Cu hi :1. sao li phi nn d liu khi truyn 2 2. Ti sao ko dng chun
MP3 nn d liu trong VOIP 3 3. Ti sao li dng chng giao thc trong cc
VOIP 4 4. Gii thch cc giao thc dng trong bc to cuc gi trong ng dng
SIP- asterisk 5 5. K thut streaming l g 6 6. Gii thch k thut dng
trong streaming khi truyn 1 file nhc 7 7. Ti sao li dng h mu YUV
trong chun MPEG vs IUT 8 8. Gii thch cch ly mu dng h mu YUV trong
nn video 9 9.cho 1 mn hnh kch thc l 366x722 30h/s nn ra chun MPEG
tc 1Mbit/ s cho m=18,n=3 tnh t s nn?10 10. khi m=12,n=3 th t s nn
thay i nh noBi gii :1. Ti sao phi nn d liu khi truyn ?Thng thng, cc
tp tin u c rt nhiu thng tin d tha, vic nn cc tp tin thc cht l m ha
li cc tp tin loi b thng tin d tha . Vic nn cc tp tin c th p dng cho
file vn bn, file nh, file nh phn v.vTrong k thut truyn tin ni tip,
cc bit d liu c truyn i ni tip , li b gii hn v bng thng ca knh truyn
v gii hn ca cc chun kt ni nn tc truyn tp tin tng i chm. tng tc
truyn ta c th dng nhiu phng php nh s dng k thut iu ch pha nhiu mc,
iu ch QAM, TCM...Nn d liu trc khi truyn i cng l mt trong cc phng
php nhm tng tc truyn d liuMt s phng php nn hay dng : nn c di thay
i, Huffman, LZW v.v2. Ti sao ko dng chun MP3 nn d liu trong VOIP (
20 cu 1.1)Do c tnh m thanh thoi trong cc ng dng VOIP :Tn hiu m
thanh c chia lm 2 loi da trn di tn l : m thanh di tn c s : l ting
ni thoi c di tn t 300Hz-4KHz. m thanh di tn rng : l ting ht, m nhc,
... c di tn t 100Hz-20KHzTheo d liu truyn trong cc dch v Voip ch yu
l ting ni thoi c c im yu cu khi s ha nh sau : Gii hn di ph trong
khong 4KHz Tn s ly mu Fs = 8 KHz Lng t ha gi tr : m ha bi 8 bt Tc
cn thit : 8bit x 8KHz = 64KHzMP3 l mt trong nhng chun nn m thanh da
trn phng php SBC (sub-band coding) c s dng ch yu trong m ha m thanh
di rng do phng php ny c xy dng da trn m hnh cm th m thanh thnh gic
ca con ngi. m thanh khi c nn theo chun MP3 c th c tn s ly mu (32
KHz, 44.1 KHz, 48 KHz) v tc bit rt a dng (32 Kbps, 48 Kbps, 56 Kbps
). Do MP3 khng thch hp s dng trong cc ng dng VOip.(Ch thm : GSM
(GSM Audio Codec) l chun nn dnh ring cho m thanh thoi s dng phng
php m ha lai: kt hp x l v phn tch ting ni da trn m hnh pht m ca con
ngi v m ha tn hiu dng sng. m thanh khi c nn theo chun GSM thng c tn
s ly mu l 8KHz c tc bit cn thit thp, nm trong khong 6.5-13Kbps.m
thanh thoi c nn theo chun GSM c yu cu thp hn v bng thng ng truyn do
chun nn ny thng c s dng nhiu hn trong dch v VoIP: G723.1
(6.4kbits/s); G728, G729 (8kbits/s) )3. Ti sao li dng chng giao thc
trong cc VOIP ( 21 cu 1.1)Mng h thng truyn thng a phng tin cn phi p
ng yu cu c s lng thu bao ln, din rng v kh nng tng tc thi gian thc.
D liu a phng tin l thng tin kt hp ca nhiu hnh thc bao gm vn bn, m
thanh, ting ni, nh tnh, nh ng, video Do yu cu truyn thng phi c kh
nng kt hp truyn nhiu loi thng tin c c tnh vt l khc nhau trn ng
truyn: nh, video, m thanh, ting ni, vn bn c di ph khc nhau nn c yu
cu khc nhau v tc , tr K thut truyn video khc nhau v a dng theo yu
cu ng dngBn cnh mt phin truyn thng l mt qu trnh bao gm nhiu cng on,
ch yu bao gm: Thit lp kt ni Thit lp mt phin truyn d liu Kim sot, iu
khin Qun l Kt thc truyn d liuDo , vic truyn thng d liu a phng tin
cn phi s dng chng giao thc trong kt hp nhiu tng giao thc khc
nhau.4. Gii thch cc giao thc dng trong bc to cuc gi trong ng dng
SIP- asterisk
lung cuc gi trong ch SIP proxy server
Cc bc tin hnh trong ch Proxy server thc hin cuc gi hai chiu thnh
cng: 1. Bn gi gi mt INVITE cho proxy server do n khng bit chnh xc v
tr ca bn c gi.2. Proxy server xc nh v tr cn gi bng cch s dng c s d
liu ca n hoc dch v nh v. 3. Proxy server pht mt yu cu INVITE ti a
ch ca v tr m proxy server nhn c t dch v nh v.4. Da vo trng via
trong yu cu INVITE nhn c, bn c gi bit rng INVITE c gi t proxy
server, bn c gi s cnh bo server v chp nhn cuc gi cho nn n gi mt 200
OK cho server.5. Proxy server nhn c tr li , check trng via u tin
trong thng ip 180 Ringing cha a ch ca n, s dng xc nh giao dch trong
trng Via, sau n xa trng Via , v chuyn tip tr li cho a ch c trong
trng Via th 2: 100.101.102.103 port 5060. Kt qu l tr li c proxy
server gi cho bn gi tc l Schroedinger.Tng t i vi thng ip 200 OK,
proxy server cng xa trng Via thnht i v gi cho bn gi:6. Bn gi xc nhn
tr li bng vic pht ra mt yu cu ACK. Da vo URI trong trng Contact
trong Header nhn c t tr li 200 OK ca server gi, bn gi c th gi trc
tip cho bn c gi thng qua proxy server. Thng ip ny v nhng thng ip
tip sau vn s dng trng tag c trong To ca yu cu:7. Phin phng tin c
thit lp gia hai UA. Cuc ci c thc hin.8. Khi mun kt thc cuc gi, mt
bn gi mt yu cu BYE cho bn cn li.9. Bn nhn c yu cu BYE chp nhn ngng
cuc gi bng cch gi mt tr li 200 OK cho bn gi BYE.
Th nghim : Cc bc to cuc gi t my 101 sang my 102 Cu hnh thng tin
cc thu bao trong
/etc/asterisk/sip.conf[101]type=friendhost=dynamicsecret=123[112]type=friendhost=dynamicsecret=123Vi
cc tham s dnh ring cho tng thu bao: [101] : Tn thu bao cn cu hnh,
tn thu bao c th l c ch v s nhng thng l s v bn phm in thoi ch bao gm
cc phm s Type : Kiu kt ni thng tin user, peer hay friend+ Kiu user
l ch gi+ Kiu peer l ch nghe+ Kiu friend l kt hp ca c hai Host : a
ch IP hay tn min m client kt ni vi knh ny.Gi tr l "dynamic" cho tt
c client kt ni n. Ngoi ra, gi tr ny c th c nh a ch ny bng a ch ip
server. Secret mt khu sdng knh ny Cu hnh DialPlan trong tp
/etc/asterisk/extensions.conf[default]exten =>
101,1,Dial(SIP/101)exten => 112,1,Dial(SIP/102)
Vi : 101 : s in thoi c quay s Priority : Th t thc hin cc ng dng
trong dialplan. y l 1 c ngha l ng dng ny c thc hin u tin
Dial(SIP/109) : ng dng thc hin. C th y l s dng giao thc SIP gi n s
109 Kt qu :
5. K thut streaming l g ? ( 21 cu 1.2)Truyn dng l mt phng php
truyn thng tin lin tc, bn nhn khi nh n dng thng tin s c th th hin
ngay ni dung theo thi gian m khng cn phi i n khi ton b ni dung thng
tin c truyn xong.Truyn dng ch yu m ta gp l truyn dng Video v
Audio.Truyn dng cho php chng ta th hin cc dng video,audio thi gian
thc m khng ph thuc vo di ca video,audio . iu ny rt c ngha khi truyn
cc file c kch thc ln hay cc dng c di khng xc nh.Truyn dng thi gian
thc, c bit hn, c ngha l nhn video v truyn i thi gian thc qua mng.
Tin trnh ny cn 1 camera thu, mt b m ha s, mt b pht video ln trn mng
ngi dng cui, hoc mt mng phn tn pht tn cho nhiu ngi dng cui c theo
di. Khi media c th c ngi dng theo di theo ng thi gian thc
6. Gii thch k thut dng trong streaming khi truyn 1 file nhc ? ((
21 cu 1.2)
7. . Ti sao li dng h mu YUV trong chun MPEG vs IUT ( 20 cu
2.1)
8. Gii thch cch ly mu dng h mu YUV trong nn videoTrong YUV c tn
s ca tn hiu Y l 6.5MHz, tn hiu U v V l khong 2 MHz, do cho php gim
tn s ly mu tn hiu U,V so vi tn s ly mu tn hiu Y m cht lng nh khng
i.Trong nn video, khng gian mu RGB c chuyn i sang khng gian mu
TCrCb hoc YUV theo cc cu trc s ha 4:2:2 hoc 4:1:1, 4:2:0 cho php m
bo cht lng video s.Gii thch r hn : (Bi tp ln K53 8 trang 17)Tiu
chun 4:2:2: Gm 8 khi: 4 khi Y, 4 khi Cb v 4 khi Cr
Tn hiu chi c ly mu ti tt c cc im ly mu trn dng tch cc ca tn hiu
video. Tn hiu mu trn mi dng c ly mu vi tn s bng na tn s ly mu tn
hiu chi .
Cu trc ly mu theo chun 4:2:2Tiu chun 4:2:0: Gm 6 khi: 4 khi Y,
mt khi Cb, mt khi Cr
Tn hiu chi c ly mu ti tt c cc im ly mu trn dng tch cc ca tn hiu
video. Cch mt im ly mu mt tn hiu mu. Ti dng chn ch ly mu tn hiu mu
CR, ti dng chn ly mu tn hiu CB. Nh vy, nu tn s ly mu tn hiu chi l
fD, Th tn s ly mu tn hiu mu s l fD/2.
Cu trc ly mu theo chun 4:2:0Tiu chun 4:1:1: Tn hiu chi c ly mu
ti tt c cc im ly mu trn dng tch cc ca tn hiu video. Tn hiu mu trn
mi dng c ly mu vi tn s bng mt phn t tn s ly mu tn hiu chi (Hnh
2.1.32) Nh vy, nu tn s ly mu tn hiu chi l fD, th tn s ly mu tn hiu
mu CR v CB s l fD/4.
Cu trc ly mu theo chun 4:1:1
9. cho 1 mn hnh kch thc l 366x722 30h/s nn ra chun MPEG tc
1Mbit/s cho m=18,n=3 tnh t s nn?Do video chun MPEG c nn s dng h mu
YUV nn cu trc ly mu c s dng c th l : 4:2:0 , 4:1:1, 4:2:2 .Nh vy nu
video cha nn , dung lng video phi truyn l :Cu trc ly mu 4:2:2 366 *
722 *30 * 8 * (1+1/2+1/2) = 126840960 Mbit/sNh vy t s nn l :
126,84:1Cu trc ly mu 4:2:0 hoc 4:1:1 366 * 722 *30 * 8 * (1+1/2+0)
= 87333120 Mbit/sNh vy t s nn l : 87,33:1
10. Khi m=12,n=3 th t s nn thay i nh no ?Khi m = 12, n =3 tc l
khong cch gia I v P khng thay i, khong cch gia 2 frame I gim xung
do s frame I trong on video tng ln. M cc frame I ch m ha bng k thut
intra-frame loi b d tha khng gian , d tha thi gian vn cn tn ti gia
cc frame .Do t s nn gim xung , tnh chu li tng ln.
