VoLTE IMS Network Architecture tutorial

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VoLTE IMS Network Architecture tutorial


hi there this is a voltage architecture
tutorial my name is Vikas and i am subject matter expert in the telecom
industry here I am going to simplify volte mystery for you
are you finding 3gpp specs extremely complicated clumsy and hard to
understand yeah it’s complex well not to worry at all
this is a right place for you I am going to simplify and make it easy for you so
that you are going to remember it forever I assure you you will feel much
more comfortable with VoLTE architecture within the next few minutes
let’s start our journey with VoLTE Hi , Along with explaining the VoLTE
architecture , I am also going to provide you two bonus tips on Volte network in
the end . You will not find these practical tips anywhere else since I
have learned these things over the time during past few years . This video
contains Gist or Summary of all my learnings . I believe in building concepts
without solid foundation you will trouble creating anything of a value . I am
going to work on building your fundamentals in this particular video
. This video will we will cover up high level nodes in VoLTE and also covers
deconstruction of telephony services along with the integration of IMS
functions . Let’s quickly jump onto the fundamentals here Let’s discuss about high level Volte connectivity what is volte ? . As diagram suggests it’s
all about carrying voice over LTE Data Network Since IMS is part of the Third Generation
Partnership Project also called as 3GPP, this makes IMS as Standard solution with Pre-Defined
Interworking with underlying 3G & 4G Services. Along with LTE Network, Operators can harness
the true power of IMS which stands for IP Multimedia Subsystem. IMS is a standalone system which resides out of the LTE network and connected to PDN Gateway or PGW through SGi Service . Now , We can see this all IP Network which is LTE as access is being used for carrying voice calls Now , Let’s discuss something about IMS , Now IMS as it have been shown in center in the Orange color You can clearly see like IMS is sitting out of LTE Network , This is standalone network which is sitting in the middle Now , Let me tell you something about the IMS IMS is a very old concept , It was was very very popular in Year 2003 – 2004 almost 12-13 years back, this used to be a failed Concept What happened at that point of time When IMS have came into the picture and when it was introduced in year 2003 Operators have conceived this as failed technology because it is too too complex Operators have seen it as far too expensive and burdensome to be introduced and to maintain With VoLTE Launch, This again got popular Now , VoLTE requires IMS to be there and to be sitting on top of core LTE network This have made IMS popular again and Its in Demand now a days again OK , In coming slides , We are going to discuss the VoLTE Building blocks As we discussed earlier , Its all about voice over LTE and it’s more or less exactly what it says on the tin . Rather than the 2G or 3G connections which are usually used for voice calls , VoLTE offers voice calls over all IP to IP based 4G Network . We
tend to think of 4G as mostly being about downloading, streaming and web browsing, and
indeed that’s primarily what it’s been used for so far, but it can also be used to
next Level of Voice Services with IMS Now , Let’s quickly see what all Benefits which Users will get along with VoLTE Services The First benefit which user is going to get is Superior call Experience – The Greatest advantage
of VoLTE is that Voice call quality is superior from 2G or 3G connections . Essentially it’s
an HD voice call and It’s a much richer experience over for end user Similarly , VoLTE also provides faster call setup VoLTE can connect call 2wise or 3rice as fast as currently used in 2G or 3G Network Our typically 2G or 3G call takes 4 to 6 seconds for maturing , While VoLTE requires only a typically less than 1 seconds or 1-2 seconds for getting connected end to end Other benefits of VoLTE for End users are better battery life Better battery life – Anyone who currently
uses 4G could also find their battery life increased with VoLTE . For a Non-VoLTE call
, Handset needs to switch from 4G to underlying 3G or 2G network to receive voice call . This
is done using CSFB or Circuit Switch Fall Back technology . This frequent switching
of Mobile from 4G to 3G & Vice versa consumes lots of Battery . In VoLTE call , Users Need
not to Go to 2G or 3G Network to receive voice call Now , That saves battery for end users , Similarly , There is one benefit which user is going to get That one is the Video calling , Just like OTT Providers such as Whatsapp , Facebook ,Google Duos and Skype VoLTE also gives you Video calling service , Now the benefit of this Video calling service is Its available on the stock dialer from the main handset itself , You need not to download any 3rd party Application from Google Play store or may be from the Apple Store , Its directly available from the handset itself you put a SIM , If its VoLTE enabled handset , You can straight away make a Video Call .. Just as simple as that .. As , next step , We are also going to De-Construct the entire network which is going to be used for VoLTE, Where we are going to discuss about Handset , The LTE Network , The IMS Core and the voice PSTN in coming slides On Left hand , We can see SIP Enabled User Equipment . This Handset or Smartphone needs
to be VoLTE Compliant where SIM should support ISIM & SIP User Agent for making voice calls over VoLTE The 2nd one is the LTE Network which typically shows the ENodeBs and the LTE core network , We will also discuss this in brief in coming slides The 3rd one is the IMS Network which is sitting on top of underlying LTE Network The last one is the PSTN Network which is used for Break-in and Break-out of calls from IMS We are going to discuss in detail in the coming slides Let’s quickly jump to the VoLTE handset component , The “1”st one We will start our Journey with User Handset
. For VoLTE to work , User handset must support two critical things : 1st is the SIM Support which is there on left hand side and then the VoLTE Binary Application in Handset which is also called as user Agent Which is on the Right hand side .. So , Let’s start with the SIM piece SIM plays the crucial role in the VoLTE Support SIM is known by the name of UICC which stands for Universal Integrated Circuit . There are primarily 3 types of SIM Cards :- The 1st one is typical SIM Card which is used for 2G or 3G This is a traditional SIM , It securely
stores the international mobile subscriber identity (IMSI) and the related key used to
identify and authenticate subscribers on mobile telephony The 2nd type of SIM is USIM . The USIM was introduced with 3G and 4G Services This is new Age SIM card which came to us The last one is ISIM which is used for VoLTE Services . ISIM is also known as IP Multimedia services identity module This is used by IMS or VoLTE subsystem For Authentication and rest of SIM functions . There are 2 Types of entities which are stored on this ISIM
SIM :- 1st one is IMPI , IMPI stands for IP Multimedia Private Identity . IMPI is a global identity allocated by home network. IMPI contains home operator’s domain information , The 2nd one is IMPU . IMPU stands for IP Multimedia Public Identity IMPU acts like a telephone number which can either be a SIP URI or a tel URI as defined in RFC 39664 , Now Let’s see whats there on Right hand side Its our SIP User Agent , which we are going to discuss . Unlike , 2G or 3G or LTE , Where all Handsets are default supporting all available Network
, VoLTE requires detailed Handset testing and Certification for Every Handset Make & Model
. This testing is required as handset manufacturers and operators will be seeking to assure themselves
that the underlying protocol features are present and functioning correctly . Operator
want to ensure that the underlying IMS and protocol is functioning, and performing as
intended, in order to deliver the expected quality of service . SIP user agent is typically a Binary which resides inside the handset The main function of this binary is to transmit & receive SIP messages . It Provides basic telephony functionality & It is acting as a client or a Server for exchanging the SIP Messages withe the IMS core Now , That’s all about Handset . Now , Let’s quickly jump to the Next slide where in like we are going to Cover High level building blocks of LTE Network Ok , Now we have covered the 1st Piece , Now quickly jump onto the 2nd one OK , This is s LTE Architecture slide I assume you already know LTE Architecture
& will not spend too much time here . All Yellow Nodes are control Plane & are responsible
for User Authentication . Control plane nodes include EnodeB , MME , HSS , OCS & PCRF The Green ones are User Plane which includes EnodeB , SGW & PGW . EnodeB is Radio cell
site of LTE Network & is used in both Control & User Plane . PGW is also used for both User & Control Plane traffic Now , We have covered User Handset & LTE Core Network . Its time to demystify IMS Core network . VoLTE Voice Call Traffic is originated by
User Handset & Terminates on IMS Network . LTE Network is just used as Bridge acting as IP
Supporting Carrier for this communication . Now , We will jump on to IMS Core without
wasting any further time This is 3GPP IMS VoLTE Architecture , I am going to simplify this in another few Minutes & you
will be able to understand every individual component shown in this complicated diagram . The VoLTE IMS Architecture is covered under @ 3GPP 23.228 . Similarly , You can also study GSMA Document : GSMA FCM.01 and GSMA IR.92 These 2 documents are really Good , If you want to further study VoLTE and want to Go in depth , You can refer to GSMA documents We will start with High level concept of LTE
Network & Going to Stitch it together with IMS This is typical LTE Network which Provides
high Speed Internet access . Traffic flows all the way from Mobile handset to EnodeB
to SGW to PGW to Internet Cloud Signaling is controlled by MME and PGW with
support of HSS , PCRF You can see User Payload Traffic flow along
with Arrow on Screen LTE Network is part of Evolved Packet System
as shown in diagram here . Our IMS Core Integration begins here . IMS
stands for IP Multimedia Subsystem IMS Core is directly connected to PGW as shown
on screen . This is connected over sGi Interface For LTE Network , IMS or SIP traffic is just
another Payload similar to Internet Traffic . LTE PGW doesn’t Decode or Interfere this
traffic . LTE is used as Carrier for accessing IMS Services
which provides IP Telephony or Voice Services over IP Network , There could be multiple
Access type for IMS Network such as Wifi , or Fix Broadband or DSL Networks SIP Server From This Slide onwards , I will start breaking
down IMS Cloud in various Bits & Pieces to make it more clear to understand
IMS Core consists of broadly Two Type of Network Elements , First is SIP Infrastructure & Another
type of Nodes are Media Gateways . These are tightly coupled under IMS Core Network .
Let’s understand SIP Server in detail Users are going to register with SIP Server
which is Part of Control Plane & Signaling This SIP Server will setup Media Part or Payload
of Voice Call with Media Gateway shown in in Blue Color which is further connected to
PSTN SIP Server is anchoring point for Session
set-up, session tear-down, session control along with BCDR Generation
SIP Server is consisting of various sub Modules covered in upcoming Slides Media Gateway Now , Let’s Understand Media Gateway which
is Highlighted in Green Color Media Gateway is Responsible for carrying
actual voice traffic & handing it over to other Networks . These Media Gateways handles
interworking of Voice traffic between IMS & PSTN Networks .
Any call coming from traditional 2G or 3G or Fix line network is using this Media Gateway
as Entry point to IMS Network . Media Gateway is Interconnect point to circuit-switched
networks and Other IMS Network . It also handles Transcoding of the media plane . Broadly it the gateway between the LTE and other PSTN Networks For the sake of easiness , I am using only
One Box depicting LTE Network This Consists of MME , SGW & PGW. There can be multiple access networks for providing IMS Services . This could be Wifi , Fix Broadband , DSL & LTE Networks As long as user handset supports SIP Device
& has IP connectivity to reach IMS Network , We can offer IMS Services to User
IPCAN Stands for IP Connectivity Access Network This is equivalent to Bearer of a IP Connectivity established from user towards IMS or Service Network Now , I am going to discuss most critical
Element of IMS Core Network . We call this as CSCF , You can see it here in central SIP Server You can also see it here along with SIP Proxy in the diagram . It stands for Call Session Control Function & act as heart of the IMS system The CSCF performs all the signaling operations,
manages SIP sessions and coordinates with other network entities for session control,
service control and resource allocation. It consists of three different entities: the
Proxy-CSCF (P-CSCF), the Interrogating-CSCF which is also called as I-CSCF and the last one is Serving-CSCF (S-CSCF). Let’s start with First Entity : P-CSCF also
known as Proxy-CSCF or Proxy Call Session Control Function
This SIP Proxy is sitting in Front of SIP Server always
SIP Proxy is first Point of Entry point to IMS Network . This Proxy SIP Server is typically located in Visitor PLMN Network ( I mean to say the roaming network ) . This SIP Proxy has several important functions such as a) Validation of the correctness of SIP messages coming from the user b) It also ensure the security It also does the function of Authentication & asserts the identity of the UE
In last , It also finds out Home SIP Server IP & Routes Traffic to same The Interrogator I am adding up two things here One is Interrogator
& Another is Home Subscriber Server Let’s discuss the Interrogator .. I-CSCF or Interrogator CSCF performs below Critical functions :- SIP Proxy forwards requests to Interrogator
which in-turns finds out which SIP Server to Select User may be served by Various SIP Servers , While Only One SIP Servers serves customer
While SIP Proxy is usually located in Roaming or Visited Network , Interrogator sits in
Home Network So this is visited network , This is home network All the main tasks of Interrogator is Select Correct SIP Server or Registrar which is going to Server User
All this communication between Proxy , Interrogator & SIP is always on SIP Protocol between all these Nodes . I-CSCF queries the HSS using the DIAMETER Cx interface to retrieve the user location
and then routes the SIP request to its assigned S-CSCF during Registration of a call. It is also used during During MT Call routing HSS ( Home Subscriber Server ) Now , Lets discuss HSS which is also know
as Home Subscriber Server The HSS is the database of all subscriber
and service data HSS contains the subscription-related information
(user profiles), used by the control layer HSS is the master user database that is used
for Registration , Authentication in both LTE Network , As well as in VoLTE IMS Network
This allows the user to access the packet and circuit domains of the network initially,
via IMSI authentication. HSS Communication is via Diameter protocol
For super easy understanding, HSS is used as profile or database of User where all information
is saved . This HSS will decide whether you are allowed to use VoLTE or Not , This will decide Whether You are even allowed to LTE Network or 4G Network or Not ? Etc.. That’s the role of HSS, It’s a significant stuff after LTE launch Telephony App Server (TAS) Also called MMTEL or TAS .. Till now , We discussed IMS Core which is great for handling SIP but this requires App Servers to provide various supplementary services
. For example , Voice calls in Mobile network requires Telephony App Server (TAS) . We need
similar App servers for other Multimedia services such as Video , Gaming etc.. Application Servers host and execute services, and interface with the S-CSCF using SIP. This allows third party providers to provide an easy integration and deployment of their value added services to the IMS infrastructure. IMS was never indented to be limited for voice
services , Rather it was designed for multimedia services & voice is only One part of same
IMS is not a service but service facilitator network . Its enables user to access various
Applications such as Telephony service App Server handles Telephony part
While SIP Server is still controlling Session such as Initiation , Maintain & Tear down
of sessions .The TAS the provides telephone brain to the App server TAS is a telephony app server used for Telephony Service such as Call waiting, Call hold, Call
pick up , Call forwarding, Call transfer , Call blocking services, Malicious Caller Identification
, Lawful interception , Announcements, Digit collection , Conference call services etc.. We can check various other App Servers such as Video App Server , Conference Bridge Service etc.. Its an addon to the IMS just like TAS we can add on all these application servers onto the existing IMS Network App Servers are not considered as part of Core IMS Service Media Control In this Slide , We have split Media functions
into two part – Control & Payload function The top one is the control function and lower one is user payload function Media Control is performing critical control plane function for VoLTE Call . Let;s discuss this one in detail . User can make call to PSTN Network or Fix Line Network where Media Control & Gateway
is supposed to Speak to PSTN Network over ISUP / SS7 Protocol
User can also make call to another IMS Networks Media Control handles the parts of the call
state that pertain to connection control for media channels
Media Control communicates with the SIP Server and selects the appropriate node depending
on the routing number for incoming calls from legacy network
Media Control also Performs protocol conversion between ISUP and the IM subsystem call for control protocols It also Controls the MGW resources with a
H.248 interface It does all the control function of voice call Now , Let’s discuss Media Gateway
Media Gateway is element responsible for Payload Handling of VoLTE Call . Here is media Gateway It Interacts with Control and terminates bearer
channels from a switched circuit network and media streams from a packet network (e.g.,
RTP streams in an IP network) Interfaces the media plane of the CS network,
by converting between RTP and PCM It can also perform media transcoding, when
the codecs used do not match (e.g. IMS might use AMR, PSTN might use G.711) . These 2 are different codecs .
Other than Media conversion , It also performs bearer control and payload processing using
codecs, echo cancellers and Managing other on-board resources Net , Net .. This is the guy who is going to ensure both the nodes , One side on LTE speaks to another side on PSTN Here , I am going to further Expand Media
Control & Media Gateway into actual Node functions IMS Gateway which communicates with Other IMS Networks 3GPP Names of these Nodes are IBCF & TrGW . IBCF is control Plane Entity
& TrGW is user Plane Entity Similarly , PSTN Gateway is the node which communicates with PSTN Networks . 3GPP Names of these Nodes are MGCF and IMS-MGW MGCF is control Plane Entity & IMS-MGW
is user Plane Entity ATCF stands for Access Transfer Control Function
ATGW stands for Access Transfer Gateway As name suggest , These both Access nodes
reside in serving IMS network (i.e. visited network in the roaming case) , They interact
with LTE Network for Media Control & Payload handling . This is the node who is going to speak to the LTE network down the line
ATCF handles the Access side Media Control for handing Voice calls with LTE PGW
ATCF also plays vital role in enabling SRVCC Session Transfer mechanisms
The ATGW is a media function that is used to anchor the media with LTE PGW . Net Net .. Its job is to interface with LTE and handover or receive the calls from the underlying 4G or LTE Network Now , I am showing you all 3 Media entities in One Screen
While MGCF & IM-MGW is designed to communicate to PSTN Networks such traditional Voice network
running on SS7 or ISUP IBCF & TrGW is designed to interwork with
Other IMS Networks running on SIP This is prime reason for Breaking them into various network Elements . You can also see PCRF (Policy and Charging Rule Function) PCRF : This is used in both IMS & LTE Network
. The main purpose of PCRF is to apply Policy to PDN or IPCAN Session . The PCRF provides
priority of Bearers used for Making VoLTE calls & SIP Signaling . The QOS Provided by
PCRF provides Highest priority to VoLTE voice traffic . This prioritization helps Voice
call to pass thru even in case of Radio Congestion . PCRF also anchors communication between
PCSCF & PGW over Gm Protocol reference Point Now , We are going to discuss .. MRF ( Media Resource Function ) MRF stands for Media resource function
MRF is both Gateway & Control function It deals with Call conferencing & things associated
with RTP Mixing It is also used for Announcement & Media related
functionalities MRF provide media plane processing independent
of application types, e.g. transcoding, multiparty conferencing, network announcements/tones,
etc. Now , Let’s discuss our last component which is BGCF .. B Number routing Table BGCF stands for Breakout Gateway Control Function
Serving CSCF Needs to make decision where to route SIP Message ( i.e. To PSTN or to Other SIP Network ) , BGCF Simplifies
this Job by making these decisions about routing the call to destination Network . This is
similar to our B Table of MSC BGCF is responsible for determining the next hop for routing of SIP messages
For PSTN terminations, the BGCF determines the network in which CS domain breakout is
to occur and selects the appropriate MGCF For terminations in peer IMS networks, the
BGCF selects the appropriate IBCF to handle the interconnect to the peer IMS domain This is the Guy who is going to take routing decisions and who is going to route the calls to respective IBCF and MGCFs Ok , So Now We have covered almost everything out there on IMS , Let’s see complete VoLTE IMS Picture The IMS architecture involves a clear separation of three layers :- This Green one is Transport layer The blue one is Application and Services layer And now , This red one is Session and Control layer The entire IMS is divided into these 3 Layers Now , let’s see the Big picture . We have covered all the way from user handset to the LTE network to the IMS Core to the Voice PSTN This is Complete network diagram for VoLTE or IMS services Ok , Now let’s see .. What happens when there is a congestion on Radio , Have you ever thought what will happen to Voice Calls during these Congestion ? .. As General principal , Data Internet can wait for some time , But Voice call Can’t . Voice has to be real-time LTE QoS provide us flexibility to provide
priority of Bearer or Connection during Radio Congestions . Since Voice can’t be compromised
and can’t afford delays , VoLTE is always provided Highest Priority over Internet traffic
. with help of this QCI , We prioritize VoLTE over the generic Internet . We will be using typically 2 QCIs . 1st one is the QCI=5 is used for VoLTE IMS Signaling . The another one is QCI=1 which is used for voice IMS Call Rest of QCIs which belongs from=6 to 9 belongs to Internet . They are always
low priority as compared to IMS They never provide Guaranteed BIT rate , Instead they work on best effort basis . Please refer to my another Video on VoLTE
IMS Registration for detailed understanding of this along with other concepts & Call flows
of VoLTE Registration , IMS APN Establishment & Default EPS bearer setup . I am going to cover all these concepts in detail in coming Videos . Here is another Bonus tip for you Now , Let’s understand what all it takes to deploy VoLTE in existing 4G Network Lets see what all is required on PGW or SGW or on existing LTE network On PGW , The 1st piece is to enable default and dedicated bearers The default and dedicated dedicated Bearer concept is required when we are making multiple bearers for having SIP Signalling for making a additional voice calls on existing LTE network The 2nd Piece is IMS APN IP Pool obviously , We are going to new IP Pool for IMS APN , I am also going to connect my PGW to IMS The last one is , We also need to disable Online charging of the SIP signalling and voice calls for Data Piece I am going to charge , the voice calls as voice but not going to charge them for payload carried over data in terms of MB , I am going to do the charging per minute basis , but not on per MB basis , Similarly , Let;’s see what all changes are required in MME , The 1st critical thing which we are going to do is SRVCC , I am going to cover SRVCC in coming Videos The 2nd One is again the IMS APN has be created , The 3rd is paging policy for VoLTE Calls , Every incoming call to the VoLTE customer require paging . We need to define high priority paging for VoLTE then the last one is QCI definition in MMEs , This is something which we recently discussed in our previous slides Now , Let’s see what is required is voice core or MSS Network in order to support VoLTE . The 1st one is SRVCC which is required to handover the in case customer goes of coverage area , I am going to cover this particular element in the coming Videos The 2nd one is again the reach-ability between CS Core Network and IMS . The 3rd one is the configuration for breakin and breakout calls for IMS or vice versa The last one is the Codec negotiation which is going to occur between MSS and IMS Again .. ANOTHER BONUS TIP , Let’s Assume there is operator who don’t have ISIM Do existing 3G or LTE customer using USIM need to go for SIM Swap for availing VoLTE Services ? How do I launch VoLTE ? If i am having USIM type of SIMs available in the network , Do i Need to go for SIM Swap Of all the entire base with ISIM , Well no , There is a workaround available . USIM can be used to Avail VoLTE Services also , USIM with help of the binary with help of SIP user agent can drive IMPU and IMPI for authentication

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