MAG, A framework for seamless roaming across cellular and wireless local area networks.pdf

7/27/2019 MAG, A framework for seamless roaming across cellular and wireless local area networks.pdf

http://slidepdf.com/reader/full/mag-a-framework-for-seamless-roaming-across-cellular-and-wireless-local-area 1/8IEEE Wireless Communications • June 200550 1536-1284/05/$20.00 © 2005 IEEE

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13. Auth_user_ack N

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A u t h_ r e q u e s t


A s s o c_ r e p l y


2 1 . H a n d e d_ o v e r

1 5 . H O_ r e p l y

1 4 . H O_ r e q u e s t

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AAA/PS

AP

T OWARD SEAML ESS IN T ERN ET W ORK I N G OF

W I R EL ESS LAN AND CEL L UL AR NET W ORK S

INTRODUCTIONIn the near future, with the deployment of IMT-2000, fourth-generation (4G) networks, and theproliferation of wireless LANs (WLANs), it is tobe expected that wireless or mobile users willplace greater demands on support for differenttypes of services, quality of service (QoS) guar-antees, and seamless roaming across differenttypes of wireless networks. The advantages of combined services, rich in features and attractivecost-wise, that can be achieved by integratingcellular and WLAN is spurring numerous activi-ties in this direction. Besides supporting call ini-

tiation from any network, seamless roamingshould also provide for roaming across different

wireless networks during an active voice call (ordata connection). This requires new conceptsand approaches in location management schemesand strategies [1, 2] with QoS mapping and sup-port.

The existing location management schemes incellular networks have certain limitations. First,the two-database approach based on home loca-tion register (HLR) and visitor location register(VLR) will not be efficient in supporting seam-less roaming. The centralized location database(i.e., HLR) is a potential bottleneck of the network with a growing mobile user population.Databases of more than one network have to beinvolved to provide seamless roaming across net-

works. Second, to hand over calls smoothly asthe user roams across wireless networks requiressome interface units between the two networks toprovide message translation and QoS mapping,and facilitate predictive handoff. Lastly, usermobility spans are highly variable requiring hier-archical mobility control. Taking these factorsinto consideration, a suitable global roamingframework has to be hierarchical and distributedin architecture, and span wireless networks par-ticipating in global roaming. The distributednature of the framework has further advantagesof being a robust solution while imposingreduced loads on the participating entities.

Numerous efforts toward integrating dis-parate wireless networks can be found in the lit-erature [3, 4–6]. Seamless handoff is also animportant topic of research when resolving inte-gration issues [7–9]. However, the work pro-posed in this article addresses this problem byproviding solutions that have the features enu-merated above. The basic concepts of this framework were introduced in [10]. Introduced in thisarticle are features of the framework specific toroaming support across cellular and WLAN, wh ic h ca n be ac co mp li sh ed un de r th e in fr a-structure provided by the global roaming framework. They are:• An entity that performs message translation

and QoS mapping

NIRMALA SHENOY, ROCHESTER INSTITUTE OF TECHNOLOGY

RAFAEL MONTALVO, CISCO SYSTEMS

ABSTRACT

In the future, wireless and mobile users willhave increased demands for seamless roamingacross different types of wireless networks, quali-ty of service guarantees, and support for a vari-ety of services. This awareness has led toresearch activities directed toward intersystemand global roaming, and can be noticed innumerous products like multimode handsets,interworking gateways, and ongoing standardsand research work on intersystem roaming. Theauthors of this article proposed a global mobilitymanagement framework to support seamlessroaming across heterogeneous wire less networks .In this article we provide details on the use of the framework to support roaming across cellu-

lar and wireless local area networks. Highlightsof the framework include a robust architecturefor mobility management for varying user mobil-ity spans, provisioning for QoS mapping, inter-system message translation, and mechanisms inthe WLAN to support user-subscribed services.Performance aspects related to handoff delays,data redirection, and processing overheads arepresented and discussed. Performance compari-son of intersystem roaming between cellular andWLAN with and without the framework is pre-sented.

A FRAMEWORK FOR SEAMLESS ROAMING ACROSS

CELLULAR AND WIRELESS LOCAL AREA NETWORKS

Wireless and mobileusers will haveincreased demandsfor seamless roamingacross different typesof wireless networks,QoS guarantees, andsupport for a variety

of services. Theauthors propose aglobal mobilitymanagement framework tosupport seamlessroaming acrossheterogeneouswireless networks.

This work was funded by a Cisco URP grant.


http://slidepdf.com/reader/full/mag-a-framework-for-seamless-roaming-across-cellular-and-wireless-local-area 2/8IEEE Wireless Communications • June 2005 51

• A predictive handoff scheme for smooth hand-off of calls across cellular and WLANs

• A bicasting mechanism via mobility agents toachieve layer 3 handoff with the least qualitydisruption

• Swap databases at often roamed visiting networks of the mobile user to speed up handoff

• A mechanism to support user-subscribed services in WLAN

The scope of the rest of the article is as fol-lows. To ease understanding, the framework of

[10] is explained briefly. The features of theframework for integrating cellular and WLANare given. Simulation details of the frameworkare presented. A performance comparison of integrating cellular and WLAN with and withoutthe framework is given. We then present ourconclusions.

THE FRAMEWORK

Figure 1 shows the framework used for integrat-ing cellular and wireless LANs.

Two cellular networks, cellular1 and cellular2, wi th mi ni ma l ne tw or k el em en ts li ke mo bi le

switching center (MSC)/VLR, radio network con-troller (RNC), and base transceiver station (BTS)are shown. In the figure HLR and gateway func-tions are assumed to be collocated with the MSC. A WLAN with an access point (AP) connected via a gateway to the IP core network is included. A prof ile server , details of which are providedlater, is shown collocated with the authentication,authorization, and accounting (AAA) server.

The framework functions are implemented inselected entities (nodes) in the core network and wireless networks. These entities are the whiteand blue oblong blocks in Fig. 1.1 The softwarein these selected nodes (represented in blue)perform the framework functions to execute

seamless roaming. The hierarchical intersystemmobility agents (HIMAs) act as interface units,mentioned earlier. The HIMA functions areshown collocated with routers in the IP core network to ease their deployment. The hierarchy of HIMAs shown interact with each other to per-form hierarchical handoff for a wider span of user mobility. The mobile user has to register with an appropriate primary HIMA and requestits services as a mobility agent. Highly mobileusers and those receiving a number of calls froma wider footprint of networks should select anHIMA at a higher level. HIMAs under the pri-mary HIMA, if any, will facilitate the mobilitymanagement functions of the primary HIMA.

The software modules at the selected nodesare the components (GMCP) of a global mobili-ty management protocol. GMCP-macro performsmacromobility-related functions and will interact with MobileIP. GMCP-macro-micro shown atgateways and MSCs relate micro- and macromo-bility functions for smooth handoff. GMCP- mi cr o shown at the RNC will trigger themacromobility management function (at theGMCP-macro-micro in the MSC) based on itsinteraction with the legacy micromobility func-tions at the RNC. The interaction between theGMCP and the legacy mobility managementschemes in the wireless and core networks is

indicated via thick arrows.

The HIMA may be configured to perform:• Intersystem message translation• Predictive handoff • QoS mapping and negotiations• As a proxy HLR• As an anchor or crossover point to forward

data as the user moves from one wireless network to another

Some other important features of HIMA are:• The duration for which HIMA acts as a prima-

ry agent to any mobile user is timed to accom-modate a changing user roaming profile.

• The registration/preliminary handoff process wi ll be init iated at the HIMA, by messages

from a mobile user who enters the boundarycells of his/her current network that border aneighboring network if the mobile user has aservice profile for traveling across the con-cerned networks and has an active call.

• As part of the preliminary handoff process,once the HIMA receives preliminary handoff messages from the mobile, it will trigger bicas-ting2 and act as an anchor or crossover point.

REGISTRATION AND PRELIM-HANDOFFThe edge BTS or transceiver systems will beused to broadcast the presence of an adjacentnetwork different from the current one.3 Onhearing this broadcast, and based on its mobilityprofile, the mobile makes the decision to initi-ate a request for intersystem handoff/registra-tion if it has an active call . The request isforwarded to the HIMA. On receiving thisrequest the HIMA gets ready to perform QoSmapping and initiate bicasting to facilitatesmooth and seamless call handoff across the dif-ferent wireless networks.

THE INTEGRATION FEATURES

In this section we present the details of thetopology, the predictive handoff scheme with-out HIMA, the predict ive handoff with

HIMA, a data redirection process for TCP

n Figure 1. The framework.

GMCP - Global mobility component protocols

HIMA - Hierarchical intersystem mobility agentAAA - authentication, authorization and accountingRNC - Radio network controllerMSC - Mobile switching centerBTS - Base transceiver stationAP - Access point

GMCP-macro IP HIMA

HIMA HIMA

APBTS WLAN

AAA/profile

server

Mobile movement

BTS

Cellular 2

Cellular 1

GMCP microRNCGMCP microRNC

GMCP-macro

GMCP-macr/micro

MSC/VLR1GMCP-macr/micro

Gateway

GMCP-macr/micro

MSC/VLR2

MobileIP GMCP-macro MobileIP

IP core network

1 Not all entities are

shown for the sake of pic-

ture clarity.

2 Bicasting was first pro-

posed in [11] to avoid

delays in layer 3 handoff,

especially in IP networks.

3 This is possible if multi-

mode handsets are able to

perform switching across

the two technologies.


http://slidepdf.com/reader/full/mag-a-framework-for-seamless-roaming-across-cellular-and-wireless-local-area 3/8IEEE Wireless Communications • June 200552

data that is required if the framework is notimplemented, the AAA/profile server, and theswap databases.

THE TOPOLOGYFigure 2 shows the topology used with predic-tive handoff messages for roaming without HIMA. The topology shows a minimal configu-

ration with only three networks and a singlehierarchy of HIMA to limit presentation. Thetopology has two cellular networks, one of which is the home network, the other the visit-ing network. A WLAN is the future network to which the mobile will roam. Each network andits components are shown within the darkerellipse. Although part of the core network, thegateway General Packet Radio System (GPRS)support node (GGSN) and gateway are shown within the coverage of the wireless networks.The broad double-edged arrows show the gen-eral communications flow between the differentnetwork entities. For data call studies, a GPRS with serving GPRS support node (SGSN) andGGSN is superimposed on the 3G cellular network. For simplicity, in each cellular networkonly one SGSN and GGSN are shown. TheBTS and base station controller (BSC) are col-located. So are the HLR and VLR in the cellu-lar networks and the AAA server and profileserver in the WLAN. For testing the topology without the HIMA, the HIMA functions weresuppressed and the HIMA was used as a simplerouter. Hence, as the user moves from one net- wo rk to an ot he r in Fi g. 2, da ta is fo rwar de dfrom the old GGSN to the gateway of theWLAN via the thicker arrows.

With efforts on QoS support in 802.11 net-

works via the latest standards like 802.11e, it

is expected that QoS guarantees and supportfor user-subscribed services in WLAN willbecome important. Works proposing solutionsfor QoS guarantees and channel reservations4

in 802.11 networks can be found in the litera-ture [12, 13]. Hence, a pr of il e server to storeand handle user/service profiles is essential atthe WLAN.

The profile server is an entity introduced tostore the user service profile and allow themobile user access to his/her subscribed servicesin a WLAN. When the mobile roams into theWLAN, the user’s profile from his/her home network can be downloaded into the profile server.

PREDICITIVE HANDOFF: NO HIMAThe handoff scheme proposed in this workapplies for roaming across different cellular networks and across cellular to WLAN. Figure 2shows the information flow for predictive hand-off as the user roams from the visiting cellularnetwork to the WLAN superimposed on thetopology picture. Not all messages could beshown in Fig. 2, so Fig. 3 is included to clearlyshow all messages in time sequence. The readercan trace the corresponding messages in Figs. 2and 3.

The information flow diagram given in Fig. 3is self-explanatory. As the handoff is predictive, when the HO_req messages are forwarded fromthe current visiting network to the futureWLAN, authentication with the home network isinitiated, before resource allocation to themobile user is done. After authentication,reserved channel and frequency details are forwarded to the mobile in an HO_reply message.Before changing its frequencies and channels to

the future network, the mobile sends an

n Figure 2. Topology and predictive handoff without HIMA.

8. Auth_request

3G cellular

9. Auth_reply

1 0 . A u t h_ r e p l y

7 . A u t h_

r e q u e s t

Home

network

GGSN

SGSN

HLR/VLR /AUC

BTS/BSC

1 7 . H O_ r e p l y

2 . H O_ r e q u e s t

H O_ c m p

H O_ c m p

1 . H O_ r e q u e s t

1 8 . H O_ r e p l y

Visiting network

19. HO_reply20. HO_cmp

GGSN

HIMA 16. HO_reply

3. HO_request

11. Auth_reply

Internet

SGSN

HLR/VLR /AUC

Triggerspacket storage

Start dataredirectionBTS/BSC

13. Auth_user_ack WLAN

4. Auth_user5. Auth_request

6. Auth_request

Data to/from internet

12. Auth_reply





2 1 . H a n d e d_ o v e r

1 5 . H O_ r e p l y

1 4 . H O_ r e q u e

s t

2 2 . H a n d e d_ o v e r

GWAY

AAA/PS

AP

4 In this work, it has been

assumed that some chan-

nel reservation mecha-

nisms are available in the

WLAN.

With efforts for QoS

support in 802.11

networks via the

latest standards like

802.11e, it is

expected that QoS

guarantees andsupport for

user-subscribed

services in WLAN will

become important.



HO_cmp message to the current network andthen makes the frequency changes.

In Fig. 2 the HO_request or HO_reply mes-sage is used to trigger packet storage for redirec-tion in the old network.5 The HO_cmp messagetriggers the data redirections process in the oldnetwork. Details about the redirection processare given in the next subsection.

In Fig. 4 the HO_cmp messages sent up to

the GGSN in the previous visiting cellular network help to clear al l the data packets storedtemporarily for redirection. The handed_overmessage is used for location update at the homenetwork and to clear the user profile from theVLR of the previous visiting network.

DATA REDIRECTION: NO HIMAThe rationale behind introducing a data redirec-tion process was to avoid TCP packet loss duringhandoff and subsequent retransmission over theInternet.

Figure 4 shows a process designed to handletemporary packet storage and redirection to thenew network. Timers are used to time the dura-tion of handoff and packet storage. Packets fromthe queues are discarded on expiration of thetimers, which could happen if the handoff werenot successful. The entities in the cellular net- wo rk th at we re in vo lv ed in th e sto ra ge an dretransmission of the IP packets are the BTS,SGSN, and GGSN.6

A number of f irst-in first-out (FIFO) queues were used for this purpose . While there is onlyone queue at the BTS, two queues are shown atthe SGSN and GGSN. One queue is for storingdata as it flows down, the second is for storingthe data as it is cleared from the entities down-stream to be forwarded to the new network. This

facilitates sequential data packet delivery from

the old network, which is important for TCPtype data, as packets not in sequence can triggerretransmissions.

The numbers in small ovals along the arrowsindicate the sequence in which the differentprocesses are invoked. Activities having doubledigits are invoked by the activities with singledigits that match the first digit of the double-digit activities. For example, activity 1 invokes

activity 11, which in turn invokes 12, which theninvokes 13.

On initiating an HO_request, the BTS startsdirecting the packets into a queue for themobile (activit ies 11, 12, and 13). TheHO_request is forwarded to the SGSN andGGSN, which in turn start redirecting the pack-ets to Q1. When the mobile receives aHO_reply with the new network channel details,it sends an HO_cmp message to the old network elements that were storing the data pack-ets. On receiving the HO_cmp message, theBTS clears its queued packets to Q2 of theSGSN (activities 41, 42, and 43). The SGSN waits unt il it has received all the packets fromthe BTS for that mobile, after which it will firstclear Q2, as they are the earlier packets, andthen clear Q1, sending these packets to Q2 inthe GGSN. The GGSN adopts a similar proce-dure, but starts clearing its queues only afterreceiving the handed over message from thenew network. Thus, the packets are streamed insequence to the new network.

The data redirection process may or may notbe implemented; this is up to the discretion of the network and service provider. It is a neatsolution to avoid loading the Internet heavily with retransmission packets. There is, however, anon-zero probability that the remote TCP server

may time out and resend a few packets.

n Figure 3. Signal flow for predictive handoff without HIMA.

T i m e f l o w

HO_reply

HP_cmpHP_cmpChange

channels

Visiting cellularnetwork Home cellularnetwork WLAN

MS

HO_reply

Handed_over

Handed_over

Handed_over

Handed_over

Handed_over Handed_over

Handed_over

Assoc_req

Handed_over

HP_cmpAuth_req

HO_reply HO_replyHO_reply

Auth_user_ack Auth_reply

Auth_req

Auth_user

Auth_reply

HO_req

Auth_req

Auth_reply

Auth_reqAuth_req

Auth_reply

HO_reqHO_reqHO_req

Auth_reply

Assoc_reply

BTS SGSN HLR GGSN GWAY AAA/PS AP GGSN SGSN HLR

HP_req/reply : Handoff request/replyAuth_req/reply : Authentication request/replyAuth_user/ack : Authenticate user/acknowledgeHO_cmp : Handoff completeAssoc_req/reply : Association request/reply

KEY

Channelallocation

Get new channeland reservation

details

5 In the study conducted

by us, HO_request was

used, which results in

more packet handling at

the old network. If

HO_reply were used, it

was noted that some data

packets were en route over

the wireless media, while

the HO_reply, if carried

on a signaling channel,

arrived early at the

mobile. This resulted in

lost data packets en route

as the mobile changed fre-

quencies on receiving the

HO_reply packet.

6 In the WLAN, the gate-

way and AP provide such

facilities.

While there is only

one queue at the

BTS, two queues are

shown at the SGSN

and GGSN. One

queue is for storing

data as it flowsdown, the second

queue is for storing

the data as it is

cleared from the

entities downstream

to be forwarded to

the new network.



PREDICTIVE HANDOFF: WITH HIMA

Figure 5 shows the information flow superim-posed on the topology picture for predictivehandoff with the HIMA functions turned on.

The time relational flow for these messages issimilar to that shown in Fig. 3 and hence is notprovided. The noticeable differences are:• The HIMA acts as proxy HLR, so the authen-

tication details can be forwarded to the newnetwork along with the HO_request or in aseparate message.

• The HIMA bicasts messages (indicated bythick arrows) in both networks once it receivesan HO_reply7 from the new network.

• On an HO_cmp message being sent by themobile, the old BTS stops sending packetsover the air.

• When the AP receives the handed_over mes-sage it starts forwarding the packets for themobile.

• The handed_over message sent from the newnetwork also travels to the previous network,stops bicasting, and discards the bicastedpackets. Wasteful transmission of packets overthe air has been avoided.

SWAP DATABASES Ano th er co ntr ib uti on of th e wo rk wa s sw ap databases. The concept of swap databases [14] issimple and yet very powerful, especially if mobileusers have typical roaming routines.

At the VLRs and AAA/profile servers in vis-ited networks, the mobile’s service profile is

stored in a secondary database when it leaves

the network. When it re-enters the network, itsuser’s profile can quickly be retrieved from sec-ondary to primary storage on receiving a predic-tive HO_request from the previous network. Theduration for maintaining the secondary database

can be timed. This will save the HIMA process-ing capacity in terms of its functions as a proxyHLR. As the HIMA is acting as a proxy HLR,the update of the mobile’s latest network is doneat the HIMA, and no information flow to theHLR is required. This mechanism also reducesthe signaling and processing overheads at theentities forwarding the signaling messages.

SIMULATIONS

Simulations using the OPNET tool were con-ducted on the presented integration scenarios.For voice calls, the SGSN performs the functionsof the MSC and the GGSN acts as the gateway.The trajectory of the mobile covers a span acrossthree networks, with the first cellular acting asthe home network. The HIMA functions can beturned off for it to perform as a normal router.In the simulation model, the HIMA also simu-lates the remote end server, generates packetstreams for the mobile, and sinks packets deliv-ered from the mobile.

The mobile was modeled to set up a GPRSdata session and also an association/ authentica-tion-based session with WLAN. Each data ses-sion duration is modeled with a negativeexponential distribution. Packets of fixed size8

are sent at exponentially distributed interarrival

times during a data session. The mobile was also

n Figure 4. Data redirection process (no HIMA).

BTS

H a n d e d_

o v e r f r o m n

e w

n e t w o r k

H O_

c m p

H O_

c m p

H O_

r e q u e s t

H O_

r e q u e s t

H

O_

r e q u e s t

Internet

GGSN

SGSN

Start Q Clear Q2

Clear Q2

Clear Q2

End Q2,clear Q1

End Q2,clear Q1

Start Q

Start Q

Triggers

Forward to nextnetwork

3

31

32

33

65

64 63

62

61

6

51

5

41

4

53

5423

22

13 43

52

4212

11

1

2

21

Packet flow

Q1

Q1 Q2

Q2

55

7 Note that using the

HO_reply in this case will

not result in problems

faced in the normal hand-

off scheme, and less pro-

cessing capacity will be

wasted processing the

extra packets during

bicasting.

8 Variable sizes could be

used without affecting the

performance of the pro-

cesses discussed in this

article.

As the HIMA is

acting as a proxy

HLR, the update of

the mobile’s latest

network is done at

the HIMA and no

information flow tothe HLR is required.

This mechanism also

reduces the signaling

and processing

overheads at the

entities forwarding

the signaling

messages.



modeled to start voice calls of varying time dura-tion with Poisson arrivals.

MODELING DELAYSThe handoff and data redirection delays wereestimated by implementing queues in theOPNET model. Four delays were considered forstudying the performance of the scenarios. Thesedelays are explained in the following subsections.

The delay models for all packet/protocol pro-cessing, databases, and so forth were assumed tobe M/G/1 queue [7], where the service time isconsidered a general distribution and the arrivalof jobs (packets) Markovian.

For an M/G/1 queue, system time Γ can beobtained using the following equation:

Γ = 1/ µ + ω, (1)

where ω is the waiting time and can be obtainedusing the Pollaczek-Khinchin equation,

(2)

In the equations σ is the variance, η is thearrival rate of jobs or packets and is given inpackets per second, and µ is the service rate of the queue server.

Packet and Protocol Processing Delays — As thepacket is handled by different nodes involved inthe handoff or redirection process, there is “pro-tocols or packet” processing delay, as the packetflows via various protocol layers. For simplicity,all the protocol delays in one node were lumpedinto one queue.

Database Delays — These are the delays incurred while accessing/storing information from/to the

databases (HLR, VLR, and profile server).

Store and Retrieve Delays — Delays incurred instoring and retrieving from the queues duringdata redirection were modeled using a separatequeue, because the loads and service time forthis process could be different from the databaserelated processes.

Channel and Allocation Delay — Channel alloca-tion delay was introduced to model the timetaken by the BTS/BSC or AP in making deci-

sions for channel reservation for the new mobilethat was getting handed off.

PERFORMANCE COMPARISON

The performance presented in this article isrestricted to one scenario where the load inevery node is uniformly varied from 70 to 95percent in steps of 5 percent. The offered loadis η*100/ µ and in the graphs the ratio is used.The first set of performance is discussed for theintegration scenario without the framework, with predictive handoff and redi rect ion. Theperformance includes the handoff delays andredirection delays for various offered loads atthe nodes for data sessions with varying dataarrival rates. This is followed by the processingoverheads if the data redirection process wereimplemented, which is estimated in terms of wasted processing times at the different nodes. An es ti ma te of lo st da ta pa cke ts if th e da taredirection were not implemented is then provided. The performance for handling voice ses-sions is then presented. This is followed by theperformance of the same integration scenariobut with the framework and HIMA implement-ed. In this case, there are no redirection delays.However, processing overheads due to bicastingand QoS loss in handling voice sessions are pre-

sented.

ϖ η µ ση

µ

= ⋅ +

⋅ −

( / ).

1

2 1

2 2

n Figure 5. Topology and predictive handoff with HIMA.

SWAP

Homenetwork

HLR/VLR /AUC

SWAP

SWAP3G cellular

HLR/VLR /AUC

GGSN

Data to/from internet

Internet

SGSN

Triggersbicasting

BTS/BSC

GGSN GWAY

APWLAN

AAA/PS

HIMA

SGSN

Bicasting

4. HO-req/auth9. HO-reply

17. Handed_over

5. HO-request

BTS/BSC

Visiting network

13. HO-reply

1 2 .

H O - r e p l y

6 .

H O -

r e q u e s t

A u t h - r e q u e s t

A u t h - r e q u e s t

A s s o c - r e p l y

A s s o c - r e p l y

7 .

H O - r e p l y

1 6 .

H a n

d e d

_ o v e r

1 5 .

H a n d e d

_ o v e r

2 0 .

H a n d e d

_ o v e r

1 .

H O - r e q u e s t

1 1 .

H O

- r e p l y

1 9 .

H a n

d e d

_ o v e r

1 8 .

H a n d e d

_ o v e r

2 .

H O - r

e q u e s t

1 0 .

H O -

3 .

H O -

14. HO-complete

8. HO-reply

Discardbicastedpackets

Stopsendingpacketsover air

C h a n n e l

a l l o c a t e

S t a r t s e n d i n g

p a c k e t s o v e r

a i r

Delays incurred in

storing and retrieving

from the queues

during data

redirection were

modeled using a

separate queue,because the loads

and service time for

this process could be

different from

the database

related processes.



SIMULATON PARAMETERS

For the simulations, the values for σ were main-tained at 0.01 in Eq. 2, and the service rates µ were uni formly assumed to be 1 job/ms for allqueue models. The values for η we re va ri edfrom 0.7 to 0.95 in steps of 0.05 to study the per-formance under varying load conditions.9 Thedata arrival from the source was varied from100, 200, 500, and 1000 packets/s. The packets within a voice session were maintained at a con-stant bit rate of 64 kb/s.

HIMA OFFData Session Handoff and Redirection Delays — Figure 6 shows thehandoff and redirection delays under the statedoffered loads. The graphs were obtained basedon simulations repeated with different seeds.The effect of the different seeds is not notice-able in the handoff delay graph but clearlynoticeable in the redirection delay graph. Themaximum handoff delay suffered under highload conditions is around 170 milliseconds. Theredirection delay accordingly peaked at 300 mil-liseconds. The handoff delay depends only onthe load at the different nodes and not on thedata arrival rates.

The redirection delay plot shows the trend of the delay variation for increasing loads. A numberof graph points can be noticed for this plot, whichare for varying data arrival rates. The spread,however, is primarily due to the different simula-tion seeds used. For example, the points close tothe x-axis indicate 0 redirection delay. This isbecause for these particular seed values andunder the given load conditions, there were nopackets for redirection. This happens as the pack-et arrivals were exponentially distributed and theprobability of a high interarrival time is non-zero.The handoff took place in one of these high inter-arrival periods; hence, there were no packets for

redirection. Another interesting observation made

was that the redirection delays depend only onthe loads in the network, not at the data arrivalrate. This is because of the way the redirectionprocess was implemented. However, the time wasted in redirecting the packets will depend onthe number of packets and hence on the dataarrival rates. An estimate of this follows.

Processing Overheads — The processing overheadsaddressed are primarily due to the redirectionprocess. This has been calculated as wasted pro-

cessing time to handle the redirection of packetsover the handoff interval. For a given offeredload of 75 percent (i.e., η = 0.75 at all nodes)the wasted time increases from BTS to SGSN toGGSN. This is because the packets processed atthe BTS have to be processed at the SGSN andGGSN. Similarly, the packets processed at theSGSN have to be processed at the GGSN. Overthe handoff interval, for a data arrival rate of 200 packets/s, 5 ms of SGSN time and 25 ms of GGSN time were wasted. While for a data arrivalrate of 100 packets/s, only 10 ms of GGSN time was wasted. There were no packets for redirec-tion in the BTS and SGSN.

The total number of packets retransmitted was 25 packets and 10 packets for arrival rates of 200 and 100 packets/s, respectively.

Voice Session — The handoff delays for voice ses-sions will be the same as for data sessions underthe given load conditions. However, redirectionof voice packets is not feasible, and packets sentto the old network must be discarded. The esti-mated voice packets lost varied from 11, 6, 8, 8,16, and 31, respectively, over load variation from70–95 percent (in steps of 5 percent). Packet lossdependence on the load was felt only under highload conditions (i.e., beyond 90 percent).

HIMA ONData Session Handoff Delays — Figure 6 also has the graph forhandoff delay with the framework implemented.The maximum delay suffered is seen to be 100ms compared to a maximum delay of 170 ms without the framework. However, even this willnot be felt by the mobile as it experiences seam-less and packet lossless handoff due to bicasting.The throughput may go down slightly during thehandoff period, as the remote server may backoff when it does not receive acknowledgmentsfor packets sent.

Processing Overheads — The processing overheadsin this case is due to the bicasting process and was calculated as wasted processing time at thedifferent nodes. It was noted that there is con-stant wasted time of 4 ms and 10 ms at each of the nodes (i.e., BTS, SGSN, and GGSN) fordata arrival rates of 100 and 200 packets/s,respectively, over the handoff period. The valueis constant because all the packets sent to theold network reach the BTS and are eventuallydiscarded. However, before reaching the BTS,the packets are processed at the SGSN andGGSN. These readings were for a typical load of 75 percent on all nodes. Data arrival rate affectsthis parameter profoundly; offered load does

not.

n Figure 6. Handoff and redirection delay vs. offered load.

Offered load at GGSN/SGSN/BTS

0.9 0.950.65

0

0.05

D e l a y i n

s e c o n d s

0.1

0.15

0.2

0.25

0.3

0.850.80.750.7

“Handoff delay (HIMA)”“Handoff delay (no HIMA)”“Redirection delay”

9 Note that the perfor-

mance studies were con-

ducted primarily under

heavy load conditions.



Voice Sessions Handoff Delays — In voice sessions, although thereis a handoff delay (as for data sessions), the userdoes not feel the effects of the handoff as no voice packets are lost or delayed. Packet num-bering was implemented at the application levelfor the voice sessions to help the mobile discardpackets duplicated due to bicasting. For uniformload of 75 percent on all nodes, the number of duplicated packets was 3.

CONCLUSIONSIn this article we have introduced a frameworkthat can be used for integrating cellular andWLANs. The main features of this frameworkare its hierarchical and distributed architecture, which provides robust and scalable solution toseamlessly roam across a number of WLANsand cellular networks while supporting call con-tinuity through predictive handoff, QoS map-ping, intersystem message translation, andprovision in the WLAN for user-subscribed services. The framework has been evaluated usingOPNET simulations and performance compar-

isons made for varying load scenarios for inte-gration with and without the framework. If amobile had a larger mobility span and a primaryHIMA at a higher hierarchical level, the com-parative performance improvement achieved would be much better.

ACKNOWLEDGMENTSThe authors would like to acknowledge Ms.Punita Mishra, who helped in data collection forthis study.

REFERENCES[1] V. W. S. Wong and V. C. Leong, “Location Management for

Next Generation Personal Communications Networks,” IEEE Network, vol. 14, no. 5, Oct 2000, pp. 18–24.

[2] J. S. M. Ho and I. F. Akyildiz, “Dynamic HierarchicalDatabase Architecture for Location Management in PCSNetworks,” IEEE/ACM Trans. Net., vol. 5, no. 5, Oct.1997, pp. 646–60.

[3] 3GPP, “Group Services and System Aspects: 3GPP Sys-tems to Wireless Local Area Network (WLAN) Interwork-ing; System Description (Release 6),” TS 23.234 v6.0.0.

[4] K. Ahmavaara, H. Haverinen, and R. Pichna, “Interwork-ing Architecture Between 3GPP and WLAN Systems,”IEEE Commun. Mag., Nov 2003, pp. 74–81.

[5] M. Jaseemuddin, “An Architecture for Integrating UMTSand 802.11 WLAN,” Proc. 8th IEEE Int’l. Symp. Comp.and Commun., 30 June–3 July, 2003, pp. 716–23.

[6] K. S. Apostolis, C. Fors, and R. Pazhyannur, “WLANGPRS Integration for Next Generation Mobile Data Net-works,” IEEE Wireless Commun., Oct 2002, pp. 112–24.

[7] W. Wenye and I. F. Akyildiz, “A New Signaling Protocol forIntersystem Roaming in Next Generation Wireless Sys-tems,” IEEE JSAC , vol. 19, no. 10, Oct 2001 pp. 2040–52.

[8] G. P. Pollini, “Trends in Handover,” IEEE Commun.Mag., Mar. 1996, vol. 34, no. 3, pp. 82–90.

[9] I. F. Akyildiz et al., “Mobility Management in Next Gen-eration Wireless Systems,” Proc. IEEE , vol. 87, no. 8,Aug. 1999, pp. 1347–84.

[10] N. Shenoy, “A Framework for Seamless RoamingAcross Heterogeneous Next Generation Wireless Net-works,” accepted for ACM Wireless Networks.

[11] H. Soliman et al., Internet draft on HIMIPv6, Oct.2002, http://www.ietf.org/internet-drafts/draft-ietf-mobileip-hmipv6-08.txt

[12] S. Mangold et al., “Analysis of IEEE 802.11E for QoSsupport in Wireless LANs,” IEEE Wireless Commun., dec.2003, vol. 10, no. 6, pp. 40–50.

[13] J. Jun and L. S. Mihail, “The Nominal Capacity of Wire-less Mesh Networks,” IEEE Wireless Commun., Oct.2003, vol. 10, no. 5, pp. 8–14.

[14] K. H. Chaing and N. Shenoy, “Swap Technique forLocation Management in Mobile Networks,” Proc. IEEE ICC 2002, New York, 28 May–2 June, 2002.

ADDITIONAL READING[1] N. Shenoy et al., “Mobility Prediction for Optimal Han-

dover and Connection Management in Mobile Multime-dia,” Proc. 3rd Asia Pacific Conf. Commun. ’97 , Sydney,Australia, 7–10 Dec.. 1997, pp. 1236–40.

BIOGRAPHIESNIRMALA SHENOY ([email protected]) is an associate professorwith the Information Technology Department at RochesterInstitute of Technology. She has several years of teachingand research experience while working in Germany, Singa-pore, and Australia before she moved to the United States.She is an avid researcher in the wireless networking areaand has technically led several wireless network projects tosuccess. She holds a Ph.D. in computer science from theUniversity of Bremen, Germany, as well as a Master’s inapplied electronics and a Bachelor’s in electronics andtelecommunications engineering, both from Madras Uni-versity, India. She is interested in research in the area ofmobility management and modeling for wireless networks,quality of service in wireless networks and the Internet,and protocol for mobile ad hoc networks and sensor net-

works.

RAFAEL MONTALVO ([email protected]) studied digitalcommunications and signal processing at Stanford Univer-sity, California, and received his Ph.D. degree in 1985. InAugust 1985 he joined AT&T Bell Laboratories, Murray Hill,New Jersey, as a member of technical staff. In June 1987he joined IBM T. J. Watson Research Center, Hawthorne,New York, as a research staff member. In June 1993 he

joined Cisco Systems, Research Triangle Park, North Caroli-na, as a hardware engineer. He is currently involved in thedesign and implementation of Cisco products for mobilewireless applications.

Among the main

features of this

framework is its

hierarchical and

distributed

architecture, which

provides robust andscalable solution to

seamlessly roam

across a number

of WLANs and

cellular networks.

Documents

MAG, A framework for seamless roaming across cellular and wireless local area networks.pdf