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.
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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
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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
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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)
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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.
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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.
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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.
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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 .
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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
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