Gioi Thieu Mang WAN

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GII THIU V MNG WAN

I. Gii thiu chung

1. WAN l g?

Wide Area Networks WAN, l mng c thit lp lin kt cc my tnh ca hai hay nhiu

khu vc khc nhau, khong cch xa v mt a l, nh gia cc qun trong mt thnh ph, hay giacc thnh ph hay cc min trong nc. c tnh ny ch c tnh cht c l, n cng tr nn kh xc

nh vi vic pht trin mnh ca cc cng ngh truyn dn khng ph thuc vo khong cch. Tuy

nhin vic kt ni vi khong cch a l xa buc WAN ph thuc vo nhiu yu t nh: gii thng v

chi ph cho gii thng, ch qun ca mng, ng i ca thng tin trn mng. WAN c th kt ni thnh

mng ring ca mt t chc, hay c th phi kt ni qua nhiu h tng mng cng cng v ca cc cng

ty vin thng khc nhau. WAN c th dng ng truyn c gii thng thay i trong khong rt ln t

56Kbps n T1 vi 1.544 Mbps hay E1 vi 2.048 Mbps,....v n Giga bt-Gbps l cc ng trc ni

cc quc gia hay chu lc. y bps (Bit Per Second) l mt n v trong truyn thng tng ng

vi 1 bit c truyn trong mt giy,v d nh tc ng truyn l 1 Mbps tc l c th truyn ti a

1 Megabit trong 1giy trn ng truyn .

Do s phc tp trong vic xy dng, qun l, duy tr cc ng truyn dn nn khi xy dng

mng din rng WAN ngi ta thng s dng cc ng truyn c thu t h tng vin thng cng

cng, v t cc cng ty vin thng hay cc nh cung cp dch v truyn s liu. Ty theo cu trc ca

mng nhng ng truyn thuc c quan qun l khc nhau nh cc nh cung cp ng truyn ni

ht, lin tnh, lin quc gia, chng hn Vit Nam l cng ty Vin thng lin tnh VTN, cng ty Vin

thng quc t - VTI . Cc ng truyn phi tun th cc quy nh ca chnh ph cc khu vc c

ng dy i qua nh: tc , vic m ha. Vi WAN ng i ca thng tin c th rt phc tp do vic

s dng cc dch v truyn d liu khc nhau, ca cc nh cung cp dch v khc nhau. Trong qu trnh

hot ng cc im nt c th thay i ng i ca cc thng tin khi pht hin ra c trc trc trn

ng truyn hay khi pht hin c qu nhiu thng tin cn truyn gia hai im nt no . Trn WAN

thng tin c th c cc con ng i khc nhau, iu cho php c th s dng ti a cc nng lc ca

ng truyn v nng cao iu kin an ton trong truyn d liu.

Phn ln cc WAN hin nay c pht trin cho vic truyn ng thi trn ng truyn nhiu

dng thng tin khc nhau nh: video, ting ni, d liu...nhm lm gim chi ph dch v. Cc cng ngh

kt ni WAN thng lin quan n 3 tng u ca m hnh ISO 7 tng. l tng vt l lin quan n

cc chun giao tip WAN, tng data linklin quan n cc giao thc truyn thng ca WAN, v mt s

giao thc WAN lin quan n tng mng. Cc quan h ny c m t trong hnh sau:

a ch: 615, F4, TM Yn Ha, Cu Giy HN Tel: 04-62691959 Fax: 04-62698538

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Cc chun v giao thc WAN trong m hnh ISO 7 tng

2. Cc li ch v chi ph khi kt ni WAN.

X hi cng pht trin, nhu cu trao i thng tin cng i hi vic x l thng tin phi c tin

hnh mt cch nhanh chng v chnh xc. S ra i v pht trin khng ngng ca ngnh cng ngh

thng tin gp phn quan trng vo s pht trin chung . Vi s ra i my tnh, vic x l thng

tin hn bao gi ht tr nn c bit nhanh chng vi hiu sut cao. c bit hn na, ngi ta nhn thy vic thit lp mt h thng mng din rng - WAN v truy cp t xa s lm gia tng gp bi

hiu qu cng vic nh vic chia s v trao i thng tin c thc hin mt cch d dng, tc th(thi

gian thc). Khi khong cch v mt a l gia cc vng c thu ngn li. Cc giao dch c din

ra gn nh tc th, thm ch ta c th tin hnh cc hi ngh vin m, cc ng dng a phng tin...

Nh c h thng WAN v cc ng dng trin khai trn , thng tin c chia s v x l bi

nhiu my tnh di s gim st ca nhiu ngi m bo tnh chnh xc v hiu qu cao. Phn ln ccc quan, cc t chc, v c cc c nhn u nhn thc c tnh u vit ca x l thng tin trong

cng vic thng qua mng my tnh so vi cng vic vn phng da trn giy t truyn thng. Do vy,

sm hay mun, cc t chc, c quan u c gng trong kh nng c th, u c gng thit lp mt mng

my tnh, c bit l WAN thc hin cc cng vic khc nhau.

Vi s pht trin nhanh chng ca cng ngh thng tin, cng ngh vin thng v k thut my

tnh, mng WAN v truy cp t xa dn tr thnh mt mi trng lm vic cn bn, gn nh l bt buc

khi thc hin yu cu v hi nhp quc t. Trn WAN ngi dng c th trao i, x l d liu truyn

thng thun ty song song vi thc hin cc k thut mi, cho php trao i d liu a phng tin nh

hnh nh, m thanh, in thoi, hp hi ngh,... qua tng hiu sut cng vic, v lm gim chi ph


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qun l cng nh chi ph sn xut khc. c bit i vi cc giao dch Khch Phc v(Client

Server), h thng kt ni mng din rng t cc LAN ca vn phng trung tm (NOC) ti LAN ca cc

chi nhnh(POP) s l h thng trao i thng tin chnh ca c quan hay t chc . N gip tng cng v

thay i v cht cng tc qun l v trao i thng tin, tin bc vng chc ti mt nn kinh t in t

(e-commerce), chnh ph in t(egoverment) trong tng lai khng xa.

II. THIT B V CNG NGH MNG WAN

1) Router:

- Router l mt thit b mng thng minh, ng vai tr rt quan trong trong hot ng ca h thng

mng.

- Router iu khin vic truyn d liu trn mng, la chn hng tt nht truyn d liu gia hai

hoc nhiu h thng mng khc nhau.

- Router c th coi nh mt my vi tnh v c y cc thnh phn c bn ca my tnh (vd: CPU,

BIOS, OS, I/O port...)

2) Modem (modulating and demodulating):

- Modem l thit b lm nhim v chuyn i tn hiu s (Digital) thnh tn hiu dng tng t (Analog)

- modulating.- Cho php d liu truyn qua ng dy in thoi.

- Bin i tn hiu t dng Analog tr li thnh tn hiu Digital.

3) CSU/DSU


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- CSU/DSU Channel Service Unit / Digital Service Unit: l thit b c giao din digital dng kt ni

gia router v tng i. Thng thng CSU/DSU c tch hp trong modem. Cng c khi CSU/DSU

l nhng thit b tch ri nhau v c lp t ti cc im thu v pht tn hiu.

- TA - ISDN Terminal Adapter: dng kt ni giao din theo chun ISDN vi cc giao din khc .

4) WAN Switch:

- WAN Switch l thit b mng c nhiu cng, thng c lp t ti cc tng i truyn s liu.

- Lm nhim v chuyn cc gi d liu da trn a ch ch ca mi gi tin.

- C cc loi switch tiu biu nh Frame Relay, X.25 hoc Switched Multimegabit Data Service

(SMDS).

5) Access Server:

- Access Server l im tp trung tip nhn v x l cc kt ni dial-in v dial-out.


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III. Mt s cng ngh kt ni c bn dng cho WAN

1) Mng chuyn mch (Circuit Swiching Network)

Mng chuyn mch thc hin vic lin kt gia hai im nt qua mt ng ni tm thi hay

ginh ring gia im nt ny v im nt kia. ng ni ny c thit lp trong mng th hin di

dng cuc gi thng qua cc thit b chuyn mch.

Mt v d ca mng chuyn mch l hot ng ca mng in thoi, cc thu bao khi bit s ca

nhau c th gi cho nhau v c mt ng ni vt l tm thi c thit lp gia hai thu bao.

M hnh kt ni WAN dng mng chuyn mch


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Kt ni PSTN

Thit b:

Dng modem tng t loi truyn khng ng b, hay truyn ng b,

kt ni thit b mng vo mng in thoi cng cng.

Phng thc kt ni:Dng kt ni PPP t my trm hay t thit b nh tuyn qua modem, qua

mng in thoi cng cng.

M hnh kt ni dng mt ng in thoi

Kt ni ISDN

Dch v s ISDN - Intergrated Services Digital Network: ISDN l mt loi mng vin thng s

tch hp a dch v cho php s dng cng mt lc nhiu dch v trn cng mt ng dy in thoi

thng thng. Vi c s in thoi c nh h tng hin c, ISDN l gii php cho php truyn dn

thoi, d liu v hnh nh tc cao. Ngi dng cng mt lc c th truy

cp WAN v gi in thoi, fax m ch cn mt ng dy in thoi duy nht, thay v 3 ng nu

dng theo kiu thng thng. Kt ni ISDN c tc v cht lng cao hn hn dch v kt ni theo

kiu quay s qua mng in thoi thng (PSTN). Tc truy cp mng WAN c th ln n 128 Kbps

nu s dng ng ISDN 2 knh (2B+D) v tng ng 2.048 Mbps nu s dng ISDN 30 knh

(30B+D).

Mng knh thu ring (Leased lines Network)

Cch kt ni ph bin nht hin nay gia hai im c khong cch ln vn l Leased Line (tm

gi l ng thu bao).

M hnh kt ni WAN dng cc knh thu ring


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Cc cng ngh xDSL

Vic kt ni WAN c thc hin u tin dng modem tng t qua mng in thoi, n nay

phng thc ny ch dng li tc truyn ti rt thp, ti a l 56kbps/line , iu ny c cha

ca ngnh l thuyt thng tin Claude Shannon a ra gii hn dung lng cho knh truyn c nhiu

l 35 kbps v thc t t c 33.6kbps. Hn ch ca knh truyn in thoi vi tc t thng tintruyn s liu do i dy cp ng nh ngi ta ngh m l khi qua mch m ha PCM (Pulse Code

Modulation) dy tn truyn dn ch cho qua cc tn hiu t 300hz n 400hz. Sau ny Modem X2 ca

hng US Robotics v modem ca hng Rockwell c thng nht bi tiu chun V90 ca ITU-T (lin

minh vin thng quc t) nhm mc ch lch khi mch lc ny trong chiu t ISP v n ngi s

dng (downtream) t c tc 56kbps nhng tc chiu t ngi dng ln ISP (uptream) vn l

33.6kbps v y l tc cao nht c th t c ca modem. n nay ci tin thnh chun V92 thc

hin kt ni nhanh hn. Khng t c tc nh ng T1: 1544kbps hay E1: 2048 kbps. vc

qua ngng tc ngi ta chuyn sang dng k thut s xDSL. Trn ng dy in thoi th thc t

ch dng mt khong tn s rt nh t 0KHz n 20KHz truyn d liu m thanh (in thoi). Cng

ngh DSL tn dng c im ny truyn d liu trn cng ng dy, nhng tn s 25.875 KHz

n 1.104 MHz .

2) Mng chuyn gi (Packet Switching Network)

Mng chuyn mch gi hot ng theo nguyn tc sau : Khi mt trm trn mng cn gi d liu

n cn phi ng d liu thnh tng gi tin, cc gi tin c i trn mng t nt ny ti nt khc ti

khi n c ch. Do vic s dng k thut trn nn khi mt trm khng gi tin th mi ti nguyn ca

mng s dnh cho cc trm khc, do vy mng tit kim c cc ti nguyn v c th s dng chng

mt cch tt nht.

Ngi ta chia cc phng thc chuyn mch gi ra lm 2 phng thc:

Phng thc chuyn mch gi theo s ri rc.

Phng thc chuyn mch gi theo ng i xc nh.

M hnh kt ni WAN dng chuyn mch gi


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Kt ni dng ATM

Mng ATM (Cell relay), hin nay k thut Cell Relay da trn phng thc truyn thng khng

ng b (ATM) c th cho php thng lng hng trm Mbps. n v d liu dng trong ATM c

gi l t bo (cell). cc t bo trong ATM c di c nh l 53 bytes, trong 5 bytes dnh cho phn

cha thng tin iu khin (cell header) v 48 bytes cha d liu ca tngtrn.

Trong k thut ATM, cc t bo cha cc kiu d liu khc nhau c ghp knh ti mt ng

dn chung c gi l ng dn o (virtual path). Trong ng dn o c th gm nhiu knh o

(virtual chanell) khc nhau, mi knh o c s dng bi mt ng dng no ti mt thi im.

ATM kt hp nhng c tnh tt nht ca dng chuyn mch lin tc v dng chuyn mch

gi, n c th kt hp di thng linh hot v kh nng chuyn tip cao tc v c kh nng qun l ng

thi d liu s, ting ni, hnh nh v multimedia tng tc. Mc tiu ca k thut ATM l nhm cung

cp mt mng dn knh, v chuyn mch tc cao, tr nh dp ng cho cc dng truyn thng a

phng tin (multimedia).

Chuyn mch cell cn thit cho vic cung cp cc kt ni i hi bng thng cao, tnh trng tt

nghn thp, h tr cho lp dch v tch hp lu thng d liu m thanh hnh nh. c tnh tc cao l

c tnh ni bt nht ca ATM.

Kt ni dng mng Frame Relay

Frame relay - mng chuyn mch khung:

Bc sang thp k 80 v u thp k 90, cng ngh truyn thng c nhng bc tin nhy vt

c bit l ch to v s dng cp quang vo mng truyn dn to nn cht lng thng tin rt cao. Vic

x dng th tc hi p X25 thc hin truyn s liu trn mng cp quang lun t c cht lng

rt cao, v v th khung truyn t 128 byte cho X25 c m rng vi khung ln hn, th l cng ngh

Frame Relay ra i. Frame relay c th chuyn nhn cc khung ln ti 4096 byte, v khng cn thi

gian cho vic hi p, pht hin li v sa li lp 3 (No protocol at Network layer) nn Frame Relay

c kh nng chuyn ti nhanh hn hng chc ln so vi X25 cng tc . Frame Relay rt thch hp

cho truyn s liu tc cao v cho kt ni LAN to LAN v c cho m thanh, nhng iu kin tin

quyt s dng cng ngh Frame relay l cht lng mng truyn dn phi cao.


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M hnh kt ni WAN dng mng Frame relay

Kt ni dng chun X.25

Mng X25 c CCITT cng b ln u tin vo 1970, lc lnh vc vin thng ln u tin

tham gia vo th gii truyn d liu vi cc c tnh: X25 cung cp quy trnh kim sot lung gia cc

u cui em li cht lng ng truyn cao cho d cht lng mng li ng dy truyn thng

khng cao. X25 c thit k cho c truyn thng chuyn mch ln truyn thng kiu im ni im.

c quan tm v trin khai nhanh chng trn ton cu.

Trong X25 c chc nng dn knh (multiplexing) i vi lin kt logic (virtual circuits) ch lm nhim

v kim sot li cho cc frame i qua. iu ny lm tng phc tp trong vic phi hp cc th tc

gia hai tng k nhau, dn n thng lng b hn ch do tng ph x l mi gi tin tng ln. X25 kim

tra li ti mi nt trc khi truyn tip, iu ny lm hn ch tc trn ng truyn c cht lng rt

cao nh mng cp quang . Tuy nhin do vy khi lng tch ton ti mi nt kh ln, i vi nhng

ng truyn ca nhng nm 1970 th iu l cn thit nhng hin nay khi k thut truyn dn t

c nhng tin b rt cao th vic tr nn lng ph. Do vy cng ngh X25 nhanh chng tr thnh

lc hu. Hin nay khng cn ph hp vi cng ngh truyn s liu.


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Kt ni WAN dng mng X25

3)Kt ni WAN dng VPN

VPN (Virtual Private Network) l mt mng ring c xy dng trn nn tng h tng mng

cng cng (nh l mng Internet). Mng IP ring (VPN) l mt dch v mng c th dng cho cc ng

dng khc nhau, cho php vic trao i thng tin mt cch an ton vi nhiu la chn kt ni. Dch v

ny cho php cc t chc xy dng h thng mng WAN ring c quy m ln ti Vit Nam.

Gii php VPN cho php ngi s dng lm vic ti nh hoc ang i cng tc xa c th thc

hin mt kt ni ti tr s chnh ca mnh, bng vic s dng h tng mng thng qua vic to lp mt

kt ni ni ht ti mt ISP. Khi , mt kt ni VPN s c thit lp gia ngi dng vi mng trung

tm ca h. Kt ni VPN cng cho php cc t chc kt ni lin mng gia cc NOC ca h

t ti cc a im khc nhau thng qua cc kt ni trc tip (Leased line) t cc a im ti mt

ISP. iu gip gim chi ph gi ng di qua dial-up v chi ph thu ng Leased line cho khong

cch xa. D liu chuyn i c m bo an ton v cc gi d liu truyn thng trn mng c m

ho.

M hnh WAN dng VPN ni POP vi NOC


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Gioi Thieu Mang WAN