GSM on the Net. Background. System concept. Olle Granberg

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1 GSM on th Nt Oll Granbrg GSM on th Nt introducs an ntirly nw concpt for businss communications, offring voic, data and multimdia srvics ovr corporat intrants. Th voic srvic can b ithr fixd or mobil (in th lattr cas using GSM accss). GSM on th Nt provids oprators and businss usrs with a mans of optimizing prsonal and organizational communications and work procsss by adding trminal and usr mobility to thir xisting IP ntwork. Th author dscribs GSM on th Nt and diffrnt solutions to its implmntation. H also prsnts a rviw of ITU-T commndation H.323, which undrlis this nw businss communication systm. Box A Trminology Endpoint A trminal, gatway or multipoint control unit (MCU) that gnrats and trminats information strams. An ndpoint can initiat and rciv calls. Payload Th actual usr information transportd btwn ndpoints. (A packt contains addrss information and payload.) Trminal mobility A trminal can b movd around gographically without losing contact with th systm. Usr mobility A usr can opt to accss GSM on th Nt in diffrnt ways. gardlss of whthr a phon or a PC is chosn as a mans of accss, th systm will kp track of th usr and th trminal s capabilitis. Background Th handling and sharing of information is of vital importanc to th businss world. To this nd, many companis hav installd a local ara ntwork (LAN) to mak in-hous ntworking and communication mor fficint. Intrnt/intrant, -mail and databass ar a fw xampls of applications and ntitis accssibl from a LAN. Voic communication is handld by a privat branch xchang (PBX) on th company prmiss or by a cntrx srvic in th public switchd tlphon ntwork (PSTN). Diffrnt solutions hav bn applid to th problm of nhancing wirlss tlphony within th offic. Th systms most frquntly usd ar digital nhancd cordlss tlcommunications (DECT) and th GSM macro ntwork (Figur 1). In DECT, all usr srvics rsid in th PBX, whil all wirlss-rlatd srvics (for xampl, roaming and handovr) ar handld by a mobility srvr. In th cas of GSM, nw faturs ar introducd in th macro ntwork in ordr to support typical offic srvics. For xampl, hom zons mak it possibl to distinguish btwn xtrnal calls and calls mad in th offic; virtual privat ntworks (VPN) support privat numbring plans. adio covrag is achivd ithr by mans of radio bas stations (BS) or diffrnt forms of indoor arrangmnts, such as indoor BSs with multicasting antnnas. So far, all switching is prformd in th macro ntwork. Th PBX markt is changing rapidly, howvr, and soonr or latr today s solutions, which ar basd on propritary hardwar and a circuit-switchd nvironmnt, will b rplacd by PBXs oprating according to th Intrnt protocol (IP). Clarly, thrfor, a sustainabl, long-trm solution for GSM in th offic will hav to b basd on IP. Systm concpt Th purpos of GSM on th Nt is to srv as a ral-tim systm in a convrgd ntwork that handls all communications within a company. Voic, data and vido will b procssd ovr a unifid IP ntwork: th company s intrant. Th systm can b accssd in a varity of ways (for xampl, by mans of GSM phons, IP phons and PCs) to provid full usr mobility. Its scond ky fatur trminal mobility supports roaming to th GSM macro ntwork. In othr words, th systm offrs th convninc of on phon and on numbr. Th usr and trminal mobility providd mans that GSM on th Nt kps track of a usr and his or hr trminal s capabilitis, rgardlss of whthr usrs ar in or outsid of th offic and whthr thy can b rachd ovr th phon or on a PC. Macro GSM with VPN srvics DECT Mobility srvr PBX PSTN Figur 1 Prsnt-day offic solutions. 184 Ericsson viw No. 4, 1998

2 WOLD WOLD WOLD PBX GSM on th nt IP phon PSTN Intrnt... Gatway Application Srvic Accss IP Intrant SMS mssag cntr MSC/BSC EI HL AUC Figur 2 A GSM on th Nt systm. Systm architctur Gnral GSM on th Nt is basd on ITU-T commndation H.323 (Box D), which dscribs trminals, quipmnt and srvics for multimdia communication ovr a packtbasd ntwork. Th basic concpt of H.323 is that of sparating call control (signaling in th call stup phas) and connction control (data flows btwn ndpoints). Whn th path has bn stablishd, th actual data flow constituts a ral-tim connction btwn th two (or mor) trminals concrnd. All s in GSM on th Nt ar connctd to th corporat intrant, and communication btwn systm s and trminals is handld ovr th Intrnt protocol. IP addrsss and ports nsur that data and signaling flows ar routd to th corrct dstinations in th systm (or to xtrnal ntworks). Th corporat intrant can hav a vry complx topology with many routrs and widly scattrd locations. Evn rmot trminals, such as modm-connctd hom trminals or IP-tunnld connctions, ar includd in th intrant, thrby forming part of th systm (Figur 2). Box B Abbrviations AN API AUC DECT EI GPS GSM HL IETF IP ISDN ISP Accss Application program intrfac Authntication cntr Bas transcivr station Digital nhancd cordlss tlcommunications Equipmnt idntity rgistr Gnral packt radio srvic Global systm for mobil communication Hom location rgistr Intrnt Enginring Task Forc Intrnt protocol Intgratd srvics digital ntwork Intrnt srvic providr ITU Intrnational Tlcommunication Union LA Location ara LAN Local ara ntwork MAP Mobil application part MCU Multipoint control unit MSC Mobil switching cntr PLMN Public land mobil ntwork PSTN Public switchd tlphon ntwork QoS Quality of srvic BS adio bas station SVP sourc rsrvation protocol SMS Short mssag srvic SoLSA Support of local srvic aras VL Visitd location rgistr VPN Virtual privat ntwork WAN Wid ara ntwork Ericsson viw No. 4,

3 Figur 3 A usr s srvic profil is stord in th srvic, not in ach typ of accss. This mans that all spcifid srvics ar availabl to usrs, no mattr what typ of trminal thy us to accss th systm. GSM accss Applications Srvic Fixd phon accss API Fixd trminal accss,.g. PC Srvic Th srvic which is th cor of GSM on th Nt is rsponsibl for call stup, usr administration, scurity and othr systmrlatd srvics. Th functionality of this is what nabls usr mobility. Th purpos of a call is to rach anothr usr; th typ of trminal usd by th calld party and th location of that party ar lss important. Call stup Th srvic controls all calls mad btwn diffrnt typs of trminal and translats addrsss btwn diffrnt typs of accss (for instanc, btwn ordinary PSTN phon numbrs and addrsss in th systm). Th stup procdur also involvs ngotiating trminal capabilitis, such as bandwidth limitations and man-machin intrfacs. Usr administration Usr administration is anothr important function. Evry usr has a systm idntity and a srvic profil that ar rgistrd in th srvic (Figur 3). Th profil lists vry srvic that th usr wants to accss; for xampl, mailbox and call forwarding. Usrs accss th systm by choosing a trminal: a PC, an IP phon or a GSM phon (locally or in th macro ntwork). If accss is allowd, th usr is automatically connctd to all srvics rgistrd in his or hr usr profil. Howvr, th man-machin intrfac or limitations in bandwidth in th chosn accss may rduc or inhibit th us of som srvics. Othr basic functions prformd by th srvic ar authntication, rsourc managmnt, and last-cost routing. Th lattr function is rsponsibl for finding th most cost-ffctiv rout. For xampl, a call from an offic in Swdn to an offic in th US can b routd through a gatway in Swdn and via th PSTN to th US subscribr; through th company intrant to a gatway in th US and on to th calld subscribr. Th srvic includs an application program intrfac (API) to high-lvl applications, such as voic mail and -mail convrgnc, wb-initiatd dialing, or vrtical applications, which includ sals support, customr-car systms, and ordrsrvicing and logistics systms. Accss Th functionality of th (wirlss) accss nabls GSM trminals to accss th systm. In this sns it rsmbls th mobil switching cntr (MSC), th visitd location rgistr (VL), and th bas station controllr (BSC). (Th accss will also contain functions for supporting gnral packt radio srvics GPS whn ths bcom availabl in th GSM macro ntwork.) Box C Bnfits of GSM on th Nt For th oprator, GSM on th Nt offrs: incrasd GSM traffic nw subscribrs and lowr churn. Furthrmor, th businss usr s bhavior pattrn, in trms of calling frquncy and call duration, will transfr to global us as wll; profit margins if supplid by th oprator, systm quipmnt (srvrs and phons) will boost profit margins through incrasd product sals and mor srvic agrmnts; nw srvics th provision of convrgnc srvics will gnrat mor rvnu. Exampls includ srvic packags, which can b tailord to suit individual organizations and vrtical markt applications. For th usr, GSM on th Nt offrs: frdom at work th sam look and fl whrvr th systm is accssd (whthr th usr is in th offic, at a subsidiary, at hom, or travling); convninc having GSM intgratd into th offic givs th convninc of on phon and on numbr. Th sam phon is usd, rgardlss of usr whrabouts, within or outsid of th offic; cost-ffctivnss implmntation costs ar low bcaus xisting PCs and LANs ar usd. Similarly, opration costs ar low thanks to low-tariff, local traffic (through local switching); diffrntiatd usr authorization; availabl rsourcs for administration (for xampl, th IT support tam); adaptability th systm is a smoothly fitting part of th xisting ntwork nvironmnt (and applications) and can asily b xpandd and improvd with rspct to capacity, functional contnt, and quality; scalability flxibl srvic and application dsign (srvr applications), asy-to-add bas stations and handsts. 186 Ericsson viw No. 4, 1998

4 Box D ITU-T commndation H.323 ITU-T commndation H.323 dscribs trminals, quipmnt and srvics for multimdia communication ovr packt-basd ntworks (for xampl, an intrant or th Intrnt). H.323 trminals and quipmnt can carry ral-tim voic, data and vido, or any combination including vido tlphony. Th scop of H.323 dos not includ th ntwork intrfac, th physical ntwork, or th transport protocol usd on th ntwork. Th ntwork ovr which H.323 trminals communicat can b a singl ntity or multipl sgmnts with complx topologis. H.323 trminals can b intgratd into prsonal computrs or implmntd in stand-alon dvics, such as vido tlphons. Support of voic is mandatory, whil data and vido ar optional. Th gnral approach to th us of th undrlying transport ntwork in H.323 is to mploy diffrnt transport channls for voic, data, vido and control, and to provid a mans of synchronizing packts at th ndpoints (Figur D1). Functional ntitis Four diffrnt functional ntitis ar dscribd in H.323: Trminal Gatkpr Gatway unit Multipoint control unit (MCU) Trminal An H.323 trminal is an ndpoint in th ntwork that provids ral-tim, two-way communication with anothr H.323 trminal a gatway unit or an MCU (in H.323, th trm ndpoint rfrs to trminals, gatways and MCUs as lmnts capabl of rciving or initiating calls). Gatkpr Th purpos of a gatkpr is to provid th following srvics to th H.323 ndpoints: Addrss translation th gatkpr translats alias addrsss into transport addrsss. Admission control th gatkpr authorizs ntwork accss, basd on call authorization, bandwidth or som othr critria slctd by th manufacturr. Bandwidth control. Zon managmnt. Th gatkpr provids th abovmntiond functions for trminals, MCUs and gatways that hav rgistrd with it. Th gatkpr may also prform optional functions, such as call-control signaling th gatkpr may choos to complt call signaling with th ndpoints and procss th call itslf. Altrnativly, th gatkpr may dirct th ndpoints to connct th call signaling to ach othr; call authorization th gatkpr can rjct calls from a trminal on th grounds of authorization failur. Othr rasons for rjction may includ rstrictd accss to or from a spcific trminal or gatway, and rstrictd accss during crtain priods; bandwidth managmnt th gatkpr chcks th numbr of H.323 trminals that ar allowd simultanous accss to th ntwork. Th gatkpr can rjct calls from a trminal if it judgs availabl bandwidth to b insufficint; call managmnt th gatkpr can kp a list of ongoing H.323 calls. This information may b ncssary to indicat that a calld trminal is busy and to provid bandwidth managmnt data. Gatway unit A gatway unit is an ndpoint in th (packtizd) ntwork that provids ral-tim, two-way communication btwn H.323 trminals in th ntwork and othr ITU-T trminals in a wid ara ntwork (WAN) or anothr H.323 gatway. Th gatway unit provids th ncssary convrsion btwn diffrnt typs of trminal. Th gatway provids th appropriat translation btwn transmission and communication procdurs. It is rsponsibl for call stup and claring on both th intrant sid and th circuit-switchd ntwork sid. Th gatway also translats btwn audio, vido and data formats. Multipoint control unit Th multipoint control unit (MCU) consists of two logical ntitis: th multipoint controllr (MC) and th multipoint procssor (MP). Th multipoint controllr controls thr or mor trminals that participat in a multipoint confrnc. It ngotiats capability with th trminals, in ordr to provid common lvls of communication; it may also control confrnc rsourcs, such as multicasting vido. Th multipoint procssor is rsponsibl for mixing, switching and othr procssing of mdia strams undr th control of th multipoint controllr. Basic call a summary Th gatkpr function controls calls. A call is initiatd by a usr (A) at on ndpoint. Usr A maks th call by ntring usr B s nam or som alias. Trminal A conncts to th gatkpr and rqusts a connction to usr B. Th gatkpr sts up th othr part of th connction (btwn th gatkpr and usr B) and supplis ach trminal with its countrpart s addrss (IP addrss and port numbr). Th two trminals opn at last two ral-tim data strams (on in ach dirction) btwn thm. Unlss som charactristics of th sssion chang, th gatkpr prforms no tasks whil a call is in progrss. Exampls of changd charactristics includ cass whr mor bandwidth is rquird (du to a chang from, say, voic only to voic and vido) or whr a nw party is addd to th sssion. H.323 ndpoint Vido transport Voic transport Data transport H.323 ndpoint Figur D1 Data strams for voic, data and vido ar transportd on sparat channls according to H.323. Ericsson viw No. 4,

5 WOLD PSTN Intrnt... HL MSC BSC Small offics ISP Gatway Intrant Srvic Accss Appl. Figur 4 Small and mdium-siz company solution, with an ISP as th carrir. Th accss manags radio rsourcs, mobility, and communication functions. It can also intrrogat (with MAP/SS7) th hom location rgistr (HL), and handls roaming numbrs and radio ntworks; for xampl, paging and handovr control. Th accss is involvd whn calls ar st up to (or from) a GSM trminal. Th itslf dos not carry any traffic, howvr; it mrly controls communication btwn GSM trminals. In trms of th GSM macro ntwork, th accss and its undrlying structur of bas transcivr stations () and trminals is prcivd as an arbitrary MSC/VL srvic ara in th ntwork. GSM trminals accss th systm through th, which srvs as a gatway btwn th air intrfac and th LAN nvironmnt. In GSM on th Nt, a GSM subscription is rgardd as a trminal, not as a usr subscription. In principl, any usr who is rgistrd in th systm could b loggd on by mans of an arbitrary GSM mobil phon, but normally a GSM trminal is associatd with a usr. Gatway A gatway is th intrconnction btwn GSM on th Nt and othr ntworks. Exampls includ intrfacs to th PSTN, th public land mobil ntwork (PLMN), PBXs and th Intrnt. Th gatway translats btwn transmission formats and communication procdurs. It also translats btwn audio, vido and data formats, including voic codrs and, as an option, vido codrs (Ericsson s gatway supports H.323 mandatory codrs and GSM full-rat and nhancd full-rat codrs). Extrnal intrfac to PSTN/ISDN Th public switchd tlphon ntwork can b rachd via ISDN accss, ithr ovr th basic rat intrfac (2B+D) or th primary rat intrfac (ITU-T: 30B+D, ANSI: 24B+D). Extrnal intrfac to th PLMN Th intrfac to th PLMN can ithr includ signaling only, or both signaling and payload. Basically, signaling btwn GSM on th Nt and th PLMN supports GSM mobil trminals in th systm. Exampls of ntitis that can b rachd in th PLMN includ: MSC/VL; HL and authntication cntr (AU); quipmnt idntity rgistr (EI); mssag cntr (for th support of shortmssag srvics). Extrnal intrfac to a PBX Th systm communicats with th PBX across a primary rat intrfac, using th 188 Ericsson viw No. 4, 1998

6 Qsig signaling protocol. Latr, othr PBX protocols may also b supportd along with a limitd st of supplmntary srvics. Th Intrnt Th Intrnt intrfac maks it possibl to rach othr H.323 ntworks and trminals. Hr it should b obsrvd that a multisit company is rgardd as on GSM on th Nt systm. Application Th application nabls othr (highlvl) applications to intract with GSM on th Nt ovr an API. This is in tun with th growing trnd toward intgrating information procssing into tlcommunications, which will lad to th dvlopmnt of a nw class of intgratd businss applications. Som xampls of nw, advancd convrgnc faturs mad possibl by th unifid ntwork that could b implmntd into a GSM on th Nt nvironmnt ar listd blow. Wb-initiatd dialing ( Click-to- Talk ) Usrs surfing on th nt can click on a phon icon on th scrn to rtriv from a dirctory th phon numbr of th prson or organization thy want to call. Instad of dialing th numbr, thy simply click on it to initiat th call. A typical banking application for popl who mak thir transactions ovr th Intrnt nabls usrs to gt in touch with a bank clrk to discuss a financial mattr or to sk assistanc. Dirctory-assistd dialing GSM on th Nt maks it possibl to initiat a call from th company s tlphon dirctory (or th public dirctory) instad of having to ntr th digits manually. Unifid mssaging All mail srvics (voic mail, -mail, fax mail, vido mail) can b convnintly providd in a singl mailbox. This srvic can b supplmntd with a mdia convrsion fatur, such as voic-to-txt and txt-tovoic. Simplifid intgration with LANbasd businss support systms Intgration with customr-car systms, billing systms, ordr-srvicing and logistics systms, sals applications, and so forth. On xampl is th automatic display of customr data and dlivry status whn a customr calls. Physical implmntation Th srvic and accss s ar implmntd on standard srvrs. This is fasibl bcaus th s, as spcifid in H.323, do not carry any ral-tim traffic. In a small systm, th functionality of th two s can b co-locatd on on physical srvr, which may also srv othr applications. Larg systms might nd mor than on srvic and accss to cop with traffic load. Systm configurations Th GSM on th Nt concpt can b configurd ithr as a systm ownd by a company or as a srvic providd by a mobil oprator or an Intrnt srvic providr (ISP). Th following xampls show two diffrnt configurations: on for small and mdiumsizd companis and on for larg companis. Small and mdium-sizd companis Small and mdium-sizd companis ar likly to choos a solution whr a carrir an oprator or an ISP provids th systm as a srvic. Th ISP s offring to th small or mdium-sizd company is a combind data/intrnt and fixd/mobil tlphony solution. Th physical transmission to th offic (which is normally rathr costly) can b shard with othr srvics providd by th ISP. Ndlss to say, such an arrangmnt is advantagous in trms of data transmission and vn mor so from a cost prspctiv. In th xampl shown in Figur 4, th ISP oprats all srvr-basd s in GSM on th Nt. Th ISP is rsponsibl for intracting with th GSM oprator in mattrs rlatd to SS7 transmission links and frquncy handling. Multimdia is offrd within th rang of srvics providd by th ISP; for xampl, Intrnt accss, nws, and -mail. In th way of nw quipmnt, all that is rquird in th offic ar accss dvics; for xampl, IP phons, GSM bas stations and, of cours, mobil trminals. Larg companis As a rul, larg companis oprat thir own systm and ar also capabl of handling thir rlation with th PLMN oprator. In most cass, a company s multipl sits ar intrconnctd by mans of an intrant. Ericsson viw No. 4,

7 WOLD WOLD WOLD Offic 3 HL PSTN Intrnt... MSC BSC Offic 1 Offic 2 Gatway Gatway Figur 5 Larg-company solution. Srvic Accss Appl. Srvic Accss In th xampl shown in Figur 5, th larg offic srvs its satllit offics with diffrnt systm functions. An intrant provids th mans of intra-company communication. This typ of larg intrant is also supportd by advancd systm functions that ar capabl of finding last-cost routs and locating usrs roaming btwn offics. Accssing th systm Gnral Diffrnt typs of accss ar includd in GSM on th Nt and nw ons will b implmntd in th futur. Basically, accss can b dividd into four catgoris: Fixd trminals connctd to th intrant. Exampls includ IP phons and PCs with H.323 trminal softwar. Trminals connctd to th xtrnal ntwork (th Intrnt). This typ of trminal is normally, but not ncssarily, accssd via a firwall or proxy srvr. Trminals outsid of th systm; for xampl, PSTN phons. This typ of trminal is accssd via th gatway. Wirlss accss (GSM accss). A fundamntal ida is that usr prcption of srvics accssd on diffrnt typs of trminal should b th sam. At th sam tim, it should b born in mind that an accss mthod or a trminal may hav limitations that rstrict or inhibit th us of a srvic. Som typs of accss might introduc a srvic or function that is uniqu from othr typs of accss. In such a cas, th inhrnt srvic or function will b hiddn if a similar on xists in th systm (for instanc, voic mail). Th aim is to support in-systm srvics rgardlss of whr accss has bn mad. On xampl is short numbrs, which rmain valid vn whn a GSM mobil trminal is roaming in th macro ntwork. If a srvic is uniqu to a spcific typ of accss, thn th systm will transport th srvic as transparntly as possibl to th trminal. GSM GSM accss is a ky fatur of GSM on th Nt. It offrs wirlss accss that can b usd both within and outsid of th offic. Howvr, th introduction of GSM in an offic nvironmnt introducs svral nw rquirmnts. Aras of particular intrst ar 190 Ericsson viw No. 4, 1998

8 subscribr handling, radio-ntwork handling, frquncy handling, charging, and opration and maintnanc. Without going into dtail, th basic idas of th GSM on th Nt solution ar as follows: Th usr should not notic any diffrnc, no mattr whthr th trminal is usd in or outsid of th offic. Srvics and systm bhavior ar, to th gratst possibl xtnt, basd on GSM on th Nt not on th xtrnal GSM systm. This mans that GSM on th Nt is in control of srvics in and outsid of th offic. A spcial, low-tariff charg (or no charg) is applid within th systm. oaming oaming btwn intrnal and xtrnal aras taks plac in th rgular way. A location updat is snt to VL/HL whn a usr movs to anothr location ara (LA). If a mobil trminal is within th offic, it is rgistrd in th intrnal VL (that is, th VL functionality in th accss ) and th HL will indicat that VL. If th trminal is outsid of th offic, it is rgistrd in an xtrnal VL. Evn in th cas of handovr, GSM on th Nt acts as any MSC/VL, from th xtrnal GSM systm s point of viw. Srvic aras and SoLSA Low tariffs and mor advancd srvics man that mobil trminals should, whnvr possibl, b rgistrd in th offic radio ntwork. A mobil trminal in activ mod is controlld by th BSC locating function. In this cas it would b possibl to chang th locating function to kp trminals within th srvic ara of GSM on th Nt. Howvr, this cannot b don with mobil trminals in idl mod. Ths trminals slct clls by signal strngth only, and th srvic ara will thus coincid with th cll bordr. A function calld support of local srvic ara (SoLSA) will b addd to furthr strngthn th bond btwn th mobil trminal and th systm location ara. This function maks th mobil trminal slct clls blonging to GSM on th Nt (almost) indpndntly of signal strngth. It forcs trminals to stick to th location ara dfind by th systm. SoLSA functionality will soon b addd to th GSM standard. Quality of srvic Th rsourc rsrvation protocol (SVP) rprsnts on mthod of controlling quality of srvic (QoS); diffrntiatd srvic is anothr mthod. In ithr cas, th function must b initializd in th ntwork. Ntwork lmnts, such as routrs, ar normally capabl of handling SVP and diffrntiatd srvic, but in a typical ntwork ths functions ar not activatd. SVP a complx protocol that mor or lss mulats a circuit-switchd systm rsults in long call-stup tims and will not b supportd by GSM on th Nt. Diffrntial srvic, by contrast, is basd on a simpl principl and is bing standardizd by th Intrnt Enginring Task Forc (IETF). It has alrady bn implmntd in svral routrs. GSM on th Nt will us diffrntiatd srvic to control quality of srvic. Conclusion Ericsson has dvlopd a nw IP-basd businss communication systm GSM on th Nt that offrs voic, data and multimdia srvics ovr corporat intrants. Th systm nabls oprators and businss usrs to optimiz thir communications by adding trminal and usr mobility to thir xisting IP ntwork. GSM on th Nt srvs as a ral-tim systm in a convrgd ntwork that handls all communications within a company. Its multi-accss fatur nsurs usr mobility, whil trminal mobility supports roaming to th GSM macro ntwork. That way, th systm offrs th convninc of on phon and on numbr. Th intgration of information procssing into tlcommunications will lad to th dvlopmnt of a nw class of intgratd businss applications, and this is whr th nw systm coms in handy. A wid rang of advancd convrgnc faturs mad possibl by a unifid ntwork can b implmntd in a GSM on th Nt nvironmnt. Ericsson viw No. 4,