EPS-Mobility and handover.pdf

Study Program

Master Telecommunications and Internet Technologies

Course

Application Prototyping

LECTURE NOTE

Version: 1.1

Datum: 25. 06. 2010

EVOLVED PACKET

SYSTEM (EPS) Mobility and Handover

DI Franz Edler

EPS: Mobility and Handover

Author: DI Franz Edler page: 2 / 21

CONTENTS:

1. Overview................................................................................................................................ 3

1.1. Content of the course ....................................................................................................... 3

1.2. Structure of the course ..................................................................................................... 3

1.3. Preconditions and further readings and exercises.............................................................. 3

1.4. Questions and exercises ................................................................................................... 3

1.5. Target audience................................................................................................................ 3

2. Mobility Management Procedures .......................................................................................... 4

3. Inter-LTE mobility ................................................................................................................. 5

3.1. Inter-LTE mobility without S-GW and MME relocation .................................................. 5

3.2. Inter-LTE mobility with S-GW relocation........................................................................ 6

4. Inter-3GPP mobility ............................................................................................................... 8

4.1. Data-Mobility between EPS and Pre-Release 8 UMTS networks ..................................... 8

4.2. Data-Mobility between EPS and Release 8 UMTS networks............................................ 9

5. Inter-Technology Mobility.................................................................................................... 12

5.1. Mobility Techniques ...................................................................................................... 12

5.1.1. GPRS Tunneling Protocol (GTP) ............................................................................ 12

5.1.2. Mobile IP (MIPv4, MIPv6) ..................................................................................... 13

5.1.3. Proxy Mobile IPv6 (PMIPv6).................................................................................. 14

5.1.4. Dual Stack MIPv6 (DSMIPv6)................................................................................ 16

5.2. Non-3GPP Interworking Interfaces ................................................................................ 16

5.3. Optimized and Non-Optimized Handovers..................................................................... 17

6. Voice over LTE.................................................................................................................... 19

6.1. Why is it a problem? ...................................................................................................... 19

6.2. VCC and SR-VCC......................................................................................................... 19

6.3. CS-Fallback................................................................................................................... 19

6.4. VoLGA.......................................................................................................................... 19

7. Exercises and Questions ....................................................................................................... 20

8. References............................................................................................................................ 21

8.1. Books ............................................................................................................................ 21



1. OVERVIEW

1.1. CONTENT OF THE COURSE

… to be added …

1.2. STRUCTURE OF THE COURSE

… to be added …

1.3. PRECONDITIONS AND FURTHER READINGS AND EXERCISES

… to be added …

1.4. QUESTIONS AND EXERCISES

At the end of each part the student can find some questions which should help to get feedback on

the core points of the course. The student should be able to answer the questions and exercises at

the end of the course.

1.5. TARGET AUDIENCE

The target audience of this course are students on bachelor degree in the upper classes on

telecommunications systems and students for the master degree of “Telecommunications und

Internet-technology”.



2. MOBILITY MANAGEMENT PROCEDURES

Mobility management is one of the key aspects of all cellular networks. EPS has been defined as

an evolution of 3GPP based networks, 3GPP2 based networks and non 3GPP based networks.

This leads to different levels of mobility management procedures depending on the networks

involved. The following mobility situations can be distinguished (Figure 1):

Figure 1: Classification of mobility management procedures

The main purpose of the mobility management procedures is to hide the user mobility from the

services, so that applications need not be aware of the movement of the user.

The most homogeneous and optimized mobility management procedures are possible within the

LTE network (Inter-LTE access).

If other 3GPP access networks (e.g. 2G/3G) are involved specific interworking procedures

between these networks have been defined (Inter-3GPP access) and there are two options

depending on an upgrade of the existing SGSN to Release 8.

The most complex situations occur when 3GPP and non-3GPP based access networks are

involved during mobility. The ePDG (evolved Packet Data Gateway) is used for providing a

secure and authenticated connection. Interworking procedures are defined for WiMAX, WiFi

and CDMA 2000 networks.

To make things more flexible (or worse to learn) different IP mobility techniques are defined:

• GTP (Generic Tunneling Protocol)

• MIPv4 (Mobile IP for IP v4)

• PMIPv6 (Proxy MIP for IPv6)

• DSMIPv6 (Dual Stack MIPv6)

Within 3GPP networks GTP is used as the most efficient protocol including all information

elements required for tunnel management and QoS. The IETF based mobility protocol are a used

in case of interworking with non-3GPP access networks.

Besides the type of access networks involved in mobility another aspect has a major influence on

the procedure: the actual state of the UE. If the UE is involved in an active session a handover

procedure is applied which allows the sessions to be maintained and the same IP address to be

kept. If the UE is idle the mobility management care for keeping the IP addresses and enable

efficient paging in case of an incoming session.



3. INTER-LTE MOBILITY

When a UE moves within the LTE network different situations have to be distinguished

depending on the impact of S-GW and MME. In the most simple case the S-GW and MME do

not change during change of the LTE (EUTRAN) cell.

3.1. INTER-LTE MOBILITY WITHOUT S-GW AND MME RELOCATION

The intra-LTE mobility without S-GW and MME relocation is the most frequent handover

procedure in an EPS. In this scenario the new eNodeB (eNB2) and the old eNodeB (eNB1) are

connected to the same S-GW and the same MME. This scenario is shown in Figure 2.

S-GW

MME

Radio

beare

r

UE

eNB1 eNB2

P-GW

UE

Radio

beare

r

S1-U bea

rer

S1-U

bearer

S5 be

arer

PDN1

1

1

3

forwarding tunnel (X2)

4

5

6

2

8

8

7

Figure 2: Inter LTE handover without S-GW and MME relocation

Let’s start the procedure with the UE being connected to eNodeB 1 (eNB1) with an active

session via S-GW and P-GW towards a PDN (step 1).

To determine when to perform handover the UE measures regularly the signal strength of

neighbour cells. As the UE cannot send or receive data at the same time when it measures on

neighbour cells it receives instructions from the network on suitable neighbour cells that are



available and which the UE should measure on. The eNodeB creates measurement time gaps

where no data are sent or received to/from the UE. The measurement gaps are used by the UE to

tune the receiver to other cells and measure the signal strength. If the signal strength is

significantly stronger on another cell, the handover is initiated by the UE.

When the UE in Figure 2 now moves and comes close to the radio area of eNB2 the UE requests

from eNB1 handover to eNB2 (step 2). The eNB1 communicates with eNB2 via the X2 interface

and sends information about all the EPS bearers of the UE like QoS and security related

information. Based on this information, eNB2 does admission control and reserves the radio

resources for all the EPS bearers.

A forwarding tunnel is also created between the two eNodeBs (step 3). The forwarding path is

used to forward the packets buffered at eNB1 to eNB2 during handover. This reduces the loss of

packets during handover.

Next eNB1 asks the UE to handover to eNB2 and now the UE acquires eNB2. Acquiring a new

cell means that it gets time and frequency synchronized in the new cell. Then it establishes the

signaling bearer, default and dedicated radio bearers (step 4).

At the end of the handover execution the eNB2 requests the MME to switch the downlink data

path from eNB1 to eNB2 (step 5) and the MME in turn requests the S-GW to switch the data

path towards eNB2 (step 6). The tunnel endpoints of the S1-U bearer are then updated to go to

eNB2 (step 7). In case there are uplink and downlink packets to be sent from/to the UE before

the S-GW has switched the S1-U bearer path towards eNB2 the eNB1 will forward these packets

over the X2 tunnel.

After switching the S1-U bearer path towards eNB2 all downlink packets for the UE are now

flowing from the P-GW to the S-GW1, then from the S-GW1 to eNB2 and then from eNB2 to

the UE.

Finally the MME requests the eNB1 to cleanup all the resources it was using for the UE (step 8).

The eNB1 releases all the resources. It also deletes the forwarding path between the two

eNodeBs after forwarding all the buffered packets.

In case the X2 interface is not available between both eNodeBs the eNodeB can initiate the

handover involving signaling via the core network (S1-MME). This is called S1-based handover.

3.2. INTER-LTE MOBILITY WITH S-GW RELOCATION

Let’s now elaborate the scenario when the new eNodeB cannot communicate with the S-GW

currently used by the UE for reasons such as load balancing or network geography. Compared

with the previous scenario additional steps are required to take care of the S-GW relocation. The

scenario is depicted in Figure 3.

The handover scenario starts similar to the previous case. Based on the measurement report from

the UE, eNB1 initiates the handover process towards eNB2 using the X2-tunnel. The eNB2 sets

everything ready on the air interface for the UE. Then the UE acquires eNB2. On notification of

the successful creation of the radio bearers by eNB2 (step 4), the MME selects the new S-GW

(step 5). The MME has a mapping of the S-GW service area and tracking area. Based on this list,

the MME picks an S-GW service area. Within the S-GW service area it picks the S-GW based

on the current load status.

The P-GW of course does not change. The same P-GW continues to serve the UE and the IP

address of the UE is anchored at the P-GW. User mobility is hidden from the services. The MME

also directs S-GW2 to update tunnel information at the P-GW. This will create a new S5 tunnel



between the P-GW and S-GW2 (step 6). Once the new S5 tunnels are ready, the MME

commands the establishment of new S1 bearers between S-GW2 and eNB2 (step 7). This moves

the EPS bearers to S-GW2. Then the resources in S-GW1 and eNB1 for that UE are released

(step 8).

As a result the EPS bearers are still anchored at the P-GW and the UE continues to use the same

IP address that was initially assigned by the P-GW even when the S-GW has changed.

Radio

beare

r Radio

beare

r

S1-U

beare

rS5 be

arer

S1-U

beare

r

S5 bearer

Figure 3: Inter LTE handover with S-GW relocation

As already mentioned the S5 interface may be based on GTP or PMIPv6. The above scenario

applies to both because it is a general handover concept of network mobility. Differences can be

found at the message level details. If the S5 is GTP-based then mobility GTP-C signaling

messages are used and GTP-U tunnels are established between the new S-GW and P-GW. If S5

is PMIPv6 based, PMIPv6 signaling messages are used and GRE tunnels are established between

the new S-GW and the P-GW.



4. INTER-3GPP MOBILITY

LTE technology will in the beginning mainly be used to cover high traffic areas of a mobile

network, at least this is the plan of most of the mobile operators. The rest of the coverage area

will be served by existing 3G/2G network technology. A handover scenario between LTE

(EUTRAN) and UMTS (UTRAN) will therefore be most common.

The EPS supports inter-working with other 3GPP access systems like UMTS and GPRS. 3GPP

has defined an interworking architecture between EPS and release 8 UMTS networks. But there

is also an interworking option if the UMTS network has not been upgraded is still a legacy

UMTS/GPRS network (pre-relase 8).

Please be aware that the mobility procedures described in the next two chapters only cover the

mobility of data connections. The situation of voice connections is more complex as this also

requires a change of domain (from PS- to CS-domain). This will be covered in a later section.

4.1. DATA-MOBILITY BETWEEN EPS AND PRE-RELEASE 8 UMTS NETWORKS

In real network it will probably be difficult to upgrade all SGSNs of an UMTS network to

release 8 in time. Therefore an interworking architecture has been defined which allows an

existing (pre-relase 8) SGSN to interwork with an EPC (see Figure 4).

EPC

MME

UE

UMTS-PS

UTRAN

NodeB

RNC

SGSN

Gn/Gp

P-GW acts

like a GGSN

PDN

P-GW

Gn/Gp

MME acts

like an SGSN

Figure 4: Interworking between EPC and pre-release 8 UMTS

In case of interworking with pre-release 8 UMTS no changes to the SGSN are required, but

additional functions are required for the MME and the P-GW. The MME acts like a peer to the



SGSN and the P-GW acts like a GGSN to support mobility. The mobility call flow would be

very similar to an inter-UMTS mobility call flow. The SGSN uses the existing Gn/Gp1 interface

to interact with the MME and the P-GW.

The UE always communicates with the IP network (PDN) using the same IP address

independently of the access network where it is attached.

4.2. DATA-MOBILITY BETWEEN EPS AND RELEASE 8 UMTS NETWORKS

The interworking architecture between EPS and a release 8 UMTS network is shown in Figure 5.

Figure 5: Interworking between EPC and release 8 UMTS

The UE again always communicates with the IP network (PDN) using the same IP address

independently of the access network where it is attached.

In case of interworking with release 8 UMTS some enhancements have to be implemented in the

SGSN. The serving gateway (S-GW) of EPC is an anchor point for the data path. It relays traffic

between the SGSN of 2G/3G systems and the P-GW. Both PMIP and GTP options are defined.

The S3 interface is defined between the SGSN and the MME for signalling. The S4 interface is

defined between the SGSN and the S-GW for the user traffic. When a UE is using the UMTS

access system, the uplink packets go from the UE to the NodeB, from the NodeB through the

RNC, from the SGSN to the S-GW and from the S-GW to the P-GW. The downlink packets go

from the P-GW to the UE along the opposite route.

1 Gn and Gp protocol are based on GTP and differ only in case of roaming.



The mobility between different access technologies is also called inter-RAT (radio Access

technology) mobility. Let’s look into more details of a handover from LTE (EUTRAN) to

UMTS (UTRAN) based on the interworking scenario of Figure 5.

The flow of activities during handover is shown in Figure 6.

The handover is triggered by measurement of the signal strength by the UE. The eNodeB

broadcasts the information about all neighboring cells (also UTRAN cells) and the measure-

ments to be taken for other technology cells. The UE performs inter-RAT neighbor cell

measurements during DL/UL idle periods that are provided by the serving eNodeB.

The UE is then aware of its UMTS neighbouring cells. It measures the neighbouring cells and

sends the measurement report to eNodeB. Based on this the eNodeB decides to handover UE to

UMTS. It sends the handover initiation message to the MME. The MME determines the SGSN

serving that UMTS cell. The MME then requests the SGSN to prepare for the handover.

eNodeB MME P-GWS-GWUERNC

NodeB SGSN

radio-bearer S1-U bearer S5-bearer

eNodeB decides to

handover to UMTS

handover initiation

MME selects an

SGSN for the cell

handover preparation

relocation

request

forwd. tunnel GTP tunnels

admission control

One-to-one mapping of EPS bearers to PDP contexts in UMTS

IP addresses are anchored at the P-GW and the S-GW is the anchor point for the data path

radio resources

reserved

UTRAN-Iu access procedure

Radio bearer

update bearer

request

GTP tunnel

relocation

completehandover command

Figure 6: Handover from LTE to UMTS



Each EPS bearer of the UE has to be handled. This means that for each EPS bearer the SGSN

creates a so called PDP context2. All QoS parameters of the EPS bearers have to be mapped to

QoS parameters of an equivalent PDP context and also the security parameters of the bearers

have to be mapped accordingly. This mapping is done by the MME. The SGSN and the RNC

take care of reserving the resources and creating the context information for the UE. The RNC

creates the required radio resource blocks for all the EPS bearers.

The MME and the SGSN coordinate and create forwarding tunnels between the eNodeB and the

RNC. It serves the same purpose as the tunnel between the two eNodeBs during intra-EUTRAN

handovers.

After admission control for the UTRAN the required radio bearers and the bearers between the

SGSN and the RNC are all kept ready. Next the UE acquires the UMTS radio bearers.

The eNodeB asks the UE to handover to UMTS and the UE acquires the UTRAN cell and this

ends the creation of the radio resources for all the PDP contexts. The UE also indicates the

successful acquisition of the UMTS radio to the SGSN and the SGSN does an update toward the

S-GW. Now, the bearers going from the S-GW towards the eNodeB are moved to the SGSN.

GTP tunnels are created between the SGSN and the S-GW.

The IP addresses of the UE are anchored at the P-GW and so the downlink packets keep coming

to the P-GW. Next, the S-GW is the anchor point for the data paths for interworking of LTE with

UMTS. Therefore the S5 bearer between S-GW2 and the P-GW is intact.

As the last step the radio bearer in EUTRAN, the forwarding tunnel and the S1-U bearer are

cleaned up (see the X symbols in Figure 6).

2 A PDP context is the equivalent term for a bearer channel in UMTS (PDP = Packet Data Protocol).



5. INTER-TECHNOLOGY MOBILITY

Before digging deeper into the details of mobility and interworking with other non 3GPP access

networks two important principles should be mentioned:

• The IP-address is always provided and maintained (anchored) by the P-GW.

• The non-3GPP based access networks may use any of the IETF defined mobility techniques

(MIPv4, PMIPv6, DSMIPv6) as long as the IP address is anchored at the P-GW.

This requires the P-GW to be very flexible in supporting the different methods.

5.1. MOBILITY TECHNIQUES

The main issue of mobility within an IP network is to keep the IP address of a user despite of its

actual location (access network) where it is connected. This problem is solved by using tunneling

and encapsulation techniques.

In case of 3GPP networks the GTP protocol is used but if non-3GPP networks are involved

mobility protocols from IETF have to be used (MIPv4, PMIPv6, DSMIPv6). Figure 7 gives an

overview on the mobility protocols used in EPS.

Figure 7: Mobility protocols used in EPS

The protocols can be divided into two groups, depending on the involvement of the client.

In host-based mobility management, the UE has a Mobile IP (MIP) stack and is actively

involved in Mobile IP registration as well as tunnel establishments and re-establishments. The

disadvantage of this is the overhead over the air to establish/reestablish the MIP tunnels and the

UE needs the ability to support MIP.

In network-based mobility management, the UE does not need to support MIP. A Proxy Mobile

Agent sitting on the anchor node within the network performs MIP messaging on the UE's

behalf. The advantage of this is that no overhead over the air exists to maintain the MIP tunnels,

and it supports UEs that do not have MIP protocols.

5.1.1. GPRS TUNNELING PROTOCOL (GTP)

GPRS (General Packet Radio Service) was the first packet oriented service in mobile networks

and to enable mobility for packet oriented connections the GPRS Tunneling Protocol (GTP) was

defined. It was deployed on a large scale and a lot of experience has been gathered also with



UMTS. Therefore it was clear that GPRS will also be used for EPS but some enhancements were

required.

The GPRS Tunneling Protocol comes in two flavors:

• GTP-C: to handle signaling tasks, and

• GTP-U: for encapsulating “user plane” traffic

GTP is a very compact and specific protocol which has been defined to cover all issues of mobile

networks. For EPS only the control part of GTP has been updated and therefore EPS uses

GTPv2-C and GTPv1-U.

The principal structure of a GTPv2-C message is shown in Figure 8.

Figure 8: Principal structure of GTPv2-C

The GTP-C header field contains the message type and common parameters like tunnel endpoint

identifiers and sequence numbers and the GTP-C information part contains all parameters of a

specific message. Typical messages of GTPv2-C are Create/Delete/Update bearer request and

responses.

The structure of a GTP-U packet is similar to GTP-C and contains the user packet encapsulated

for a specific tunnel.

5.1.2. MOBILE IP (MIPV4, MIPV6)

Mobile IP v4

The main principle of MIPv4 is based on two key nodes (functions), see also Figure 9 :

- the Home Agent (HA) and

- the Foreign Agent (FA).

The HA can be thought of as a router in the home network of the mobile station (MS). The HA

assigns the IP address to the MS and keeps track of the current location of the MS.

The FA can be thought of as a router on the network where the mobile is currently attached (the

visited network). The FA is responsible for delivering packets from the HA to the MS.

When a MS first attaches to a Visited Network, it gets a local IP address from the FA in the

network. This is called a Care-of-Address (CoA). Then the MS registers at the HA with his CoA

and the HA assigns an external IP address to the MS. The MS is identified in the Internet with

this external IP address.

A MS therefore has two IP addresses, one is a CoA and the other is the external IP address. The

MS is identified in the Internet with this external IP address. A tunnel is setup between the HA

and the FA. All the packets routed to the mobile would first go to the HA. The HA forwards

them to the FA based on the association of the external IP address and the CoA address. The FA

would then route the packets to the MS.

When the MS moves to another visited network, it retains its external IP address assigned by his

HA. The MS gets a new CoA address from a new FA. The MS registers this new CoA address

with the HA. A new tunnel gets established between the HA and the FA in the visited network.

The MS is always identified in the Internet by the same IP address even when it moves between

networks. The packets destined to the MS are correctly routed to the MS because of a change in



the association of external IP address to CoA address at the HA and the tunnel between the HA

and the new FA.

Visited

Network 1

Home

Network

MS

HA

Tunnel

Visited

Network 2MS

External IP address

of mobile station

CoA 1 &

external IP

Tunnel

CoA 2 &

external IP

FA

FA

Figure 9: Principle of Mobile IP (IPv4)

Mobile IP v6

In case of IPv6 the mobility technique makes use of inherent features if IPv6.

Conceptually MIPv6 is the same as MIPv4, but a FA is not required in MIPv6. Also, when a host

moves from one network to another, the first few packets are tunneled from the HA and the rest

are directly routed to the host without the intervention of the HA. This reduces the delay in

routing the packets and avoids the triangular routing problem.

5.1.3. PROXY MOBILE IPV6 (PMIPV6)

If the UE doesn't support Mobile IP the session continuation and IP address retention can be

achieved by using the Proxy MIP (PMIP) method. The principle of Proxy Mobile IPv6 is shown

in Figure 10.

Figure 10: Principle of Proxy Mobile IPv6



A node in the access network acts as a proxy UE regarding mobility. This node is called the

Mobile Access Gateway (MAG). It takes the responsibility of getting an IP address for a UE

from the anchor during the UE registration. When the UE moves out of the scope of one MAG,

the new MAG updates the anchor node with the UE's new location. The node that does the IP

address anchoring and acts like an HA is called the Local Mobility Anchor (LMA).

In EPS the Serving Gateway is the MAG and the PDN Gateway is the LMA.

The principle of MIPv6 registration and address assignment is shown in Figure 11.

Figure 11: Registration and address assignment in PMIPv6

When the UE registers with an access network, the MAG will know about the registration. The

MAG then does a PMIPv6 registration with the LMA. The LMA assigns and anchors the UE's IP

address. A GRE tunnel is also established between the MAG and the LMA to carry the messages

to and from the UE.

In case of mobility of the UE the situation is shown in Figure 12.

MAG 2

LMA

UE

MAG 1UE

GRE tunnel

GRE tunnel

Access bearer

Access bearer

UE still uses the same IP address

Figure 12: UE mobility with PMIPv6

Let us assume that initially the UE registers with MAG 1 and MAG 1 does the PMIP registration

for the UE with the LMA. The LMA anchors the UE and provides an IP address to the UE.

Please note that the UE is not aware of the PMIP messaging between MAG 1 and the LMA. At

the end of the access registration process of the UE to the MAG 1, an access bearer and a GRE

tunnel is setup for the UE.



Let us assume that the UE moves out of the scope of MAG 1, and goes to a new MAG. Now, the

new MAG, becomes the UE's proxy. It does a PMIP registration with the LMA and requests the

LMA to retain the same address for the UE. It also directs the LMA to tunnel IP packets for the

UE to the new MAG. So the UE still retains its IP address and can continue with the session

when it is mobile. The GRE tunnel between the LMA and the old MAG is deleted.

5.1.4. DUAL STACK MIPV6 (DSMIPV6)

Dual stack MIP is a combination of MIPv4 and MIPv6. A common set of messages are used to

achieve MIP support for a host that supports IPv4 and IPv6 and needs to achieve IPv4 and IPv6

address retention. The summary of features of DSMIPv6 is shown below.

5.2. NON-3GPP INTERWORKING INTERFACES

EPS defines two ways of non-3GPP access system interconnection. Trusted non-3GPP access

systems directly interconnect with EPS, P-GW (PDN Gateway). Untrusted non-3GPP access

systems interact with the ePDG (evolved Packet Data Gateway). The ePDG in turn interacts with

the P-GW.

3GPP and trusted non-3GPP access systems may be operated by the same or different operators.

Here, an IPSec tunnel between the UE and the 3GPP Evolved Packet Core (EPC) is not required.

However to interwork with an untrusted non-3GPP access system an IPSec tunnel is required

between the UE and the 3GPP network to provide adequate security mechanisms acceptable to

the 3GPP network operator.

There are three different options for EPS to interwork with non-3GPP IP access: s2a, s2b and s2c

reference points. The reference points are based on the support for different Mobile IP options.

The S2a and S2b reference points are shown in Figure 13. The s2a reference point between the

trusted non-3GPP access system and P-GW supports PMIPv6 and MIPv4 FA mode. The s2b

reference point between the ePDG and P-GW for non-3GPP access supports the PMIPv6 option.

Figure 13 also shows a AAA-server which is used if non-3GPP access networks are involved.

The interfaces between the AAA server and various other entities (dashed lines) support AAA

functionality and charging-related information for the interworking of EPS with non-3GPP IP

access.

Figure 14 shows the application of DSMIPv6, where also the UE is involved in handling

mobility (MIP stack in UE). The DSMIPv6 option is supported over the s2c reference point and

can be used for interworking with both the trusted and untrusted access technologies.



Figure 13: S2a and S2b reference points

Wa

Figure 14: S2c reference point

5.3. OPTIMIZED AND NON-OPTIMIZED HANDOVERS

Besides the three Mobile IP techniques that are used in EPS (MIPv4, PMIPv6 and DSMIPv6)

and the classification into trusted and untrusted system there is also the notion of handover types.

Two handover types are defined: Optimized and Non-Optimized Handovers. The handover types

characterize the delays that can be expected during handover.

With optimized handovers, registration is done with the target access system by tunneling the

target access messages over the source access system. An example could be a dual mode mobile

capable of using both LTE and WLAN access systems. When it is currently using LTE for its

services it can pre-register with the WLAN system. This pre-registration is not performed by



acquiring the WLAN radio instead it is done by tunneling the registration messages over the LTE

and EPC nodes to the WLAN nodes. This pre-registration reduces the time required for inter-

technology handovers. Optimized handovers require special interfaces defined to support this

pre-registration. Optimized handovers can be used when an operator owns more than one type of

access technology or when an operator has a suitable agreement with the other operator.

In non-optimized handovers, registration to the target access system is done during the handover

execution phase. It is done using the radio resources of the target system.



6. VOICE OVER LTE

6.1. WHY IS IT A PROBLEM?

• EPS (LTE and EPC) only supports packet oriented traffic, no circuit switched domain.

• Voice is handled in EPS via IMS.

• LTE will not have full coverage for a long period of time.

• Handover between eUTRAN and UTRAN will happen frequently.

• Seamless handover of voice calls has been specified by 3GPP using IMS and SR-VCC3

• IMS should be available when migrating to LTE, but that is often not the case.

• 3GPP has defined a workaround solution for that: CS-Fallback4 which has some

drawbacks

• Some operators and vendors developed a solution called VoLGA5

• The majority of operators (GSM Association) promote a functionally limited introduction

of IMS: VoLTE6

6.2. VCC AND SR-VCC

6.3. CS-FALLBACK

6.4. VOLGA

3 SR-VCC: Single-Radio Voice Call Continuity 4 CF-Fallback: fallback to circuit switched network (UMTS) before starting or accepting a call

5 VoLGA: Voice over LTE via Generic Access (http://www.volga-forum.com/)

6 VoLTE: Voice over LTE (http://gsmworld.com/our-work/mobile_broadband/volte.htm)



7. EXERCISES AND QUESTIONS

After studying this part of the lecture you should be able to answer the following questions:

… to be added …



8. REFERENCES

8.1. BOOKS

Magnus Olsson: System Architecture Evolution (SAE):

Evolved Packet Core for LTE, Fixed and other Wireless Accesses

Gebundene Ausgabe: 464 Seiten

Verlag: Academic Press (24. August 2009)

Sprache: Englisch

ISBN-10: 0123748267

ISBN-13: 978-0123748263

Gottfried Punz: Evolution of 3G Networks:

The Concept, Architecture and Realization of Mobile Networks Beyond

UMTS

Gebundene Ausgabe: 306 Seiten

Verlag: Springer, Wien; Auflage: 1., st Edition. (14. Februar 2010)

Sprache: Englisch

ISBN-10: 3211094393

ISBN13: 978-3211094396

EPS-Mobility and handover.pdf

Documents

Transcript of EPS-Mobility and handover.pdf