What is Voice over LTE?

1. VOICE OVERLTE Simnovus Tech Private Limited 520-523, City Centre,Plot No.05, Sector-12, Dwarka, New Delhi-110075

2. SIMNOVUS.COM INTRODUCTION In November 2009, 3GPP released the first document regarding VoLTE, and this technology is widely accepted. Therefore it is still trending in the world. The Voice over Long Term Evolution is an industry-recognized technology for providing voice over IP. When this technology comes up, the circuit-switched based voice communication got replaced by all IP based voice communication, which leads to it creating a huge standalone market for operators in the world. When 3gpp designed LTE, the main aim was to provide better throughput with low latency as much as possible, and since LTE also needed to support CS(circuit-switched) services, it can be achieved by IMS(IP Multimedia Subsystem). The LTE network provides for signaling QoS; these signaling not ensured to provide call quality. To ensure the call quality communication providers need to verify the actual quality experienced by subscribers so that they can take immediate action if voice quality is not as expected. Like, they use dedicated bearers for better user experience and QCI value for prioritizing the packets.

3. SIMNOVUS.COM WHATISVOICE OVERLTE? Voice over LTE delivers the packet voice over IP using IMS(IP Multimedia Subsystem) via an LTE network. The IMS is designed for multimedia services, and via LTE networks, it makes the delivery of voice packets over IP, which is capable of doing with low latency. VoLTE call can be achieved as a process of both support of LTE and IMS to create an environment so that it is capable of supporting high-quality voice traffic. It supports high data rates with better voice quality. As you can see in the above image, the IMS can be found between the caller and receiver but before the LTE core network. The IMS network can be utilized as other services like 3G/2G.

4. SIMNOVUS.COM EPC (EVOLVEDPACKET CORE) NETWORK As we already discussed, EPC is the LTE network, we will be now discussing components of EPC below. The EPC network mainly consists of five elements. MME (Mobility Management Entity) Following are the points of functionality of MME. It deals with the control plane(signalling). Involved in the bearer activation and deactivate process. It is responsible for the UE tracking and the paging(it means waking up UE which is idle) of UE in idle-mode. Authenticate the user. It is the termination point for ciphering/integrity for NAS signalling.

5. SIMNOVUS.COM SGW (SERVINGATEWAY) SGW (Serving Gateway) Following are the points of functionality of MME. It deals with the user plane (user data). Interconnection point between eNodeB and EPC. All IP packets in uplink and downlink flow through SGW. When the UEs are in idle state, it terminates the DL data path and when the DL data arrives it triggers the paging. PGW (Packet Data Network Gateway) It also deals with the user plane. It is the interconnection point between the EPC and other external IP networks. Provides the entry and exit point of traffic for the UE. Single UE can have multiple simultaneous PGW for using multiple PDN(Packet Data Network) It also handles packet filtering, charging support, and packet screening.

6. SIMNOVUS.COM HSS (HOME SUBSCRIBERSERVER) HSS (Home Subscriber Server) It is a database server that contains user-related and subscriber information. It also provides user profile information to the MME and IMS. PCRF (POLICYCHARGINGANDRULESFUNCTION) PCRF (Policy Charging and Rules Function) It basically provides the policy control decision and flow based charging control. It also sends QoS setting information for each user session. To ensure the user plane data mapping is according to the user’s profile.

7. SIMNOVUS.COM WHYDOWENEED VOLTE? There is no denying the fact that the evolution of Voice over LTE has strongly affected the mobile network operator. Having VoLTE can boost the requirement of operators needs so that they can provide the best traffic service to the users. There are many reasons to discuss why we need VoLTE in today’s scenarios, some of them listed below. It provides the high data rates with better voice quality. Able to deploy in parallel with the video calls over LTE and other services also. Increase the battery life of smartphones. VoLTE call setup time is higher than the 3G Circuit Switched call setup. It uses less network resources. When leaving the LTE network, 3G circuit switched voice continues to handle VoLTE calls without any interruption. Provides the richer and reliable service. Can use IMS as a common platform.

8. SIMNOVUS.COM DEFAULTAND DEDICATEDBEARERS Before we move to default and dedicated bearers, let’s understand what bearers are. Actually, the bearer is simply a virtual concept in which UE data flows among the different network protocols. Taking this into account, we can say that EPS(Evolved Packet Switched System) is a pipeline in which traffic data flows from one point to another within the EPS. When UE gets attached initially to the network, then the default bearer is set up, which will remain as UE connected to the network. A UE can have an additional default bearer with the condition of a unique IP address. The default bearer provides the Non-GBR(Guaranteed Bit Rate), not GBR, the range of QCI values for this bearer is between 5 to 9.

9. SIMNOVUS.COM DEFAULTAND DEDICATEDBEARERS In the case of VoLTE call, the MME initiates the request of the Create Session Bearer to the SGW so that it can create a default bearer for the IMS signaling and this message contains QCI value of 5. The PGW allocates the IP address for UE. The dedicated bearer is created on top of the default bearer. It is specifically a dedicated tunnel for different services like voice, video, real-time gaming, etc. It can be of GBR or non-GBR, but in the case of VoLTE, we only use GBR because we have to provide better conversational services. The range of QCI values for this bearer is between 1 to 4. The PGW sends the request message of Create Bearer Request to the SGW to create the dedicated bearer for VoLTE media. This message contains a QCI value of 1, Linked Bearer Identity to identify the default bearer and TFT(Traffic Flow Template) then the SGW sends the request to the MME. The MME sends the Bearer Setup Request to the eNodeB to activate the dedicated bearer for voice traffic.

10. SIMNOVUS.COM QUALITYOFSERVICE INVOLTE The QoS(Quality of Service) concept in VoLTE comes up when premium subscribers need better quality and higher bandwidth voice. In order to meet the above expectations, operators need to implement a traffic handling policy. By adopting this policy, the operator can guarantee the bitrate of voice service and precision to jitter and delay. Actually, the time of high congestion of network QoS defines the priority for allowed subscribers. In the LTE network, QoS exists between the PDN Gateway and UE. A dedicated bearer is created when there is a requirement of QoS to service like voice and video, etc. If we going to divide the dedicated bearer then it will be of two types: GBR: It does provide the Guaranteed Bit Rate for uplink and downlink. NON-GBR: It does not provide the Guaranteed Bit Rate for uplink and downlink. As you can see in above table QCI value 1 is referred to as conversational voice and other QCI values used for other services such as video, real time gaming, etc.

11. SIMNOVUS.COM VOLTECALLFLOW It is interesting to know the VoLTE call flow when someone uses VoLTE call, it connects to the LTE network and then IMS core, in that way making a VoLTE call uses signaling and media protocols to make this call in the right direction. We will be discussing how the VoLTE call flow works. To make it easy, let’s take two users, user A and user B, and user A initiate the call. Primarily, registration will happen between them, and user A will send a request message to the P-CSCF server, and it will 401 authorization response to user A. Again, user A will send a REGISTER message with an Authorization header, and IMS will send 200 OK. The user A sends a SIP invite message to User B to initiate a new call and informs IMS core and user B. SDP is used for carrying AMR codecs and bandwidth information. In IMS, a SIP message is first sent to the P-CSCF then assigned to the user during the registration process.

12. SIMNOVUS.COM VOLTECALLFLOW The P-CSCF has a connection with the S-CSCF, which is responsible for the originator. S-CSCF forwards the message to I-CSCF.I-CSCF will locate the S-CSCF of the receiver side. Here, S-CSCF will know the context of the recipient. When user B sends “183 In Progress,” it means that it informs about codec supported in the response of SDP, and the dedicated bearer is created at both ends (**PCRF set up the dedicated bearer for IMS services after checking the VoLTE subscription ). User A sends SIP PRACK (Provisional Response Acknowledgment) to User B to communicate on the selected codec, and it is also a response of “183 In Progress”. User B sends 200 OK(PRACK) to tell User A that the codec that you have sent is accepted. User A sends SIP UPDATE to user B to confirm the resources to voice call. In the same way, user B is also confirmed by sending 200 OK (UPDATE) to user A. After all the pre rules are satisfied, then user B sends an alert message to user A about the ringing of a call, i.e., “180 RINGING”. “200 OK” happened when user B answered the call.

13. SIMNOVUS.COM References https://www.gsma.com/futurenetworks/wp- content/uploads/2014/10/FCM.01-VoLTE-Service-Description-and- Implementation-Guidelines-Version-2.0.pdf https://en.wikipedia.org/wiki/Voice_over_LTE

14. SIMNOVUS.COM CONTACTUS https://simnovus.com Contact Information Product Enquiries sales@simnovus.com Job Enquiries hr@simnovus.com Support Enquiries support@simnovus.com Marketing Enquiries marketing@simnovus.com Registered Office Simnovus Tech Private Limited 520-523, City Centre, Plot No.05, Sector-12, Dwarka, New Delhi-110075, India Phone: +91 1128085036 R&D Office Simnovus Tech Private Limited 104, Regent Prime, #48, Whitefield Main Road, Whitefield Bengaluru – 560066, INDIA Phone+91 8041273808