Interworking in heterogeneous wireless networks: comprehensive framework and future trends

IEEE Wi eless Communica ions • Ap il 2010 22 1536-1284/10/$25.00 © 2010 IEEE Con ol plane (Ou 802.11 adio s ack Da a plane IP laye IP ou e Access poin s WLAN access ne wo k ACCEPTED FROM OPEN CALL RAMON FERRUS, ORIOL SALLENT, AND RAMON AGUSTI, UNIVERSITAT POLITECNICA DE CATALUNYA INTERWORKING IN HETEROGENEOUS WIRELESS NETWORKS: COMPREHENSIVE FRAMEWORK AND FUTURE TRENDS INTRODUCTION An in insic cha ac e is ic in cu en and u u e wi eless communica ion scena ios is he e ogene- i y, which e e s o he coexis ence o mul iple and di e se wi eless ne wo ks wi h hei co e- sponding adio access echnologies (RATs). He - e ogenei y is di ec ly associa ed o he ac ha no single RAT is able o op imally co e all he di e en wi eless communica ions scena ios. Hence, a adio echnology op imized o p o ide ou doo co e age o high mobili y use s may ail o mee mo e demanding da a a es in low mobili y indoo scena ios and ice e sa. He e o- genei y is also inhe en o echnological e olu- ion since many new wi eless ne wo ks a e deployed while suppo ing legacy in as uc u es. Despi e RAT he e ogenei y, he se ice model pu sued unde nex -gene a ion wi eless ne wo ks is in ended o acili a e he deploymen o applica ions and se ices independen o he unde lying RAT. Hence, i is expec ed ha mobile use s could e en ually enjoy uly seam- less mobili y and ubiqui ous se ice access in an always bes connec ed mode, employing he mos e icien combina ion o a ailable access sys ems a any ime and anywhe e. In his con- ex an app op ia e in e wo king o di e en wi eless access sys ems is c ucial o mee mobile use s’ expec a ions while making possible he coexis ence o di e se RATs. The de elopmen o in e wo king solu ions o he e ogeneous wi eless ne wo ks has spu ed a conside able amoun o esea ch in his opic, especially in he con ex o IEEE 802.11 wi eless local a ea ne wo ks (WLANs) and cellula ne - wo k in eg a ion. In e wo king is linked o many echnical challenges such as he de elopmen o enhanced ne wo k a chi ec u es [1, 2], new mechanisms and p o ocols o seamless hand- o e [3], and ad anced managemen unc ionali- y o he join exploi a ion o he e ogeneous wi eless ne wo ks [4, 5]. Acco dingly, in e wo k- ing aspec s a e ecei ing a lo o a en ion in s anda diza ion o ums such as he Thi d Gene - a ion Pa ne ship P ojec (3GPP), 3GPP2, In e - ne Enginee ing Task Fo ce (IETF), WiMAX Fo um, and IEEE 802 LAN/MAN Commi ee, he new IEEE 802.21 s anda d [6] o media- independen hando e (MIH) being a clea exponen o such an e o . While mos published wo k is ocused on pa - icula in e wo king solu ions o speci ic wi e- less echnologies, his a icle es ablishes a comp ehensi e amewo k aimed a ca ego izing and analyzing in e wo king solu ions. The p o- posed amewo k is based on he de ini ion o a gene ic se o in e wo king le els along wi h a classi ica ion o he ela ed key in e wo king mechanisms en isioned so a o he e ogeneous wi eless ne wo ks. The elabo a ed amewo k is hen used o analyze om a common pe spec i e some o he mos ele an in e wo king solu ions p oposed o 3GPP, WLAN, and WiMAX ne - wo ks. In pa icula , conce ning he in eg a ion o WLAN and cellula ne wo ks, wo in e wo k- ing a chi ec u es speci ied by 3GPP, in e wo k- ing WLAN (I-WLAN) [7] and gene ic access ne wo k (GAN) [8], a e discussed. Nex , in he con ex o coexis ing mobile b oadband access ne wo ks, in e wo king solu ions o Mobile WiMAX and 3GPP Long Te m E olu ion/Sys- em A chi ec u e E olu ion (LTE/SAE) ne - ABSTRACT In e wo king mechanisms a e o p ime impo - ance o achie e ubiqui ous access and seamless mobili y in he e ogeneous wi eless ne wo ks. In his a icle we de elop a comp ehensi e ame- wo k o ca ego ize in e wo king solu ions by de ining a gene ic se o in e wo king le els and i s ela ed key in e wo king mechanisms. The p oposed amewo k is used o analyze some o he mos ele an in e wo king solu ions being conside ed in di e en s anda diza ion bodies. Mo e speci ically, I-WLAN and GAN app oach- es o WLAN and cellula in eg a ion, solu ions o WiMAX and 3GPP LTE/SAE in e wo king, and he o hcoming IEEE 802.21 s anda d a e discussed om he common poin o iew p o- ided by he elabo a ed amewo k. The au ho s de elop a comp ehensi e amewo k o ca ego ize in e wo king solu ions by de ining a gene ic se o in e wo king le els and i s ela ed key in e wo king mechanisms. SALLENT LAYOUT 4/8/10 12:57 PM Page 22 Au ho ized licensed use limi ed o: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 a 14:58:00 UTC om IEEE Xplo e. Res ic ions apply. IEEE Wi eless Communica ions • Ap il 2010 23 wo ks a e analyzed [9–11]. Finally, he MIH solu ion elabo a ed wi hin IEEE 802.21 [6] is add essed. The es o his a icle is o ganized as ollows. Fi s , we desc ibe a gene ic in e wo king sce- na io o he e ogeneous wi eless ne wo ks and b ing up some majo conside a ions abou ne - wo k a chi ec u es and mul imode e minals. F om such a basis, he p oposed in e wo king amewo k is elabo a ed and he abo e men- ioned in e wo king solu ions a e analyzed. The inal sec ion includes ou main concluding ema ks and discusses u u e ends. WIRELESS HETEROGENEOUS INTERWORKING SCENARIO Figu e 1 illus a es a gene ic in e wo king sce- na io o wo coexis ing wi eless ne wo ks wi h pa ially o e lapped co e age. I is assumed ha he RAT used in each wi eless ne wo k can be di e en , and e minals ha e mul imode capabil- i ies. Conce ning se ice p o isioning, a common se o se ices can be o e ed h ough bo h wi e- less ne wo ks (e.g., oice calls o/ om public swi ched elephone ne wo ks), bu he e can also be some speci ic se ices only a ailable when connec ed o a gi en wi eless ne wo k (e.g., ins an messaging and p esence se ices). Besides, as o ne wo k access igh s, i is conside ed he mos gene al iew whe e wo wi eless ne wo ks could belong o di e en adminis a i e domains (e.g., ne wo ks ope a ed by di e en ne wo k se ice p o ide s) bu use s can po en ially be a ached o ei he ne wo k (e.g., p ope oaming ag eemen s exis ). No ice ha a mo e pa icula case would be whe e he wo ne wo ks o m pa o he same adminis a- i e domain (e.g., a mobile ope a o wi h Global Sys em o Mobile Communica ions [GSM] and Uni e sal Mobile Telecommunica ions Sys em [UMTS] ne wo ks). In any case, in his a icle we deno e as home ne wo k he ne wo k om which a use has ob ained his/he c eden ials, and as isi ed ne wo k any o he ne wo k o which he use can be connec ed. In such a con ex , di e en in e wo king mechanisms would be needed o a end o he se o equi emen s imposed in e ms o ubiqui- ous and seamless se ice access as well as o o e all ne wo k esou ce op imiza ion. WIRELESS NETWORKS CHARACTERISTICS A ending o cu en a chi ec u al ends in nex - gene a ion ne wo ks [12], wi eless ne wo ks a e mainly de o ed o p o iding ne wo k connec i i y se ices (i.e., bea e se ices) ha may be cha ac- e ized by a gi en quali y o se ice (QoS) p o ile. Then, end use se ice p o isioning is suppo ed by means o specialized se ice pla o ms (e.g., IP mul imedia subsys em [IMS]) ha become acces- sible o he use s ia hose bea e se ices. Acco dingly, Fig. 2 illus a es a gene ic wi e- less ne wo k a chi ec u e in e ms o i s main ne wo k nodes and p o ocol laye alloca ion. As shown in he igu e, he wi eless ne wo k p o- ides ne wo k laye connec i i y (e.g., IP connec- i i y) o ex e nal ne wo ks and se ice pla o ms ia some ype o ne wo k ga eway (NG). In addi- ion, his NG can alloca e mechanisms o dynam- ically acqui e ope a o policies ela ed o QoS and accoun ing, and en o ce hem on a packe - by-packe basis o each mobile use . On he o he side, a RAT-speci ic adio link p o ocol s ack would be used in he ai in e ace. This adio p o ocol s ack can be en i ely alloca ed in base s a ions (BSs) o dis ibu ed in a hie a chi- cal manne be ween BSs and some ype o adio con olle s. The adio link p o ocol s ack com- p ises physical, medium access con ol, and adio link con ol laye s. Th ough his adio p o ocol s ack, da a ans e in he adio in e ace can be managed, a ending o each mobile use ’s speci - ic needs while simul aneously pu suing an e i- cien usage o adio esou ces by means o app op ia e adio esou ce managemen (RRM) mechanisms. Hence, BSs and NGs cons i u e he wo key elemen s wi hin he da a plane unc ions (i.e., hose unc ions ha a e execu ed di ec ly on he low o da a packe s). Addi ionally, he da a plane be ween BSs and NGs can also com- p ise mobili y ancho ing unc ions in cha ge o ecei ing da a des ined o a gi en mobile and edi ec ing he da a (usually h ough unneling) o he mobile’s se ing BS. The managemen o he o e all connec i i y se ice is achie ed by a ne wo k con ol plane. Unlike he da a plane, he con ol plane unc- ions a e hose ha do no di ec ly ope a e on he da a low. This ne wo k con ol plane would be in cha ge o handling mechanisms such as ne wo k access con ol (e.g., au hen ica ion and au ho iza ion), accoun ing and cha ging unc- ions, mobili y managemen (e.g., loca ion and paging), secu i y managemen , and session man- agemen . Fo he sake o b e i y, all o he abo e men ioned con ol plane mechanisms a e e e ed o as wi eless ne wo k con ol (WNC) mechanisms h oughou his a icle. Thus, as Figu e 1. Gene ic in e wo king scena io o he e ogeneous wi eless ne wo ks. In e wo king mechanisms Speci ic se ices Speci ic se ices Common se ices Wi eless ne wo k A Wi eless ne wo k B SALLENT LAYOUT 4/8/10 12:57 PM Page 23 Au ho ized licensed use limi ed o: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 a 14:58:00 UTC om IEEE Xplo e. Res ic ions apply. IEEE Wi eless Communica ions • Ap il 2010 24 shown in Fig. 2, a se o WNC se e s, along wi h some ne wo k da abases (e.g., subsc ibe p o ile da abases), a e assumed o alloca e all hese unc ions. Finally, gene al-pu pose packe -swi ched ne - wo ks would cons i u e he backbone anspo ne wo k ha in e connec s he di e en ne wo k nodes. MULTIMODE TERMINAL CHARACTERISTICS Focusing now on he key e minal cha ac e is- ics, a simpli ied p o ocol s ack o a mul imode e minal is depic ed in Fig. 2. This p o ocol s ack consis s o RAT-speci ic p o ocols o he lowe laye s (i.e., physical and link laye s) and a com- mon se o p o ocols o he highe laye s (i.e., ne wo k, anspo , and applica ion laye s). Con- ce ning he RAT-speci ic p o ocols, hey would comp ise he co esponden adio link p o ocol s ack o handle da a ans e in he ai in e ace along wi h he p o ocols used o WNC- ela ed unc ionali y in each wi eless ne wo k. As o he common p o ocol laye s, he ne wo k laye (e.g., IP) has a undamen al ole in he in e wo king model since i p o ides a uni o m subs a e o e which anspo (e.g., T ansmission Con ol P o- ocol [TCP] and Use Da ag am P o ocol [UDP]) and applica ion p o ocols (e.g., Session Ini ia ion P o ocol [SIP]) can e icien ly un independen o he used access echnologies. In addi ion o p o ocol s ack conside a ions, whe he he e minal is able o ansmi and ecei e simul aneously on bo h adio links (dual- adio ope a ion) o only on one a a ime (single- adio ope a ion) has impo an implica ions on equi ed in e wo king mechanisms, as discussed la e . INTERWORKING LEVELS Bea ing in mind all he abo e conside a ions, se e al in e wo king le els can be en isioned wi h a di e en ange o in e wo king equi e- men s. The de ini ion o in e wo king le els can be conduc ed a ending o, say, ne wo k a chi- ec u e aspec s o he le el o suppo o speci - ic se ice and ope a ional capabili ies. In his wo k a de ini ion exclusi ely based on he le el o se ice in eg a ion among ne wo ks is consid- e ed because o i s independence om unde ly- ing ne wo k echnologies and a chi ec u es. Hence, in a gene al case, ou in e wo king le - els a e dis inguished. LEVEL A: VISITED NETWORK SERVICE ACCESS This le el o in e wo king would allow a use o ge access o a se o se ices a ailable in a isi ed ne wo k while elying on his/he home ne wo k c eden ials. As well, he use could be cha ged o se ice usage in he isi ed ne - wo k h ough i s own home ne wo k billing sys- em. An example could be he case o a cellula subsc ibe , equipped wi h a lap op wi h bo h cellula and WLAN ne wo k in e aces, able o log in o a public WLAN ho spo using i s cellu- la sma ca d c eden ials and ge high-speed In e ne access om he ho spo se ice p o ide . LEVEL B: INTERSYSTEM SERVICE ACCESS In his le el, use s connec ed h ough a isi ed ne wo k would also be able o ge access o spe- ci ic se ices loca ed in his/he home ne wo k. Hence, coming back o he example gi en in le el A, he cellula subsc ibe using a cellula /WLAN lap op would also enjoy his/he cellula IMS se ices while a ached o he pub- lic WLAN ho spo . Nei he le el A no le el B would suppo se ice con inui y when he use mo es be ween ne wo ks. LEVEL C: INTERSYSTEM SERVICE CONTINUITY This le el ex ends he p e ious ones so ha he use is no equi ed o e-es ablish ac i e ses- sion(s) when mo ing be ween ne wo ks. How- Figu e 2. Gene ic a chi ec u e o a wi eless access ne wo k and p o ocol s ack o a mul imode e minal. WNC p o ocol s ack RAT B ne wo k in e ace Radio link p o ocol s ack WNC p o ocol s ack RAT A ne wo k in e ace Radio link p o ocol s ack Common p o ocols RAT-speci ic p o ocols Applica ion laye Ne wo k laye T anspo laye Base s a ions/ adio con olle s Con ol plane Ne wo k laye Wi eless access ne wo k Mul imode e minal T anspo ne wo k Da a plane Ne wo k ga eway Ne wo k da abases Ex e nal ne wo ks, se ice pla o ms WNC p o ocol s ack WNC se e s Radio link p o ocol s ack SALLENT LAYOUT 4/8/10 12:57 PM Page 24 Au ho ized licensed use limi ed o: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 a 14:58:00 UTC om IEEE Xplo e. Res ic ions apply. IEEE Wi eless Communica ions • Ap il 2010 25 e e , a empo a y QoS deg ada ion can be ol- e a ed du ing he ansi ion ime. As an exam- ple, i he cellula use equipped wi h he cellula /WLAN lap op begins o download a la ge ile when a ached o he public WLAN ho spo and he use mo es so ha WLAN co - e age is los , he downloading se ice will con- inue h ough he cellula ne wo k wi hou use in e en ion e en hough a sho ans e in e - up ion migh be obse ed du ing he ne wo k change. LEVEL D: INTERSYSTEM SEAMLESS SERVICE CONTINUITY This le el is aimed o sa is y se ice equi e- men s also du ing mobili y (i.e., o o e a seam- less mobili y expe ience). Seamless se ice con inui y can be achie ed by enabling mobile e minals o conduc seamless hando e s ac oss di e se access ne wo ks. A seamless hando e is commonly ela ed o he achie emen o low hando e la encies (e.g., less han 300 ms could be equi ed o eal- ime se ices in in e ech- nology hando e s) so ha his in e wo king le el is he one ha imposes he ha des equi emen s on he in e wo king mechanisms. As an example, ocusing again on he dual-mode cellula /WLAN use , a oice o e IP (VoIP) call es ablished when a ached o he WLAN ho spo should be seamlessly handed o e o he cellula ne wo k. Finally, i is wo h no ing he e ha seamless se - ice con inui y is dependen no only on in e - wo king mechanisms bu also on he consis ency o QoS cha ac e is ics p o ided by in ol ed ne - wo ks. INTERWORKING MECHANISMS A ending o he ou in e wo king le els iden i- ied, he ea e we p o ide a classi ica ion o hose in e wo king mechanisms ha cons i u e he basic enable s/building blocks in each le el. Figu e 3 illus a es he add essed in e wo king le els and mechanisms, and ela es hem o he main ne wo k nodes and p o ocol laye s wi hin wi eless ne wo ks p e iously illus a ed in Fig. 2. LEVEL A: INTERSYSTEM AAA Mechanisms included he e aim a ex ending au ho iza ion, au hen ica ion, and accoun ing (AAA) unc ions among wi eless ne wo ks, allowing use s o pe o m au hen ica ion and au ho iza ion p ocesses in a isi ed ne wo k a ending o secu i y sui es and subsc ip ion p o- iles p o ided by hei home ne wo ks. As well, he deploymen o one bill solu ions ad oca es o he exis ence o mechanisms o ans e accoun ing and cha ging da a be ween wi eless ne wo ks. All hese unc ionali ies a e basically achie ed by: • Adop ion o lexible AAA amewo ks able o suppo mul iple au hen ica ion me hods (e.g., he Ex ensible Au hen ica ion P o ocol [EAP] de ined in IETF RFC 3748 p o ides suppo o he eliable anspo o di e en au hen i- ca ion p o ocols). • Deploymen o addi ional unc ionali y such as AAA p oxy/ elay unc ions and ela ed signal- ing in e aces be ween ne wo ks (e.g., he Diame e p o ocol de ined in RFC 3588 p o- ides he minimum equi emen s o an AAA p o ocol be ween ne wo ks). Figu e 3. In e wo king le els and ela ed in e wo king mechanisms. Wi eless ne wo k A Wi eless ne wo k B WNC p o ocol s ack A WNC se e s Radio link p o ocol s ack RAT A Base s a ions/ adio con olle s Ne wo k laye Ne wo k nodes (e.g. ne wo k ga eway, mobili y ancho ing nodes) WNC p o ocol s ack B WNC se e s Radio link p o ocol s ack RAT B Base s a ions/ adio con olle s Ne wo k laye Ne wo k nodes (e.g. ne wo k ga eway, mobili y ancho ing nodes) In e sys em AAA ( lexible AAA amewo ks; in e sys em AAA p o ocols; iden i y selec ion suppo ) Link laye hando e op imisa ion (in e -RAT con igu a ion in o ma ion; in e -RAT measu emen s con ol and epo ing; in e -RAT esou ce ese a ion; in e -RAT esou ce a ailabili y knowledge) Visi ed ne wo k se ice access (Le el A) In e sys em use da a ans e In e sys em se ice access (Le el B) Ne wo k laye hando e In e sys em se ice con inui y (Le el C) Ne wo k laye hando e op imisa ion In e sys em seamless se ice con inui y (Le el D) SALLENT LAYOUT 4/8/10 12:57 PM Page 25 Au ho ized licensed use limi ed o: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 a 14:58:00 UTC om IEEE Xplo e. Res ic ions apply. IEEE Wi eless Communica ions • Ap il 2010 26 • Enhanced ne wo k disco e y mechanisms o iden i y selec ion so ha mobile e minals could know in ad ance whe he hei home ne wo k’s c eden ials a e alid o AAA con- ol in a isi ed ne wo k. No ice ha nowa- days, wi eless ne wo ks do no p o ide such in o ma ion o e he ai in e ace, and he ypical app oach is o ha e mobile e minals p econ igu ed wi h a lis o allowed access ne wo ks. LEVEL B: INTERSYSTEM USER DATA TRANSFER These mechanisms enable he ans e o use da a be ween ne wo ks in o de o gi e access o speci ic se ices p o ided in a ne wo k o he han he se ing one. A common app oach o en o ce use da a ans e be ween ne wo ks elies on unneling p o ocols such as he Laye 2 Tunneling P o ocol (L2TP) de ined in RFC 2661 o he IPsec unnel mode de ined in RFC 2401. Tunnels may be es ablished ei he di ec ly be ween mobile e minals and emo e NGs o may equi e addi ional dedica ed ne wo k nodes. As an example, 3GPP speci ica ions o WLAN access o 3GPP packe -swi ched se ices [7] manda es he suppo o he IPsec Encapsula ing Secu i y Payload (ESP) p o ocol desc ibed in RFC 4303 o in e sys em use da a ans e . LEVEL C: NETWORK LAYER HANDOVER These mechanisms a e equi ed when se ice con inui y be ween wi eless ne wo ks elies on he main enance o a pe manen mobile e mi- nal IP add ess. In his ega d, Mobile IP (MIP) desc ibed in RFC 3344 was he ini ial p oposed s anda d o se e he needs o globally mobile use s who wish o connec o he In e ne and main ain connec i i y as hey mo e om one ne wo k o ano he . MIP is a ne wo k laye mobili y solu ion ha co e s bo h hando e and loca ion managemen aspec s. MIP is based on a edi ec ion app oach achie ed by a home agen (HA) unc ionali y ha main ains a binding be ween he global IP add ess assigned o he e minal (i.e., home add ess [HoA]) and he p o- isional IP add ess (i.e., ca e-o add ess [CoA]) alloca ed empo a ily wi hin he se ing wi eless ne wo k. Wi hin he MIP solu ion, he mobile e minal i sel is in cha ge o upda ing he add ess binding o i s HA as he CoA changes (i.e., hos -based mobili y). On such a basis, se - e al IP mobili y p o ocols ha e been p oposed o e he pas se e al yea s o complemen o enhance MIP o e IP 4 ne wo ks (e.g., e e se unneling in RFC 3024) as well as IP 6 ne wo ks (e.g., MIP 6 desc ibed in RFC 3775 and Hie a - chical MIP 6 o localized mobili y desc ibed in RFC 5380). Mo e ecen ly, ne wo k-based IP mobili y solu ions whe e he e minal is no di ec ly in ol ed in managing IP mobili y (e.g., P oxy MIP 6 de ined in RFC 5213) a e also being in oduced in wi eless ne wo ks [13]. As an example, he 3GPP LTE/SAE ne wo k allo- ca es HA unc ionali y o MIP-based mobili y ancho ing (ei he hos -based o ne wo k-based) be ween LTE/SAE and o he non-3GPP access ne wo ks [10]. I is impo an o men ion he e ha , when ocusing on se ice con inui y o com- mon se ices (i.e., hose a ailable in bo h home and isi ed ne wo ks as illus a ed in Fig. 1), se - ice con inui y can also be p o ided by he se - ice i sel (e.g., applica ion-based mobili y ely- ing on SIP as conside ed in [14]) wi hou he need o ne wo k laye hando e solu ions. LEVEL D: NETWORK LAYER HANDOVER OPTIMIZATION While simple ne wo k o applica ion laye hand- o e solu ions may su ice o in e sys em se ice con inui y, hey may no be able o sa is y he equi emen s o seamless mobili y. In pa icula , du ing a hando e , la encies ela ed o adio link laye s (e.g., new adio link es ablishmen ) and ne wo k laye ope a ion (e.g., mo emen de ec- ion, new IP add ess con igu a ion, and binding upda es) can u n in o a pe iod du ing which he e minal is unable o send o ecei e packe s. In his espec , se e al op imiza ion mechanisms ha e been p oposed o educe he hando e la ency due o ne wo k laye ope a ions. As an example, RFC 4068 de ines as hando e ex en- sions o MIP 6 so ha e minals can acqui e a new alid CoA be o e a hando e occu s, and whe e unneling be ween he old and new CoAs educes he binding upda e la ency. Besides, when hando e akes place be ween ne wo ks in di e en adminis a i e domains, p e-au hen ica- ion schemes such as he one p oposed in [15] can help educe non-negligible au hen ica ion and au ho iza ion delays. Also, con ex ans e mechanisms (e.g., Con ex T ans e P o ocol [CXTP] de ined in RFC 4067) can be used o p econ igu e di e en ne wo k (and also link) laye pa ame e s in he a ge ne wo k and so a oid e-ini ia ion o some signaling o and om he e minal. Finally, ne wo k disco e y mecha- nisms (see RFC 5113 o a de ailed discussion) also ha e a c ucial ole in hando e op imiza ion since hey can con ey speci ic in o ma ion need- ed o op imized mobili y. LEVEL D: LINK LAYER HANDOVER OPTIMIZATION Radio link laye ope a ions could also in o- duce delays in he hando e p ocess. Hence, se - ing up he new adio link could ake se e al s eps (e.g., scanning, au hen ica ion, and associa- ion in 802.11) ha hando e op imiza ion mech- anisms should ha e o ei he bypass o minimize he la ency o while connec ed o he p e ious link, especially o he single- adio ope a ion case. A he same ime, hando e op imiza ion mechanisms can allow o mo e e icien adio esou ce usage (e.g., ne wo k-con olled in e - RAT mobili y). Hence, among he main aspec s co e ed by such mechanisms we ha e: • P o ision o in e -RAT con igu a ion in o ma- ion abou neighbo ing BSs o enhance he adio scanning p ocess and guide ne wo k selec ion decisions. • In e -RAT measu emen s con ol and epo - ing o imp o e hando e ini ia ion. • In e -RAT esou ce ese a ion. This could imply he p econ igu a ion o some adio link con ex s (e.g., QoS con ex s) in he a ge RAT. • In e -RAT esou ce a ailabili y knowledge (e.g., check o epo ing p ocedu es) in o de o enhance hando e decisions. When ocusing on se ice con inui y o common se ices, se ice con inui y can also be p o ided by he se ice i sel (e.g., applica ion- based mobili y elying on SIP as conside ed in [14]) wi hou he need o ne wo k laye han- do e solu ions. SALLENT LAYOUT 4/8/10 12:57 PM Page 26 Au ho ized licensed use limi ed o: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 a 14:58:00 UTC om IEEE Xplo e. Res ic ions apply. IEEE Wi eless Communica ions • Ap il 2010 27 As an example, in e -RAT hando e mecha- nisms speci ied be ween 3GPP GSM and UMTS ne wo ks co e mos o he a o emen ioned aspec s. DISCUSSION ON RELEVANT INTERWORKING APPROACHES Acco ding o p e ious in e wo king le els and ela ed mechanisms, in his sec ion we analyze some o he mos ele an p oposed solu ions. I-WLAN I-WLAN a chi ec u e [7], commonly e e ed o as loose coupling in e wo king, is a ge ed o co e in e wo king le els A and B o packe se ices (i.e., Gene al Packe Radio Se ice [GPRS]). Following he basic a chi ec u e ep e- sen a ion o a gene ic wi eless ne wo k in o- duced in Fig. 2, Fig. 4 illus a es bo h he main ne wo k elemen s wi hin 3GPP UMTS and WLAN ne wo ks and he new main ne wo k unc ions (ma ked wi hin ci cles) and in e aces (highligh ed in i alics) added o I-WLAN in e - wo king suppo . As o he 3GPP UMTS ne wo k, ne wo k ga eway unc ions a e p o ided by he ga eway GPRS suppo node (GGSN); WNC unc ions such as mobili y managemen (MM) and session managemen (SM) a e mainly handled wi hin he con ol plane o he se ing GSN (SGSN) and wi h he suppo o a home subsc ibe se e (HSS) da abase; and UMTS adio link p o ocol s ack and ela ed unc ions (e.g., RRM) a e dis- ibu ed among NodeBs (i.e., naming con en ion o UMTS BSs) and adio ne wo k con olle s (RNC). As o he WLAN access ne wo k, he IEEE 802.11 p o ocol s ack (e.g., MAC and physical laye ) is en i ely handled by access poin s (APs); WNC unc ions such as access con- ol can be suppo ed by means o AAA se e s using IETF p o ocols (e.g., RADIUS p o ocol be ween APs and AAA se e s o EAP/IEEE 802.1X access con ol); and gene ic ou e s can p o ide NG unc ions o ex e nal ne wo ks o se ice pla o ms. Thus, in e wo king le el A is achie ed by he alloca ion o a 3GPP AAA se e in he 3GPP ne wo k and AAA p oxy unc ions wi hin he WLAN ne wo k. The in e ace be ween hem is based on he Diame e p o ocol, which can be used o he ans e o , say, EAP messages o au hen ica ion and au ho iza ion. In his espec , speci ic EAP ex ensions o 802.1X access con ol ha e been de ined o allow au hen ica ion based on UMTS c eden ials (i.e., EAP-AKA de ined in RFC 4187). As well, he lis o a ailable in e - wo king UMTS ne wo ks can be p o ided h ough he WLAN connec ion by means o he ne wo k disco e y mechanism de ined in RFC 4284. Rega ding le el B, IPsec ESP is used o p o- ide secu e unnels be ween e minals connec ed o he WLAN and a new node named packe da a ga eway (PDG) wi hin he 3GPP ne wo k. The PDG basically beha es as a GGSN o WLAN use s. F om he e minal side, he I-WLAN model basically equi es a con en ional 3G ne wo k in e ace, a 802.11 ne wo k in e ace suppo ing EAP/802.1X au hen ica ion (e.g., Wi eless P o- ec ed Access [WPA] ce i ica ion) and suppo o IPsec wi hin he IP p o ocol s ack. GENERIC ACCESS NETWORK The GAN [8], also e e ed o as igh coupling in e wo king, p o ides a solu ion o ex end cellu- la ci cui and packe se ices o e IP b oadband Figu e 4. I-WLAN in e wo king model. Le el B IPsec IP laye GGSNPDG 3GPP AAA se e HSS WNC se e s (SGSN con ol plane) IP laye 802.11 adio s ack EAP o e RADIUS/ diame e Diame e - based in e ace IPsec-based in e ace WNC se e s (e.g. AAA se e /p oxy) T anspo Le el B Le el A Dual- adio ope a ion IPsec 802.11 in e ace 3G in e ace EAP- AKA EAP- AKA SM/ MM SM/ MM 3GPP adio s ack RNC and NodeBs UMTS ne wo k 802.11 adio s ack Te minal equipped wi h 802.11 and 3G ne wo k in e aces 3GPP adio s ack IP laye Applica ion Da a plane WLAN access ne wo k IP ou e Access poin s Con ol plane Con ol plane Da a plane Ex e nal ne wo ks, se ice pla o ms Ex e nal ne wo ks, se ice pla o ms IP laye Rega ding Le el B, IPsec ESP is used o p o ide secu e unnels be ween e minals connec ed o he WLAN and a new node named Packe Da a Ga eway (PDG) wi hin he 3GPP ne wo k. The PDG basically beha es as a GGSN o WLAN use s. SALLENT LAYOUT 4/8/10 12:57 PM Page 27 Au ho ized licensed use limi ed o: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 a 14:58:00 UTC om IEEE Xplo e. Res ic ions apply. IEEE Wi eless Communica ions • Ap il 2010 28 access ne wo ks. Hence, he GAN model is no es ic ed o 802.11 access ne wo ks, ye deploy- ing GAN o e 802.11 is he mos common app oach. Mo eo e , dual-mode cellula /802.11 e minals complian wi h GAN speci ica ions a e mo e widely known as unlicensed mobile access (UMA)-enabled e minals. The GAN model p o ides in e wo king le els B, C, and D. Figu e 5 illus a es he main aspec s o he GAN in e wo king app oach wi hin he same UMTS/WLAN scena io p e iously desc ibed o I-WLAN. The h ee key elemen s o he GAN model, as shown in Fig. 5, a e a new ne wo k elemen named GAN con olle (GANC) loca ed wi hin he 3GPP ne wo k, a UMA-enabled e minal, and a new in e ace be ween bo h elemen s speci ied by 3GPP. The GANC se es much like an RNC, eusing legacy 3GPP in e aces owa ds he co e ne - wo k. Rega ding he UMA-enabled e minal, i is a 3GPP e minal wi h embedded 802.11 com- munica ions. Hence, he in e ace de ined be ween GANC and UMA-enabled e minals comp ises new p o ocols wi h unc ions simila o he UMTS adio esou ce con ol (RRC) p o- ocol (e.g., Gene ic Access RRC [GA-RRC]) along wi h legacy 3GPP con ol plane p o ocols (e.g., SM and MM). The ans e o all he in o - ma ion be ween he e minal and he GANC h ough he WLAN ne wo k uses IPsec unnel- ing mechanisms. On his basis, in e wo king le el B is p o id- ed by he ac ha GANC ac ually cons i u es a ga eway owa d 3GPP se ices. Also, le el C is buil up on legacy 3GPP hando e p ocedu es so ha es ablished da a sessions o calls could be ans e ed be ween he GANC, ac ing as an RNC om he ne wo k side, and he co e- sponding RNCs wi hin he UMTS ne wo k wi h- ou se ice disconnec ion. Conce ning le el D, hando e la encies simila o hose be ween UMTS cells can be achie ed by he ac ha UMA-enabled e minals suppo dual- adio ope a ion. Hence, a UMA-enabled e minal can connec o he GANC ia a WLAN while main- aining he UMTS ne wo k connec ion so ha he a o emen ioned in e wo king mechanisms o ne wo k o link laye hando e op imiza ion a e no necessa y. On he con a y, le el A is ou o he scope o he GAN speci ica ions since he GAN model only equi es IP connec i i y om he WLAN access ne wo k and does no deal wi h 802.11-speci ic issues such as access con ol. WIMAX AND 3GPP NETWORKS INTERWORKING The ini ial in e wo king solu ion conside ed wi hin he WiMAX Fo um and 3GPP is based on he I- WLAN a chi ec u e by 3GPP. Hence, such a solu- ion is al eady a ailable o he i s elease o he WiMAX a chi ec u e (WiMAX NWG Release 1 [9]), and co e s in e wo king le els A and B. Then, in he con ex o 3GPP LTE/SAE ne wo ks, a chi ec u e enhancemen s a e being speci ied o p o iding IP connec i i y using non-3GPP accesses [10]. In pa icula , suppo o MIP and PMIP- based mobili y is being in oduced o achie e le el C. As well, unc ionali y in ended o p o ide ne - wo k disco e y and selec ion assis ance da a is being speci ied (i.e., access ne wo k disco e y and selec ion unc ion [ANDSF]). Besides, op imized in e wo king solu ions be ween WiMAX ne wo ks and 3GPP LTE/SAE Figu e 5. GAN in e wo king model. 3GPP adio s ack RNC and NodeBs HSS GGSN GANC IPs ack GA-RPC SM MM IP laye Da a plane WNC se e s (SGSN con ol plane) UMTS ne wo k Con ol plane Da a plane Ex e nal ne wo ks, se ice pla o ms UMA-enabled e minal IP s ack GA-RRC 3GPP SM/MM Le els B,C Le els B,C Dual- adio ope a ion Con ol plane 3GPP s anda dised in e ace AAA p o ocols (Ou o scope o GAN) WNC se e s (e.g., AAA se e ) 802.11 adio s ack Da a plane IP laye IP ou e Access poin s GAN in e ace 802.11 s ack 3GPP adio s ack T anspo IP laye Applica ion Le el D WLAN access ne wo k The ini ial in e wo k- ing solu ion consid- e ed wi hin WiMAX Fo um and 3GPP is based on he I-WLAN a chi ec u e by 3GPP. Hence, such a solu ion is al eady a ailable o he i s elease o he WiMAX a chi ec u e (WiMAX NWG Release 1) and co e s in e wo king Le els A and B. SALLENT LAYOUT 4/8/10 12:57 PM Page 28 Au ho ized licensed use limi ed o: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 a 14:58:00 UTC om IEEE Xplo e. Res ic ions apply. IEEE Wi eless Communica ions • Ap il 2010 29 ne wo ks a e also unde conside a ion o achie e in e wo king le el D [11]. In his ega d, an app oach a ge ed o co e in e wo king le els A, B, C, and D is illus a ed in Fig. 6 and discussed nex o a 3GPP LTE/SAE ne wo k coexis ing wi h a WiMAX access se ice ne wo k (ASN). As shown in Fig. 6, ega ding he 3GPP LTE/SAE ne wo k, adio link laye p o ocols and unc ions a e now en i ely alloca ed in an enhanced NodeB (eNB), and an e ol ed packe co e (EPC) han- dles WNC unc ions ia so-called MME se e s and p o ides NG unc ionali ies h ough PDN ga eways (PDN-GWs). As o he WiMAX ASN, he IEEE 802.16e adio p o ocol s ack is also en i ely alloca ed o BSs, and a sepa a e ne wo k node named ASN ga eway (ASN-GW) could hold bo h da a plane ancho ing unc ionali ies (e.g., o mobili y ancho ing wi hin WiMAX ASN) and con ol plane WNC unc ions (e.g., au hen ica ion based on an EAP amewo k). Acco ding o his in e wo king solu ion, in e - wo king le el A is achie ed as in I-WLAN solu- ion by means o AAA p oxy unc ions in he ASN-GW, a 3GPP AAA se e loca ed in he 3GPP LTE/SAE ne wo k, and he use o speci ic EAP au hen ica ion mechanisms. Then le els B and C a e buil up on MIP-based mobili y solu- ions al eady suppo ed wi hin he WiMAX ASN (e.g., he ASN-GW da a plane can alloca e o - eign agen [FA] unc ions o MIP 4 o access ou e [AR] unc ions o MIP 6) and on adding da a plane ancho ing unc ions wi hin he 3GPP LTE/SAE ne wo k (e.g., HA unc ions o MIP 4 o MIP 6 a e alloca ed in he PDN-GW). The MIP clien can be loca ed in he e minals, as shown in Fig. 6, o in he ASN-GW i PMIP solu ions a e used ins ead be ween he ASN-GW da a plane and he 3GPP PDN-GW. Conce ning in e wo king le el D, in e sys em hando e op imiza ion mechanisms a e dis- cussed in [11] o p o ide seamless mobili y o single- adio ope a ion e minals, a condi ion ha imposes he ha des equi emen s on he in e wo king solu ion. In his espec , as illus- a ed in Fig. 6, bo h WiMAX and 3GPP LTE/SAE adio link p o ocol s acks would need o be modi ied o add suppo o in e -RAT measu emen con ol and epo ing along wi h he deli e y o in e -RAT BS in o ma ion. Besides, suppo o esou ce ese a ion, as well as p e- egis a ion (i.e., co e ing aspec s such as access con ol, con ex ans e s and de aul bea e se ice es ablishmen ), is en is- aged by means o unneling legacy signaling messages be ween e minals and BSs o WNC se e s in he a ge ne wo k while being a ached o he se ing ne wo k. Hence, as example, a e minal connec ed o an eNB can ini ia e a hando e o WiMAX by unneling a 802.16e Hando e Reques message h ough he 3GPP LTE/SAE ne wo k owa ds he a ge WiMAX BS. T anspa en ans e o in e sys em hando e signaling messages equi es each adio p o ocol s ack o p o ide anspo unc ionali y o signaling messages o a di e en echnology. Mo eo e , anspa en signaling ans e be ween ne wo ks equi es he deploymen o an in e ace among MME and ASN-GW Con ol Plane as shown in Fig. 6 (e.g., S101 in e ace in [11]) o in e sys em hando e signaling. Finally, i is wo h no ing ha o dual- adio ope a ion, p e ious mechanisms would no be necessa y. Ne e heless, dual- adio ope a ion may no be easible due o e.g., adio equency coexis ence issues when adio equencies used in he wo RATs a e close o each o he . Figu e 6. WiMAX and 3GPP LTE/SAE in e wo king model. SM MM 3GPP adio s ack 3GPP cells measu emen s Le el D IP laye PDN-GW EAP- AKA HA o non-3GPP IP laye T anspo Applica ion Le el D Le el A MIP Dual-mode ne wo k in e ace Dual-mode WiMAX/3GPP LTE e minal EAP Le el B,C Le els B,C Le el A IP laye EAP o e diame e 802.16 adio s ack 802.16 adio s ack ASN-GW da a plane In e sys em hando e signaling in e ace Single- adio ope a ion Diame e - based in e ace 3GPP AAA se e HSS eNB MIP-based in e ace WNC se e s (MME) WNC se e s (ASN-GW con ol plane) WiMAX ASN Con ol plane 802.16 BS Radio s ack modi ica ions o accoun o in e -RAT issues T ans e capabili ies o in e sys em hando e signalling 3GPP neighbou ing BS in o ma ion Con ol plane Da a plane Da a plane 3GPP LTE/SAE ne wo k Ex e nal ne wo ks, se ice pla o ms Ex e nal ne wo ks, se ice pla o ms 3GPP adio s ack SM/MM Le el D I is wo h no ing ha o dual- adio ope a ion, p e ious mechanisms would no be necessa y. Ne e heless, dual- adio ope a ion may no be easible due o, say, adio equency coexis ence issues when adio equencies used in he wo RATs a e close o each o he . SALLENT LAYOUT 4/8/10 12:57 PM Page 29 Au ho ized licensed use limi ed o: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 a 14:58:00 UTC om IEEE Xplo e. Res ic ions apply. IEEE Wi eless Communica ions • Ap il 2010 30 IEEE 802.21 INTERWORKING The s anda d IEEE 802.21 “Media-Independen Hando e (MIH) Se ices” [6] is mainly con- cei ed as an enable o in e wo king le el D be ween he e ogeneous link-laye echnologies (e.g., IEEE 802 and cellula ne wo ks) and ac oss IP subne bounda ies. The s anda d de ines a new link laye unc ionali y (i.e., MIH Func ion [MIHF]) o be added wi hin e minals and ne wo ks oge he wi h he p o ocol equi ed o message exchanging be ween hem. This MIHF en i y is allowed o ha e some con ol on link laye beha io (e.g., link ac ions such as powe -up and link con igu a ion) and can collec link in o ma ion (e.g., link s a us polling o e en - igge ed in o ma ion). To ha end, adio link laye s ha e o add suppo o a media spe- ci ic in e ace wi h he MIHF unc ion. On he o he hand, he MIHF en i y p o ides a media independen in e ace o uppe laye s o he p o- ocol s ack (deno ed as MIH use s in 802.21 s anda d’s no a ion) and o e s a se o gene ic se ices classi ied as e en , command and in o - ma ion se ices [3]. Th ough hese se ices, uppe laye s can con ol lowe laye ’s beha io (e.g., emo e/local link ac ions and hando e commands) and acqui e ele an in o ma ion o mo e e icien hando e decisions (e.g., local/ emo e link s a us and in o ma ion abou di e en a ailable ne wo ks and hei se ices). The 802.21 se ice model o e s a lexible ame- wo k o acili a e di e en hando e app oaches. Hence, while se ed by a gi en wi eless ne wo k, he MIHF en i y o he mobile e minal could in e ac wi h a MIHF en i y in he se ing ne - wo k in o de o e ie e in e -RAT ne wo k in o ma ion and ini ia e an in e - echnology han- do e by indica ing a p e e ed lis o candida e access ne wo ks. As well, hando e could also be ini ia ed om he ne wo k side. In bo h cases, 802.21 signaling enables in e sys em adio esou ce a ailabili y check and esou ce p epa a- ion (e.g., in e sys em esou ce ese a ion) be ween in ol ed ne wo ks and p o ides he e - minal wi h he equi ed con igu a ion o he ese ed esou ces a he a ge ne wo k. I ’s wo h no ing ha he scope o 802.21 is limi ed o hando e ini ia ion and p epa a ion phases, while he execu ion phase is no co e ed (e.g., mobili y handling in uppe laye s is s ill equi ed o se ice con inui y be ween ne wo ks). The adop ion o he 802.21 solu ion in a WiMAX-3GPP LTE/SAE in e wo king scena io such as he one desc ibed in Fig. 6, would equi e he alloca ion o unc ional MIHF en i- ies in he e minal and wi hin bo h wi eless ne - wo ks along wi h he co esponden anspo capabili ies o exchange MIH p o ocol messages. In his sense, MIH signaling o/ om e minals could be ans e ed by using speci ic mecha- nisms in oduced in he adio link laye s (IEEE 802.16g ex ension adds such a suppo bu 3GPP does no conside i ye ). Ano he possibili y would be he ans e o MIH signaling op op o he IP bea e se ice p o ided by he ne wo k (e.g., IETF MIPSHOP wo king g oup is speci y- ing suppo o sending 802.21 messages o e IP ne wo ks). CONCLUDING REMARKS AND FUTURE TRENDS This a icle has elabo a ed a comp ehensi e amewo k o analyze and ca ego ize in e wo k- ing solu ions in he e ogeneous wi eless ne - wo ks. O e such a basis, some o he mos ele an in e wo king solu ions being conside ed in di e en s anda diza ion bodies ha e been discussed. As o WLAN and cellula in e wo king solu- ions, I-WLAN has been no ed o cons i u e a simple bu e sa ile solu ion o ex end cellula packe se ices access. I-WLAN equi es li le changes in e minals and ne wo ks, hough i is no able o p o ide seamless mobili y and ne - wo k-con olled mobili y. Cu en ly, I-WLAN in e wo king a chi ec u e is being ex ended o handle mobili y be ween I-WLAN and 2G/3G by means o adding suppo o MIP-based mobili- y, hus con e ging o he ype o solu ions being conside ed wi hin LTE/SAE o in e wo king wi h non-3GPP access ne wo ks. On he o he hand, 3GPP GAN solu ion equi ing dual- adio ope a ion UMA-enabled e minals was i s ly de eloped o 2G, hen ex ended o 3G bu no ex ension is en isioned so a wi hin LTE/SAE, whe e PDN-GW in e sys em mobili y ancho ing wi h MIP and PMIP-based solu ions a e a o ed. Ne e heless, UMA- e minals a e a eali y oday as some ope a o s a e emb acing such a echnol- ogy bo h in business and in he home 802.11/cel- lula access. Conce ning possible solu ions o achie e op i- mized seamless hando e wi h single- adio e - minals be ween LTE/SAE and WiMAX, wo main app oaches ha e been discussed. While 3GPP en isions a ailo ed solu ion, IEEE 802.21 e o s a e ying o push o a gene ic solu ion o in e - echnology hando e . Ne e heless, he adop ion o a gene ic solu ion such as IEEE 802.21 in 3GPP ne wo ks is no as s aigh o - wa d as wi hin IEEE-based echnologies, which al eady sha e a common a chi ec u al ame- wo k. In his ega d, he 802.21 s anda d is becoming a cen al piece o an IEEE 802 wide ini ia i e so ha , in addi ion o de ining he new 802.21 s anda d i sel , IEEE is making changes o exis ing access echnologies speci ica ions (e.g., WLAN, WiMAX) o suppo 802.21 ela - ed hando e unc ionali y. Mo eo e , IEEE 802.21 solu ion is expec ed o cons i u e a co - ne s one wi hin IEEE 802’s p oposal o IMT- Ad anced [3], whe e enhanced IEEE 802.16 and IEEE 802.11 adio echnologies will be in eg a - ed unde he in e wo king model o e ed by 802.21. ACKNOWLEDGMENTS This wo k has been pa ly unded by he Span- ish Minis y o Science and Inno a ion unde he TelMAX p ojec , belonging o he Ingenio 2010 p og am. This wo k has also been pa ly suppo ed by he Spanish Resea ch Council and FEDER unds unde a COGNOS g an ( e . TEC2007-60985. The au ho s a e e y hank ul o he e iewe s o hei cons uc i e commen s. IEEE 802.21 solu ion is expec ed o cons i u e a co - ne s one wi hin IEEE 802’s p oposal o IMT-Ad anced, whe e enhanced IEEE 802.16 and IEEE 802.11 adio echnologies will be in eg a ed unde he in e wo king model o e ed by 802.21. SALLENT LAYOUT 4/8/10 12:57 PM Page 30 Au ho ized licensed use limi ed o: UNIVERSITAT POLITÈCNICA DE CATALUNYA. Downloaded on July 22,2010 a 14:58:00 UTC om IEEE Xplo e. Res ic ions apply.