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© Tech Mahindra Limited 2010

802.16e & 3GPP Systems

Network Handover

Interworking

Venkat Annadata

Tech Mahindra (R&D) Services Ltd.

Abstract:

Next Generation Mobile Networks are paving its way

towards the ubiquitous wireless access abilities which

provide the automatic handovers for any moving

devices in the heterogeneous networks combining

different access technologies.

In this technical paper, intend to present a possible

Mobile WIMAX ‐3GPP/2 Network Interworking

architecture based on the 3GPP/2 standards and

propose the seamless inter ‐system handover schemewhich enables the service continuity with low

handover latency and packet loss.

This technical approach of enabling handover feature

is purely based on the IEEE 802.16e Mobile WiMAX &

3GPP/2 Standards.

APRIL ‐2010



1 © Tech Mahindra Limited 2010

Introduction

Today Mobile WiMAX (IEEE 802.16e) is one of

the wireless broadband standards capable of

providing the Quadruple play Technologies ‐

Data, Voice, Video & Mobility using a single

network. The introduction of the 802.16e

WiMAX flavor is creating new markets for

mobile broadband services. Abilities of the

802.16e standard to provide seamless mobility

to end users in their homes, offices, and during

transit, are spurring the demand for innovative

mobile services. Users can now take advantage

of complex IP‐based data‐intensive applications

while traveling at vehicular speeds. This is made

possible by IP‐specific optimizations of 802.16

and its built‐in support for high‐speed handoffs.

Mobile customers shall now be able to

download full‐length DVD‐quality movies

quickly or host multi‐party video‐conferencing

sessions from their WiMAX–enabled handheld

devices. Regardless of the deployment of this

IP‐based version of WiMAX, it is clear that

802.16e is providing a strong mobility platform

to help accelerate convergence.

In the current paper Handover Interworking

scenarios for Mobile WiMAX and 3GPP/2 are

presented with Network Interworking

Architecture along with the call flows from

WiMAX to 3GPP and vice –versa.

Roadmap for evolution of WiMAX‐IMS (3GPP2)

interworking architecture is also presented as

per the NWG WiMAX stage 2 specifications.

Handover Solution Architecture

Architecture Description

At the onset, let us understand the differences

between 3GPP‐WLAN interworking and 3GPP‐

WIMAX interworking. The WLAN in hot‐spot

areas forms the micro‐cells within the 3GPP

macro‐cells. The mobility between 3GPP and

WLAN can be referred to fully overlapping

handover. Accordingly, the required time for

switching from 3GPP to WLAN connection can

be tolerantly long. Moreover, when the mobile

is connected to WLAN, it can maintain

simultaneously the Packet Data Protocol (PDP)

context of 3GPP so that it can reconnect

immediately to 3GPP without need of PDP

context re‐activation.

On the contrary, from the below figure 1, the

mobility between 3GPP and WIMAX is referred

to partially overlapping handover since the

WIMAX coverage is in order of 3GPP coverage

area. Consequently, the handover should be

done quickly to maintain the connection

particularly when the speed of the mobile

terminal is high. In order to enable the mobility

between two access networks 3GPP and

WIMAX, we propose a solution under some

following conditions: minimum change of the

existing network infrastructure of these two

technologies and feasible solution for short

term.

By

using

IP

as

the

common

interconnection protocol, the mobile can

connect to multiple networks seamlessly

ignoring the heterogeneity of access

technologies. This is achieved by using Mobile IP

mechanism that hides the heterogeneities of

lower‐layer technologies. The proposed

architecture for 3GPP‐WIMAX interworking,

depicted above is based on interworking

architecture models of 3GPP standards.

Figure: 1




The mobile subscriber (MS) is a mobile node

that can communicate with both 3GPP network

and WIMAX network. However, note that it can

connect to only one access network at a time.

Therefore, the handover between 3GPP‐WIMAX

must be the hard handover. The WIMAX Access Network (AN) provides the WIMAX access

services for the MS. The mobility inside WIMAX

network is managed by the WIMAX Home Agent

(HA) located between the ASN gateway and the

WAG. The WIMAX HA is not necessarily included

in 3GPP core network to keep its independence

from 3GPP system. The Foreign Agents (FA)

located in ASN Gateway is considered as the

local FAs in the interworking architecture. The

WIMAX AN is connected to the 3GPP network

via WAG and to the 3GPP AAA server for the

WIMAX

authentication

process.

The

WAG

is

a

gateway through which the data from/to

WIMAX AN is routed to provide MS with 3GPP

services. The functions of WAG include

enforcing routing of packets through PDG,

performing accounting Information and filtering

out packets. The main functions of PDG are to

route the packets received from/sent to the

PDN to/from the MS and to perform the FA

functions.

The mobility within the 3GPP network is

managed by its own mobility mechanism and

the FA functions implemented in the GGSN. In

order to enable the vertical handover between

these two technologies, the HA is placed in the

PDN and manages FAs of both WIMAX and

3GPP networks.

IP Address Management

In WIMAX network, each time the mobile

changes its ASN gateway; it will obtain a new

local IP address through the DHCP server. The

ASN GW can learn this new local IP address and

also ask to the DHCP server the WIMAX HA’s

address since it plays the role of the DHCP relay

agent in the DHCP discovery process. The ASN

GW then informs the serving BS the MS’s new

local IP address and sends the Mobile IP (MIP)

registration to the WIMAX HA. A generic IP‐in‐IP

tunnel such as Generic Routing Encapsulation

(GRE) may be used to transport the IP packets

between the WIMAX HA and the FA. Each time

the mobile switches the connection to the 3GPP

network, it will initiate the PDP context

activation procedure. No IP address is allocated

to the MS at the PDP context activation. The

remote address provided by HA or an external

entity in PDN will be kept unchanged and will be

informed to the GGSN via PDP context activation. The remote IP address is a global

home address that is used to address to the

external network and the correspondent node.

It may be a static address or a dynamic address

acquired from the HA or another external entity

when the mobile first time connects to the

network, discovers and registers with the HA.

The PDG/GGSN is then responsible for relaying

MS’s remote allocated IP address to the MS.

Handover Procedure

To reduce the interruption time during the

handover, we have specified a forward

handover procedure. That is to say, before

leaving the serving network, the mobile

prepares a new attachment in the target

network. In order to reduce the packet loss

during handover, the old FA notifies the HA the

MS’s movement so that the HA can buffer the

packets and forward them to the MS as soon as

the HA receives the MIP update from the MS.

Handover Call Flows

Scenario 1: Handover from WIMAX Access

Network to 3GPP Network

From the Figure:2 below, before the handover is

initiated, the mobile is connected to the 3GPP

services through WIMAX access network. When

the MS enters to an overlapped zone, the MS

can measure signal quality from the 3GPP

neighboring cells. If the triggering conditions for

vertical handovers are satisfied, the handover

decision is then taken. The target 3GPP‐UTRAN

will be notified the imminent handover from the

WIMAX network via the HO request message

routed through the core network. The MS will

perform the GPRS attach procedure with the

3GPP‐UTRAN. Mobility management contexts

are established at the MS and SGSN. The MIP

registration between the HA and new GGSN/FA




can be updated after the PDP context is

activated between GGSN and MS. The details of

handover procedure from a WIMAX cell to a

3GPP cell is depicted in below call flow:

1. The WIMAX BS sends periodically the

topology advertisement message to inform the

MS of neighboring WIMAX BSs and NodeBs.

Alternatively, the MS can scan different

channels to discover the neighboring topology.

However, it is not a good solution and it will be

our future work. Throughout our study here, we

assume that there exists a total cooperation

between the 3GPP and WIMAX networks

operators. Thus, the 3GPP NodeB can transmit

to the MS the WIMAX neighboring cell

information and vice versa.

2. Based on the topology advertisement, the MS

performs synchronization and measurement

procedure. The event‐triggered inter‐system

measurement may be based on the degradation

of

current

signal

quality

or

on

the

necessity

of

switching between access technologies to

support higher QoS requirements or low cost.

Since the WIMAX operates in TDD mode, during

the downlink frame duration, only some OFDM

symbols are addressed to the mobile.

Accordingly, the remaining time can be used to

measure neighboring cell signals. Note that, to

facilitate the measurement on 3GPP cell, the

information such as scrambling code, carrier

frequency, should be included in advertisement

messages.

3. After the measurement step, the mobile shall

send the measurement report to the WIMAX

BS. The report must contain the signal quality

level of each candidate 3GPP cell.

4. The WIMAX BS initiates the handover

procedure by notifying the potential target

3GPP via handover (HO) request message. The

PDG will perform a DNS request to know the

addresses of GGSNs which serve the current

MS’s Access Point Name (APN). The PDG then

selects one GGSN in the result list of GGSNs

from the DNS request phase and sends the HO

request to this selected one. If the PDG does

not receive any response from the GGSN for a

certain time, it will select another GGSN in the

found list and resend the HO request message.

5. The GGSN then sends the HO request

message to the SGSNs who serve the indicated

nodeBs. In order to be able to retrieve the

address of SGSN that serves a specific nodeB,

we assume that the DNS server or the Home

Location Register (HLR) stores this routing

information.

6. The target RAN establishes bearer resources,

including radio resources, for the MS. This step

aims to check if the candidate 3GPP NodeBs can

accept the MS handover with the required QoS.

Figure: 2




7. The NodeB which supports the MS handover

will send a HO support message to the ASN GW

which contains the handover decision function.

8. Upon receiving HO support messages, the

ASN GW selects the best target 3GPP cell and

then returns the HO command to the MS. This

message must include the recommended target NodeB and all the required information for

setting up a new connection. The above

exchange may require a large amount of

information and add more latency to handover,

it is therefore Preferable to use a pre‐

configuration mechanism. It means that only a

reference number corresponding to a

predefined set of 3GPP‐3GPP‐3GPP‐UTRAN

parameters is inserted in the handover

command. The MS should download the

predefined radio configurations before. During

this

temporary

connection,

the

MS

can

reconfigure the connection into a suitable one.

9. Right after that the ASN GW sends the

handover confirmation which includes the

target NodeB identifier to the PDG/FA. The

allocated resources in the WIMAX network will

be then released.

10. Upon reception the handover confirmation

message, PDG/FA will send a MIP update

message to HA to notify the MS’s movement.

The HA then stops sending the packets to the

MS via this PDG/FA and buffers the inbound

packets until it receives the MIP update from

the target 3GPP network.

11. The MS performs the GPRS attachment

procedure to 3GPP‐3GPP‐3GPP‐UTRAN

network. The GPRS attachment procedure

consists of accessing to SGSN, authenticating

with the AAA server and updating the location.

12. After performing successfully the GPRS

attachment, the MS starts the PDP context

activation through which the MS informs its

remote IP address (its global home address) to

the GGSN.

13. After the connection is established between

a new GGSN/FA and MS, the GGSN/FA will

perform the MIP registration with the HA

including the MS’s remote IP address and its

care of address (address of GGSN/FA). The data

will then be transmitted to MS via the new

NodeB

and

the

handover

procedure

is

completed.

Scenario 2: Handover from 3GPP to WIMAX

Access Network

Before the handover is initiated, the MS is in the

3GPP network. When the MS moves to an

overlapped zone, it can measure the signal

quality from the neighboring WIMAX BSs. When

the network decides to handover to WIMAX,

the MS will set up the connection with WIMAX

AN, perform the authentication and MIP

registration update, etc. The handover scheme

from a 3GPP cell to a WIMAX cell is depicted in

below figure 3:

Figure: 3




1. The 3GPP‐3GPP‐UTRAN is responsible for

detecting the handover need and initiating the

inter‐system measurement process by sending

the measurement control message to the MS.

This message contains the neighboring WIMAX

cell information, the compressed mode pattern,

etc.

2. While the MS has an on‐going

communication in FDD mode, in order to

perform the measurement on the neighboring

WIMAX cells, it must enter in the compressed

mode. Note that the measurement on WIMAX

cell is performed on the preamble of each

WIMAX frame.

3. After the measurement period, the MS sends

the measurement report to the network. The

report

must

contain

the

parameters

indicating

the signal quality level of the neighboring

WIMAX BSs.

4. The RNC initiates the handover procedure by

notifying the potential target WIMAX BSs where

the mobile may handover. The HO request

message including the MS’s APN, the candidate

BS identifiers, the required QoS of MS’s current

applications, etc. will be sent to the GGSN. The

GGSN performs the DNS request to learn the

addresses of the PDGs which serve the MS’s

current APN. The GGSN selects one PDG in the

result list and sends it the HO request message.

If the GGSN does not receive any response from

the PDG after a certain time, it will send the HO

request to another PDG in the list. The HO

request message will then be transmitted to the

potential WIMAX BSs based on the routing

information at the PDG. This step aims to check

if the target WIMAX BS can accept the MS

handover with the required QoS.

5. The WIMAX BSs which support the MS

handover will return a HO support to the RNC.

6. The RNC will select the best target WIMAX BS

among the supporting BSs and then sends the

HO command to the MS. This message includes

all the required information for setting up the

connection to the selected target WIMAX BS.

7. Right after that the RNC sends the HO

confirmation. The mobile is then disconnected

from the 3GPP network and starts the

connection setup to the target WIMAX BS.

8. Upon receiving the handover confirmation,

GGSN/FA sends a MIP update message to the

HA to notify the MS’s movement. The HA then

stops sending the packets to the MS via this GGSN/FA and buffers the inbound packets until

it receives the MIP update from the target

WIMAX network.

9. Based on the information included in the HO

request message, the WIMAX BS can provide a

non‐contention based initial‐ranging

opportunity to the MS by placing a Fast Ranging

Information Element in the UL MAP. This

information will facilitate the RAN connection

setup of the MS. If not, the MS must perform

the

normal

ranging

procedure

which

takes

more

time.

10. The MS initiates the connection setup by

exchanging Ranging Request (RNGREQ)/

Ranging Response (RNG‐RSP) with the target

WIMAX BS.

11. In the WIMAX AN, the MS will perform

DHCP request to obtain new local IP address. In

this scenario, we describe an address allocation

procedure based on IPv4 mechanism. If IPv6 is

used, the local address can be allocated by

Stateless Address Autoconfiguration mechanism

without the presence of the DHCP server.

Through this procedure, the ASN GW will also

learn the WIMAX HA address which serves for

MIP registration in the following step.

12. The MS will perform the MIP registration to

associate the MS’s local address with its care of

address.

13. The MS performs DNS resolution for PDG

address. MS uses APN to indicate the network

service it wants to access. The DNS request will

be relayed to ASN GW which in turn relays the

request to the DNS server. The MS will select

one suitable PDG among the list of PDGs given

in DNS response. Note that the selected PDG

here may be different from the PDG selected

by GGSN during HO request/support step.




14. The MS then establishes an end‐to‐end

tunnel with the selected PDG using IKEv2

protocol. Through this process, the MS will

inform the PDG about its local and remote IP

address. Each time the mobile changes its ANS

network, it obtains a new local IP address and

therefore a new tunnel should be correctly

configured. Regarding inter‐WIMAX mobility,

the time required for setting up a new IPSec

tunnel when changing of ASN may be too long

that the seamless mobility cannot be achieved.

To speed up this kind of IPSec tunnel relocation,

we can use the MOBIKE mechanism proposed

by the IETF MOBIKE WG.

15. The PDG performs the MIP registration with

the HA as soon as it will be notified the MS’s

remote IP address. The data packets will be

transmitted

to

MS

via

the

WIMAX

AN.

The

handover procedure is completed.

Proposed WiMAX –IMS interworking

Network Reference Model

NOTE: Figure:4 below architecture diagram is

evolving. It may contain old representations

that will be resolved at a later stage of

investigation. Also, this assumes that the CSN

functionality is provided by the 3G Network.

This includes IP address space allocation.

Solution benefits

This network architecture approach offers

the following benefits:

• Offers interworking handover

functionality between Mobile WiMAX &

3GPP Networks.

• Seamlessly integrates wireless

technology specific functionality with IP

networking equipment

• Allows for the use of a common IP

network for multiple wireless access

technologies

• Enables cost‐effective implementation

for deployments ranging from small to

large scale

• Enables the use of mobile devices and

optimizes handovers

• Scalable to 3GPP2 IMS Networks for

feature rich multimedia applications &

quadruple play technologies.

• Enables new types of transport

networks such as metro Ethernet, and

wireless point‐to‐point for backhaul

• Enables distribution of “application

level” functionality such as content

delivery networks

Figure: 4




Conclusion & Future Work

In this paper, we have introduced a practical

3GPP‐WIMAX interworking architecture based

on 3GPP standards and proposed a handover

procedure which promises a low packet loss and

low interruption time during the switching of

the communication. The mobility between two

access networks is achieved by the MIP

mechanism at the network layer. The packet

loss during handover is reduced since the old FA

notifies the HA the MS’s movement and

consequently

the HA buffers the data packets destined to the

MS. The proposed interworking architecture

does not require lot of changes on existing

network infrastructures which is a big

advantage. The proposed handover scheme

needs the exchange of messages between the

PGD and the GGSN which serve the same APN

with the help of the DNS server. In case the MS

connects to multiple APNs, the handover

preparation phase may be more complex, which

will be our future work with some performance

evaluation of our proposed mechanisms in a

large scale network. Moreover, we aim to

consider the tightly‐coupled interworking

architecture approach for 3GPP‐WIMAX and

3GPP‐WLAN interworking which allows a

seamless handover with less handover latency

and less packet loss. The future interworking

architecture should be based on the

architecture evolution proposed by 3GPP

standards. We therefore plan to consider the

roaming architecture as well as the mobility

scheme in the context of multiple operator

environments.

References

1. An Architecture for UMTS‐WIMAX Interworking ‐ Quoc‐Thinh

Nguyen‐Vuong, Lionel Fiat and Nazim Agoulmine

2. IEEE P802.16e/D11, ”Part 16: Air Interface for fixed and mobile

broadband wirelessaccess system”, Sept. 2005.

3. Salkintzis,A.K.; Fors, C.; Pazhyannur, R.; ”WLAN‐GPRS

integration for nextgeneration mobile data networks”, IEEE

Wireless Communication, Vol.9, pp. 112‐124, Oct. 2002.

4. Shiao‐Li Tsao; Chia‐Ching Lin; ”Design and evaluation of 3GPP‐

WLAN interworking strategies”, Proceedings. VTC 2002‐Fall, IEEE

56th Volume 2, pp.777 ‐781, Sept. 2002

5. Hyun‐Ho Choi; Song, O.; Dong‐Ho Cho; ”A seamless handoff

scheme for 3GPPWLAN interworking”, Globecom, 2004, pp.1559 ‐

1564, Vol.3. Dec. 2004

6. V. Varma, S. Ramesh, K.D. Wong, M. Barton, G. Hayward and J.

Friedhoffer. ”Mobility Management in Integrated 3GPP/WLAN

Networks”, IEEE ICC, Anchorage, Alaska, USA, May 2003.

7. Muhammad Jaseemuddin. ”An Architecture for Integrating

3GPP and 802.11 WLAN Networks”, 8th IEEE ISCC,p.716, 2003.

8. N. Vulic, S.H. Groot, I. Niemegeers, ”A comparison of

Interworking Architectures for WLAN Integration at 3GPP Radio

Access Level”,ConWIN05, July 2005.

9. G. Cunningham, P. Perry and L. Murphy, ”Soft, Vertical

Handover of Streamed Multimedia in a 4G network”,5th

International Conference on 3G mobile communications

Technologies, London, UK, Oct. 2004.

Glossary

AAA Authentication, Authorization & Accounting

E2E End to End

IMS IP Multimedia Subsystem

IP Internet Protocol

MGC Media Gateway Controller

MGW Media Gateway

P‐CSCF Proxy‐Call Session Control Function

PDF Policy Decision Function

POC Proof Of Concept

QoS Quality of Service

RADIUS Remote Access for Dial‐In User Services

SBC Session Border Controller

TM Tech Mahindra

VoIP Voice over IP

VSA Vendor Specific Attributes

WISP Wireless Internet Service Provide

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