CHNG I

H THNG MNG XBEE

2.1 Tng quan v mng XBee

2.1.1 Th trng m XBee nhm ti

Chun mng XBee wireless ph hp vi nhu cu th trng m cc k thut wireless

khc khng th p ng (xem Hnh 2.1-1).

Cc giao thc wireless khc XBee

Tc Truyn nhanh Truyn data thp

Tnh nng Nhiu tnh nng Nhm ti mt tiny stack

Nhu cu Media nt cao iu khin thit b

Thi gian s dng Pin Vi gi, vi ngy Hng nm

Vng i sn phm 1 n 2 nm Hng thp k

Tnh bo mt D b xm nhp Bo mt cao

Bng 2.1-1: So snh xu hng ng dng gia cc giao thc wireless khc v XBee.

Trong khi hu ht cc chun wireless hng ti tc nhanh hn th XBee nhm ti tc

truyn data thp. Trong khi cc giao thc wireless khc thm cng nhiu tnh nng th

XBee nhm ti mt tiny stack m ph hp vi cc vi iu khin 8-bit. Cc k thut wireless

khc hng ti cung cp truyn data ti Internet hay phn phi dng media nt cao (high-

definition) th XBee hng ti iu khin n hoc gi d liu nhit t cc cm bin. V

thit k chy trong vi gi hoc c th vi ngy bng pin th XBee chy ti hng nm. Mt

iu ng ch na l cc k thut wireless khc cung cp 12 n 24 thng vng i cho

mt sn phm th cc sn phm XBee c th dng trong hng thp k hoc hn trong cc ng

dng c trng.

Cc dch v m XBee hng ti l vic kt ni cm bin khng dy v iu khin hay

n gin l iu khin khng dy.

Hnh 2.1-1: So snh cc k thut khng dy

Th trng iu khin khng dy c nhiu yu cu m ch c XBee mi ph hp:

- tin cy cao

- Chi ph thp

- Nng lng cn rt thp

- Bo mt cao

- Mt chun m

c c nng lng tiu th thp v chi ph thp, XBee thm mt rng buc k

thut l tc truyn thp.

2.1.2 XBee l g?

XBee c tin cy cao.

S truyn thng khng dy c nh gi l khng c tin cy cao. C th dn

chng iu ny bng vic s dng in thoi di ng: bt c ai s dng in thoi u gp s

c cuc gi b ngt hoc ng truyn yu. Tn hiu chy qua cc vt cn, c th b chn bi

kim loi, nc hoc khi b tng v ph thuc vo nhiu yu t phc tp nh thit k ng-

ten, s khuch i nng lng, v thm ch l cc iu kin thi tit.

Tuy nhin, iu khin khng dy thng khng c cng vn nh trong s dng

thoi, vic di chuyn tm im nhn sng tt hn hay vic i c quay li sau. Hip hi

XBee hiu iu ny, v vy s c t XBee th hin iu ny. XBee c nh gi c tin

cy cao trong nhiu cch kt ni:

- IEEE 802.15.4 vi O-QPSK v DSSS

- CSMA-CA

- 16-bit CRCs

- C bit bo nhn ti mi chng (hop)

- Vic ni mng li (mesh) tm ra ng i tin cy

- C phn hi khi kt ni im - im kim tra d liu n ch

iu u tin l da vo mt k thut wireless rt tin cy, khong cch thp, s c t

IEEE 802.15.4. c t ny l mt k thut mnh, rt hin i c xy dng trn 40 nm

kinh nghim ca IEEE. N dng nhng k thut c gi l Offset-Quadrature Phase-Shift

Keying (O-QPSK) v Direct Sequence Spread Spectrum (DSSS), mt s kt hp ca cc k

thut c hiu sut lm vic tuyt vi trong cc mi trng t l tn hiu truyn nhiu (signal-

to-noise) thp.

XBee dng Carrier Sense Multiple Access Collision Avoidance (CSMA-CA)

tng kh nng tin cy. Trc khi truyn, XBee kim tra knh truyn. Khi knh truyn trng,

XBee bt u truyn. iu ny ngn cc sng khi vic gy ra xung t d liu. CSMA-CA

tng t nhng g con ngi lm trong cc hi thoi. Chng ta ch ngi khc ni xong mi

ni.

XBee dng 16-bit CRC trn mi gi d liu (packet), c gi l mt Fame

Checksum (FCS). iu ny m bo cc bit d liu chnh xc.

Mi gi d liu c th li 3 ln trong ton b 4 ln truyn. Nu c gi khng th

truyn qua sau ln truyn th t th XBee thng bo cho node gi tn hiu bo li cho ln

truyn ny.

Mt cch khc m XBee c c kh nng tin cy l kt ni mng li. Mng li c

bn cung cp 3 kh nng tng cng cho mt mng wireless: m rng khong cch bng

nhiu chng (multi-hop), vic to mng ad-hoc v quan trng l kh tm ng i t ng v

t phc hi.

Vi mng li, d liu t node u tin c th n bt c node no khc trong mng

XBee, nh gi khong cch bng cc sng gi tn hiu (xem Hnh 2.1-1).

Hnh 2.1-1: Kt ni mng li trong XBee

Node 1 mun giao tip vi node 3, nhng n ra khi vng ph sng ca node 3. XBee t

ng tm ra ng tt nht v node 1 s gi thng tin cho node 2 ri truyn tip n node 3.

By gi gi s rng, c vi th xy ra n ng i ny. C th node 2 hon ton b loi

b hoc cht hay vi vt cn nh mt bc tng b-tng hoc mt thng nc ln. iu ny

khng h g vi Xbee, n s t ng pht hin s tht bi ca ng i v i vng (xem Hnh

2.1-2).

Hnh 2.1-2: Vic tm li ng i trong mng li XBee

Thm vo mng li, XBee cung cp vic truyn tn hiu mt cch tin cy, mt k thut

cho vic phn phi mt tn hiu n nhiu node trong mng. XBee cng cung cp nhiu

ng truyn c th gi mt tn hiu n bt k cc nhm node. V nh mt k thut tm

ng h tr, XBee c kh nng tm ng trong mng cy (tree routing) tng mng li

XBee trong cc h thng gii hn RAM. XBee cng cung cp thng bo t ng t kt ni.

ng dng c th bit mt gi c th c nhn bi node khc hay khng. Vi tt c cc th

li, XBee lc ra cc gi trng, nn ng dng khng cn bn tm.

XBee c chi ph thp

C nhiu nh cung cp stack v silicon, cc module XBee v nhiu ti nguyn phn

phi vi chi ph thp cho cc thit b XBee.

XBee s dng nng lng thp

Cc thit b trong mt mng XBee c th chy trong nhiu nm ch cn 1 cp pin AA

ph thuc ng dng.

Hnh 2.1-3: Thi gian s dng Pin ca XBee

XBee bo mt cao

Cho vic bo mt mng, XBee dng National Institute of Standards and Technology

(NIST) Advanced Encryption Standard (AES). Chun AES-128 l mt khi m ha (block

cipher). N m ha v gii m cc gi d liu trong mt phng thc kh b kha. y l

mt trong nhng chun ni ting. Nguyn nhn m n c dng bi XBee l:

- Chun c xc thc quc t.

- Min ph.

- C th hin thc trn mt vi iu khin 8-bit.

XBee l mt chun m ton b

C nhiu nh cung cp XBee stack, silicon v cc gii php ng dng.

c t XBee c th c ti min ph t: http://www.Xbee.org

XBee c tc d liu thp

c c chi ph thp v nng lng tiu hao thp v vic xem xt khng gian v th

trng ng dng m XBee nhm ti, hip hi XBee quyt nh gi giao thc trong mt

mi trng tc truyn d liu thp.

XBee nm trn cc trm thu pht theo chun IEEE 802.15.4, trong khng gian 2.4GHz

truyn thng ti 250kbps, nhng do s ln lp li, s m ha, gii m v giao thc kt ni

mng li y c dng nn dung lng knh truyn thc s khong 25kbps.

Vic thu pht bn song cng cng l mt yu t gim dung lng knh truyn t 250 ti

25kbps.

Cc ng dng s dng XBee

- ng dng trong qun s an ninh.

- ng dng trong gim st xe c v cc thng tin lin quan.

- ng dng cho vic iu khin cc thit b trong nh.

- ng dng cho cc ta nh t ng.

- ng dng trong qu trnh qun l t ng trong cng nghip.

- ng dng trong y sinh hc.

Hnh 2.1-4: Cc mng th trng ca XBee

2.1.3 Kin trc mng c bn ca XBee

Kin trc mng XBee chia lm 3 phn chnh:

- ng dng

- XBee stack

- Radio

V c chia thnh cc lp, mi lp khng bit g v lp trn n. Lp trn c xem

nh mt master m gi yu cu cho slave bn di lm.

XBee khng ging m hnh mng OSI 7-layer, nhng n c vi thnh phn ging

gm PHY (physical), MAC (link layer), NWK (network). Cc lp 4-7 (transport, session,

presentation v application) c chuyn qua cc lp APS (APplication support) v ZDO

(XBee Device Object) trong m hnh XBee

Hnh 2.1-5: Kin trc ca XBee

Gia cc lp l Service Access Points (SAPs). SAPs cung cp API tch bit bn

trong lp khi cc lp trn v bn di. Ging nh c t IEEE 802.15.4, XBee dng 2 SAPs

cho mi lp, mt cho d liu v mt cho s qun l. V d, tt c cc s truyn thng d liu

n v t lp network i qua Network Layer Data Entity Service Access Point (NLDE-

SAP). Cc yu cu trong c t XBee ging nh APSDE-DATA.request. Mt yu cu gi d

liu ra nhng ch c khi to lp APS.

Hai lp thp nht, MAC v PHY c nh ngha bi c t IEEE 802.15.4. Lp

PHY n gin dch cc gi thnh cc bit OTA v ngc li. Lp MAC cung cp khi nim

ca mt network, gm mt PAN ID v kt ni thng qua cc tn hiu yu cu v phn hi. N

cng cung cp cc lnh cho mi chng v mt vi lnh cho vic tham gia v to mt mng.

Lp MAC khng c nhiu chng hay li.

Lp NWK c trch nhim cho hnh thnh mng li, gm vic truyn pht cc gi tn

hiu qua mng, xc nh cc ng i cho cc gi a chiu v m bo chng c gi mt

cch tin cy t mt node n node khc. Lp network cng c mt tp cc lnh cho mc ch

bo mt, gm bo mt tham gia v ti tham gia mng. Tt c cc mng XBee c bo mt

lp NWK, v ton b tn hiu i qua ca khung NWK c m ha.

Lp APS c nhim v cho ng dng. N hot ng nh mt b lc cho ng dng

chy pha trn n. Cn cc im kt thc (endpoints) n gin l logic trong cc ng dng

ny. Lp APS cng lc cc tn hiu trng m hon ton c gi ln bi lp NWK. Lp APS

gi mt bng lin kt cc b, mt bng ch cc nodes hoc cc nhm nodes trong mng li

m node mun giao tip n.

Lp ZDO (bao gm XBee Device Profile, ZDP) c trch nhim cho qun l cc b v

ton khng gian ca mng. N cung cp cc dch v khm ph cc nodes khc v cc dch

v trong mng, v c trch nhim trc tip cho trng thi hin ti ca node trn mng.

Cc dch v bo mt c dng bi nhiu lp, v c th c dng bi cc lp ZDO,

APS or NWK, do n nm cnh nhau (xem Hnh 2.1-5).

2.2 Network Layer

2.2.1 XBee v IEEE 802.15.4

Tn XBee v 802.15.4 thng c dng qua li ln nhau nhng n khng ging nhau.

c t 802.15.4 c to v h tr bi IEEE. c t ny nh ngh cc lp vt l v MAC

cho mt mng khng dy, c nhn, nng lng thp.

IEEE 802.15.4 nh ngha:

- Cc c ch cho vic tm mng

- Cc c ch cho vic thit lp v kt ni vo mng

- Cc cc c ch cho thay i knh truyn

- Cc c ch vic pht hin vt cn v nhiu trn mt knh

- Phng php phn phi gi d liu, cht chng n, ACK, vic dng CSMA-CA

trnh ng

- Phng php truyn pht d liu, cht chng n, khng c ACK

IEEE 802.15.4 khng nh ngha v truyn thng a im, gn a ch hay kh nng

tng tc vi mc ng dng.

Nu mng xy dng ch vi mt cht chng th 802.15.4 MAC/PHY c th lm tt c

nhng g cn. Cc nh cung cp h tr mt mi trng ng dng 802.15.4 m khng yu cu

XBee.

Ngoi XBee, c nhiu h thng protocol hon ton c xy dng trn tiu chun

802.15.4, mt s l dng li hay mng hnh sao.

Nhng XBee l protocol chnh m c xy dng trn tiu chun 802.15.4, thm vo

mt lp network c kh nng ca mng li, peer-to-peer; mt lp bo mt c kh nng x l

cc trng hp bo mt phc tp v mt lp ng dng cho cc ng dng c th tng thch

vi nhau.

Trong s kin trc XBee chun, cc lp MAC v PHY th hin di IEEE, trong

khi phn cn li th hin di XBee.

Cng vic ca lp MAC l chuyn cc gi t chui byte thnh ph RF v ngc li.

Lp MAC cho php mt mng c thit lp, cc knh c chia s, v d liu c truyn

mt cch thc tinh cy, hp l.

XBee c t tt c cc lp trn MAC v PHY, gm NWK, APS, ZDO v cc lp bo

mt. XBee cung cp mng li, tng cng kh nng tin cy ca phn phi gi d liu v c

t tng thch gia cc ng dng.

XBee khng dng tt c s c t 802.15.4 MAC/PHYm ch dung mt phn nh.

iu ny cho php cc nh cung cp stack c cc gii php nh hn (dng t RAM v flash)

bng cch cung cp mt lp MAC gii hn cho XBee stack ca h.

XBee cng c mt s iu chnh trong 802.15.4 chun. Mt trong s l m hnh

bo mt. MAC nh ngha l CCM, c vit tt t counter-mode cipher-block chaining-

message authentication code. CCM yu cu bo mt khc nhau cho mi lp. Do rng buc

hiu sut trn cc vi x l nh, XBee khng lm vy. M hnh bo mt XBee cng c gi

l CCM (mt iu chnh nh ca bo mt MAC CCM).

Mt trong cc phn th v hn m XBee khc vi s c t 802.15.4 l time-out

cho cc tn hiu phn hi.

Trong cc mng vi hn 30 nodes trong cng vng nghe, cc time-out 802.15.4 mc

nh cho cc phn hi cc tn hiu yu cu khng cho php thi gian cho tt c cc node

phn hi. c t 802.15.4 khng c xy dng vi cc mng ln nhng XBee th c th.

c t 802.15.4 MAC hon ton n nh t thng 11 nm 2003. Mt s nh cung cp chip

thm ch cung cp 802.15.4 MAC trong ROM. Nhng IEEE vn cha dng ti . Nm

2006, IEEE ra mt c t 802.15.4 khc c gi l 802.15.4-2006.

Thay i ln nht trong IEEE 802.15.4-2006 l mt PHY tt hn cho cc di 1GHz.

Trong c t 802.15.4-2003, 868MHz v 900 MHz b gii hn 20kbps v 40kbps. Tc

truyn d liu ti tn s di 1GHz qu chm cho XBee; cho XBee l 2.4GHz, hot ng vi

250kbps. IEEE 802.15.4-2006 thay i tt c. c t ny thm 2 PHY ty chn mi

cho tn s di 1GHz cho php truyn ln ti 250kbps.

2.2.2 Thit lp, kt ni v kt ni li cc mng XBee

Trc khi bt c cc node XBee c th giao tip trn mt mng, n phi thit lp mt

mng mi hay kt ni vo mt mng ang tn ti. Ch XBee Coordinator c th thit lp mt

mng. Ch XBee Routers v XBee End-Devices c th kt ni vo mng. Nhiu nh cung cp

stack cung cp kh nng mt node c ch nh nh mt ZC, ZRm ZED ti lc bin dch

(tit kim code v RAM) hay ti lc chy (gim cc phn OEM-manufactured)

Mi node bt u vi mt a ch 64-bit IEEE (MAC), c gn bi OEM trong sn

xut. Trong qu trnh kt ni mng, mi node c gn mt a ch 16-bit duy nht

(NwkAddr) dng khi giao tip vi cc node khc qua mng.

Thit lp mng

XBee Coordinator thit lp mng. Qu trnh thit lp mt mng l xc nh mt nh

danh duy nht cho mng, c gi l PAN ID v chn mt trong 16 knh 802.15.4 (11-16)

iu hnh mng.

Trong sut qu trnh thit lp mng, mt gi n c gi over-the-air trn mi knh:

mt MAC qut t ng.

Mt XBee Coordinator c nhim v sau:

- Thit lp mng

- Thit lp knh 802.15.4 trn mng s hot ng

- Thit lp PAN ID ngn v di cho mng

- Quyt nh stack profile dng

- Hot ng nh Trust Center cho cc ng dng bo mt v mng

- Hot ng nh ngi ng gia cho vic End-Device-Bind

- Hot ng nh mt router trong li routing

- Hot ng nh l gc ca mng hnh cy, nu n c s dng

XBee Coordinator thc s ch l mt router nu h thng mng khng c y thc.

V c nhiu cch thay th XBee Coordinator sau khi mt h thng mng chy nu thit b

ZC trc trc do mt s nguyn nhn.

ng dng chy trn mt node XBee Coordinator thc s quyt nh khi no thit

lp mt h thng mng, t mt tp cc knh v t tp cc PAN ID. ng dng trn ZC c th

l bt c th g: mt gateway c kt ni vi Internet, mt hp iu khin, mt b n nhit,

n hay cng t in. Cc kh nng l v hn. Khi nng lng c cp n thit b m cha

XBee Coordinator, n c th tc khc hnh thnh mt mng hay c th i mt vi s kin

trc khi to mng. N thm ch kim tra xem nhng mng no sn sng v quyt

nh tr thnh XBee Router hn l mt Coordinator, nu mt node khc hon ton to

mng mong mun. Tuy nhin ng dng c lp trnh, mt XBee Coordinator s to mt

mng.

NLME-NETWORK-FORMATION c khi to bi ZDO to mt mng bi ng

dng. Trong Z-stack:

NLME_NetworkFormationRequest();

Tip theo XBee gi lp MAC th hin 2 hot ng: energy scan v active scan.

Energy scan c dng xc nh knh no l knh tnh nht t tp cc knh c c th

trong bin thng tin c bn APS, apsChannelMask. Energy scan mt 0.5 giy cho mi knh.

Knh c th thc s nhiu trong 1 gi trc v qu trnh ny khng pht hin ra iu ny.

Vic qut tt c 16 knh mt khong 8 giy.

Tip theo l active scan, n gin mt MAC tn hiu yu cu c tr v khng hoc

hn cc tn hiu phn hi, c dng tm mng khc trong vng. Active scan m bo

XBee khng to mt mng c cng PAN ID. Hot ng qut c th mt thi gian.

Hnh 2.2-1: Qu trnh XBee to mng

Tham gia mng

XBee Router v XBee End-Device tham gia mng. XBee Router thng c cp

ngun chnh, lun bt, lng nghe cc gi tm ng. ZED thng dng c 2 trng thi

dng ngun Pin v ng, n ch hot ng giao tip mt cch nhanh gn trc khi tr v

sleep.

XBee Router c nhim v:

- Tm v tham gia mng

- Duy tr cc trm pht sng thng qua mng

- Tham gia vic tm ng, gm khm ph v duy tr ng i

- Cho php cc thit b khc tham gia mng

- Lu cc gi thay cho cc children ang sleep

XBee End-Device c nhim v:

- Tm v tham gia mng

- Kim tra ln lt cc node cha ca n xem c bt k cc tn hiu c gi ti

chng khi chng sleep hay khng

- Tm mt node cha mi nu kt ni ti node cha c b mt (NWK rejoin)

- Sleep hu ht thi gian tit kim Pin

Vic tham gia mng l mt qu trnh ca tm mng v node no trong vng v sau

chn mt trong chng tham gia. S kt hp c cung cp c th chp nhn bi network,

vic tham gia hon thnh v node tham gia s c 1 a ch trn mng.

Qu trnh tham gia s dng tn hiu yu cu. Bt k ZCs v ZRs trong vng tr li

bng tn hiu tr v. Cc ZCs v ZRs c cng knh vi node mun tham gia, v khng phn

bit PAN ID.

Cc tn hiu tr v cha kh nhiu thng tin v mng XBee, gm PAN ID, PAN ID

m rng, s cho php tham gia v node c kh nng cho router hay end-device tham

gia hay khng. Cho iu ny, mt node ang tm kim u tin phi tham gia mng, tm kim

ng dng ph hp, nu khng thy th ri mng v th cc mng khc.

ZR v ZED tham gia mt node c th, khng phi mt mng, dng 64-bit MAC cho

a ch ch v ngun ca MAC yu cu ca h thng. Node thc hin vic tham gia c gi

l node con. Node nhn yu cu c gi l node cha.

ZR v ZC c th l node cha khc nhng ZED lun l node con. Mi quan h cha/con

khng l g trong mng kiu li. Bt k cc routers no cng c th tm ng qua bt k

router khc trong vng nghe trn cng mng. Tt c cc routers l ngang hng. Nu mt node

cha hay con ca router ra khi vng nghe (thm ch ri mng) khng tc ng ti cc ng

i, ng i khc s ti cc thit b c th ny. XBee Router khng tm ng thng qua cc

mng khc, ch trong cng PAN ID v knh.

Tuy nhin, i vi ZEDs, mi quan h node cha/con rt c bit. ZEDs trong khi

chng c th giao tip vi cc node khc trong mng, chng ch giao tip trc tip vi node

cha. Hop k tip ca ZED lun l node cha ca n. Nu mt ZED mt kt ni vi node cha,

n phi tm node cha khc gi lin lc vi mng, c gi l tham gia lai (rejoin).

Khng thng xuyn ZED mt lin lc vi node cha. S giao tip 2.4GHz b nh hng bi

nc v lin kt b mt. Trong vi giy, ZED s tm node cha mi, thng bo n mng rng

n hon ton di chuyn v cc giao tip tip tc.

Hnh 2.2-2: Qu trnh XBee tham gia mng

Qu trnh tham gia ca ZR v ZED c m t trong c t XBee. u tin mt lnh

qut (beacon request) gi ra trn mi knh. ZC hoc ZED ch mt thi gian cho lnh tr v.

Thi gian c ci bi ng dng nhng mc nh l 0.5 giy trn knh. Khi cc tn hiu c

thu thp, chng c phn tch knh v PAN ID. Ngoi ra cn c lnh cho php tham gia

ngn cc node tham gia vo v cng c th c dng bt buc cc node c mt node cha

c th.

Sau khi active scan hon thnh v mt node cha ph hp c chn, qu trnh xc

thc bt u. Ch rng node c mt a ch trn mng trc thi gian xc thc bt u. S

xc thc ch c mng bo mt v a thng tin quyn t chi/cho php node tham gia. Mt

node gi mo m ch gi mo mt a ch trn PAN s khng nhn key mng v nn khng

th giao tip vi cc node khc. Nu xc thc khng hon thnh thnh cng, node cha s

thng bo vi node con cha c xc thc ri v nh du a ch c th dng bi node

khc mun tham gia.

Khi mt node hon ton tham gia mt mng, n c th giao tip vi bt k node khc

trong ton b mng. Khng c yu cu cho vic lin kt hoc cc c ch khc. n gin gi

d liu t node ny n node khc, min l bit a ch ngn ca node . Tt nhin, ng

dng mun thy gi th Application Profile phi ging nhau hai pha v im cui ca

ngun trn node gi v im ch trn node nhn phi c ng k vi ZDO.

2.2.3 Ti tham gia mng

Vic ti tham gia gi s node sn sng tham gia mng, c mt PAN ID, extended

PAN ID, security key, v a ch ngn. C nhiu nguyn nhn mt node cn phi ti tham gia

mng:

- Mt ZED mt lin lc vi node cha ca n

- Nng lng b thay th v nhiu hay tt c cc node trong mng ti tham

gia silently

- Tham gia mt mng bo mt nu permit-joining tt.

ZED lun giao tip trc tip vi node cha ca n. Nu node cha khng tr li, child

phi tm node cha mi gi giao tip trn mng. Node con t quyt khi no n mt node

cha. XBee khng c th s ln th kt ni li hay truyn tn hiu trc khi ZED cho rng n

khng th giao tip vi node cha.

Qu trnh ti tham gia bt u vi mt tn hiu yu cu tm node cha ph hp. N

ch quan tm ti kh nng cn v tr trong mng nhng vic cho php tham gia mng ph

thuc vo node cha. Sau khi yu cu tham gia mng, ZED ly mt node (cng PAN) lm

node cha, thc hin ti tham gia, nhn mt a ch ngn mi (ch trong stack profile 0x01) v

cui cng pht mt device-annouce ni cho mng rng node hon ton di chuyn. Bc

cui rt quan trng bo v rng buc trong mng.

Mt loi khc ca vic ti tham gia l silent rejoin. Silent rejoin khng c c t

trong XBee, nhng tt c cc nh cung cp stack c v n cn thit trong mt mng hin thc

vi bt k kch thc no. V d, tng tng rng nng lng b ti cung cp n tt c cc

router trong mt mng 1000 node. Khi nng lng c tr li, nu tt c cc node u c gng

tham gia (ti tham gia) mng cng mt lc th mng s tht bi v qu nhiu lu lng trong

mng cng lc. Nhng khi cc routers bit thng tin mng ca n (PAN ID, extended PAN

ID, NwkAddr, security key), chng t ng kt ni. Cc node XBee khng cn ni chuyn

duy tr trng thi mng. Khi mt in c th xem nh mng khng ni chuyn trong mt

lc. Khi in c tr li, mi router nhn ch trn PAN ID, extended PAN ID, NwkAddr,

m an ton chnh xc nh th mng khng bao gi tt. iu ny gi l silent rejion

Silent Rejion cng c dng khi mng chuyn n knh mi, mt c tnh mi trong

XBee 2007 v Pro. Silent rejoin ch lm vic nu cc node c mt vi loi lu tr vnh vin

(non-volatile memory).

Mt cch dng khc ca ti tham gia l dng NWK-Rejoin tham gia mt mng m

c cho php kt ni tt. iu ny thnh thong c dng nu qu trnh y nhim c network

key, PAN ID, sn sng lp trnh trong node. Qu trnh NWK-Rejoin s a thit b v

a ch trn mng v ZDP: DeviceAnnce s cho php tt c cc node trong mng bit n hon

ton tham gia.

2.2.4 Gn a ch XBee

nh a ch l quan trng trong mt mng. a ch mi node phi duy nht trong

XBee. XBee dng hai a ch duy nht trn mt node: a ch di (IEEE hay MAC address)

v a ch ngn (NwkAddr).

a ch di (64-bit), cng c gi l IEEE hay a ch MAC, c gn bi nh sn

xut thit b dng 802.15.4 (khng phi nh sn xut chip) v khng thay i trong qu trnh

s dng thit b. a ch di c nh ngha mt cch duy nht thit b vi tt c cc thit b

khc trn th gii.

a ch ngn (16-bit) c gn ti mt node ti thi im node tham gia mng.

Ch rng c MAC layer header v NWK layer header c c mt a ch ngun v

mt a ch ch. Nu a ch MAC 8-byte c dng, th s dng ti 32 byte ca gi 127

byte over-the-air. Do , XBee dng mt a ch mng 2-byte, gim cc trng ny n 8

bytes, cho php hn 24 bytes cho ng dng dng.

Ti sao phi dng cc trng a ch ny trong c MAC header v NWK header? Nu

vic gi gi t node A n node Z, cht chng u tin t A ti B, tip theo t B

ti C, cho n cht chng cui cng t Y ti Z. NwkSrc v NwkDst lun ch t A

ti Z, trong khi MacSrc v MacDst l a ch ti mi hop.

Khi ni v vic gn a ch XBee l lin quan ti a ch a ch ngn. a ch ngn

c mng XBee gn ti thi im mt node tham gia mng hoc thit lp mng v khng

lin quan ti a ch IEEE. XBee dng hai m hnh gn a ch ngn:

- Cskip

- Stochastic (random)

Gn a ch Stochastic, mt node tham gia vo mt mng chn ring cho mnh mt

a ch. Sau n gi mt thng bo rng buc n mng xem c node no khc c a

ch hay khng. Nu c node chn mt a ch khc, nu khng node gi a ch .

Trong stack profile 0x01, cc a ch c gn vi mt mi quan h node cha/con to

mt mng hnh cy i xng. M hnh gn a ch trong stack profile 0x01 dng mt s c

tnh ton cho mi depth (s hop t ZC), c gi l Cskip (Child skip).

Trong Cskip, ZC l node 0 (0x0000). Node tip theo tham gia mng s nhn mt a

ch t node cha. a ch m node ny c gn ph thuc vo node con l mt router, m c

th c cc node con ca ring n hay mt thit b.

Bng 2.2-1: Cskip c tnh ton cho stack profile 0x01

Cskip dng ba thng s xc nh vic a ch: maxDepth, maxChildren v

maxRouter. Vic dng cc thng s ny, Cskip c th xc nh mt cch ton hc nhng a

ch node con mi tham gia v nh th no tm ng mt gi theo mng hnh cy i

xng. Stack profile 0x01 dng gi tr maxDepth(5), maxChildren(20), v maxRouter(6), m

gii hn tng s node trong mng ti 31,101 nodes.

Khi nim c bn l: mng hnh cy chia thnh level 0 l ZC, level 1 l node con ca

n, level 2 l node con ca node con ca n Router u tin tham gia mng ZC nhn a

ch 0x0001. Router tip theo tham gia ZC nhn a ch (0x0001 + Cskip ti level ), nh vy

l 0x143e, bi v Cskip ti level 0 l 0x143d. S 0x143d, ln cho router ny v tt c cc

node con v node chu ca n ph ht mng hnh cy i xng.

ZED u tin tham gia ti level 0 nhn mt a ch sau tt c cc router, l 0x796f.

Cng thc: 1[ZC nhn 1 a ch] + 6[maxRouter] x 0x143d[Cskip ti level 0] = 0x796f

Khi nim ca Cskip c gii thch d hn vi mt mng dng cc thng s Cskip

nh, mc d n khng tng thch vi stack profile 0x01. Cc thng s l maxDepth(3),

maxChildren(5) v maxRouter(3). iu ny ch cho php 66 node trong mng. Cc thng s

ny ch gii thch tree i xng cho d hiu m thi.

Hnh 2.2-3: Gn a ch Cskip trong cy i xng

hiu mng hnh cy nh a ch, d nht bt u ti y ca mng hnh cy

(maxDepth). Xt node 24. Bi v n nm maxDepth, c ba nt t ZC, n khng th c bt

k node con, nn Cskip ti mc ny l 0. Node cha ca n l node 23. Node ny ti mc 3, c

hai nt t ZC, nn n c Cskip l 1. Mi node con ca n s dng mt a ch. Node 23 c

nm node con, nn a ch ca n (23) cng vi nm node con ca n (24-28) dng ht su

a ch. Do , Cskip ca node cha (node 22) ca n l 6. Node 22 v tt c cc node con v

chu dng tng cng 21 a ch: 1 + 3x6 + 2 = 21. y 2 l s ZED c th tham gia vo

node:

2 = maxChildren(5) - maxRouter(3) = maxChildren(2)

Do , Cskip ti mc 0, cho ZC trong mng ny l 21. Mi router child ti mc 0

dng 21 a ch cho chnh n v tt c cc nhnh ca n. Cc ZED khng bao gi c node

con, nn cc ZED dng hai a ch. Cng thc cho tng cc node cho php trong mng vi

cc thng s trn l: 1 + 3x21 + 2 = 66

Bng cch gi s mt mng hnh cy i xng, XBee c th bit, dng cc cng thc

ton hc n gin, a ch node l mt node con (bao gm node chu) hay khng. Nu n l

mt node con, gi tn hiu c gi n hop tip theo bn di, n mt router hoc a ch

chnh n. Nu a ch khng phi l mt node con th tn hiu c gi ti node cha. Node 22

bit rng bt k a ch t 23 n 42 l child. Cn li l khng phi nn n gi ti node cha.

Vn ca Cskip v nguyn nhn n khng c dng trong XBee Pro (stack profile

0x02) l n khng th co gin ngoi maxDepth(5), cho php ln nht 10 hops trong mng (2x

maxDepth). Nu mun nhiu chng ch thay i maxDepth n 6 v maxChildren(20) v

maxRouter(6) cho php 186,621 node, mt s khng th cha trong 16-bit a ch ngn.

Trong hnh Hnh , bao gm c kt ni mng hnh cy (ng en) v li (ng

t). XBee lun lun l mt mng li. Mng hnh cy c th c dng nh mt m hnh

tm ng backup nu li qu ti nhng khng thay th mng li.

2.2.5 Tm ng cho gi tn hiu trong XBee

XBee dng nhiu phng php cho vic tm ng cho cc packet t mt node n

node khc:

- Broadcast (t mt node n nhiu node)

- Li routing (unicast t mt node n node khc)

- Tree routing (unicast t mt node n node khc, ch trong stack profile 0x01)

- Source routing (unicast t mt node n node khc, ch trong stack profile 0x02)

Mi phng php c u im v nhc im

Bng 2.2-2: So snh cc phng tm ng trong XBee

Broadcast cho php mt node vn ti nhiu node khc vi ch mt yu cu. Phng

php ny khng cn ACK v cn nhiu ti nguyn.

Li routing l bng iu khin v rt hu dng (time, bandwidth, ti nguyn b

nh), khi ng i c thit lp. Packet c gi theo mng li c ACK, nn node gi bit

packet nhn hay cha.Tree routing, cng c ACK, ch c trong stack profile 0x01. N

c din t Cskip. Mng hnh cy c hiu qu bng thng nh mng li v hiu qu hn

v b nh. Nhng khi lin kt gia node cha v child t th n khng th phc hi. Vy nn

XBee dng li nh mc nh.

H thng ngun nh mng hnh li v mng hnh cy, c ACK, ch c trong stack

profile 0x02. c s dng chnh khi mt data concentrator (hoc gateway) cn

giao tip vi nhiu node. Vi li routing, mi ng i cn mt bng ng i c

th v cc node XBee khng RAM cho mt ngn ng i. Trong source routing,

mt node n (t hn) c nhiu RAM lu tt c cc ng i. ng i cho bt

k giao tip c th no c gi nh mt over-the-air packet. Hn ch ln nht l n

gii hn ln nht 5 hop