gateway performance for network-controlled wlan ip mobility

INTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMSInt. J. Commun. Syst. 2007; 20:1337–1365Published online 1 February 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/dac.873

Gateway performance for network-controlledWLAN IP mobility

Moshiur Rahman1 and Fotios C. Harmantzis2,*,y

1AT&T Labs, 200 Laurel Avenue, Middletown, NJ 07748, U.S.A.2Stevens Institute of Technology, Hoboken, NJ 07030, U.S.A.

SUMMARY

‘Always on’ broadband-accessed network gateway (GW) control can facilitate inter-WLAN IP mobility,with seamless connectivity. The GW server plays a critical role in the overall WLAN IP (WIP) mobilityarchitecture (IEEE Wireless Communications and Networking Conference, WCNC, Atlanta, GA, 21 March2004; Int. J. Wireless Inf. Networks 2006; 13(3):173–192). This paper provides a comparison of WIP withcellular IP (CIP) and mobile IP (MIP), and identifies the main requirements for a broadband-accessednetwork-based GW that supports WIP mobility. The paper then evaluates GW-contributed handoffmessage processing delay in the WIP architecture through an analytical system model and OPNETsimulation model, and provides a comparison of the GW-contributed handoff message processing delaysfor non-preemptive vs preemptive queuing schemes. Both analytical and simulation results show that WIPhandoff message processing delay at the GW has negligible impact on the overall system delay. Finally, thispaper presents the simulation results of the fast routing table lookup and forwarding speed on the GWoverall performance, which can assist service providers in the challenging implementation issues that theyface. Copyright # 2007 John Wiley & Sons, Ltd.

Received 15 March 2006; Revised 2 October 2006; Accepted 15 November 2006

KEY WORDS: WLAN; gateway; broadband; QoS; mobile IP; handoff

1. INTRODUCTION

Current broadband access technologies have made WLANs an effective extension of backbonenetworks. An ‘always on’ broadband access mechanism, where an end-user device, such as awireless access point, and a network gateway (GW) are connected over a permanent virtualconnection, can be used to send handoff control messages between the GW and the mobile node(MN) for inter-WLAN IP mobility. Here, ‘always on’ refers to the broadband (such as DSL or

*Correspondence to: Fotios C. Harmantzis, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, NJ07030, U.S.A.yE-mail: [email protected]

Copyright # 2007 John Wiley & Sons, Ltd.

cable access) network pre-provisioned connection where there is no separate session establish-ment phase needed. With an appropriate GW control mechanism, it is possible to detect the lossof connectivity to the control point of the first WLAN and establish a connection to the nextavailable control point of a different WLAN over the broadband access network [1]. Reference[2] provides a network-controlled architecture and suggests the use of a network control forWLAN IP (WIP) mobility, but it does not provide any specific network service controlfunctionalities, such as handoff control messages in the network layer, using a GW.

A GW server plays a critical role in the broadband-accessed network-controlled WIPmobility performance. The term ‘network-controlled’ refers to the GW supporting mobilitymanagement that is off the high-speed core infrastructure network, which could potentially beprovided by broadband network service providers. This network-controlled entity wouldbe shared by many end-users. (The terms GW and server have been used alternatively inthis paper.)

The GW in WIP has all the required intelligence to support mobility management, includinglocation management needed in micro- and macro-mobility and route translations [1] of IPpackets, unlike other wireless mobility GWs, such as cellular IP (CIP) GWs.

This paper identifies the main requirements of a network-based GW supporting WIPmobility. It then evaluates GW-contributed handoff message processing delay in WIP BANCarchitecture through an analytical system model and an OPNET simulation model. The paperalso provides a comparison of the GW-contributed handoff message processing delays for non-preemptive vs preemptive queuing schemes. Both analytical and simulation results show thatWIP handoff message processing delay at WIP GW has negligible impact on the overall systemdelay. The paper compliments the previous works published [1, 3–5].

The rest of the paper is organized as follows: Section 2 provides a brief overview of the CIPGW capabilities. Section 3 provides related work on CIP GWs and handoff priority queuing.Section 4 describes the WIP GW requirements and performance analysis with an analyticalmodel. Section 5 provides simulation results of CIP vs WIP and the packet routing andforwarding effect on WIP GW CPU utilization. Finally, Section 6 concludes the paper.

2. RELATED RESEARCH

2.1. Cellular IP (CIP)

Use of the CIP GW requires MIP to complete end-to-end handoff management.Thus, the functionalities alone in a CIP GW are not adequate for the WIP GW to supportinter-WLAN (inter-BCP) mobility, as the CIP GW and WIP GW are similar in micro-mobility.

Mobile IP (MIP) is the only current means of offering seamless mobility to mobile devices inthe Internet [6]. In order to meet performance needs, the MIP protocol has already gonethrough major enhancements. However, it does not have any GW involved in managingmobility. In MIP [7–12], on arrival at a new network, a mobile host contacts the local FA, whichsupplies it a care-of-address which may be an address of the FA itself. After that, the mobile’sHA is informed that all IP datagrams destined to the mobile host must be forwarded (tunnelled)to the new care-of-address in order to reach it. This tunnelling causes latency, delay and packetloss in IP mobility. A major problem of scale is the frequent change of IP care-of-addresses.

M. RAHMAN AND F. C. HARMANTZIS1338

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Commun. Syst. 2007; 20:1337–1365

DOI: 10.1002/dac

MIP addresses the problem of wide area mobility, but it has the problem of seamless mobility.A number of protocols have been proposed to address that MIP weakness. CIP is one of thoseprotocols for internet host IP Mobility. Figure 1 shows two communication scenarios between ahost and a mobile terminal in CIP.

CIP inherits principles of cellular systems for mobility management, passive connectivity andhandoff control and it is designed based on the IP paradigm [13–19]. CIP is a wireless Internetaccess technology that operates on mobile terminals, base stations (BSs) and Internet GWs. TheBS serves a wireless access point and a simultaneous routing of IP packets. It integrates thecellular control, functionality traditionally found in a mobile switching centre, with that in theBS controller. The BS is built on a regular IP forwarding engine, but IP routing is replaced byCIP routing and location management. The CIP network is connected to the Internet via a GWrouter [13]. Mobility between GWs (i.e. CIP access networks) is managed by MIP, whilemobility within access networks is handled by CIP. A mobile terminal attached to the networkuses the IP address of the GW as its MIP care-of-address. Assuming mobile IPv4 and no routeoptimization, a packet intended for the mobile terminal will travel first to the host’s HA, whereit is tunnelled to the GW. The GW de-tunnels the packet and forwards it to the BS. Note thatMIPv6 is still not entirely tunnel free. Inside the CIP network, the mobile terminal is identifiedby its home address and so the data packet is routed without tunnelling or address conversion.A packet transmitted by the mobile terminal to a host connected on the network is first routedto the GW and from there into the Internet to its intended destination.

As mentioned above, the CIP protocol is intended to provide local mobility and handoffsupport. The loss of downlink packets when the mobile host moves between access points (BSs)(local mobility) is reduced by customized handoff procedures. The three types of handoffprocedures in CIP are: CIP hard handoff ; CIP semi-soft handoff ; and CIP indirect handoff.Maleej and Farikha [20] provide details on these three types handoff. In these CIP handoffschemes, the CIP GW is involved only in micro-mobility, unlike a WIP GW that supports bothmicro- and macro-mobility [1]. This is the fundamental difference between the CIP GW andWIP GW.

Mobile IP enabledInter network

GW

Cross-overnode

OldBS

New BS

CNHA

MN

MIP enabled

HostHA

GW

MN

BS

IP Tunneling

CIP Routing

BS

BS

IP Routing

(a) (b)

Figure 1. CIP network.

GATEWAY-CONTROLLED WLAN MOBILITY MANAGEMENT 1339


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With the standard CIP network topology used in [13, 15–20], a local GW is not able tomanage handoff between different access networks for inter-CIP mobility. The CIP networktopology, proposed in [21] for the inter-CIP mobility, still uses a dedicated local GW for eachCIP access network and relies on MIP for macro-mobility which itself suffers significant delayand packet loss during handoff [1]. The semi-soft handoff extension architecture proposed in [21]with route optimization will not guarantee improved packet loss and delay, as it needs toforward a handoff request from the old BS to the new BS via two separate GWs. This handoffpacket travel time could potentially be subjected to large variations due to unexpected networktraffic congestion. Lee [19] proposes an advanced semi-soft handoff algorithm that improves themicro-mobility but has a dependency on MIP for macro-mobility and other issues, such astunnelling of user packets during the entire session in standard MIP and tunnelling until thebinding update completion in enhanced MIP still exist.

2.2. Priority queuing handoff

The way handover (HO) calls or messages are handled has a direct impact on the quality ofservice (QoS) provided to wireless mobile users. This problem is well known in the researchcommunity as significant research has been done in this area [22–31]. Since dropping calls inprogress is less desirable than blocking new calls, various methods have been devised toprioritize handoff calls over new calls. These issues are not unique to the WIP mobilitymanagement scheme.

For completeness sake, in assessing the complexity and issues of priority HO management, weprovide a brief overview of the classical problems of HO priority schemes. These problems arenot specific to our WIP mobility scheme.

To employ dynamic queuing for HO calls, Xhafa and Tonguz [22] have proposed a novelapproach with two classes of priority for HO calls and two queues; first priority and secondpriority are used for the two priority classes. They also incorporate a priority transition betweenHO calls in queue; specifically, a second-priority HO call in the second-priority queue canbecome a first-priority HO call and join the first-priority HO queue. In addition, the event that aHO call could finish its call while waiting in the queue is incorporated in the analysis. One canalso use the generalized framework proposed in [22] to analyse the HO performance ofintegrated voice/data networks, including the proposed BNAC architecture.

Xhafa and Tonguz [22] have provided an exhaustive list of research work in this area.An overview of priority schemes for HO calls was done by Posner and Guerin as early as 1985[32]. For later developments on HO priority schemes and work, the reader can referto the excellent references by Pollini [23], Katzela and Nagashineh [24], Jabbari [25] andTripatti et al. [26].

One of the earliest analytical frameworks for the guard channel method (GCM) wasdeveloped by Guerin [27]. Guerin proposed a novel approach, where a certain number ofchannels is used exclusively for HO calls and only the queuing of originating (new) calls isinvestigated. This approach not only minimizes the HO blocking probability for HO calls, butalso increases the total carried traffic in the network. Simple closed-form expressions areprovided for state probabilities; hence, the evaluation of the performance of the wirelessnetwork is straightforward [27]. Daigle and Jain [33] reconsider the approach proposed byGuerin [27] and propose a novel and alternative analysis based on Neuts’s matrix approach.Heffes and Ryan [29] develop an analytical framework for GCM with first-in-first-out (FIFO)



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queuing of HO calls and no queuing of originating (new) calls. Results show that the guardchannel priority scheme with FIFO queuing of HO calls achieves a smaller forced terminationprobability compared to other schemes, thus reducing the number of dropped HO calls [29].Chang et al. [30] investigate a new cutoff priority scheme that allows finite queuing of both newand HO calls. In this approach, the HO and new calls are queued in two separate FIFO queues.In addition, Chang et al. [30] consider reneging of new calls and dropping of queued calls as theymove out of the HO area before the HO call is successful. Optimal cutoff parameters andappropriate queue sizes that minimize overall blocking probability are found numerically. Misicand Tam [34] provide a complete analytical framework on fixed channel allocation (FCA) and[35] provide a description on the issues in dynamic channel allocation (DCA) for handoffmanagement.

The aforementioned studies [27, 28, 30] dealt with the analytical framework for GCM and itsvariants, i.e. with and without FIFO queuing. While one could use FIFO queuing for new calls,this is not a good idea for HO calls because mobile users move with different speeds; so HOrequests need to be queued in such a way that the priority changes dynamically to account forthe dynamics of the user motion. Therefore, the FIFO queuing scheme is not suitable for dealingwith HO calls.

Tekinay and Jabbari [31] study via simulations the performance of non-preemptive priorityqueuing for HO calls where, if a channel is released, the HO call in the queue that has the lowestreceived signal strength (RSS) gets served. It is shown that the proposed scheme, which is calledmeasurement-based priority scheme (MBPS), outperforms the FIFO queuing scheme under alltraffic conditions. However, the study in [31] does not take into account the dynamic usermotion. Ebersman and Tonguz [36] investigate the dynamic queuing of HO calls using a signalprediction priority queuing (SPPQ) discipline, where the order of HO calls is not only based onthe RSS but also on the rate of change of RSS. The performance (i.e. new call blockingprobability, forced termination probability, etc.) of a cellular system that uses the SPPQ schemeis evaluated via extensive Monte Carlo simulations. The results show that the SPPQ schemeachieves a smaller forced termination probability than FIFO queuing and MBPS, at the expenseof a slight increase in the new call blocking probability.

The channel holding time of a call plays a major role in the QoS of the handoff managed bythe GW. A new and general mobility model is presented and analysed in [29] to study the impactof user mobility on HO. This model is valid for a wide variety of different types of user mobilityand is used to derive the Laplace transform of the probability density function for the channelholding time.

In summary, the above studies in priority management are geared towards reducing theblocking probabilities for handoff calls and new calls. These are classical issues in cellularnetworks and not unique to our proposed WLAN mobility management. However, ourmobility management scheme could leverage some of the above promising priority queuingwork, including multi-level priority scheduling [37], the SPPQ discipline [36, 38] and dynamicpriority queuing [22] recommended for the cellular network HO to handle the blockingprobabilities of the HO calls and the new calls. This is one of the potential future research areasfor WLAN mobility and beyond the scope of this work. We will focus, however, on the GW’scontribution to the WIP HO message processing delay. The GW is the main network controlcomponent of the WIP mobility management scheme. As mentioned before, our objective hereis to show that WIP HO message processing at the GW has little, hence, negligible impact on theoverall system delay.



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3. WLAN IP

The main idea of the WIP architecture is to show how a GW of a high-speed backbone networkcan be used to facilitate inter-WLAN communication to support seamless WLAN mobility. TheGW is the main component of the proposed concept of inter-WIP mobility using high-speedbroadband access. It provides the intelligence needed in network-controlled IP mobility. TheGW differentiates the proposed architecture from the current IP mobility scheme that uses MIP.MIP does not have any central control, like WIP GW, to provide location management anddirect packet forwarding. The GW could be the performance bottleneck of this networkarrangement but that could be mitigated by using a high-performance GW. Our analytical andsimulation results show that the high-performance GW’s contribution to handoff delay isnegligible compared to other components of end-to-end delays, such as the access point’scontributed handoff message processing delay.

In planning to deploy a WIP network, a system architect must have a good estimate of thenumber of users, i.e. MNs, that are supported by each GW router. The number of MNs woulddepend on the type of application, traffic patterns, network stability factors and the effect ofhidden terminals. The capacity of the GW router should be planned based on the above factors.In practice, service providers can scale the WIP GW by using standard means, such as addingmore central processing units (CPUs) or adding more servers to share the given traffic load. Theassumption is that the GW will incorporate a fast routing table lookup algorithm availabletoday to mitigate the packet discriminating and forwarding task.

As shown in Figure 2, WLANs are connected to the backbone network via a broadband,always on, access network, such as DSL, cable or wireless. In this architecture, each WLAN ismanaged by the respective broadband control point (BCP) and can communicate with anymobile terminal of a different WLAN through the backbone network. An MN in the sameWLAN communicate with each other via the local BCP, without going through the backbonenetwork. In this architecture, inter-WLANs means inter-BCPs, as each WLAN can interact withother WLAN through their respective BCPs connected to the backbone network via the high-speed broadband access. When a mobile MN (voice or high-speed data) connected to the

MN x MN y

PSTN

A

CC

E

S

S

DSL Loop or

Cable Loop orWireless Loop

ATM

/IP

PVCBCP1 BCP2

Server

/Gateway

Internet

DB

Figure 2. WIP mobility management network topology.



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backbone via any one of these WLANs, moves away from the current WLAN service area toanother WLAN service area, the connection with the backbone will be maintained under thecontrol of the backbone-based network server, termed as GW in this paper. The BCP uses thepre-established PVC to forward the registration message to the GW. Thus, all the BCPs,provisioned and connected to the GW over a PVC, would be under the same virtual network,regardless of the BCPs’ geographic locations.

This broadband-facilitated WIP network model mobility [3] has gone beyond the traditionalWLAN mobility. When the GW receives the registration message, it updates the locationmanagement and routing table, by associating the MN’s IP address with the BCP IP address. Ifthe BCP IP address is not the MN’s home BCP address, the server would notify the home BCPabout the move (this is not mandatory for the given architecture) as the home BCP is notexpected to tunnel the traffic to the new BCP (unlike in MIP). With this network arrangement,all the connected WLANs appear to belong to a single virtual network. The GW’s packet-redirection functionality would send all the packets to a new BCP serving the MN. Compared tothe MIP tunnelling, where packets destined to MN have to come to HA first and then forwardedto MN via the current FA, the proposed architecture has straightforward routing and reducedpayload size unlike MIP. Note that the difference between the HA in MIP and GW in BANCWIP is that GW has its own intelligence to manage the mobility and handoff. In MIPconfiguration, HA has no local intelligence built in and it forwards the packet based on thenotification received from MN’s current access point.

For the incoming traffic, if the GW knows about the move of the MN based on the table, thenthe packet will be sent to the BCP serving the MN. If the GW is not aware of the move, thepackets will be sent to the home BCP, assuming that the MN did not move out of its ownWLAN area. When the MN moves to the new BCP while it is connected to the network, itinitiates the handoff by sending a route update message. This could be specific protocoldependent. For example, for IEEE 802.11, the MN will send a re-association message to the newBCP, and would forward this to the GW. The GW updates the routing table reflecting the moveof MN. All packets from now on are sent to the new BCP, which in turn forwards these packetsto the MN. The GW updates the routing table when the association time has expired for thevisiting MN.

3.1. Comparison: CIP, MIP and WIP

Table I summarizes some of the main differences between CIP, MIP and WIP. Note that IPv6 isalso applicable for WLAN and HMIPv6 and FMIPv6 [39, 40] are the enhanced versionsof IPv6.

An MIP-like mobility management approach is not needed when the WIP BANC topology isimplemented [1]. In Section 6, we show the comparison of WIP vs CIP (with the inter-CIParchitecture proposed in [21] and the semi-soft handoff algorithm proposed in [19]) and WIP vsMIP through simulation.

3.2. WIP requirements

Based on the WIP architecture description in Section 3.1, the GW router needs the followingmain functions: (a) location management; (b) routing; (c) handoff; (d) security; and (e) capacityfor required QoS.



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3.3. A location management

The GW maintains a table for location management and routing, where each entry is identifiedby the tuple: permanent MN address, home BCP address, new BCP address, association lifetime. On demand, the GW server can consult a database for any address resolution via the high-speed backbone network.

The difference between the GW and the HA (in MIP) is that the HA does not have anyintelligence to keep track of the MN’s mobility and to provide service level QoS. The HA, inMIPV4, tunnels the packets after being notified about the MN’s current location, and in MIPv6and FMIPv6, the HA keeps tunnelling the packets until the binding is completed. Note thatMIPv6 and FMIPv6 environments are not entirely tunnel free as the incoming packets aresubject to triangular routing until the binding with the correspondent node is complete. Inaddition to providing direct routing, the GW in the proposed network-controlled architecture[1] provides the target BCP selection capability.

3.4. Routing

Based on the location management scheme described above, the WIP GWmust have an efficientrouting capability. This capability is similar to HA’s capability in MIP. In CIP, location andhandoff support are integrated in routing. To route downlink packets addressed to the MN, thepath used by recent packets, transmitted by the MN to the GW, is reversed; the path taken bythe uplink packets to the GW is cached in BSs. In WIP, after the GW is informed that all IPdatagrams destined to the mobile host must be forwarded to the new BCP in order to reach theMN, the GW forwards the incoming packets directly to the MN via the new BCP, by employinga fast route look scheme and a routing table. The WIP routing avoids the tunnelling used inMIP/CIP. The BCPs and the MNs in WIP do not need to cache (routing cache) the path takenby the uplink packets, unlike in CIP.

3.5. Handoff

The execution of the handoff operation by the GW, including the update the routing mappingtable, would affect the overall system performance. One of the main criteria of the implementedhandoff strategy would be to reduce the handoff latency which is the time that elapses betweenthe handoff and the arrival of the first packet through the new route to the new BCP to the MN.The frequency of the handoff, that is the speed of the mobility of the MN from one WLAN to

Table I. CIP, MIP and WIP main functionalities.

CIP MIP WIP

Layer 3 mobility management Layer 3 mobility management Layer 3 mobility managementLocal GW for micro-mobility No GW involved GW micro- and macro-mobilityOptimization: MIP macro-mobility Optimization: HMIPv6

and FMIPv6Direct routing with GW

Mobility agent: HA and FA(local GW)

Mobility agent: HA and FA Mobility agent: central GW

Tunnelling Tunnelling until HO completes No tunnelling



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another one, would also affect the performance of the GW and this requires an effective trafficengineering capacity planning. The GW must support the handoff signalling messages shown inFigure 4 to support mobility in WIP. The proposed seamless handoff scheme uses minimumsignalling messages compared to MIP protocol [8] to decrease latency and signalling costs.Moreover, in order to prevent packet loss during handoff, the proposed scheme allows thenetwork server to manage MN’s CoAs assigned by two WLANs’ access points (BCPs)participating in the handoff. By using this ability, the network server can buffer the data packetsdestined to the MN during handoff. The network server also registers the CoA assigned in thenewly handed-off network as it allocates extra space to store the MN’s next CoA in the addresstable, as in Table II.

WIP has lower handoff latency than IAPP. Figure 3 shows how the WIP-employed modifiedIAPP reduces the handoff latency. While still within the range of the current access point, MNstarts re-association with the new access point at time T1 and completes at time T2: Note thatby time T2 all the required message transactions between the GW server and the new accesspoint (BCP) are complete over the high speed and always-on broadband distribution system. Bytime T2; the GW server knows the location of the MN and starts sending data directly to theMN via the new BCP. No tunnelling is involved.

The basic difference between the WIP and the enhanced IAPP is that the IAPP does not haveany central control to assist any specified QoS. Secondly, the enhanced IAPP suffers from thedelay encountered in redirecting traffic during the handoff period. The WIP mobility schemeavoids this redirect routing by using network control of location management. Handoff latencymeasurement results for micro-mobility (IAPP vs WIP) are shown in Table III.

Table II. Address management.

Home IP Address Current CoA Next CoA

IP MN1 IP C1 IP N1IP MN2 IP C2 IP N2... ..

. ...

LL

IPT1 T2

T5 T6

MN

BCPold

BCPNew

B-updatestarts

B-updateends

time

BANC HO latency

Enhanced IAPP HO latency

Original IP - layer HO latency

Reasso _RESMOVE -notify/MOV -response/MOVE_Forward

T4T3

Figure 3. Reduced handoff latency with modified IAPP in WIP handoff scheme (the handoff phase issmaller than the IAPP and MIP).



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The detailed inter-WLAN handoff operation, depicted in Figure 4, can be explained bydividing the process into two parts: handoff initiation phase and handoff execution phase. Thesemessages are the modified version of enhanced IAPP messaging [41].

Figure 4 shows the required signalling message flow for the proposed handoff procedure.Additional messages are added to IAPP to support the mobility management using theproposed scheme. The flowing additional messages, referred to as Protocol Data Units (PDUs)in IEEE 802.11, are defined in the modified IAPP: HO ACK, RSRC REQ and RSRC RES.As the MN migrates away from the old BCP and towards the WLAN coverage area of the newBCP, it listens to the beacon signals from both BCPs. Based on its measurements of the BCPbeacons, the MN can decide when to initiate handoff from the old BCP to the new BCP. Thenetwork server is also able to initiate MN handoff [2], where the MN detects that the radioquality is falling below a given threshold and collects quality measurements with neighbouringAPs and reports them to the network server. After analysing the measurement data, the networkserver can decide when to initiate handoff. However, the HO decision is a classical problem forwireless networks, including cellular and WLANs. The beacon measurements and HO decisionsare specific vendor implementation dependent. In our architecture, we have assumed that theMN has received the HO decision from the network server. After receiving this HO indication,

Table III. Handoff latency comparison: enhanced IAPP vs WIP.

Confidence interval

Handoff scheme Mean (ms) Std dev. (ms) 90% 95% 99%

IAPP 107 10 104.6–109.3 104.2–109.8 103.3–110.7WIP 101 12 107.1–112.80 106.67–113.32 105.6–114.37

BCPOld

Networkserver

MN BCPNew

HO_REQ

HO_ REQReasso_R EQ

HO_RES

Rea sso_RES

WLAN

RSRC_REQ

RSRC_RES

HO_RES

HO-ACK

Extended IAPP HO procedureOver the wired part

MOVE-notify

MOVE-response

Figure 4. IAPP modified messaging in WIP.



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the MN sends a re-association request (Reasso REQ), an existing IEEE 802.11 MAC message,to the new BCP using the broadcast radio channel, specifying all its audible frequencies andtheir corresponding signals to interference ratio (SIR) status. Upon receiving this HO indicationmessage (Reasso REQ), the new BCP sends a HO request (HO REQ), an existing IAPPmessage to the network-based server over the broadband (such as DSL/ATM) interface. Basedon this message (user device ID, potential handoff BCP}the new BCP}and the call connectionID of the existing call) from the new BCP, the server establishes a correlation between theexisting call and the old BCP that is serving the MN. The BCPs maintain a permanent virtualcircuit with the network server through the broadband access. The network server wouldmaintain a connection table of all the existing calls (data or voice) using BCPs served by theserver. The server then sends a query resource message, RSRC REQ, to the new BCP askingabout its resource availability. This would be the first new message to IAPP. In response, thenew BCP sends an RSRC RES (the second new message), providing information about theresource availability (in this message flow it is assumed that the resource is available). Thenetwork server could use an inter-BCP (WLAN) protocol to determine the best possible newBCP (WLAN) for the migrating remote mobile terminal. After receiving an RSRC RES fromthe new BCP, the network server sends an acknowledgement, HO ACK (the third newmessage), to the new BCP. Assured of the establishment of the data path to the MN via the newBCP, the sever sends a HO REQ message to the old BCP. The old BCP then sends a HO RESto the server, and in turn, the server sends this HO RES to the new BCP. Then the new BCPsends this Reasso RES to the MN to complete the HO initiation phase. After receiving theReasso RES from the new BCP, the MN changes its operating frequency and startscommunicating through the new BCP. Now, the resource allocation tables in both the BCPsand the network server are updated. The address table in the server is also updated. WithMOVE notify and MOVE response packets, the IAPP provides context transfer betweenBCPs. It assumed that a single MOVE response could carry all buffered data. The MOVEforward message identified in the enhanced IAPP could also be used.

3.6. Security

WLAN mobile systems are open to numerous security problems that do not exist in wiredsystems. In a fixed network, the prefix of a subnet is usually configured manually. The locationof the prefix is communicated between routers that either have some form of an inherent trustmodel or use a secure protocol. This makes it hard to impersonate someone. On the other hand,mobile hosts must update their location while moving. These location messages makeimpersonation possible, unless they are properly secured. WIP networks compound thisproblem because packets can be snooped over the air interface. WIP faces impersonation andsnooping attacks because it is wireless and mobile. Therefore, in WIP GW these problems mustbe addressed. During handoff, the new BCP can acquire a session key from the GW router.Rather than defining a new IP security algorithm for the WIP, the security technique used inCIP could be modified for the WIP in the following way.

The session key is secure hash, which combines:

* The IP address of the MN.* The random value ðRMNÞ assigned to an MN when the GW router receives a handoff

request message (HO REQ) from a BCP.* A network secret ðKðnetworkÞÞ known by all BCPs within the subnet.



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The session key is calculated as in CIP security using the MD5 hash function: (Ksession ¼MD5ðIPMN;RMN;KnetworkÞ). A session key is first calculated and transmitted to an MN when it firstcontacts the GW router from a new location (a different WLAN area) during mobilityauthentication and authorization. A random value, RMN; is assigned to the MN at this point.Control packets (handoff messages defined above) carry this random value RMN together withtheir authentication information. The session key is used for authentication purposes. First, theGW router can quickly calculate the session key by combining the IP address and the randomvalue found in the control packet with the network secret. Second, the GW router can validatethe authentication easily with the session key. Third, the GW router performs the validationprocess without any further communication or pre-distributed subscription databases. Thiswould result in a fast and secure handoff. To enhance security, the network key could beperiodically replaced. Triggering session key change makes brute force attacks more difficult.

3.7. Capacity

To plan the deployment of a WIP network, the system architect must have a good estimate ofthe number of users (MNs) that are supported by each GW router. The number of MNs woulddepend on the type of application, traffic patterns, the network stability factor, the effect of thehidden terminals and the pattern off. The capacity of the GW router should be planned based onthe above factors. Note that capacity management is just one of the several factors in order tomaintain service quality.

4. GW PERFORMANCE ANALYSIS

While supporting all the functionalities described in the previous section, in planning to deploythe WIP GW, its capacity must be sized and scaled based on traffic patterns and volume tomaintain specified QoS levels. By using OPNET tools, we have developed the interactionbetween the IP and the MIP manager to reflect the required control capabilities needed in theGW-assisted IP mobility between different WLANs’ access points.

4.1. Effect of handoff on GW performance

The assumption is that the GW in this handoff will incorporate a fast routing table lookupalgorithm available today to mitigate the packet discriminating and forwarding task. Handoff isa critical attribute of wireless mobility management performance.

While supporting all the functionalities described in previous section, in planning to deploythe WIP GW, its performance must be analysed based on traffic patterns and volume tomaintain a specified QoS level. In this section, we have assessed the impact of HO on the GWthrough a queuing system model for the proposed BANC mobility scheme.

The way HO calls or messages are handled has a direct impact on QoS provided to wirelessmobile users. This problem is well known in the research community [22–31] as we have seen inSection 2. Since dropping calls in progress is less desirable than blocking new call, variousmethods have been devised to prioritize handoff calls over new calls. These issues are not uniqueto the proposed mobility management scheme. We will focus on the GW’s contribution to theBANC HO message processing delay. The GW is the main network control component of theproposed BANC mobility management scheme. To assess the impact of the BANC HO



DOI: 10.1002/dac

messaging on the overall delay, we leverage the M/G/1 queuing systems model described in [42].Wu et al. [42] implicitly assume that output of an M/G/1 system is also Poisson. In order tovalidate the model, we have compared the analytical results and the simulation results of theGW-contributed HO message processing delays shown in Table V.

4.2. System model

Handoff message processing is a critical aspect of BANC mobility management performance. InWIP, the HO process requires processing of the WIP HO messages by the GW. The HO delay istypically the time required for the processing and completion of the first four mandatorymessages in WIP HO messaging. The HO delay is calculated as dho ¼ dtr þ %TGW þ %Tbcp; wheredho is the total delay incurred due to handoff to different WLAN access points, dtr is the totaltransmission and propagation time, %TGW is the mean packet processing time at the GW, and%Tbcp is the mean packet processing time at the BCP.We focus on the contribution of the GW and BCP to the HO message processing delay, which

is related to the packet (HO messages) arrival rates. HO packets are a fixed portion of the totalserved packets.

Based on [42], our proposed GW-supported BANC system can be modelled as a single servicedelay system with generalized service time (M/G/1), where we would like to give high priority tothe HO packets in the GW. For the BCP, all packets are treated with the same priority. Thereare three types of packets flowing through the system GW: (i) HO-related packets; (ii) packetsthat are re-directed by the GW; and (iii) normal packets.

The packets entering the GW are classified into two levels of priorities: voice HO-relatedpackets have high priority; other packets, including data HO, have lower priority. We evaluatethe delay for both the non-preemptive and preemptive cases where we have only one class ofhigh priority (voice HO packet). As mentioned above, unlike the GW, the BCP treats packetswith the same priority as the first-come-first-served (FCFS) method.

Let us denote the arrival rate of the packets to the GW as l ¼ l1 þ l2 þ l3; where l1 is thearrival rate of (voice) HO-related packets, l2 is the arrival rate of packets to be redirected, andl3 is the arrival rate of other packets, including data HO. As mentioned above, TGW is the meanservice time for all the classes in the GW.

We will derive our system model queuing formulas based on [43]. The delay for priority 1customers (voice HO packet), at the GW, can be expressed as follows:

D1GW ¼Wp þ TGW ð1Þ

where Wp is the total waiting time in the queue for the tagged customer (voice HO packet) andTGW is the mean service time in the GW. Following [43], we can write Wp as follows:

Wp ¼W0 þXP

i¼p

TGWðNip þMipÞ ð2Þ

where W0 is the delay seen by our tagged customer due to packet in service when our taggedcustomer enters the GW. This delay will be equal to the serviced packet’s residual life of distributionthat depends upon the priority group to which the service packet belongs. From [43], we can write

W0 ¼XP

i¼p

liT2GW

2ð3Þ



DOI: 10.1002/dac

PPi¼p ðNipÞTGW is the second component of the delay, which is due to packets found in the

queue by our HO packet that receive service before HO packet does. Nip is the number ofpackets from group i found in the queue by the tagged packet (HO packet) (from group p) andthat receive service before the HO packet (tagged packet) does.PP

i¼p ðMipÞTGW is the third component of the queuing delay for the HO packet due to thehigher priority packet. Mip is the number of customers from i who arrive at the system while ourtagged HO packet (from group p) is in the queue and who receive service before the HO packetdoes.

Because of the strict order of queuing and under the assumption that customers within thesame priority group get served according to an FCFS rule, it is clear that Nip ¼ 0; wherei ¼ 1; 2; . . . ; p� 1 and Mip ¼ 0; where i ¼ 1; 2; . . . ; p:

All customers from group p and higher who are present in the queue upon our HO packet’sarrival must be served before our tagged packet (HO packet) does. From Little’s result [43] weknow that (on average) there will be liWi customers from the ith group present in the queuewhen our HO packet arrives, therefore, Nip = liWi , where i ¼ p; pþ 1; . . . ;P:

Similarly, all customers from groups pþ 1; pþ 2; . . . ;P who join the system while our HOpacket is in the queue will also be served before our tagged packet (HO packet) does. Since itspends (on average) Wp s in the queue and since each group’s arrival process is independent ofqueue size we know that there will (on average) be liWp customers arrivals from the ith groupwhile our tagged packet waits on queue. Therefore,

Mip ¼ liWi; i ¼ pþ 1; pþ 2; . . . ;P

Thus, for the non-preemptive HOL system Equation (2) becomes

Wp ¼W0 þXP

i¼p

TGWiliWi þ

XP

i¼pþ1

TGWiliWp ð4Þ

where p ¼ 1; 2; . . . ;P:According to Kleinrock [43], Equation (4) can be written as follows:

Wp ¼W0 þ

PPi¼pþ1 riWi

1�PP

i¼p rið5Þ

Solving the above equation recursively [43], we have

Wp ¼W0

ð1�PP

i¼p riÞð1�PP

i¼pþ1 riÞð6Þ

where p ¼ 1; 2; . . . ;P:In the above equation, we see the effect of customers of equal or higher priority present in the

queue when our HO packets arrives is given by the denominator term 1�PP

i¼p ri while theeffect of the customers of higher priority arriving during our HO packet’s queuing time is givenby the denominator term 1�

PPi¼pþ1 ri: Furthermore, we notice that Wp does not depend on

customers from lower priority groups (that is, for i ¼ 1; 2; . . . ; p� 1) except for theircontribution to the numerator term W0:

In our case, we have only one class, i.e. HO, and the HO packet has the highest priority.Therefore, the average waiting time for the voice HO packet (tagged packet) in HOL



DOI: 10.1002/dac

non-preemptive queuing discipline, Wp; can be written [43] as

Wp ¼W0

1� rpð7Þ

where rp is the GW server utilization of HO-related packets.Thus, for the HOL non-preemptive queuing scheme, the delay D1

GW of the HO messages (withpriority 1) at the GW can be expressed as

D1GW ¼

PPi¼p liT

2GW

2ð1� r1Þþ TGW ð8Þ

where r1 ¼ l1TGW; T2GW is the second moment of the GW service time, and T2

GW ¼ d2GW þ

ð %TGWÞ2 with d2GW the variance of the service time.

Since the mean service time at the GW, TGW; is the same (constant) for all classes (packets)and l ¼ l1 þ l2 þ l3; we can write D1

GW in (8), as

D1GW ¼

lT2GW

2ð1� l1TGWÞþ TGW ð9Þ

Similarly, based on the M/G/1 FCFS queuing scheme [43], the average waiting time W at theBCP will be

W ¼W0

ð1� rÞð10Þ

where r is the GW server utilization of HO-related packets the BCP.Thus, the delay of the HO messages at the BCP, can be expressed as

Dbc ¼laT2

bc

2ð1� rbcÞþ Tbc ð11Þ

where Tbc is the mean service time of the BCP, rbc ¼ laTbc is the BCP server utilization, with lathe arrival rate of packets to a BCP. Here, we assume la ¼ l=N as described in [42]. Since theGW connects to a number of BCPs, la is only a fraction of l:

Here T2bc is the second moment of the BCP service time, and T2

bc ¼ d2bc þ ðTbcÞ2; where d2bc is

the variance of the service time.The total delay of HO message processing in both the GW and the BCP would be

DTHO ¼ D1

GW þDbc ð12Þ

where Dbc ¼ Dbcu þDbcd ; Dbcu is the uplink delay and Dbcd is the downlink delay.Considering the different down link and up link processing delays in access points [44], the

total delay, DTHO; incurred in completing BANC scheme HO messages (Figure 4), with non-

preemptive scheme in GW and FCFS scheme in BCPs, can be expressed as

DTHO ¼ 3ðD1

GWÞ þ 2ðDbcu Þ þDbcd ð13Þ

Now, the total delay, DTHO; for the BANC HO messages (Figure 4) for the HOL non-

preemptive queuing scheme (two nodes, GW and BCP) can be written as

DTHO ¼

3ðlT2GWÞ

2ð1� l1TGWÞþ 3TGW þ

ðlaT2bcuÞ

ð1� la2TbcuÞþ 2Tbcu þ

laT2bcd

2ð1� laTbcdÞþ Tbcd ð14Þ



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For the preemptive-resume scheme, based on [43, Equation 3.39], we have the followingexpression for the highest priority packet delay at the GW:

DpGW ¼

lpT2p

2ð1� rpÞþ Tp ð15Þ

In our case, we can write the HO message delay of (voice) with highest priority at the GW,D1

GW; with the preemptive scheme as

D1GW ¼

3ðl1T2GWÞ

2ð1� r1Þþ 3TGW ð16Þ

So the total delay, DTHO (i.e. delay in GW and in BCP), for the preemptive queuing scheme will be

DTHO ¼

3ðl1T2GWÞ

2ð1� l1TGWÞþ 3TGW þ

ðlaT2bcuÞ

ð1� laTbcuÞþ 2Tbcu þ

laT2bcd

2ð1� laTbcdÞþ Tbcd ð17Þ

We will evaluate the numerical results, based on Equations (14) and (17), in the context of ourBANC system.

4.3. Numerical results

As shown in Equation (14), the HO message processing delay is related to the packet arrivalrate, which is the portion of the packets out of the total served packets and service time of bothGW and BCP. We assume that the packet arrival rate to a BCP is la ¼ l=N if a GW connects toN number of BCPs. Denoting the portion of the HO-related packets of the total packets servedby the GW by a, we have l1 ¼ al as the HO traffic is a small fraction of the total GW traffic.From the simulation test bed, the mean service time at the GW is 10 ms: Based on [44], thedownlink and uplink service times of BCP can be estimated as 1087 and 395 ms for our analysis.Table IV shows the parameters used for the performance analysis.

Figure 5 shows the delay of the HO-related packets, as various portions of the total packetsserved by the GW. The results show that the delay of HO-related messages as different portionsof the total served packets is almost the same. The reason is that the processing delay at the GWis negligible compared with the processing delay in the BCP. The total delay is dominated by theservice delay of BCPs, which can also be observed from Equation (14) and Table IV. Since theHO messages are generated when the mobile users move to the new WLAN areas, the resultsimplicitly show that the performance of the proposed mobility management scheme is not

Table IV. Input parameters in numerical analysis.

Parameters Value

TGW 10 ms

TGW2 100 ms

Tbcpup 395 ms

Tbcpdown 1087 ms

T2bcpup 156425 ms2

T2bcpdown 118517 ms2



DOI: 10.1002/dac

severely affected by the user mobility; a high-performance GW is capable of supporting aconsiderable number of such users.

Figure 6 shows the comparison of the non-preemptive vs preemptive-resume queuingdisciplines for the proposed mobility scheme. As seen from this figure, the delay in thepreemptive-resume scheme is smaller. For the preemptive-resume queuing scheme, we have seenin [43, 45] and in Equation (17) that the customer with the highest priority (a class 1 customer)will be completely unaffected by customers of other classes and more generally, class j customersare only affected by those in classes 1 through j: In this case, we have only one high-priority class(HO messages). So, in the preemptive-resume scheme, arriving (highest priority) HO packets donot have to wait for the packet that is in service to be finished, i.e. there is no queuing delay forthe HO packets due to other lower priority packets. The other lower priority packets will bebumped from the service and put back in the queue. Thus, the HO packets delay is significantlysmaller in the preemptive case than in any non-preemptive scheme. This is good for the HOtransactions with improved response time but may not be good for the time-sensitive services,such as VoIP, as the other lower priority packets will experience larger delay. This is the penaltyto be paid in the preemptive queue scheme. This is a classical issue and more theoretical detailscan be found in [22, 43].

Table V shows delay comparison of HO messages in WIP GW for analytical vs simulationresults of non-preemptive-resume scheme. The simulation model is described in Section 5.

Figure 6 shows numerical results for the delay in the non-preemptive-resume scheme. We seethat a break-point is reached at 900 packets/s. From Table V, we see that the simulation andanalytical results have negligible differences: below 2% at the beginning with small traffic(packets) and as the received traffic increases the differences become narrower, below 1%. In

2 4 6 8

2

3

4

5

6

Del

ay o

f Pr

oces

sing

HO

Mes

sage

s (m

sec)

Packet Arrival Rate (packets/sec)

N=10

a=0.1

a=0.2

a=0.3

a=0.4

Figure 5. Handoff message processing delay (GW and BCP together) vs packet arrival rate ðN ¼ 10Þ:



DOI: 10.1002/dac

both cases, the break-point occurs almost at the same time (at 900 packets/s). Both simulationand analytical results get closer with the increase of packet numbers and this validates theaccuracy of the analytical model used in evaluating the GW-contributed HO message processingdelay in large network.

As shown in Equation (14), priority class p customers are affected directly by higher prioritycustomers because of the terms in the denominator, and indirectly by customers of all classesbecause of the numerator. Adding a new priority class of lowest priority (in our case priority 2for data HO) to a preemptive-resume system will have no effects on the performance statisticsfor any of the existing priority classes (voice HO packets that have priority 1), but can have aprofound effect on the performance of all existing classes in a non-preemptive priority system.

Our BANC mobility management scheme could leverage some of the promising priorityqueuing work, including multi-level priority scheduling in [37], the SPPQ discipline [36, 38] and

200 400 600 800 1000

2.5

5

7.5

10

12.5

15

Del

ay o

f Pr

oces

sing

HO

Mes

sage

s (m

sec)

Packet Arrival Rate (packets/sec)

N=10

a=0.1

Non preemptive

Preemptive resume

Figure 6. Handoff message processing delay comparison at GW only for voice with priority 1 (one priorityclass): non-preemptive vs preemptive-resume.

Table V. HO messages processing delay in GW of analytical results vs simulation results.

Packets/s Analytical (ms) Simulation (ms) Difference (%)

100 0.046 0.045 1.902200 0.067 0.066 1.315300 0.094 0.093 1.060400 0.130 0.128 1.044500 0.180 0.178 0.993600 0.255 0.254 0.348700 0.380 0.379 0.233800 0.630 0.629 0.140900 1.380 1.379 0.064



DOI: 10.1002/dac

dynamic priority queuing [22] recommended for the cellular network HO to further improve theHO QoS, such as lower blocking probabilities.

4.4. Effect of IP routing lookup time on GW performance

With this network arrangement, the GW server could be a bottleneck of the system. One of themajor functions of the GW is packet forwarding. This is basically a routing table lookup basedon the IP destination field in the packet header and identification of the next hop to which theincoming packet should be sent. A GW router that supports the described IP mobility schememust have an efficient routing table lookup to maintain QoS. The implementation of the WIPinter-WIP mobility scheme is assumed to be with a managed IP network with broadband access,where packets coming from a correspondent node to a MN are routed through the GW thatknows the current location of the MN. Two issues here with the GW server: route lookup andmemory storage. We assume that with an advanced fast route lookup and compressiontechnique, which are beyond the scope of this paper, the location update and packet forwardingtasks of the GW could be mitigated. Packet lookups in a GW can be expedited by variousapproaches, generally classified as software based or hardware based [46, 47]. Tzeng [47]describes a technique for fast route lookup that could be used in a WIP GW for betterperformance. In the proposed mobility scheme, the WIP GW supports mobility at layer 3 packetprocessing level. An efficient route lookup algorithm to forward the packets in the GW wouldsignificantly improve the processing time. This fast route lookup impact has been demonstratedthrough simulation in Section 5.2. As mentioned earlier, this GW will be off a high-speedbackbone network. Our study on backbone routers’ routing table trace analysis shows thatbackbone routers have ‘default-free’ routing tables, i.e. they are supposed to recognize all theincoming packets with various IP destination addresses [46], so they do not need to use defaultrouting for incoming data packets. Ji and Srinivasan [48] present a fast route lookup algorithmand an efficient update algorithm that supports incremental route updates. Here, a configurableprocessor is used to achieve fast IP route lookup (66 million lookups per second (MLPS)) for theprocessor at 200 MHz: Depending on the traffic size, a service provider needs to assess the givenMLPS capability.

In WIP architecture [3], all the incoming packets are routed through the GW to facilitatemobility management. Packet forwarding requires routing table lookup based on the IPdestination field in the IP header. Therefore, one of the key issues in GW performance is the IProuting lookup mechanism used for the ferrying of a large number of incoming IP packets toMN via respective links of the GW. The GW uses the destination IP address encoded in theincoming packets to look up the next hop node to which the packets have to be forwarded. Theproblem involves two steps: search the routing database to obtain the longest matching prefixfrom all the possible prefixes that match the particular destination IP address and header toretrieve the next hop port for the longest matched prefix.

5. SIMULATION EXPERIMENTS AND RESULTS

Figure 7 shows the network topology that we have used in our simulation to investigate thebasic characteristics of the proposed WIP architecture HO mechanism and GW’s role in overallperformance. For the CIP, we have modelled the network topology shown in Figure 1(b) in



DOI: 10.1002/dac

Section 2. Inter-WLAN mobile simulation network consists of access points (configurable up to50 access points) and 11 MNs roaming between the access points to measure WIP data loss andGW-contributed HO message processing delay.

Parameters in Table IV (Section 4) are used in the simulation for GW-contributed HOmessage delay. The GW is configured with multiple queues for each type of packet. Queues areserved using a priority queuing mechanism. The priority queuing mechanism differentiatesbetween queues according to its priority. The GW-contributed HO delay priority queuingsimulation results are compared with the analytical results in Table V of Section 4. Table VIshows the network’s links values used in the simulation.

WLAN is configured as IEEE 802.11, with 11 Mbps data rate and no RTS/CTS orfragmentation (Table VI). Each WLAN radio coverage is set to 250 m that ensures non-overlapping radio coverage of separate BCPs (eventually requiring a sort of hard HO uponcrossing the coverage boundaries). The mobility pattern of the MN is characterized by ahorizontal linear path with pedestrian speed of 1 m=s; the speed has been varied from 1 to 8 m=swhen needed to observe the impact of the moving speed on various performance measures. The8 m=s moving speed implies that the MN moves faster than typical pedestrians but also slowerthan typical passenger vehicles in a metropolitan area. Consequently, this choice of mobilitypattern would result in moderate HO rates.

MN1

GW

IP

BCP1 BCPx

MNx

….

CN

Tunneling with MIPv6

WIP directrouting

Figure 7. Simulation platform.

Table VI. Link values.

Links (mbps)

Wired: CN-IP Router 100Wired: P Router-HA 1.5Wired: IP Router-FA 1.5Wireless: 802.11 11



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The application traffic exchanged between the CN and MN is configured to represent IP andTCP/IP. We have used the IEEE 802.11 MIP reference model provided by the OPNET version11.0. Our simulation model is built on this OPNET provided standard MIP roaming model andis customized accordingly. More specifically, we have developed the interaction between the IPand the MIP manager to reflect the required control capabilities needed in the central networkserver assisted IP mobility between multiple WLAN (BCPs). We have utilized 802.11b WLANinterface with roaming capability to simulate hand-offs between MIP agents who are alsoWLAN access points. We have modified the OPNET 11.0 MIPv6 process model code tocalculate the layer 3 delay for WIP (control messages in Figure 4).

In our simulation test bed, each experiment had 50 runs with 1000 simulation minutesduration. The parameters we examined in our experiments to observe QoS performance of thetwo alternatives are:

* Packet size.* Interval rate.* MN speed.* Application type.

In the following sections, we have evaluated data loss for WIP, MIP and CIP. We thenevaluated the impact of routing table lookup time on GW’s delay. We also have evaluated theimpact of MN traffic load, server packet forwarding rate and the datagram processing schemes,such as central vs slot-based processing, on the CPU utilization to assist the WIP serviceprovider’s capacity and traffic planning strategy.

5.1. Data loss comparison: WIP MIP CIP

Figure 8 shows a WIP vs MIPv6 data loss comparison during handoff. Handoff delay is theinter-arrival time of two consecutive packets before and after handoff. When packet loss occurs,the packet delay is meaningless. The data loss in WIP is smaller than that of MIPv6. This isbecause the incoming data packets in MIPv6 are tunnelled until the binding is complete with thecorrespondent node during handoff. The mobility management scheme in WIP is providedthrough the central GW intelligence. The handoff phase in WIP is smaller than that in MIPv6.The larger the mobility handoff time, the higher the data loss.

Figure 9 shows a data loss comparison between CIP semi-soft handoff and WIP handoff. Wesee from Figure 9 that CIP has more data loss than WIP. As mentioned earlier, CIP uses MIPfor macro-mobility and we see that MIP itself suffers from triangular routing (tunnelling)causing data loss until the handoff is complete. Using CIP/MIP simultaneously is inefficient andredundant, especially for mobility management since CIP uses two IP packets (ICMP forregistration and IP for ack) and MIP uses four UDP/IP packets. In addition to this data lossduring macro-mobility, CIP suffers from data loss during its own semi-soft handoff phase thatadds an additional procedure, before performing the actual handoff. In this CIP semi-softhandoff method, a semi-soft handoff creates the routing cache mapping associated with the newBS by sending a semi-soft packet before handoff in order to reduce handoff latency. This packetitself introduces another delay which causes additional data loss. Table VII shows outputstatistics of WIP vs MIPv6 vs CIP data loss comparison.



DOI: 10.1002/dac

Figure 8. Data loss comparison: WIP vs MIPv6.

Figure 9. Data loss comparison: WIP vs CIP.



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5.2. Gateway performance improvement with routing table lookup and data forwarding speed

To assess the impact of routing table lookup time on a GW’s delay, we have carried outsimulation experiments with 2 million lookups per second (MLPS) first, and then 4 MLPS.Figure 10 shows the delay of TCP traffic where the GW was configured with 2 MLPS for twocases: (a) with load balancing and (b) without load balancing. Figure 11 shows the delay of TCPtraffic where the GW was configured with 4 MLPS for the similar two cases as in the previousscenarios: (a) with load balancing and (b) without load balancing. The higher the MLPS, thebetter the performance. In Figure 10, we observe larger TCP delay compared to TCP delay inFigure 11. Tables VIII and IX show the simulation output statistics for the 2 MLPS and 4MLPS comparisons, respectively.

With the increase of HO frequency, the processing task is increased, as seen from Equation (1).If the GW’s capacity is not scaled accordingly or the traffic is not treated with some other means

Table VII. Simulation output statistics of data loss: WIP vs CIP vs MIPv6.

Confidence interval

Scheme Mean (bps) Std dev. (bps) 90% 95% 99%

WIP 110 34 104–115 103–117 101–119MIPv6 132 32 127–138 126–139 124–141CIP 1524 530 1436–1612 1419–1628 1386–1661

Figure 10. GW delay with 2 MLPS route lookup time.



DOI: 10.1002/dac

accordingly (e.g. as intelligent routing during node or link congestion or load balancing), theapplied traffic’s QoS gets degraded. There are many ways that a service provider can choose tomaintain or improve the level of QoS: for example, by increasing the processing speed ormemory size, or by shifting the load, or using intelligent routing when the capacity of the systemreaches a specified threshold. In the real world, a service provider could do detailed traffic

Figure 11. GW delay with 4 MLPS route lookup time.

Table VIII. Simulation output statistics of 2 MLPS: load balancing vs w/o load balancing.

Confidence interval

Scheme Mean (ms) Std dev. (ms) 90% 95% 99%

W/o load balance 0.079 0.0007 0.078836–0.079116 0.078805–0.079199 0.078740–0.079255Load balance 0.076 0.00071 0.074342–0.077657 0.074031–0.077968 0.073410–0.078586

Table IX. Simulation output statistics of 4 MLPS: load balancing vs w/o load balancing.

Confidence interval

Scheme Mean(ms)

Std dev.(ms) 90% 95% 99%

W/o load balance 0.021 0.00003 0.020992–0.021007 0.020991–0.021008 0.020990–0.021010Load balance 0.020 0.00001 0.0199976–0.0200023 0.0199923–0.0200028 0.0199913–0.0200036



DOI: 10.1002/dac

engineering to maintain high QoS in order to be competitive, or adapt other techniques such aspriority handoff queuing and load sharing. Proper QoS is the most important step in ensuringsuccess of the service. Both Figures 10 and 11 show the effect of load balancing. With a loadbalancing which demands additional resources, such as a load balancing server and secondarymain GW server where the primary server will not have to operate at its full capacity, the latencyimproves.

Packet forwarding speed plays an important role in the server’s utilization and the over-allcapacity. Table X has the output statistics of the packet forwarding impact experiment. Figure12 shows the impact of packet forwarding scheme with 2500 and 5000 packets/s on the serverutilization. The higher the forwarding speed, the higher the CPU power, the higher thethroughput.

Table XI has the output statistics of the packet processing scheme impact experiment in WIP.

Table X. Simulation output statistics of packet forwarding speed impact.

Confidence interval

Packet forwarding speed Mean Std dev. 90% 95% 99%

2500 packets/s 19.4 7.4 17.6729–21.1270 17.3485–21.4514 16.7038–22.09625000 packets/s 2.7 0.9 2.4899–2.9100 2.4504–2.9495 2.3720–3.02798000 packets/s 1.2 0.39 1.1089–1.2918 1.0918–1.3081 1.0579–1.3420

Figure 12. Datagram forwarding speed impact on CPU utilization.



DOI: 10.1002/dac

Figure 13 shows the impact of slot-based vs central processing impact on server CPU. Toobtain a defined CPU utilization, a service provider can plan for slot-based for high-volumetraffic with QoS. The server processing scheme attribute controls the number of servers(processors) and queues used by IP for packet forwarding. In ‘central processing’, a single serverwith a single queue is used to process all packets.

In ‘slot-based processing’, in addition to a single central server (with its own queue) Nadditional servers}each with a queue}are used to process packets. N is determined by thenumber of slots that have been configured. The ‘slot info’ model attribute controls the numberof slots used by an IP process instance. With multiple servers ‘slot-based processing; will delivergreater packet throughput. The ‘slot-based processing’ scheme would be preferred for betterQoS for high traffic. But it comes with extra cost compared to the ‘central processing’ scheme.The service provider can choose the appropriate scheme based on their capacity and trafficvolume to be supported.

Figure 13. Packet processing scheme impact on CPU utilization.

Table XI. Simulation output statistics of packet processing scheme impact.

Confidence interval

Processing scheme Mean % Std dev. 90% 95% 99%

Central 1.7 0.9 1.48995–1.91004 1.45044–1.94950 1.37208–2.02792Slot-based 0.03 0.003 0.02929–0.03070 0.02916–0.03083 0.02891–0.03109



DOI: 10.1002/dac

6. CONCLUSION

This paper, after providing a comparison of WIP with CIP and MIP, identifies the mainrequirements for a broadband-accessed network-based gateway (GW) that supports WLAN IPmobility. It then evaluates GW-contributed handoff message processing delay in the WIPBANC architecture through an analytical system model and an OPNET simulation model. Italso provides a comparison of the GW-contributed handoff message processing delays for non-preemptive vs preemptive queuing schemes. Both analytical and simulation results show thatWIP handoff message processing delay at GW has little, negligible impact on the overall systemdelay. The proper level of defined QoS could be maintained at the GW by priority treatment ofthe handoff load and proper sizing of the GW. Another key issue in WIP GW performance isthe IP route lookup mechanism for ferrying large numbers of incoming communication packetsto respective outgoing links. Through our simulation study we observed that by speeding theGW routing table lookup time, GW processing delay could be reduced significantly. Finally, interms of mobility management, our OPNET simulation results show that WIP has betterperformance than MIP and CIP. The GW described in this paper could be used for inter-WLAN IP mobility employing wired broadband such as DSL or cable and wireless broadbandsuch as WiMAX compliant broadband wireless loop, supporting hot spots and the residentialmarket.

REFERENCES

1. Rahman M, Harmantzis F. Broadband-facilitated inter-WLAN mobility architecture. International Journal ofWireless Information Networks 2006; 13(3):173–192.

2. Bertin P, Guillouard K, Rault J-C. IP based network controlled handover management in WLAN access networks.Communications 2004; 7(1):3906–3910.

3. Rahman M, Harmantzis F. IP mobility with high speed access and network intelligence. IEEE WirelessCommunications and Networking Conference, WCNC, Atlanta, GA, 21 March 2004.

4. Rahman M, Harmantzis F. IEEE 802.11 inter-WLAN mobility control withband supported distribution systemintegrating WLAN and WAN. Eleventh International Conference on Telecommunication, Ceara, Brazil, 1 August2004.

5. Rahman M, Harmantzis F. Low-latency handoff inter-WLAN IP mobility with broadband network control.Computer Communications 2007, in press.

6. Perkin CE. Mobile IP. Communications Magazine IEEE 2002; 40(5):66–82.7. Kim Ha, Won H, Sung J, Kim J. Adaptive location management scheme for mobile IP. Pervasive Computing and

Communications Workshops, PerComWorkshops. Third IEEE International Conference, 8–12 March 2005; 263–267.8. Koodli R. Fast Handover for Mobile IPv6. IETF Internet Draft, draft-ietf-mishop-fsat-mipv6-01.txt, February 2004.9. Perkins CE. IP mobility support. RFC 3220, January 2002.10. Perkins C, Johnson C. Mobility support in IPv6. IETF Internet draft, draft-ietf-mobileip-optim-24.txt, June 2003.11. Saha D, Mukherjee A, Misra IS, Chakraborty M, Subhash N. Mobility support in IP: a survey of related protocols.

Network, IEEE 2004; 18(6):34–40.12. Soliman H et al. Hierarchical mobile IPv6 mobility management (HMIPv6). IETF Internet draft draft-ietf-mipshop-

hmipv6-01.txt, February 2004.13. Cambell AT, Kim S. Cellular IP. IETF Mobile IP Working Group, 1999.14. Valko AG. Cellular IP: a new approach to Internet host mobility. ACM SIGCOMM Computer Communications

1999; 29(1):45–54.15. Edwards G, Suryakumar N. Cellular IP performance. IEEE Wireless Communications and Networking (WCNC)

2003; 3:2081–2085.16. Lee J, Kim Y, Lee HS. Fast route recovery methods for cellular IP access networks. IEEE 61st Vehicular Technology

Conference, VTC, vol. 4, May 2005; 2580–2584.17. Masajedian SMS. A novel approach to track mobile hosts in cellular networks. Fifth IEEE International Conference

on 3G Mobile Communications Technologies, vol. 1, 2004; 594–595.



DOI: 10.1002/dac

18. Wu ZD. Optimal method for using multiple gateways in cellular IP networks. 12th IEEE International Conference,ICON, vol. 1, November 2004; 184–190.

19. Lee J. Advanced semisoft handoff method of cellular IP access networks. IEEE International Conference onAdvanced Information Networking, AINA, vol. 2(1), June 2004; 407–412.

20. Maleej L, Farikha M. Handovers in micro-mobility for the WLANs. IEEE International Conference on IndustrialTechnology, ICIT, vol. 1, December 2004; 267–271.

21. Carli M, Neri A, Pecci AR. Mobile IP and Cellular IP integration for inter access network handoff. IEEEInternational Conference on Communications, vol. 8(1), June 2001; 2467–2471.

22. Xhafa AE, Tonguz OK. Dynamic priority queueing of handover calls in wireless networks: an analytical framework.IEEE Journal on Selected Areas in Communications 2004; 22(5):904–916.

23. Pollini GP. Trends in handover design. IEEE Communications Magazine 1996; 34:82–90.24. Katzela I, Nahgshineh M. Channel assignment schemes for cellular mobile telecommunication systems: a

comprehensive survey. IEEE Personal Communications Magazine 1996; 1:10–31.25. Jabbari B. Teletraffic aspects of evolving and next-generation wireless communications networks. IEEE Personal

Communications Magazine 1996; 1:4–9.26. Tripatti N, Reed H, VanLandighan. Handoff in cellular systems. IEEE Personal Communications Magazine 1998;

1:26–37.27. Guerin RA. Queueing-blocking system with two arrival streams and guard channels. IEEE Transactions on

Communications 1998; 36:153–163.28. Hong D, Rappaport SS. Traffic model and performance analysis for cellular mobile radio telephone systems with

prioritized and nonprioritized handoff procedures. IEEE Transactions on Vehicular Technology 1986; VT-35:72–92.29. Heffes H, Ryan KM. User mobility and channel holding time in mobile communications. IEEE 47th Vehicular

Technology Conference, vol. 2, May 1997; 577–581.30. Chang C-J, Su T-T, Chiang Y-Y. Analysis of cutoff priority radio system with finite queueing and reneging/

dropping. IEEE/ACM Transactions on Networking 1994; 2(1):166–175.31. Tekinay S, Jabbari B. A measurement-based prioritization scheme for handovers in mobile cellular networks. IEEE

Journal on Selected Areas in Communications 1992; 10(1):1343–1350.32. Posner EC, Guerin R. Traffic policies in cellular radio that minimize blocking of handoff calls. Eleventh Teletraffic

Congress, Kyoto, Japan, vol. 1, 1985; 2.4B-2-1–2.4B-2-5.33. Daigle JN, Jain N. A queueing system with two arrival streams and reserved servers with application to cellular

telephones. IEEE INFOCOM 1992; 1:2161–2167.34. Misic J, Tam YB. Adaptive admission control in wireless networks under non-uniform traffic conditions. IEEE

Journal of Selected Areas in Communications}Wireless Series 2000; 18(11):2429–2442.35. Misic J, Tam YB. Non-uniform traffic issues in DCA wireless multimedia networks. ACM/Kluwer Wireless

Networks Journal (WINET) 2003; 9(6):605–622.36. Ebersman HG, Tonguz OK. Handoff ordering using signal prediction priority queueing in personal communication

system. IEEE Transactions on Vehicular Technology 1998; 48:20–35.37. Madani M, Light J. Multi-level queue scheduling algorithm for critical packet loss Elimination during handoff.

Proceedings of the Third Annual Communication Networks and Services Research Conference, 2005.38. Fantacci R. Performance evaluation of prioritized handoff schemes in mobile cellular networks. IEEE Transactions

on Vehicular Technology 2000; 1:485–493.39. HeeYoung J, SeokJoo K. Fast handover support in hierarchical mobile IPv6. Advanced Communication Technology,

The Sixth International Conference, vol. 2(1), 2004; 551–554.40. Jaiswal S, Nandi S. Simulation-based performance comparison of TCP-variants over mobile IPv6-based mobility

management schemes. 29th Annual IEEE International Conference on Local Computer Networks, vol. 1(1), 2004;284–291.

41. Chou C-T, Shin KG. An enhanced inter-access point protocol for uniform intra and inter subnet handoffs. IEEETransactions on Mobile Computing 2005; 4(4):321–334.

42. Wu W, Banerjee N, Basu K, Das SK. Network IP mobility support in wireless LANs. Second IEEE InternationalSymposium on Network Computing and Applications, vol. 1(1), 2003; 257–264.

43. Kleinrock L. Computer applications. Queueing System, vol. 2. Wiley.44. Khatib IA, Maguire GQ, Ayani R, Forsgren D. Wireless LAN access points as queueing systems: performance

analysis and service time. Sigmobile’s Mobile Computing and Communications Review Publication, 2002.45. Allen AO. Probability, Statistics, and Queueing Theory with Computer Science Applications (2nd edn). Academic

Press: New York, 326–329.46. Sangireddy R, Futamura N, Aluru S, Somani AK. Scalable, memory efficient, high-speed IP lookup algorithms.

IEEE/ACM Transactions on Networking 2005; 3(4):802–812.47. Tzeng N-F. SPAL: a speedy packet lookup technique for high performance routers. International Conference on

Parallel Processing, 2004.48. Ji HM, Srinivasan R. Fast IP routing lookup with configurable processor and compressed routing table. IEEE

Global Telecommunications Conference, GLOBECOM, vol. 4, November 2001; 2373–2377.



DOI: 10.1002/dac

AUTHORS’ BIOGRAPHIES

Moshiur Rahman received his MS in Electrical and Computer Engineering fromWayne State University, MI, in 1985. He joined AT&T Bell Labs, Napperville IL, in1987, at the switching division. He is currently a Senior Technical Member in AT&TLabs, Middletown, NJ, at the voice and data services division. He is also pursuing hisPhD degree at Stevens Institute of Technology, Hoboken, NJ, in Telecommunica-tions Management. His research areas of interest include mobility management forvoice and data, WLAN, broadband and signaling protocols.

Dr Fotios C. Harmantzis was born in Greece, where he completed his bachelor’s andmaster’s degrees in Computer Science at the University of Crete, in 1995 and 1997,respectively. He received the MSE degree from the University of Pennsylvania,Philadelphia, in 1998, focusing on operations research. In September 1998, he joinedthe Communications Research Group at the University of Toronto, as a researchfellow and instructor. He worked on stochastic modelling, pricing and simulation incomputer networks, towards his PhD degree in Electrical and Computer Engineering(graduated in November 2002). During his academic studies, Dr Harmantzis held theConnaught Scholarship and the Ontario Graduate Scholarship at the University ofToronto, a Teaching Fellowship at the University of Pennsylvania and a ResearchFellowship from the Foundation for Research and Technology-Hellas, for out-standing academic performance and research contributions. His research activities

are in mathematics and economics of computer networks and systems, as well as in mathematics of financeand risk. His teaching interests include computer networks, probability, stochastic processes andsimulation modelling and analysis. Since September 2002, he is with the School of TechnologyManagement, Stevens Institute of Technology, as an Assistant Professor.



DOI: 10.1002/dac

gateway performance for network-controlled wlan ip mobility

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