Lte Volte and Srvcc

7/26/2019 Lte Volte and Srvcc

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Regardless of the pace of LTE networks deployment around the world (faster in someareas, slower in others), the number of users with 4G deices is intensiely growing!

Thanks to factors such as lower costs " due to the gain of production scale, and also by

encouraging migration to 4G plans " offered by operators who already hae an aailablenetwork, more and more people hae access to new serices and benefits that thistechnology offers!

#oweer, as much as the current data serices are improed, and that progress in thearea lead to the adoption of new serices, a basic necessity should still continue to e$ist

at least for a while% oice calls&

'hile making a oice call may seem simple, largely depends on the scenario where theuser is, and alternaties aailable for its completion! o it is necessary to understandwell what are the possibilities and the most important concepts of these key scenarios!

n the first generation of cellular networks, the communication through oice calls wasthe main goal, and was based on a circuit switched topology or *channels* (+ +ircuited

witched)!

er time, the need for other serices (data&) has emerged! -oice calls hae come into

e$istence with these new serices! .s demand increasead, these new serices were

supported by a new domain, the /"based packet"switched (/ /acket witched)! Thefollowing figure shows how these two domains work!


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.nd in LTE (4G) system we had another great change% the + domain has beene$tinguished& LTE networks are based e$clusiely on the / domain, and oice sericesshould be carried out in other ways (as we shall see)!

0ut as we mentioned, regardless of network topologies, oice serices are still needed!(f course, they slightly decreased compared to a few years ago, but are still significant,ie their demand continues)!

'ith the continue growth of LTE networks, let*s try to understand a little more theconcepts, alternaties and solutions for any user to make a oice call on an LTE network1

Note: All telecomHall articles are originally written in Portuguese. Following we translateto English and Spanish. As our time is short, maybe you find some typos sometimes we

!ust use the automatic translator, with only a final and "#uic$" re%iew&. 'e apologi(e andwe ha%e an understanding of our effort. )f you want to contribute translating * correctingof these languages, or e%en creating and publishing your tutorials, please contactus: contact.

How, when and where?

2irst of all, we need to understand how, when and where oice calls can occur!

n the 3G legacy networks, oice calls are made practically only on circuits " for each call

(+ domain)!

n G legacy networks, oice serices can use the + domain, but can also be made

through TT (er The Top) solutions, applications that encapsulate the oice andtransport ia an / domain (/), but who lack the 5o re6uirements needed to ensure

good communication " with the 7on G0R type serices (no bit rate guarantee)! E$ample%kype! 7ote% t is ery unusual, but it is also possible to make TT oice calls on 3Gnetworks! n fact, there may be TT solutions in any technology " it can be used inlegacy networks, and also in others such as 'i2i " which are already commonly used for-o/!

.nd in LTE networks, oice calls can be fully /"based, can use TT solutions ia 4G, or

be transferred to the legacy 3G8G!
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.s we begin to see, there are many alternaties! .s usual, we will easily see each one!

7ote% n this tutorial, we will always refer to oice calls (originating and8or terminating)9#oweer, : serices are also included!

Alternative to voice calls in a generic 2G-3G-4G Network Topology

.nd the best way to understand the alternaties or possibilities of making oice calls inLTE network (4G), it is to start from a 3G"G"4G network topology ery simplified "

considering only the main elements inoled!

.s we can see in the following figure, the LTE (E/+) has no direct *link* to the + network

" as we hae seen, it is designed to take care of purely / (/) calls! t has no :ediaGateway directly connected, so no + call is supported by the ::E!


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n other words, if the user or ;E (;ser E6uipment) is on a LTE network, as shown in thetopology aboe, we can not make a oice call!

7ote% .s mentioned before and according to the topology aboe, the only way to hae

oice serices in LTE would be through TT serices such as kype! #oweer, thissolution is not discussed today!

f we understand this, it is also easy to reali


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To delier oice serices in LTE network is necessary to hae an infrastructure thatsupport it! n other words, there needs to e$ist an : (/ :ultimedia ubsystem or /:ultimedia +ore 7etwork ubsystem)! f an : is aailable, then the oice oer LTEmay be proided as long as a minimum set of : functionality and entities also arepresent!

7ote% : is much more complete, and hae more other purposes than the oice! The

oice is =ust another *application* of :, as we*ll see soon!

This minimum set of features and entities of the : (called -oLTE or ne -oice) was

standardi


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'hen we hae a new call (be it originated or terminated), it is managed by the :+ oflegacy network! -7+ is who mediates the oice signal and its related messages betweenthe :+ and the LTE network!

.lthough it is possible to carry out the deliery of oice and : serices to users LTE,the -olga was unsuccessful! This is because, as we hae seen, e$clusie inestment are

needed for this purpose! .t the same time howeer, global efforts to -oLTE increased (eginestments in :), and thus this alternatie eentually falled into disuse!

!"#$

0ut if in one hand operators follow seeking a complete LTE infrastructure (with full :)to meet multimedia serices and also purely LTE oice, this is not a topology that is

aailable in the short and een medium term!

'hile that reality doesn*t come, we must use the legacy network when there is the need

of oice and : deliery to LTE users!

.nd the most common alternatie to this is the +20 (+ 2all 0ack), an interim solutionuntil we hae full support for oice oer LTE!

.t +20 scheme, wheneer there is a demand for a new oice call, the LTE user is*backed* for a + legacy network, assuming that this proides an oerlapping coerage!n other words, with +20, a oice call is neer actie in LTE, but in legacy networks!

.t the end of the call in the legacy network, the ;E can re"register the LTE network!


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t goes something like this% the ;E is registered (also) in the legacy network! 'hen it gota call, the legacy network tells to LTE network% * hae a call to the ;E, can you ask it tocome here and make the call1*

To +20 be possible, users must be using dual mode deices, ie able to operate both inLTE network and in the legacy network!

To support +20, a new interface is introduced% the Gs, connecting the ::E to the

legacy :+!

.s the +20 is currently the most widely used option by seeral operators, let*s seesome basic scenarios of it (+20)!

!"#$ - %egistration and ocation'hen the +20 ;E is turned on, it registers itself in the two networks% LTE and legacynetwork (+)!

.nd to allow 6uick transfer to the legacy network (either 3G or G) when necessary, the

LTE network needs to know the location of the ;E!

2or this, the ::E, which tracks the location of the ;E in the LTE network, continuouslyproides location information to the legacy :+, using the new Gs interface!

The set of Gs messages then supports management of mobility, paging and :!


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!"#$ - &riginated !all

'e will continue, and assume that the ;E is initially coered by the LTE network, and

that there is an actie / connection!

'hen the ;E decides to originate a oice call, it sends an R: (erice Re6uest:essage) to the ::E (more specifically the ER " E$tended erice Re6uest)!

The ::E checks whether the ;E is +20 capable, and notifies the e7ode0 to transfer the;E to the legacy network!

0efore performing the ;E transfer, the e7ode0 can ask it to make R2 measures on

neighboring 3G8G network! The e7ode0 then decides the best network for the ;E andperforms the transfer!

nce the ;E camp in 3G8G network, it starts the call procedure as usual " the ;E startsthe call control procedures in legacy network!

!"#$ ' !all ( )ata !onnection in T*

.nd what happens if hae an actie data connection in the / LTE network, and decideto make a oice call1

There are two options%

The data are also transferred to the legacy network, or

The data are temporarily suspended, until return to the LTE network!

.lthough the first option seems the best, we must take into account that thetransmission of / data is also transferred% it can operate at much lower speeds (legacy

systems)! n addition, it may be that the legacy networks deny the / session due to lackof resources or for not being able to process it!

The interface is used to carry out the / session handoer for G (in this case, the

>T: " >ual Transfer :ode must e$ist, but this details escapes form our theme today)!

There are no 4G data handoer supported to 3G " in this case, the data is suspended!

The eR.0s 4G are released when the ;E performs the +20!

.n important information is that the is a *new* interface between ::E and G7 onGT/+3! .nd to support it, the G7 needs to be updated (most carriers do not want todo this without a strong =ustification)!

.nd Gn interface is already on GT/+?, which is the natie GT/ ersion for G networks!

o in this case only the ::E needs to be updated, and as it is a relatiely new node, it isprobably easier to do! 7ot to mention that the new G7 may hae natie support for

!


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!"#$ - Ter+inated !all

2inally, we hae the case of a terminated call for LTE user!

The call re6uest arries first to the :+ where the ;E was preiously registered!

'hen the :+ the receies call re6uest, it sends paging messages to the related ::Eia Gs interface!

This message is forwarded to the ;E, which is still connected to the LTE network!

f the user accepts the call, it sends an R: (erice Re6uest :essage) to the ::E!

The then ::E notifies the e7ode0 to transfer the ;E for the legacy network, and the

e7ode0 then decide the best network for the ;E to make the call!

!"#$ ' hat happens ater the end o the !" call?

'e hae seen that the 4G eR.0s are released when the ;E performs the +20! 0ut whathappens when the ;E ends the + call1

.bout what should follow ne$t (if the ;E should return or not to LTE as soon end the call+), there is no specific rule!

.nyway, the main possibilities are%

The upper layers forcing the *reselection* to LTE so that the ;E enters idle mode in legacy

network!

The operator send LTE *redirection* information in RR+ connection release message of legacy G

network after the call is finished! This will result again in reselection to LTE!

The lower layers (. " .ccess tratum in this case ;RR+ or GRR) reselect to LTE if the

reselection criterion is satisfied! n most cases, operators hae their parameters set such that

the reselection to LTE happen if there is a good LTE coerage area oerlapping the legacy

network!

VoT*

Eerything we hae seen so far is based on the making of oice call in the legacynetwork! 0ut as we hae seen these are *temporary* solutions until the *final* solution "-oLTE " is aailable!

.nd the final LTE oice solution (-oice oer /, or more specifically -oLTE) uses the :

backbone! .n e$ample of network topology supporting -oLTE is shown in the followingfigure!


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To make oice calls, LTE networks need to hae an :! 'hen the first LTE networksappeared, they had no :, and without :, it was not possible to make any calls toany /T7 or +!

'e hae spoken of the : before, but let*s remember!

./"

: is a backbone (network) at the application leel, which works on top of otherwireless networks and not =ust the LTE (as G, 3G, 'i2i and others)!

ts concept is 6uite broad, and to understand it with all its entities, possibilities,interfaces, protocols, and possibilities is an e$tremely difficult task, een for the moste$perienced in the sub=ect!

The : is not new% it already e$isted before the LTE (as well as other entities, such asthe E/+ /R+2, which also is not new&)!

ts complete specification consists of thousands and thousands of G// standards! 0ut

let*s try to understand in a simpler way than that found there!

.s its name suggests (/ :ultimedia erices), : offers seeral multimedia /serices, including -o/ (-oice oer /)! n :, oice is =ust *another* serice&

: brings together oice features such as authentication, serice authori


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these e$ist in the E/+% this is the reason why the pure E/+ without : can not processa oice call!

n other words, for -oLTE, access is made by the .E (e;TR.7 @ E/+), while oiceserice lies in the :!

.n analogy we can do is to consider the : being a car! .nd the LTE oice, as ourshuttle serice (to go from one place to another)!

'e can buy a ery basic car " 0asic ?!A engine, wheels, steering wheel and other minimum

parts% yes, we can go from one place to another!

r we can buy a *connected* car " ultra modern, powerful, tetra"fuel, with all the safety features,

.0, .ir bag, connected to the nternet, etc% we also go from one place to another !!! but we

can make seeral other things as well&

That*s more or less what happens with the :! t is used in con=unction with the LTE

network to support oice% both full : implementation and also the minimum :suggested implementation for -oice oer LTE!

0ut the telecommunications industry would rather not inest in a full :, or at least didnot hae sufficient reason to do it immediately! .nd for the adoption of the simpler :

oice solution, appear the -oLTE initiatie, which specifies a minimum set of features,and selects a simple choice when multiple options e$ist for certain features!

#oweer, not all of these features are re6uired for deliery of basic oice serices by theLTE network!

o let*s illustrate with a diagram (e$tremely simple) the implementation of a oice in :(-oLTE)!

Let*s assume that we will make a -oLTE call with a + network whatsoeer, for e$ample the

/T7 (/ublic witch Telephony 7etwork)!

.nd consider in the : only two simple elements, one for the control plane (with signaling) and

one for the user plane (with oice)!


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.nd the entry being the .E, or LTE network!

n :, the control element would be a / serer (soon we will talk about / " for now =ust

understand that when we hae a call re6uest to this serer, it sets up the call!)9 and the user

element would be a :edia Gateway!

n comparison with the legacy networks, the / erer is e6uialent to the :+ in themobile network topology and the media gateway is e6uialent to a typical :ediaGateway on any network topology, which is common in irtually any oice network tohandle calls!

The aboe concept is alid, but in practice the : consists of much more entities, asseen below! 7ote% 7ot all possible8e$isting entities and interfaces are shown in the

figure!


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Let*s (6uickly) see a little about these key elements!

7ote% >o not worry or try to understand eerything now about these elements!

Remember that our goal here today is not that! .nyway, it*s worth a read!

The :G+2 (:edia Gateway +ontroller 2unction) is the control element that

communicates with other /T7 networks! t is significant because it has to inter"networking function% can speak /, can speak ;/, can speak other signaling protocols!

The :":G' (: :edia Gateway) is the element that takes care of oice functions fore$ample making protocol translation re6uired to support the call! :ore specificallybetween the Real Time Transport /rotocol (RT/) to analog format or basic /+: in the +network9 and ice ersa!

The # (#ome ubscriber erer) is an element that also e$ists in the LTE E/+

(although appeared first in :), and its functions are similar!

The :R2 (:edia Resource 2unction) proides many serices related to oice, such asconferences, announcements, oice recognition and so on! t is always diided into twoparts, the :R2/ (:edia Resource 2unction /rocessor), for media streams, and the :R2+(:edia Resource 2unction +ontroller) that functions basically as an RT/ *mi$er*!

.n important concept, and that*s worth stand out here is the /ro$y, for e$ample to makefilters, identify where the users come from, the cases of roaming, etc! Remember that

we are talking about an / network! nstead of the users to speak directly with the /serer, they use the pro$y!

The ++2 (+all ession +ontrol 2unction) has some ariations!


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/"++2 (/ro$y ++2) among other tasks, proides 5o information related to the LTE

network! .cccess an .2 to oice serice, and proides the control functions *policy* and

*charging* to the /+R2!

"++2 (/ro$y ++2) is an interrogator!

.nd the "++2 (ering ++2)% the ++2 serer acts as a central node!

The 0G+2 (0order Gateway +ontrol 2unction) functions as a routing table (or table 0)and acts to help the "++2! t has basically routing decisions!

.s we speak, the : oice is a *serice* " the : is a serices *facilitator*! The :serices are proided through . (.pplication erers)!

ne such application is the oice! .nd there are also ideo serices, conference, etc!

n fact, sometimes the . are not considered as part of : (when we understand the

: as a +RE)!

.nd in :, the standard . for oice is the ::Tel (:ultimedia Telephony erice),

sometimes called :T. (:ultimedia Telephony .pplication erer)!

The 0+ (ession 0order +ontroller) is an element of the edges of the : to controlsignaling and often the media streams inoled in calls!

The "++2 will be responsible for call routing depending on where the other user (the

other party) are%

. 0G (ession 0order Gateway) if the the other party is in / domain9

. :G+8:G' if the other party is in the + domain (/T78/L:7)!

0+2 and TrG' are not shown in our figure, but are respectiely the control and userplane for other : networks, other / networks in general! They are similar to the:G+28:":G' " the re6uirements for reaching one or another type of network aredifferent, so also hae separate parts for performing the same functions but withdifferent networks!

".0

To support telephone signaling between the LTE network and telephone networks, the: uses / (ession nitiation /rotocol)! / is a standard protocol for establishingoice calls oer / networks!


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The code is open, and uses the *re6uest"response* model to allow communicationsessions!

There is a set of standard commands that can be used to initiate, manage and terminate

calls between two / deices!

The / has been adopted by : standardi


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nitially, the ;E sends a / message like *nite*, containing the description of one or more

measures for the oice session (nitial >/ " ession >escription /rotocol " ffer)!

Then the /"++2 forwards this same message to the "++2 (which has been identified during

the registration process)!

.ll going well, the termination network will hae sent a message of type *offer response* to the

"++2, and this sends this message to the /"++2, authori


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eeral other diagrams e$ist, with far more comple$ scenarios, but the basic idea can beseen aboe, and e$tended if necessary!

Let*s complete the tutorial today, now seeing the case where an initially established callon : has to be *transferred*!

"%V!!

2inally we come to our last alternatie listed at the beginning of this tutorial% R-++(ingle Radio -oice +all +ontinuity)!

The R-++ howeer is not an alternatie for deliery, but a rather a handoer process ofa oice call preiously started in the LTE (whether ne -oice " -oLTE LTE or : 2ull-oice)!

t is a call transfer method (handoer), in a simplified and reliably way, when an LTE userhas an actie oice session in : and is moing to areas without LTE coerage, but withlegacy 3G8G coerage!

The main adantage is that the call will not drop " will only be transferred to the +domain of the legacy networks!

f in the aboe case the ;E moes out of LTE coerage area with an actie call (but goesto a legacy 3G8G coerage), we must maintain the continuity of this actie oice call! nthis case, the R-++ is used% the procedure where the conte$t of an actie oice call onthe : is transferred to the + legacy network (e!g! : node conte$t transfer to the:+)!

The challenge with R-++ is to perform the handoer while the ;E is connected to only asingle radio at any gien moment!


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There are two ersions of R-++%

R-++ handoer to G: or ;:T, defined by G//9

R-++ #andoer to ?$RTT networks defined by the G//3!

To allow R-++ both the ;E and LTE networks, as also the legacy, must support R-++!2or this, a new special - interface is introduced between the ::E and the :+, whichruns on GT/3 protocol!

To support R-++, the : network should also include an application serer, called ++. (erer +entrali


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Reali


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.n efficient +20 solution re6uires the the T.+ "D L.+ mapping is so that the fallback toan e$ternal :+8L.+ be aoided, since this will further increase the call setup time!

Call quality% call 6uality in LTE is better when compared with the same third"partyapplications (TT)! This is due to specific 5o allocated to the call :, which may notbe present in common data applications!

Resource limitations for VoLTE% .:R"7' LTE re6uires much less resources and

datarate than G:, and we will hae many more users on the same bandwidth (spectralefficiency)!

Investment x Current et!or"% if eerything is *working well*, what would be thereason for inestment, since surely such inestments generate resistance fromcommercial and business areas1

The comparison that must be done is% nestment ersus (all) 0enefits of:8:G'80G+2!

Future#n any way all that discussion hereafter will more significance! +urrently we still

hae e$tensie legacy networks, capable of supporting these oice calls!

n this case, it is no problem to continue using this aailable infrastructure! Resistancewill only decrease when such capacity also decrease! 0ut in an LTE network, if the : issupported can make a -o/ call! o why would we need to make a + oice call1

CSFB x SRVCC%

t is not necessary to implement both solutions (+20 and R-++) at the same time, if the

network has a wide LTE coerage and a complete : backbone!

o f we implemente +20, it means we will not make the call setup using e$isting :

+ore, and that could take care of that call in LTE!

o n respect to the R-++% assuming the 0ackbone : is aailable! n this case, if the

register in the : is successful, the user do not need to do +20 " . oice call can be

simply initiated in LTE network using :!

+20 is a serice handoer procedure while R-++ is a coerage handoer procedure!

!ase "tdies and Analogies

'ith all that we hae seen today, let*s imagine some scenarios!

2irst, imagine that you are in a network that does not hae LTE :! Then the only wayto make a oice call, whether originated or terminated, is through using legacy 3G8G!

Cou need to be redirected8released from LTE to legacy 3G8G network to make a oicecall! Like a *reselection* from cell LTE to the 3G8G! nce the legacy network, you can

make the call normally, as you*re already used to!

.nd so, you =ust saw the +20 in practice&


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7ow suppose you are watching a ideo stream on 4G network, and receie a oice call!n this case, you need to go to the G network (in idle mode), and get the resources forto make that call in G!

.fter you end your oice call, you keep watching the ideo stream, but now in the Gnetwork (the handoer from G to 4G is not yet defined)!

Cou =ust saw the +20 with an actie data call&

7ow let*s imagine that you are in another LTE network, this time with :! n this case,you can make a oice call using / packets!

'e hae =ust seen a -oLTE call&

2urther, imagine that you are in one of these oice calls using packets in 4G! upposefurther you reach your 4G cell coerage edge! o the only option to keep your call is tohandoer it to the G (assuming this is the e$isting coerage)! Cour call will then

continue on the G network, but now as one + oice call! R-++&

f the R-++ is not supported, the call is dropped as soon as it leaes the LTE coerage

area!

f the R-++ is supported, a set of messages are e$changed, and the oice call istransferred (handoer) from LTE : to + domain of the 3G8G network!

.nd so, we hae =ust seen an e$ample of R-++ handoer&

.nd that*s all for today! 'e hope that the tutorial has managed to be useful for you thatsomehow are interested oice in LTE networks!

!onclsion

'e saw in this tutorial today, in a ery general way, the main ways to make oice calls(and :) in LTE networks!

The options or alternaties depend on seeral factors, such as aailable networktopology and the operator*s strategy!

>epending on the situation, the call can be originated in LTE ia data applications (TT-o/), be purely originated on LTE : (-oLTE), sent to be performed on other networks

through mechanisms deeloped for this purpose (+20) or transferred ia handoer " ifactie -oLTE call " to a legacy network (R-++)!

o, for a user who is a LTE coerage area, a number of considerations should bechecked, as the type of deice that it uses (whether supports +20), if the LTE networkhas an : that allows outgoing calls, if the cells supports R-++, etc!

0ased on the concepts seen here today, we hope you hae a position to fully understand

what happens when a user performs a oice call from an LTE network!

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