1208 ieee journal on selected areas in …vleung/journal_papers/jsac/yu0997.pdf · wide handoffs,...

1208 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 15, NO. 7, SEPTEMBER 1997

Adaptive Resource Allocation for Prioritized CallAdmission over an ATM-Based Wireless PCN

Oliver T. W. Yu and Victor C. M. Leung,Member, IEEE

Abstract— In future personal communications networks(PCN’s) supporting network-wide handoffs, new and handoffrequests will compete for connection resources in both themobile and backbone networks. Forced call terminations dueto handoff call blocking are generally more objectionable thannew call blocking. The previously proposed guard channelscheme for radio channel allocation in cellular networks reduceshandoff call blocking probability substantially at the expenseof slight increases in new call blocking probability by givingresource access priority to handoff calls over new calls in calladmission control. While the effectiveness of a fixed number ofguard channels has been demonstrated under stationary trafficconditions, with nonstationary call arrival rates in a practicalsystem, the achieved handoff call blocking probability maydeviate significantly from the desired objective. We proposea novel dynamic guard channel scheme which adapts thenumber of guard channels in each cell according to the currentestimate of the handoff call arrival rate derived from the currentnumber of ongoing calls in neighboring cells and the mobilitypattern, so as to keep the handoff call blocking probabilityclose to the targeted objective while constraining the new callblocking probability to be below a given level. The proposedscheme is applicable to channel allocation over cellular mobilenetworks, and is extended to bandwidth allocation over thebackbone network to enable a unified approach to prioritizedcall admission control over the ATM-based PCN.

Index Terms—Adaptive bandwidth resource management, dy-namic guard channel scheme, mobile virtual circuit, personalcommunication networks, wireless ATM.

I. INTRODUCTION

FUTURE personal communications networks (PCN’s) willlikely employ ATM-based backbone networks to intercon-

nect cellular mobile networks [1]–[3]. To support network-wide handoffs, new and handoff call requests will competefor connection resources in both the mobile and backbonenetworks. Handoff calls require a higher congestion relatedperformance, i.e., blocking probability, relative to new callsbecause forced terminations of ongoing calls due to hand-off call blocking are generally more objectionable then newcall blocking from the subscriber’s perspective. When theresource capacity is shared among the new and handoff callsfor better resource utilization, differential congestion-relatedperformances can be supported by a resource access priorityscheme [4]. The access priority can be established via differen-

Manuscript received September 1, 1996; revised April 1, 1997. This workwas supported by a grant from the Canadian Institute for TelecommunicationsResearch under the NCE Program of the Canadian Government. This paper isbased in part on a paper presented at IEEE VTC’96, Atlanta, GA, May 1996.

The authors are with the Department of Electrical Engineering, Universityof British Columbia, Vancouver, B.C., V6T 1Z4 Canada.

Publisher Item Identifier S 0733-8716(97)05858-7.

tial treatments of new and handoff calls by one or more of thefollowing methods: resource capacity limit, congestion controldiscipline, and admission criterion [5]. In the mobile networks,one common bandwidth resource access priority scheme is theguard channel (or bandwidth) scheme [6]–[9] which gives ahigher access priority to handoff calls by assigning them ahigher capacity limit. Resource access priority can also beestablished via differential congestion controls. For example,servicing handoff calls and new calls with blocked-calls-queued and blocked-calls-dropped disciplines, respectively,would further enhance access priority for handoff calls, or anexplicit queueing priority can be established between differentcall classes [9].

These schemes are developed under the assumption ofstationary call arrivals or loading condition, and may notbe able to adapt to nonstationary loading while meetingthe congestion-related performance objectives. Future PCN’swill employ microcells and picocells to support a highercapacity [10], thus increasing the frequency of call handoffsand dramatizing the effects of nonstationary traffic condi-tions due to fluctuations in new call arrivals and mobilitypattern, and the achieved handoff call blocking probabilitymay deviate significantly from the targeted objective. Tominimize subscriber dissatisfaction, the requirement to keepthe handoff call blocking probability at a targeted objectivein the mobile and backbone networks under nonstationaryloading condition is becoming even more important for futurePCN’s. In [5], a new call admission control takes into accountthe projected future handoff call blocking probabilities in theoriginating and the neighboring cells, which are kept at thetarget objective by blocking new calls even if capacity iscurrently available to service these new calls. A theoreticalanalysis was presented for a simple three-cell configurationunder stationary loading. Similarly, this paper proposes a noveldynamic guard bandwidth or channel scheme which adaptsthe guard bandwidth in each cell according to the number andmobility patterns of current active mobile terminals (MT’s)in the neighboring cells so as to keep the current handoffcall blocking probability within the targeted objective. But incontrast to [5], we demonstrate the effectiveness of our methodby simulations using a more complex and realistic multicellconfiguration, and we present performance evaluations underboth stationary and nonstationary loadings. In the dynamicguard bandwidth scheme, a change in channel assignmentstatus in any cell will trigger neighboring cells to performpreventive congestion controls for handoff calls by increasingor decreasing the capacity limits for handoff calls. By changing

0733–8716/97$10.00 1997 IEEE

YU AND LEUNG: RESOURCE ALLOCATION FOR PRIORITIZED CALL ADMISSION 1209

the parameters of the adaptation control policy, the targetedobjective can be maintained strictly, or loosely by ensuringthat new calls are not denied resource access completely.

The problem of maintaining differential congestion-relatedperformances associated with setting up new and handoffcalls in the backbone network has received little attentionin the literature. Under nonstationary loading conditions, thisproblem is compounded by the need to estimate the in-stantaneous handoff call arrival rate for adaptation purposes.While handoffs into a radio cell can only originate from theneighboring cells, handoffs to a wired link in the backbonenetwork are dependent on network topology and call routing.Estimation of instantaneous rerouting call arrival rate for eachnetwork link becomes a difficult proposition. The problemis eased considerably in ATM-based PCN’s by employingthe mobile virtual circuit (MVC) per-call connection treearchitecture [3] to support handoff processing. The MVCof an MT links the base stations (BS’s) of the current andneighboring (potential handoff) cells to a common root nodein the network. Consequently, the number of MT’s that caninitiate handoff calls to a network link is determined by thenumber of potential handoff connections of the MVC’s passingthrough it.

The paper is organized as follows. Section II presents thedynamic guard bandwidth scheme. Section III discuss theextension of the proposed scheme from the cellular mobile net-work to the ATM-based backbone network. Section IV com-pares the performance of the proposed dynamic guard band-width scheme to that of the fixed guard bandwidth scheme un-der stationary and nonstationary loading conditions. Section Vconcludes the paper.

II. M ODEL FOR DYNAMIC GUARD BANDWIDTH SCHEME

The fixed guard bandwidth scheme is a prioritized resourceaccess scheme which allows new calls and handoff callsto share capacity, while giving resource access priority tohandoff calls by assigning a larger resource capacity limitto handoff calls than new calls, with the guard bandwidthbeing the difference between the capacity limits. Thus, calladmission control is based on the current bandwidth usageand respective assigned capacity limit of each call class.Although in this paper we only consider congestion controlbased on the blocked-calls-dropped discipline, the resourceaccess priority for handoff calls may be further increased byemploying the blocked-calls-queued discipline. For a givenstationary call arrival rate, the targeted long-term congestionrelated performance (i.e., blocking probability) for handoffcalls can be satisfied by choosing an appropriate value offixed guard bandwidth.

In general, given the total resource (channels or bandwidth)that may be allocated to the new and handoff calls, blockingoccurs during call admission control when the call requiresbandwidth over the radio channel in a cell or the links traversedover the backbone network in excess of what is available.In the ATM-based backbone, admitting the call under suchconditions would degrade the connection performance of allcalls sharing the capacity with the setup call. Without prior-

itized resource allocation, handoff and new calls would havethe same blocking probability. However, handoff calls canexperience a more favorable blocking probability than newcalls by prioritizing bandwidth allocation during call admissionusing the guard channel scheme: with guard channels ina pool of channels where a handoff call isallocated any free channel, while a new call is allocated achannel only when the number of free channels is greater than

It has been shown [6] that under stationary traffic, evenfor a small value of handoff call blocking probability issubstantially reduced at the expense of only a slight increasein the new call blocking probability.

The proposed dynamic guard bandwidth scheme adaptsthe guard bandwidth according to the current estimate ofthe instantaneous handoff call arrival rate so as to keep thehandoff call blocking probability close to the targeted objectivewhile not deteriorating the new call blocking significantly.The proposed scheme can be implemented in the BS’s in amobile network or the bandwidth controller of each wiredlink in the backbone network. An implementation model ofthe proposed scheme is illustrated in Fig. 1. In addition tothe admission and congestion control functions as requiredby the fixed scheme, the proposed scheme also requires theinstantaneous handoff call arrival rate estimation and capacitylimits adaptation functions described below.

The arrival processes of new and handoff calls at a BSor link bandwidth controller are assumed to be Poisson withtime-varying rates of and respectively. The calldeparture process is assumed to be Poisson with a constant rateof The congestion-related performance parameters at time

are the new call blocking probability and the handoffcall blocking probability For a given total number ofchannels the number of guard channels at timeis employed as a control parameter which can be set withinthe range withand

Assume that changes in the arrival rates occur at a moderaterate, and the time required to reach the steady state after achange is short compared to the time to the next change inthe arrival rate, then

(1)

(2)

where andis the probability that all channels are unoccupied at

time

(3)


Fig. 1. Dynamic guard bandwidth scheme.

A. Instantaneous Handoff Call Arrival Rate Estimation

The instantaneous handoff call arrival rate at a test cell forthe next estimation interval depends on the handoff initiationprocess, the number of active MT’s with ongoing calls in theneighboring cells, the mobility patterns of the active MT’s interms of speed and direction during the estimation interval, thesizes of the cells currently resided by the active MT’s, and theremaining call durations of the ongoing calls.

The handoff initiation process employs a decision algorithmbased on the received signal level measurements with orwithout hysteresis. With hysteresis, a handoff is initiated whenthe average signal level from the new base station exceeds thatfrom the current BS by a threshold amount specified by thehysteresis level, and the decision at any instant depends onprevious decisions. As the hysteresis level increases, the meantime between handoffs per call or the average cell residingtime for a call increases, and the probability of back-and-forthhandoffs (i.e., the ping-pong effect) decreases. In this study,to facilitate analysis, we assume that radio propagation effectson the handoff initiation process are taken into account in thechannel holding time statistics. How this is done in practice isoutside the scope of the present investigation.

Let be the probability of handoff call arrival at thetest cell due to an active MT in the neighboring cellat the

th estimation interval, let be the cdf of the channelholding time in the neighboring cellassociated with the activeMT (the channel holding time depends on the cell residingtime and the remaining call duration), letbe the estimationinterval length, let be the cdf of the unencumbered callduration (i.e., the time an assigned channel would be held if

no handoff is required) in the neighboring cellletbe the (mobility-pattern-dependent) typical transit probabilityof an active MT from the neighboring cellinto the test cell

given a call handoff occurs at theth estimation interval,let be the handoff call arrival rate at the test cell atthe th estimation interval, let be the number of activeMT’s in the neighboring cell and let be the number ofneighboring cells. Then

(4)

(5)

If and are assumed to be exponential, andchanges slowly relative to the estimation interval,

then can be carried forward to the next thestimation interval if the associated call is still active in theneighboring cell. Instead of exchanging all call state or activeMT information among neighboring BS’s regularly after eachestimation interval, each BS can just signal changes in the callstate to the neighboring cells for updating Then theupdate of would become a simple exercise of additionand subtraction for the respective increase and decrease of thenumber of active MT’s or ongoing calls in the neighboringcells. As illustrated in Fig. 2, the number of active MT’s orongoing calls in a cell changes at the following events: callcompletion, call handoff, new call arrival, and handoff callarrival. Upon each call completion or call handoff, the BSwould signal a decrease in the call state to the neighboringcells. Upon each new call arrival or handoff call arrival, theBS would signal an increase in the call state to the neighboringcells.

After obtaining at an estimation interval, the number ofguard channels that is required to satisfy a given targetobjective for either or can be determined via (2) and(1), respectively. Fig. 3 illustrates the required to maintaina targeted or as varies for a given

The accuracy of estimating depends on the precision ofthe channel holding time and cell transit statistics. To facilitateanalysis, we assume that precise knowledge of these quantitiesenables us to track the changes in accurately. In practice,estimates of will be biased due to incomplete knowledgeof the above quantities or inaccuracy of the mobility model.These errors can be minimized by experimentally adjusting therelationship between and the number of ongoing calls inneighboring cells. Note that the fixed guard channel scheme isalso susceptible to inaccuracies in estimating the handoff callarrival rate, and the lack of adaptiveness means that it is moredifficult to correct for such inaccuracies.

B. Capacity Limit or Guard Bandwidth Adaptation

Adaptation control policy can be formulated under a strictor a relaxed criterion. A strict criterion would be to maintainthe handoff call blocking probability at the targeted objective


Fig. 2. Estimation of instantaneous handoff call arrival rate at a BS.

at all cost (hard target) by compromising the new call blockingprobability indefinitely. A relaxed criterion would be to keepthe handoff call blocking probability close to the targetedobjective (soft target) while ensuring that new calls are notdenied resource access completely at any given time. Tosupport the relaxed criterion, the proposed adaptation controlpolicy allows deviation of the handoff call blocking probabilityfrom the targeted objective and restricts the guard bandwidthfrom taking up the whole resource capacity.

Let be the targeted objective for new call blockingprobability, and let and be the hard and softtargeted objectives, respectively, for handoff call blockingprobability, where with At a giventime instant can be obtained by either ongoing localmonitoring or by setting it to a time-independent value ifstationarity is assumed, is assumed to be time independent,and is estimated by (5) based on call state informationobtained from neighboring cells.

Let be the greatest value that can assumeto satisfy accordingto (2), and let and be the smallest values that

can assume to satisfyand respec-

tively, according to (1), whereand

The proposed policy as illustrated in Fig. 4 is described asfollows.

1) Determine and via (2) and (1),as shown in Fig. 3.

2) If then else

A strict control criterion to maintain handoff call blockingprobability at a targeted objective can be realized by setting

and Relaxed control criteria can be realized byincreasing and decreasing and the limiting case wouldcorrespond to the strict control criterion to maintain new callblocking probability at a targeted objective.

III. EXTENSION TO ATM-BASED BACKBONE NETWORK

The fixed or dynamic guard channel methods can be ex-tended to the nodes in the backbone network for link band-


Fig. 3. Congestion-related performances.

Fig. 4. Adaptation control policy.

width allocation to enable prioritized call admission control.Different bandwidth assignment schemes may be employeddepending on the traffic characterization and multiplexingscheme. Cellular mobile systems employ per-call multiplexingand deterministic bandwidth or channel assignment, whereas

ATM-based backbone networks may employ statistical multi-plexing and per-call peak or statistical bandwidth assignment.Since statistical bandwidth assignment can be mapped into per-call equivalent bandwidth assignment [11], [12], the conceptof guard channels can be directly applied to set aside reserved


bandwidth for handoff calls. In this paper, the model forthe dynamic guard bandwidth scheme is based on per-callequivalent bandwidth or deterministic channel assignment.

To extend the dynamic guard channel scheme to the back-bone network, estimation of the instantaneous handoff callarrival rate is required to adapt the resource access priorityto changing handoff call arrival rates. The estimation processis based on the accounting of all potential handoff connections(associated with active MT’s with ongoing calls) passingthrough a network link, and the determination of the reroutingprobabilities from the ongoing connections to the potentialhandoff connection. In the mobile network, handoffs intoa radio cell can only originate from the neighboring cells,and the accounting of all potential handoffs at a radio cellcan be performed by monitoring the state information of theneighboring cells. In the backbone network, however, handoffsto a wired link are dependent on the network topology andcall routing.

With fixed routing, a route is selected for eachsource–destination pair of access nodes within the ATM-based backbone network. Under this situation, potentialhandoff connections can be extended from the radio cellsto a given destination access node through the backbonenetwork. Consequently, the accounting of all potentialhandoff connections passing through a wired link can beperformed. However, fixed routing is typically not used inlarge networks because of its lack of flexibility to adapt tonetwork congestion or failures.

With adaptive routing, a route is selected on a per-callbasis within the ATM-based backbone network. Consequently,potential handoff connections cannot be extended from theradio cells into the backbone network to reach the destinationnode, as it is not possible to predict what routing decisionwill be made in the future. Thus, adaptation to handoff ratefluctuations is not possible because of the inability to estimatethe instantaneous handoff call arrival rate at each network link.We propose to overcome this by employing the mobile virtualcircuit (MVC) [3].

A. Extension via Mobile Virtual Circuit (MVC)

The MVC is a dynamic connection tree which supportsnetwork-wide terminal mobility across cells connected to anypoints of the backbone network while allowing sharing ofnetwork resources between mobile and nonmobile traffic,maximizing connection reuse, and minimizing routing tableupdates and logical link identifiers reservation during connec-tion rerouting. It forms the basis for rerouting protocols [3]which optimize resource utilization while enabling fast andseamless handoff operations.

The neighboring cells of a given cell determine the potentialhandoff connections in the backbone network, associated witheach ongoing call in this cell. For example, there are six poten-tial handoff connections for an MT in a six-sided regular cellpattern (Fig. 6). To enable fast handoffs over the ATM/B-ISDNbackbone while minimizing resource allocation, a possibleapproach is to determine the routes for all potential handoffconnections at call setup via intelligent network (IN) control,and to employ a fast resource allocation scheme for completing

the establishment of the selected handoff connection duringcall handoff, thus enabling fast handoff processing withoutIN involvement. This approach calls for a multipoint-to-point or connection tree configuration to link the current andthe potential handoff connections between the MT and thenetwork, and the MVC is a realization of this approach.Implemented at a new level of the ATM layer hierarchicallyabove the VC and the VP levels (Fig. 5), each MVC connectsthe BS in the current cell serving an MT and the BS’s in all theimmediate neighboring cells to which the MT may potentiallyhand off its call to the root node called the tethered point (TP).After a handoff, the TP may shift to another node as the MVCis reconfigured. Because of the continuous route updates forpotential handoff connections during the lifetime of a mobilecall, the MVC naturally accommodates the proposed dynamicguard bandwidth scheme.

The MVC connection establishment scheme employs theIN for route determination in configuring the MVC. Whilefast handoff is easily accomplished in the MVC architectureby preallocating resources over all potential handoff connec-tions, this method is inefficient in resource utilization sincemost connections will remain unused. To optimize resourceutilization while enabling a fast handoff operation, MVCconnection establishments defer resource allocation until thehandoff operation, when establishment of the actual handoffconnection is completed without IN involvement, by using afast resource allocation scheme supported by robust source-routing over an associated signaling transport network thatshares the same physical and logical transmission networkwith the user transport network.

All logical links of the multipoint-to-point connection be-tween the multiple endpoints and the TP are supported by thegroup virtual channel (GVC). At the GVC level, all logicalconnections between the multiple endpoints and the TP aredefined by a single logical link identifier associated with all ofthe concatenated links that is unique at the TP. Consequently,the employment of GVC allows a TP to associate a singleGVC identifier with a group of endpoints associated withan MT. The GVC identifier is encoded in the standard VCidentifier field of the cell header. The GVC is supported overa group of virtual paths (VP’s) which correspond to the setof mobile-specific links employed by the current and potentialhandoff connections between the TP and the endpoints. TheVP’s are fixed by network topology and bind the BS’s tothe TP. Since VP’s are usually established permanently or ona long-term basis, GVC’s can be established over them ondemand. Consequently, the support of the GVC over a groupof VP’s allows the TP to identify the MT via the GVC, andto identify the mobile-specific link sets from the MT to theTP via the VP’s.

The MVC architecture introduces the connection controlservices of potential handoff connection setup and release tosupport fast seamless handoff and optimal resource utilization,with the setup service involving route determination andlogical links reservation (but no bandwidth reservation) and therelease service involving logical links release. To extend theproposed dynamic guard bandwidth scheme into the backbonenetwork via the MVC, we propose to enhance these MVC


Fig. 5. MVC connection architecture to support mobile communications.

connection control services with dynamic guard bandwidthmanagement functions. This involves, at the time of MVCconnection establishment, disconnection or reconfiguration,interpreting the potential handoff connection setup or releaseat each mobile-specific link, respectively, as an increase or de-crease in the number of potential handoff calls for the respec-tive link, and propagating the estimated probability of handoffcall arrival as derived from the MT mobility pattern overeach link set during setup. Consequently, the dynamic guardbandwidth controller would update the estimated instantaneoushandoff call arrival rate at the wired link as discussed below.

B. Instantaneous Handoff Call ArrivalRate Estimation with MVC

By employing MVC to support connection rerouting duringcall handoff, the number of active MT’s that can initiatehandoff calls at a wired link is limited by the number ofpotential handoff connections of the MVC’s passing through itas illustrated in Fig. 6 (e.g., MVC’s 1 and 2 pass through link

which is associated with two active MT’s). An active MTsituated at the cell terminating the current ongoing connectionof an MVC may generate a handoff call to the neighboring


Fig. 6. Estimation of instantaneous handoff call arrival rate at a wired link with MVC.

cells terminating the potential handoff connections of theMVC. Consequently, the probability of a handoff call initiatedby the active MT at the wired links along a potential hand-off connection is determined by the corresponding handoffprobability between the respective cells. As discussed inSection II-A, the probability of a handoff call arrival at acell generated by an active MT in a neighboring cell duringan estimation interval depends on its mobility pattern, thecell size, the remaining call duration, and the length of theestimation interval.

To enable the continuous update of the estimated instanta-neous handoff call arrival rate at a wired link, the signaling of

changes in the number of ongoing calls that may hand off tothe wired link occurs during potential handoff connection setupand release associated, respectively, with the establishmentand release/reconfiguration of an MVC. Sections III-B1 andIII-B2 below detail how the dynamic guard bandwidth man-agement functions can be integrated into the potential handoffconnection control services associated with the MVC.

1) Potential Handoff Connection Controls During MVCSetup and Release:In a general connection-oriented packet-switched network, connection establishment involves thefollowing steps: 1) reservation of logical link identifier ateach switch associated with the connection, 2) establishment


of routing information for translating incoming logicallink identifiers into outgoing logical link identifiers, and3) reservation of communication resources (buffer andphysical link bandwidth) at each switch. The same proceduresare employed in the ATM/B-ISDN which employ fastpacket switching with IN support, where logical links areidentified by virtual circuit identifiers (VCI’s) or virtual pathidentifiers (VPI’s) reserved via the IN. The MVC connectionestablishment scheme is based on reserving logical linkidentifiers via the IN for handoff connections, but delayingresource reservation until handoff processing (which does notinvolve the IN), when a fast resource reservation scheme isinvoked to complete the setup of the handoff connection.The establishment of the MVC connection during initial callprocessing is decomposed into the following tasks, with logicallink identifiers (GVCI, VCI, and VPI), routing information,and resource requirements provided by the IN.

1) Establishment of the fixed common connecting linksshared by the original connection and the potentialhandoff connections:

a) Standard VC connection establishment along thefixed common connecting links—VCI’s and routinginformation entries in VC switch tables are reservedby the ATM switches along the fixed common con-necting links.

b) Communication resources are reserved to satisfyconnection performance requirements.

2) Establishment of the mobile-specific link sets betweenthe multiple BS’s and the TP:

a) GVC connection establishment between the BS’sgroup and the TP—a single unique logical linkidentifier in the form of GVCI is reserved by theTP.

b) Standard VP connection establishment between eachBS and the TP (since the VP’s are usually preestab-lished permanently by the network, this step maynot be necessary); each BS stores its outgoing VPIaccordingly.

c) Bandwidth for the GVC connection is reserved fromthe VP associated with the mobile link set specificto the original connection.

d) For those VP’s associated with the mobile link setsspecific to the potential handoff connections, actualbandwidth is not reserved immediately; instead, theguard bandwidth is increased according to the proba-bility that the associated ongoing call will hand off tothe wired-link. A fast bandwidth reservation schemewill be employed to reserve actual bandwidth if theanticipated handoff does occur.

The dynamic guard bandwidth management functions canbe incorporated as follows. During MVC setup, the TP wouldsignal an increase in the number of potential handoff callsalong the predetermined routes to enable updates of instan-taneous handoff call arrival rate at the wired links alongthe predetermined routes, so that the guard bandwidth canbe increased accordingly by the dynamic guard bandwidthcontrollers of the wired links. During MVC release, the TP

would signal a decrease in the number of potential handoffcalls along the predetermined routes to enable updates of theinstantaneous handoff call arrival rate at the wired links alongthe predetermined routes, so that the guard bandwidth canbe decreased accordingly by the dynamic guard bandwidthcontrollers of the wired links.

2) Potential Handoff Connection Controls during MVC Re-configuration: To support successive handoffs so as to allowan MT to have an unrestricted range of movement or network-wide terminal mobility, the MVC connection tree is reconfig-ured after each handoff to account for the new set of immediateneighboring cells into which the MT can potentially enter.MVC reconfiguration after handoff is illustrated in Fig. 7,where the MT has roamed from cell 0 to cell 6, and is accessinga new BS controlled by a different mobile switching center(MSC). Cell 0 is an immediate neighbor to cells 1 to 6, whilecell 6 is an immediate neighbor to cells 0, 1, 5, 7, 8, 9.Before the handoff, the MVC is configured as follows: TP atnode ; fixed common links , , and ; currentongoing connection associated with cell 0 has mobile-specificlinks , and ; and the potential handoff connectionassociated with cell 6 has mobile specific links , , and

.The MVC reconfiguration after each handoff involves IN

signaling control, and it is described as follows.

1) The potential mobile endpoints are reconfigured to ac-count for the new set of immediate neighboring cells intowhich the MT can potentially enter in the next handoff.

2) One of the nodes (node) along the new connection(i.e., among nodes , and ) is se-lected to be the reconfigured TP. The selection is basedon minimizing the number of mobile-specific links ormaximizing the number of fixed common links of thecurrent and the potential handoff connections. If thereconfigured TP remains at the same node (i.e., node

), no TP reconfiguration is required.3) Potential handoff connection setup:

a) Predetermine the route for the mobile-specific linksfrom each potential mobile endpoint (i.e., those asso-ciated with cells 1, 0, 5, 7, 8, 9) to the reconfiguredTP in the form of source-routing information. Thesource-routing information may be stored in the MT,the associated BS, or the MSC.

b) Signal an increase in the number of potential handoffcalls along the predetermined routes to enable up-dates of instantaneous handoff call arrival rate at thewired links along the predetermined routes, so thatthe guard bandwidth can be increased accordinglyby the dynamic guard bandwidth controllers of thewired links.

4) If TP reconfiguration is required, encode the route for themobile-specific links from the current mobile endpointto the reconfigured TP in the form of source-routinginformation. The reconfigured TP is requested to reservethe same current ongoing GVC identifier to supportthe multipoint-to-point connection between the new setof multiple endpoints and the reconfigured TP. If the


Fig. 7. MVC reconfiguration after handoff.

particular GVC identifier is being employed by anotherMVC at the reconfigured TP, then a new GVC identifiermust be reserved.

5) Potential handoff connection release:

a) Remove information storage of the predeterminedroutes for the mobile link sets of all obsolete poten-

tial handoff connections (i.e., those associated withcells 2, 3, and 4).

b) Signal a decrease in the number of potential handoffcalls along the predetermined routes to enable up-dates of instantaneous handoff call arrival rate at thewired links along the predetermined routes, so that


Fig. 8. Simulation model.

the guard bandwidth can be decreased accordinglyby the dynamic guard bandwidth controllers of thewired links.

6) Routing table entries are established in the connectingnodes along the reconfigured route of the new currentongoing connection. The predetermined and the recon-figured routes of the new current ongoing connectioninclude the same connecting nodes and physical links.

a) If the ongoing GVC is available at the reconfiguredTP, the routes only differ in the point-to-point logicallinks between the previous and the reconfigured TP.

b) If the ongoing GVC is not available at the recon-figured TP, the predetermined and the reconfiguredroutes of the new current ongoing connection differin the point-to-point logical links between the currentmobile endpoint and the reconfigured TP’s.

7) The current ongoing connection is switched over to thereconfigured route from the predetermined route. Sincethe predetermined and the reconfigured routes includethe same connecting nodes and physical links, there is noneed to reserve the bandwidth again, and packet orderingwould still be maintained.

IV. PERFORMANCE ANALYSIS

We simulate the operations of the proposed dynamic guardbandwidth scheme under a strict criterion in a mobile wirelesssystem with radio cells having diverse traffic characteristics.Fig. 8 shows the cell coverage over downtown, city areas, sub-urbs, and rural areas configured as follows: 1) the downtowncell is surrounded by six city area cells; 2) each city area cellis surrounded by the downtown cell, two city area cells andthree suburbs cells; and 3) each suburb cell is surrounded byone city area cell, two suburb cells, and one rural area cell.Performance results obtained from a city cell will be illustratedand discussed.

As shown in Fig. 1, when a new call is originated in a celland assigned a channel, the call holds the channel until the callis completed in the cell or the call is handed off to anothercell as the mobile moves out of the cell. The probability of

requiring a handoff depends on the cell coverage area, theMT’s movement, and the call duration. A call handoff mustbe directed to one of the neighboring cells. The probability ofeach neighboring cell receiving the call handoff depends on theamount of common boundary area and the mobile directionalpattern.

A. Call Blocking Probabilities

1) Stationary Traffic: The condition with stationary trafficis simulated with the following system parameters: the newcall and handoff call interarrival times are assumed to beexponential and stationary with mean new call arrival rate

and , respectively, the call duration is assumed to beexponential with mean s, the channel holding timeis assumed to be exponential with mean s andmean service rate the total number of channelschannels, the adapted number of guard channels at estimationinterval is set within the rangewith and the targeted objective of newcall blocking probability the hard targetedobjective of handoff call blocking probabilityand the transit probability of an active MT from one cellto another cell depends on the time of day. There is agreater tendency to transit toward the downtown cell duringmorning traffic rush hours, and to transit outwards away fromthe downtown cell during evening traffic rush hours. Forexample, during morning traffic rush hours, is set asfollows: downtown-to-city 0.167, city-to-city 0.1, city-to-downtown 0.7, city-to-suburb 0.1, suburb-to-suburb0.167, suburb-to-city 0.5, suburb-to-rural 0.167, rural-to-rural 0.5, and rural-to-suburb 0.167. Long-term handoffcall and new call blocking probabilities and areobtained for a 120 h simulation time duration.

A reference scheme with a fixed guard size of two channelsis also employed. Comparing the fixed guard scheme and theproposed dynamic guard scheme, Figs. 9 and 10 showand

, respectively, as functions of for different values ofwhereas Figs. 11 and 12 show and respectively as

functions of for different values of It can be seen thatand are more sensitive to increases in compared

to as increases in will also increase Whenresource access priority (via the number of guard channels)given to handoff calls is adapted according to the instantaneoushandoff call arrival rate, decreases significantly with onlynegligible or small increases in

Let be the ratio of under dynamic guard to thatunder fixed guard, and let be the ratio of under fixedguard to that under dynamic guard, we define adaptive gain

which measures the relative tradeoff between the decreasein and the increase in as It isimperative that to justify the switch from fixed guardto dynamic guard, i.e., an increase in is compensated witha relatively larger decrease in Fig. 13 shows the proba-bility distribution function of based on the experimentalsamples, and it can be seen that with a mean gainof about 1.3. If it is true that dropped calls due to handoff callblocking are more costly to the service provider than blocked


Fig. 9. Effect of handoff probability on new call blocking probability.

Fig. 10. Effect of handoff probability on handoff call blocking probability.

calls, in terms of customer dissatisfaction, then the average30% adaptive gain attributed to the proposed dynamic guardscheme may translate to a more substantial cost saving for theservice provider.

Fig. 14 shows and as functions of the channelloading factor which is varied froma medium level to overloading It can beseen that resulting from the fixed guard scheme and the


Fig. 11. Effect of new call arrival rate on new call blocking probability.

Fig. 12. Effect of new call arrival rate on handoff call blocking probability.

proposed dynamic guard schemes are very similar under bothmedium and heavy loadings. On the other hand, resultingfrom the dynamic guard scheme is generally lower than thatresulting from the fixed guard scheme. The difference in

decreases as loading increases, and becomes negligible underheavy loading or overloading.

When a call handoff results in connection rerouting over thebackbone network, the overall blocking probability increases


Fig. 13. Adaptive gain.

Fig. 14. Effect of channel loading factor on blocking probabilities.

as the number of rerouting links is increased. Letbe thenumber of links, including the wireless link and the wiredlinks in the logical virtual path. Assume that all links havethe same bandwidth capacity and are subjected to similar

and due to fixed call routing. Let be theprobability of per-connection handoff call blocking; letbe the probability of per-connection new call blocking. Then,

and Fig. 15shows and as functions of whencalls/s and To maintain the per-connection handoffcall blocking probability at the targeted objective of 0.005, onewireless link and one wired link can be accommodated underthe fixed guard scheme, whereas one wireless link and fourwired links can be accommodated under the dynamic guard

scheme. This shows that the dynamic guard scheme allowsthe handoff call blocking probability objective to be met overa network with a greater diameter than the fixed guard scheme,without reducing the loading of the links as would be requiredfor the fixed guard scheme to meet the above objective.

2) Nonstationary Traffic:Actual traffic and the associatedcall arrival rate are seldom stationary or have the same levelas the nominal since the new call arrival rate changes withthe hours of the day, and the handoff call arrival rates dependon the number and movements of callers in adjacent cells.Based on the simulation model as specified previously, anonstationary traffic condition is introduced by increasing theaverage new call arrival rate during morning and afternoontraffic rush hours from the nominal rate.


Fig. 15. Per-connection blocking probabilities.

Fig. 16 shows the instantaneous (averaged over 20 min timesegments) and running average values (averaged from time 0)of the new and handoff call blocking probabilities over a 24 hduration, with the average new call arrival rate increasing to0.6 call/s during rush hours from the nominal rate of 0.2 call/s.The average handoff call arrival rate would also increase asthe average new call arrival rate increases. Under the fixedguard scheme, the handoff call blocking probability increasesto 0.015, from the targeted objective of 0.005, while the newcall blocking probability increases to 0.25 from the targetedobjective of 0.05. In contrast, under the proposed dynamicguard scheme, the handoff call blocking probability stayswithin the targeted objective at 0.0033, which is substantiallylower than the corresponding value for the fixed guard scheme,while the new call blocking probability deviates from thetargeted objective by increasing to 0.265, 6% higher than thecorresponding value for the fixed guard scheme.

By varying the deviation from the nominal rate and keepingother parameters constant, Fig. 17 shows the call blockingprobabilities as functions of the ratio of deviation of new callarrival rates from the nominal rate of 0.2 call/s. It shows thatthe proposed dynamic guard scheme is able to maintain thehandoff call blocking probability at the targeted objective, butthe fixed guard scheme fails to accomplish that when the ratioof deviation is more than 1.5, while the new call blockingprobabilities are similar under the two schemes.

B. Forced Call Termination and OverallCall Failure Probabilities

Various system traffic performance characteristics canbe derived from the new call and handoff call blockingprobabilities. When the mobile handoff probability is smalldue to large cell area or slow mobile movement,would be a good indication of system traffic performance.However, when the mobile handoff probability is larger

due to microcells or high speed mobile movement,and the resulting forced call termination probability havean increasing influence on system traffic performance. Theforced call termination probability is the probability thatan ongoing call is forced into termination, e.g., as a resultof handoff failure. The overall call failure probability, or theprobability that a call cannot be completed because of eitherblocking or unsuccessful handoff, would be a good unifiedperformance measure of the effects of both and

Let be the probability of forced call termination, andlet be the overall call failure probability. The probability

that either a new call which is not blocked or a call whichhas already been handed off successfully will require at leastone more handoff before completion is given as follows:

(6)

where and are the cdf and pdf of the unen-cumbered call duration and the channel holding timerespectively.

Then, due to handoff call blocking or resourceunavailability is given as follows:

(7)

and is given as follows:

(8)

Substituting (7) into (8)

(9)

Since and


Fig. 16. Example of nonstationary call traffic condition.

with being the number of links

(10)

(11)

We can see that when or the probability of cell tran-sition of MT is large due to small cell size area, high MTroaming speed, or large call duration, would have amore dominant effect on At the extreme with

it means that successive handoffs continue indefinitely,resulting in Conversely, when the probability ofcell transition is small, would have a more dominant


Fig. 17. Effect of nonstationary condition on call blocking probabilities.

Fig. 18. Forced call termination and overall call failure probabilities.

effect on At the extreme with

Fig. 18 shows and as functions of whencalls/s and It illustrates that, while the

fixed and the dynamic guard schemes offer similar thedynamic guard scheme offers a lower

V. CONCLUSIONS

The proposed dynamic guard bandwidth scheme adapts theresource access priority via guard bandwidth adjustments in

response to changes in instantaneous handoff call arrival rates,estimated by observing the state information of neighboringcells. Under stationary traffic conditions, the proposed dy-namic guard scheme offers better resource utilization than thatof the fixed guard scheme by providing lower and comparablelong-term handoff call blocking probabilities under respectivemedium and heavy bandwidth loadings. Under nonstationarytraffic conditions, the proposed dynamic guard scheme enablesthe handoff call blocking probability to stay close to itstargeted objective, without significantly increasing the new


call blocking probability relative to the fixed guard scheme.The distributed adaptive algorithm is simple, and only requirescells to exchange a small amount of information (numberof ongoing calls or, equivalently, call arrival and departureevents) with its neighbors whenever a call arrives at or leavesthe cell. The required guard bandwidth can be calculated offline and loaded into lookup tables to facilitate the dynamicallocation. With adaptive call routing within the ATM-basedbackbone network, estimation of instantaneous handoff call ar-rival rate for each network link is not feasible, and the dynamicguard scheme cannot be applied. We propose to overcomethis difficulty by employing the MVC connections within theATM-based backbone network to support terminal mobility.In so doing, the proposed dynamic guard scheme is applicableto channel allocation over cellular mobile networks, and isextended to bandwidth allocation over backbone networks toenable a unified approach to prioritized call admission controlover the ATM-based PCN.

REFERENCES

[1] A. D. Malyan, L. J. Ng, V. C. M. Leung, and R. W. Donaldson, “Net-work architecture and signalling for wireless personal communications,”IEEE J. Select. Areas Commun.,vol. 11, pp. 830–841, Aug. 1993.

[2] A. S. Acampora and M. Naghshineh, “An architecture and methodologyfor mobile-executed handoff in cellular ATM network,”IEEE J. Select.Areas Commun.,vol. 12, pp. 1365–1375, Oct. 1994.

[3] O. T. W. Yu and V. C. M. Leung, “Connection architecture andprotocols to support efficient handoffs over an ATM/B-ISDN personalcommunications network,”ACM/Baltzer J. Mobile Networks Appl., vol.1, pp. 123–139, Oct. 1996.

[4] A. Gavious and Z. Rosberg, “A restricted complete sharing policy for astochastic knapsack problem in B-ISDN,”IEEE Trans. Commun., vol.42, pp. 2375–2379, July 1994.

[5] M. Naghshineh and M. Schwartz, “Distributed call admission controlin mobile/wireless networks,”IEEE J. Select. Areas Commun., vol. 14,pp. 711–717, May 1996.

[6] D. Hong and S. S. Rappaport, “Traffic model and performance analysisfor cellular mobile radio telephone systems with prioritized and non-prioritized handoff procedures,”IEEE Trans. Veh. Technol., pp. 77–92,Aug. 1986.

[7] J. Daigle and N. Jain, “A queueing system with two arrival streams andreserved servers with application to cellular telephone,” inProc. IEEEINFOCOM, Apr. 1992, pp. 2161–2167.

[8] R. Guerin, “Queueing-blocking system with two arrival streams andguard channels,”IEEE Trans. Commun., vol. 36, pp. 153–163, Feb.1988.

[9] C. H. Yoon and C. K. Un, “Performance of personal portable radiotelephone systems with and without guard channels,”IEEE J. Select.Areas Commun., vol. 11, pp. 911–917, Aug. 1993.

[10] W. C. Y. Lee, “Smaller cells for greater performance,”IEEE Commun.Mag., vol. 29, pp. 19–23, Nov. 1991.

[11] J. Y. Hui, M. Gursoy, N. Moayeri, and R. Yates, “A layered broadbandswitching architecture with physical or virtual path configurations,”IEEE J. Select. Areas Commun., vol. 9, pp. 1416–1426, Dec. 1991.

[12] D. Hong and T. Suda, “Congestion control and prevention in ATMnetworks,” IEEE Network Mag., vol. 5, pp. 10–16, July 1991.

Oliver T. W. Yu received the B.A.Sc. (1981),M.A.Sc. (1991), and Ph.D. (1997) degrees in elec-trical engineering from the University of BritishColumbia. His Ph.D. dissertation was on the emerg-ing area of wireless-ATM-based personal commu-nications networks; it proposes the connection ar-chitecture, real-time rerouting services, signalingprotocols and resource management architecture tosupport terminal mobility over an ATM-based back-bone network.

He worked in the telecommunications industry forover ten years, employed by Microtel Pacific Research Ltd., Bell NorthernResearch Ltd., and Hughes Aircraft of Canada Ltd. He had also been employedas a Software Engineering Manager in Soltech Industries Ltd. His pastresearch and development work included telecommunications and signalingnetworks architectures, communications protocols designs and conformancetesting, satellite and aviation communications systems, and the interworkingof heterogeneous networks and network management. His current researchinterests are in the areas of wireless ATM and broad-band ISDN networks,with applications in network management, personal mobile, and multimediacommunications.

Victor C. M. Leung (S’75–M’81) received theB.A.Sc. (Hons.) degree in electrical engineeringfrom the University of British Columbia (UBC) in1977, and was awarded the APEBC Gold Medalas the head of the graduating class in the Facultyof Applied Science. He attended graduate schoolat UBC on a Natural Sciences and EngineeringResearch Council Postgraduate Scholarship, and re-ceived the Ph.D. degree in electrical engineering in1981.

From 1981 to 1987, he was a Senior Member ofTechnical Staff at Microtel Pacific Research Ltd., specializing in the planning,design, and analysis of satellite communication systems. He also held a part-time position as Visiting Assistant Professor at Simon Fraser University in1986 and 1987. In 1988, he was a Lecturer in the Department of Electronics,Chinese University of Hong Kong. He joined the Department of ElectricalEngineering at UBC in 1989, where he is an Associate Professor and a memberof the Centre for Integrated Computer Systems Research. He is also a ProjectLeader in the Canadian Institute for Telecommunications Research, a Networkof Centres of Excellence funded by the Canadian Government. His researchinterests are in the areas of architectural and protocol design and performanceanalysis for computer and telecommunication networks, with applications insatellite, mobile, and personal communications and high-speed networks.

Dr. Leung is a member of the ACM.

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