A Two-Cell-Lookahead Call Admission and Handoff Management Scheme for Multimedia LEO Satellite...

Proceedings of the 36th Hawaii International Conference on System Sciences - 2003

A Two-Cell-Lookahead Call Admission and Handoff Management Scheme for Multimedia LEO Satellite Networks

Petia Todorova†, Stephan Olariu‡ and Hoang Nam NguyenŦ †FhG-Fokus, Kaiserin-Augusta-Allee 31, D-10589 Berlin, Germany

‡Department of Computer Science, Old Dominion University, Norfolk, VA 23529, U.S.A. Ŧ Institute of Communication Networks, TU Vienna, Favoritenstrasse 9-388, Vienna, Austria

Abstract

The main contribution of this work is to propose a novel call admission and handoff management scheme for LEO satellite networks. A key ingredient in our scheme is a companion predictive bandwidth allocation strategy that exploits the topology of the network and contributes to maintaining high bandwidth utilization. Our bandwidth allocation scheme is specifically tailored to meet the QoS needs of multimedia connections. The performance of our scheme is compared to that of various bandwidth allocation schemes proposed in the literature. Simulation results show that, compared with the other bandwidth allocation schemes, our scheme offers low call dropping probability (CDP), providing for reliable handoff of on-going calls and acceptable call blocking probability (CBP) for new calls. 1. Introduction

In response to increasing demands for truly global coverage needed by Personal Communication Services (PCS), a new generation of mobile satellite networks intended to provide anytime-anywhere communication services was proposed in the literature [Aky97, Del00, Lug99]. Low Earth Orbiting (LEO) mobile satellite networks, deployed at altitudes ranging from 500 km to 2000 km, are well suited to handle bursty Internet and multimedia traffic and to offer low cost access to end-users equipped with hand-held devices. LEO satellite networks offer numerous advantages over terrestrial networks, such as global coverage, low cost-per-minute access time, etc. Since LEO satellite networks are expected to support real-time interactive multimedia traffic they must be able to provide their users with Quality-of-Service (QoS) guarantees including

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andwidth, delay, jitter, call dropping and call blocking robability.

While providing significant advantages over their errestrial counterparts, LEO satellite networks present rotocol designers with an array of daunting challenges, ncluding handoff, mobility and location management Com97, Ngu00]. Because LEO satellites are deployed at ow-altitude, Kepler’s third law implies that these atellites must traverse their orbits at very high speed. The overage area of a satellite – a circular area of the surface f the Earth - is referred to as its footprint. For spectral fficiency reasons, the satellite footprint is partitioned into lightly overlapping cells, called spotbeams. As their overage area changes continuously, in order to maintain onnectivity, end-users must switch from spotbeam to potbeam and from satellite to satellite, resulting in requent intra- and inter-satellite handoffs. For example, iven a spotbeam diameter of 50-500km and an rotational eriod of 110min, the time interval between two onsecutive handoffs ranges between 10s-100s [Pro01]. ue to the large number of handoffs experienced by a

ypical connection during its lifetime, resource anagement and connection admission control are very

mportant tasks if the system is to provide fair bandwidth haring and QoS guarantees. In particular, a reliable andoff mechanism is needed to maintain connectivity nd to minimize service interruption to on-going onnections, as end-users roam about the system. In fact, ne of the most important QoS parameter for LEO atellite networks is the call dropping probability, uantifying the likelihood that an on-going connection ill be force-terminated due to an unsuccessful handoff

ttempt. In this work we focus on intra-satellite handoffs,

eferred to, simply, as handoffs. Our main contribution is o propose a novel call admission control and handoff anagement scheme for LEO satellite networks,

pecifically targeted at the meeting the QoS needs of

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multimedia connections. A key ingredient in our scheme is a novel predictive bandwidth allocation and de-allocation scheme that frees up bandwidth allocated to connections that are unlikely to use it. Our proposed solution is predicated on the view that the wireless networks of the future will have to provide support for multi-service traffic with different QoS requirements. In particular, our call admission and handoff management protocols strive to achieve a fair sharing of resources between various service classes.

The performance of our scheme is compared with that of various fixed reservation, adaptive reservation, and predictive reservation schemes proposed in the literature. Simulation results confirm that, when compared with other bandwidth allocation schemes proposed in the literature, our scheme offers low call dropping probability, providing for reliable handoff of calls in progress, acceptable call blocking probability for new call attempts and high resource utilization.

The remainder of this work is organized as follows. Section 2 reviews recent bandwidth allocation and call admission control strategies. Section 3 discusses the mobility and traffic model assumed in this work. Sections 4 and 5 discuss, respectively, the details of our call admission and handoff management schemes. Section 6 offers a performance evaluation of our scheme in terms of Call Dropping Probability (CDP) and Call Blocking Probability (CBP). Finally, Section 7 offers concluding remarks and maps out directions for further work

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2. Related work

In wireless mobile networks, the radio bandwidth is shared by a large number of users. An important property of the network is that a user would changes its access points several times. This fact causes technical problems in which fair sharing of bandwidth between handoff connections and new connections is required. Resource allocation and Call Admission Control (CAC) are very important tasks to achieve a high resource utilization and QoS provisioning for both handoff and new connections. We refer the reader to Table 1 for a succinct comparison between several resource allocation strategies and CAC algorithms for LEO satellite networks found in the recent literature.

In [Cho00], location information of users is used for adaptively bandwidth allocation for handoff resource reservation. In a beam, bandwidth reservation for handoff is allocated adaptively by calculating the possible handoffs from neighboring beams. A new call request is accepted if its originated beam has enough an amount of available bandwidth for new calls. The reservation mechanism provides a low handoff blocking probability comparing with fixed guard channel strategy. However, using user location information seems not reasonable because updating locations would cause high processing load to on-board handoff controller and increase the complex of terminals. The method seems only suitable for fixed users.

Table 1: A synopsis of recent resource allocation and admission control strategiesTable 1: A synopsis of recent resource allocation and admission control strategiesTable 1: A synopsis of recent resource allocation and admission control strategiesTable 1: A synopsis of recent resource allocation and admission control strategies

Papers Multi-services

Reservation Strategy

Resource allocation

Necessary information

CAC criteria QoS issues

[Cho00] No Adaptive DCA User location Available Bandwidth

No

[Mer97] Yes Fixed per connection FCA Handoff residual time

Available Bandwidth

No

[Elk01] Yes Time-dependent probabilistic reservation

FCA Handoff residual time

Available bandwidth and

reservation

Yes

[Uzu99] No No FCA User location Threshold of Pho-

blocking Yes,

Pho-blocking [Del95] No FCA DCA Handoff residual

time Available bandwidth

No

[Tod02] Yes Predictive reservation

DCA Handoff residual time

Available bandwidth and

reservation

Yes

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In [Mer97], the authors have introduced two different mobility models for satellite networks. In the first model, only the motion of satellites is taken into account where as in the second model, other motion components like earth rotation and user movements are considered. To design a CAC algorithm for mobile satellite systems, the authors introduced a new metric so called “mobility reservation status” which provides the information about the current bandwidth requirements of all active connections in a specific spotbeam in addition to the “possible” bandwidth requirements of mobile terminals currently connected to the neighboring spotbeams. A new call request is accepted in its originated beam, say beam m if there are enough available bandwidth in the beam and the mobility reservation status of particular neighboring beams have not exceeded a predetermined threshold TNewCall. If a new call is accepted, mobility reservation status of particular S number of spotbeams will be updated. A handoff request is accepted if there is available bandwidth in the new spotbeam and also the handoff threshold (THO) is not exceeded. The key idea of the algorithm is that to prevent dropped handoff during a call, bandwidth will be reserved in a particular number S of spotbeams that the call would handoff into. The balance between new call blocking and handoff call blocking depends on the selection of predetermined threshold parameters for new and handoff calls. However, during simulation implementation, we found that the scheme has a problem of determining threshold points for the case of LEO satellite networks.

In [Elk01], a probabilistic resource reservation strategy for real-time services was proposed. A new CAC algorithm is introduced where real-time and non-real-time service classes are differently treated. The concept of sliding windows is proposed to predict the necessary amount of reserved bandwidth for a new call in its future handoff spotbeams. For real-time services, a new call request is accepted if the originated spotbeam has available bandwidth and resource reservation is successful in future handoff spotbeams. Non real-time service new call requests are accepted if the originated spotbeam satisfies its maximum required bandwidth. Handoff requests for real-time traffic are accepted as long as their minimum bandwidth requirement can be satisfied. Handoff requests from non real-time traffic are honored as long as there is some residual bandwidth in the cell.

In [Uzu99], CAC is based on the location information of users. A new call is accepted only if the handoff call blocking probability of the system is below the target blocking rate at all times. A contention area is defined as the area swept by an spotbeam serves a new user. The CAC ensures that the handoff blocking probability averaged over the contention area is less than a target handoff blocking probability PQoS (QoS of the contention area). The system always has to trace location of all users

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in each spotbeams and to update the handoff blocking parameter of users. The algorithm requires a high computational load to the satellite so that it seems not suitable in a high capacity systems where a satellite footprint consists of many small size spotbeams and serves at them same times thousand active users.

In [Del95], the authors proposed a mobility model and different resource allocation strategies for LEO satellite networks. The queuing of handoff request is proposed aiming to reduce the handoff blocking. Dynamic channel allocation scheme is expected as the best scheme to optimize system performances.

In [Tod02], the authors proposed a selective look-ahead strategy specifically tailored to meet the QoS needs of multimedia connections where real-time and non-real-time service classes are differently treated. The handoff admission policies introduced distinguish between both types of connections. Bandwidth allocation only pertains to real-time connections handoffs. To each accepted connection bandwidth allocation is performed in a look-ahead horizon of k cells along its trajectory, where k is referred to as the depth of the look-ahead horizon. The algorithm offers low CDP, providing for reliable handoff of on-going calls and acceptable CDP for new calls.

3. Mobility model and traffic parameters

As customary, we assume a one-dimensional

mobility model where the end-users move in straight lines and at constant speed, essentially the same as the orbital speed of the satellite [Del95,Del00]. We assume an orbital speed of 26,000Km/h. All the spotbeams (i.e. cells) are identical in shape and size. Although each spotbeam is circular, we shall approximate spotbeams by regular hexagons, since they provide an exact tiling of the satellite’s footprint. The diameter of a cell is taken to be 425Km [Del95, Del00]. Further, end-users traverse each cell along its maximum diameter. Thus, the time ts it takes an end-user to cross a cell is, roughly, 65 seconds.

t C tf

t s

N N+1 N+2

ts ts

Figure 1: Illustrating some of the mobility and cell Figure 1: Illustrating some of the mobility and cell Figure 1: Illustrating some of the mobility and cell Figure 1: Illustrating some of the mobility and cell pppparametersarametersarametersarameters

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Referring to Figure 1, assume that a connection C was accepted in cell N at time tC. The connection remains in cell N for tf time, where tf is uniformly distributed between 0 and ts. Thus, tf is the time until the first handoff request, assuming that the call does not end in the original cell. After the first handoff, a constant time ts is assumed between subsequent handoff requests until call termination.

The traffic offered to the satellite system belongs to

two classes: • Class I traffic -- real-time multimedia traffic, such as

interactive voice and video applications, and • Class II traffic -- non real-time data traffic, such as

email or ftp. When a mobile user requests a new connection C in a

given cell, it provides the following parameters: • the desired class of traffic for C (either I or II), • MC the desired amount of bandwidth for the

connection. If the request is for a Class I connection, the smallest amount mC of bandwidth that the source requires in order to maintain acceptable quality, e.g. the smallest encoding rate of its codec is also specified.

A Class I connection C in a generic cell N is said to be: • regular if C has confirmed bandwidth reservations in

cells N+1 and N+2, • 1-short if C has a confirmed bandwidth reservation in

cell N+1 but not in cell N+2, • 2-short if C has no confirmed reservation in cells N+1

and N+2. We note here that 2-short connections are liable to be

dropped at the next handoff attempt, while 1-short connections are in no imminent danger of being dropped. The stated goal of our bandwidth allocation scheme is to minimize the likelihood of a dropped connection, by striving to render regular as many Class I connections as possible. The principle vehicle for achieving this goal is a judicious priority-based bandwidth allocation strategy.

4. Our call admission control strategy Connection admission control is one of the

fundamental tasks performed by the satellite network at call setup time in order to determine whether a connection request can be accepted into the system without violating prior QoS commitments. The task is non-trivial since the traffic offered to the system is a heterogeneous mix of new call attempts and handoff requests.

Consider a request for a new connection C in cell N. Our new call admission strategy involves two criteria.

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• The first call admission criterion, which is local in scope, ensures that the originating cell N has sufficient resources to provide the connection with its desired amount of bandwidth MC. Both Class I and Class II connections are subject to this first admission criterion.

A Class II connection request that satisfies the first admission criterion is accepted into the system and placed into the queue Q(N) of Class II connections currently in cell N. On the other hand, if the first admission criterion is not satisfied, the connection request is rejected outright. • The second admission control criterion, which is non-

local in scope, applies to Class I connections only, attempting to minimize the chances that, once accepted, the connection will be dropped later due to a lack of bandwidth in some cell into which it may handoff. Specifically, a minimum reservation of mC units of bandwidth must be made on the connection’s behalf in cells N+1 and N+2. Assuming that C was admitted at time tC and that the holding time of C in cell N is tf, these reservations are, respectively, for the time intervals IN+1 =[tC + tf , tC + tf + ts] and IN+2 =[tC + tf + ts, tC + tf + 2ts].

If both these conditions are satisfied, the Class I connection C is admitted as a regular connection in cell N and, unless the connection terminates in cell N (an unlikely event), it is placed in the queue R(N) as discussed in Section 5.

5. Our handoff management scheme

A well-known strategy for reducing handoff failure is

to reserve bandwidth for the exclusive use of handoff connections [Cho00,Mer97,Oli98,Del95]. In the fixed reservation approach a fixed percentage of the available bandwidth in a cell is permanently reserved for handoff. In the predictive reservation strategy, bandwidth is reserved dynamically using a probabilistic approach [Elk01, Lev97, Nag96, Tod02].

The main goal of this section is to spell out the details of our novel handoff management scheme. For each cell the satellite maintains four different queues R, Q, S1 and S2 as described below. The management of these queues is the workhorse of our scheme and will be presented in detail in Subsection 5.1.

Before discussing the details of managing Class I handoff requests, we note that the handoff request of a Class II connection in Q(N-1) attempting to transit into cell N is accepted as long as cell N has some residual bandwidth available, and placed in the queue Q(N) of Class II connections in cell N. It is important to observe that for Class II connections no minimum bandwidth guarantees are honored. This is consistent with the view that Class II connections do not have hard delay

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requirements: a lower bandwidth allocation will naturally translate into a longer end-to-end delay.

Now, consider a regular Class I connection C in cell N-1 that performs a handoff into cell N. Recall that, since C is regular, there is bandwidth reserved for C in both cells N and N+1and, thus, the handoff is guaranteed to be successful. Two cases may arise at this point: • If a minimum of mC units of bandwidth can be reserved

on C’s behalf in cell N+2 all is well, C becomes a regular connection in cell N, and is placed in queue R(N)

• If bandwidth cannot be secured for C in cell N+2, C becomes 1-short in cell N and is placed in queue S1(N).

Next, assume that a 1-short Class I connection C in cell N-1 performs a handoff into cell N. Since C is 1-short in N-1, the handoff is successful, as there exists bandwidth reserved on its behalf in cell N. The following three cases may arise: • If a minimum of mC units of bandwidth can be reserved

on C’s behalf in cells N+1 and N+2, C becomes a regular connection in cell N and is inserted in queue R(N),

• If a minimum of mC units of bandwidth can be reserved on C’s behalf in cell N+1 but not in cell N+2, C becomes 1-short in N and is placed in queue S1(N),

• If bandwidth cannot be secured for C in either of the cells N+1 and N+2, C becomes 2-short in cell N and is placed in queue S2(N).

Finally, consider a 2-short Class I connection C in cell N-1 that is attempting a handoff into cell N. Since there is no reservation for C in cell N and, as we shall see, there is not enough bandwidth to satisfy the minimum of mC units, C will be dropped.

5.1. Queue management For a generic cell N, the four queues Q(N), R(N),

S1(N) and S2(N) are maintained as priority queues. This means that each connection within the queues is processed based on the residual time it will spend in cell N. The priority regimen is suggested by the fact that connections closest (in time) to handoff have to get attention first. It is important to note that, except for the connections that terminate in cell N, all other connection in the cell must occur in exactly one of the queues above. The priority rules introduced between the different types of queues are explained later on.

Consider an arbitrary group of cells N-2, N-1, and N. Bandwidth in cell N is reclaimed as a result of call terminations (occurring in cell N) or bandwidth de-allocation as discussed in Section 6. Let Avail(N) denote the amount of bandwidth currently available in cell N.

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The processing specific to R(N-1) is as follows: when a connection C in R(N-1) is about to handoff into cell N, the satellite attempts to reserve bandwidth for C in cell N+1. If this reservation attempt is successful then, at the handoff point, C is removed from R(N-1) and added to R(N), in other words, C is now a regular connection in cell N. If, on the other hand, the reservation attempt described above fails, at the handoff point C is removed from R(N-1) and added to S1(N), as in cell N, connection C is 1-short.

The processing of queue Q(N-1) is straightforward: when a connection C in Q(N-1) is about to handoff into cell N, the satellite checks if Avail(N) > 0, that is, if some bandwidth is available in cell N. If such is the case, D is removed from Q(N-1) and added to Q(N). If Avail(N)=0, that is, no bandwidth is available in cell N, C is removed from Q(N-1) and dropped.

The first priority in queue management is to reserve bandwidth in cell N on behalf of connections in the queue S2(N-1) that are in imminent danger of being dropped, since there is no reserved bandwidth for them in cell N. The details of managing the queue S2(N-1) are spelled out in protocol Handle_S2 below. Protocol Handle_S2; while S2(N-1) is not empty do let C be the head of S2(N-1) and let t be the remaining dwell time of C in cell N-1; no_bandwidth_reserved = true;

while (t > 0) and no_bandwidth_reserved(C) do adjust available bandwidth Avail(N);

if Avail(N)>= mC then reserve mC units of bandwidth for C in cell N;

Avail(N) = Avail(N) - mC no_bandwidth_reserved(C)= false; remove C from S2(N-1); add C to S1(N-1) endif t = t-1 endwhile; // since we are out of the while loop either t=0 or //no_bandwidth_reserved(C)=false. // the next statement purports to test implicitly if t=0 if no_bandwidth_reserved(C) then // thus, it must be the case that t=0

drop connection C; remove C from S2(N-1)

endif endwhile end. The second priority in queue management is to reserve bandwidth in cell N on behalf of connections in queue

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S1(N-2). These connections are in no imminent danger of being dropped, for they are guaranteed bandwidth in cell N-1. However, as already mentioned, our philosophy is to ensure that there are as many regular connections as possible. The details of managing the queue S1(N-2) are spelled out in protocol Handle_S1 below. Protocol Handle_S1 while S1(N-2) is not empty do let C be the head of S1(N-2) and let t be the remaining dwell time of C in cell N-2; no_bandwidth_reserved = true;

while (t > 0) and no_bandwidth_reserved(C) do if S2(N-1) is empty then adjust available bandwidth Avail(N);

if Avail(N)>= mC then reserve mC units of bandwidth for C in cell N;

Avail(N) = Avail(N) - mC no_bandwidth_reserved(C)= false; remove C from S1(N-2); add C to R(N-2) endif endif t = t-1 endwhile; // since we are out of the while loop either t=0 or //no_bandwidth_reserved(C)=false. // the next statement purports to test implicitly if t=0 if no_bandwidth_reserved(C) then // thus, it must be the case that t=0

remove C from S1(N-2); // because C just left N-2 and is now in cell N-1 add C to S2(N-1) unless C ends in cell N-1

endif endwhile end.

The third priority goes to new call admission requests. Recall that, as discussed in Section 4, a new Class I request C in cell N is accepted into the network only if cell N can honor the desired bandwidth requirement of the connection and, in addition, bandwidth can be reserved on C’s behalf in cells N+1 and N+2.

The fourth priority goes to Class II handoff connection.

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It is important to understand the factors that impact the acceptance of C in cell N. Since our first obligation goes to keeping on-going connections alive, the acceptance of C hinges on the status of the queues S2(N-1) and S2(N). If these queues are empty, the new connection request is processed immediately and if the admission criteria (as specified in Section 4) are met, the connection is accepted.

6. Performance evaluation

6.1 Simulation model

Deriving from the above description of queue management and CAC algorithm, we have developed a simulation program to evaluate the performances of the proposed scheme. Each spotbeam is modeled as a queue system, consisting of four queues and a server, shown in Figure 2. Queue properties: As we can see from Figure 2 and according to subsection 5.1, the following queues are maintained in cell N as priority queue: • S2(N) -- for 2-short connections • S1(N) -- for 1-short connections • R(N) -- for regular Class 1 connections • Q(N) -- for Class 2 connections

Server functions: The server functions implemented are: • To monitor available bandwidth of the spotbeam • To reserve bandwidth required by individual

connection • To accept or reject a new call request • To accept or reject a handoff request

System parameters used in our simulation experiments are described in Table 2 based on the well-known Irridium satellite system. We assume that each spotbeam has been loaded by homogeneous traffic. New call arrival rate is following Poisson distribution and connection duration is exponentially distributed. We define six types of services with different QoS requirements. We assume that the mean arrival rate of each service type is equal and that each spotbeam is assumed to have a fix amount of radio capacity.

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Server

R(N)

S1

S2

Q(N)

N-1 N N-2

Server

R(N)

S1

S2

Q(N)

Server

R(N)

S1

S2

Q(N)

New Call Request

Handoff Request

New Call Request

Handoff Request

New Call Request

Handoff Request

Figure 2. Simulation modelFigure 2. Simulation modelFigure 2. Simulation modelFigure 2. Simulation model

Table 2. SimulaTable 2. SimulaTable 2. SimulaTable 2. Simulation parameterstion parameterstion parameterstion parameters

Spotbeam parameters

Radius Capacity Speed 212 km 30000 Kbit 26000 km/h

Service parameters

Class 1 Class2 Parameters Type1 Type 2 Type 2 Type 1 Type 2 Type 3

Mean duration (s) 180 300 600 30 180 120 Maximum bandwidth (kbps) 30 256 6000 20 512 10000 Minimum bandwidth (kbps) 30 256 1000 5 64 1000

6.2 Simulation results

The simulation results are shown in Figures 3-6. We compare the CDP and CBP performances of three recently proposed protocol: the adaptive allocation-ADCA from [Cho00], the fixed reservation FRES from [Oli98], where 5% of the spotbeam capacity is reserved for real-time handoff traffic and the selective look-ahead allocation-SILK from [Tod01] where bandwidth is allocated to each accepted Class I connection in a look-ahead horizon of k cells along its trajectory. We call our novel call admission and handoff management scheme Queued-PRDT because of the queuing management strategy introduced in the proposed predictive algorithm.

Figure 3 and Figure 4 show that the CDP for Class I of Queue-PRDT is better than all the other cases because of the well-determined horizon limited to two cells. CDP for Class II gains better performances than FRES and ADCA and is virtually the same as SILK. The FRES gains a better performance in terms of CBP for Class I

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services only. In comparison with the original SILK and ADCA, the new scheme Queued-PRDT provides better new CBP for Class I and Class II services, as showed in Figure 5 and Figure 6, respectively.

Call arrival rate (calls/spotbeams/minute) 10 15 20 25 30 35 40

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FFFFigure 3. CDP of Class 1 servicesigure 3. CDP of Class 1 servicesigure 3. CDP of Class 1 servicesigure 3. CDP of Class 1 services

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Figure 4. CDP of Class 2 servicesFigure 4. CDP of Class 2 servicesFigure 4. CDP of Class 2 servicesFigure 4. CDP of Class 2 services

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Figure 5. New CBR of Class 1 servicesFigure 5. New CBR of Class 1 servicesFigure 5. New CBR of Class 1 servicesFigure 5. New CBR of Class 1 services

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Figure 6. New CBP of Class 2 servicesFigure 6. New CBP of Class 2 servicesFigure 6. New CBP of Class 2 servicesFigure 6. New CBP of Class 2 services

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7. Conclusions and open problems

LEO satellites are expected to support multimedia traffic and to provide their users with the appropriate QoS. However, the limited bandwidth of the satellite channel, satellite rotation around the Earth and mobility of end-users makes QoS provisioning and mobility management a challenging task. One important mobility problem is the intra-satellite handoff management. The main contribution of this work was to propose a novel call admission and handoff management scheme for LEO satellite networks. Our scheme was specifically tailored to meet the QoS needs of multimedia connections where real-time and non-real-time service classes were differently treated.. In Queue-PRDT, bandwidth allocation for a connection with stringent CDP requirements differs significantly from that of a more tolerant application.. We compared the performance of Queue-PRDT to that of various bandwidth allocation schemes proposed in the literature. Our simulation results showed that Q-PRDT offers low CDP, providing for reliable handoff of on-going calls and acceptable CBP for new calls. Our future work will be concentrated on the development of a bandwidth de-allocation strategy as part of the handoff management scheme proposed. Acknowledgments The work was support in part, ny NATO grand PST.CLG979033. References [Aky97] I. F. Akyldiz and S. H. Jeong, Satellite ATM networks: A survey, IEEE Communications, 35, July 1997. [Cho00] S. Cho, Adaptive dynamic channel allocation scheme for spotbeam handover in LEO satellite networks, Proc. IEEE VTC 2000, 1925-1929. [Com97] G. Comparetto and R. Ramirez, Trends in mobile satellite technology, IEEE Computer, 1997, 44-52. [Del95] E. Del Re, R. Fantacci, and G. Giambene, Efficient dynamic channel allocation techniques with handover queuing for mobile satellite networks, IEEE Journal of Selected Areas in Communications, Feb. 1995, 397-405. [Del00] E. Del Re, R. Fantacci, and G. Giambene, Characterization of user mobility in Low Earth Orbiting mobile satellite systems, Wireless Networks, 6, 2000, 165-179. [Elk01] M. El-Kadi, St. Olariu and P. Todorova, Predictive resource allocation in multimedia satellite networks, Proc IEEE, GLOBECOM 2001, San Antonio, TX, November 25-29, 2001. [Hua98] A. Huang, M-J. Montpetit and G. Kesidis, ATM via satellite: A framework and implementation, Wireless Networks, 4, 1998, 141-153.

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[Lev97] D. Levine, I. Akyildiz and M. Naghshineh, A resource estimation and call admission algorithm for wireless multimedia networks using the shadow cluster concept, IEEE/ACM Transactions on Networking, 5, (1997), 1-12. [Lug99] M. Luglio, Mobile multimedia satellite communications, IEEE Multimedia, 6, (1999), 10 - 14. [Mer97] I. Mertzanis, R. Tafazolli, and B. G. Evans, Connection admission control strategy and routing considerations in multimedia (Non-GEO) satellite networks, Proc. IEEE VTC, 1997, 431-436. [Nag96] M. Naghshineh and M. Schwartz, Distributed call admission control in mobile/wireless networks, IEEE Journal of Selected Areas in Communications, 14, (1996), 711-717. [Ngu00] H. N. Nguyen, J. Schuringa, and H. R. van As, Handover schemes for QoS guarantees in LEO-satellite networks, Proc. ICON'2000. [Oli98] C. Oliveria et al., An adaptive bandwidth reservation scheme for high-speed multimedia wireless networks, IEEE Journal of Selected Areas in Communications, 16, 1998, 858-874.

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