LTE_Tech_Ov_Sec06_131009_v01

Informa Telecoms & Media

Service Provision in LTE

Service ProviSion in LTe



Service ProviSion in LTe

Introduction to LTE Services 4CS Fallback 6Disadvantages of CS Fallback 8IMS, IP Multimedia Sub-System 10Voice over LTE via Generic Access (VoLGA) 12Operation of VoLGA 14Quality of Service in VoLGA 16Handover in VoLGA 18

4Service Provision in LTE


introduction to LTe Services

LTE is designed as a flat IP network explicitly to support data services, whether these services are simple internet connectivity or more complex operator defined services is not defined in the standard, LTE architecture and protocols do not define any specific services. Whilst the provision of data services is not a great problem the lack of an explicitly defined voice and text service is currently seen as a potential limitation of future network expansion, particularly for those operators who may potentially skip a technology generation, i.e. GSM/GPRS to LTE (no UMTS). This section will outline the possible voice solutions and discuss the advantages and disadvantages of each proposal.

The 3 current voice solutions being proposed are;

CS fallbackIMS voiceVoLGA

There is one other possible method of delivering voice in an LTE network and that is the so called over the top VoIP method.

5 Informa Telecoms & Media

Packet Data ServiceLTE was designed for this

Simple internet access

Operator defined services

Over the top or Application based services

voice and SMSNo specified method of supporting these

CS (circuit switched) Fallback (2G/3G)

IMS SIP based

VoLGA, Voice over LTE via Generic Access

Over the top VoIP e.g. SKYPE

Fig. 1 Services in LTe



cS Fallback

CS Fall back requires that the MME is connected to the MSC Server via the SGs interface, the MME thus acts as an SGSN from the perspective of the MSC Server.

When the UE first register with the LTE network it will indicate that a combined update is required, this instructs the MME to pass the registration messages to the 2G/3G network. This method does not require major changes to the network architecture.

For incoming CS calls the MSC will signal the MME via the SGs interface, the paging message can then be forwarded directly to the mobile if it is currently in an active state, otherwise and LTE paging procedure will be carried out, the CS paging information can be forwarded to the UE once the radio connection is re-established.

The UE will then be required to select a 2G/3G cell, the eNB may instruct the UE to take measurements from the 2G/3G network in order to determine which cell to use or may blindly send instructions about a preconfigured cell, this may be the case when the LTE and 2G/3G cells are co located.

The paging response from the UE is carried out in the new 2G/3G cell.

There is the additional problem however of the case when the UE has an active data connection in LTE when the CS call arrives, the data session would have to be suspended or handed over to the 2G/3G network, this begins to get somewhat complex.

MME

Gs

SGs

A

lu-cs

lu-ps

Uu

Um

LTE-Uu S1-MME

S3Gb

UE

UTRAN

E-UTRAN

GERAN

SGSN

MSCserver


Fig. 2 cS Fallback Architecture



Disadvantages of cS Fallback

This method of supporting voice calls in LTE seem to be favoured by many 3GPP members, at least in the short term, however there a number of disadvantages that can be identified,

The SGs interface is similar to the Gs interface specified originally in GPRS however implementation of the SGs interface requires the addition of new software on the critical MSC nodes in the network, operators may be hesitant to interfere with the operation of these devices. As ever there is the issue on inter-operability between the various vendors in the operators network, the SGs interface may have to link diverse vendors or MSC and MME.

Perhaps a more tangible problem is the additional time taken for the UE to fallback to the 2G/3G network, this would manifest itself as longer call setup time for the user, some estimates quote and additional 1.5 second could be added to the current call setup time.


Disadvantages of cS FallbackModification to MSC nodes, SGs interface

Longer Call Setup time +1.5s

Fig. 3 cS Fallback Disadvantages

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iMS, iP Multimedia Sub-System

IMS offers a potential solution for voice in the LTE network. IMS is a standard that has existed for a number of years now, however has perhaps not seen the major growth of service supported on IMS that was predicted a few years ago. Cost and complexity are a couple of the issues faced by the operators.

IMS is an IP based network based on the SIP (Session Initiation Protocol) which allows the operators to deployed a diverse range of services such as text, media, video and other communication application on the basis of a single IP based platform. The main platform in the IMS is referred to as the Call Session Control Function (CSCF) or SIP Server. The CSCF in the case of IMS supporting VoIP would act as the call control function during the voice call setup.

VoIP via IMS requires that the UE support an IMS (SIP) client on the handset, many medium to high end smart phones already support the SIP protocol, however the operator would need to be more explicit in their support of the VoIP application an procedures.

IMS was originally conceived to be an IP only deployment with no connection between the IP and CS networks, however this turned out to be less than practical therefore the 3GPP have included the feature of Voice Call Continuity (VCC) which allows call to be handed over from CS to IP based networks.

MME

eNB

S1

S5 SGi

S11 IMS

CS network

CSCFSIP server

Serving GW PDN GW

Limited CSconnectivity

VCC


Fig. 4 iMS and voiP

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voice over LTe via Generic Access (voLGA)

VoLGA is in many respects similar to the Generic Access Network (GAN) previously specified by the 3GPP, which enable WiFi to be added as an access network to 3GPP systems, dual mode UEs are then able to access network services via the standard access network and also via WiFi. And network controller provides a secure connection to the operators network allowing voice and text calls to be made via the alternative RAN.

The VoLGA concept uses the principle of GAN but replaces WiFi with LTE, this feature was originally examined in 3GPP TR 23.879 and the VoLGA forum was created to encourage the development of detailed standards.

The basic architecture specifies the VoLGA Access Network Controller (VANC) which connects the MSC devices in the CS network to the PDN-GW in the LTE EPC. The VANC transports both user data packets and signalling transparently from UE to the CS network.

The VANC appears to be a BSC/RNC device from the point of view of the MSC and supports both the A and Iu interfaces. Support of these interfaces means the MSC devices do not have to be modified.

GSMaccess

A Iu

D

S6a SGi

UMTSaccessGSM

MSC

HLR/HSS

MME S-GWP-GW

UMTSMSC

VANC

LTEaccess


Fig. 5 voLGA Architecture

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operation of voLGA

The UE will register with the MME on the LTE network then establishes a connection with the VANC. The connection with the VANC can only be done if a suitable IP connection exists, e.g. the default bearer or some specific bearer used for the purpose. The host name (or IP address) of the VANC may be stored in the UE and DHCP principles can be used to resolve the IP address of the VANC. Once the IP address of the VANC is known an secure IPsec connection is established. The VANC will authenticate the UE with the help of the HSS via the D interface. The UE is then able to register on the MSC using standard DTAP protocols.

Both UE terminating and originating calls are carried out using the standard call protocols via the IPsec tunnel.

The voice packets are delivered over the LTE network via the RTP protocol to the VANC, the voice stream is then converted to ATM-IP or TDM 64Kbps depending on the type of connection required.

UE MSCVANC

1. GA-CSR Connection Establishment

4. Authentication

5. Ciphering conguration

GA-CSR Dedicated

11. Activate second EPS bearer for user data

2. GA-CSR Uplink Direct Transfer(CM Service Request)

6. GA-CSR UL Direct Transfer(Setup)

7. GA-CSR DL Direct Transfer(Call Proceeding)

14. GA-CSR DL Direct Transfer(Alerting)

15. GA-CSR DL Direct Transfer(Connect)

16. GA-CSR UL Direct Transfer(Connect Ack)

17. Voice traffic(via second EPS bearer)

8. Assignment Request

9. GA-CSR Activate Channel

10. GA-CSR Activate Channel Ack

13. Assignment Response

12. GA-CSR Activate Channel Complete

3. Complete L3 Info(CM Service Request)


Fig. 6 call Setup Procedures for voLGA

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Quality of Service in voLGA

VoLGA supports QoS for voice calls by reusing IMS components to determine the manner in which the voice IP packets will be handled by the network nodes. The VANC will contact the Policy Charging Rule Function (PCRF) during the voice call setup and requests that exchanged to and from certain IP addresses and UDP port number are given a higher priority over other packets. Based on the subscriber profile the request can be granted or denied. If the call is allowed to go ahead the PCRF will establish a secondary bearer through the LTE network with the corresponding QoS attributes.

GSMaccess

A Iu

RxD

S6a SGi

Gx

UMTSaccessGSM

MSC

HLR/HSS

MME

PCRF

S-GWP-GW

UMTSMSC

VANC

LTEaccess


Fig. 7 QoS in voLGA

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Handover in voLGA

Handover to and from exiting CS networks is important, it is also a feature that may not be present for the over the top VoIP providers. The VoLGA handover mechanism is based on the Single Radio Voice Call Continuity (SRVCC) specification. The single radio, feature here is the ability of the UE to manage the handover using only a single active radio, which is important to simplify UE design.

When the UE registers on the LTE network is will signal its SRVCC capability to the MME. The MME should then be aware that this procedure needs to be executed when the device leaves LTE coverage during an active bearer session.

The eNB can indicate to the UE that is should take 2G/3G measurements for the handover and can instruct the UE to select a pre-configured cell.

The MME will inform the VANC of the imminent handover via the Sv interface, including the target cell id, based on this information the MSC will prepare the handover in the standard fashion toward the new target BSC/BTS or RNC/NB. Again there are no modification that need to be carried out to the existing MSC nodes.

GSMaccess

A Iu

D

S6a SGiSv

UMTSaccessGSM

MSC

HLR/HSS

MME S-GWP-GW

UMTSMSC

VANC

LTEaccess


Fig. 8 Handover in voLGA

Documents

LTE_Tech_Ov_Sec06_131009_v01