Bi kim tra gia k Mn: Truyn thng a phng tinH v tn: L Vit Anh M SV: 0800204 Lp: Mng my tnh v truyn thng K49

I. Cc phng php m haI.3. M ha JPEG v JPEG20001. M ha JPEG1.1. Quy trnh m ha JPEG M ha D 1 file JPEG c th c m ha bng nhiu cch, cch thng dng nht l n c m ha JFIF. Qu trnh m ha bao gm nhng bc sau : M t nhng mu ca nh c chuyn t RGB sang YcbCr, bao gm 1 thnh phn chi (Y), biu din sng v 2 thnh phn sc biu din mu sc. Bc ny i khi c th b lc b. phn gii ca d liu mu sc c gim i thng l thng qua tha s 2. Vic thc hin ny t gy nh hng n cm nhn ca mt hn l thng tin v chi. Bc nh c chia thnh nhng khi 8x8 im nh, v cho mi khi, mi d liu Y, Cr, Cy cn bin i bin i cosin ring (DCT). Mt DCT tng t nh bin i Fourier chuyn khng gian tn s Bin ca thnh phn tn s c lng t ha. Mt ngi nhy cm vi cng sng tn s thp nhiu hn mc thay i sng tn s cao. V vy, thnh phn tn s cao c lu tr t chnh xc hn tn s thp. Cht lng thit lp ca b m ha tc ng ti phm vi phn gii ca mi thnh phn c gim xung. Ti mc thit lp cht lng thp, thnh phn tn s cao b loi b hon ton. D liu u ra ca nhng khi 8x8 sau c nn vi thut ton mt mt thng tin, cc bin th m ha Huffman.

Qu trnh gii nn o ngc cc qu trnh trn. Hu ht phn mm nh hin nay dng nh dng JFIF khi to file JPEG. y l cch m ha vi u vo l nh 24 bit/pixel v l phng thc nn nh mt mt thng tin. Chuyn i khng gian mu u tin 1 nh cn c chuyn t dng RBG sang 1 khng gian nh khc l YCbCr. N c 3 thnh phn l Y, Cb v Cr : vi Y l thnh phn biu din sng v Cb,Cr l thnh phn mu sc. Khng gian mu YCbCr cho php vic nn tt hn vi cng 1 cht lng nh. Vic chuyn i ny dng chun JFIF v c thc hin tng thch hon ton vi file JPEG. Tuy nhin, nhng nh JPEG cht lng cao b qua bc ny v vn gi khng gian mu RGB v mi mu c nn v lng t ha ring bit Downsampling Mt ngi c th nhn thnh phn Y chi tit hn thnh phn Cb v Cr. Nh vic nn hiu qu hn. Bng cch gim cc thnh phn Cb v Cr (downsampling hay subsampling). T l gim mu ca nh JPEG thng l 4:2:0 (gim 2 ln theo chiu ngang v dc). Vic gim mu gim 35%-50% khng gian lu tr nh. Vic x l Y, Cb v Cr c thc hin ring bit v tng t nhau. Chia khi. Sau khi gim mu, mi knh phi c chia thnh nhng khi 8x8 im nh.Nu d liu cho 1 knh khng phi l cc s nguyn th b m ha phi chuyn i nhng khi c d liu khng ph hp vi nhng d liu tha. X l nhng im nh bin bng nhng im nh en Vic thay th nhng im nh bin ph bin nhng khng phi l k thut ti u trnh vin nhn thy nhng vn to ra nhng artifact vi thit b o mu ca ca nhng ph y. Mt chin lc tt hn l ph nhng im nh s dng nhng mu duy tr h s DCT ca nhng im nh nhn thy, t nht l cho nhng thnh phn tn s thp. (v d : ph vi nhng mu trung bnh ca phn nhn thy s duy

tr h s DC u tin, nhng vic khp tt nht cho 2 h s AC tip theo s cho ra nhng kt qu tt hn vi t nhn thy hn nhng cnh (theo ng vin). nh con 8x8 trn sau khi cn bng histogram (vd : 154 trng, 55 en). Ch rng iu ny c hon thnhch v mc ch th gic v vic cn bng khng c thc hin d liu th. Tip , mi thnh phn (Y,Cb,Cr) ca mi khi 8x8 c chuyn i thnh biu din trn min tn s, s dng bin i DCT 2 chiu, bnh thng bin i. M ha Entropy M ha entropi l 1 dng c bit ca nn d liu khng mt mt thng tin. N l dng nn da trn m ha di (RLE) to nhm nhng tn s tng t vi nhau, thm di nhng s 0 v s dng m ha Huffman cho phn khc 0. Chun JPEG thng s dng m ha Huffman thay v m ha s hc bi v thut ton m ha s hc chm hn rt nhiu thut ton m ha Huffman k c m ha v gii m d cho php nn tt hn khong 5%. Nu khi th I l Bi v v tr mi khi l (i,j) vi p,q=1..7 th mi h s DCT biu din bi Bi(p,q). V vy, vi lc trn, th t m ha im nh l Bi(0,0), Bi(0,1), Bi(1,0), Bi(2,0), Bi(1,1), Bi(0,2), Bi(0,3), Bi(1,2) v tip tc nh th Qu trnh nn s c thc hin c gi l m ha tun t (sequential encoding). ng c s ca JPEG cng h tr m ha tng dn. Khi tin hnh m ha tun t, tng im nh theo th t nh trn c a vo v c th tip tc cho n khi cn li ton b cc im nh cn li c gi tr 0, kt thc bng k t EOB. Vic m ha nh trn s chia cc h s thnh 2 phn - Nhng h s c ngha c du. - Nhng h s 0 i theo sau cng. Nh v d cc h s trn, hu ht nhng h s lng t ha l nhng s nh. Nhng trng hp ny c ngha s c m t bi nhng t m ngn hn. Chun JPEG cung cp nhng bng Huffman khc nhau; b m ha c th la chn to ra bng Huffman ti u cho nhng tn s c th cho nhng nh c m ha.

1.2. Gii thiu chun nn JPEG 2000 JPEG 2000 l 1 chun nn nh da trn dng sng. N c to ra bi y ban Joint Photographic Experts Group trong nm 2000 vi mc ch thay th chun nn JPEG da trn bin i DCT (to ra vo nm 1992). Chun nn ny c phn m rng tn file l .jp2 cho nhng file hp chun ISO/IEC 15444-1 v .jpx cho phn m rng th 2, thuc ISO/IEC 15444-2 khi kiu MIME l nh/jp2. JPEG 2000 i hi thi gian gii nn ln hn nhiu so vi JPEG v cho php vic ti xung c ci tin cng phu. JPEG 2000 tng cng hiu ng m vi t l nn cao hn vic to blocking v ringing ca JPEG, phc tp vic nn trc tip ca nhng t l nn ring ca chng. Nhng nh nn bng chun JPEG 2000 c cht lng tt hn (vi con ngi) t l bit thp. JPEG 2000 c ph bin l chun ISO/IEC 15444-1:2000. Hin nay, JPEG 2000 khng c h tr rng ri bi cc trnh duyt v v th khng dng trn WWW. D cho nhng nh cung cp tch hp mi kiu siu d liu bn ngoi, d liu Exif vn khng c nhng.

I.4. Gii thiu v m ha video2.M ha VideoQu trnh m ha video theo chun MPEG-1 2.1. S lc qu trnh nn Qu trnh nn frame I : p dng php bin i DCT ln cc khi 8x8 biu din chi v sc ca frame. u ra ca DCT (64 h s) c a qua b lng t ha Q. Qu trnh lng t ha c iu khin bi hai tham s : bng lng t ha v cc h s lng t. Dng bit sau khi lng t ha c m ha entropy v a vo b m M ha nn cc frame P,B: Frame tham chiu cho qu trnh nn frame P l frame I hoc frame P trc . 2 frame tham chiu cho qu trnh nn frame B l frame I trc v frame P sau . Tnh ton vec t chuyn ng bng cch tm kim mt khi ph hp vi khi ca frame hin ti trong ca s frame tham chiu. Vec t chuyn ng s c x l trn cc khi DCT, Q ,m ha entropy v a vo b m

Tnh sai s d on (frame d on b chuyn ng) bng cch tr tng im nh ca Macroblock trn frame ang x l cho bn sao ca n trn frame tham chiu. Tnh c s sai khc gia frame hin ti vi frame d on b chuyn ng. Kt qu c x l qua cc khi DCT, Q, m ha entropy trc khi a vo b m Ni dung d liu c nn cho chui nh video { I(i,j,n)}: D liu nh frame I Cc vec t chuyn ng c lng cho tng frame D liu sai khc gia frame d on b chuyn ng so vi frame thi im hin ti. B m v khi iu chnh ng vai tr gi tc dng bit l khng i Ch , cc frame s c chuyn t h mu RGB sang h YUV (YCBCR) 2.2. Cc khi chc nng 2.2.1. Bin i Cosine ri rc (DCT) DCT hai chiu (c th khng cn ghi cng thc)

u, v, x, y = 0, 1,2, .,7

DCT-1

c im: Bin i t min khng gian sang min tn s L php bin i gii tng quan (decorrelation) : loi b cc thnh phn d tha cc im nh ln cn ti thiu ha bin dao ng ca cc hm t tng quan. Tp trung nng lng min tn s thp (gc trn bn tri ca nh) khi lng t ha cho php a cc gi tr v 0 m khng lm bin dng nh L php bin i trc giao v i xng gim chi ph tnh ton bng cch thay th bi php bin i Cosine nhanh FCT

Ti u bng cch p dng ln cc khi 8x8 v: Cho kt qu tt nht khi p dng FCT. Tng chnh xc khi lm vic vi du phy tnh, gim sai s lm trn Cc im nh ln cn c tng quan cao hn, khi p dng DCT cho tng khi nh s tp trung nng lng vo mt s t cc h s bin i ch gy ra mt mt thng tin cc b Vai tr trong s : gim bt nh (entropy) ca nh, tng xc sut xut hin ca mt s gi tr nht nh tng t s nn. 2.2.2. Lng t ha Khi nim : nh x khng tuyn tnh c tn hao t khong biu din lin tc bin tn hiu vo khong biu din bi cc gi tr ri rc ca cc mc gi tr hay t m. S dng bng lng t Q(m x n) lng t ha: Lng t ha Q: Xq[i,j] = Round( X[i,j]/Q[i,j] ) Lng t ha ngc Q-1: X'[i,j]=Xq[i,j]*Q[i,j] Vn : thit k bng lng t sai s lng t l nh nht, khng lm bin dng nh. Vai tr trong s : Do cc gi tr sau php bin i l s thc nn s dng b lng t ha chuyn v s nguyn biu din n gin vi s bit t hn Thu hp di gi tr, gim s bit biu din cho mi gi tr. 2.2.3. M ha entropy (Entropy coding) c im: Mi mt phng bit c m ha ring bng m ha s hc thch nghi. M ha mi mt phng bit c chia thnh cc khi Mi mt phng bit ca mi khi m ha da theo nguyn tc lan truyn bit Cc bc thc hin : Tin x l :sp xp li theo chui Zigzag cc gi tr 0 nm cnh nhau

M ha DPCM (Difference Pulse Coded Modulation) cho thnh phn h s mt chiu C (0,0) M ha lot di (RLE) cho phn cn li ca chui (cc h s xoay chiu) p dng cc thut ton m ha vi di thay i VLC ( Huffman , Shanon, Shanon Fano )

2.2.4. c lng v b chuyn ng Cc k thut d on : D on tin nu d on chuyn ng da trn nh c m ha D on tin nu d on chuyn ng da trn nh sp c m ha D on 2 chiu (ni suy nh) nu d on chuyn ng da trn c d on tin v d on li. Phng php d on chuyn ng gm hai giai on: nh gi chuyn ng xc nh vec t chuyn ng cho mi khi B chuyn ng xc nh nh b sai s d on S dng phng php i snh khi xc nh vec t chuyn ng. Vec t chuyn ng chnh l vec t c ta tnh t tm khi hin ti n tm khi tham chiu. Thay v phi m ha nh gc vi t l ln bit chun MPEG m ha vec t chuyn ng v nh sai s d on (nh b chuyn ng).

I.5. Chun Mpeg1, Mpeg2, Mpeg4 v H.264Hip hi vin thng quc t (ITU) v t chc tiu chun quc t/ U ban k thut in t quc t (ISO/IEC) l hai t chc pht trin cc tiu chun m ho Video. Theo ITU-T, cc tiu chun m ho Video c coi l cc khuyn ngh gi tt l chun H.26x (H.261, H.262, H.263 v H.264). Vi tiu chun ISO/IEC, chng c gi l MPEG-x (nh MPEG-1, MPEG-2 v MPEG-4). MPEG hay gn hn l MPG c xut x t Moving Picture Experts Group (Nhm cc chuyn gia in nh). T chc ra i hi nm 1988 ny chuyn pht trin cc tiu chun v ci v nn phim s (digital video) v m thanh s (digital

audio). N hot ng di s u ca T chc Quc t v Tiu chun ha (ISO). Mpeg-4 l chun cho cc ng dng MultiMedia. Mpeg-4 tr thnh mt tiu chun cho nn nh k thut truyn hnh s, cc ng dng v ho v Video tng tc haichiu(Games,Videoconferencing) v cc ng dng Multimedia tng tc hai chiu (World Wide Web hoc cc ng dng nhm phn pht d liu Video nh truyn hnh cp, Internet Video...). Mpeg-4 tr thnh mt tiu chun cng ngh trong qu trnh sn xut, phn phi v truy cp vo cc h thng Video. N gp phn gii quyt vn v dung lng cho cc thit b lu tr, gii quyt vn v bng thng ca ng truyn tn hiu Video hoc kt hp c hai vn trn. Vi MPEG-4, cc i tng khc nhau trong mt khung hnh c th c m t, m ho v truyn i mt cch ring bit n b gii m trong cc dng c bn ES (Elementary Stream) khc nhau. Cng nh xc nh, tch v x l ring cc i tng (nh nhc nn, m thanh xa gn, vt, i tng nh video nh con ngi hay ng vt, nn khung hnh ), nn ngi s dng c th loi b ring tng i tng khi khun hnh. S t hp li thnh khung hnh ch c thc hin sau khi gii m cc i tng ny. H.264 ( MPEG-4 AVC hay MPEG-4 part 10), hin ang l phng thc tin tin nht trong lnh vc nn video. H.264 cho cht lng hnh nh tt nht khi c cng dung lng so vi cc chun nn khc. H.264 cng c ng dng nh thut nn chnh trong video phn gii cao (HD) Mc tiu chnh ca chun nn H.264 ang pht trin nhm cung cp Video c cht lng tt hn nhiu so vi nhng chun nn Video trc y. iu ny c th t c nh s k tha cc li im ca cc chun nn Video trc y. Khng ch th, chun nn H.264 cn k tha phn ln li im ca cc tiu chun trc l H.263 v MPEG-4. Trong ti ny chng ti xin php c trnh by k thut m ha video nn theo hng i tng MPEG-4 m hin nay ang uc s dng rt nhiu nh 1 cng c m ha vi nhiu tnh nng u vit.

II. VoIPII.1. Cc giao thc dng trong VoIP

1. Giao thc thi gian thc RTP Lung tn hiu thoi VoIP v cc tn hiu Video c mt s yu cu chung phn bit vi cc dch v internet truyn thng khc mt s c im sau : S lin tc: Cc gi phi c sp xp li theo th t thi gian thc bn nhn, c th cc gi b mt trn ng truyn, khi phi thc hin d tm v b li cho s mt ny m khng truyn li. S ng b bn trong phng thc truyn thng: thi gian truyn gia cc gi phi u n. Ngha l nu khng c tn hiu th cng phi tm cch lp y cc khong im lng . S ng b gia cc phng thc truyn thng: Nu c nhiu phng thc truyn thng cng c s dng trong mt phin truyn thng th cc phng thc phi ng b vi nhau. V d nh vic truyn ng thi tn hiu m thanh v tn hiu hnh th hai tn hiu ny phi ng b vi nhau, gi l s ng b hnh v ting. S nhn din payload: Trong internet, thng thng cn s thay i s m ho cho cc phng thc truyn ti (payload ) trn ng truyn hiu chnh thay i bng thng sn sng hoc kh nng cho ngi dng mi lin kt vo. Do cn c mt vi c ch nhn din s m ho cho mi gi. Nhn din khung: Tn hiu m thanh v tn hiu truyn hnh c gi trong mt n v d liu logic l cc khung. Tn hiu nhn dng khung dng ch cho bn nhn ch no bt u v kt thc ca cc khung, gip cho s phn phi ng b ti cc tng cao hn. thc hin iu ny th cn c cc giao thc thi gian thc. Thc t cc dch v ny c cung cp bi cc giao thc tng truyn ti ( transport layer ). Cc giao thc thi gian thc bao gm: giao thc dng thi gian thc RTSP ( Real Time

Stream Protocol ), giao thc iu khin thi gian thc RTCP ( Real Time Control Protocol ), giao thc khi u phin SIP ( Session Initiation Protocol ), giao thc gi trc ti nguyn RSVP ( Resources Reservation Set - up Protocol ). Cc giao thc ny a ra mt vi chc nng lin tc v tm s mt gi rt tt. a pht p thun tin (Multicast of friendly): RTSP v RTCP l cc k thut cho pht p (Multicast). Trn thc t, chng c thit k hot ng trong c cc cc nhm pht p nh ph hp cho cc cuc gi in ba ngi. i vi cc nhm ln th phi dng pht p qung b. c lp thit b: RTSP cung cp cc dch v cn thit cho cc dch v thi gian thc ni chung nh thoi v video, bt k mt b m ho v gii m no v gii m c th no c thm cc trng thng tin tiu v ng ngha s nh ngha cho mi phng thc m ho v gii m theo mt tiu chun k thut ring. Cc b trn v chuyn i: cc b trn l cc thit b nm gi phng thc truyn t mt vi ngi s dng trn v ni chng vo dng phng thc truyn thng, v gi kt qu n dng u ra.. b chuyn i s nm gi mt dng cc phng thc truyn thng, chuyn i chng vo cc khun dng khc nhau v gi i. Cc b chuyn i c vai tr thu nh bng thng yu cu ca dng s liu so vi yu cu bng thng trc khi c chuyn i m khng cn ngun pht thu nh tc bit ca n, iu ny cho php bn nhn thc hin kt ni nhanh m khng b gim v cht lng dch v so vi bng thng ban u. Cc b trn s gii hn bng thng nu c vi ngi ng thi gi mt lc, p ng chc nng cu hi thoi. RTPTS h tr c b trn ln b chuyn i. Cht lng dch v phn hi: RTCP cho php bn nhn c th a ra phn hi v cht lng nhn c n tt c cc thnh vin ca nhm. Cc ngun pht c th dng thng tin ny iu chnh tc bit s liu, trong khi cc bn nhn khc

c th xc nh xem cht lng ny l ton mng hay cc b. Ngi quan st ngoi c th dng thng tin ny thc hin qun l cht lng dch v. iu khin phin lng: RTCP cng cho php cc bn tham gia c th trao i thng tin nhn din nh tn, email, s in thoi v cc thng ip vn tt. M ho thnh mt m: cc dng phng thc truyn thng RTSP c th m ho thnh mt m dng cc kho. Cc kho l s trao i mt vi phng php khng phi RTP v d nh SIP hoc giao thc m t phin SDP ( session Describe Protocol ) 1.1 Giao thc dng thi gian thc RTSP( Real Time Stream Protocol ) RTSP ni chung c dng trong lin kt vi UDP(User Datagram Protocol). Nhng c th li dng bt k mt giao thc no ca tng thp hn trn c s gi tin. Khi mt trm ch mun gi mt gi, th cn phi bit phng thc truyn thng c th to khun dng gi, thm vo phn tiu ca gi phng thc truyn thng . Cng vic ny phi thc hin quyt nh trc tiu ca RTSP v a vo phng thc truyn tng thp hn. Sau chng c gi vo mng (dng mt trong hai cch a pht p hay n pht p) n cc thnh vin khc tham d.

V P X CSR C

M type

payload Timestamp

Sequence number

Synchronization source identifier (SSRC) Contributing source identifiers (CSRC) Header extension Payload ( audio, video ... )

Hnh 2.1 Cu trc tiu c nh RTSP Tiu ca gi RTSP di 12 bytes (nh hnh di). Trng V ch r phin bn ca giao thc, c X bo hiu c trng ca tiu m rng gia tiu c nh v s liu i theo. Nu bit P l 1, phn s liu c m thm m bo lin kt y cho s m ho thnh mt m. Danh sch ngun ng b phn tn CSRC (contributing SSRC), trng chiu di ca n c biu th bi trng chiu di ca ng b phn tn, ghi danh sch tt c cc ngun ng b phn tn vo ni dung ca gi. i vi thoi th trng ny ghi vo danh sch ton b ngi ang tham gia hi thoi. RTSP h tr khi nim phng thc truyn thng ph thuc vo vic t tn gip trong qu trnh xy dng li v pht gi ra. Bit ngi ghi M cung cp thng tin cho mc ch ny. i vi m thanh, gi u tin trong ting ni pht ra c th c a vo lp thi gian biu pht i mt cch c lp, nu gi nm trong ting ni pht ra trc . Bit M trong trng hp ny dng nh du gi u tin trong ting ni. i vi video, mt khung video ch c th c pht i khi gi cui cng n. Do trong trng hp ny M dng nh du gi cui cng trong mt khung video. Trng payload type nhn dng phng thc m ho trong gi. Trng sequence number tng mi khi gi c pht ra d s mt gi v khi phc th t. Trng Timestamp, s tng lin tc theo tn s ca phng tin ly mu ch ra rng khi no khung c to ra. 1.2 Giao thc iu khin thi gian thc RTCP ( Real Time Control Protocol ) Giao thc iu khin thi gian thc RTCP l giao thc iu khin i km vi RTSP. Phng thc truyn thng ngi gia ngi gi v ngi nhn thc hin

theo chu k cc gi RTCP n cng mt nhm a pht p nh dng phn pht cc gi RTSP. Mi gi RTCP c cha mt s cc phn t thng thng l cc bn tin ngi gi hoc bn tin ngi nhn tip theo ngay sau m t ti nguyn. Mi loi phc v mt chc nng ring. Cc bn tin ngi dng: c to ra bi ngi s dng ng thi cng km theo cc phng thc truyn thng ( cc ngun RTSP ). Chng m t s lng d liu c gi ging nh tng quan vi gn nhn thi gian ly mu RTSP v thi gian tuyt i cho php ng b gia cc phng tin khc nhau. Cc bn tin ngi nhn: c to ra bi cc thnh phn tham gia phin RTSP chng l phng thc truyn thng nhn. Mi bn tin nh vy c cha mt khi cho cc ngun RTSP trong nhm. Mi khi m t h s mt tch lu tc thi v s tri t ngun . Khi cng ng thi ch ra nhn cui cng v tr t lc nhn mt bo co ngi gi, cho php cc ngun c lng khong cch ca chng h thp dn. Cc gi k hiu ngun ( source describe - SDES ) dng cho iu khin phin. N c cha tn chun ( Canonical name CNAME ), mt nhn dng duy nht ging nh khun dng ca mt a ch trn mng internet. Tn chun dng gii quyt xung t trong gi tr ngun ng b v cc dng phng thc truyn thng lin kt khc nhau c to ra bi cng mt ngi s dng. Cc gi SDES cng nhn dng cc thnh vin qua tn ca n, email, s in thoi. iu ny cung cp dng iu khin phin n gin. Cc ng dng client c th hin th v tn v email trong cc giao din ngi s dng. iu cho php cc thnh vin tham gia phin nghe c cc thnh vin khc cng tham gia, n cng cho php chng thu c cc thng tin lin lc phc v cho cc dng khc ca truyn thng. V d nh khi to mt cuc hi thoi ring. Chnh iu ny cng lm thun tin hn trong vic lin lc vi mt ngi s dng. Nu mt ngi ngng tham gia phin

thng tin, ngi s c thng ip BYTE. Cui cng cc phn t ng dng (APP) c th dng a thm cc thng tin c th vo cc gi tin RTCP. Cc bn tin ngi nhn v ngi gi v cc gi SDES c cha cc thng tin, cc thng tin ny c th thay i thng xuyn do phi gi cc gi ny mt cch nh k. Nu cc thnh phn phin RTSP n gin gi cc thng ip theo mt chu k c nh s gy nn hu qu l bng thng ca nhm a pht p s ln t l tuyn tnh vi kch thc ca nhm - khng c li. khc phc iu ny, mi thnh vin m s thnh vin m n nghe c ( theo cc goi RTCP ). Chu k gia cc gi RTCP t mi ngi s dng sau c cn i t l vi vi s cc thnh vin ca nhm, m bo bng thng dnh cho cc bn tin RTCP vn c nh, khng ph thuc vo kch thc ca nhm. T khi kch thc ca nhm c lng thu c bi m s cc thnh phn khc, n chim gi thi gian cho mi thnh vin tham gia mi tin gn n kch thc nhm mt cch chnh xc. Nu nhiu ngi s dng cng tham gia nhp vo mt nhm, mi ngi s dng s c sai lch trong vic c lng kch thc ca nhm. Do c th dn n s trn cc bn tin RTCP. Mt gii thut back -off c a ra gii quyt vn ny. 1.3 Cc nh dng payload Cc c cu trn ca RTSP a ra dch v cn thit cho vic truyn ti chung ca m thanh v video. Tuy nhin cc b m ho v gii m ring s c tc dng tng thm cc thng tin cn thit truyn. h tr iu ny, RTSP cho php cc nh dng payload nh dng cc kiu m ho v gii m ring. Cc nh dng payload m t cc c php v ng ngha cho payload RTSP. Ng ngha c bit ca payload c truyn trong cc bit nhn dng kiu payload RTSP. Cc bit ny cho bit b m ho v gii m thc t ang dng v cc nh dng ny rng buc bi cc cc tn c cc nh cp php s iANA ( internet assigned number authority ) gn cho v c truyn ngoi bng. Cc ch nh ny

c th c truyn ngoi bng cho php mt ngun RTSP c th thay i khun dng m ho m khng cn s bo hiu r rng. Hn na bt k ai cng c th thm vo cc kiu m ho v gii m mi , c ngha l RTSP l h thng m c th c pht trin bi bt k ai. Cc nh dng s liu payload, phng thc truyn thng RTSP c nh ngha trong cc khuyn ngh H.263, H261. Cc b m ho -gii m video theo chun JPEG, MPEGG v mt trm ch ca cc b m ho -gii m video v m thanh khc c h tr vi cc nh dng payload n gin hn. Hn na, cc nh dng payload RTSP c nh ngha cung cp cc dch v chung. Mt nh dng s liu khc cng c nh ngha cho php cung cp c ch sa li, khi phc cc gi b mt theo nh ngha c lp vi b m ho v gii m. Ngoi ra cn mt nh dng khc c gii thiu ghp cc phng tin truyn thng t nhiu ngi s dng vo mt gi. y l hiu qu c th ring thay th cc trung k gia cc gateway VoIP, l ni thu nh ng k thng tin mo u trong tiu cc gi. 1.4 Giao thc gi trc ti nguyn RSVP Da vo s quan trng ca cc dch v thoi cng vi s on trc t l cn thit bng thng trong internet dng bi thoi v video ngi ta s dng giao thc RSVP. cng l bin php m bo cho cht lng ca dch v thi gian thc ( QoS - Quality of Service ). Tuy nhin cc xut xung quanh RSVP cn rt phc tp, cha trin khai c trn din rng. Ngi ta xut b giao thc gi trc ti nguyn v n gin l dng cc thng ip RTCP thng bo cho cc router dc theo ng truyn gi trc ti nguyn. nh ra s lng bng thng cn gi trc, cc bn tin bn gi RTCP c th dng bng s quan st s khc nhau gia hai s n trc (cc s m byte ca bn tin ngi gi), hoc c th chn thm vo mt trng mang theo cc c trng th hin mong mun cht lng dch v di dng chi tit hn.

Cc thng ip RTCP mang cc yu cu gi trc c nh du s dng ring bit bi mt tu chn linh hot ca router. Cc bn tin ngi nhn s quay li xem s gi trc c thc hin hon tt hay tng phn. Cc xut tng t cng c a ra nhm n gin ho cc giao thc gi trc ti nguyn trn c s bn gi c th thy (cho d khng dng cc bn tin RTCP) trong giao thc cn bng SRP ( Scalable reservation Protocols). Vn gi trc cho cc dch v VoIP li c kha cnh khc, nu vic gi trc b hng th cuc gi vn din ra nhng phi dng truyn ti best effort. iu ny ph hp vi mt tn hiu fast busy, ni khng c tt c cc truyn thng c thit lp v a ra kh nng quyt nh chp nhn hay loi b hon ton tt c cc end to end. Trn mi kt ni trong mng, bng thng cho lu lng thoi c th xc nh. Bng vic nghin cu k thut mng in thoi truyn thng c th dng xc nh s lng cp pht cn thit. Phn bng thng cn d trn lin kt c dng cho cc dch v khc. Cc gi n c phn bit l thoi v phi thoi. S c nhiu kt ni thoi hot ng mt lc ti thi im tr hon bt k (c th t khi khng c kh nng thu nhn), cc cuc gi ph tr c th n gin tr thnh best effort. Do mt ngi qun l tt c th t cc kh nng thp hp l trong trng hp nh vy v s dng trn tn hiu bng thng best effort c th c lng thu nhn iu khin vn ny khng xy ra cc s c. Cc c ch ni trn xem nh chia cc dch v VoIP thnh cc lp khc nhau.

II.2 Gii thiu chung v cng ngh VoIPVoIP (Voice over Internet Protocol) l mt thut ng dng ch mt phng thc truyn thoi mi, l truyn tn hiu thoi qua mng gi s dng giao thc Internet. y l mt ng dng m rng ca b giao thc IP tin ti mt mng a phng tin hi t ton IP trong tng lai. Trong cng ngh in thoi IP thng tin thoi s c ng trong cc gi v truyn i trn mt ng truyn chung thay v cc knh ring nh trong mng PSTN truyn thng. Chng ta c th nhn nhn VoIP nh l kh nng thit lp cc cuc gi in thoi v gi nhng bn fax qua mng d liu IP vi cht lng dch v c th chp nhn c v vi cc ph thp hn rt nhiu so vi dch v ca

mng PSTN thng thng. thc hin truyn thoi qua mng IP, trc ht ti pha pht tn hiu thoi phi c s ho, nn v ng gi truyn qua mng IP nh nhng gi d liu thng thng. Ti pha thu, cc gi thoi c m gi, gii nn v chuyn t tn hiu s sang tn hiu tng t a n tai nghe ca ngi dng. V c im ca mt mng gi l mt mng tn dng ti a hiu qu s dng bng tn m t quan tm n thi gian tr lan truyn trong mng, trong khi tn hiu thoi li l mt dng thi gian thc. Do , ngi ta phi b sung vo mng cc phn t mi v thit k cc giao thc ph hp c th m bo c cht lng dch v cho ngi dng. Cc thnh phn ny to thnh mt mng gi l mng VoIP. Cng ngh VoIP v ang m ra mt vin cnh mi trong ngnh Vin Thng trn ton th gii. Vi u im rt ln v gi cc, dch v VoIP thc s ang thu ht rt nhiu s quan tm ca cc nh kinh doanh bao gm c cc nh u t cung cp dch v v cc nh tiu dng dch v. Vic a dch v VoIP vo phc v c th coi l mt cuc cch mng trong ngnh Vin Thng v thc t cho thy sc cnh tranh rt ln ca cng ngh ny so vi cng ngh thoi chuyn mch knh truyn thng. Mc d mi ra i v c trin khai cha lu, nhng dch v VoIP chim mt th phn rt ng k trn th trng quc t ni chung v nc ta ni ring. im ct li to nn cc u im ca mng VoIP l kh nng s dng hiu qu v tit kim rng bng tn. l do s kt hp cc c tnh vn c ca mt mng s liu gi vi cc k thut m ho nn s hin i. K thut ny cho php gim c ng k s bt phi truyn i. Do gim c bng thng truyn dn m vn m bo c cht lng dch v theo yu cu.

Ni chung VoIP l mt hnh nh mi cho mng Vin Thng hin i. N ang c nghin cu v trin khai v ang ha hn rt nhiu trin vng trong tng lai gn.