VOLTE-Part 1: Overview

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VOLTE-Part 1: Overview

Hi guys, we are starting a new series on VoLTE. This is the first post, which introduces the concept of Voice over LTE. Hope you find it informative. Stay tuned for more posts. 

Brief introduction of VoLTE

Mobile telephony was invented as a technology to transmit human or artificial voice either through wired or wireless medium over long distances. 1G was completely analog in nature. With 2g, wireless communication became digital. This foray into the digital world also allowed mobile devices to transmit not just voice, but data packets to be consumed through mobile screens. Still, the primary focus of this standard was voice and not data. With 3G, 3GPP started taking data seriously and implementation standards were introduced to increase data speeds.  

But similar up gradations were not introduced for voice and is still continued to be transmitted using the now called legacy network. In 4G, everything became compatible with the internet protocol. Every node and entity in the network now had an IP address using which data was addressed to it. But this all-IP network was all about data. 

What about voice? Was it still supposed to travel through the derelict and outdated circuit-switched legacy network?

The answer is NO. Now, voice was also divided into IP packets and routed through a packet-switched network called IMS, which stands for IP Multimedia Subsystem. Like “internet”, “IMS” is also a PDN. If a UE wants to use data, it should be connected to the APN “internet”. Similarly, if a UE wants to make/received packet-switched IP calls called VoLTE, it needs to connect to IMS.

How does a UE establish PDN connection with IMS?

After the initial attach is successful, and the UE has established a default bearer with the default APN, which usually is internet, the UE sends a PDN Connectivity Request to IMS. 
This request is sent so that the UE can use services like making a VoLTE call. Once this request is accepted, a default bearer with QCI = 5 is created.

What are the steps taken by the UE and NW to establish a bearer to IMS?

  • The UE sends a PDN Connectivity Request to MME (QCI9 already establish). In this request the APN requested is “ims”.
  • Upon receiving the PDN Connectivity Request for IMS (QCI-5), the MME sends Create Session Request to the SGW.
  • The SGW forwards this request to the PGW.
  • The PGW allocates an IP Address to the UE at this point.
  • The PGW then sends a Credit Control Request to the PCRF.
  • The PCRF makes a decision on Policy and Charging rules for the UE.
  • In response to the Credit Control Request, the PCRF sends Credit Control Answer to the PGW.
  • Post this, the Policy and Charging rules are enforced on the UE.
  • The PGW/SGW sends the Create Session Response to the MME.
  • The MME sends the Activate default EPS bearer context request message to the UE.
  • In response, the UE sends Activate default EPS bearer context accept message to the MME.
  • Now a default bearer has been established between the UE and PGW for IMS.

What is the EPS id of the default EPS bearer of IMS?

EPS Bearer ID-6 (* Here for QCI9= EPS bearer id is 5 and for QCI5= EPS bearer id is 6)

What is the QCI of the default EPS bearer of IMS?



  1. In some networks, policy control requests and answers are exchanged between PCRF and PGW. Especially in prepaid markets where credit control requests and answers are managed by PGW and OCS. Thanks for the very simple and concise explanation.

    1. Thank you so much for your valuable feedback @Abiodun, Stay tune with us for more updates and please share article with your friends and other contacts.

  2. Nice explanation. Pls keep posting.


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