cs services in lte

CS services in LTE- An Overview

Prepared by: Darshan Patil Sr. Engineer, Alcatel Lucent Managed Solutions India

Summary

• What? To support CS services in LTE

• Why? Need to support CS services in LTE

• When? Different Phases of evolution of Voice over LTE (VoLTE)

• How? Network methodologies to implement CS services in LTE network

WHAT?

• LTE is All- IP data only support technology using packet switching

• This subjects to challenges for CS services in LTE as referred to all the legacy networks which support them currently

• Solutions to provide CS services in LTE network

• Inter-networking solutions to provide CS services in LTE network

WHY?

• Current legacy networks increase expectations to improve the efficiency of the services they are providing

• This includes Voice, SMS, Video Call, High data rate internet and streaming etc. Services

• The evolution of LTE technology is to significantly improve the user experience, substantially improving end-user throughputs, increasing sector capacity, and reducing user plane latency

• Though, due to lack of Circuit switching, the technology has a need to find some way outs to provide CS service solutions at par with the current legacy networks

• Also, till the time when LTE completely swaps the legacy networks there is a need for internetworking with them

Comparison between

Dual Radio Solutions

o use two always-on radios (and supporting chipsets), one for packet switched LTE data and one for circuit switched telephony, and as a data fallback where LTE is not available

o have emerged for LTE-CDMA2000 network interworking driven by time-to-market pressures

o Battery gets drained faster due to two radio antennas and chipsets

Single Radio Solutions

o use one radio to handle both types of traffic, and use network signaling to determine when to switch from the PS network to the CS network

o this solution is universally accepted for LTE-3GPP network interworking solutions

o Power consumption of the battery is fairly reduced due to single antenna – single chipset.

Phases of Single radio solutions for VoLTE

Phase I:

Circuit Switched Fall Back (CSFB) addresses the requirements of the first phase of the evolution of mobile voice services, which began on commercial scale in 2011

primarily due to inherent cost, size and power advantages of single radio solutions on the device side.

CSFB is the solution to the reality of mixed networks today and throughout the transition to ubiquitous all-LTE networks in the future phases of LTE voice evolution

All the voice traffic is handled by current legacy CS networks (UMTS/GSM/CDMA)

All the data traffic is handled by LTE PS network wherever and whenever available

The traffic will be able to fallback to 2G/3G network in non-LTE network


Phase II:

The second phase in LTE voice evolution introduces native VoIP on LTE (VoLTE) along with enhanced IP multimedia services such as video telephony, HD Voice and Rich Communication Suite (RCS) additions like instant messaging, video share and enhanced/shared phonebooks

This phase also uses a single radio solution with Single Radio Voice Call Continuity (SRVCC) that seamlessly maintains voice calls as mobile users move between LTE and non-LTE coverage areas

CSFB continues to be deployed during phase 2, to provide voice services for roamers and CSFB only devices

Without SRVCC, a VoLTE call on a device moving out of LTE coverage will be dropped, since no operators currently support VoIP on 3G


Phase III:

converges the native power of IP to deliver enhanced capacity, value-added services (e.g., voice and video over IP and rich communication services) and interoperability across network access methods and operators (LTE, 3G/ HSPA, Wi-Fi and legacy telephony domains)

HOW?

• Phase I: CSFB

The architecture in Figure below shows a simplified view of the parallel LTE and 2G/3G networks.

Phase I: CSFB – CN procedures

• The legacy network (2G/3G) and LTE network co-exists

• To support CS Fallback signaling and SMS transfer for LTE devices, the MME connects to the MSC Server

• MME is the mobility management entity serving users while in LTE access. It is responsible for maintaining SGs association towards MSC, initiating paging procedure towards eNodeB during CSFB

• MME Support SMS procedures

• MSC is responsible for maintaining SGs association towards MME for EPS/IMSI attached UE

SGs interface

• It 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.

• It enables the user’s device to be both CS and PS registered while on the LTE access network. This interface also enables the delivery of CS pages via the LTE access, as well as SMS, without having the device leave LTE.

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

• 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.

CSFB- Incoming Call• With the default LTE data network connection in operation, a

mobile terminating (incoming) CS voice call triggers a page via LTE to the user’s device, as shown in below Figure

CSFB- Incoming Call continued..• This page initiates CSFB, as the device sends an extended

service request to the network to transition to 2G/3G, as shown in below Figure

• Once transitioned, the legacy call setup procedures are followed to setup the CS call.

CSFB- Incoming Call continued..

• When the voice call ends, the device returns to LTE via idle mode or connected mode mobility procedures, as shown in below Figure

CSFB- Outgoing call

• Mobile originating (outgoing) calls follow the same transition from LTE (PS) to 2G/3G (CS), except for the paging step, which is not needed.

Simultaneous PS and CS services in CSFB

• Depends on the DTM feature availability with the legacy network

• In 3G networks, PS data sessions can also move for simultaneous voice and data services.

• In 2G networks, PS data sessions may be suspended until the voice call ends and the device returns to LTE, unless the 2G network supports dual transfer mode (DTM), which permits simultaneous voice and data.

Phase I: CSFB – RAN proceduresAcquisition of the 2G/3G network and setup of the call can employ one of two

procedures: handover or redirection.

Handover• Target cell is prepared in advance• Device enters in connected mode

during IRAT• Signal strength measurements

are done prior to HO to 2G/3G (i.e in LTE access mode)

• Process require more time to identify the best cell

• Under changing IRAT environments, this method proves to be not much efficient

• Therefore it is not widely followed method currently

Redirection• Target frequency is indicated• UE may choose other IRAT

frequency cell if no cell could be found out in target frequency

• After UE has access to legacy network, call setup procedure has to be initiated

• IRAT measurements are not required prior to Redirection

• As per statistics, a marginal loss in performance in this method as compared to Handover based IRAT can be accepted to compensate variable IRAT RF environment

CSFB w/ ReselectionThree types of procedures according to System Information

Blocks sent to the UE

1. Release 8 – Basic Redirection: UE reads all the SIB’s before having an access to the

target cell

2. Release 8 – with SIB skipping Mandatory SIB’s like 1,3,5,7 are only read by the UE. After

it accesses the target cell, in connected mode it reads the neighbor info present in SIB 11

3. Release 9 – Enhanced (SI tunneling) where SIB information can be tunneled from the target

Radio Access Network (RAN) via the core network to the source RAN and be included in the redirection message sent to the device. This can avoid reading any SIBs on the target cell.

CSFB Performance Indices

• After commercial deployments, CSFB have been par with the 3GPP performance requirements for 3G UMTS.

• These basic requirements includes Call setup time (mobile originated and mobile terminated), Call reliability, LTE to 3G cell handover time, PS data interruption time.

• Call reliability in case of Handover based CSFB process is poor due to variable RF environment

• Call reliability in case of Redirection based procedure is better than Handover based as per statistics due to no delay in between identification and accessing the target cell.

• Rel9- SI tunneling has slightly better call reliability (about 99%- 100%) than Rel8- Basic Reselection.

CSFB Outgoing Call setup penalty time

Sr. No RAN procedure Transition to Sub category Call setup time penalty

1 Handover Based 3G UMTS NA 0.4S > CST of 3G

2 Redirection 3G UMTS Rel8- Basic 2.5S > CST of 3G

Rel8- SIB skip 0.9S > CST of 3G

Rel9- SI tunnel 0.5S > CST of 3G

3 Handover 2G GSM NA 2.6S > CST of 3G

2.7S > CST of 2G

4 Redirection 2G GSM Rel9- Basic 2.6S > CST of 2G

2.5S > CST of 3G

Rel8- SIB skip NA


0.5S > CST of 3G

CSFB Incoming Call

Sr. No RAN procedure Transition to Sub category Call setup time penalty

1 Handover Based 3G UMTS NA 0.4S > CST of 3G

2 Redirection 3G UMTS Rel8- Basic 2.5S > CST of 3G

Rel8- SIB skip 0.9S > CST of 3G


3 Handover 2G GSM NA NA

4 Redirection 2G GSM Rel9- Basic

Rel8- SIB skip

Rel9- SI tunnel

CSFB Data Interruption timeThe active session PS data interruption time during LTE PS to

2G/3G CS and reverse

Sr. No RAN procedure Transition to Sub category Data interruption time

1 Handover Based 3G UMTS NA 0.3S

2 Redirection 3G UMTS Rel8- Basic 6.8S

Rel8- SIB skip 5.1S

Rel9- SI tunnel 4.8S

3 Handover 2G GSM NA NA

4 Redirection 2G GSM Rel9- Basic

Rel8- SIB skip

Rel9- SI tunnel

LTE to 3G cell Handover time

• Time taken to handover is never identical due to varied Rfenvironment

• Delay added by LAU may increase Ho time. Further if the Lte cell is overlapped with multiple LA’s, the delay can increase up to 1-2 seconds

• Fewer times, apart from updating LA’s TA’s, there would be need to update HLR data between diff overlapping MSC’s.

• More uncertainties can add to the delay further.• Solutions to the above:

– IU/A Flex– MTRF– MSC pooled architecture planning

Phase II: VoLTE and SRVCC

• Voice over IP can be implemented in two ways:1. using “Over the top” (OTT) VOIP applications such as skype, Ekiga,

Goober etc..2. using native VOIP using additional entities such as IMS and MMTel in

core network

• Using OTT services is not recommended as the Qos is similar to NRT services with no special priorities to voice. Therefore, even the voice quality of current legacy CS services is not matched

• VoLTE, in contrast, operates as a native application in the user’s device, enabling prioritization over other data streams to deliver quality of service levels consistent with established user expectations

• In non-LTE network, this quality is taken care by 2G/3G networks which can be accessed using SRVCC (Single Radio Voice Call Continuity) method of IRAT HO

SRVCC Network Architecture

MMTel and Sv

• The 3GPP/NGN IP Multimedia Subsystem (IMS) multimedia telephony service (MMTel) is a global standard based on the IMS, offering converged, fixed and mobile real-time multimedia communication using the media capabilities such as voice, real-time video, text, file transfer and sharing of pictures, audio and video clips. With MMTel, users have the capability to add and drop media during a session.

• Sv is defined in TS 23.216, where it is defined as the reference point between the MME/SGSN and MSC Server.

SRVCC Procedure

1. User (UE) is in active voice call in LTE network using VoLTE with the help of IMS as shown in below figure

2. User moves from a LTE network to a non-LTE network (3G-UMTS), the active call remains under the control of IMS throughout the handover process

SRVCC Procedure

3. Two step process of SRVCC from LTE to 2G/3GIRAT Handover

IRAT handover is the traditional handover of the user’s device from LTE radio access to WCDMA/GSM radio access

Session transfer

Session transfer is a new mechanism to move access control and voice media anchoring from the LTE Evolved Packet Core (EPC) to the legacy CS core

SRVCC Procedure

4. The handover process is initiated by a session transfer request to the IMS/MMTel as shown below

SRVCC Procedure

5. The IMS/MMTel responds simultaneously with two commands

i. Sending an “IRAT HO execution command” to the LTE network on which the voice call is in progress through MME & Lte RAN to instruct the UE to prepare to move to CS network for the voice call

ii. Sending a “session transfer response” to the CS n/w where the users voice call is being sent asking to prepare to accept the call in progress

SRVCC Procedure

• Both the n/w send acknowledgements respectively and under the control of the IMS/ MMTel, the call is switched to 2G/3G n/w

Network upgrades required for SRVCC methodology

Sr. No Entity Up gradation required?

1 LTE E-UTRAN Yes

2 LTE EPS Yes

3 IMS Core/ MMTel Yes

4 2G/3G Core (MSC) Yes

5 2G/3G RAN No

6 UE (user equipment) Yes

PI for SRVCC

o Voice interruption time:1. IRAT HO execution process requires more time than session

transfer procedure (of the order 0.01S). Therefore voice interruption time is influenced primarily by the IRAT HO execution time.

2. Further, we need to consider addition of time required to confirm the HO execution after HO process

3. The total voice interruption time is at par with the 3GPP guidelines of 0.3S

o Call Retention rate depends on:1. Probability of IRAT failure2. Probability of Session transfer failure3. Current stats show it as 99%

References

• 3GPP specs

• Qualcom (VoLTE phases)

• China Mobile (CSFB)

• Nokia Seimens (CSFB)

• Ericsson (VoLTE phases)

• Alcatel-Lucent (LTE Network Architecture)

Future Scope of VoLTE

• Enhanced SRVCC – Next article

• Video SRVCC – Next article

• CSFB and VoLTE Roaming (Phase III)

cs services in lte

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