tìm hiểu về giao thức chuyển giao

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Tìm Hiểu Về Giao Thức Chuyển Giao

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Tm hiu v giao thc chuyn giao (handover procedure)

Tm hiu v giao thc chuyn giao (handover procedure)

1. Handover l g?

V d trong mng t bo, mi mt trm pht sng BTS c mt vng ph gii hn (vi km). Khi bn ngi trn t/ xe bus v in thoi cho bn gi, th chc chn bn khng th ni kt vi cng 1 trm BTS c. Khi bn i ra khi vng ph sng ca mt trm BTS A v i vo vng ph ca mt trm BTS B, lc bn s kt ni vi trm B. D nhin l cuc gi vn din ra bnh thng. Qu trnh chuyn i kt ni t mt trm pht sng ny sang mt trm pht sng khc c gi chung l chuyn giao (handover).

Trong hu ht cc loi hnh mng khng dy: t bo, wifi, wimax, DVB-H..., u c tnh n khi nim chuyn giao. Ch l khi nim chuyn giao lun gn lin vi vic c communication ang din ra.

2. Phn loi cc loi hnh chuyn giao (HO).

Thc t c nhiu khi nim khc nhau lin quan n "Handover" (HO).

- Trong mng t bo, ngi ta phn bit ra Hard HO, Soft HO (khi nim ny khng c GSM) v intersystem HO (inter-RAT).- Trong IETF/IEEE th hay nhc n cc khi nim Fast HO, smooth HO, horizontal HO, vertical HO, intra/inter-technology (system) HO, L2 HO, L3 HO ... Cc bn c th tham kho cc terminology y http://www3.tools.ietf.org/html/draf...amoby-terms-00- Trong WiMAX (IEEE) th c khi nim Hard HO, Fast BS Switching (FBSS) v Macro diversity HO.

Tm li, theo mnh th l th ny.

- Trc ht gia Soft HO v Hard HO, ci khc nhau c bn l new connection (ni kt mng n) c thit lp trc hay sau khi old connection (ni kt hin ti) chm dt. V th i khi bn nghe ni n make-before-break (tc l SOFT) v break-before-make (gi l HARD) HO. Khi nim Hard v soft l c lp (independent of) vi technology v operator domain.

- Vi cch phn bit nh trn th inter-system HO c th l Hard cng c th l Soft. System y khng hn ch l khc nhau v technology (RAT) m c th khc nhau v administrative domain (gia cc operator/owners khc nhau, tuy nhin cng nn phn bit vi khc nim roaming).

- V khi nim horizontal v vertical HO. Horizontal y tc l HO gia cng 1 system, cng 1 technology. Cn Vertical l ch HO gia 2 technology khc nhau (c th thuc cng hoc khng cng thuc 1 owner). VD: Handover gia WLAN v UMTS l vertical.

3. Cc bc c bn trong mt qu trnh chuyn giao

Chuyn giao ngang (horizontal HO) trong mng t bo c th phn ra thnh 3 bc chnh nh sau:

1. HO measurement (o c/theo di cht lng ng truyn radio)2. HO decision (ra quyt nh chuyn giao)3. HO Execution (tin hnh chuyn giao)

C th tng bc nh sau:

Bc 1: Trong sut qu trnh lin lc, thit b cm tay (gi nm na l UE (user equipment)) s lun o nng lng thu c t BTS ang ni kt v ca c cc BTS xung quanh n. UE thng xuyn gi bo co v tnh hnh o c ny v trm pht sng (Node B trong UMTS/BTS trong GSM) v thit b qun l radio (RNC trong UMTS/BSC trong GSM).

Bc 2. Nu cht lng ng truyn t trp pht hin ti kt ni n UE b gim mnh di mt mc no , th mng s quyt nh tin hnh HO. Mng s chn ra mt ci BTS ln cn m cho cht lng ng truyn tt nht UE ni kt vo.

Bc 3. UE thit lp ni kt vi trm mi v ti nguyn dnh ring cho UE trm c s c gii phng. Ty theo soft/hard HO m cc qu trnh ny s c thc hin mt cch khc nhau. 1. Trc ht l lm th no o c cht lng ng truyn cng nh cc thng tin lin quan ca mt BS ln cn trong khi UE vn ang lin lc vi BS n ang kt ni? Cc bn ng tng l vn ny n gin! Nu BS ln cn hot ng trong cng 1 tn s vi BS bn ang ni kt th bn c th "o" c nng lng pht ra t n. Nu by gi BS ln cn khng cng tn s, v d nh trong UMTS inter-frequency BS. Nu UMTS hot ng mode FDD th UE phi dng "compressed mode" dng lin lc ang din ra v shift n tn s ca BS ln cn c th o c nng lng pht ra t BS ln cn. Nu BS ln cn li thuc mt cng ngh khc (v d bn ang kt ni vi GSM m mun chuyn giao sang WLAN). Nu UE c 2 radio interface, 1 dng lin lc vi GSM, mt ci khc dng d thng tin WiFi. Vn c v khng phc tp lm. Nu UE ch c 1 radio interface m c kh nng t cu hnh (SDR) li hot ng c 2 mode, th sao?

2. Trong trng hp VHO, bn cnh thng tin v RSS bn cn cn cc thng tin khc v d "cost" ca tng BS ln cn (gi bn phi tr nu thc hin cuc gi thng qua BS y/ mt operator khc). V cn nhiu thng tin khc . Liu BS c broadcast thng tin y khng? Cc bn quan tm c th xem chun IEEE 802.21 Media Independent Handover...

3. Khi no th UE cn thit phi thc hin chuyn giao? Vn khng hn n gin l khi UE sp mt connection vi BS ang kt ni (serving BS). C th UE thc hin chuyn giao sang mt BS c cht lng tt hn/ gi r hn ngay khi n vn ang kt ni vi serving BS....

4. Ai s ra quyt nh trong qu trnh chuyn giao? UE hay BS?

5. Mt khi c tt c thng tin cn thit thc hin chn la BS ln cn tt nht. Tt nht theo ngha no? Tt nht i vis UE A s khng ging vi tt nht ca UE B???

6. Mt cu hi kh l liu BS ln cn c m bo c QoS (Quality of Service) m UE yu cu (expect) ko?

7. Lm th no UE, serving BS v target BS (c chn trong s cc BS ln cn) trao i tay ba vi nhau thc hin qu trnh authentication?

8. Nu l VHO th vic tnh tin (billing) s phn chia v gii quyt th no?

9. Trong qu trnh thc hin chuyn giao, cuc gi ang din ra khng c gin on qu lu: low handover latency + less packet loss. Qu trnh HO ny phi khng c nhn bit bi ngi dng (transparency).

10. v nhiu vn khc na