I GOi phat la
1. 3 Ki~n true QoS
QoS ca ban bao g6m 3 phan chinh:
• Dinh dang QoS va ki thuat danh dfru cho phep phoi hop QoS nr diem dfiu toi
diem cudi giira rung thanh phfin mang.
• QoS trong tung thanh phan mang dan( cac cong cu hang doi dinh dang, l~p
lich, dinh dang luu luong)
• Cach giai quyet, dibu khien QoS, cac chirc nang tinh toan d6 dibu khien va
giam sat luu luong dfiu cuoi qua mang.
Chuong I. Tang quan vJ QoS Db an tat nghi~p
20
Phtpn Van Ng(JC /Op: H07VTTD
1. 3. 2 QoS trong m{jt thi~t b] mang
Bao g6m .Quan ly tftc nghen, quan ly hang doi, hieu suat ket n6i va cac cong cu
dinh hinh/xir ly cung dp QoS trong mot thiet bi mang,
a. Quan ly t~c nghen
Do sir bung n6 nr nhien cua luu hrong thoai/video/dtr lieu, nen vai luong luu luong se vuot qua t6c d(> cua m(>t lien ket. Tai diem nay, cac router se lam gi? Se tao mot b(> dem trong hang doi va cho phep g6i d§u tien vao se ra d§u tien? Hay lit se d?t cac g6i vao cac hang doi khac nhau. Cong cu quan ly tftc nghen bao g6m hang doi U'U tien PQ (Priority Queuing), hang doi khach CQ (Custom Queuing), hang doi hop ly theo trong
s6 WFQ (Weighted Fair Queuing) va hang doi hop ly theo trong s6 dua tren co so 16p CBWFQ (Class-Base Weighted Fair Queuing).
b. Quan ly hang dc;ri
Do cac hang dqi khong co kich thuoc vo han nen chung co th~ bi d§y va tran. Khi hang do'i d§y, cac goi them vao khong duce d?t trong hang dqi va se bi d:iy ra ngoai.
D6 la hien tuong "tail drop" -roi duoi (tire la cac goi cuoi se bi d:iy ra tung g6i mot). vfin d€ xay ra voi hien nrong "tail drop" la cac router kh6ng th~ ngan chan cac goi mi
ra ngoai (k~ ca cac goi co d(> uu tien cao ). Vi th~ dn giai quyet hai vfin d€ sau:
• Thu tao mot hang doi va dam bao la no khong bi d§y, a hang doi d6 chira cac g6i co d(> uu tien cao.
• Thi6t l~p tieu chuan d6i voi cac g6i bi mi ra ngoai.cac goi co d(> uu tien thfip se bi d:iy ra ngoai truce cac g6i co d(> uu tien cao.
Thuat toan WRED(Weighted Early Random Detectj-phat hien sorn ng~u nhien theo
trong s6 co th€ thuc h ien duce ca hai di€u tren.
c. Hi~u suit lien k~t
Nhieu lien k€t t6c d(> thap da d?t ra mot vftn d€ d6i voi cac g6i nho nhat. Vi du, thoi
gian tr€ cua mot buci phat thanh cua m(>t g6i 1500 byte, t6c d(> 56kb/s la 214ms (Kich
thu6c g6i la 1500*8 bit =12000 bit, t6c d(> duong day 56000 bps nen k~t qua la
12000bit/56000=214 ms). Neu m(>t g6i tl10?i dugc d?t trn6c g6i l6n thi tr€ cl'.ia g6i tho?i c6 th~ da VU'Q't qua so v6i cac g6i a phia trai router. Cac g6i l6n se duqc pban
manh thanh cac g6i nh6 va x€p xen v6i cac g6i tho?i. Vi~c ghep xen nay cling quan
tr9ng nhu vi~c phan do?n .
d. Chinh sach va djnh hinh lu·u lu9·ng
Dinh hinh duqc su d\mg d~ t?o m(>t lu6ng lm1 luqng ma 116 h?n ch€ kha nang cl'.1a bang thong. :Di€u nay duqc su dl,lng nhi€u l§n khi tran luu luqng. Vi dl,l, nhi€u topo tn?ng su dl,lng Frame Relay trong m(>t thi€t ke hub-and-spoke. Trong tnrong hgp nay, vj tri trung tam thuong c6 k€t n6i v6i bang thong cao(say, Tl), trong khi cac vi trJ tach
Chuong I. Tang quan vJ QoS
21
Phtpn Van Ng{JC /Op: H07VTTD
ffinh 1. 6 Cac muc QoS
H9·p d6ng QoS vach ra mong muon thuc hien nhiem v1,1 dam bao ch.it luong dich v1,1 theo mot k€ hoach cu th~ va thong qua M thong bao hieu QoS d~ ra lenh cho cac ca
ch€ chsp hanh tai cac nut mang thuc hien nhiem V1,l do. Trong mang IP truyen thong chi Cung cfrp chfrt hrong dich vu CT IDUC "best-effort", tire la IDUC "116 luc tfii da" ma khong co bfrt ky m(>t sv cam k€t hay rang bu(>c nao. Cac goi du9·c chuy~n tt'r di~m nay t&i di~m khac kh6ng co bfrt kl m(>t s\r dam bao nao v€ bang th6ng hay thO-i gian tr~ t6i thi~u. V&i mo hlnh luu lugng "best-effort", tfrt ca cac yeu cfru cua nguO'i su d1,1ng co cung m(>t U'U tien Va dll'Q'C Xlr [y ke ti€p nhau theo ki~U ai den tfU'CTC thi p[}\lC V\l tnrac.
Certllm OllPlicotlO"' roqliro :;pocrhc n!MwOfll. 1eflOUtCe.!:I ou .. anto&d (l>end'Mdll), llel8\'. pnet) Somotr.tllclo ~ moro import.:nll ~-------'-------! 11\an ttlo rll$1 Dlfforvoli31od
(firot. -CioM) oo"'"""·
$olv..,.IP 1111omot ~iquiltlu.!l eoMecllvlly Sc&t effort OP IP)(. lll>Ple 1"1<}
Ba rmrc do duce mo ta trong hinh ve sau:
biet co k€t n6i voi bang thong thftp (say, 384Kbps). Vi the, hru hrong nr vi tri trung tam trim sang k€t n6i voi bang thong thfrp tai di~m cu6i khac. Dinh hinh la mot phuong phap hoan chinh xac dinh luu hrong gin 384 Kbps d~ ngan chan tran tai cac k€t n6i tach biet. T6c d(> dinh dang luu hrong la t6c d(> truyen dfin o b(> dem d~ duy tri t6c d(> dinh dang do.
Chinh sach ciing tuong tu nhu dinh hinh nhung khac 6 mot diem dt quan trong: Tfic d(> dinh dang hru hrong vuot qua khong phai ab(> d~m (va thuong la no duce loai b6).
1. 3. 3 Cac rmrc QoS
Xet nr dftu cu6i toi dftu cuoi, ch.it hrong dich vu chia thanh 3 rmrc:
i. Best-effort Service: La cac dich vu khong dn co mot su bao dam nao v€ ch.it luong dich vu (d(> tr~, jitter ... ).
ii. Differentiated Service (con g9i la soft QoS): Mot vai luu luong cua dich V1,1 duce uu tien hon nhung dong hru luong con lai (Duoc XU ly nhanh hon, bang thong trung binh nhieu hon, ty I~ mfrt goi ft hon ... ). No duce cung dp boi b(> phan loai hru luong va su dung cac c6ng cu cua QoS nhu PQ, CQ, WFQ va WRED
iii. Guaranteed Service (Con ilu'{J'c so! la hard QoS): nhirng dich vu duce dam bao tuyet d6i v€ tai nguyen mang danh cho no. No duce cung dp boi cac giao thirc RSVP
va CBWFQ.
Chuong I. Tang quan vJ QoS
22
Phtpn Van Ng(JC /Op: H07VTTD
Tfit ca dbu dong y r~ng mang Internet la mot mang toan du nhung lai khong c6 mot
chut bao dam QoS nae. QoS tren mang Internet nhu mot cau chuyen cuoi ma mot RFC
ra vao ngay Ca thang tu voi tieu db "IP qua Avian Carriers voi QoS" (RFC 2549). Cau
chuyen cuoi n6i rang QoS co th~ them vao IP de nhu la sir dung chim b6 can d€ chuyen cac g6i IP.
N~u mot mang thieu QoS thi nguoi sir dung phai tu them vao cac phuong phap cua
minh d€ co QoS dn thi€t. Trong tfit ca cac tham s6 QoS, tham s6 ma nguoi Slr dung kho nr them vao nhfit la tre. Thuc t~ se don gian hon nhieu neu khac phuc duoc nhuoc
di~m cua QoS bang each them bang thong hon la bfit cir each nao khac. Them vao du
bang thong thi it nhat tre va jitter se duce cai thien, N€u bang thong duce them vao dung dftn thi tham chi ca 16i, tinh s~n sang va bao mat ciing se duce cai thien. Ket qua
cua viec them bang thong la co th~ du doan duce. Diu tu vao bang thong d€ nhan
duoc cac k€t qua bi~t truce thi t6t hon la diu tu vao m<)t each m6i nao d6 d~ v?n chuy~n I6i di vong quanh va sau d6 nMn ra la khOng c6 gi t6t hon tru6c d6.
Nhu v?y la c6 hai l\ra ch9n d€ nh?n duqc QoS cho nguoi sfr dl,lng va cho frng d\mg
d6 la tich hqp QoS hay la them QoS len tren m:;tng. Di~u nay th\rc s\r la khong c6 gi khac hon la vi~c can nhftc gili"a vi~c c6 s~n QoS trong m:;tng va vi~c them QoS 6 dp
l'.mg dl,lng ben ngoai m:;tng.
Tuy nhien, voi nhimg ung dung thoi gian thuc nhu voice, audio hay video, cac dich vu moi nhu IP VPN, hay khi nguoi sir dung thuc hien mot tac V\1 quan trong lien quan toi thuong mai di~n nr (e-commerce) thi khi do mang bftt buoc phai cung dp nhtrng dich vu tin cay. Mot s6 loai hru hrong dn chat Iuong dich vu cao hon cac loai khac.
Cung voi str phat trien rfit nhanh ch6ng cua Internet hien tai cling nhu tuong lai lam cho nhu du luon vuot qua kha nang dap ung. Do do, ma rong bang thong khong phai
la giai phap giai quyet moi vfin db cua mang.