VoLGA adds 3G, new LTE features

The VoLGA Forum has announced a set of new features for delivering mobile voice and SMS services over LTE. New features included are:

• support for SIM-less emergency calling
• optimized voice-bearer routing
• host APIs for LTE handsets

In addition to above VoLGA has extended its architecture to support 3G/HSPA in phase 2 specifications. New specifications now support Voice (and other CS services) over LTE via Generic Access (VoLGA) and "VoLGA" service over UTRAN packet access.

Addition of 3G via VoLGA is surprising however it gives another option to provide seemless voice service via existing 3G packet switched networks. With this enhancement now voice calls can be placed via 3G packet network using VoLGA and handover to CS is also possible. With VoLGA over UTRAN packet access, following new options are possible for voice calls.

• handover from E-UTRAN to a UTRAN PS domain (Voice Call remains VOIP)
• handover from UTRAN PS to a GERAN or non-VoIP capable UTRAN target cell (Voice Call switches from PS voice to CS Voice)

See below the roaming architecture in UTRAN PS case, for E-UTRAN refer VoLGA wiki.

It would be interesting to know why VoLGA Forum has extended VoLGA services to 3G, as of now it just gives another option for operators to use VoLGA for 3G PS as well. Concept is similar to UMA/GAN, radio access network has been replaced from WiFi to 3G UTRAN.

Just to mention VoLGA does not support CS to PS handover from GERAN/UTRAN CS domain to E-UTRAN or UTRAN PS.

source: LteWorld

What Next For VoLGA?

Operators are getting ready for LTE deployments in second half of 2010. However data only deployment is initially expected, followed by voice later. So how does operators are planning to launch voice services - 3GPP CSFB or IMS based or VoLGA ?

Last year there were discussions all around about inability of 3GPP LTE to handle voice from day 1. Kineto Wireless came forward with VoLGA. The VoLGA (Voice over LTE via Generic Access) Forum was initiated last year to promote an approach for extending traditional GSM and UMTS circuit-switched services over LTE access networks. It was an evolution from UMA/GAN but for LTE.

VoLGA claimed to solve voice problem, as operators around the world would adopt IMS voice on different timelines.

3GPP backers came back strongly and major mobile and converged operators such as AT&T, Verizon, France and Telecom/Orange along with powerful wireless equipment vendors such as Ericsson and Nokia-Siemens announced (November 2009) their plan to follow the IMS-based approach for delivery of voice and messaging services over Long Term Evolution (LTE) broadband mobile radio access.

For more visit LteWorld

Understanding CS Fallback in LTE

LTE technology supports packet based services only however 3GPP does specifies fallback for circuit switched services as well. To achieve this LTE architecture and network nodes require additional functionality, this blog is an attempt to provide overview for same.

In LTE architecture, the circuit switched (CS) fallback in EPS enables the provisioning of voice and traditional CS-domain services (e.g. CS UDI video/ SMS/ LCS/ USSD). To provide these services LTE reuses CS infrastructure when the UE is served by E UTRAN.

A CS fallback enabled terminal, connected to E UTRAN may use GERAN or UTRAN to connect to the CS domain. This function is only available in case E UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage.

The figure above provides architecture for CS fallback in EPS.

CS Fallback and IMS based services can co-exist in the same operator’s network. Although its not very straight forward to support CS fallback, all participating elements i.e UE, MME, MSC & E-UTRAN needs to support additional functionalities.

The support CS fallback in EPS a new interface SGs is added in LTE architecture. SGs interface is the reference point between the MME and MSC server. SGs interface is used for the mobility management and paging procedures between EPS and CS domain, and is based on the Gs interface procedures.

The SGs reference point is also used for the delivery of both mobile originating and mobile terminating SMS.

The CS fallback enabled network elements need to support the following additional functions:

UE

• supports access to E-UTRAN/EPC as well as access to the CS domain over GERAN and/or UTRAN.
• Combined procedures for EPS/IMSI attach, update and detach.
• CS fallback and SMS procedures for using CS domain services.

MME

• Deriving a VLR number and LAI from the GUTI received from the UE or from a default LAI.
• Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE.
• Initiating IMSI detach at EPS detach.
• Initiating paging procedure towards eNodeB when MSC pages the UE for CS services.
• Support of SMS procedures
• Rejecting CS Fallback call request (e.g. due to O&M reasons)
• Use of the LAI and a hash value from the IMSI to determine the VLR number when multiple MSC/VLRs serve the same LAI.

MSC

• Maintaining SGs association towards MME for EPS/IMSI attached UE.
• Support of SMS procedures as provided in 3GPP specification

E-UTRAN

• Forwarding paging request and SMS to the UE.
• Directing the UE to the target CS capable cell.

At MME - MSC Server interface a new protcol SGsAP is being added to support CS fallback. SGsAP protocol is based on the BSSAP+. Stream Control Transmission Protocol (SCTP) is used to transport SGsAP signaling messages.

A CS Fallback and IMS capable UE would follow the procedures for domain selection for UE originating session/calls according to 3GPP specification 23.221.

If a UE is configured to use SMS over IP services and it is registered to IMS then it would send SMS over IMS, even if it is EPS/IMSI attached.

The home operator has option to activate/deactivate the UE configuration to use SMS over IP by means of device management in order to allow alignment with HPLMN support of SMS over IP.

When UE is performing CS fallback procedure for Mobile Originating Call for the purpose of emergency call, it needs to indicate to the MME that this CS fallback request is for emergency purpose. MME also indicates to the E-UTRAN via the appropriate S1-AP message that this CS fallback procedure is for emergency purpose.

Contents of this blog are mostly derived from 3GPP specification 23.272, for better and detailed understanding same should be referred.

Although there had been talks about another approach for CS Fallback by VoLGA which does not require any enhancement in existing CS elements like MSC but for VoLGA another set of additional nodes are needed. to know more about VoLGA refer one of our earlier blog LTE needs VoLGA.

LTE needs VoLGA

LTE (Long Term Evolution) is a data only technology but it seems that we can not live without voice or SMS services. VoLGA forum claims that VoLGA specifications fill this gap. A number of telecom companies (including Ericsson, Alcatel-Lucent, Huawei) are showing interest in VoLGA and are part of VoLGA forum.

According VoLGA stage 1 specification, The aim of VoLGA is to make traditional GSM/UMTS circuit switched (CS) services available to UEs accessing the EPS network via E-UTRAN.

The lack of native SMS over LTE support is a show-stopper for LTE dongle service. Mobile operators rely on SMS for back office customer care, provisioning and management of HSPA-dongle services. Clearly those systems need to be available to an LTE-based terminal service at launch.

read more at LteWorld

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