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THE MOBILE IP HANDOFF BETWEEN HYBRID NETWORKS

YE Min-hua, LIU Yu, ZHANG Hui-min

School of Information Engineering Beijing University of Posts and Telecommunications, Beijing 100876, China,mhye@bupt.edu.cn

Abstract - In the future, the mobile data networks mustconsist of several tiers, which overlap each other. In this

paper, we introduce two main technologies of them: GPRSnetwork and IEEE 802.11 WLAN, which will be widelydeployed. We propose the schemes to support mobility inthese two networks. We also analyze the Mobile IP handoff

between hybrid networks and propose our handoff scheme,which reduces the handoff delay, packet loss and preventsfrom the ping-pong effect by utilizing the dwell timer andmulti-tunnel technology.

Keywords - Mobile IP; Handoff; Hybrid Network; GPRS;IEEE 802.11;

I. INTRODUCTION

Future mobile data networks will consist of severaloverlapping tiers: satellite, macro, micro and pico-cellular segments [1]. Each network has its own characteristics suchas geographic coverage and data rate supporting. For example, pico cells with data rate in excess of 25Mbit/s willcover a building; micro cells with 2Mbit/s data rate,typically cover the dense urban areas; macro cells withseveral hundred kbit/s data rate cover wide area; satellitenetwork with data rate 144kbit/s or more covers thecontinents.

The main important character of the tiered data networks isthat several networks coverage can be overlapped. Thesame user can be under the coverage of several kinds of networks at the same time. For example, a mobile terminalin a building can access the wireless LAN in the buildingand simultaneously it is under the coverage of GSM (GlobalSystem for Mobile Communication) network and thesatellites. To achieve flexibility in communication, it can beanticipated that future terminal will have different radio

interfaces for different wireless networks. We call this kindof terminal multi-mode terminal. When the multi-modeterminal is under the coverage of several networks, thecontrol software will select one interface to access Internetdepending on specific policies. If the mobile terminal wantsto roam between different networks when it is incommunication, the handoff mechanisms between thehybrid networks become a critical issue.

II. WIRELESS DATA COMMUNICATIONTECHNOLOGIES

A. GPRS technology

GPRS (General Packet Radio System) is the packet modeextension of GSM [2]. It uses the same air interface withGSM. The voice traffic and the data packet share the same

physical channel, but the new logical GPRS radio channelsare defined. Because the GPRS can utilize the current GSMinfrastructures, it can be evolved from GSM networkssmoothly. It is sure that the GPRS technology will be widelydeployed.

Fig. 1. GPRS network architectureGPRS network architecture is illustrated in Figure 1. As inGSM, the mobile host (MH) accesses the GPRS network through the base transceiver station (BTS), which coversspecific area. Each BTS and the MHs that it services form acell. Each GPRS base station controller (BSC) and one or more BTSs that it controls form one base station subsystem(BSS). Two service nodes are defined in GPRS: servingGPRS support node (SGSN) and gateway GPRS supportnode (GGSN). The GGSN acts as the interface to publicdata networks such as Internet and contains the routinginformation to be used to tunnel packets to the MH throughSGSN. Each SGSN is in charge of one or more BSS and it isresponsible for location management through HLR andMSC/VLR and responsible for delivery of packets. TheSGSNs and GGSN are interconnected through an IP basedintranet backbone. When a data packet reaches the GGSN,GGSN determines the MH the packet belongs to and theSGSN the MH is served. Then the packet is forwarded to theSGSN and is further delivered to MH by the SGSN.

B. Wireless LAN technology

The Wireless LAN (WLAN) [6] defined by IEEE 802.11committee is a wireless extension of the LAN technologies.

GGS

N

IntranetBSCS

GS

NBSSS

GS

N

MSC/VLR

HLR

BTSMH

BTS

Internet

0-7803-7589-0/02/$17.00 2002 IEEE PIMRC 2002

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Its network architecture and protocol stacks are illustrated inFigure 2.

MH AP1

Fixed network assuming as 802.3

Fixed Host

Router

Internet

AP3

AP2

BSS1

BSS3

BSS2

APPLICATIONTCPIP

802.11 MAC802.11 PHY

802.11 MAC 802.3 MAC802.11 PHY 802.3 PHY

MH

AP1

Fig. 2. WLAN network architecture and protocol stack architecture

In the WLAN, each service access point (AP) covers a cell.In IEEE 802.11, each single cell is defined as a basic service

set (BSS). Several BSSs can form an extended service set(ESS). IEEE 802.11 only defines the communication between MH and AP (the physical layer and data link layer).The organization of ESS is not defined yet.

As the representatives of wireless wide area and local areadata communication networks respectively, the GPRS andthe WLAN will be deployed widespread. MH can accessInternet through each of them. If the MH wants to roam

between the two networks freely while not disrupting thecurrent connection, these two networks should support hostmobility. In the next section we will introduce how toachieve this goal through Mobile IP [5].

III. ADOPTING MOBILE IP TO SUPPORT MOBILITY

A. Mobile IP

Traditional IP technology cant support mobility in IP layer.IETF defines Mobile IP. Mobile IP introduces two network entities: home agent (HA) and foreign agent (FA) to managemobility. When MH is in its home subnet (its initial subnet),it uses normal IP protocol to communicate. While it entersthe foreign subnet (other subnets except home subnet), itacquires an IP address, called care of address (COA). It then

sends registration message to HA to inform HA its currentlocation, COA.

The data packets sent by correspondent host (CH) to MHarrive at MHs home subnet by normal IP routing. HAcaptures these packets on behalf of MH, and encapsulatesthem with new IP header, whose destination address isCOA, source address is HA (it is tunneling). Then theencapsulated packets are forwarded to MHs COA. FA or MH restores the original IP packets. Data packets from MHto CH are routed normally. The flow of data transmission isillustrated as Figure 3.

Internet

MH

HA

FA

CH

tunnel

Fig. 3. Mobile IP principle

When MH enters new subnet, it needs handoff. It acquires anew COA and registers it to HA again, so that HA cancorrectly forward IP packets to it. During the time betweenMH leaving its old foreign subnet and HA receiving MHsnew registration request message, because HA doesnt knowMHs current COA, it still forwards those packets whosedestination address is MH to the old FA, and these packetswill be dropped by the old FA. It is possible that the

connection will be disrupted. If the distance between theMH and HA is a bit long, the disruption time will be large.In this case, decreasing handoff delay and packet loss is thecrucial issue for Mobile IP handoff.

Mobile IP is proposed to support mobility in computer network. But because of its characteristic of easy realization,Mobile IP can be used in many wireless networks to supportmobility.

B. Adopting Mobile IP to support mobility between GPRS and WLAN

In order to support mobility between the two networks, we

use a peer network structure. That is, GPRS and WLANaccess Internet as peer networks, and implement thefunction of Mobile IP respectively.

In GPRS network, we propose to implement the HAfunction at GGSN. When MH whose home network isGPRS moves to a foreign network (it is possibly not GPRS,such as WLAN), it registers to HA (GGSN) its current COAthrough the FA at the foreign network. GGSN checks all theIP packets that came from outside Internet. Once there aresome packets whose destination is MH, it acts as HA, that is,it re-encapsulates these IP packets and forwards them to MH

by tunnel.

We can also implement the FA function at GGSN, but we propose to implement it at SGSN. Then the FA function can be distributed to the SGSNs, but not centralized at GGSN,which can alleviate the burden of GGSN.

When MH moves to GPRS network, which is a foreignnetwork to it, GGSN will assign an IP address to it(assuming IPG). IPG can be a private IP address, but alsocan be a public one. At this time, SGSN acts as the FA of MH, so it broadcasts the Agent Advertisement messages toMH [5]. MH registers the IP address of SGSN (assuming asIPS) as its COA to the HA. SGSN relays this registration

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message, and records a mapping: in its database.When HA receives the registration message, it forwards thedata packets belonging to MH to SGSN. When SGSNreceives these packets, it looks up in the database and finesthe mapping of MH. It de-encapsulates these packets, andre-encapsulates them to new IP packets, whose destination

address is IPG and source address is IPS. SGSN forwardsthe new packets to MH using GPRS routing mechanisms. Atlast, MH de-encapsulates the IP packets and restores theoriginal IP packets. The packets from MH to CH are sent toSGSN firstly, and then are tunneled to GGSN. They areforwarded to Internet by the GGSN at last.

When MH moves in the service area of a SGSN, it onlyconsiders handoff between different BSSs (This is a

problem of link layer handoff.); when it moves betweendifferent SGSNs, it should take the Mobile IP handoff. Itshould register the new SGSN to its HA.

The mobility support in WLAN is comparatively simple.

WLAN itself defines physical layer and data link layer, so itonly need add the layer 3Mobile IP function to WLANnetwork: adding the HA and FA module using hardware or software in the fixed network it connects. The HA and FAfunction can be implemented in a router or a host (Illustratedin Figure 2).

So, these two kinds of networks can both provide mobilitysupport while MH moves between them arbitrarily.

IV. MOBILE IP HANDOFF BETWEEN WLAN ANDGPRS

A. Handoff policyBefore Mobile IP handoffs, MH or network should firstlydecide the handoff depending on certain policy. Handoff

policies can be classified into three categories [7]: network controlled handoff; network controlled and MH assistedhandoff; mobile-controlled handoff. In Mobile IP, handoff isentirely controlled by MH.

Handoff policy is to detect network situation and to decidewhether it matches the handoff criterion. The basic network situation is received signal strength-RSS or received signal

power-P. Under wireless circumstances, the RSS is arandom process. If handoff decision is only dependent on

RSS, there will be ping-pong effect, that is, MH is keepingon handoff between the two base stations to and forth. So itneeds some complicated methods to make handoff decision.

Traditional decision algorithms include [8]:

(1) RSS: The BS whose signal is being received with thelargest strength is selected (choose the new BS if RSS new>RSS old )

(2) RSS plus Threshold: A handoff is decided if the RSS of a new BS exceeds that of the current one and the signal

strength of the current BS is below a threshold T(choose the new BS if RSS new>RSS old and RSS old RSS old+H).

(4) RSS, Hysteresis and Threshold: A handoff is decided if the RSS of a new BS exceeds that of the current BS bya hysteresis margin H and the signal strength of thecurrent BS is below a threshold T (choose the new BS if RSS new>RSS old+H and RSS old

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We separate the entire handoff process into two phases: pre-handoff phase and handoff confirm phase.

(1) Handoff from WLAN to GPRS network

If the signal strength that MH receives from the WLAN BSis lower than the threshold T OFF , and this status persists for T

DWELL1, then a handoff will be performed.

If MH detects that RSS WLAN TOFF , the timer is resetedto zero; when RSS WLAN TOFF . So the MH leaves the pre-handoff status and communicates with WLAN normally.

If the value of the timer is greater than zero but lower thanTDWELL1 , it means RSS WLAN TOFF in the last interval. The signal strength is still not stable. TheMH sends the pre-handoff registration message to HA againand remains in the pre-handoff status. Once the value of timer is lager than T DWELL1 , it enters the handoff confirm

phase.

If the value of dwell timer is equal to or greater thanTDWELL1 , it means the MH has dwelled in the status for TDWELL1 interval. Then MH moves to the handoff confirm

phase.

Once the MH decides to handoff formally, it changes itsinterface to GPRS and sends HA the formal registrationmessage through FA GPRS to confirm the handoff. Whenreceiving the formal registration message, the HA stopsmulti-tunneling, and only tunnels the IP packets to FA GPRS .The FA GPRS sends the content of the buffer to MH.

Whatever the HA or FAGPRS

, if they have not received anyregistration message (pre-handoff or formal message) for 2T DWELL1 interval since the last pre-handoff registrationmessage, they will leave the pre-handoff status andcommunicate normally.

(2) Handoff from WLAN to GPRS network

The handoff procedure is similar as mentioned above. Butthe MH checks whether the RSS WLAN is greater than T ON . If so, the MH enters into pre-handoff phase. If the MH dwellsin this status for T DWELL2 interval, it moves into the handoff confirm phase.

(3) Several note:

To use the WLAN as long as possible, we set T DWELL1 >TDWELL2 ;

To be more stable, we set T OFF

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This work is supported by NSFC (National Natural ScienceFoundation of China) (grant number: 60072036).

REFERENCES

[1] Larry Taylor, et al., The Challenges of Seamless

Handover in Future Mobile Multimedia Networks,IEEE Personal Communications, April 1999, p32-p37.

[2] Sarikaya, B., Packet mode in wireless networks:overview of transition to third generation, IEEECommunications Magazine, Volume: 38 Issue: 9, Sept.2000, Page(s): 164 172.

[3] Pahlavan, K. Krishnamurthy, P., et al., Handoff inhybrid mobile data networks, IEEE PersonalCommunications, Volume: 7 Issue: 2, April 2000,Page(s): 34 47.

[4] Stephane, A.; Mihailovic, A.; Aghvami, A.H.,Mechanisms and hierarchical topology for fasthandover in wireless IP networks, IEEE

Communications Magazine, Volume: 38, Issue: 11, Nov.2000, Page(s): 112 115.

[5] Perkins C., IP Mobility Support, IETF RFC 2002, Oct.1996.

[6] Pahlavan, K., et al., Wideband local access: wirelessLAN and wireless ATM, IEEE CommunicationsMagazine, Volume: 35 Issue: 11, Nov. 1997 Page(s): 34

40.[7] Akyildiz, I.F., et al., Mobility management in next-

generation wireless systems, Proceedings of the IEEE,Volume: 87 Issue: 8, Aug. 1999, Page(s): 1347 1384.

[8] G. P. Pollini, Trends in Handover Design, IEEECommunications Magazine, Mar. 1996.