai giai on ca CFP v CP, HCCA cho php CFPs

c khi ng ti gn nh bt c lc no trong mt CP. y l mt loi CFP c

gi l giai on kim sot truy cp (Coltrolled Access Phase - CAP) trong IEEE

802.11e. Mt CAP c khi to bi cc AP, bt c khi no n mun gi mt frame

n mt trm, hay nhn c mt frame t mt trm, trong mt kiu cnh tranh t do.

Trong thc t, CFP cng l mt CAP. Trong mt CAP, cc iu phi Hybrid (HC) -

cng l AP - kim sot truy cp im gia. Trong CP, tt c cc trm chc nng bn

trong EDCA. S khc bit vi PCF l lp lu lng (TC) v lung lu lng (TS)

c nh ngha. iu ny c ngha l HC khng gii hn hng i trn tng trm vc th cung cp mt loi dch v cho mi phin. Ngoi ra, HC c th phi hp cc

lung hoc cc phin vi kiu bt k m n chn (khng ch round-robin). Hn na,

cc trm cung cp thng tin v di ca hng i cho tng lp lu lng (TC). HC

c th s dng thng tin ny u tin cho mt trong nhng trm pht qua cc trm khc,

hoc tt hn cchiu chnh lch trnh ca n. S khc bit khc l cc trm a ra

mt TXOP: chng c th gi nhiu gi tin trong mt hng, trong mt thi gian nht

nh c la chn bi cc HC. Trong CP, HC cho php cc trm gi d liu bi vic

gi cc khung tin CF-Poll.

HCCA thng c coi l chc nng phi hp tin tin v phc tp nht . Vi

HCCA, QoS c thc cu hnh vi chnh xc tuyt vi. QoS cho php cc trm

c kh nng truyn ti cc thng s yu cu c th (t l jitter d liu, vv) m c th

cho php cc ng dng nng cao nh VoIP v video lm vic hiu qu hn trn mt

mng khng dy.

H trHCCA khng bt buc cho cc im truy cp theo chun 802.11e. Thc

t, c rt t( thm ch khng c) cc AP h trHCCA. Tuy nhin, thc hin HCCA

khng i hi nhiu cc yu cu trn, v v cbn s dng cch DCF hin ti cho

truy cp knh (khng c thay i DCF hoc hot ng EDCA l cn thit). c

bit, vic thc hin pha im pht sng l rt n gin nh trm ch cn c th tr

li tin nhn thm d. V pha AP, l dnhin, vic lp lch v s dng hng i l cn

thit. C kin cho rng, AP c trang b tt hn l mt trm thu pht, iu ny

nn l mt vn khc.


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2.3. Truyn video streaming trong mng khng dy IEEE 802.11e

Vic truyn video streaming trong mng khng dy trc y gp nhiu vn

do phi tranh chp vi cc lung d liu khc. Do vic truyn video rt b hn ch

v cho kt qu thp. Vi vic p dng chun IEEE 802.11e vic truyn videostreaming trong mng khng dy c nhiu ci thin v t c hiu qu cao hn.

Vic IEEE 802.11e phn bit cc dch v khc nhau tng MAC to phn

chia chng vo cc AC khc nhau c u tin khc nhau dn ti vic cc dch v

truyn video cng nh audio khng b tranh chp bi cc dch v khc nh cc ng

dng web hay l cc ng dng truyn file

Chun IEEE 802.11e nh ngha bn loi truy cp tng ng vi bn u tin

khc nhau dnh cho cc nhm dch v. Dch v truyn video streaming c xp vonhm dch v c u tin cao. Tc l cc khung d liu ca n t tng trn y

xung sc xp vo hng i tng ng vi AC c u tin cao. T dn n

vic xc xut c truyn i ca cc khung ny l c tng ln. V kt qu l cht

lng dch vc tng ln. Bn cnh do c tnh cht chi truyn (TXOP) nu

nh AC c cp pht TXOP m sau khi gi khung thnh cng AC vn c th truyn

tip nhng khung d liu tip theo trong hng i. iu ny gip cho dch v video

streaming c th gi mt lot cc gi tin lin tip nhau. iu ny s lm cho s duy tr

dch v l c tng cng.

Tuy nhin chun IEEE 802.11e li ch hng vo vic m bo cht lng dch

v cho nhm dch v. Tc l mt khung d liu bt k khi c d liu y xung tng

MAC th ng vi dch v ca khung MAC s cp pht cho n u tin cnh v

y n vo hng i tng ng vi u tin . Nhng vi video streaming cch

phn tng ca n s to ra nhiu lung d liu c tc khc nhau cng nh quan

trng khc nhau. Vic gii m d liu ca cc tng m rng trong video streaming

phi lun ph thuc v d liu ca tng cbn v tng cbn l nhn tu tin dch v video streaming c thc duy tr v th tng cbn lun lun phi c u

tin cao nht sau th tu tin gim dn theo th t lung m rng 1, lung m

rng 2 V th nu nh vic phn phi u tin ca cc lung ny lun lun duy

tr bng nhau nh trong chun MAC th vic phn bit mc quan trng ca cc

lung d liu xem nh l khng c h tr.


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Do vy phi c mt cch no gii quyt vn ny nhm h trtt hn

cho vic m bo cht lng dch v video streaming. V cng s l ni dung ca

chng tip theo.


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Chng 3. Gii php gn u tin ng cho cc lungvideo streaming trong mng IEEE 802.11e

3.1. Vn nghin cuVic truyn video streaming trong mng khng dy gp nhiu kh khn, bi v

nhng rng buc ca video streaming rt kh p ng trong iu kin mng khng

dy. Tuy chun IEEE 802.11e phn no khc phc c mt s vn bng

cch cho php cc dch v gn u tin khc nhau. Tuy nhin, nh th l cha

m bo c cht lng truyn video, bi v vi vic phn tng trong video

streaming, cc tng khc nhau s c u tin khc nhau khi truyn.

Trong video streaming tng cslun lun phi c u tin u tin sau lcc tng mrng theo th t tng dn ca chng. Chun IEEE 802.11e ch dng li

mc h trcht lng dch v hng nhm dch v. Tc l, mi mt dch v nht nh

nh video streaming chng hn th ch tng ng vi mt mc u tin. Dn ti vic,

cc tng khc nhau trong video streaming vn s cng u tin, v dnhin l cc

gi tin ca chng t tng trn gi xung s cng nm trong mt hng i trong s bn

hng i tng MAC (tng ng vi 4 AC). iu ny sng nht cc khung d liu

ca tt c cc tng trong vic phn tng ca video streaming l nh nhau khi truyn.

Vn ny sinh khi tnh trng mng l bn, khi mng bn cc tng video vi cc

lung d liu tng ng li cng c truyn vi u tin ging nhau, hu qu l

tr ca d liu t tc tng gi i s tng ng nhau v dnhin l tr ny s ln,

ng thi thng lng ca tt c cc lung d liu cng u gim xung. iu ny i

ngc vi tinh thn ca vic phn tng trong video streaming, vic phn tng l gip

cho vic truyn video c th tng thch vi s thay i tnh trng mng, nhm c mt

hiu qu truyn tt nht trong iu kin c th. Tc l nu tnh trng mng bn th vic

truyn d liu ca tng csphi c t ln hng u. V nht thit l phi truynd liu ny vi tr nh v thng lng m bo sao cho vn duy tr c dch

v. Cn vi cc tng khc th c th truyn mt cch c gng ti a.

Sau y ta si xt mt vi th nghim kim chng xem s bt ny sinh khi

truyn video streaming trong IEEE 802.11e nh ni trn.

Mi trng thc nghim y l da trn b m phng ns2-2.28 ( ci thm

b h tr c th s dng c chun IEEE 802.11e). M hnh m phng s gm 3

my trm v mt AP. Trong 3 trm ny ta s xt 1 trm nh gi cn hai trm kia


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ch l hai trm lm mi trng. Trm c xt l trm 1, n s bao gm ba lung

d liu tng ng vi 3 tng ca dch v video streaming (tng cbn, tng mrng 1

v tng mrng 2). Cc lung ny s c tc truyn tng ng l 128kb/s, 256kb/s

v 512kb/s. Hai trm cn li l trm 2 v trm 3 mi trm s bao gm ba lung d liu

tng ng vi ba u tin nhn gi tr t 1 ti 3. S th nghim nh sau :

Hnh 3.1. M hnh th nghim

a. Trng hp mng bn

u tin, ta s i xt trng hp mng bn. mng bn th tng s tc

truyn phi ln hn bng thng cc i c th ca knh truyn (2mb/s), nn ta s cho

cc lung d liu c u tin 1, 2, 3 trong trm 2 v 3 nhn tc truyn bng nhau

v bng 400kb/s. Trong trng hp mng bn, ta si xem xt hai trng hp con, l

trng hp trm 1 cc lung video streaming nhn cng gi tru tin l 1 (tun

th chun IEEE 802.11e), v trng hp khc l ta s gn cc lung tng ng vi ccu tin khc nhau. Lung cbn s nhn u tin l 1 lung mrng 1 s l 2 v

lung mrng 2 s l 3. Sau khi chy chng trnh m phng ta c mt s nhn xt

sau.

Th nht v tr. Khi gn u tin ca cc lung nh nhau. tr trung bnh

ca cc gi tin trong cc lung ny s nhn c s xp x nhau v xp x gi tr

0.778s. Trong khi , vi trng hp gn u tin khc nhau, th tr trung bnh


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ca hai lung tuy kh cao, nhng tr trung bnh ca lung cbn ch l 0.04s. V ta

cng c th so snh tr lung cbn ca hai th nghim nh sau:

Hnh 3.2. Biu so snh tr lung c bn khi mng bn trong hai trng hp gn u

tin bng nhau v gn u tin khc nhau

Ta c th thy c rng trong trng hp mng bn nu gi nguyn u tin

cc lung bng nhau v mc u tin cao th cc lung s cng c gi tr tr v gi

tr ny l kh ln. Trong khi nu phn phi cc lung mc u tin khc nhau th

tr ca lung cbn sc m bo mc thp v gi phi tr l trcc

lung kia phi tng ln.

Tip theo, ta xt n thng lng ca cc lung. Vi trng hp u tin ca

cc lung nh nhau s gi tin nhn c trn cc lung theo th t lung c bn,

lung mrng 1 v lung mrng 2 l 1110 (packet), 1718(packet), 2876(packet). V

trong trng hp u tin khc nhau th kt qu l 1440(packet), 2228(packet),

601(packet). M tng s gi tin gi i t trm 1 tng ng vi cc lung cbn, lung

mrng 1, lung m rng 2 ln lt l 1440(packet), 2880(packet) v 5760(packet).

Nhn vo s liu ny ta c: vi trng hp u tin ca cc lung nh nhau, s

lng gi tin ca tng lung nhn c tnh theo phn trm th t l 77,1%, 59,65%

v 49,93% tng ng vi lung cbn, lung m rng 1 v lung m rng 2. Vi

dch v video streaming mt mt ny l kh c th chp nhn c. Cn trng

hp u tin ca cc lung khc nhau, tuy lung m rng 2 ch gi ti ch c

mt s t gi tin so vi s lng gi tin m bn nhn gi. Nhng b li lung cbn


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nhn c 100% s gi tin v lung m rng hai cng nhn c nhiu gi tin hn

(77,36%). iu ny m bo duy tr dch v video streaming.

Mt khc, ta cng c thng lng ca cc lung c biu th nh sau.

(a)

(b)

Hnh 3.3. Thng lng trong trng hp mng bn

(a) Trng hp u tin nh nhau ; (b) Trng hp u tin khc nhau


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D dng thy rng, thng lng ca lung cbn trong trng hp u tin

ca cc lung bng nhau l khng n nh v ch yu l nm di 100kb/s. Cn vi

trng hp u tin khc nhau th thng lng lun duy tr mc 128kb/s mt khc

lung mrng 1 trong trng hp u tin khc nhau cng c thng lng kh cao

(nm t khong mt na thi gian duy tr mc ln hn 200kb/s v t khi thng lng

tt xung di ngng 150kb/s).

Thng qua vic xem xt hai th nghim trn ta c th thy c rng, vi

trng hp mng bn th vic gn u tin ca cc lung khc nhau ty theo mc

quan trng ca n l tt hn so vi vic gn chung u tin cho tt c cc lung.

iu ny th hin r trn c hai phng din tr cng nh thng lng. Vy trong

trng hp mng ri th sao? Ta si xem xt thng qua th nghim cc sau.

b. Trng hp mng ri :

mng ri th ngc li vi trng hp mng bn tng s tc truyn phi

nh hn bng thng cc i c th ca knh truyn (2mb/s) nn ta s cho cc lung d

liu c u tin 1, 2, 3 trong trm 2 v 3 nhn tc truyn bng nhau v bng

100kb/s. Ta cng si xem xt hai trng hp con nh xt trong trng hp mng

bn

Th nht v tr. Khi gn u tin ca cc lung nh nhau. tr trung bnh

ca cc gi tin trong cc lung ny s ln lt nhn gi tr l 0.00855(s), 0.01233(s) v

0.01554(s). Trong khi vi trng hp gn u tin khc nhau th tr trung bnh

ca cc lung l 0.00894(s), 0.01383(s) v 0.02191(s). tr trung bnh ca cc gi

tin ca lung cbn c hai trng hp l tng ng nhau. Nhng tr cc gi

tin hai lung mrng l c s khc bit. Ta c th nhn thy r hn thng qua th

so snh tr lung mrng 2 trong hai trng hp nh hnh di y.


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Hnh 3.4. Biu so snh tr lung m rng hai khi mng bn trong hai trng hp gn

u tin bng nhau v gn u tin khc nhau

T biu trong hnh 3.3 ta c th thy rng, tr ca cc gi tin lung m

rng 2 trong trng hp gn u tin cc lung nh nhau (mu ) hu nh lun

nm di ngng 0.02s, v ch yu l nh hn 0.01s. Trong khi , tr ca cc gi

tin cng lung mrng 2 nhng vi trng hp gn u tin cc lung khc nhau

(mu xanh), phn na c gi tr ln hn 0.02s v c nhng lc vt ln ti gi tr 0.08s.

iu ny chng t rng khi mng rnh ri nu xp u tin ca cc lung

video streaming l khc nhau, th trcc lung pha di (cc lung m rng bc cao) s b tng ln v iu ny l hon ton khng hp l.


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(a)

(b)

Hnh 3.5. Thng lng trong trng hp mng ri

(a) Trng hp u tin nh nhau ; (b) Trng hp u tin khc nhau

D dng thy rng vi c hai trng hp lung cbn u c thng lng nh

nhau. Tuy nhin vi trng hp u tin khc nhau hai lung mrng u c thng

lng l khng n nh bng thng lng ca chng trong trng hp u tin

ging nhau. Vi lung m rng 1 trng hp u tin ca cc lung bng nhau


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thng lng lun duy tr mc 256kb/s, cn trng hp u tin cc lung khc

nhau, mt s thi im thng lng ca lung ny b thay i mt t. Cn vi lung

m rng hai, sai lch ln xung ca thng lng trong trng hp u tin cc

lung ging nhau l t hn, so vi trng hp u tin cc lung khc nhau.

T vic xt hai th nghim trong trng hp mng ri ny ta c th nhn ra

c rng trong trng hp mng ri cc lung d liu streaming vic gn cng u

tin mc cao cho kt qu tt hn l gn chng cc u tin khc nhau.

Vy ta c th kt lun rng trong trng hp mng bn ta nn gn u tin

ca cc lung d liu tng ng vi cc tng trong c ch phn tng ca video

streaming l khc nhau. m bo duy tr c lung d liu quan trng hn t

m bo cho vic duy tr dch v. Cn vi trng hp mng ri nn gn u tin ca

cc lung ny l bng nhau v mc cao c thm bo cung cp c dch v

video streaming vi cht lng tt hn.

3.2. Gii php gn u tin ng cho cc lung video streaming

Dch v truyn video streaming lun lun bnh hng bi tc ng ca mi

trng bn ngoi. C lc th cc lung video tng ng vi cc tng nn c c cng

u tin mc cao, c lc li nn c u tin khc nhau. M trong chun IEEE

802.11e khng h trvic thay i u tin cho cc lung d liu.Nhu cu m bo dch v cng nh cht lng video streaming l rt cn thit,

do phi c mt gii php no h tr su hn cho vic truyn video streaming

trong mng khng dy. Trong kha lun ny ti mun tp trung cho vic duy tr s tn

ti ca dch v, cng nh tng cng cht lng dch v cho vic truyn video

streaming trong mng khng dy. V gii php ti a ra y l gn u tin ng

cho cc lung video streaming. Gii php ny ca ti c hnh thnh trong qu trnh

nghin cu, o t v th nghim trn cc lung video streaming trong IEEE 802.11e

m rt ra.

Trong chun IEEE 802.11e, mi lung d liu lun duy tr mt u tin c

nh t khi bt u khi to cho n khi kt thc vic truyn. Gi tru tin ny

ph thuc vo dch v s dng tng ng dng. Vic lm ny to ra c s phn bit

gia cc nhm dch v vi nhau. Vi vic p dng chun IEEE 802.11e, dch v video

streaming s gn u tin ca cc lung d liu mc cao, t chi truyn ca

cc khung d liu tng ng vi dch v ny l cao hn mt s dch v khc nh

web,fpt


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Tuy nhin vi s phn tng trong video streaming, mi mt video gc sc

m ha thnh nhiu tng video vi tc nn v mc quan trng khc nhau. Vi

vic mi dch v ch c th tng ng vi mt u tin, hu qu l tt c cc lung

d liu ca dch v video streaming tt c cc tng u cng c mt u tin. Vic

ny lm gim i tnh nng mm do ca sm bo cht lng dch v, v mc

quan trng ca mi tng trong video streaming l khc nhau. Cng vi mi trng

truyn khng dy lun trng thi c th thay i bt c lc no. Phng php gn

u tin ng cho cc lung video streaming n gin l kim tra gim st mi trng

mng, t tnh ton mt cch hp l thay i gi tru tin ca cc lung d

liu tng ng vi cc tng trong video streaming mt cch ph hp vi s thay i

ca mi trng truyn.

Nh ta xt, tc nhn gy ra s mt n nh trong vic truyn video streamingl s thay i ca mi trng truyn. Vy mun khc phc iu ny trc ht ta phi

c mt cch kim tra mi trng truyn, t c cch thay i u tin ph

hp. Vy vic u tin l lm sao bit c mi trng ri hay bn, mc ri, bn

nh th no. Sau da vo cc tham s ty chnh u tin cho cc lung d liu

mt cch ph hp.

3.2.1. Cc tham snh gi mi trng truyn.

Trong mi trng khng dy cc trm hay ni ng hn l cc AC (chun IEEE

802.11e) u tranh chp vi nhau ginh c chi truyn. Vic tranh chp ny

da vo cch chn thi gian backoff. AC no c gi tr backoff gim ti 0 nhanh nht

th c quyn chim knh truyn. Khi mng rnh ri cc AC c th ch backoff mt

hay mt s nh ln ginh c chi truyn, ngc li khi mng bn, mt khung

d liu trn mt AC c th phi backoff nhiu ln c th truyn i. iu ny th

hin r nht trn cc AC c u tin thp. Vy tham s tng thi gian backoff trung

bnh ca mt khung d liu mt AC l mt gi tr gin tip th hin nhn ri ca

mi trng.

Mt mt na, trong nhng th nghim mc 3.1 ta c th nhn thy rng, trong

khi mi trng truyn ri, th hu nh tt c cc gi tin gi t bn gi u n c

bn nhn. Trong khi vi mng bn th cc gi tin gi i c th b hy v khng bao

gin c bn nhn. iu ny l do bng thng ti a ca mng khng p ng

yu cu ca cc trm nn mt s gi tin phi b hy. Do , tiu ch tip theo nh

gi mi trng truyn l t l gi tin b hy trong khi truyn.


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Cc thng s khc nh tr gi tin v thng lng cng phn nh tnh trng

mi trng truyn nhng chng khng xc nh c bn gi cho nn cc tham s

ny s khng c cp trong phng php.

Vy ta c thnh gi mi trng truyn thng qua hai tham s l tng thi gianbackoff ca mt khung d liu v t l mt gi tin. Da vo chng ta s nhn ra c

nhng thay i ca mi trng truyn, t iu chnh vic truyn d hp l.

3.2.2. M t gii php.

nh ngha mt s tham s :

-t : khong thi gian s dng xem xt ng truyn. Ta s tnh cc thng s

v thi gian backoff trung bnh, cng nh t l gi tin b hy trong khong thi gian

ny nh gi mi trng.

- ][ACebackoffTim : L gi tr trung bnh backoff time ca mi khung tnh trong

khong thi gian t trn mi AC.

-dropRatio[AC] : L t l khung b hy trn mi AC trong khong thi gian t.

-backoffThresholdDown[AC] : l ngng backoff time trn mi AC. Nu gi tr

][ACebackoffTim tng vt qu ngng ny th ta phi thc hin c ch chuyn cc

lung d liu video streaming xung AC c u tin thp hn AC hin thi.

-backoffThresholdUp[AC] : Cng l ngng backoff time trn mi AC. Nu gi

tr ][ACebackoffTim gim xung qu ngng ny, th ta s thc hin cch chuyn cc

lung d liu video streaming t AC c u tin nh hn ln AC hin thi.

-dropThresholdDown[AC] : L ngng ca t l hy gi tin trn mi AC. Nu

gi tr ratioDrop tng vt ngng ny, ta phi thc hin cch chuyn cc lung d

liu video streaming xung AC c u tin thp hn AC hin thi.

-dropThresholdUp[AC] : Cng l ngng ca t l hy gi tin trn mi AC. Nu

gi tr ratioDrop gim xung qu ngng ny, th ta s thc hin cch chuyn cc

lung d liu video streaming t AC c u tin nh hn ln AC hin thi.

-numberFlow[AC] : l s lng lung d liu video streaming ang c u

tin tng ng vi AC.

Trong phng php ny. Ban u vn gi nguyn nh trong chun IEEE

802.11e, ng dng video streaming s gn u tin ca tt c cc lung ng vi cc


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tng ca n cng mt gi tr (v l video streaming nn u tin l l mc cao

prio=1).

Mi mt AC cc trm, s lun lun theo di gi tr backoff time trung bnh ca

cc khung d liu v t l gi tin b hy trong mt khong t v xem xt xem nu nhgi tr backoff time trung bnh vt qu ngng backoff time cho php, hoc t l hy

gi tin vt qu ngng cho php ca AC , th s thc hin cch chuyn u

tin ca cc lung d liu. Cch c m t trong gii thut sau

for (int i = 1; i < 3; i++){

if( ][iebackoffTim > backoffThresholdDown[i] ||

dropRatio[i] < dropThresholdDown[i]) {

if( numberFlow[i] >1 || (numberFlow[i] == 1 && i != 1)){Chuyn lung ng vi tng c mc quan trng nh nht

ang cn tn ti trong AC[i] xung AC[i+1]

}

}

}

Tc l sau mi thi gian t, kim tra tt c cc AC xem nu AC c mt

hoc nhiu hn mt lung d liu ca dch v video streaming chy, v gi tr backofftime trung bnh ca AC vt ngng backoffThresholdDown[AC] ca n, hoc t l

hy khung d liu vt qu gi tr dropThresholdDown[AC], th mt lung trong

sc chn y xung AC c u tin thp hn. Tiu ch chn l lung c

quan trng thp nht vi dch v video streaming. quan trng ny sp theo th

t gim dn theo th t: lung cbn, lung mrng 1, lung mrng 2 Nhng

nu AC l AC c u tin nhn gi tr l 1 (prio = 1) v n ch c mt lung d liu

cn li th khng c y lung ny xung v l lung cbn(tnh trng ny ch

gp khi mng qu bn, v dch v video streaming khng th duy tr c vi bt ccch thc no).

Nhng nu ch c cch chuyn cc lung xung cc AC c u tin thp hn

khi mng c xc nh l bn th cha . Bi v nh kt lun phn 3.1 ta c: vi

trng hp mng ri nn gn u tin ca cc lung l bng nhau v mc cao

c thm bo cung cp c dch v video streaming vi cht lng tt hn. Nh

vy song song vi cchy cc lung xung cc AC c u tin thp khi mng


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bn th phi c cch ko cc lung ln cc AC c u tin cao hn khi tnh trng

mng l rnh ri hn. Do gii thut trn cn vit li nh sau :

for (int i = 1 ; i < 3; i++){

if( ][iebackoffTim > backoffThresholdDown[i] ||

dropRatio[i] < dropThresholdDown[i]) {

if( numberFlow[i] >1 || (numberFlow[i] == 1 && i != 1)){

Chuyn lung ng vi tng c mc quan trng nh nht

ang cn tn ti trong AC[i] xung AC[i+1]

}

}

if( ][iebackoffTim < backoffThresholdUp[i] ||

dropRatio[i] < dropThresholdUp[i] ){

Chuyn lung ng vi tng c mc quan trng cao nht ang

cn tn ti trong AC[i+1] ln AC[i]

}

}

Vic chn cc ngng chuyn cc lung ln hay xung cc AC c u tin

cao hay thp hn AC hin ti l khc nhau thay v gn chung mt ngng quy nhlung d liu tt xung hay ko ln gia cc AC. Bi v iu ny lm gim thiu kh

nng mt lung no c b chuyn i chuyn li lin tc gia cc AC do cc thng

cc thng snh gi tng gim quanh ngng chung.

Mt khc khi c thay i th hin thng qua tham snh gi ta khng chuyn

mt cch t ngt v t cc lung d liu xung cc tng thp m chuyn mt cch

t t tng lung mt hp l dn s phn b cc lung cc AC sao cho ph hp

nht vi thay i ca mi trng truyn.

3.3. Phn tch nh gi

Gii php gn u tin ng cho cc lung video thc s c tc dng trong

mi trng mng khng n nh nh trong mng khng dy. S bin i ca ca mi

trng truyn l thng xuyn din ra. Do vic thay i linh ng u tin cc

lung vi mc ch m bo duy tr dch v cng nh cht lng dch v l cn thit

v c hu ch.


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Nu nh mi trng mng ban u ang ri. cc lung d liu ca video u

c truyn AC c u tin cao, iu ny lm cho dch v video streaming c th

t c cht lng tt do c s kt hp ca cc lung mrng v lung cbn.

Tuy nhin nu nh tnh trng mng chuyn sang bn, lc ny nu cc lung vngi nguyn u tin ca n. Th tnh trng tranh chp s xy ra thng xuyn ng

thi l vic tranh chp gia cc trm vi nhau. iu lm cho tt c cc lung k c

lung cbn cng nh cc lung mrng u b gim tc truyn cng nh tng

tr mt cch mnh m. iu ny gy ra vic dch v video streaming s khng duy tr

c ngay c vi cht lng thp.

Vic p dng gii php gn u tin ng s gii quyt c trng hp ny

bi v khi tnh trng mng l bn, cc lung mrng s b chuyn dn xung cc AC

c u tin thp hn. Do lung cbn v c th thm mt s lung mrng thp

(cc lung mrng ng vi cc tng gn tng cbn nht) s khng phi chu tranh

chp vi cc lung mrng cao (cc lung mrng ng vi cc tng xa so vi tng

cbn). Do chng c iu kin truyn tt hn. T dch v sc m bo hn.


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Chng 4. nh gi gii php u tin ng cho cclung video streaming trong mng IEEE 802.11e

4.1. Mi trng m phng

4.1.1. Xy dng chng trnh m phng.

M phng thc hin trn b m phng NS-2. NS-2 l phn mm m phng

mng iu khin s kin ring r hng i tng, c pht trin ti UC Berkely,

vit bng ngn ng C++ v OTcl. NS rt hu ch cho vic m phng mng din rng

(WAN) v mng local (LAN). Bn li ch ln nht ca NS-2 phi kn u tin l:

Kh nng kim tra tnh n nh ca cc giao thc mng ang tn tiKh nng nh gi cc giao thc mng mi trc khi a vo s dng

Kh nng thc thi nhng m hnh mng ln m gn nh ta khng th thc thi

c trong thc t

Kh nng m phng nhiu loi mng khc nhau

NS-2 (Network Simulator version 2.xx) l mt trong cc b m phng mng m

ngun m, t do, c s dng rng ri nht trong cc trng i hc (c ging dy v

mng) trn th gii. NS-2 c bit mnh trong vic m phng nghin cu v cc

giao thc mng, t tng MAC cho n tng Transport. S dng NS-2 c th m phng

cc mng c dy (Wired Networks), khng dy (Wireless Networks), mng di ng

khng dy c bit - MANET (Mobile Adhoc Networks), hn hp, v.v. Cc s kin

xy ra trong mng m phng v mt s tham s cn nghin cu thng c kt xut

ra tp vt (Trace file) hoc mt s tp dng vn bn. nhn c cc kt qu

mong mun cn phi x l cc tp kt qu ny v biu din kt qu thu c, thng

l di dng th.NS thc thi cc giao thc mng nh Giao thc iu khin truyn ti (TCP) v

Giao thc gi ngi dng (UDP); cc dch v ngun lu lng nh Giao thc truyn

tp tin (FTP), Telnet, Web, Tc bit cnh (CBR) v Tc bit thay i (VBR) ;

cc k thut qun l hng i nh Vo trc Ra trc (Drop Tail), D sm ngu nhin

(RED) v CBQ; cc thut ton nh tuyn nh Dijkstra NS cng thc thi

multicasting v vi giao thc lp iu khin truy cp ng truyn (MAC) i vi m

phng LAN.


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Hnh 4.1. Tng quan v NS di gc ngi dng

OTcl Script Kch bn OTcl

Simulation Program Chng trnh M phng

OTcl B bin dch Tcl mrng hng i tng

NS Simulation Library Th vin M phng NS

Event Scheduler Objects Cc i tng B lp lch S kin

Network Component Objects Cc i tng Thnh phn MngNetwork Setup Helping Modules Cc m un Trgip Thit lp Mng

Plumbling Modules Cc m un Plumbling

Simulation Results Cc kt qu M phng

Analysis Phn tch

NAM Network Animator Minh ha Mng NAM

Trong hnh trn, NS l B bin dch Tcl mrng hng i tng; bao gm cci tng B lp lch S kin, cc i tng Thnh phn Mng v cc m un Tr

gip Thit lp Mng (hay cc m un Plumbing).

s dng NS-2, ngi dng lp trnh bng ngn ng kch bn OTcl. User c

th thm cc m ngun Otcl vo NS-2 bng cch vit cc lp i tng mi trong

OTcl. Nhng lp ny khi sc bin dch cng vi m ngun gc.

B m phng NS-2 s dng trong kha lun l phin bn ns2-2.28 v c ci

thm b h trchun ieee 802.11e. Da trn b m phng ns2 ny ta snh gi


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hiu qu ca gii php gn u tin ng cho cc lung video streaming so vi

chun 802.11e. lm vic ny ta cn phi xy dng mt kch bn r rng v ph hp

sao cho vic nh gi l cng bng v cho ra kt qu l chun xc.

4.1.2. Kch bn m phngM hnh m phng s bao gm 3 my trm v mt access point s dng mng

khng dy theo chun ieee 802.11e nh hnh di.

Hnh 4.2. M hnh th nghim

Vi m hnh m phng ny ta si xt nhng th nghim so snh hiu qu

ca gii php u tin ng cho cc lung so vi chun c trong 802.11e. u tin

ta s quy nh mt s tham s chung thc hin m phng.

4.1.2.1. Cc tham smi trngDa trn m hnh 802.11 MAC, cc tham s PHY trong m hnh m phng

EDCF c thit lp nh sau:

- SlotTime: 16s - SIFS: 8s

- PIFS: 12s - DIFS: 16s

Bng thng chung ca mng l 2Mb/s. kch cca mi gi tin ca tng ng dng

l 1kB.

Tham s v CW, PF, AIFS ca IEEE 802.11e chun dng m phng tun theo

cc gi trbng sau


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Bng 4.1. Cc tham s m phng

AC PF AIFS CW_MIN CW_MAX TXOPLimit

0 2 2 7 15 0.003264s

1 2 2 15 31 0.006016s

2 2 3 31 1023 0

3 2 7 31 1023 0

4.1.2.2. Thng sti cc trmTrm th nht (station 1) s l trm s dng thc hin vic streaming video

v cng l trm ta quan st v nh gi. trm ny, tng ng dng s s dng 3

lung d liu cbr gi d liu cho AP cc lung ny tng ng l lung cbn,

lung m rng 1 v lung m rng 2 c to ra do c ch phn tng ca dch v

video streaming. Cc lung d liu ny tng ng s c tc l 128kbps, 256kbps v

512kbps.

Trm th hai (station 2) v trm th 3 (station 3) s l hai trm s dng lm

mi trng. Hai trm ny s gy ra cho mng tnh trng nghn hay rnh ri bng cch

thay i tc truyn d liu ca cc lung cbr ca n. Mi trm s c 3 lung d liucbr gi cho AP cc lung ny ban u s c tc truyn bng nhau v nhn gi tr l

100kbps, cc lung d liu ny sc xp vo 3 AC c u tin khc nhau vi gi

tr l 1,2 v 3.

Trm 2 v trm 3 s truyn d liu trong sut thi gian chy m phng sau 10

giy (giy th 10 trong khong thi gian m phng) trm 1 s bt u truyn d liu.

n giy th 250 th trm 1 dng vic truyn d liu v m phng s kt thc giy

th 300 gy s mt n inh cho mng, trong qu trnh truyn tc ca cc lung d

liu trm 2 v trm 3 sc thay i ng lot ti mt s thi im. C th l :

- giy th 40 : Tng tc truyn t 100kbps ln 200kbps

-Giy th 70 : Tng tc truyn t 200kbps ln 400kbps




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-Giy th 160 : Gim tc truyn t 700kbps xung 150kbps

-Giy th 190 : Gim tc truyn t 150kbps xung 50kbps


By gi, da trn m hnh trn ta s thc hin hai th nghim. Tng ng vi hai

trng hp : s dng cch IEEE 802.11e chun v cch p dng gii php gn

u tin ng cho cc lung d liu. Kt qu sc so snh v phn tch trn hai

phng din chnh l bng thng v tr.

4.2. Kt qu m phng

4.2.1. nh gi v thng lng

Sau khi thc hin hai th nghim vi m hnh nh m tphn 4.1.2 tng ng

vi hai trng hp s dng cch chun 802.11e v cch p dng gii thut gn

u tin ng cho cc lung d liu. Ta c th m t thng lng ca cc lung d

liu station 1 nh sau :

(a)


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(b)

Hnh 4.3. Thng lng ca cc lung d liu video streaming

(a) S dng c ch 802.11e chun ; (b) S dng c ch gn u tin ng

Da vo hai th trn ta c d dng thy rng. Vi phng php gn u tin

ng lung d liu ng vi tng cbn (lung cbn) lun lun duy tr mc thng

lng cnh l 128kbps m bo rng bn nhn lun lun duy tr c dch vvi bt k thay i no ca mi trng truyn.

Trong khi vi trng hp s dng cch 802.11e chun th thng lng ca

tt c cc lung trong c lung cbn l khng n nh v tng, gim ph thuc

vo ri, bn ca bn ca mi trng. Vic ny gy ra tnh trng dch v video

streaming s khng n nh. Do gn cng u tin v cnh chng trong sut thi

gian truyn do k c nhng lc mng bn lu lng ca cc lung mrng vn ln

hn lung cbn (trng hp cch 802.11e chun) iu ny l mt lng ph bi v

trong khi lung cbn c thng lng nh, vic gii m cc lung m rng li ph

thuc vo lung cbn. Do thng lng ca cc lung mrng trnn hoang ph

v khng c ch g.

Trong hai th trn ta cng c th nhn thy rng trong iu kin mng rnh ri

(t giy th 10 n giy th 40 v t giy th 190 n giy th 210) c hai phng

php u cho kt qu l 3 lung d liu video u c thng lng t mc cao do

cht lng video bn nhn c c c cht lng tt. Song khi mng bt u trnn


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bn v d nh khong thi gian t giy th 40 n giy th 70 th trng hp s

dng cch 802.11e chun c ba lung d liu sng lot gim thng lng, trong

khi trng hp s dng cchu tin ng. lung mrng 2 s gim nhanh

thng lng trong khi lung m rng 1 v lung cbn vn duy tr c thng

lng mc cao. Tc l cht lng video bn nhn s vn nhn c tn hiu tt do

s kt hp gia lung cbn v lung mrng 1.

Ta cng c biu ct so snh tng s gi tin nhn c ca mi lung d liu

nh sau.

Hnh 4.4. Biu so snh tng s gi tin

Biu so snh tng s gi tin cho ta thy rng. vi phng thc gn u tin

ng lung mrng 2 bn nhn thu c s lng gi tin t hn so vi trng hp

s dng cch 802.11e chun. Tuy nhin b vo hai lung cn li (lung cbn v

lung mrng 1) ca th nghim trong trng hp s dng u tin ng li c kt

qu l s lng gi tin truyn ti ch ln hn so vi trng hp s dng c ch

802.11e chun. M trong video streming th hai lung ny l quan trng hn lung m

rng 2 do c th nhn thy vic phng php u tin ng hng ti vic duy tr

dch v hn l phng php 802.11e chun (t l gi tin truyn c ch t l vi tc

truyn ca lung d liu m khng quan tm ti vic m bo cht lng cho cc

lung quan trng).

4.2.2. nh gi v tr

Ta si so snh tr ca cc gi tin trong c hai trng hp; s dng cch

802.11e chun v c chu tin ng cho cc lung. nh gi xem phng

php u tin ng c em li hiu qu v tr hay khng.


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tr ca cc lung d liu video tng ng vi c hai trng hp c th

hin bi hai th di y.

(a)

(b)

Hnh 4.5. tr ca cc lung d liu video streaming

(a) S dng c ch 802.11e chun ; (b) S dng c ch gn u tin ng


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