CSFB & SRVCC

8/15/2019 CSFB & SRVCC

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What is CSFB and SRVCC in LTE?Regardless of the pace of LTE networks deployment around the world (faster in some areas,slower in others), the number of users with 4G de ices is intensi ely growing!

Thanks to factors such as lower costs " due to the gain of production scale, and also byencouraging migration to 4G plans " offered by operators who already ha e an a ailablenetwork, more and more people ha e access to new ser ices and benefits that thistechnology offers!

#owe er, as much as the current data ser ices are impro ed, and that progress in the arealead to the adoption of new ser ices, a basic necessity should still continue to e$ist at leastfor a while% oice calls&

'hile making a oice call may seem simple, largely depends on the scenario where the useris, and alternati es a ailable for its completion! o it is necessary to understand well whatare the possibilities and the most important concepts of these key scenarios!

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

witched)!

er time, the need for other ser ices (data&) has emerged! -oice calls ha e come intoe$istence with these new ser ices! .s demand increased, these new ser ices were

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



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

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

'ith the continued growth of LTE networks, let*s try to understand a little more theconcepts, alternati es 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 translate toEnglish 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 and weha%e an understanding of our effort.

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 ser ices can use the + domain, but can also be madethrough TT ( er The Top) solutions, applications that encapsulate the oice and transport



ia an / domain (/ ), but who lack the 5o re6uirements needed to ensure goodcommunication " with the 7on G0R type ser ices (no bit rate guarantee)! E$ample% kype!7ote% t is ery unusual, but it is also possible to make TT oice calls on 3G networks! nfact, there may be TT solutions in any technology " it can be used in legacy networks, andalso 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 betransferred to the legacy 3G8 G!

.s we begin to see, there are many alternati es! .s usual, we will easily see each one!

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

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

.nd the best way to understand the alternati es or possibilities of making oice calls in LTEnetwork (4G), it is to start from a 3G" G"4G network topology ery simplified " consideringonly the main elements in ol ed!

.s we can see in the following figure, the LTE (E/+) has no direct *link* to the + network "as we ha e seen, it is designed to take care of purely / (/ ) calls! t has no :edia Gatewaydirectly connected, so no + call is supported by the ::E!



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

7ote% .s mentioned before and according to the topology abo e, the only way to ha e oiceser ices in LTE would be through TT ser ices such as kype! #owe er, this solution is notdiscussed today!

f we understand this, it is also easy to reali<e that in order for us to ha e oice ser ices inLTE, changes need to be made! There are some alternati es, and below we ha e the mainones%

• VoLGA (-oice o er LTE ia Generic .ccess)% ;se legacy 3G8 G as a generic access, *packaging*

oice ser ices, and deli ering ia LTE!

• CSFB (+ 2all 0ack)% whene er the ;E ha e the need to place a call, make it re ert (fallback) for

legacy networks!

• VoLTE (-oice o er LTE)% make oice o er LTE itself! n this case, the oice is pure / " -o / LTE!

o SRVCC ( ingle Radio -oice +all +ontinuity)% ensure that purely LTE (-oLTE) calls are

transferred ( ia hando er) to the legacy networks in a transparent manner!

7ote% notice that the R-++ is an option when the oice call has been established in LTE! eit is a conditional alternati e " considering that -oLTE option has been used!

E en without knowing ery well the options presented, it is easy to imagine that the *best*solution would carry oice o er their own LTE network! 0ut like e erything in life, it alsoha e the other side, the pros and cons!



To deli er oice ser ices in LTE network is necessary to ha e an infrastructure that supportit! n other words, there needs to e$ist an : ( / :ultimedia ubsystem or / :ultimedia+ore 7etwork ubsystem)! f an : is a ailable, then the oice o er LTE may be pro idedas long as a minimum set of : functionality and entities also are present!

7ote% : is much more complete, and ha e more other purposes than the oice! The oiceis =ust another *application* of : , as we*ll see soon!

This minimum set of features and entities of the : (called -oLTE or ne -oice) wasstandardi<ed to enable LTE operators to pro ide oice ser ices without ha ing to make eryradical changes in the network (without ha ing to in est in a complete : , with all entitiesand functionality)!

n any case, it re6uires in estment!

.nd therefore the first two alternati es become attracti e% based on legacy network +infrastructure! 0ut if on the one hand such alternati es re6uire less in estment in LTE

network, these alternati es depend on the e$isting 3G8 G networks!

Let*s talk a little more about each of these possibilities, but always trying to maintain theo er iew, in the simplest possible way to understand! Remember that our goal is to learnthe concept, in order to enable a deepening on the sub=ect, if desired, more easily!

Vo GA

The first implementation alternati e that emerged was the -oLG. (-oice o er LTE iaGeneric .ccess), or% try to use what are already a ailable, with minimal changes re6uired!

To use the infrastructure of legacy 3G8 G networks, -oLG. introduces a new network entity,the -7+ (-oLG. 7etwork +ontroller), which basically functions as a 3G 0 +, communicatingwith a G : : + (:obile witching +enter) and as one G R7+ communicating with a ;:T: + (:obile witching +enter)!



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

.lthough it is possible to carry out the deli ery of oice and : ser ices to users LTE, the-olga was unsuccessful! This is because, as we ha e seen, e$clusi e in estment are neededfor this purpose! .t the same time howe er, global efforts to -oLTE increased (e!g!in estments in : ), and thus this alternati e e entually falled into disuse!

!"#$

0ut if in one hand operators follow seeking a complete LTE infrastructure (with full : ) tomeet multimedia ser ices and also purely LTE oice, this is not a topology that is a ailable inthe short and e en medium term!

'hile that reality doesn*t come, we must use the legacy network when there is the need of oice and : deli ery to LTE users!

.nd the most common alternati e to this is the + 20 (+ 2all 0ack), an interim solution

until we ha e full support for oice o er LTE!

.t + 20 scheme, whene er there is a demand for a new oice call, the LTE user is *backed*for a + legacy network, assuming that this pro ides an o erlapping co erage! n otherwords, with + 20, a oice call is ne er acti e in LTE, but in legacy networks!

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



t goes something like this% the ;E is registered (also) in the legacy network! 'hen it got acall, the legacy network tells to LTE network% * ha e a call to the ;E, can you ask it to comehere and make the call1*

To + 20 be possible, users must be using dual mode de ices, i!e! 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 se eral operators, let*s see somebasic 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 LTEnetwork needs to know the location of the ;E!

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

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



!"#$ - &riginated !all

'e will continue, and assume that the ;E is initially co ered by the LTE network, and thatthere is an acti e / connection!

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

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

0efore performing the ;E transfer, the e7ode0 can ask it to make R2 measures onneighboring 3G8 G network! The e7ode0 then decides the best network for the ;E andperforms the transfer!

nce the ;E camp in 3G8 G network, it starts the call procedure as usual " the ;E starts thecall control procedures in legacy network!

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

.nd what happens if ha e an acti e data connection in the / LTE network, and decide tomake 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 the transmission 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 lack of resources orfor not being able to process it!

The interface is used to carry out the / session hando er 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 hando er 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 G 7 onGT/+ 3! .nd to support it, the G 7 needs to be updated (most carriers do not want to dothis without a strong =ustification)!



.nd Gn interface is already on GT/+ ?, which is the nati e GT/ ersion for G networks! oin this case only the ::E needs to be updated, and as it is a relati ely new node, it isprobably easier to do! 7ot to mention that the new G 7 may ha e nati e support for !

!"#$ - Ter+inated !all

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

The call re6uest arri es first to the : + where the ;E was pre iously registered!

'hen the : + the recei es call re6uest, it sends paging messages to the related ::E iaGs 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: ( er ice Re6uest :essage) to the ::E!

The then ::E notifies the e7ode0 to transfer the ;E for the legacy network, and thee7ode0 then decide the best network for the ;E to make the call!

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

'e ha e 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 ha e their parameters set such that the reselection toLTE happen if there is a good LTE co erage area o erlapping the legacy network!



Vo T*

E erything we ha e seen so far is based on the making of oice call in the legacy network!0ut as we ha e seen these are *temporary* solutions until the *final* solution " -oLTE " isa ailable!

.nd the final LTE oice solution (-oice o er /, or more specifically -oLTE) uses the :backbone! .n e$ample of network topology supporting -oLTE is shown in the followingfigure!

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

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

./"

: is a backbone (network) at the application le el, which works on top of other wirelessnetworks 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$tremely difficult task, e en for the most e$perienced inthe sub=ect!



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

ts complete specification consists of thousands and thousands of G// standards! 0ut let*stry to understand in a simpler way than that found there!

.s its name suggests ( / :ultimedia er ices), : offers se eral multimedia / ser ices,including -o / (-oice o er /)! n : , oice is =ust *another* ser ice&

: brings together oice features such as authentication, ser ice authori<ation, callcontrol, routing, and interoperability with / T7, billing, additional ser ices and -. ! 7one of these e$ist in the E/+% this is the reason why the pure E/+ without : cannot process a

oice call!

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

.n analogy we can do is to consider the : being a car! .nd the LTE oice, as our shuttleser ice (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 canmake se eral other things as well&

That*s more or less what happens with the : ! t is used in con=unction with the LTEnetwork to support oice% both full : implementation and also the minimum :suggested implementation for -oice o er LTE!



0ut the telecommunications industry would rather not in est in a full : , or at least did notha e sufficient reason to do it immediately! .nd for the adoption of the simpler : oicesolution, appear the -oLTE initiati e, which specifies a minimum set of features, and selectsa simple choice when multiple options e$ist for certain features!

#owe er, not all of these features are re6uired for deli ery of basic oice ser ices by the LTEnetwork!

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 whatsoe er, 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)!

•

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

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

understand that when we ha e a call re6uest to this ser er, it sets up the call!)9 and the userelement would be a :edia Gateway!



n comparison with the legacy networks, the / er er is e6ui alent to the : + in themobile network topology and the media gateway is e6ui alent to a typical :edia Gateway onany network topology, which is common in irtually any oice network to handle calls!

The abo e concept is alid, but in practice the : consists of much more entities, as seenbelow! 7ote% 7ot all possible8e$isting entities and interfaces are shown in the figure!

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

7ote% >o not worry or try to understand e erything now about these elements! Rememberthat 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 communicateswith other / T7 networks! t is significant because it has to inter"networking function% canspeak /, can speak ;/, and 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 specifically betweenthe Real Time Transport /rotocol (RT/) to analog format or basic /+: in the + network9and ice ersa!

The # (#ome ubscriber er er) is an element that also e$ists in the LTE E/+ (althoughappeared first in : ), and its functions are similar!



The :R2 (:edia Resource 2unction) pro ides many ser ices related to oice, such asconferences, announcements, oice recognition and so on! t is always di ided 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 weare talking about an / network! nstead of the users to speak directly with the / ser er,they use the pro$y!

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

• /"+ +2 (/ro$y + +2) among other tasks, pro ides 5o information related to the LTE network!

.ccess an .2 to oice ser ice, and pro ides the control functions *policy* and *charging* to the /+R2!

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

•

.nd the "+ +2 ( er ing + +2)% the + +2 ser er acts as a central node!

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

.s we speak, the : oice is a *ser ice* " the : is a ser ices *facilitator*! The : ser icesare pro ided through . (.pplication er ers)!

ne such application is the oice! .nd there are also ideo ser ices, 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 er ice),sometimes called :T. (:ultimedia Telephony .pplication er er)!

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

The "+ +2 will be responsible for call routing depending on where the other user (theother 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 respecti ely the control and user planefor other : networks, other / networks in general! They are similar to the :G+28 :":G' " the re6uirements for reaching one or another type of network are different, so alsoha e separate parts for performing the same functions but with different networks!



".0

To support telephone signaling between the LTE network and telephone networks, the :uses / ( ession nitiation /rotocol)! / is a standard protocol for establishing oice callso er / networks!

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

There is a set of standard commands that can be used to initiate, manage and terminatecalls between two / de ices!

The / has been adopted by : standardi<ation as the protocol to allow signaling betweentelephone networks and -o / networks!

/ is te$t"based and was de eloped " in the BAs " in order to be simple and efficient, =ustlike the #TT/ protocol (in fact, was inspired by #TT/ and other protocols such as :T/)!

. good analogy is to compare the / with #TT/!

Cou probably can understand well the #TT/ interaction principle, which allows audioconnection, te$t, ideo and other elements on a web page! 'ith / is pretty much thesame thing% it allows the establishment, management and calls endings (or sessions) for /multi"users without knowing the content of the call! . session can be a simple telephone callbetween two users, or a multi"user multimedia conference!

0oth ( / and #TT/) take the control of the application to the end user, regardless of thetransport protocol ( / is a control protocol in the application layer), so there is no need forswitching centers8ser ers!



The / howe er is not a resource reser ation protocol, and has nothing to do with 5o !

. short break% our tutorial today is already 6uite e$tensi e, but we*ll keep a little more withthis issue because these concepts are ery important, and you*ll be hearing a lot of them!

To try to understand it better, let*s see a simplified e$ample for a oice call establishmentprocess using : platform and / signaling!

• nitially, the ;E sends a / message like * n ite*, 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 ha e sent a message of type *offer response* to the "

+ +2, and this sends this message to the /"+ +2, authori<ing the allocation of the resourcesnecessary for this session!

• 2inally, the /"+ +2 forwards the *offer response* message back to the ;E, which confirms the

receipt of the *offer response* message and the resource reser ation is started!

This is a ery simplified e$ample of how you can be getting (origination) of a oice ser iceby the ;E, ia : !

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

Let*s complete the tutorial today, now seeing the case where an initially established call on

: has to be *transferred*!

"%V!!

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

The R-++ howe er is not an alternati e for deli ery, but a rather a hando er process of aoice call pre iously started in the LTE (whether ne -oice " -oLTE LTE or : 2ull -oice)!

t is a call transfer method (hando er), in a simplified and reliably way, when an LTE userhas an acti e oice session in : and is mo ing to areas without LTE co erage, but withlegacy 3G8 G co erage!

The main ad antage is that the call will not drop " will only be transferred to the + domainof the legacy networks!

f in the abo e case the ;E mo es out of LTE co erage area with an acti e call (but goes toa legacy 3G8 G co erage), we must maintain the continuity of this acti e oice call! n thiscase, the R-++ is used% the procedure where the conte$t of an acti e oice call on the :is transferred to the + legacy network (e!g! : node conte$t transfer to the : +)!



The challenge with R-++ is to perform the hando er while the ;E is connected to only asingle radio at any gi en moment!

There are two ersions of R-++%

• R-++ hando er to G : or ;:T , defined by G//9

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

To allow R-++ both the ;E and LTE networks, as also the legacy, must support R-++! 2orthis, a new special - interface is introduced between the ::E and the : +, which runs onGT/ 3 protocol!



To support R-++, the : network should also include an application ser er, called ++ .( er er +entrali<ation and +ontinuity .pplication er er)!

This application ser er is who manages the signaling re6uired for the process!

Let*s see a simplified e$ample of some R-++ procedures from LTE to G :!

• 'hen an ;E that supports -oLTE is in an LTE co erage area, it starts oice sessions ia the :network, which will host the session and pro ide applications and session control based on /!

• 'hen the ;E mo es from an LTE co erage area for a + 3G8 G co erage area, with the acti e :

session, the : switches the session to the + domain, maintaining both parts aware of thehando er session!

*1a+ple o "%V!! Handover

Reali<ing that its LTE signal le el begins to decrease, the ;E with an acti e : oicesession signals it to the e7ode0, initiating the R-++ hando er!

The e7ode0 then identifies the best a ailable network to recei e the ser ice, and sends thehando er re6uest (specifying that it is the R-++ type) to the ::E!

The new oice call re6uest is then sent to the : , using a R T7 ( ession Transfer7umber for R-++) " a uni6ue number that is generated by each ;E, and is stored in # !



This uni6ue number is sent by the ::E to the # when the ;E firts comes into contactwith the network!

;pon recei ing the T7 R number, the ++ . belie es that the corresponding call shouldbe transferred to a different network network, and starts the redirecting process for thetransfer point (hando er) to the legacy network!

.fter resource preparation is completed, the ::E confirms the hando er re6uest, pre iouslypro ided by the e7ode0!

The e7ode0 then transmits this acknowledgment to the ;E, while still pro iding the re6uiredinformation about the target network!

n the final stages, the ;E is detected in legacy networks, and the call is re"established in it!

Thus we ha e the completion of the R-++ hando er!

-oice packets and also packets that are not oice can be transferred using this method, butthe data rates will be limited by the capabilities of the legacy networks!

nce the R-++ is a procedure for inter"R.T hando er based on : LTE network to the +legacy network 3G8 G, it is much more comple$ than that of hando ers legacy networks G

8 3G! The 6uestion is how to maintain a hando er performance comparable to or betteracceptable!

n order to impro e the performance of the R-++ hando er, one ' ('ork tem) callede R-++ ( R-++ enhancement) was established in the G// .3 in Release ?A! Theanchoring solution is based on the : , and introduces new entities .T+2 (Transfer +ontrol.ccess 2unction) and .TG' (Transfer .ccess Gateway)!

.gain, the deepening of this sub=ect escapes from our goal today!

2inally, we will enumerate some of the main ad antages and disad antages (or pros andcons) of each alternati e!

Advantages and disadvantages o each alternative

Call setup time % 'hen operators use + 20, one of the biggest problems faced (and one of

the ma=or disad antages of + 20) is the increase in call setup time due to retuningprocedures in 3G8 G radios!

.n efficient + 20 solution re6uires the T.+ "D L.+ mapping is so that the fallback to ane$ternal : +8L.+ be a oided, 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 not bepresent in common data applications!



Resource limitations for VoLTE % .:R"7' LTE re6uires much less resources and data ratethan G :, and we will ha e many more users on the same bandwidth (spectral efficiency)!

Investment x Current et!or" % if e erything is *working well*, what would be the reasonfor in estment, since surely such in estments generate resistance from commercial andbusiness areas1

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

Future# n any way all that discussion hereafter will more significance! +urrently we stillha e e$tensi e legacy networks, capable of supporting these oice calls!

n this case, it is no problem to continue using this a ailable infrastructure! Resistance willonly 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 co erage and a complete : backbone!

o f we implemented + 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 a ailable! n this case, if the

register in the : is successful, the user do not need to do + 20 " . oice call can besimply initiated in LTE network using : !

• + 20 is a ser ice hando er procedure while R-++ is a co erage hando er procedure!

!ase "t dies and Analogies

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

2irst, imagine that you are in a network that does not ha e LTE : ! Then the only way tomake a oice call, whether originated or terminated, is through using legacy 3G8 G!

Cou need to be redirected8released from LTE to legacy 3G8 G network to make a oice call!Like a *reselection* from cell LTE to the 3G8 G! nce the legacy network, you can make thecall normally, as you*re already used to!

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

7ow suppose you are watching a ideo stream on 4G network, and recei e a oice call! nthis case, you need to go to the G network (in idle mode), and get the resources for tomake that call in G!

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



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

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

'e ha e =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 co erage edge! o the only option to keep your call is tohando er it to the G (assuming this is the e$isting co erage)! Cour call will then continueon the G network, but now as one + oice call! R-++&

f the R-++ is not supported, the call is dropped as soon as it lea es the LTE co eragearea!

f the R-++ is supported, a set of messages are e$changed, and the oice call istransferred (hando er) from LTE : to + domain of the 3G8 G network!

.nd so, we ha e =ust seen an e$ample of R-++ hando er&

.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!

!oncl sion

'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 alternati es depend on se eral factors, such as a ailable network topologyand 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 networksthrough mechanisms de eloped for this purpose (+ 20) or transferred ia hando er " if acti e -oLTE call " to a legacy network ( R-++)!

o, for a user who is a LTE co erage area, a number of considerations should be checked, asthe type of de ice that it uses (whether supports + 20), if the LTE network has an : thatallows outgoing calls, if the cells supports R-++, etc!

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