Volte call flow: Made simple

Dear Folks, today we will discuss about one of the basic things in Volte, which is Volte call flow.

The Normal Volte call flow is look like below:

We will discuss each step one by one:

Step 1: SIP Invite: Direction:- A Party---B Party
Calling party(A) informs IMS network and Called party(B) about New call. Session Description protocol(SDP) is used for carrying information such as Bandwidth and codec.

Step 2: 100 Trying: Direction:- B Party----A party
100 Trying is the response to the calling party to stop the retransmission of SIP invite. Every Node in IMS send the 100 trying message.

Step 3: 183 session in progress: Direction:- B Party----A Party
Called party informs about codec supported in response of SDP. Dedicated bearer(QCI=1) are created at both ends i.e. Calling end as well as called end.

Step 4: SIP PRACK(Provisional Response Acknowledgment):  Direction:-A Party----B Party
It is a provisional response acknowledgment to 183 session in progress message received. Calling party uses this PRACK to communicate final selected Voice codec.

Step 5: SIP 200 OK (PRACK): Direction:- B Party----A Party
Called party accepts final selected Voice codec by Calling party.

Step 6: SIP Update: Direction:- A Party----B Party
Calling party confirms the Resources to voice call.

Step 7: SIP 200 OK (Update):- Direction:- B Party----A Party
Called party confirms the resources to voice call.

Step 8: 180 Ringing:- Direction:- B Party----A Party
All pre conditions are satisfied now and called party alerts the Calling party about ringing of call.

Step 9: SIP 200 OK(INVITE): Direction:- B Party----A Party
Finally called party Answers the call. 

VoLTE KPIs

Today we will discuss about the Various types of VoLTE KPIs. 
VoLTE KPI is mainly divided into three parts:

1.IMS KPI

2.EPC(Packet core) KPI

3.RAN KPI.


IMS KPI is further divided into 2 parts:- IMS Control plane KPI and IMS User Plan KPI.

IMS Control plane KPIs are as follows:

1.RSR:- Registration success rate.         

Formulae = Count of 200OK for Registration completed/Count of SIP register sent from UE excluding 401 error attempts*100

2.CSSR- Call setup success rate%           

Formulae = Count of (Normal end call+ call failed with user behaviour)/Sum of all call attempts*100

3.Call setup time:-          

Formulae= Avg of(Time(180 ringing)- Time(SIP INVITE request))


IMS User plane KPIs are as follows:

1.Mute rate(%)

Formulae= % of calls muted (samples >2 or 5s RTP loss in both direction)

2.MOS score

3.RTP packet loss %

Formulae = % of RTP packets lost in the uplink or downlink direction.

4.One way call %

Formulae = % of calls having no voice packets count for 2 or 5 Sec in either the downlink or uplink direction.


EPC(Packet core KPI):- This also classified further in 2 parts- MME generated and SGw/PGw generated.

MME generated KPI are as follows:-

1.VoLTE attach success rate% = % of PDN connect request responded successfully by MME for IMS APN.

2.VoLTE bearer Activation success rate:% of create request got successful.

3.Paging success rate%-= % of paging responses on QCI=5 received at MME.
SGw/PGw generated KPI are as follows:-
1. IMS IP Pool Utilization%


RAN KPI are as follows: 
1.Call drop rate % = % of calls getting dropped abnormally.
2.SRVCC Success Rate % = % of calls successfully transferred with SRVCC from VoLTE to legacy NW

3.Handover Success rate %(S1 and X2).

SRVCC in LTE

SRVCC( Single Radio Voice call continuity):-  SRVCC is a feature/process by which a voice call(VoIP) in LTE can be continued in legacy RAT(2G/3G, CS call) via handover when UE moves out of LTE coverage. 

Why its called Single Radio?- The process is called single radio is due to only a single radio is required in the handset.

Call Flow for SRVCC:- Here below I m indicating the simplest way of SRVCC call flow.

The process:- When UE moving away from LTE coverage then UE notifies the eNodeB(By sending measurement reports). Then eNodeB informs MME to make a decision for handover to legacy RAT network.

We are defining thresholds of SRVCC for serving cell by Inter-RAT B1 and B2 thresholds.

Note: We can set a greater threshold in case of TTI bundling feature is enabled. As with TTI bundling feature VoLTE services can be improved at cell edges.

https://mytechnew2019.blogspot.com/2019/02/tti-bundling-in-lte.html

New Interface: For SRVCC implementation  3GPP introduces a new interface called Sv. The Sv interface is from MME to MSC.

Elements of SRVCC Network Architecture:- Here we are discussing about some important network elements which play a big role in SRVCC.

Sv interface:- Signalling interaction for SRVCC handover as an interface between MME and MSC server.

eNodeB:- SRVCC indication to MME with help of UE measurement reports.

MME:- Bearer splitting for Voice and Non-Voice services and MSC selection.

Capacity of Volte User per cell

Now days there is massive shift for voice call to VoLTE calls from conventional CS calls. Here i am discussing about Max theoretical VoLTE User per cells.

For this first we have to calculate VoLTE call Packet size and Radio resource required for VoLTE packets . 

VoLTE call packet size mainly depends on below factors:
     > Codec used by UE
     >  UE Radio conditions
     > EnodeB Scheduling Algorithm
     > Protocol used.
Lets try to find out the VolTE call packet size with below assumption.

Assumptions:-
LTE System Bandwidth: 5MHZ(25 PRB)
Duplex Mode: FDD
MIMO: 2*2
Codec used: AMR-Wide band codec(12.65)
RoHC: Enabled.

Calculation: AMR-WB(12.65) coder generates about 253 bits of speech every 20ms( Voice data is genearted in every 20ms for VoLTE). In order to deliver each voice samples to UE needs to add protocol headers such as RTP header(12 bytes), a UDP header(8 bytes) and an IP header(40 bytes).

        So VolTE packet length with all headers are:
 =Codec bits + RTP Header + UDP Header + IP Header
 = 253bits + 12*8(=96bits) + 8*8(=64 bits) +40*8(=320bits) = 733bits/20ms.

Note: When RoHC is enabled it will replaced the RTP, UDP and IP header with a much smaller RoHC header(24 bits).

Further RLC and MAC layers will add their own headers hence approx. 300 bits are needed for every VoLTE voice call.

Radio resource calculation for VoLTE packets: In LTE one PRB has 12 subcarrier and 14 symbols(Normal CP) over 1ms duration. 
 Total number of REs over 1ms = 12*14=168 but some of the REs are used by control symbols(PDCCH and RS). So final count is around 120 REs.

LTE DL modulation supports QPSK, 16QAM, and 64 QAM for PDSCH which means each resources can carry 2 bits, 4 bits and 6 bits. But some of these bits used for error control also.

Consideration: Lets consider CQI 15= Good ,provides 64 QAM, CQI 7= Average ,provides 16 QAM and CQI 1= Poor, provides QPSK.

When CQI =15:-
 Modulation: 64 QAM(6 bits)
 Effective coding rate = 0.926
 Total data bits in 1 RE = 6*0.926 = 5.55
 Total data bits in 1 PRB = 120*5.55 = 666 data bits or equivalent to 2 VoLTE calls.
But LTE schedular can not allocates < 1PRB so 1 PRB is needed for VolTE calls with CQI=15.

When CQI = 7:-
Modulation: 16 QAM(4 bits)
 Effective coding rate = 0.369
 Total data bits in 1 RE = 4*0.369 = 1.476
 Total data bits in 1 PRB = 120*1.476 = 177 data bits. So 300 VolTE bits required 2 PRB.

When CQI =1:-
Modulation: QPSK(2 bits)
 Effective coding rate = 0.076
 Total data bits in 1 RE = 2*0.076 = 0.152
 Total data bits in 1 PRB = 120*0.152 = 18 data bits. So 300 VolTE bits required 16 PRB.

Now we are ready to calculate max theoritacal VoLTE user per cell:- In VoLTE voice data is generated every 20ms so if everything is good then about 20 VoLTE calls can share the same set of PRB one after another.

The max VolTE call that can be carried = (No of avg PRB)/ No of PRB required per VoLTE calls *20.
Hence VolTE calls per CQI and Bandwidth is as below:

Bandwidth             1.4MHZ         3 MHZ         5MHZ             10MHZ             15MHZ           20MHZ
No of total PRB         6               15               25                     50                   75                100
CQI15(1 PRB)         120             300              500                 1000                 1500            2000
CQI7(2 PRB)            60              150              250                  500                  750             1000
CQI1(16 PRB)           8                18               31                    63                    94               125

The above is the theoretical max value but in practical there are some issues. We will discuss it in next..