VoLTE Characteristics: A Brief Overview

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VoLTE Characteristics: A Brief Overview

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Voice over LTE (VoLTE) is expected to become the mainstream solution for providing voice services in commercial LTE networks in the coming years.  VoLTE integrates voice over IP (VoIP), LTE radio network (i.e., E-UTRAN), LTE core network (i.e., EPC), and the IMS (IP Multimedia Subsystem) to support voice services.  Let’s summarize the main characteristics of VoLTE from the perspectives of IMS capabilities, media codecs, LTE radio and core capabilities, and IP functionalities [1]. 

IMS plays an essential role in VoLTE.  The UE performs SIP (Session Initiation Protocol) registration with the IMS network.  IMS-AKA (IMS- Authentication and Key Agreement) procedures are followed for authentication.  Integrity protection, whereby integrity of SIP signaling messages is ensured, is mandatory.  The use of ISIM (IP Multimedia Services Identity Module) or USIM (UMTS Subscriber Identity Module) is required during the IMS authentication.  Alphanumeric or MS ISDN based identities can be used for the consumer.  SIP signaling messages are ASCII text messages and could thus be quite large.  Hence, signaling compression is mandatory to reduce the bandwidth requirements, especially for over-the-air transmission.  A variety of supplementary services such as communication forwarding and communication hold are supported. 

As far as speech codecs are concerned, the basic Adaptive Multi Rate (AMR) speech codec with all the eight modes is mandatory; the popular data rate for good speech quality is 12.2 kbps.  VoLTE optionally supports the AMR wideband codec with nine modes, where the data rate of 12.65 kbps (often called the anchor bit rate) is expected to be popular.  The Real-time Transport Protocol/ Audio Video Profile (RTP/AVP) must be supported.  Furthermore, RTP over UDP is used to transport AMR speech.  While Real-time Transport Control Protocol (RTCP) is turned off during the active speech, it is temporarily enabled for link aliveness when the media are on hold. 

LTE radio and core networks need to use optimal configurations for VoIP calls.  For example, to make the over-the-air transmission of VoIP packets efficient, the IP-related overhead is reduced using Robust Header Compression (RoHC) at the eNodeB and the UE.  A default EPS bearer with QCI (QoS Class Identifier)=5 toward the IMS Access Point Name (APN) is established to carry SIP signaling.  This default EPS bearer requires an Acknowledged Mode Data Radio Bearer (DRB) between the eNodeB and the UE.  A dedicated EPS bearer with QCI=1 is required to carry the VoIP traffic.  Such dedicated EPS bearer requires an Unacknowledged Mode DRB.  The network initiates the dedicated EPS bearer setup.  The UE battery life is improved through the use of DRX (Discontinuous Reception), where the UE receives (and transmits) information intermittently and saves the battery power and the processing power between the subframes carrying the UE’s VoIP frames. 

VoLTE supports both IPv4 and IPv6, with IPv6 expected to be quite popular.  The UE conveys its preference for IPv4v6 (i.e., dual-stack IPv4 and IPv6) while connecting to the IMS APN.  When the operator’s network supports IMS-based E911 calls, Release 9-compliant UE and the network are utilized. 

MetroPCS is offering VoLTE to its subscribers.  VoLTE is expected to be deployed by AT&T and Verizon in 2013. 



[1] GSMA, “IMS Profile for Voice and SMS,” Version 3.0, PRD IR.92, December 22, 2010, http://www.gsma.com/newsroom/wp-content/uploads/2012/06/IR9230.pdf.

  • Hi

    Are there any challenges that will be faced on deploying VoLTE if the networks are optimized for data?