1

MC LC

Chng 1. VoiceOverIP and SIP ................................................................................. 3

1.1. Tng quan v mng VoIP ......................................................................................... 3

1.2. c tnh ca mng VoIP ........................................................................................... 5

u im ............................................................................................................... 5

Nhc im ......................................................................................................... 6

1.3. Yu cu cht lng i vi VoIP ............................................................................. 7

1.4. Giao thc bo hiu VoIP .......................................................................................... 7

Giao thc bo hiu H.323 .................................................................................... 7

Thit lp cuc gi VoIP s dng giao thc H.323 ............................................ 12

1.5. Giao thc SIP .......................................................................................................... 16

Cc thnh phn trong mng SIP ........................................................................ 17

Bn tin SIP ......................................................................................................... 19

M t cuc gi SIP ............................................................................................ 22

1.6. So snh gia giao thc H.323 v SIP ..................................................................... 23

Chng 2. Video over Peer-to-Peer Networks .......................................................... 26

H THNG LUNG VIDEO QUA MNG NGANG HNG P2P ......................... 26

2.1. Phn tch mt s im cn tn ti ca k thut streaming truyn thng ................. 26

2.2. Mng ngang hng Peer-to-Peer ............................................................................... 26

Khi nim ......................................................................................................... 26

u th ca mng ngang hng .............................................................................. 27

Phn loi mng ngang hng ............................................................................... 27

Cc vn cn xem xt trong P2P streaming ................................................... 29

2.3. Cc phng php tip cn xy dng lp ph ........................................................ 30

Phng php tip cn dng cy ......................................................................... 30

Cc h thng lung trc truyn da trn cy n lung ................................... 32

Phng php tip cn da trn cy a lung ..................................................... 36

Cc h thng lung trc truyn da trn cy a lung ..................................... 38

Phng thc tip cn da trn li ..................................................................... 38

Cc h thng lung trc truyn ph bin da trn li...................................... 41

2.4. Video theo yu cu trn P2P ................................................................................... 49

H thng VoD da trn dng cy ...................................................................... 51

2

H thng VoD da trn li ............................................................................... 54

2.5. Kt lun chng 2 ................................................................................................... 56

Chng 3. TNG QUAN V MPEG DASH ............................................................ 57

3.1. Gii thiu DASH .................................................................................................... 57

3.2. Tng quan v MPEG DASH .................................................................................. 58

M t h thng ................................................................................................... 58

M hnh my khch DASH ............................................................................... 59

M hnh d liu DASH ..................................................................................... 60

3

Chng 1. VoiceOverIP and SIP

1.1. Tng quan v mng VoIP

u nm 1995 cng ty VOCALTEC a ra th trng sn phm phn mm thc

hin cuc thoi qua Internet u tin trn th gii. Sau c nhiu cng ty tham gia

vo lnh vc ny. Thng 3 nm 1996, VOLCALTEC kt hp vi DIALOGIC tung ra

th trng sn phm kt ni mng PSTN v Internet. Hip hi cc nh sn xut thoi

qua mng my tnh sm ra i v thc hin chun ho dch v thoi qua mng

Internet. Vic truyn thoi qua internet gy c ch ln trong nhng nm qua v

dn c ng dng rng ri trong thc t.

C th nh ngha: Voice over Internet Protocol (VoIP) l mt cng ngh cho

php truyn thoi s dng giao thc mng IP, trn c s h tng sn c ca mng

Internet. VoIP l mt trong nhng cng ngh vin thng ang c quan tm nht hin

nay khng ch i vi cc nh khai thc, cc nh sn xut m cn c vi ngi s dng

dch v. VoIP c th va thc hin cuc gi thoi nh trn mng in thoi knh truyn

thng (PSTN) ng thi truyn d liu trn c s mng truyn d liu. Nh vy, n

tn dng c sc mnh v s pht trin vt bc ca mng IP vn ch c s dng

truyn d liu thng thng.

c th hiu c nhng u im ca VoIP mang li, trc ht chng ta i vo

nghin cu s khc bit gia mng knh PSTN hin c vi mng chuyn mch gi ni

chung v mng VoIP ni ring.

K thut chuyn mch knh (Circuit Switching)

Mt c trng ni bt ca k thut ny l hai trm mun trao i thng tin vi

nhau th gia chng s c thit lp mt knh (circuit) c nh, knh kt ni ny

c duy tr v dnh ring cho hai trm cho ti khi cuc truyn tin kt thc. Thng tin

cuc gi l trong sut. Qu trnh thit lp cuc gi tin hnh gm 3 giai on:

1. Giai on thit lp kt ni: Thc cht qu trnh ny l lin kt cc tuyn

gia cc trm trn mng thnh mt tuyn (knh) duy nht dnh ring cho cuc gi. Knh

ny i vi PSTN l 64kb/s (do b m ha PCM c tc ly mu ting ni 8kb/s v

c m ha 8 bit).

2. Giai on truyn tin: Thng tin cuc gi l trong sut. S trong sut th

hin qua hai yu t: thng tin khng b thay i khi truyn qua mng v tr nh.

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3. Giai on gii phng (hu b) kt ni: Sau khi cuc gi kt thc, knh s

c gii phng phc v cho cc cuc gi khc.

Qua , ta nhn thy mng chuyn mch knh c nhng u im ni bt nh cht

lng ng truyn tt, n nh, c tr nh. Cc thit b mng ca chuyn mch knh

n gin, c tnh n nh cao, chng nhiu tt. Nhng ta cng khng th khng nhc ti

nhng hn ch ca phng thc truyn d liu ny nh:

S dng bng thng khng hiu qu: Tnh khng hiu qu ny th hin qua hai

yu t. Th nht, rng bng thng c nh 64k/s. Th hai l knh l dnh ring cho

mt cuc gi nht nh. Nh vy, ngay c khi tn hiu thoi l lng (khng c d liu)

th knh vn khng c chia s cho cuc gi khc.

Tnh an ton: Do tn hiu thoi c gi nguyn bn trn ng truyn nn rt

d b nghe trm. Ngoi ra, ng dy thu bao hon ton c th b li dng an trm

cc vin thng.

Kh nng m rng ca mng knh km: Th nht l do c s h tng kh nng

cp v tng thch vi cc thit b c. Th hai, l hn ch ca h thng bo hiu vn

c s dng t trc khng c kh nng ty bin cao.

K thut chuyn mch gi (Packet Switching).

Trong chuyn mch gi mi bn tin c chia thnh cc gi tin (packet), c

khun dng c quy nh trc. Trong mi gi cng c cha thng tin iu khin: a

ch trm ngun, a ch trm ch v s th t ca gi tin, Cc thng tin iu khin

c ti thiu, cha cc thng tin m mng yu cu c th nh tuyn c cho cc

gi tin qua mng v a n ti ch. Ti mi node trn tuyn gi tin c nhn, nh v

sau th chuyn tip cho ti chm ch. V k thut chuyn mch gi trong qu trnh

truyn tin c th c nh tuyn ng truyn tin. iu kh khn nht i vi chuyn

mch gi l vic tp hp cc gi tin to bn tin bn u c bit l khi m cc gi tin

c truyn theo nhiu con ng khc nhau ti trm ch. Chnh v l do trn m cc

gi tin cn phi c nh du s th t, iu ny c tc dng, chng lp, sa sai v c

th truyn li khi hin tng mt gi xy ra.

Cc u im ca chuyn mch gi:

Mm do v hiu sut truyn tin cao: Hiu sut s dng ng truyn rt cao v

trong chuyn mch gi khng c khi nim knh c nh v dnh ring, mi ng

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truyn gia cc node c th c cc trm cng chia s cho truyn tin, cc gi tin sp

hng v truyn theo tc rt nhanh trn ng truyn.

Kh nng tryn u tin: Chuyn mch gi cn c th sp th t cho cc gi

c th truyn i theo mc u tin. Trong chuyn mch gi s cuc gi b t chi t

hn nhng phi chp nhn mt nhc im vi thi gian tr s tng ln.

Kh nng cung cp nhiu dch v thoi v phi thoi.

Thch nghi tt nu nh c li xy ra: c tnh ny c c l nh kh nng nh

tuyn ng ca mng.

Bn cnh nhng u im th mng chuyn mch gi cng b l nhng nhc

im nh:

Tr ng truyn ln: Do i qua mi trm, d liu c lu tr, x l trc

khi c truyn i.

tin cy ca mng gi khng cao, d xy ra tc nghn, li mt bn tin

Tnh a ng c th gy l lp bn tin, loop lm tng lu lng mng khng

cn thit.

Tnh bo mt trn ng truyn chung l khng cao.

1.2. c tnh ca mng VoIP

u im

Gim chi ph: y l u im ni bt ca VoIP so vi in thoi ng di thng

thng. Chi ph cuc gi ng di ch bng chi ph cho truy nhp Internet. Mt gi

cc chung s thc hin c vi mng Internet v do tit kim ng k cc dch v

thoi v fax. S chia s chi ph thit b v thao tc gia nhng ngi s dng thoi v

d liu cng tng cng hiu qu s dng mng. ng thi k thut nn thoi tin tin

lm gim tc bit t 64Kbps xung di 8Kbps, tc l mt knh 64Kbps lc ny c

th phc v ng thi 8 knh thoi c lp. Nh vy, l d ln nht gip cho chi ph

thc hin cuc gi VoIP thp chnh l vic s dng ti u bng thng.

Tch hp dch v nhiu dch v: Do vic thit k c s h tng tch hp nn c

kh nng h tr tt c cc hnh thc thng tin cho php chun ho tt hn v gim thiu

s thit b. Cc tn hiu bo hiu, thoi v c s liu u chia s cng mng IP. Tch hp

a dch v s tit kim chi ph u t nhn lc, chi ph xy dng cc mng ring r.

Thng nht: V con ngi l nhn t quan trng nhng cng d sai lm nht

trong mt mng vin thng, mi c hi hp nht cc thao tc, loi b cc im sai

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st v thng nht cc im thanh ton s rt c ch. Trong cc t chc kinh doanh, s

qun l trn c s SNMP (Simple Network Management Protocol) c th c cung

cp cho c dch v thoi v d liu s dng VoIP. Vic s dng thng nht giao thc IP

cho tt c cc ng dng ha hn gim bt phc tp v tng cng tnh mm do. Cc

ng dng lin quan nh dch v danh b v dch v an ninh mng c th c chia s

d dng hn.

Vn qun l bng thng: Trong PSTN, bng thng cung cp cho mt cuc

gi l c nh. Trong VoIP, bng thng c cung cp mt cch linh hot v mm do

hn nhiu. Cht lng ca VOIP ph thuc vo nhiu yu t, quan trng nht l bng

thng. Do khng c s bt buc no v mt thng lng gia cc thit b u cui

m ch c cc chun tu vo bng thng c th ca mnh, bn thn cc u cui c th

t iu chnh h s nn v do iu chnh c cht lng cuc gi.

Nng cao ng dng v kh nng m rng: Thoi v fax ch l cc ng dng

khi u cho VoIP, cc li ch trong thi gian di hn c mong i t cc ng dng

a phng tin (multimedia) v a dch v. Tnh linh hot ca mng IP cho php to ra

nhiu tinh nng mi trong dch v thoi. ng thi tnh mm do cn to kh nng m

rng mng v cc dch v.

Tnh bo mt cao: VOIP c xy dng trn nn tng Internet vn khng an

ton, do s dn n kh nng cc thng tin c th b nh cp khi cc gi tin b thu

lm hoc nh tuyn sai a ch mt cch c khi chng truyn trn mng. Cc giao

thc SIP (Session Ineitiation Protocol giao thc khi u phin) c th thnh mt m

v xc nhn cc thng ip bo hiu u cui. RTP (Real Time Protocol) h tr m

thnh mt m ca phng thc truyn thng trn ton tuyn c m ho thnh mt m

m bo truyn thng an ton.

Nhc im

Cht lng dch v cha cao: Cc mng s liu vn d khng phi xy dng vi

mc ch truyn thoi thi gian thc, v vy khi truyn thoi qua mng s liu cho cht

lng cuc gi khng c m bo trong trng hp mng xy ra tc nghn hoc c

tr ln. Tnh thi gian thc ca tn hiu thoi i hi cht lng truyn d liu cao

v n nh. Mt yu t lm gim cht lng thoi na l k thut nn tit kim ng

truyn. Nu nn xung dung lng cng thp th k thut nn cng phc tp, cho cht

lng khng cao v c bit l thi gian x l s lu, gy tr.

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Vn ting vng: Nu nh trong mng thoi, tr thp nn ting vng khng

nh hng nhiu th trong mng IP, do tr ln nn ting vng nh hng nhiu n cht

lng thoi.

K thut phc tp: Truyn tn hiu theo thi gian thc trn mng chuyn mch

gi l rt kh thc hin do mt gi trong mng l khng th trnh c v tr khng

c nh ca cc gi thng tin khi truyn trn mng. c c mt dch v thoi chp

nhn c, cn thit phi c mt k thut nn tn hiu t c nhng yu cu kht khe:

t s nn ln ( gim c tc bit xung), c kh nng suy on v to li thng tin

ca cc gi b tht lc... Tc x l ca cc b Codec (Coder and Decoder) phi

nhanh khng lm cuc m thoi b gin on. ng thi c s h tng ca mng

cng cn c nng cp ln cc cng ngh mi nh Frame Relay, ATM,... c tc

cao hn hoc phi c mt c ch thc hin chc nng QoS (Quality of Service). Tt c

cc iu ny lm cho k thut thc hin in thoi IP tr nn phc tp v khng th thc

hin c trong nhng nm trc y

Ngoi ra c th k n tnh phc tp ca k thut v vn bo mt thng tin (do

Internet ni ring v mng IP ni chung vn c tnh rng khp v hn hp, khng c g

bo m rng thng tin c nhn c gi b mt).

1.3. Yu cu cht lng i vi VoIP

T nhng nhc im chnh ca mng chuyn mch gi t ra nhng yu cu

cho VoIP nh sau:

Cht lng thoi phi n nh, tr chp nhn c.

Mng IP c bn phi p ng c nhng tiu ch hot ng kht khe gm gim

thiu vic khng chp nhn cuc gi, mt mt gi v mt lin lc. iu ny i

hi ngay c trong trng hp mng b nghn hoc khi nhiu ngi s dng chung

ti nguyn ca mng cng mt lc.

Vic bo hiu c th tng tc c vi bo hiu ca mng PSTN.

Qun l h thng an ton, a ch ho v thanh ton phi c cung cp, tt nht

l c hp nht vi cc h thng h tr hot ng PSTN.

1.4. Giao thc bo hiu VoIP

Giao thc bo hiu H.323

Khi cp n thoi IP, tiu chun quc t thng c cp n l H.323.

Giao thc H.323 l chun do ITU-T pht trin cho php truyn thng a phng tin

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qua cc h thng da trn mng chuyn mch gi,v d nh Internet. N c ITU_T

ban hnh ln u tin vo nm 1996 v gn y nht l nm 1998. H.323 l chun ring

cho cc thnh phn mng, cc giao thc v cc th tc cung cp cc dch v thng tin

multimedia nh : audio thi gian thc, video v thng tin d liu qua cc mng chuyn

mch gi, bao gm cc mng da trn giao thc IP.

1.4.1.1. Cc thnh phn trong mng H.323

Thit b u cui H.323 (H.323 Endpoint)

Cc thit b nm ngoi phm vi khuyn ngh H.323

- Thit b vo ra Video.

- Thit b vo ra Audio.

- Thit b vo ra s liu.

- Giao din mng LAN.

- Giao din ngi s dng.

Cc phn t nm trong phm vi khuyn ngh H.323

- B m ho v gii m Video.

- B m ho v gii m Audio.

- B m nhn d liu.

- Khi iu khin h thng.

- Khi iu khin theo chun H.245

S dng knh iu khin H.245 mang cc bn tin iu khin im - im iu

khin hot ng ca thc th H.323 bao gm : kh nng trao i, m v ng cc

knh logic, cc yu cu ch hot ng thch hp, iu khin lung bn tin, pht cc

lnh v cc ch th.

iu khin bo hiu cuc gi

S dng bo hiu cuc gi theo khuyn ngh H.225 thit lp mt kt ni gia

hai u cui H.323. Knh bo hiu cuc gi c lp vi knh RAS v knh iu khin

H.245. Trong h thng khng c Gatekeeper th knh bo hiu cuc gi c thit lp

gia hai u cui H.323 tham gia cuc gi. Cn trong h thng c Gatekeeper th knh

bo hiu cuc gi c thit lp gia cc u cui v Gatekeeper hoc gia hai u cui

vi nhau, vic la chn phng n thit lp knh bo hiu cuc gi nh th no l do

Gatekeeper quyt nh.

Chc nng bo hiu RAS

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S dng cc bn tin H.225 thc hin: ng k, cho php dch v, thay i bng

thng, trng thi, cc th tc tch ri gia cc u cui v Gatekeeper.

Hnh 1.1: S khi thit b u cui H.323

Gatekeeper

Mt min H.323 trn c s mng IP l tp hp tt c cc u cui c gn vi

mt b danh. Mi min c qun tr bi mt Gatekeeper duy nht, l trung tm u

no, ng vai tr gim st mi hot ng trong min . y l thnh phn tu chn

trong h thng VoIP theo chun H.323. Tuy nhin nu c mt Gatekeeper trong mng

th cc u cui H.323 v cc Gateway phi hot ng theo cc dch v ca Gatekeeper

. Mi thng tin trao i ca Gatekeeper u c nh ngha trong RAS. Mi ngi

dng ti u cui c Gatekeeper gn cho mt mc u tin duy nht. Mc u tin ny

rt cn thit cho c ch bo hiu cuc gi m cng mt lc nhiu ngi s dng. H.323

nh ngha c nhng tnh cht bt buc ti thiu phi c cho Gatekeeper v nhng c

tnh tu chn:

Cc chc nng bt buc ti thiu ca mt Gatekeeper gm : Phin dch a ch,

iu khin cho php truy nhp, iu khin di thng, qun l min dch v.

Cc chc nng tu chn ca Gatekeeper gm c : Bo hiu iu khin cuc gi,

cp php cho cuc gi, qun l cuc gi.

Gatekeeper hot ng hai ch :

Ch trc tip: Gatekeeper ch c nhim v cung cp a ch ch m khng

tham gia vo cc vic nh tuyn cc bn tin bo hiu.

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Hnh 1.2: Phng thc nh tuyn trc tip

Ch nh tuyn qua Gatekeeper: Gatekeeper l thnh phn trung gian,

nh tuyn mi bn tin bo hiu trong mng H.323.

Hnh 1.3: Phng thc nh tuyn qua Gatekeeper

Khi iu khin a im

Khi iu khin a im (MCU) c s dng khi mt cuc gi hay hi ngh cn

gi nhiu kt ni hot ng. Do c mt s hu hn cc kt ni ng thi, nn cc MCU

gim st s tho thun gia cc u cui v s kim tra mi u cui v tnh nng m

chng c th cung cp cho hi ngh hoc cuc gi. Cc MCU gm hai phn: B iu

khin a im (MC) v B x l a im (MP).

B iu khin a im c trch nhim trong vic tho thun v quyt nh kh

nng ca cc u cui. Trong khi b x l a im c s dng x l multimedia,

cc lung trong sut qu trnh ca mt hi ngh hoc mt cuc gi a im.

1.4.1.2. Giao thc H.323

I P

UDP

RTP

TCP

A / V

A / V

RTCP H.225.0

RAS Signaling

H.225.0 Call

Signaling

H.245 Control

Signaling

Terminal / Application Control

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Hnh 1.4: Giao thc bo hiu H.323

Giao thc H.323 c chia lm 3 phn chnh:

1. Bo hiu H.225 RAS (Registration, Admissions, and Status): bo hiu gia

thit b u cui vi H.323 gatekeeper trc khi thit lp cuc gi.

2. Bo hiu H.225 Q.931 s dng kt ni, duy tr v hy kt ni gia hai u

cui.

3. Bo hiu H.245 s dng thit lp phin truyn media s dng giao thc

RTP.

Bo hiu RAS

Bo hiu RAS cung cp iu khin tin cuc gi trong mng H.323 c tn ti

gatekeeper v mt vng dch v(do gatekeeper qun l). Knh RAS c thit lp

gia cc thit b u cui v gatekeeper qua mng IP. Knh RAS c m trc khi cc

knh khc c thit lp v c lp vi cc knh iu khin cuc gi v media khc.

Bo hiu ny c truyn trn UDP cho php ng k, chp nhn, thay i bng thng,

trng thi v hy.

Giao thc H.245

Chc nng H.245 l thit lp cc knh logic truyn audio, video, data v cc

thng tin knh iu khin. Gia hai thit b u cui c thit lp mt knh H.245 cho

mt cuc gi. Knh iu khin ny c to da trn TCP gn ng port. Chc nng

iu khin ca knh H.245 l thng lng v mt s thng s sau:

B m ha ting ni s c s dng hai pha. Ly v d, chun m ha ting

ni v tc bit tng ng nh sau: G.729 - 8 kbps, G.728 - 16 kbps, G.711 - 64 kbps,

G.723 - 5.3 hay 6.3 kbps, G.722 - 48, 56, v 64 kbps

Thng lng v Ch/t gia hai thit b u cui: xc lp vai tr ca cc thit

b trong khi thc hin cuc gi trnh hin tng xung t.

Round-Trip Delay: xc nh tr gia pha pht v pha thu. Da vo thng s

ny xc nh kt ni vn hot ng.

Bo hiu trn knh logic thc hin vic m v ng cc knh logic. Cc knh

ny c thit lp trc khi thng tin c truyn n . Bo hiu ny c th thit lp

knh n hng hoc song hng. Sau khi knh logic c thit lp, cng UDP cho

knh media RTP c truyn t pha nhn ti pha pht. Khi s dng mt hnh nh

tuyn qua Gatekeeper th Gatekeeper s chuyn hng lung RTP bng cch cung cp

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a ch UDP/IP thc ca thit b u cui. Lung RTP s truyn trc tip gia hai thit

b u cui vi nhau.

Mi knh media s dng RTP truyn thi gian thc - s c mt knh phn

hi v cht lng dch v QoS theo chiu ngc li gip pha pht kim sot c lung

media truyn i v c nhng iu chnh ph hp.

Hnh 1.5: Cu trc lung media gia cc u cui

Thit lp cuc gi VoIP s dng giao thc H.323

1.4.2.1. Bo hiu trc tip gia cc thit b u cui

Trong m hnh ny, c ch l cc thit b u cui (Endpoint) ch xin php

Gatekeeper thc hin cuc gi thng qua bo hiu RAS cn cc bc bo hiu gia cc

thit b ny c thc hin trc tip khng thng qua Gatekeeper.

H.245 Logical

H.245 Logical

Channel 8

H.245 Logical

Channel 12

Audio over RTP

QoS Feedback over RTCP Audio over RTP

QoS Feedback over RTCP

H.245 Control

H.245 Logical

H.245 Logical

Channel 8

H.245 Logical

Channel 12

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Hnh 1.6: Thit lp bo hiu H.323 trc tip gia cc u cui

Bc 1: Endpoint O ng k vi Gatekeeper yu cu cho php thc hin mt

cuc gi ti Endpoint T. Cc bc thc hin xc thc thu bao gi s c thc hin

bc ny. Gatekeeper tr li cho php Endpoint O thc hin cuc gi v a ch ca

chnh xc ca Endpoint T. Trong trng hp ny, hai Endpoint thc hin cuc gi trc

tip vi nhau.

Bc 2: Endpoint O v Endpoint T thit lp mt kt ni TCP cho bo hiu H.225

truyn cc bn tin Q.931 cho php thit lp cuc gi. Endpoint O gi bn tin Setup

ti Endpoint T yu cu thit lp cuc gi. Endpoint T tr li bng bn tin Call Proceeding

thng bo cuc gi ang c thc hin.

Bc 3: Endpoint T xin php Gatekeeper cho php thc hin cuc gi vi

Endpoint O. Gatekeeper tr li ng cho Endpoint T chp nhn cuc gi. Endpoint T

thc hin rung chung v bo cho Endpoint O bit l ang rung chung ngi b gi.

Bc 4: Ngi b gi nhc ng nghe. Endpoint T gi bn tin Conect ti Endpoint

O thng bo knh cuc gi c thit lp. Lc ny, gia hai Endpoint m mt kt

ni TCP na cho knh bo hiu H.245 thng lng, thit lp v duy tr knh media.

Bc 5: Khi thng lng xong (cc thng s c m t trong phn bo hiu

H.245), mi Endpoint yu cu m mt kt ni audio truyn thoi. Nh vy s tn ti

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hai knh cho php thc hin cuc gi hai chiu gia hai thu bao. Qu trnh thoi c

thc hin hin da trn giao thc RTP vi s kim sot ca RTCP.

1.4.2.2. Bo hiu c nh tuyn thng qua Gatekeeper

Trong hnh thc bo hiu ny th mi bn tin bo hiu c gi qua

Gatekeeper. Gatekeeper s x l v chuyn tip bo tin ti pha b gi. Khi , pha gi

khng nht thit phi bit chnh xc a ch ca pha b gi nhng qu trnh ny s b tr

nhiu hn.

Cc bn tin bo hiu trong m hnh ny gn nh ging vi trng hp bo hiu

trc tip gia hai thit b u cui nhng c mt s ch nh sau:

Gatekeeper y s gm c hai giao din: giao din vi Endpoint O v Endpoint

T. Vic phn bit nh vy s gip chng ta r rng hn trong vic gi nhn cc bn tin

v hai giao din ny hot ng c s c lp nht nh vi nhau.

Knh bo hiu H.225 c thit lp gia cc Endpoint v Gatekeeper. Khi nhn

c bn tin Setup t Endpoint O gi ti, Gatekeeper s gi bn tin ny ti Endpoint T

v gi ngay bn tin Call Proceeding v cho Endpoint O bo rng cuc gi ang trong

qu trnh thit lp.

Sau khi nhn c bn tin Connect t Endpoint T, Endpoint O v Endpoint T s

thc hin bo hiu trc tip vi nhau m knh truyn media.

15

Thit lp bo hiu H.323 nh tuyn qua Gatekeeper

1.4.2.3. Thit lp cuc gi gia hai thit b u cui hai vng dch v

Trong m hnh ny l vic thc hin cuc gi gia hai thit b u cui hai

vng dch v khc nhau cho nhau. y l m hnh bo hiu da trn vic nh tuyn ca

cc Gatekeeper.

Sau khi nhn c yu cu ca Endpoint O mun thit lp cuc gi vi Endpoint

T, Gatekeeper 1 gi ti Endpoint T yu cu thit lp cuc gi. V Endpoint T nm trong

vng dch v do Gatekeeper 2 qun l nn n phi xin s cho php c th thc hin

cuc gi (ging nh cc trng hp trc). trong trng hp ny, Gatekeeper 2 cng

gi tr li bn tin ARQ ca Endpoint T bng bn tin ACF cho php thit lp cuc gi

nhng phi thng qua n (khng cho thc hin cuc gi trc tip ti Endpoint T). Do

vy, Endpoint T gi bn tin Facility ti Gatekeeper 1 thng bo l cuc gi c chp

nhn nhng phi c nh tuyn li thng qua Gatekeeper 2. Chnh v vy, knh bo

hiu H.245 c c hy v thay bng cc knh bo hiu biu din nh trong hnh v.

16

Hnh 1.7: Thit lp kt ni gia hai vng dch v

1.5. Giao thc SIP

SIP (Session Initiation Protcol) l giao thc bo hiu iu khin lp ng dng

c dng thit lp, duy tr, kt thc cc phin truyn thng a phng tin

(multimedia). Cc phin multimedia bao gm thoi Internet, hi ngh, v cc ng dng

tng t c lin quan n cc phng tin truyn t (media) nh m thanh, hnh nh,

v d liu. SIP s dng cc bn tin mi (INVITE) thit lp cc phin v mang cc

thng tin m t phin truyn dn. SIP h tr cc phin n b (unicast) v qung b

(multicast) tng ng cc cuc gi im ti im v cuc gi a im. C th s dng

nm chc nng ca SIP thit lp v kt thc truyn dn l nh v thu bao, kh nng

thu bao, sn sng ca thu bao, thit lp cuc gi v x l cuc gi. SIP c IETF

a ra trong RFC 2543. N l mt giao thc da trn tng v cu trc ca HTTP

(HyperText Transfer Protocol) giao thc trao i thng tin ca World Wide Web v l

mt phn trong kin trc multimedia ca IETF. Cc giao thc c lin quan n SIP bao

gm giao thc t trc ti nguyn RSVP (Resource Reservation Protocol), giao thc

17

truyn vn thi gian thc (Realtime Transport Protocol), giao thc cnh bo phin SAP

(Session Announcement Protocol), giao thc miu t phin SDP (Session Description

Protocol). Cc chc nng ca SIP c lp, nn chng khng ph thuc vo bt k giao

thc no thuc cc giao thc trn.

Mt khc, SIP c th hot ng kt hp vi cc giao thc bo hiu khc nh

H.323. SIP l mt giao thc theo thit k m do n c th c m rng pht trin

thm cc chc nng mi. S linh hot ca cc bn tin SIP cng cho php p ng cc

dch v thoi tin tin bao gm c cc dch v di ng.

Cc thnh phn trong mng SIP

1.5.1.1. Gii thiu chung v cc thnh phn trong mng SIP

SIP Client: l thit b h tr giao thc SIP nh SIP phone, chng trnh chat,

y chnh l giao din v dch v ca mng SIP cho ngi dng.

SIP Server: l thit b trong mng x l cc bn tin SIP vi cc chc nng c th.

Proxy Server: l thc th trong mng SIP lm nhim v chuyn tip cc SIP

request ti thc th khc trong mng. Nh vy, chc nng chnh ca n trong mng l

nh tuyn cho cc bn tin n ch. Proxy server cng cung cp cc chc nng xc thc

trc khi cho khai thc dch v. Mt proxy c th lu (stateful) hoc khng lu trng

thi (stateless) ca bn tin trc . Thng thng, proxy c lu trng thi, chng duy

tr trng thi trong sut transaction (khong 32 giy).

Redirect Server: tr v bn tin lp 300 thng bo thit b l chuyn hng bn

tin ti a ch khc t lin lc thng qua a ch tr v.

Registrar server: l server nhn bn tin SIP REGISTER yu cu v cp nht thng

tin t bn tin request vo location database nm trong Location Server.

Location Server: lu thng tin trng thi hin ti ca ngi dng trong mng SIP.

1.5.1.2. Mi lin h gia cc thnh phn trong mng SIP

Trong v d th nht, cho ta c mt ci nhn khi qut v chc nng ca Proxy

Server, Redirect Server, SIP Phone trong mng. Gi s thu bao c tn user1 trong min

dch v do here.com mun thc hin mt cuc gi thoi ti thu bao c th l user2 (

thuc there.com)

18

Hnh 1.8: Chc nng ca Proxy, Redirect Server trong mng SIP

Khi User 1 mun gi ti User 2, trc ht n s gi bn tin INVITE 1 n Proxy

Server 1. Proxy Server 1 chuyn tip bn tin ti Redirect Server. Redirect Server ny x

l v tr v m 3xx thng bo cho Proxy Server t thc hin kt ni. Proxy Server 1 gi

bn tin INVITE 2 ti ch tr v bi Redirect Server ( chnh l Stateless Proxy Server

1). V y l Stateful Proxy nn thc cht bn tin INVITE c gi bi Stateful Proxy

l khc so vi bn tin nhn c t User1(ban u). Stateless Proxy Server chuyn tip

bn tin INVITE ti SIP Statefull Proxy 2. Do l Stateless Proxy nn cng vic ca n

n gin l chuyn tip bn tin. SIP Statefull Proxy 2 chuyn tip bn tin INVITE ti

user2. Khi user2 nhc my th n s gi bn tin 200 OK theo chiu ngc li. Sau khi

nhn c bn tin 200 OK, user1 s gi xc nhn ACK ti user2. Lung RTP trc tip

gia hai thu bao c thit lp. V cuc gi c thc hin.

Trong v d th hai s m t qu trnh mt SIP Phone ng k vi vi Registrar

Server qun l n,hot ng ca Location Server, Proxy Server.

19

Hnh 1.9: Chc nng ca Location, Registrar Server trong mng SIP

Khi mt SIP Phone c kt ni vi mng. N lin tc gi bn tin REGISTER

ti Registrar Server thng bo v tr hin ti ca n. Gi s trong min dch v c tn

chicago.com th qu trnh REGISTER (ng k) c tin hnh nh sau:

Thu bao c tn Carol gi bn tin REGISTER ti Registrar Server. Server ny

tin hnh xc thc. Nu hp l th cc thng tin c lu trong Location Server.

Khi mt thu bao khc (c tn l Bob) gi bn tin INVITE ti Proxy Server

xin kt ni ti thu bao Carol. Proxy Server s truy vn cc thng tin v thu bao b gi

thng qua Location Server.

Proxy Server gi bn tin INVITE ti thu bao Carol thit lp cuc gi.

Bn tin SIP

1.5.2.1. Cc loi bn tin SIP

Bn tin yu cu (Request): c gi t client ti server. RFC 3261 nh ngha 6

kiu bn tin request cho php UA v proxy c th xc nh ngi dng, khi to, s i,

hy mt phin.

Bn tin INVITE: yu cu thit lp mt phin hoc thay i cc c tnh ca

phin trc .Trong bn tin ny c s dng SDP nh ngha v cc thng s media

ca phin. Mt response thnh cng c gi tr 200 c tr li cc thng s m ngi

c gi chp nhn trong phin media.

Bn tin ACK xc nhn rng client nhn c response cui cng ca bn tin

INVITE. ACK ch c s dng km vi bn tin INVITE. ACK c gi t u cui

20

n u cui cho response 200 OK. ACK cng c th cha phn thn bn tin vi m t

phin cui cng nu bn tin INVITE khng cha.

Bn tin OPTIONS: UA s dng request ny truy vn ti server v kh nng

ca n.

Bn tin BYE: UA s dng bn tin ny yu cu hy mt phin c thit

lp trc .

Bn tin CANCEL: cho php client v server hy mt request, v d nh INVITE.

N khng nh hng ti request hon thnh trc m server gi response.

Bn tin REGISTER: Mt client s sng REGISTER yu cu ang k v tr ca

n ti AOR (address of record) ca ngi dng vi SIP server.

Bn tin p ng (Response): server gi bn tin SIP p ng (SIP response) ti

client bo v trng thi ca SIP request m client gi trc . Cc SIP response

c nh s t 100 n 699, c chia thnh cc lp ngha khc nhau

1.5.2.2. Cu trc bn tin SIP

Bn tin Request:

INVITE sip:bob@proxy.company.com SIP/2.0

Via: SIP/2.0/UDP

ph1.company.com:5060;branch=z9hG4bK83749.1

From: Alice ;tag=1234567

To: Bob

Call-ID: 12345601@ph1.company.com

CSeq: 1 INVITE

Contact:

Content-Type: application/sdp

Content-Length: ...

v=0

21

INVITE sip:bob@proxy.company.com SIP/2.0

o=alice 2890844526 28908445456 IN IP4 172.18.193.102

s=Session SDP

c=IN IP4 172.18.193.102

t=0 0

m=audio 49170 RTP/AVP 0

a=rtpmap:0 PCMU/8000

Bn tin Response:

SIP/2.0 200 OK

Via: SIP/2.0/UDP

ph1.company.com:5060;branch=z9hG4bK83749.1

From: Alice ;tag=1234567

To: Bob ;tag=9345678

Call-ID: 12345601@ph1.company.com

CSeq: 1 INVITE

Content-Length: ...

v=0

o=bob 3800844316 3760844696 IN IP4 172.18.193.109

s=Session SDP

c=IN IP4 172.18.193.109

t=0 0

m=audio 48140 RTP/AVP 0

22

SIP/2.0 200 OK

a=rtpmap:0 PCMU/8000

M t cuc gi SIP

1.5.3.1. Cuc gi c nh tuyn qua Proxy Server

Hnh 1.10: Thit lp cuc gi SIP vi Proxy Server

Proxy server nhn c bn tin INVITE t client.

Proxy server lin lc vi Location server xc nh a ch ca ngi b gi.

Location server xc nh v tr ca ngi c gi v cung cp a ch server

ch.

Bn tin INVITE c chuyn tip ti a ch m Location server tr v. Proxy

server s thm tiu Record-Route vo bn tin INVITE chc rng tt c cc

bn tin tun t sau c nh tuyn qua proxy. iu ny cn thit cho qu

trnh tnh cc hoc cc ng dng khc cn thit kim sot cc bn tin cho

dialog ny.

Pha c gi rung chung. Ngi c gi nhc my.

Pha c gi gi bn tin 200 OK thng bo cuc gi bt u.

Bn tin 200 OK c chuyn tip qua proxy server ti pha gi.

Pha gi tr li bn tin 200 OK nhn c bng bn tin ACK ti proxy-server (

khi proxy chn tiu Record-Route vo trong bn tin INVITE) hoc gi trc

tip ti pha ngi c gi.

Proxy chuyn tip ACK ti ngi c gi.

Cuc gi thoi c thit lp.

23

1.5.3.2. Bo hiu trc tip gia cc thit b u cui

Thit lp cuc gi vi Redirect Server

Redirect server nhn c bn tin INVITE t pha UA gi.

Redirect server lin lc vi Location server ly thng tin a ch ca UA c

gi.

Location server tr li a ch ca UA c gi.

Redirect server tr a ch trc tip v UA gi vi bn tin 3xx vi trng Contact

c cp nht. Khng ging nh Proxy server, Redirect server khng chuyn

tip bn tin INVITE.

UA gi gi bn tin ACK ti Redirect server xc nhn v bn tin 3xx.

UAC gi gi trc tip bn tin INVITE vi trng Contact: l a ch tr v bi

Redirect server ti UA c gi.

UA c gi rung chung v ngi dng nhc my. UA c gi gi bn tin 200

OK ti UA gi.

UAC gi gi bn tin ACK xc nhn.

1.6. So snh gia giao thc H.323 v SIP

Gia H.323 v SIP c nhiu im tng ng. C hai u cho php iu khin,

thit lp v hu cuc gi. C H.323 v SIP u h tr tt c cc dch v cn thit, tuy

nhin c mt s im khc bit gia hai chun ny.

H.323 h tr hi ngh a phng tin rt phc tp. Hi ngh H.323 v nguyn tc

c th cho php cc thnh vin s dng nhng dch v nh bng thngbo, trao i d

liu, hoc hi ngh video.

24

SIP h tr SIP-CGI (SIP-Common Gateway Interface) v CPL (Call Processing

Language).

SIP h tr iu khin cuc gi t mt u cui th 3. Hin nay H.323 ang c

nng cp h tr chc nng ny.

SIP H.323

Ngun gc IETF ITU-T

Quan h mng Ngang cp Ngang cp

Khi im K tha cu trc HTTP. K tha Q.931, Q.SIG

u cui SIP H.323

Server

Proxy Server

Redirect Server

Location Server

Registrar Servers.

H.323 Gatekeeper

Khun dng Text, UTF-8 Nh phn

Tr thit lp

cuc gi 1.5 RTT 6-7 RTT hoc hn

Gim st trng

thi cuc gi

C 2 la chn:

trong thi gian thit lp

cuc gi

sut thi gian cuc gi

Phin bn 1 v 2: my ch phi

gim st trong sut thi gian cuc

gi v phi gi trng thi kt ni

TCP. iu ny hn ch kh nng

m rng v gim tin cy

Bo hiu

qung b C h tr Khng

Cht lng

dch v

S dng cc giao thc khc

nh RSVP, OPS, OSP

m bo cht lng dch v

Gatekeeper iu khin bng

thng. H.323 khuyn ngh dng

RSVP lu d ti nguyn mng.

Bo mt

ng k ti Registrar

server, c xc nhn u

cui v m ho

Ch ng k khi trong mng c

Gatekeeper, xc nhn v m ho

theo chun H.235.

25

nh v u

cui v nh

tuyn cuc gi

Dng SIP URL nh a

ch. nh tuyn nh s

dng Redirect v Location

server

nh v u cui s dng E.164

hoc tn o H.323 v phng php

nh x a ch nu trong mng c

Gatekeeper. Chc nng nh tuyn

do Gatekeeper m nhim.

Tnh nng

thoi

H tr cc tnh nng ca

cuc gi c bn

c thit k nhm h tr rt

nhiu tnh nng hi ngh, k c

thoi, hnh nh v d liu, qun l

tp trung nn c th gy tc nghn

Gatekeeper

To tnh nng

v dch v mi

D dng, s dng SIP-CGI

v CPL H.450.1

Kh nng

m rng D dng Hn ch

26

Chng 2. Video over Peer-to-Peer Networks

H THNG LUNG VIDEO QUA MNG NGANG HNG P2P

2.1. Phn tch mt s im cn tn ti ca k thut streaming truyn thng

K thut streaming truyn thng da trn h thng Client - Server cung cp

hiu sut tt v t l sn sng cao nu s lng khch hng c gii hn. Tuy nhin,

vic trin khai v chi ph bo tr ca cc h thng ny thng rt cao. c tnh hin ti

cho chi ph ca YouTube l 1 triu USD mi ngy v cc chi ph c th tng ln

nhanh nu nhiu video tip tc c chuyn sang cht lng cao hn. Trong thc t,

nhiu dch v lung trc tuyn hin nay u c phn gii tng i thp tit

kim bng thng. Cht lng ca cc dch v lung trc tuyn thng khng th so

snh c vi mng li truyn hnh truyn thng. Do qun l ti nguyn l mt vn

quan trng trong vic trin khai k thut streaming trn Internet.

Mt khc, kh nng x l, dung lng lu tr, lu lng ca my ch I/O c

th to thnh nt tht c chai. Mt khc, s lng ln cc kt ni mng ng di cng

c th dn n tc nghn lu thng. Do , h thng khng th p ng yu cu

thc hin ca cc ng dng lung a phng tin quy m ln, thi gian thc. Tuy

nhin, k thut IP multicast c cung cp gii quyt nhng vn ny cn s h

tr t phn cng c bit v cc chi ph thit lp c s h tng v qun l l rt tn

km. V bn cht, cc k thut truyn thng khng th gii quyt hiu qu nhng vn

ca video streaming. Do , m hnh phn phi lung video quy m ln qua

mng Internet ang ni ln. Mng P2P c chp nhn rng ri nh l mt cch

gii quyt cc vn ti nguyn vi cc ng dng lung Internet nh VoD , live

streaming v cung cp mt s thay th cho h thng client - server.

2.2. Mng ngang hng Peer-to-Peer

Khi nim

Mt mng my tnh ngang hng (Peer to Peer hoc P2P) ch yu da trn

sc mnh tnh ton v bng thng ca cc my tham gia trong mng hn l tp trung

vo mt s lng nh cc my ch (server). Mng P2P c s dng in hnh cho

vic kt ni cc nt thng qua nhng kt ni ad hoc ln. Nhng mng nh vy c ch

cho nhiu mc ch s dng. Chia s file cha audio, video, data hoc mi th nh

dng s, cc d liu thi gian thc, v d nh truyn ti ging ni, video streaming u

c th thc hin vi cng ngh P2P.

27

Mt mng P2P thun ty s khng c khi nim v khch (clien) v ch (server),

m ch c nhng nt ngang hng thc hin c hai chc nng ca mt my ch v my

khch i vi nhng nt khc trong mng. M hnh mng ny khc vi m hnh mng

khch ch (server client) m vic giao tip thng l vi cc my ch trung tm.

Mt v d in hnh cho vic truyn file theo m hnh khch ch l gia mt FTP

Client v mt FTP Server, hai chng trnh FTP Client v FTP Server c vai tr rt

khc nhau, client khi to vic download/upload file cn server th tip nhn v phc

v cc yu cu .

u th ca mng ngang hng

Mc ch quan trng ca mng ngang hng l trong mng tt c cc my tham

gia u ng gp ti nguyn, bao gm bng thng, lu tr, v kh nng tnh ton. Do

khi cng c nhiu my tham gia v mng th kh nng tng th ca h thng mng

cng ln. Ngc li, trong m hnh my khch ch, nu s lng my ch l c nh

th khi s my khch tng ln th kh nng chuyn d liu t cc my ch cho mi

my khch s gim xung.

Tnh cht phn tn ca mng ngang hng cng gip cho mng hot ng tt

khi mt s my gp s c. i vi cu trc tp trung, ch cn my ch gp s c th c

h thng s ngng tr.

Phn loi mng ngang hng

Mng ngang hng P2P c chia lm hai loi chnh: mng ngang hng thun

ty v mng ngang hng lai ghp ( Hnh 2.1)

Hnh 2.1: Phn loi mng ngang hng

28

2.2.3.1. Mng ngang hng thun ty

Mng ngang hng thun ty c chia lm 2 loi: Mng ngang hng c cu trc

v mng ngang hng khng cu trc.

Mng ngang hng khng cu trc: l khi cc lin kt gia cc nt mng trong mng

ph c thit lp ngu nhin (tc l khng theo quy lut no). Nhng mng nh th

ny d dng c xy dng v cc my mi khi mun tham gia mng c th ly cc lin

kt sn c c mt my khc ang trong mng v sau dn dn t bn thn n s

thm vo cc lin kt mi ca ring mnh. Khi mt my mun tm mt d liu trong

mng ng ng khng cu trc, yu cu tm kim s c truyn trn c mng tm

ra cng nhiu my chia s cng tt. S hiu bit v mng ca cc nt trong mng khng

cu trc l rt t, mi nt ch nm bt thng tin v nhng nt lin kt trc tip v mt s

t cc nt khc, thng tin v nhng nt cn li hon ton thng qua Broadcast. Chnh v

vy, vi nhng ng dng cn c s lin kt cht ch gia cc nt vi nhau nh multicast

th mng ngang hng khng cu trc khng phi l s la chn ti u.

Mng ngang hng c cu trc: Khc phc nhc im ca mng khng cu trc

bng cch s dng h thng DHT (Distributed Hash Table Bng bm phn tn). H

thng ny nh ngha lin kt gia cc nt mng trong mng ph theo mt thut ton c

th, ng thi xc nh cht ch mi nt mng s chu trch nhim i vi mt phn

d liu chia s trong mng. Vi cu trc ny, khi mt my nh tuyn thng bo, n

ch cn p dng mt giao thc chung xc nh nt cn thng bo v sau lin lc

trc tip n nt mng . Bi vy vic to cy multicast s din ra rt d dng v vic

qun l cy cng c nhiu thun li khi cc nt u c lin kt cht ch vi nhau.

Mt s mng ngang hng c cu trc ni ting bao gm Chord, CAN, Kademlia,

Pastry v Tapestry.

2.2.3.2. Mng ngang hng lai ghp

Trong m hnh mng ngang hng lai ghp, tn ti mt server trung gian c

trch nhim iu khin hot ng ca mng. Server ny lu tr cc ch mc bao gm

thng tin v cc nt m n qun l v v tr cc cp key value trn mng. Cc nt

trong mng phi to lin kt vi server ny. Mt nt mun trao i thng tin vi mt

nt khc th n s phi lin lc trc tip vi server, sau server s tm kim trong c

s d liu v gi li a ch nt ch. Qu trnh trao i thng tin sau c din ra

29

trc tip gia 2 nt. Vic trin khai truyn tin multicast trn mng ngang hng lai ghp

gn ging nh m hnh proxy base ALM. Nhc im chnh ca n vn l chi ph

cao cho cc server trung gian. ng dng in hnh cho m hnh mng ny l Napster.

Cc vn cn xem xt trong P2P streaming

Trong vi nm qua, mng P2P xut hin nh l mt phng php thun li

cho vic cung cp cc ni dung a phng tin trn nn mt mng ph ln. Cc c

tnh bn trong mng P2P lm cho cc m hnh P2P tr thnh mt ng c vin tim nng

gii quyt cc vn khc nhau trong truyn thng a phng tin trn mng Internet.

P2P Streaming l gii php tt hn v hai l do. u tin, mng P2P khng cn s h

tr t cc b nh tuyn Internet v do chi ph hiu qu v n gin trin khai.

Th hai, mt peer ng thi hot ng nh mt client cng nh server, do c th

ti xung mt lung video v cng mt lc ti ln lung video cc peer khc

xem chng trnh . Do , truyn ti P2P lm gim ng k nhu cu bng thng ca

nt ngun. Mc tiu ca c ch P2P streaming l ti a ha cht lng phn phi ni

dung cho cc im peer ring l . Tng hp ngun ti nguyn c sn ca cc peer v

c kh nng m rng n s lng bt k cc im tham gia. Mi peer c th tip tc

cung cp ni dung ph hp vi cc peer kt ni vi n trong lp ph bng cch s

dng bng thng ca cc im peer ang tham gia. Tuy nhin, vic cung cp cc dch

v P2P streaming video cho mt s lng ln ngi xem to ra nhng thch thc

cng ngh rt kh khn trn c h thng v ti nguyn mng.

Trong khi cc ng dng phn phi file P2P truyn thng nhm mc tiu truyn

d liu linh hot, P2P streaming tp trung vo vic phn phi hiu qu cc ni

dung audio v video theo yu cu thi gian rng buc. Lung d liu c nhn, pht

ngay lp tc v truyn cho cc im peer lin quan khc. V d: ng dng chia s file

P2P, BitTorrent cho php cc im peer trao i on bt k ca cc ni dung c

phn phi m n l khng quan trng th t. Ngc li, cc k thut nh vy l khng

kh thi trong ng dng streaming. Cc tp tin video c pht trc tip trong lc n

ang c ti xung. Ngoi ra, cc peer hn ch kh nng ti ln, m xut pht t

thc t l Internet c thit k cho cc m hnh client server v cc ng dng. Hn

na, h thng streaming chu vic rt gi tin hoc chm tr do tc nghn mng.

Trong P2P streaming, tr end-to-end t ngun n ni nhn ln bi v ni dung

c th phi i qua mt s ni nhn trung gian. Hnh vi ca ngi nhn l khng th

30

on trc, h c t do tham gia v ri b bt c lc no, do loi b cc peer k

tip ca h. Ngi nhn c th phi lu mt s cu trc d liu cc b v trao i

thng tin trng thi vi nhau bo v cc kt ni. Cc chi ph kim sot mi im

nhn p ng mc ch nh vy s nh gi cho khng s dng qu nhiu ti

nguyn mng v khc phc hn ch ti nguyn mi im nhn. iu ny l quan

trng i vi kh nng m rng ca h thng vi mt s lng ln cc im nhn.

Vic cu trc cc nt thnh lp mt mng ph cung cp video m bo QoS.

Mt khc, h thng c quy m m rng nn c th cha hng chc ngn ngi nhn

ti cng mt thi im. ng thi, cc chi ph qun l lin quan hp l ngay c quy

m ln.

2.3. Cc phng php tip cn xy dng lp ph

K thut streaming trong cch tip cn P2P c phn loi thnh truyn video

thi gian thc (P2P live video streaming) v truyn video theo yu cu (P2P on

demand video streaming). Mt s h thng P2P streaming c trin khai cung

cp xem video theo yu cu hoc cc dch v truyn video trc tuyn qua Internet.

Hnh 2.2: Cc loi P2P Streaming

Da trn cu trc mng che ph, h thng P2P streaming c sp xp thnh

ba loi: tree-based (phng thc tip cn dng cy), mesh-based (phng thc tip

cn dng li) and hybrid schemes (phng thc tip cn dng n). Phng php tip

cn tree-based s dng phng php y d liu phn phi ni dung. Tuy

nhin, phng php tip cn mesh-based s dng trn lt d liu phn phi ni dung.

Phng php tip cn dng cy

Tng t nh mt cy IP multicast c hnh thnh bi cc b nh tuyn

lp mng, cc ngi dng tham gia vo mt phin video streaming c th to thnh

mt cy lp ng dng c gc c bt ngun t my ch video ngun nh hnh 2.3.

V nguyn tc, mi nt nhn d liu t mt nt cha, c th l nt ngun hoc mt

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peer. Cc h thng da trn cy thng phn phi video bng cch ch ng y d

liu t mt peer ti cc nt con ca n.

Mt cch tip cn chung ca P2P streaming l sp xp cc peer tham gia vo

mt lp ph c cu trc cy n lung m ni dung c y i t ngun ti tt c

cc peer. Ni dung c lan truyn nh mt dng chy lin tc ca thng tin t

ngun xung. Mi ngi dng tham gia vo cy v tr nht nh. Cc h thng loi

ny ch yu l khc nhau trong cc thut ton c s dng to ra, duy tr cu trc

cy. Cho mt tp hp ca cc im peer, c rt nhiu cch kt ni cc peer. Mc

tiu ca thut ton xy dng cy l ti a ha bng thng ti gc ca tt c cc nt. V

cc h thng ny rt gn vi IP multicast, c gng m phng cu trc cy ca n,

n c th t c di ng truyn d liu tng ng m khng chnh lch

qu nhiu t cc ng dn IP multicast.

Hnh 2.3: M hnh cy n

Xy dng v duy tr cy c th c thc hin tp trung hoc mt cch phn

tn trong h thng cy n lung. Trong hnh 2.4 & 2.5, mt my ch trung tm qun

l xy dng cy v phc hi cy. Khi mt peer tham gia h thng n giao tip vi my

ch trung tm. Da trn cc kt ni hin c v c tnh ca cc peer mi c tham

gia vo nh v tr v im truy cp mng, my ch quyt nh v tr ca peer mi trong

cy v thng bo nt cha peer mi c kt ni vo. Cc my ch trung tm c th

pht hin mt peer tch ra thng qua tn hiu mt hoc phn on da trn ch s thi

gian u ra. Trong c hai trng hp my ch s tnh ton li cu trc lin kt cy cho

s peer cn li v ch dn chng to thnh cu trc lin kt mi. i vi mt h thng

streaming ln, my ch trung tm c th tr thnh im nt tht c chai. gii quyt

vn ny, cc thut ton phn phi v d ZigZag c pht trin xy dng v duy

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tr cy streaming phn phi ni dung. Nu cc peer khng thay i mt cch thng

xuyn, h thng da trn m hnh cy i hi t chi ph nh cc gi tin c chuyn

tip t peer ti peer m khng cn phi thm bn tin thng bo. Tuy nhin trong mi

trng t l bin ng cao, cy s thng xuyn thay i v phi xy dng li. Do ,

cc peer cn phi m d liu (buffer data) t nht l bng thi gian cn thit cho sa

cha cy trnh khi mt gi.

Hnh 2.4 : Xy dng li cy Streaming

Peer 0 ri (b) M hnh cy sau khi phc hi

Hnh 2.5 Gii php tp trung cho cu trc cy v duy tr cy

Cc h thng lung trc truyn da trn cy n lung

2.3.2.1. Giao thc NICE

H thng ph bin nht s dng cch tip cn cy n lung l NICE. NICE

(The Internet Cooperative Environment) c thit k ban u cho bng thng thp v

cc ng dng truyn d liu vi mt s lng ln ngi dng. Giao thc sp xp cc

trm cui (host) vo trong mt h thng phn cp da trn thi gian thng tin chuyn

i vng quanh gia cc host hnh 2.6. Cc hot ng c bn ca giao thc l to ra

33

v duy tr cc tuyn ng trong h thng. Mi thnh vin gi chi tit trng thi v

cc thnh vin khc gn n trong h thng phn cp. V ch c kin thc hn ch v

cc thnh vin khc trong nhm. Cu trc phn cp cng rt quan trng cho xc nh v

tr ca cc thnh vin tht bi.

Hnh 2.6: Sp xp cc host trong NICE.

2.3.2.2. Giao thc SpreadIt

SpreadIt xy dng ng dng cy multicast trn cc thit lp ca khch hng.

Cc nt c t chc thnh cc cp khc nhau (hnh 2.7 v hnh 2.8). i vi mi nt

n mc 1+l; l = 0 , 1 , 2 , c mt nt p c gi l cha ca n, mc l, n c gi

l con ca p. Tt c cc nt trong cy con c gc l p c gi l con chu ca n. Mi

peer trong cy chu trch nhim vic gi cc d liu cho nt con ca mnh. Mi nt

khch hng cn phi c kch hot vi mt lp peering(lp nm gia lp ng dng

v lp vn chuyn). Cc lp peering ti cc nt khc nhau phi hp vi nhau thit

lp v duy tr mt cy multicast. Cc ng dng (RealPlayer, Windows Media Player,

etc) c c lung t cc lp peering trn cc my tnh cc b. SpreadIt s dng mt

cy n phn b v nh vy rt d b tn thng do gin on mt nt tch ra.

Hnh 2.7: SpreadIt mt ng dng cy multicast xy dng trn cc Peer

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Hnh 2.8: SpreadItKin trc phn lp ca mt Peer

2.3.2.3. Giao thc ESM

ESM (End System Multicasst) l mt c s h tng cho pht thanh truyn

hnh, thc hin bi i hc Carnegie Mellon. ESM cho php truyn d liu m thanh

hnh nh ti mt lng ln ngi s dng. H thng ESM s dng mt cu trc lp ph

da trn xy dng mt cy. Cc thng tin c phn phi mt cch truyn thng

da trn cy n lung, trong bt k peer no cng ch nhn c lung tin t duy

nht mt ngun. Mi nt duy tr thng tin v mt nhm nh ngu nhin ca cc thnh

vin khc, cng nh cc thng tin v ng i t ngun n n. Mt nt mi tham gia

pht sng bng cch lin h vi ngun v ly danh sch ngu nhin ca cc thnh vin

hin ang trong nhm. Sau chn mt trong cc thnh vin lm nt cha ca n bng

cch s dng thut ton la chn nt cha. tm hiu v cc thnh vin khc, mt bn

tin thm d c s dng. Khi mt nt tham gia vo pht sng hoc cn thc hin

thay i nt cha, n d mt tp hp ngu nhin ca cc nt n bit. Cc bn tin thm

d c u tin ti cc thnh vin khng c kho st hoc c s tr thp.

Hnh 2.9 cho thy mt vi d v nhim v ca ESM. Ngi nhn cui cng c

th ng vai tr ca nt cha hoc nt con. Cc nt cha ng vai tr l cc thnh vin

v thc hin sao chp. Cc nt con l ngi nhn trc tip d liu t nt cha. C

mt my ch iu khin v mt my ch d liu c tr trong cng mt gc ngun. Bt

k mt ngi nhn c th ng vai tr ca nt cha chuyn tip d liu cho nt con

ca n. Mi khch hng s c hai kt ni: mt kt ni qun l v mt kt ni d liu.

35

Hnh 2.9: V d v ESM

Mt li th ca ESM l n gii quyt cc vn trin khai ca IP multicast.

Tuy nhin, multicasting thc hin ti my trm cui hiu nng thp. Ni chung cc

trm cui khng x l c thng tin ging nh cc router. Ngoi ra, gii hn bng

thng v nhu cu ca vic chuyn tip bn tin t host to host s dng kt ni unicast

v do tng tr end-to-end ca qu trnh truyn tin, gp phn tng chi ph phi tr cho

cch tip cn ny. l nhng l do lm cho ESM km hn so vi IP multicast.

2.3.2.4. Giao thc ZigZag

ZigZag c xut bi trng i hc Central Florida v ci thin

giao thc NICE. Cc thut ton xy dng cu trc v bo dng kh ging vi NICE

v tt c cc thuc tnh ca giao thc NICE vn cn. ZigZag nhn vo mt h thng

cc nhm v xy dng cy munlticast trn h thng phn cp ny theo mt b quy

tc c gi l quy tCc (hnh 2.10). Mt nhm c mt nt cha ng u v mt nt

lin kt. Nt cha chu trch nhim gim st cc thnh vin ca nhm v nt lin kt

chu trch nhim truyn ti nhng ni dung cho cc thnh vin trong nhm. V vy nt

cha gp vn khng nh hng n tnh lin tc dch v ca cc thnh vin khc

hoc trong trng hp nt lin kt tch ra, nt cha vn cn hot ng v c th ch nh

mt nt lin kt mi nhanh chng. Trong khi NICE tt c mi d liu c chuyn

tip bi cc nt cha, NICE chu trch nhim chuyn tip d liu l bi nt lin kt.

Cc giao thc iu khin trn ZigZag c chi ph thp. Mt my thu cn trao i thng

tin iu khin ti O(LogN) my thu khc trong trng hp t nht. ZigZag c p

dng tt nht cho cc ng dng lung chng hn nh my ch truyn thng mt s kin

th thao di hn trc tuyn cho nhiu khch hng. N khng ph hp cho cc ng

dng truyn thng trc tuyn ni nhiu ngun c mt. Hn ch chnh ca ZigZag

l n khng xem xt dung lng bng thng ti ln ca peer tham gia vo giao thc.

36

Ngoi ra, ZigZag to ra cy n lung kt ni gia cc peer, n cng c nhng vn

chung ca cy n lung nh khng s dng bng thng ti ln ca l, cc nt bn trong

cy d b tn thng.

Hnh 2.10: T chc hnh chnh ca Peer trong ZigZag

Phng php tip cn da trn cy a lung

Cc gii php cy n lung c l l cch tip cn t nhin nht v n khng

yu cu phc tp thut ton m ha video. Tuy nhin, mt trong nhng mi quan tm

vi cch tip cn da trn cy n lung l s tht bi ca cc nt, c bit l

nhng nt c v tr cao trong cy c th lm gin on vic cung cp d liu cho mt

lng ln ngi dng dn n kt qu thu c km. Nu mt nt bn trong ti nguyn

bng thng khng p ng phc v cc nt con ca n, cc peer trong cy s b nh

hng tr cao trong tip nhn d liu hoc s khng bao gi nhn c gi tin. Cc h

thng ny khng khai thc tt tt c cc ti nguyn peer c sn v c bit bng thng.

V d, cc nt l chim mt phn ln cc peer trong h thng v h khng ng gp

bng thng vo h thng, iu lm cc peer s dng bng thng khng hiu qu.

p ng mi quan tm, cc nh nghin cu nghin cu cu trc tt hn cho vic

phn phi d liu. c bit, mt trong nhng cch tip cn tr nn ph bin l

phng thc tip cn trn cy a lung.

37

Hnh 2.11: Cy a lung trn c s Streaming

Trong cch tip cn cy a lung (hnh 2.11), mt c cu xy dng cu trc

cc peer tham gia trong cy a lung. Mi peer xc nh mt cy tham gia da trn

lin kt bng thng truy cp. Mi peer c t nh mt nt ni b trong mt cy

n lung v nt l trong cy khc m n tham gia. Khi mt peer tham gia h thng,

n tip xc vi nt khi ng (bootstrapping) xc nh mt nt cha trong cy. Trong

cy a lung da trn h thng P2P lung trc tuyn, on video c m ha thnh

nhiu lung v mi lung c truyn qua mt cy. Cht lng video nhn c ti

cc peer ph thuc vo s lng lung tin m n nhn c. Khi mt nt bn trong

ca mt cy tch ra, cc nt con ca n cng nh cc cy c gc l nt tch ra c

phn chia li v chng tham gia li cc cy. Vic cung cp ni dung l mt c ch n

gin, cc nt ni b trong mi cy ch n gin l chuyn tip bt k gi tin nhn

c cho tt c cc nt con tng ng ca n. Do , thnh phn chnh ca cc lung

P2P da trn phng php tip cn cy l cc thut ton xy dng cy.

C hai li th quan trng cho cc gii php cy a lung. u tin, nu mt mng

ngang hng b ph hoc b i, tt c cc nt con ca n mt i lung gi t cc peer,

nhng n vn nhn c cc lung phn phi d liu qua cc cy khc. Do , tt c

cc nt con ca n s nhn c lung video trong trng hp mt i lung gi t

mt lung. Th hai, mt peer ng vai tr khc nhau nh nt ni b cng nh cc nt

l trong cy khc nhau.

Nu cc peer khng thay i qu thng xuyn, h thng cy a lung trc

tuyn i hi t chi ph, v cc gi tin c chuyn tip t nt ti nt m khng cn

thng bo thm. Tuy nhin, trong mi trng bin ng cao, cy lin tc b thay i

v xy dng li. Qu trnh ny i hi ng k bn tin qun l qu trnh trn. Do ,

38

cc nt phi m d liu t nht l thi gian cn thit cy sa cha, trnh mt gi

tin.

Cc h thng lung trc truyn da trn cy a lung

Vi ng dng c xy dng trn khi nim cy a lung ph bin ngy nay.

V d nh SplitStream and Coopnet. y ta ch xem xt ti h thng CoopNet

CoopNet (Cooperative Networking) kt hp cc kha cnh ca c s h tng

da trn h tng mng P2P. N s dng nhiu m ha thc hin truyn ni dung

trn lp x l , sau truyn ni dung trong cc lp khc nhau theo nhng con ng

khc nhau. Mt my ch giu ti nguyn ng vai tr trung tm trong vic xy dng

v qun l cc cy phn phi ni dung, tuy nhin bng thng cho vic gi cc lung

d liu ni dung c ng gp bi tp hp cc peer (hnh 2.12). H thng xy

dng nhiu cy phn phi m rng ngun v tt c ngi thu. Khi mt nt mun tham

gia, n lin lc vi my ch trung tm nu p ng yu cu th mt nt cha trong cy

c ch nh cho n. CoopNet h tr c lung trc tuyn cng nh cc dch v theo

yu cu. Cch tip cn CoopNet l tt, y cc peer thu c ni dung cht lng

thp c u tin hn. Tuy nhin, cc my ch trung tm cn phi duy tr kin thc y

ca tt c cc cy phn phi, n s to ra chi ph kim sot nng trn my ch. V

vy kh nng m rng ca CoopNet khng tt. Mt vn khc l cc my ch trung

tm tr thnh mt im tht bi trn h thng.

Hnh 2.12: Streaming Media s dng ni dung CoopNet

Phng thc tip cn da trn li

chng li cc ng thi t cc peer, nhiu h thng P2P streaming s

dng phng php tip cn da trn li. Trong phng php tip cn da trn li,

peer tham gia to thnh mt lp ph kt ni ngu nhin. Trong lp ph cc ni dung gc

t ngun ban u c phn phi gia cc peer khc nhau. Mi nt c thng tin v tt

c cc nt khc nhau trong h thng. Kt qu l, mi nt duy tr kt ni vi mt vi

39

nt khc trong mng. Mi peer trao i d liu vi mt tp cc peer hng xm. Nu

mt peer hng xm ri i, cc peer vn c th ti v cc video t nhng peer hng

xm cn li. Trong khi , cc peer s b sung thm cc my khc vo tp hp peer

hng xm ca n. Khng ging nh cc h thng cy n lung trong h thng li

trn, mi peer c th nhn c d liu t nhiu peer hng xm cung cp. V vy, cc

h thng truyn da trn li khe mnh chng li cc trng thi t cc peer hng

xm. Nhng thch thc ln trong h thng lung trc truyn P2P da trn li l

hnh thnh thng tin lng ging v lp danh mc d liu.

Khi mt peer tham gia vo mng. Mt peer trong cc a ch mng (mt

nt bootstrapping) theo di peer mi v cung cp mt danh sch c cha cc thng tin

ca mt nhm ngu nhin cc peer ang hot ng. S dng danh sch ny, cc peer

c gng bt u kt ni v nu thnh cng n bt u trao i ni dung video vi

cc peer (hng xm). x l nhng peer ri khi mng khng bo trc, cc peer

trong mng thng xuyn trao i vi nhau. ng thi, ty thuc vo cu trc h thng

ca cc peer, mt peer khng kt ni duy nht ti mt peer lng ging, p ng

vic cc peer tch ra.

Hnh 2.13 Danh sch peer t Tracker Server Hnh 2.14 P2PliveVideo Streaming

Trong h thng da trn li, cc khi nim v lung video tr nn khng c

tc dng do cu trc lin kt li. Cc khi n v d liu trong h thng li l mt

on video. Cc my ch a phng tin phn chia ni dung truyn vo cc khi

truyn thng nh, mi khi c mt s th t nht n. Sau , mi khi c truyn n

tt c cc peer qua li (hnh 2.14). Mi khi d liu c cc con ng khc nhau ti

cc peer, chng c th n ch theo mt th t khng lin tc. i ph vi vn

ny khi d liu thng c thm cc b m (buffered) vo b nh v t sp xp

li trc khi cung cp cho my thu, m bo pht lin tc.

Ch yu c ba vn chnh ca thit k trao i d liu trong h thng li:

40

y (push), ko (pull) v lai y-ko (Hybrid Push - Pull) hnh 2.15. Trong mt h

thng li y, mt peer tch cc y cc khi ni dung nhn c cho cc peer hng

xm ca n (peer m cha nhn c cc khi ni dung ). Khng c quy nh r

rng mi quan h cha-con trong h thng li. Mt peer c th y m mm mt khi

ni dung ti peer va c khi . N cng c th xy ra hai peer cng y cc khi

nh nhau, s lm lng ph bng thng. gii quyt vn , lch trnh y khi ni

dung cn phi c ln k hoch mt cch cn thn gia cc peer. V lch trnh cn

phi c ti to khi cc peer n v i trong h thng.

Hnh 2.15: Trao i d liu thit k trong h thng da trn li

Mt phng php cho vic truyn d liu l phng php ko. tng chnh

ca phng php ko l mi peer yu cu cc on thiu t cc peer khc. Mi peer

c mt nhm hng xm v nh k trao i thng tin d liu sn c (buffer maps) vi

cc peer hng xm. Mt b nh m cha s th t ca cc khi hin c trong b m

ca peer. Bt c khi no mt peer nhn c thng tin t cc peer khc, n hc v cc

khi n khng c bit. Sau n yu cu cc khi cn thiu t cc peer trong

nhm. Hn na, bt k khi u c sn ti nhiu i tc, do mng li mnh trnh

tht bi khi mt nt tch ra. Tc l cc peer s s dng cc peer hng xm khc

nhn c khi d liu. Mt bt li ca k thut ko l thng xuyn trao i bn

m v ko cc sn phm yu cu nhiu hn tn hiu pha trn v tng tr khi ly mt

khi ni dung.

Phng php ko (pull) trong lp ph khng c cu trc mnh m c th lm vic

tt vi tc cao nht trong mi trng P2P trong khi ch y (Push) c th lm

gim hiu qu tr c thy cc nt ngi s dng. Phng php ko thun ty

khng th p ng nhu cu ca cc ng dng v tr v tr tch ly ln . Lai push-pull

streaming c th lm gim tr v k tha cc tnh nng tt nht nh n gin v

mnh m ca phng php ko thun ty. Mi nt s s dng phng php ko v sau

khi nhn c khi d liu mi nt s chuyn tip khi n peer hng xm m

41

khng c yu cu r rng t nhng hng xm. Cc lung d liu c phn loi nh

cc gi c ko v y nh cc gi tin. Mt gi ca nt b ko ch khi cc gi d liu

c yu cu, trong khi mt gi b y ngay sau khi nt nhn c gi . Mi nt

hot ng theo ch ko thun ty trong khong thi gian u tham gia. Trong khi

, cc gi tin b mt gy ra bi s khng ng tin cy ca lin kt mng hay nhng

peer tht bi s c ko cng t nhng peer khc. V vy, hu ht cc gi tin nhn

c s c y i.

Cc h thng lung trc truyn ph bin da trn li

Nhiu ng dng c pht trin bi cc nh nghin cu cho cc mc ch

khc nhau ca P2P streaming da trn li. Anysee l mt ng dng da trn phng

php y nhng tri li CoolStreaming, Chainsaw, PPLive, PPStream, and SopCast l

mt v d v ng dng da trn phng php ko. GridMedia and PRIME l nhng

ng dng c pht trin da trn cch tip cn s lai push-pull.

2.3.6.1. H thng AnySee

AnySee l mt ng dng da trn phng php y trong h thng li trong

cc ngun c phn chia da trn tr ca n. Cc cng vic c bn ca anysee l

nh sau. Ban u, mt lp ph c xy dng da trn li. Tt c cc peer, vi mt

nh danh duy nht, u tin kt ni vi cc nt bootstrapping v la chn mt hoc mt

s peer xy dng lin kt. Mi peer do duy tr mt nhm cc peer hng xm. Pht

hin mt v tr da trn thut ton c s dng ph hp vi lp ph v m hnh vt l

pha di lp ph. Ban u, tt c cc ng streaming c qun l bi cc lp ph

hp n duy nht d qun l vic tham gia, ri khi ca cc peer. Ngi qun l ti

u ha gia cc lp ph v khm ph con ng thch hp, xy dng cc lin kt sao lu

v ct t con ng c Q0S thp. Ngi qun l duy tr hai ng truyn hot ng,

gm ng dn streaming hin ti v cc ng dn sao lu c tnh ton trc

ca tt c cc peer trong mng. V vy, khi cc peer cht hoc ri khi mng, mt

con ng mi c chn t b d phng thay th cc lin kt b hng do khi

phc li kt ni ca mng. S h thng ca mt nt AnySee c th hin trong

hnh 2.16. C ch AnySee thun li bi v cc peer hng xm khng t hp i yu cu

khi c mt peer ra i, thay vo ngi qun l lp ph thay th mt lin kt b mt

bng cch cp n cc thit lp sao lu thay th mt peer hiu qu. Do , Anysee

khi phc cc kt ni ca mng rt nhanh. Mc tiu ca qun l nt quan trng l xc

42

nh s lng yu cu m mt peer cn phi c. Video c phn chia thnh nhiu

khi, mi khi c thi gian c nh 1s. Cc peer ly cc khi t cc ngun hoc peer

hng xm khc v b nh cache ca chng trong b nh a phng. im yu ca

Anysee l cht lng truyn ni dung khng c m bo, vic la chn mt nhm

cc peer ngu nhin c th khng c ngun lc cung cp cht lng truyn

thng mong mun.

Hnh 2.16: S h thng ca mt nt Anysee

2.3.6.2. H thng Chainsaw

Chainsaw l mt h thng li da trn phng php y m khng da trn

mt cu trc mng cng nhc. Trong s ny, cc peer c thng bo v cc gi d

liu mi ca cc peer hng xm. Bng cch ny, d liu trng lp c th c loi b

v mt peer c th m bo nhn c tt c cc gi d liu. i vi mi peer hng xm,

mt peer to ra mt danh sch cc gi mong mun, tc l mt danh sch cc gi tin d

liu peer mun m trong peer hng xm c sn. Sau n s c p dng mt s

chin lc chn mt hoc nhiu gi d liu t danh sch v yu cu ti peer hng

xm thng qua mt bn tin yu cu. Mt peer theo di cc gi d liu yu cu t

mi peer hng xm v m bo rng n khng yu cu gi tin t nhiu peer. N

cng hn ch s lng yu cu ti mt peer no , m bo rng yu cu c tri

rng trn tt c cc peer hng xm. Cc nt theo di cc yu cu t cc peer hng xm

ca h v gi cc gi tin tng ng trong iu kin bng thng cho php. H thng

khng cung cp mt c ch no thc thi vic ng gp ngun ti nguyn cng bng

ca cc nt nhng Chainsaw cho php peer t xc nh r bng thng ti ln ti a

43

ca. Chainsaw c kh nng em li hiu nng mng cao v cc chi ph CPU do mi

thng bo gi.

2.3.6.3. H thng PPLive

PPLive l mt ng dng P2P TV thng mi, n s dng mng li nh mt

giao din mng ph da trn phng thc ko, phn phi lung ni dung trc tuyn v

ni dung ghi sn. S khc bit ln PPLive vi BitTorrent l trong gi PPLive phi

p ng cc thi hn pht li. Vo thng Ging nm 2008, ng dng PPLive cung cp

gn 500 knh vi 1.000.000 ngi s dng trung bnh. S lng cc knh truyn

hnh trong thng 12 nm 2008 c bo co l tng ng nm 1775. Nn tng

PPLive bao gm nhiu lp ph. Mt lp ph tng ng vi mt knh PPLive. Mi peer

trong mt lp ph c xc nh bng cp (a ch, s port). Hnh 2.17 cho thy cc

hot ng c bn ca mt peer PPLive. Lc u, peer PPlive ti danh sch cc knh

t danh sch cc knh ca my ch thng qua giao thc http. Sau cc knh c

la chn, peer thu thp mt tp hp nh ca cc peer hng xm tham gia trong cng

mt lp ph bng cch truy vn ti cc my ch thnh vin thng qua giao thc UDP.

Mt peer giao tip vi cc peer hng xm trong danh sch c c danh sch b

sung, tp hp vi danh sch bn b hin ti ca mnh thng qua UDP. Theo cch ny,

peer gi mt danh sch ca cc peer khc xem cng mt knh. c thi gian

phn ng vi cc nt ri i v gii quyt nhng kh khn, gi tin cho php c hai

b m: mt c qun l bi PPLive v th hai ca my thu ca ngi dng. Mt nhc

im ca kin trc P2PLive l khi ng chm c tr ln. Cc hot ng ca

PPStream l rt ging nh PPLive.

Hnh 2.17: Kin trc c bn PPLive

2.3.6.4. H thng DONet

DONet (hoc CoolStreaing) l h thng P2P streaming li da trn phng

thc ko c a ra bi trng i hc Hong Kong v Vancouver. Trong DONet tt

44

c cc nt nh k trao i d liu thng tin sn c vi mt nhm i tc, v ly d

liu c sn t mt hoc nhiu i tc, hoc cung cp d liu c sn cho cc i tc.

Mt nt bao gm ba phn chnh (hnh 2.18). Mt ngi qun l thnh vin (Membership

manager) gip qun l cc nt, mt ngi qun l quan h i tc (Partnership

manager) trong thit lp v duy tr quan h i tc vi cc nt khc, mt lch trnh

(A Scheduler) ln lch trnh vic truyn ti d liu video v xc nh khi cn c

ly t i tc v ti khi ca cc i tc. CoolStreaming yu cu nt mi tham gia

lin lc vi my ch gc c c mt kt ni ban u. Mi nt cng duy tr mt

nhm con ca cc peer khc trong nhm. CoolStreaming s dng kh nng m rng

peer, qua giao thc (SGAM) pht tn thng bo ti cc peer. Mt nt

CoolSreaming c th ra i. S ra i ca peer c th d dng pht hin sau mt thi

gian n khng lm g v mt nt b nh hng c th nhanh chng phn ng thng

qua ti lp lch trnh bng cch s dng thng tin bn b m ca cc i tc cn

li. CoolStreaming cng cho php mi nt nh k thit lp quan h i tc mi vi

cc nt la chn ngu nhin t danh sch nhm ca mnh. Hot ng ny s gip mi

nt duy tr mt s n nh vi cc i tc trong iu kin c nt tch ra v khm ph

cc i tc c cht lng tt hn, v d nh nhng ngi lin tc c bng thng ti

ln cao hn v nhiu phn on c sn. CoolStreaming h tr cc loi phn mm khc

nhau ca cc my thu a phng tin, chng hn nh Windows Media Player, Real

Player... S dng cc thut ton lp lch v mt h thng m (buffering) mnh m,

CoolStreaming t c vic pht video mt m v kh nng m rng h thng rt

tt cng nh hiu sut. Tng th tc streaming v pht li lin tc ca h thng

CoolStreaming l t l thun vi s lng cc peer trc tuyn ti bt k thi im

no. Mt trong nhng nhc im ca DONet l thng bo cho cc peer v sau

c th yu cu chia khi d liu trc trao i dn n kt qu tr ln. Tng t nh

vy, do cc thut ton la chn ngu nhin, cht lng dch v c th khng c

m bo. Hn na, DONet gi nh rng tt c cc peer c th hp tc trong s lp li

ca cc lung, n c th c cc peer ch k trong cc h thng khng mun chia

s bng thng ti ln ca peer .

45

Hnh 2.18: Mt s h thng chung cho mt nt DONet

2.3.6.5. SopCast

SopCast l mt ng dng min ph BitTorrent- like P2PTV, sinh ra t mt d

n sinh vin ti i hc Fundan Trung Quc. SOP l vit tt cho Streaming trn P2P.

Trong Sopcast cc knh c th c m ha trong Windows Media Video (WMV),

Video tp tin cho Realplayer (RMVB), Real Media (RM), Advanced Streaming

Format (ASF), v MPEG Audio Stream Layer III (MP3). Mt khch hng c nhiu s

la chn knh truyn hnh, mi knh thnh lp lp ph ring ca mnh. Mi lung

knh bao gm ngun cp d liu m thanh v video trc tuyn hoc vng lp hin th

theo mt lch trnh nh sn. Ngi xem iu chnh vo mt knh la chn ca h v

Sopcast bt u hot ng ca mnh ly lung. N cng cho php ngi s dng

pht sng knh ring ca mnh. Sopcast cung cp t l tn tht khung thp. Tuy nhin,

Sopcast b tr peer, tc l cc peer xem cng mt knh c th khng c ng b.

Hn na, thi gian chuyn knh l rt cao.

Hnh 2.19: GridMedia Kin trc Da trn MSOMP

46

2.3.6.6. GridMedia

GridMedia thng qua mt c ch truyn ti y - ko ly d liu t cc nt

i tc. Ch ko trong lp ph khng c cu trc c th lm vic tt vi tc

cao trong mi trng P2P trong khi ch y c th lm gim tr tch ly

pha ngi dng. Mt my ch theo di c trin khai h tr vic xy dng cc

lp ph. Khi khi ng, mt nt tham gia lin lc u tin ti RP c c mt

danh sch cc phn ca cc nt c trong lp ph, c gi l qu trnh ng nhp.

Sau cc nt tham gia ngu nhin s la chn mt s nt trong danh sch ny nh

cc peer hng xm. GridMedia ch yu bao gm nhiu ngi gi da trn giao

thc multicast lp ph (MSOMP) v nhiu ngi gi d phng da trn thut ton pht

li (MSRRA). MSOMP bt ngun t my ch streaming l mt nt gc.

Cc MSOMP trin khai li da trn cu trc hai lp v nhm tt c cc peer vo cm

vi nhiu con ng i khc nhau t gc mi peer. Sau , vi mt hoc mt s

cc nh ch huy trong mi nhm, tt c cc nt ch huy xy dng xng sng ca lp

ph. MSOMP s dng cc dch v multicast IP hin c sn trong mng LAN. IP

Multicast min (IMD) l mt mng li a phng ca bt k kch thc mng no

h tr IP multicast. IMD c th l mt my ch duy nht, mt mng LAN. Trong

mi IMD, mt peer tiu c trch nhim ph bin cc ni dung streaming vi cc

peer ng nghip khc trong cng IMD. Ngay sau khi nt phn u ri khi, mt nt

tiu mi s c bu thay th vai tr. MSOMP kt ni vi cc IMD bng

ng hm unicast. MSOMP da trn GridMedia kin trc c th hin hnh 2.19.

gii quyt vn mt gi tin, MSRRA c xut ti cc peer gi v

gi tin b mt bng cch s dng peer nhn d on mu mt. Trong MSRRA,

mi peer nhn c gi tin truyn ti ng thi t nhiu ngi gi. Ngay sau khi c

tc nghn xy ra trn mt lin kt, peer nhn s c thng bo v tnh trng tc nghn

ny v sau n s thng bo cho ngi gi khc, nhng ngi s tip tc v cc gi

tin b mt. Cc thut ton MSRRA hiu qu lm gim tc ng ca cc nt tht bi,

tc nghn mng v cc hot ng chuyn i lin kt.

2.3.6.7. PRIME

PRIME l mt kh nng m rng k thut P2P streaming ca ni dung trc tuyn

da trn h thng li y - ko li. Mc tiu thit k quan trng nht ca PRIME l

gim tc nghn bng thng v trnh b hin tng tht nt c chai. PRIME s dng

47

trn ngp ni dung kt hp vic y ni dung bi nt cha vi ko ni dung yu cu

ca nt con. Mi peer ng thi nhn c ni dung t tt c cc nt cha ca mnh v

cung cp ni dng cho tt c cc nt con ca n. Cho cc gi d liu c sn ti nt

cha, mt chng trnh lp lch trnh gi tin ti mi peer nh k xc nh mt danh

sch lnh ca cc gi d liu cn c yu cu t cha.Nt cha ch n gin l cung

cp cc gi tin yu cu ca mi con trong trnh t quy nh v theo t gi c xc

nh bi cc c ch kim sot tc nghn. Mi phn on ca cc ni dung c gi

n cc peer c nhn tham gia trong hai giai on: giai on khuch tn v trn ngp

giai on. Trong giai on khuch tn, mi peer nhn c bt k ca mt phn on

mi t nt cha ca n mc cao hn. V vy, cc mnh ca mt phn on mi c

to ra ang dn dn ko theo cc ng nghip cc cp khc nhau. Trong giai

on lt, mi peer nhn c thp hn. Cc cha c gi l trn ngp cc nt cha.

Mi phn ca bt k phn on mi c lan ta thng qua mt khuch tn c bit

cy con trong giai on khuch tn ca phn khc . Sau , cc phn c sn c

trao i gia cc ng nghip trong cy tng, khuch tn khc nhau thng qua cc

li trn ngp trong giai on trn ngp ca phn khc ny. Cc ng dng ca hai giai

on khc nhau cho phn phi ni dung dn n hiu qu s dng ngun lc sn c

cha kh nng m rng v cng gim thiu ni dung nt c chai. Nhng bt li

ca PRIME l nu ni dung nt c chai xy ra, cc nt phi ch i lu tm ra n

v d liu cn thit ca h sau khi mt vi giai on lt, khi d liu c sn trong khu

vc ca mnh. Do , khng c s m bo cho mt mc hp l ca lung trc

tuyn cht lng. Hn na, cc thut ton khng xem xt hnh vi ca cc h thng

P2P trong s hin din ca mt thng ng.

Hnh 2.20: Hai lp li/ cy che ph trong mng lai

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HyPo l mt lp ph lai P2P cho truyn thng a phng tin trc truyn. Ti

u ha cc lp ph bng cch t chc peer vi cc phm vi c bng thng tng t

trong bt k khu vc a l no trong mt lp ph li v to thnh mt lp ph cy

bng cch chn cc peer c n nh. Hnh 2.20 minh ha mt li / cy che ph hai

lp trong HyPO. Ty thuc vo c ch ti u ha cy, trong cc peer c mt

bng thng ln s c gn vi nt ngun trong cc lp ph cy, v phn b u

trong cc cy vi cc nhnh vi mt chiu su tng t. Do , vic ti u ha cy

lm gim su trung bnh ca cy, do nng cao kh nng m rng. Li trong

HyPO khng phi l mt kt ni b tr t cc peer trong li, thnh vin lun lun cung

cp d liu cho li cho n khi n tr thnh mt thnh vin cy. Tuy nhin, tt c cc

th tc ca HyPO rt t da trn mt my ch theo di, mt khong thi gian lin tc

c th xy ra nu my ch b li. Hn na, HyPO khng cp n cch lm sao d

liu c phn b trong lp ph li ca n.

2.3.6.8. mTreebone

mTreebone l mt thit k cy li phi hp thc y c li v cu trc cy.

tng chnh ca mTreebone l xc nh mt nhm cc nt n nh xy dng mt

cy da trn xng sng, cc nt ny c gi l treebone vi hu ht cc d liu b

y trn xng sng ny. Cc nt bn vng cng vi nhng nt khc, c t chc

thnh mt lp ph li b tr, to iu kin cho treebone ph hp ng lc ca

mt nt v khai thc hon ton bng thng gia cc nt lp ph. Cc nt khng n

nh khc c gn vo xng sng nh vng bn ngoi. Hnh 2.21 cho thy mt m

hnh mTreebone. Trong s ny, kt ni li c gi ch khi c mt nt ring bit

b nh hng bi s ra i ca nt cha hay nt cha tht bi. Vic duy tr treebone v ti

u ha ch xy ra ti cc nt treebone v khng c thm chi ph cho cc peer vng

ngoi xng sng. Thng thng, cht lng truyn l tt nht cho cc nt treebone

do s n nh tt ca cc con ng cung cp d liu ca n t ngun. Thch thc

chnh l chng ta cn phi xc nh tp hp cc nt lp ph n nh v v tr ca nt

ti cc a im thch hp trong cy. Mt yu cu nh vy c th gy xung t vi

bng thng v chm tr trong xy dng cy. Mt pht sinh khc khi xem xt v s n

nh ca mTreebone l ph thuc vo cc nt treebone, l cc nt s li trong

treebone trong khong thi gian bao lu. Mt khc, CliqueStream l mt lp ph lai

tng t nh mTreebone khai thc cc thuc tnh ca mt nhm lp ph P2P (hnh

49

2.22). ClipueStream tm ra mt hay nhiu nt bn vng c bng thng cao nht trong

mi nhm v giao cho cc nt vai tr chuyn tip ni dung c bit. duy tr

hiu qu truyn ti, mt cy phn phi ni dung c xy dng trn cc nt bn vng

s dng cu trc trong nn nh tuyn c bn v ni dung s c y qua chng. Cc

nt t n nh trong mt nhm sau tham gia vo vic ph bin ni dung v ko ni

dung to ra mt li xung quanh cc nt bn vng.

Hnh 2.21: Khung mTreebone (a) mt lp ph lai (b) x l trng thi nt

Hnh 2.22: Cu trc trc tuyn trong CiqueStream

Lai ko y P2P treaming lp ph ni ln nh mt gii php thay th cho

cc phng php truyn thng nh l cy v li. K t khi thit k h thng lai lm

n gin ha vic xy dng lp ph v cc quy trnh bo dng v ng thi vn gi

c hiu qu ca mng v t c kim sot tt cc nt qu ti.

2.4. Video theo yu cu trn P2P

VoD (Video on Demand) hin c t ra mt s vn nh tnh kh thi ca giao thc

multicast, my ch hng v chi ph trin khai bo ng cc thit b nh tuyn lp

ph cao. Tuy nhin, video streaming da trn P2P cung cp mt kin trc thay th cho

cc dch v video theo yu cu. Trong mt h thng VoD P2P, tt c cc peer l host

50

c kt ni Internet, trong peer lu tr v cc lung video do khch hng ngh.

Chi ph ca cc peer truy cp Internet s c chu bi khch hng ch khng phi

bi cc nh cung cp dch v VoD. Bi v c mt ngun cung cp phong ph ca cc

peer cung cp ngun ti nguyn cha c s dng nh bng thng v lu tr, da

trn kin trc P2P nn c chi ph t hn so vi cc gii php truyn thng client- server

v gii php CDN.

p dng k thut lung trc tuyn P2P vo lung VoD khng phi l mt

cng vic d v nhiu l do. Nh h thng lung trc truyn P2P, h thng P2P - VoD

cng cung cp ni dung bng lung. Tuy nhin, cc peer c th xem cc gi khc nhau

ca mt on video cng mt lc, do lm yu i kh nng gip ln nhau. H

thng VoD c kh nng cho php ngi dng bt u xem mt on video sau khi ch

i mt thi gian, trong khi vn tip tc ti video v. Mc d, tr end-to-end ca

lung trc truyn ngn nhng vn gy kh khn cho ngi s dng. Do , mt cy

ngn bt ngun ti my ch video v ko di trn peer khng phi l iu mong mun

trong truyn ti VoD. Ngi s dng s c th xem video ti mt thi gian ty ,

khng ging nh trong truyn hnh trc tip h cn phi ng b ha thi gian xem.

Ngi s dng cng s c th thc hin cc hot ng kim sot nh tua li, li v pha

trc..vv trn video. V d, mt peer c th chn xem mt lung VoD khi QoS ca

h thng gim, nhng cc peer khc c th khng lm iu tng t cho mt lung v

n khng c ty chn xem li. Nu QoS ca cc lung video gim, s c nhiu cc

peer ri h. iu ny cho thy tm quan trng ca mt giao thc phc hi trong mt

h thng lung VoD. Cc giao thc kt ni li cc peer b b ri, do khng c

mt mt ni dung v khng tr lu.

Mt yu cu quan trng ca mt dch v VoD l kh nng m rng. Mt h thng

VoD cho php mt peer mi tham gia h thng nhanh. Thi gian tham gia ngn, yu

cu tham gia ca cc peer n h thng cc thi im khc nhau. D kin h thng

phi truyn ti cc video cho tt c cc peer m khng lm cho my ch tr thnh mt

nt c chai. H thng VOD P2P thng yu cu ngi dng ng gp dung lng lu

tr v cc h thng ny cn kch thc b m rt ln p ng cc yu cu a dng

t cc peer khc nhau. Khng gian lu tr ny thng l 1 GB trong PPLive. C hiu

lc sau khi ngi dng ci t PPLive v chy h thng ln u tin, ngi dng c

th nhn thy mt loi file cha bit 1 GB tn ti trong b nh my.

51

Nh h thng video streaming, h thng VoD P2P thng c phn loi da

trn cy v h thng da trn li.

H thng VoD da trn dng cy

Nhiu ngi dng s dng mng che ph dng cy c ng b v nhn

ni dung theo th t cc my ch s gi i. Vn chnh trong h thng VoD da

trn cy l vic thit k cy v cc th tc cho ngi dng tham gia vo h thng.

P2Cast v P2VoD l nhng v d ca h thng VoD da trn cy.

P2Cast l mt m hnh v li (patching) ban u cho h thng VoD. chng

trnh xut h tr s dng dch v VoD c ngun gc IP multicast. P2Cast gii

quyt hai vn k thut quan trng nh xy dng mt lp ph thch hp cho cc tuyn

v cung cp mt lung tin lin tc pht li khi mt khch hng ra khi h thng.

S lng khch hng n mt ngng nht nh to thnh mt phin. i vi mi phin

my ch cng vi cc khch hng P2Cast to thnh mt cy a lung lp ng dng

trn mng unicast. Cc khch hng trong P2Cast c th chuyn tip cc lung video

cho cc khch hng. Mi khch hng tch cc gp phn bng thng v khng gian lu

tr cho h thng ng thi tn dng cc ngun ti nguyn ti cc khch hng khc.

Ton b video c truyn trc tip trn cy a lung lp ng dng, n c th

c chia s gia cc khch hng. i vi khch hng tham gia mun hn cc khch

hng u tin trong phin giao dch v do b l mt on ban u ca video, on

thiu ny c th c ly t my ch hoc my khch hng khc c lu tr on ban

u. P2Cast c th phc v nhiu khch hng so vi dch v unicast client-server

truyn thng. Cc chng trnh phc hi trong P2Cast cho php cc ng nghip nhn

d liu t my ch trc tip khi cc nt cha tch ra. Tuy nhin, iu ny lm tng khi

lng cng vic ca my ch.

P2VoD l mt m hnh video theo yu cu dng cy m n c gng gii quyt

cc vn ca vic kt ni nhanh, cung cp nhanh chng v phc hi tht bi cc b,

x l hiu qu cc yu cu khng ng b ca khch hng v cung cp chi ph qun l

nh so vi P2Cast. Mi khch hng trong P2VoD c mt b m FIFO kch thc

thay i lu tr cc ni dung gn y nht ca lung video nhn c. Tn ti

nhng khch hng trong P2VoD c th chuyn tip cc lung video ti mt khch

hng mi min l h bng thng u ra v vn gi khi u tin ca tp tin video

trong b m. M hnh b nh m cho php mt nhm cc khch hng, n vi h

52

thng ti thi im khc nhau, lu tr cc ni dung video ging nhau trong tin t

ca cc b m ca n. Nhm nh vy hnh thnh mt phin. Khi mt thnh vin ca

mt phin ri h thng, bt k thnh vin cn li ca phin c th cung cp video m

khng c bin ng ti cc peer con b b ri ca cc thnh vin ri i vi iu kin

bng thng u ra quy nh l . Trong P2VoD, mt kt ni lung c gi nh l

tc bit khng i, bng vi tc pht li ca video. Qu trnh phc hi trong

VoD l phc tp hn. Ngoi ra, P2VoD khng xem xt cc bng thng khng ng

nht ca cc peer.

B nh m v chuyn tip l mt cch tip cn da trn cy, l ni m mt

khch hng VoD thng da trn ni dung c lu trong b m ca cc peer cha

ca n. Trong m hnh ny, cc b nh tuyn khng thc hin chc nng multicast.

Do , cc host cui chu trch nhim cho vic lu tr v phn b ca lung tin a

phng tin. Cc host cui c th l cc my khch hng hoc cc proxy ca n v

cc h thng ny duy tr cc i tng truyn thng trong b nh cc b tm thi

ca h. Nu mt khch hng yu cu cc i tng truyn thng v sau, my ch gc

c th chuyn tip cc yu cu ti cc host cui.

Ostream tn dng kh nng m ca host cui bng cch s dng b nh cache

v phng php lu tr v chuyn tip. M hnh s dng mt thut ton cy m rng

cho cc peer xy dng mt lp ph cho truyn thng trc tuyn. ostream lm gim

s thiu s thiu hiu qu ca topo nh tr lin kt v ko di c gii thiu bng

cch s dng multicast lp ng dng.

Hnh 2.23: Cu trc DirectStream

Mt khung lm vic c gi l DirectStream cho php khch hng tn dng

nhng li ch ca khong thi gian b nh m v dch v video theo yu cu vi s

h tr hot ng VCR . DirectStream bao gm mt my ch ch dn, my ch ni

53

dung v khch hng. My ch ch dn hot ng nh mt im trung tm hnh chnh.

N duy tr mt c s d liu theo di ca tt c cc my ch v khch hng tham gia

trong DirectStream v gip khch hng mi xc nh cc dch v cn thit. Cc my

ch ni dung cung cp cc chc nng tng t nh trong m hnh client server

truyn thng lu tr ni dung trong kho lu tr ca n v phc v yu cu ca khch

hng, min l bng thng c sn. Do khch hng trong DirectStream c chc

nng nh nt P2P. Mt peer lu tr mt ca s chuyn ng ca mt ni dung c

nhn c mi nht v phc v nhng peer n mun bng cch lin tc chuyn tip

cc ni dung c lu tr. Mt tp hp cc khch hng hot ng trong mt lp ph

P2P streaming c thnh lp c gi l mt nhm. Cc nhm trong DirectStream

pht trin theo thi gian v mi khch hng trong mt nhm chia s lung ging nhau.

Qu trnh tm kim dch v cho mt yu cu mi bao gm bn bc nh c ch ra

trong hnh 2.23. u tin cc khch hng mi s gi mt yu cu n my ch ch

dn yu cu video bt u t v tr. Cc my ch ch dn sau nhn vo c s d

liu ca mnh v tr v mt danh sch cc nt ng c vin, bao gm c my ch d

liu v khch hng c ni dung phc v yu cu ny. Khch hng mi xc nh t

cc nt ly cc lung s dng cho thut ton la chn cha QoS. S dng thut ton

ny, mt khch hng la chn mt nt cha c bng thng. Cc khch hng mi

lin h vi nt ng vin c chn v yu cu chuyn tip lung. Sau khi kt ni

c thit lp, khch hng mi gi tn hiu tr li my ch ch dn v ng k chnh

n vo c s d liu. DirectStream gim ng k khi lng cng vic t ra trn my

ch. Mt li th l quy m ca n cng nh s ph bin ca video tng ngay c khi

khch hng tham gia khng hp tc chia s. DirectStream c hai nhc im, qun

l tp trung a ra mt im n ca s tht bi. Khi nhiu nhm cao cp khc nhau tht

bi, mt peer c th nhanh thiu b m ca n.

Trong h thng VoD da trn cy, cc peer trn lp trn lun lun ng mt

vai tr quan trng trong mng che ph ton b. S ra i ca n s dn n s bin

ng lp mng thp hn. Hn na, mi peer ch c mt nh cung cp d liu, iu

s gy ra khng hiu qu s dng bng thng c sn trong mt mng khng ng

nht v mng cmi trng nng ng. ng thi, trong b nh cache v da trn h

thng chuyn tip, nu cha nhy n mt im no trong on video, n bt u

nhn d liu v khng quan tm ti nt con v nhng nt cn phi tm kim cha mi.

54

H thng VoD da trn li

Trong h thng VoD da trn li, khng c cu trc lin kt c th c to

ra. Cc peer trong mng kt ni vi mt s peer cha nhn cc gi tin video. H thng

VoD da trn li c giao thc trn lp ph, d dng thit k v linh hot hn

cho tc cc khi ni dung v do n c ph bin hn. Hin P2P da trn h

thng li c chng minh l rt tt cho phn phi ni dung video quy m ln

vi t ti nguyn my ch. Tuy nhin, trong h thng VoD kh khn trong thc t l

ngi dng mun nhn c cc khi tun t xem video trong khi ti v. Ngoi ra,

trong dch v VoD ngi s dng c th quan tm ti cc phn khc nhau ca on

video v c th cnh tranh ti nguyn ca ngi dng khc. Qua , thch thc chnh

ca h thng l m bo rng ngi dng c th bt u xem video ti bt k thi

im no, vi thi gian ch nh v tc pht li cao.

BitTorrent (BT) l mt trong nhng k thut da trn li phn phi

khi lng ln cc ni dung trn Internet. N l mt giao thc chia s tp tin m rng

m cn kt hp vi c ch truyn d liu trn lt. C mt s hn ch ca k thut BT

ban u trong vic cung cp video streaming. Trong BT, cc tp tin c phn on

trn khong trng (space). Mc d c ch la chn on mc nh ca BitTorrent l

rt hiu qu trong vic gim thiu xc sut cho cc on ming him v cung cp

cc on him cho cc peer, n khng tht vng trong thi gian lu lng tc nghn.

L do l vi thi gian d liu nhy cm th mi on s phi nhn trong mt khong

thi gian nht nh. Yu t ny khng c xem xt trong c ch la chn on ban

u ca BT v do vy n khng th cung cp dch v phn phi nhy cm thi gian, bi

v cc on c yu cu da trn s him c v khng theo thi hn ca n. Do ,

c ch la chn on hin ti cn sa i h tr dch v thi gian nhy cm nh VoD.

BASS v BiToS l cc v d ca BitTorrent da trn h thng P2P VoD dng li.

Hnh 2.24: BASS: (a) Tng quan h thng, (b)Mu khch hng

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BitTorrent h tr h thng streaming (BASS) m rng h thng BitTorrent hin

ti cung cp mt dch v gn Video-on-Demand. Bi v BASS s dng s h tr

ca BT cho cc lung, n s dng cc dch v ca mt my ch bn ngoi m c th

lu c ton b video ca cc nh sn xu v m bo rng ngi s dng c th

pht li video m khng gp s thay i cht lng. S thay i duy nht ca

BitTorrent cho thy l n khng nn ti v bt c d liu no trc thi im playback.

N c php s dng on him nht u tin v chnh sch n ming tr ming (tit

for - tat). Trong chnh on him u tin, khch hng yu cu mt phn da trn s

lng bn sao n thy c sn v chn mt ci l ph bin nht. T my ch truyn

thng, BASS ti cc phn trong trt t, b qua cc phn m c ti bi BitTorrent,

hoc ang trong qu trnh c ti v v d kin s hon thnh trc thi hn pht.

Tng quan v h thng ca BASS c a ra trong hnh 2.24. D BASS lm gim

ti ti cc my ch, thit k ca h thng vn l my ch theo nh hng v do cc

yu cu bng thng ti cc my ch tng tuyn tnh vi s lng ngi dng.

Hnh 2.25: Phng php tip cn BiToS

H thng BiToS cng da trn BitTorrent. tng chnh l phn chia cc

khi ang thiu thnh hai tp hp: tp hp u tin cao v tp hp cn li v yu

cu vi cc khi xc sut cao hn t tp hp u tin cao hn (hnh 2.25). Tp hp u

tin cao hn, c cha tt c cc phn kh gn c ti to. V vy, peer mong mun

ti v nhng phn sm hn. Tri ngc vi tp hp cn li, trong c cha cc

phn m s khng cn n trong tng lai gn. Sau khi ngi dng bt u, b m

ca ngi dng yu cu nhng phn cn thit t cc phn m cc peer nhn.

Trong BiToS, trng tm chnh c a ra cho vic lp lch cn thn ca cc khi

video. Cc phn m b l thi hn pht li ca n c gim xung n gin. Do vy,

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iu ny c th dn n suy gim cht lng pht li video. Ngoi ra do tnh cht bt

i xng ca cc kt ni Internet v khng ng nht ca cc peer, h thng khng

th m bo rng cc on video c yu cu lun lun c sn pht li ng thi

gian yu cu.

2.5. Kt lun chng 2

Chng 2 trnh by khi qut v khi nim mng ngang hng P2P, u nhc

im ca mng ngang hng. Bn cnh cn cp n vic phn loi mng ngang

hng. Qua thy mng ngang hng rt n nh v d m rng; tn dng ti a ti

nguyn ng gp ca cc nt tham gia mng; mng ngang hng d ci t; chi ph

thit b thp, trin khai mt h thng mng kh d dng.

Ni dung chnh chng 2 trnh by cc h thng P2P streaming, c th phn

chia thnh h thng P2P live Streaming v P2P VoD. Ty theo cch tip cn, h thng

P2P live streaming c th c chia thnh dng cy hoc dng li. H thng P2P

VoD cng c th chia thnh dng cy v dng li.

Tuy nhin, h thng P2P streaming cng bc l nhiu nhc im cn phi xem

xt, trong quan trng nht l tr lung v