Hu ht nhng ngi mun mua in thoi ch phi ngh v mt cng ngh vin thng duy nht c tn Global System for Mobile Communications, hoc vit ngn li lGSM. Ci tn ny c ngha l "h thng ton cu dnh cho vic lin lc di ng", v ng nh vy, n c pht trin dng ti nhiu quc gia, nhiu nh mng khc nhau. Tuy nhin th gii lun c s a dng, khng phi hng vin thng no cng chn GSM, thay vo h s dng mt chun khc gi l Code Division Multiple Access, hay vit tt lCDMA. Hai chun ny tn ti song song, thm ch mt s ni cc nh mng CDMA vn cn rt mnh. Vy ti sao li c hai chun ny? Chng khc nhau ra sao? V chun no s l tng lai?1. GSM vs CDMACDMA s dng mt cng ngh gi l tri ph (spread spectrum) ti u ha vic s dng bng thng. N cho php nhiu b gi nhn tn hiu gi thng tin cng lc thng qua mt knh duy nht. Hay ni cch khc, ngi dng s chia s chung mt di tn s rng dng cho mc ch truyn ti d liu. Mi mt cuc gi s c gn cho mt "kha c nht" (key) trc khi truyn i, sau n s c gii m bi thit b nhn tn hiu tch thnh nhng cuc gi ring l. Qualcomm l hng u tin pht trin nn CDMA.Trong khi , GSM s dng cng ngh ph nhn (wedge spectrum) cung cp mt th gi l carrier (tm dch: sng mang). Carrier ny c chia thnh nhiu "khe thi gian" khc nhau, mi ngi dng s c gn cho mt khe v khe s khng b ai khc truy cp cho n khi cuc gi kt thc. Chun mng GSM s dng c hai phng thc phn chia theo thi gian (TDMA) v theo tn s (FDMA) phc v mc ch tch ring ngi dng v trm pht sng. TDMA s "ct" knh truyn ti thng tin thnh cc "ming" thi gian, cn FDMA th tch ring cc tn s trong di tn ca nh mng.S pht trin ca GSM bt u vo nm 1987, khi Chu u ra lut bt buc cc nh mng phi xi cng ngh ny. Chi ph cc cng ty xy dng nn h tng GSM cng t hn l CDMA.Hin nay Vit Nam tt c cc nh mng ln u vn hnh mng GSM, bao gm MobiFone, VinaPhone, Viettel, VietnamMobile. Trc y CDMA tng c mt nc ta thng qua mng S-Fone v CityPhone nhng gi hai nh mng ny khng cn xut hin na. Nu bn i du lch sang Chu u th tt c cc nh mng ti y l GSM, cn nu bn sang M th s c c hai chun ny (Verizon v Sprint dng CDMA, T-Mobile v AT&T dng GSM).2. in thoi GSM vs in thoi CDMAMt im ng ch trn thit b di ng ca chng ta l khe SIM. Vit Nam th khe SIM l chuyn bnh thng, bi chng ta dng mng GSM v cc nh mng s xi th SIM xc nh ngi dng. Trong khi , cc mng CDMA th s i chiu c mt chic in thoi vi danh sch thu bao ca h, chnh v l do ny m hu ht cc in thoi CDMA khng c khe SIM

Droid X, mt chic smartphone CDMA ca nh mng Verizon, khng c khe SIMCng trn mng CDMA, vic chuyn thit b s dng s kh khn hn bi bn phi lin h vi nh mng chuyn thng tin ti khon thu bao sang my mi. Trong khi , vi GSM, bn ch vic rt SIM ra ri gn vo thit b mi l xong, bn chng phi ng k g li c, tit kim c nhiu thi gian v cng sc hn.iu ng ni l hu ht nhng in thoi ca chng ta ch h trmt trong haichun GSM hoc CDMA m thi. Chnh v th, vic la chn in thoi s ph thuc rt nhiu vo nh mng m bn ng k gi cc v khu vc bn ang . V d, Vit Nam, bn s khng th dng mt chic in thoi ch h tr CDMA.Vi mt chic in thoi CDMA m c thm khe SIM th bi v chic in thoi h tr cng ngh4G LTE. Cng ngh i hi phi dng SIM nh l mt phn trong c t k thut. Ngoi ra, mt s nh mng nh Verizon cn mun khch hng ca mnh c th s dng c my khi i n cc quc gia GSM khc trn th gii du lch hoc cng tc. Tuy nhin, nhng nh mng ny vn s dng CDMA xc thc in thoi trnmng vin thngca mnh ti qu nh khi ngi dng gi in, nhn tin. Ch khi no xi d liu Internet th mi dng ng 4G LTE qua SIM.Khi mua in thoi GSM, bn cng nn n bng tn m mng ny h tr. C nhiu bng tn ko di t 380MHz n 1900MHz. thi im hin nay, hu ht cc in thoi u h tr 4 tn s chnh l 850MHz, 900MHz, 1800MHz v 1900MHz, cng l nhng tn s h tr bi hu ht cc nh mng GSM trn ton cu.3. 3G GSM (UMTS) v 3G CDMACc mng 3G CDMA (cn c n vi ci tn "EV-DO" hay "Evolution Data Optimized") khng th thc hin cuc gi v gi nhn d liu Internet cng lc. Ni cch khc, khi bn ang gi in thoi th bn s khng th check mail hay cp nht trng thi Facebook c. Mng CDMA c mt chun khc gi l "SV-DO" hay "Simultaneous Voice and Data Optimization" h tr tnh nng ni trn, tuy nhin khng phi nh mng CDMA no cng dng SV-DO.

Palm Pre, mt trong s cc sn phm ni ting c h tr EV-DONgc li, tt c mi mng 3G GSM u h tr gi nhn ting v d liu cng lc, bi v iu c quy nh trong cu hnh ca mng 3G GSM ri. (Thc cht th 3G GSM cng l mt loi CDMA y. Cc bn bnh tnh, mnh s gii thch ngay bn di).Vy ti sao nhiu nh mng li chn CDMA? Tt c u xoay quanh vn thi gian. Khi cng ty tin nhim ca Verizon v Sprint chuyn t cng ngh analog sang k thut s vo nm 1995 v 1996, CDMA ang l cng ngh mi nht, hot nht v nhanh nht. N mng li cng sut hot ng cao hn, cht lng cuc gi tt hn v c nhiu tim nng hn l mng GSM thi by gi. Sau ny GSM bt kp, th nhng lc mt s nh mng chn xong con ng i ca mnh.Vn c kh nng chuyn i mt mng t CDMA sang GSM. Hai nh mng Bell v Telus ca Canada lm iu khch hng ca h c th xi c nhiu loi in thoi hn. Tuy nhin, mt s nh nh Verizon v Sprint th qu ln nn h c th yu cu nh sn xut lm ring cho mnh mt phin bn CDMA ca chic my mi nht, l iu din ra vi iPhone v nhiu smartphone khc. Chnh v th, nhng nh mng ny khng cn phi tn tin ch chuyn i h tng 3G, thay vo h s tp trung u t cho mng 4G ca mnh.Ni v vn 3G, cc mng 3G "GSM" hin nay s dng cng ngh CDMA, gi l WCDMA (wideband CDMA) hoc UMTS (Universal Mobile Telephone System). cng l th ang c p dng ti Vit Nam. WCDMA i hi cc knh truyn d liu rng hn so vi CDMA c, nhng n c th p ng nhu cu gi d liu tt hn.Trong qu trnh pht trin theo thi gian, GSM c nhiu bin th v bn nng cp hn l CDMA. V d, WCDMA vn c xem l mt phin bn 3G trn cng ngh GSM. C quan gim st cho GSM cn a ra thm mt phn m rng gi l HSPA y tc truyn ti d liu ca mng GSM ln nhanh nht khong 42Mbps.Trong khi , mng 3G CDMA b kt li con s 3,6Mbps. C mt s cng ngh CDMA nhanh hn, tuy nhin nhiu nh mng khng trin khai n m i thng ln 4G LTE tng thch tt hn vi ton th gii.4. 4G LTE chnh l tng lai

Mul cc quc gia trin khai mng 4G LTE thng mi. Muxanh ml cc nc ang c k hoch hoc ang trin khai 4G LTE, cn muxanh nhtl cc ni ang th nghim cng ngh nyHi thng 6 nm 2013, nh mng SK Telecom Hn Quc chnh thc trin khai mng LTE-Advanced (LTE-A) thng mi u tin ca th gii. LTE-A l mt ci tin ln ca 4G LTE. N c th cung cp tc ti v l thuyt ln n 150Mbps (18,75MB/s) v tc ti ln l 37,5Mbps (4,68MB/s), nhanh gp i so vi LTE v gp 10 ln tc ca 3G. Trong thc t, tc download trn h tng LTE-A ca SK Telecom hon ton c th t mc 102-115Mbps (12,75 - 14,4MB/s). Hin sng LTE-A ph khp th Seoul v nh mng s tip tc m rng n sang 84 tnh thnh khc ca Hn Quc trong tng lai, cn gi cc th vn gi nguyn nh chi ph s dng mng LTE. Nh vy, Hn Quc i trc th gii mt bc khi m hin nay nhiu quc gia ch mi bt u a vo s dng mng LTE m thi.

Vn l mi nh mng li s dng cc bng tn 4G khc nhau, km theo l cc h thng 3G d phng. Ngoi ra cn c mng Sprint Spark mi M dng mt bin th TD-LTE, khng ging vi bt k nh mng no nc s ti. iu cng gy ra nhng vn lin quan n tnh tng thch, cng tng t nh vic in thoi CDMA th khng xi c trn mng GSM v ngc li. Mt s cng ty nh Qualcomm, Broadcom, Samsung v Intel c gng tung ra cc gii php chip thu nhn tng thch vi nhiu bng tn 4G LTE nht c th nhm xoa du tnh trng khc bit ni trn thit b u cui.V mc d hin nay 4G LTE c mt kh nhiu nhng n vn cha ph bin. Mng 2G v 3G vn cn tn ti trong mt thi gian di na. Nh Vit Nam v nhiu nc Chu u hin vn cha c 4G LTE. Cc nh mng tng d on rng phi t nht n nm 2020 th cc h tng UMTS/WCDMA v EVDO mi ngng hot ng.Khi nhc n 4G LTE, cc bn cn bit mt iu rng hin nay cc nh mng s dng 4G LTE ch truyn ti d liu Internet m thi, cn d liu ging ni vn i con ng GSM hoc WCDMA truyn thng. Trong tng lai, chuyn ny s khng cn na nh vo cng ngh VoLTE (Voice-over-LTE). VoLTE s chuyn d liu cuc gi thnh d liu Internet gi nhn vi nh mng, tng t nh cch m tin nhn Facebook, email i vo hoc i ra khi chic in thoi ca bn.Bng cch s dng VoLTE, ngi dng s c cuc gi cht lng cao hn (mt s thit b v nh mng h tr HD Voice, chnh l VoLTE) bi cng ngh ny nn m thanh tt hn. Thi gian kt ni cuc gi cng nhanh hn, cuc gi c th c truyn thng qua Wi-Fi ( cc ni mt sng 4G), tnh tng thch gia nh mng - thit b cng cao hn. B li, thi gian u, VoLTE c th khng n nh do cn qu mi m, s lng thit b hin h tr VoLTE cng cha nhiu lm v cc ph cng c th tng ln (hoc gim xung, ci ny th chng ta kh m bit chc c).5. KtNy gi nhiu thng tin qu, chng ta hy cng im li vi im chnh nh sau:

GSM v CDMA khng c ci no tht s tt hn ci no, c hai u cung cp cc dch v tt nh nhau, cn cht lng mng nh th no th ph thuc nhiu hn vo nh mng v h tng ca h. Phn ln thit b GSM c dng SIM, CDMA khng dng SIM, vi my CDMA c khe SIM dng vi mng 4G LTE hoc bn world Khi mua in thoi, bn cn kim tra xem bng tn GSM c h tr bi thit b ca bn c tng thch vi nh mng hay khng. Hu ht thit b v nh mng u h tr di 850MHz, 900MHz, 1800MHz v 1900MHz nn chuyn ny khng qu nghim trng. 4G LTE s l tng lai, nhiu nc trn th gii ang cho trin khai chun mng ny. VoLTE s c dng nh l phng tin truyn ti cuc gi thng qua Internet trn h tng 4G LTE.Tham kho:PCMag,MakeUseOf,Radio-Electronics,CNET,Wikipedia