2009年12月25日

VoLTE (1) - Overview of Voice over LTE

VoLTE (1) - Overview of Voice over LTE

Alternatives for Delivering Voice in FTTP

1). No Native Voice and SMS in LTE

While packet switched wireless networks have many advantages, there is also a major disadvantage: Voice calls and SMS messaging, the main revenue generators of mobile network operators, are no longer available in LTE, as they are based on a circuit switched radio and core network


Alternatives for Delivering Voice in FTTP

2). Options for Voice over LTE:

1) Data Only (No Voice over LTE) - Choice of initial deployment for LTE without voice

Voice Based on Legacy MSC

2) CS Fallback (CSFB)

3) Voice over LTE using Generic Access (VoLGA)

Voice Based on New Infrastructure

4) IMS
5). Over-The-Top (like Skype)


3). 3GPP has so far adopted two different approaches:

§ 1). Fallback to 2G or 3G for Voice

§ 2). IMS As a Potential Solution For the Mid and Long Term


4-1). CS Fallback:

Pros:

§ Proven technology

§ Supports migration from CDMA as well as GSM/UMTS

Cons:

§ Call setup time delay - Minimum 30% increase* in call setup times (requires a ‘blind fallback’ to achieve 30%, delay is higher if handset must do network scan before fallback)

§ Requires MSC upgrade - Dependent on MSC vendors to implement (home and visited networks)

§ Impact on coverage and handoff for 2G and 3G

§ Standard is ‘barely adequate’

§ No support for simultaneous voice/data over LTE - No LTE during voice call (Dropped data connections)

§ CSFB Device for LTE voice (Poor fit with Multi-tasking devices)

§ CSFB fits very poorly with LTE femtocells - “Falling back” to GSM network negates value of LTE femtocell

§ No support for combinational IMS/RCS + voice over LTE

§ No validating LTE network QoS capabilities

§ No verification of IMS telephony plumbing features


4-2). IMS “One Voice” Profile

§ A solution envisaged for the mid and long term is to introduce network operator based voice services in LTE with the IMS.

§ TAS from IMS Centralized Services (ICS) - Service anchor in IMS to improve service consistency

§ Single Radio Voice Call Continuity (SR-VCC) - Swap between IMS and CS without preserving services

§ IP Multimedia Subsystem (IMS) Emergency Sessions is supported via E-CSCF. The Emergency solution should be able to provide continuity of location support the SR VCC of emergency calls.

§ The One Voice Initiative aims to achieve an industry agreement on a harmonized way to implement voice and SMS over LTE based on existing standards -“Voice over IMS Profile” specification

Pros:

§ Most feature rich

§ Enables Real time Rich Media - fully IP based platform for rich media communication

§ Doesn’t require MSC support

§ Handing-over ongoing IMS based voice calls to circuit switched networks via SRVCC (between IMS over PS access and CS access for calls is anchored in IMS)

§ Convergence via ICS – IMS based on the popular SIP is widely used in fixed line IP based networks for Voice over IP


Cons:

§ IMS is a costly and complex solution.

§ Complexity - significant complexity of the system, and it will still take several years before large scale commercial IMS deployments, and features to handle wireless specific issues such as unreliable radio connections, application servers for external application development, international roaming, scalability, security, etc.

§ IMS Market timing

§ SRVCC provides the ability to transition a voice call from the VoIP/IMS packet domain to the legacy circuit domain, but the ability to transition from the circuit domain to the packet domain is not addressed in the current generation (R8) of LTE standards

§ Limited LTE Coverage if only hotspots at the initial phase

§ SRVCC-capable mobile initiated in a voice call determines that it is moving away from LTE coverage

§ ICS-capable UE if ICS is utilized


4-3). Voice over LTE using Generic Access (VoLGA):

§ A technological approach for delivering voice and SMS services over LTE access networks

§ Leverages a mobile operator’s existing core voice network

§ VoLGA is architecture independent and uses the UMA/GAN (Unlicensed Mobile Access/Generic Access Network) protocol - Derived from the existing 3GPP GAN standard (originally adopted for Wi-Fi/3G fixed-mobile convergence)

§ VoLGA re-uses this principle by replacing the Wi-Fi (GAN-based) access with LTE access on an LTE/UE new dual mode mobile device (both GSM/UMTS and LTE).

§ VoLGA does not require modifications in the LTE RAN or Core, or the MSC, but uses a new separate gateway controller (VANC).

§ The VoLGA Access Network Controller (VANC) , as a GAN gateway between LTE and CS domain, securely connects a subscriber to the infrastructure of a network operator and voice calls and other circuit switched services such as SMS are then securely transported between the mobile device and the Gateway.

§ VoLGA is a stronger contender than CSFB. From technical view, VoLGA seems to be a much better starting point. VoLGA would further delay IMS deployment

Pros:

§ Voice and Data over LTE

§ Call setup times as good as 3G

§ Preserves CS core investments

§ External controller (VANC) minimizes impact to core network - No MSC upgrades

§ Supports simultaneous voice/ data over LTE

§ VoLGA will support all existing circuit services as well as IMS RCS - Supports combinational IMS/ RCS + Voice over LTE

§ Delivering voice over LTE validates LTE QoS capabilities

§ Voice services delivered natively through LTE femtocell

§ The VoLGA forum decided to use the SRVCC as the means to handover VoLGA calls from LTE to GSM or UMTS.

§ No VoLGA specific features required in the MSC or SGSN for VoLGA is a great plus for deployment in a running network.

Cons:

§ Not 3GPP standardized yet - VoLGA is currently not a work item in 3GPP

§ Not fully standardized yet as the stage 3 specification has not yet been finalized

§ Limited operator support

§ GAN-based dual-mode mobile phones is required

§ SRVCC-capable mobile is required

§ Only T-Mobile strongly enthusiastic right now

§ Scaling and Roaming

§ Limited LTE Coverage if only hotspots at the initial phase

§ It also requires changes to handsets, as well as a mechanism for allowing the network to trigger LTE-to-3G/2G handovers for VoLGA calls, originally defined as part of SR-VCC (single radio voice call continuity).


4-4). Over-The-Top (like Skype)

§ Some network operators might also decide to go an entirely different way and offer voice services (like Skype) over LTE with external partners.

§ UK network operator '3', for example, has partnered with Skype to deliver voice services in addition to their own circuit switched services.

Pros

§ Not limited by legacy continuity

§ No “IMS complexity tax”

§ Integration with presence and productivity apps

§ Non-traditional voice apps (Voice Twitter?)

Cons

§ Business case unproven

§ QoS - service providers have no control over quality of service

§ Relationship politics

§ What happens outside LTE footprint - no Handover (or Mobility) that calls can't be handed over to a circuit switched 2G or 3G network when a user leaves the LTE coverage area

§ Regulation


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