A Fast Handoff Scheme for Multi-Connection Calls

IEICE TRANS. COMMUN., VOL.E85–B, NO.10 OCTOBER 2002

2002

PAPER

Special Issue on Mobile Multimedia Communications

A Fast Handoff Scheme for Multi-Connection Calls in Wireless ATM Networks Sung Cheol CHANG† , Nonmember and Dan Keun SUNG†† , Regular Member

SUMMARY A dynamic pre-allocated connection (DPC) scheme is proposed to support fast handoff and to effectively utilize wireline links in a multi-connection call environment. Handoff can be quickly executed in real-time with reduced connection overhead, since the proposed scheme uses pre-allocated switched virtual connections (PSVCs). This dynamic resource management scheme increases link utilization due to statistical multiplexing effects. A path-loop elimination algorithm can be applied to remove duplicate resource usages. The DPC scheme in an environment of multi-connection calls is analyzed to obtain three probabilities; 1) new multi-connection call blocking probability, 2) multi-connection handoff call blocking probability, and 3) fast handoff failure probability. dynamic pre-allocated connection, multikey words:

connection, fast hando

1.

Introduction

Wireless ATM networks extend ATM capability over an air interface providing broadband and multimedia services to mobile stations (MSs). In particular, MSs maintain seamless connections through handoffs when the boundary of a base station (BS) coverage is crossed. A long handoff connection time is not desirable for MSs, especially with high speed connections of realtime traffic. Therefore, a fast handoff scheme is required. One important issue for mobility support of MSs is a handoff function. Handoff procedures are implemented during two phases. A re-establishing procedure of wireline connections to a new BS is initiated in the first phase. In the second phase an old radio resource is released and a new radio channel is established from a new BS to the MS. This type of radio resource management is closely related to signaling procedures and call admission control (CAC) functions at an air interface with consideration of multi-access capability. This requires physical layer, multiple access control (MAC), and logical link control (LLC) protocols [1]. There are two approaches for connection manageManuscript received February 8, 2002. Manuscript revised May 10, 2002. † The author was with the Department of Electrical Engineering, Korea Advanced Institute of Science & Technology, 373-1, Kusong-dong, Yusong-gu, Taejon, 305-701, Korea, and is with Electronics and Telecommunications Research Institute, 161 Gajeong-dong, Yuseong-gu, Taejeon, 305-350, Korea. †† The author is with the Department of Electrical Engineering, Korea Advanced Institute of Science & Technology, 373-1, Kusong-dong, Yusong-gu, Taejon, 305-701, Korea.

ment schemes of wireline networks. These are path rerouting and path extension approaches. In the path rerouting scheme a crossover switch (COS) is selected and a new connection segment is established from the COS to a new BS [2], [3]. To keep cell-sequence integrity, however, either the BSs or the COS requires data-buffering. The path extension scheme establishes a new connection segment from the current BS to a new BS and extends the pre-established communication path with this new connection segment. The current BS should have a simple function to forward data streams between the pre-established path and a new connection segment. This approach maintains the sequence integrity of ATM cells on the path without any additional data-buffering. However, duplicated use of network resources reduces the efficiency of the network. Path rerouting schemes have been the subject of past study. Acampora and Naghshineh [4] proposed a virtual connection tree (VCT) consisting a root ATM switch and BS leaves. The VCT scheme enables MSs to perform handoff within a fixed VCT without involvement of network call processors. If the root ATM switch is in an overload state, however, all requests of new calls and handoff calls within a VCT are rejected. Yu and Leung [5] proposed a source-routing mobile circuit (SRMC) scheme in which ATM switches, called tethered-points (TPs), configure dynamically VCTs, including neighboring BSs. Since one VCT is established per MS, an overhead for signaling and processing is required for every new and handoff call. Akyol and Cox [6] proposed an interzone handoff procedure, which they referred to as the nearest common node rerouting (NCNR), which performs connection rerouting at the closest ATM node common to both zones. This scheme results in a delay delays in finding the nearest common node (NCN) and causes signaling overhead during handoff. Extension schemes have also been studied. Veeraraghavan et al. [7] proposed a path extension scheme to extend a pre-established communication path in a wireless ATM LAN environment with direct links between neighboring BSs. This scheme causes delays and signaling overhead for establishment of an SVC during handoff. Lee and Sung [8], [9] proposed a fast handoff management scheme for an ATM-based wireless network environment in which bundles of permanent virtual connections (PVCs) to neighboring BSs are re-

CHANG and SUNG: A FAST HANDOFF SCHEME FOR MULTI-CONNECTION CALLS IN WIRELESS ATM NETWORKS

2003

served for fast handoff. Since the PVC bundles have a fixed bandwidth, this type of static resource management can reduce flexibility of link bandwidth usage. In an attempt to increase the usage of wireline link bandwidth, Chang and Sung [10] proposed a dynamic pre-allocated connection (DPC) scheme in which wireline links are used instead of PVCs for single connection calls based on switched virtual connections (SVCs) because handoff call requests are generated as a Poisson process and can be processed sequentially in a time domain. The dynamic resource management scheme reduces required wireline resources for fast handoff since the reserved bandwidth for fast handoff is used during establishment of an SVC instead of during call holding time. Recently there has been a trend that all the transport connections and services are provided with IPbased networks. The mobile wireless internet forum (MWIF) has currently proposed an IP-based radio access network. Currently ATM-based connections within wireline networks in 3G mobile communication systems may be replaced by IP-transport connections in the mid-term and long-term future. On the other hand, standardization efforts in wireless ATM have been discontinued in the ATM forum. A wideband code division multiple access (WCDMA) system specified at Release 99 in the 3rd generation partnership project (3GPP), however transports multi-media data upon ATM-based connections in radio access networks. The Iur interface in the WCDMA system is defined to interconnect radio network controllers (RNCs) within radio access networks and requires additional signalling procedures and physical links to implement between RNCs. If compared with the Iur interface, the DPC scheme requires that an ATM switch has routing and resource allocation functions to establish ATM connections via the ATM switch. In this paper the DPC scheme is extended to include fast handoff for multi-connection calls. Before handoff attempts are made, BSs reserve SVCs (called pre-allocated SVCs, or PSVCs) that are connected to neighboring BSs. Upon receiving handoff requests, BSs assign PSVCs to quickly extend pre-established communication paths to new BSs. This scheme removes the overhead of establishing an SVC, including call admission control (CAC) and routing processing, without involvement of ATM switches during fast connection extension to new BSs. After executing the fast connection extensions, BSs establish new SVCs to maintain the required number of PSVCs to neighboring BSs. Two wireline link usage policies and two PSVC management schemes are considered. The DPC scheme uses a path-loop elimination scheme with two path-loop detection algorithms to eliminate duplication of link resources.

2.

Dynamic Resource Managements with the DPC Scheme

A wireline network with a star topology is considered here in which several BSs are connected to an ATM switch. In this architecture the ATM switch can establish connections to the attached BSs. When a BS extends a pre-established communication path for a handoff call request, an extended connection segment to a new BS is established via an ATM switch. In the DPC scheme, wireline link resources are used in the form of SVCs instead of PVCs. Figure 1 shows an architecture in which PSVCs between neighboring BSs are reserved either for one ATM switch (between BS1 and BS2) or for several ATM switches (between BS3 and BS4). Therefore, the DPC scheme using PSVCs can perform both intra and inter-switch handoffs. 2.1 Handoff Procedure Figure 2 shows an intra-switch handoff through PSVCs. An MS establishes a new call at BS1 and moves to BS2 during the call. A communication path to the MS is established at BS1 via the ATM switch. Upon receiving a handoff call request to BS2, BS1 uses a reserved PSVC to extend the pre-established communication path and

Fig. 1

An access network with start topology and PSVCs.

(a) Before handoff Fig. 2

(b) After handoff

A fast handoff using PSVCs.

IEICE TRANS. COMMUN., VOL.E85–B, NO.10 OCTOBER 2002

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forwards ATM cells between the pre-established communication path and a new connection segment to BS2. After completing the fast handoff, BS1 establishes a new SVC to BS2 and assigns it as a PSVC. Thus, each BS maintains the required number of PSVCs to meet the quality of service (QoS) requirement for fast handoffs. Figure 3 shows a signaling procedure during establishment of an inter-switch handoff call. An MS measures and traces all the pilot signals from the current BS and neighboring BSs. When the MS requests a handoff from the current BS to a new BS, the MS sends a handoff call request (H/O request) message which includes the address information of both the current and new BSs. Upon receiving the handoff call request from the MS, the current BS checks whether there is an available PSVC to the new BS. If a PSVC is unavailable, the MS may receive a handoff reject (H/O reject) message. If a PSVC is available, a PSVC extension notification (PSVC notify) message is exchanged with the new BS to confirm path extension and the MS may receive a handoff ready (H/O ready) message. For exchange of signaling messages it is assumed that dedicated signaling channels to neighboring BSs are made. The current BS begins to forward the downlink cell stream to the new BS in which the ATM cells are buffered until the radio connection connected to the MS is established. After the MS established a radio channel to the new BS, a handoff complete message is sent to the new BS. Meanwhile, the current BS requests an SVC setup in order to maintain the required number of PSVCs to the new BS for use with future handoffs. ATM switches involved in this connection perform an SVC setup procedure. This SVC setup procedure after a handoff connection requires processing loads on ATM switches and may cause a considerable setup delay that depends on the number of involved ATM switches and their processing capacity. However, in the DPC scheme

For a comparison of resource management between the PVC scheme and the DPC scheme, Fig. 4 shows logical bandwidth occupancy of a link between a BS and an ATM switch in an environment of K heterogeneous connection types and M neighboring BSs. In the PVC scheme the bandwidth of both new calls and handoff calls for the k-th type connection is assigned in a fixed allocation mode for k = 1, · · · , K. The k-th type connections for handoff call requests to the m-th neighboring BS cause establishment of extended connection segments only within the m-th PVC bundle (Pm ) for m = 1, · · · , M . If the m-th PVC bundle for the k-th type connection has no free channels, the connection fails. For the DPC scheme two examples of resource management are considered. These are the complete partitioning (CP) policy, shown in Fig. 4(b), and the complete sharing (CS) policy, shown in Fig. 4(c). In the CP policy the total link capacity C is divided into K fixed capacity partitions. The free bandwidth is used on demand for the k-th type connection within the k-th capacity partition Ck whenever an SVC is requested by by either a new call or a handoff call. If the free bandwidth is exhausted, new call requests are blocked and handoff call requests are accepted using PSVCs. In the CS policy all the connection types share the total link capacity. The bandwidth reserved by PSVCs consists of K bundles of PSVCs. Each connection type has its own traffic descriptors and quality of service (QoS) requirements in a heterogeneous connection environment. These different connection characteristics require one

Fig. 3

Fig. 4 A logical bandwidth occupancy of a link between a BS and an ATM switch.

A signaling procedure for an inter-switch handoff call.

the processing overhead and delay related to establishing an SVC are independent of PSVC assignment procedures for handoff connections. 2.2 Wireline Bandwidth Utilization for Heterogeneous Connections

CHANG and SUNG: A FAST HANDOFF SCHEME FOR MULTI-CONNECTION CALLS IN WIRELESS ATM NETWORKS

2005

bundle of PSVCs per connection type. When handoff call requests of the k-th connection type occur, path extensions are made using the k-th bundle of PSVCs. While different bundles of PSVCs are considered according to connection types, the bandwidth of SVCs in ATM networks can be renegotiated with traffic descriptors and QoS requirements during a call. Using this renegotiation procedure, all connection requests by handoff calls share common PSVCs which are established by worst traffic descriptors (e.g. maximum peak cell rate). After a fast extension with a PSVC, a new BS renegotiates the extended connection segment according to the connection type. This PSVC sharing scheme can maximize resource utilization when the values of traffic descriptors are similar. However, this causes an additional load of connection renegotiations on ATM switches.

(a) Link duplication elimination with local path-loop detection. Fig. 5

(b) Node duplication elimination with global path-loop detection.

Two examples of path-loop eliminations.

2.3 Multi-Connection Calls Consider multi-connection calls which simultaneously require several type connections to provide several type services (for example, voice and fax services). When new multi-connection calls and multi-connection handoff calls are generated, these calls are accepted if all the connections within a multi-connection call are established. A new multi-connection call is admitted only if the free bandwidth is greater than or equal to the required bandwidth. Fast handoff is performed if all the connections within the multi-connection call have been already established using PSVCs. Thus, the admission of multi-connection handoff calls depends on the management policies of the PSVCs. For multi-connection handoff calls, two management schemes on PSVCs are considered: Scheme I reserves PSVCs for each call type and Scheme II reserves PSVCs for each connection type. In Scheme I the reserved bandwidth of the link between an ATM switch and a BS consists of L bundles of PSVCs in which the lth bundle is reserved for the l-th type multi-connection handoff calls given L multi-connection call types. All the connection types for a multi-connection handoff call are made using PSVCs within the same bundle. For K heterogeneous connection types, however, the number of multi-connection call types for Scheme I can be up to 2K −1. In Scheme II each connection type has a bundle of PSVCs with its own traffic descriptors and quality of service (QoS) requirements. Each connection within a multi-connection handoff call is accepted when a PSVC of the matched connection type is available. 2.4 Path-Loop Elimination After handoff completion, a new connection path to the MS consists of a pre-established connection segment to the current BS and an extended connection segment from the current BS to a new BS. Since PSVCs have

been reserved before a handoff request occurs and a PSVC is assigned to the extended connection segment, a path loop may be made when the connection path to the MS traverses the same link or node more than once. This path-loop wastes wireline link capacities and increases end-to-end delays. An optimization algorithm has been outlined with OAM cells [1]. Path-loop detections occur when extended connection segments are established, and path-loops are eliminated with OAM cells at ATM switches with path-loops. This path-loop detection algorithm [1], however, cannot be applied to the DPC scheme due to the independence of pre-established connection segments and extended connection segments. We consider two possible solutions after handoff completion. These are local path-loop detection and global path-loop detection. In the former case, illustrated in Fig. 5(a), the current BS (BS1) can check link duplication on connections with local information. If duplication is present a local duplication check message is sent to ATM switch 1 and the same detection procedure is applied to ATM switches recursively. Upon receiving the local duplication check message, ATM switch 3 may perform loop-elimination procedures. A path-loop is removed by sending a specially defined OAM cell to both the uplink-direction and downlink-direction path-loops. While OAM cells are traversing the path-loop, ATM switch 3 buffers incoming ATM cells locally and forwards both uplinkdirection ATM cells to an ATM network, and downlinkdirection ATM cells to ATM switch 2. The local pathloop detection procedure stops at ATM switch 3. In the latter case, after handoff completion, a new BS (BS2) that terminates a communication path sends a global duplication check (Global dupli. check) message sequentially to all the ATM switches involved in the path. This scheme can detect node and link duplications, as shown Fig. 5(b), since link and node information is carried in the global duplication check messages.

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3.

Performance Analysis

For multi-connection calls in a heterogeneous connection environment, three probabilities are analyzed for the DPC Scheme without path-loop elimination (DPC). These are the new call blocking, the handoff call blocking, and the fast handoff failure probabilities. And this analytical model is extended to the DPC with pathloop elimination (DPC-PLE). 3.1 Basic Assumptions and Notations Consider an environment of L call types and K connection types in which the k-th type connection has a peak cell rate of qk when the DPC scheme without path-loop elimination is applied as a connection management scheme in a wireline access network. A link between an ATM switch and a BS has K bundles of PSVCs. Each PSVC bundle consists of dk × M PSVCs since dk PSVCs are reserved for each of M neighboring BSs. Let τlk denote the number of the k-th type connections within the l-th type call (τlk = 0 or 1). 1. New l-th type calls follow a Poisson process with a mean rate of ηl for l = 1, · · · , L. 2. The call holding time Tlc for the l-th type call, the BS residence time Tlr of MSs with the l-th call type, and the SVC setup time T svc are exponentially distributed. 3. An MS crosses M boundaries with an equal prob1 . ability of M After new or handoff calls are established in the DPC scheme without path-loop elimination, wireline links are held until the calls are terminated due to either call completion or handoff call blocking. Under these assumptions described above the request rate of handoff calls in a BS, λhl and the link holding time Tl for the l-th type calls are given by:
−1 E[Tlr ] λhl = ηl (1 − Pln ) × Plh + . (1) E[Tlc ]
−1 E[Tlr ] r h , (2) E[Tl ] = E[Tl ] × Pl + E[Tlc ] where Plh denotes the handoff call blocking probability and Pln the new call blocking probability for the l-th type calls [8]–[11]. In Eqs. (1), (2) the last term  −1 E[T r ] Plh + E[Tlc ] indicates the mean number of handl offs during a call. A link between an ATM switch and a BS can be used by handoff call requests either from the BS or to the BS. Based on this last assumption the link request rate θl due to the l-th type handoff calls is the sum of all the handoff call request rates λhl to the link.

θl =

M  m=1

2

λhl M

= 2λhl .

(3)

3.2 Markovian Process In order to analyze link capacity between an ATM switch and a BS we describe the system state w = (x1 , · · · , xL ; h1 , · · · , hK ). In this state, xl denotes the number of the l-th type calls established by both new call requests and handoff call requests for l = 1, · · · , L, and hk represents the number of the k-th connection type PSVCs which are used by handoff call requests but which are not newly reserved with new SVCs due to a lack of free bandwidth for k = 1, · · · , K. If a BS can maintain the required number of the k-th connection type PSVCs (dk M after a handoff connection) the value of hk must be zero (from the definition of the system state w). Whenever the value of hk is not zero, the free link bandwidth is less than the required bandwidth of the k-th type connection and new k-th type connection call requests are blocked. We introduce the following notations: • Ω : sample space of a Markov process W . • Ul : subset of Ω such that both new calls and handoff calls of the l-th call type are admitted. • νlk (w) : number of k-th connection type PSVCs that are used but not reserved with new SVCs due to a lack of free bandwidth when an l-th type handoff call arrives (νlk (w) = 0 or 1). • lk (w) : number of k-th connection type PSVCs that are reserved with new SVCs when an l-th type call is released. • wln+ : state that an l-th type new call is admitted from w. • wlh+ : state that an l-th type handoff call is admitted from w. • wl− : state that an l-th type call is released from w. w = (x1 , · · · , xL ; h1 , · · · , hK ). wln+ = (x1 , · · · , xl + 1, · · · , xL ; h1 , · · · , hK ). wlh+ = (x1 , · · · , xl + 1, · · · , xL ; h1 + νl1 (w), · · · , hK + νlK (w)). wl− = (x1 , · · · , xl − 1, · · · , xL ; h1 − l1 (w), · · · , hK − lK (w)).

(4) (5) (6) (7)

Two management policies for wireline links can be used to specify sets and variables. These are the complete partition (CP) and the complete sharing (CS) policies. Case 1. CP policy (Refer to Fig. 4(b))    (Ck − qk < Gk + Dk ≤ Ck , 0 <  Ω = w hk ≤ dk M ) or (Gk + Dk ≤ Ck , . (8)   hk = 0) for k = 1, · · · , K

CHANG and SUNG: A FAST HANDOFF SCHEME FOR MULTI-CONNECTION CALLS IN WIRELESS ATM NETWORKS

2007

Ul =

 w ∈ Ω, Gk + Dk + τlk qk ≤ Ck w . (9) for k = 1, · · · , K   1,

νlk (w) =

τlk = 1, hk < dk M, and Gk + Dk + qk > Ck . o.w.

 0,

1, τlk = 1 and hk > 0 lk (w) = , 0, o.w.

(10) (11)

where Ck denotes the dedicated link capacity for the k-th type connection. The used bandwidth is Gk = L qk ( l=1 xl τlk − hk ) and the bandwidth reserved by the k-th type bundle of PSVCs is Dk = qk dk M for the k-th type connection. Case 2. CS policy (Refer to Fig. 4(c)) When l-th type calls are released, the required numbers of PSVCs are checked for all the connection types of τlk = 1. If the required number of PSVCs is not reserved, a new SVC of the k-th connection type is established and is assigned as a new PSVC. The required number of PSVCs are maintained in the ascending order of connection type indexes. Let Jlk (w) denote the indication function that given w, a k-th type PSVC of τlk = 1 is reserved when an l-th type call is released.

1, τlk = 1 and hk > 0, Jlk (w) = . (12) 0, o.w. Then,

   C − qk < G + D ≤ C, 0 < hk  Ω = w ≤ dk M orG + D ≤ C, hk = 0 . (13)   for k = 1, · · · , K

Ul = {w| w ∈ Ω, G + D + rl ≤ C} .

(14)

 1,     

τlk = 1, hk < dk M, and (G + D+ k−1 νlk (w) = . i=1 δ(νli (w))qi   +q > C  k   0, o.w.    (w) lk (w) = Min hk − Jlk (w), C−Bqlk k +Jlk (w).  Blk (w) = G + D − rl + K i=1 Jli (w)qi k−1 + i=1 li (w)qi ,

Given state w, the new call admission probability ζl (w) and the handoff call admission probability χl (w) are introduced for the l-th type calls. If hk > 0, a kth type connection for a handoff call is admitted with probability 1 − γk (hk ), for k = 1, · · · , K, where the exhaustion probability of PSVCs to a new BS, γk (hk ) is derived in Appendix A of the reference [10]. From the definition of Ul , ζl (w) and χl (w) are given by:

1, w ∈ Ul ζl (w) = . (18) 0, w ∈ Ulc  w ∈ Ul  1, (19) χl (w) = (1 − γ (h )), w ∈ Ulc , k k  k∈Ol

where the set of connection types with τlk = 1 for the l-th type call, Ol is {k|τlk = 1} and Ulc is the complementary set of Ul . Let π(w) denote the steady state probability of a state w. The Markovian equilibrium balance equations are required to derive π(w). Since states related to w in the balance equations are too complicated to express formally, we introduce a simple iteration algorithm that is equivalent to solving the balance equations. 1. Initialize πi (w) for i = 1. L 2. E(w) = l=1 (ζl (w)ηl + χl (w)θl + xl µl ) × πi (w) and F (w) = 0 for all w. 3. F (wln+ ) = F (wln+ ) + ζl (w)ηl × πi (w), F (wlh+ ) = F (wlh+ )+χl (w)θl ×πi (w), and F (wl− ) = F (wl− )+ xl µl × πi (w) for all w and l = 1, · · · , L. F (w) πi (w) for all w. 4. πi+1 (w) = E(w) 5. Stop if |πi+1 (w) − πi (w)| <  for all w ( is very small value). Otherwise, repeat from step 2 with i = i + 1,

(15)

where E(w) denotes the transition-out rate from w, F (w) the transition-in rate into w, and the departure rate in a BS is ul = [E1r ] . From the above algorithm l the steady state probability π(w) can be obtained.

(16)

3.3 Call Blocking and Fast Handoff Failure Probabilities

(17)

We focus on a wireline link between an ATM switch and a BS in a wireline access network. Links between ATM switches are assumed to be sufficient enough to support all both new calls and handoff calls. For state w, k-th type new and handoff call requests are blocked due to a lack of free bandwidth in the wireline link with probabilities of 1 − ζl (w) for new call requests and and 1 − χl (w) for handoff call requests. New call requests may be blocked at the wireline link to the current BS, and handoff call requests may be rejected due to connection failures at either the wireline link to the current BS or to a new BS. The new call blocking probability Pln and the handoff call blocking probability Plh for l-th

where C denotes the total link capacity. The total K bandwidth of the l-th type call is rl = k=1 τlk qk , the K L used bandwidth is G = l=1 rl xl − k=1 hk qk , and the  bandwidth reserved with PSVCs is D = K k=1 qk dk M . Blk (w) denotes the unavailable bandwidth to the k-th type PSVC, which is not satisfying the required number when an l-th type call is released. And δ(x) =

1 x = 0, and [x] is the largest integer equal to 0 x = 0, and less than x.

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type calls are derived from the steady state probability π(w) as follows:    n Pl = 1 − ζl (w)π(w) . (20) w∈Ω

 Plh = 1 −



2 χl (w)π(w)

.

(21)

w∈Ω

Handoff call requests may fail in fast handoffs due to exhaustion of PSVCs from the current BS to a tagged BS. This exhaustion occurs when dk or more than dk consecutive handoff requests to the tagged BS are generated during the SVC setup time T svc . To obtain the fast handoff failure probability Plf to the tagged BS, a process V = (V1 , · · · , VL ) is introduced, where Vl is the number of the l-th type call requests to the tagged BS during the SVC setup time T svc . If all the types of connections involved in the l-th type handoff call are available, a fast handoff call is successfully connected. Let Il be a subset of the sample space of a Markov process V such that the l-th type call can successfully have a fast handoff, given v = (v1 , · · · , vL ) ∈ Il .   L  Il = v| vn τnk ≤ dk − 1 for k ∈ Ol . (22) n=1



Plf

=

Plh

+

1−

L  v∈Il

where ϕn =

(ϕn )vn exp(−ϕn ) vn ! n=1

 , (23)

2λh svc n ]. M E[T

3.4 Path-Loop Elimination The call model of the DPC scheme without pathloop elimination is easily used in the DPC scheme with path-loop elimination (DPC-PLE) if E[Tl ] and θl are modified. After handoff procedures are completed, a new BS can execute a path-loop elimination algorithm for the call. Let Tlpl denote the completion time of a path-loop elimination algorithm for the l-th type call. Calls established in the wireline link hold the links either until the calls are released or until conversing MSs cross the boundary to a neighboring BS. Thus, the mean link holding time is given by: E[Tl ] = Prob(call releases)E[Tlc ] + Prob(handoff)(E[Tlr ] + E[Tlpl ]). In the wireline link the expected value of link holding time E[Tl ] is given by: E[Tl ] =

wireline link capacity during Tlpl and the latter handoff request rate affects the wireline link capacity during Tl . If E[Tl ] >> E[Tlpl ], the link request rate θl is given by: θl = λhl

(24)

Other parameters and formulae of the analytical model of the DPC scheme without path-loop elimination (DPC), except E[Tl ] and θl , are applied to the DPC scheme with path-loop elimination (DPC-PLE). 4.

Numerical Examples

Consider a single connection type environment (T0) with single call type (C0) on a wireline access network in a square-shaped cell structure. The peak rate of 0-th type connections (T0), q0 is 1 Mbps. The call holding time E[T0c ] is 2 min/call, the BS residence time E[T0r ] is 4 min/BS, and the traffic load is 36 Erlang/BS for the 0-th type call (C0). For the DPC scheme the fast handoff failure due to consecutive handoff requests depends on the number of PSVCs per boundary. Figure 6 shows the fast handoff failure probabilities versus the load of handoff call requests during SVC setup time, ϕ0 (= 2λh0 × E[T svc ]) for varying the number of PSVCs per neighboring BS, d0 = 1, · · · , 5. When the fast handoff failure probability is 0.1%, the allowable values of ϕ0 are 0.001, 0.04, 0.2, 0.4, and 0.7 Erlang/boundary for d0 = 1, · · · , 5 PSVCs, respectively. Figure 7 shows the new call blocking and the fast handoff failure probabilities of the DPC scheme, the path extension (PE) scheme, and the PVC scheme for E[T svc ] = 100 ms and d0 = 2. Since the DPC scheme reserves the minimum bandwidth for the PSVCs, the available capacity for new calls is greater than for the PVC scheme. This property yields a lower new multiconnection call blocking probability of the DPC scheme than for the PVC scheme. For the PVC scheme most new multi-connection calls are blocked even in a smaller capacity range. As the link capacity increases, handoff

E[Tlc ](E[Tlr ] + E[Tlpl ]) E[Tlc ] + E[Tlr ]

The link request rate θl for the l-th type call consists of a handoff request rate from the current BS to neighboring BSs and a handoff request rate from neighboring BSs to the current BS. In the path-loop elimination model, the former handoff request rate affects the

Fig. 6 Fast handoff failure probabilities due to consecutive handoff requests.

CHANG and SUNG: A FAST HANDOFF SCHEME FOR MULTI-CONNECTION CALLS IN WIRELESS ATM NETWORKS

2009 Table 1

Parameters.

(a) Connection types parameters

T1

T2

Peak rate qk (Mbps) Number of PSVCs per BS, dk

1 2

2 2

[ht] (b) Call types parameters Connection type Call holding time E[Tlc ] (min/call) BS residence time E[Tlr ] (min/BS) Traffic load (Erlang/BS) New call arrival rate (calls/sec)

C1

C2

C3

T1 2 4 12 0.10

T2 2 4 6 0.05

T1+T2 2 4 4 0.033

(a) New call blocking probability.

(b) Fast handoff failure probability for the DPC and PVC schemes and handoff call blocking probability for the PE schemes. Fig. 7 Call blocking/fast handoff failure probabilities versus wireline link capacity C.

calls increase and an increase in handoff calls results in a higher handoff failure probability because the link capacity reserved for handoff calls is fixed. In the DPC scheme, fast handoff failures occur due to the two factors that are reflected in Eq. (23). Fast handoff failures due to the fixed capacity for PSVCs is dominant in a large capacity range where the DPC scheme has a fixed, lower bound for the value of the fast handoff failure probability. To maintain the required QoS requirements of P0n = 1%, P0h = 0.1%, and P0f = 0.1%, the DPC and the DPC-PLE schemes require link capacities of 97 and 58 Mbps, respectively. While supporting fast handoffs, the DPC and the DPC-PLE use only approximately 80% and 50% of the required bandwidth of the PVC scheme, respectively. Next, we consider an environment of two traffic types (T1 and T2) and three call types (C1, C2, and C3) including a multi-connection call (C3), as shown in Table 1. Figure 8 shows the new call blocking probabilities and the fast handoff failure probabilities of the DPC scheme without path-loop elimination for the CS

Fig. 8 New call blocking/fast handoff failure probabilities for the CS policy with PSVC management Scheme II: analysis and simulation results.

resource management policy with PSVC management Scheme II. The new call blocking and fast handoff failure probabilities depend on the required bandwidths of the calls. Mobile users of the third type calls experience higher values of both the new call blocking probability and the fast handoff failure probability than do C1 and C2 users since the multi-connection call has two connection types. Figure 9 shows the blocking probabilities of the DPC scheme based on the CP policy with the PSVC management Scheme II for varying the first type connection (T1) to the total link capacity when the total link capacity is C = 120 Mbps. As the link capacity of T1 increases, first type (C1) calls experience a lower degree of new call blocking and fast handoff failures. Second type (C2) calls experience a higher degree of new call blocking and fast handoff failures since the link capacity for T2 is reduced. Third type (C3) calls are accepted when all the T1 and T2 connection types are established. Therefore, third type (C3) calls experience all the connection blocking probabilities of both types T1 and T2. A T1 link capacity ratio of 0.42 (50 Mbps) yields the lowest new call blocking probability for third type call MSs.

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the CP policy due to the flexibility of link capacity usage. 5.

Fig. 9 New call blocking/fast handoff failure probabilities versus the link capacity ratio of T1 for the CP policy with PSVC management Scheme II.

Conclusions

A dynamic pre-allocated connection (DPC) scheme is proposed for dynamic resource management in order to support fast handoff and to effectively use wireline links for intra and inter-switch handoff in wireless ATM networks. PSVCs, which are a set of SVCs reserved before handoff requests, are assigned for handoff calls to extend pre-established communication paths. Since the DPC scheme uses wireline link capacity on demand, link capacity is used effectively due to statistical multiplexing effects. If path-loop elimination algorithms with either local loop or global loop detection are applied, the required wireline bandwidth can be reduced. The proposed schemes are evaluated in an environment of heterogeneous connections and multiconnection calls with the two resource management policies (the complete partitioning and the complete sharing policies). Numerical examples show that the DPC scheme without path-loop elimination requires a wireline link capacity of approximately 80% due to statistical multiplexing effects, compared with the PVC scheme, and the DPC-PLE scheme requires a minimum bandwidth. The CP policy with PSVC management Scheme II may use more wireline resources than the CS policy with PSVC management Scheme II. References

Fig. 10 The required wireline bandwidth versus the BS residence time.

Figure 10 shows the required link capacity versus the BS residence time for the PVC scheme, the DPC scheme with the CP policy and PSVC management Scheme II, the DPC scheme with the CS policy and PSVC management Scheme II, and the DPC-PLE scheme with the required QoS requirements of Pkn = 1%, Pkh = 0.1%, and Pkf = 0.1% for k = 1, · · · , 3. As the BS residence time of MSs increases, the number of handoff requests during a call decreases. In the DPC scheme without path-loop elimination the required wireline bandwidth decreases because the traffic load carried to a BS, (ηk (1 − Pkn ) + θk )E[Tk ], is proportional to the handoff request rate. In the DPCPLE scheme, however, the traffic load carried to a BS is nearly constant and is independent of the BS residence time. The DPC-PLE scheme uses the wireline link capacity most efficiently for fast moving users. However, frequent handoff requests require ATM switches and BSs to process heavy handoff traffic loads which are proportional to the speed. The DPC scheme with the CS policy is more efficient than the DPC scheme with

[1] A. Acharya, J. Li, B. Rajagopalan, and D. Raychaudhuri, “Mobility management in wireless ATM networks,” IEEE Commun. Mag., vol.35, no.11, pp.100–109, Nov. 1997. [2] C.K. Toh and S.K. Dao, “Crossover switch discovery for wireless ATM: Principles and implementation outlines,” ATMF/97-0669, Montreal, Canada, July 21–25 1997. [3] C.K. Toh, “Performance evaluation of crossover switch discovery algorithms for wireless ATM LANs,” Proc. IEEE INFOCOM, pp.1380–1387, 1996. [4] A.S. Acampora and M. Naghshineh, “An architecture and methodology for mobile-executed handoff in cellular ATM networks,” IEEE J. Sel. Areas Commun., vol.12, no.8, pp.1365–1375, Oct. 1994. [5] O.T.W. Yu and V.C.M. Leung, “B-ISDN architectures and protocols to support wireless personal communications internetworking,” Proc. PIMRC, pp.768–772, 1995. [6] B.A. Akyol and D.C. Cox, “Rerouting for handoff in a wireless ATM Networks,” IEEE Personal Commun., pp.26–33, Oct. 1996. [7] M. Veeraraghavan, M.J. Karol, and K.Y. Eng, “Mobility and connection management in a wireless ATM LAN,” IEEE J. Sel. Areas Commun., vol.15, no.1, pp.50–68, Jan. 1997. [8] S.J. Lee and D.K. Sung, “A new fast handoff management scheme in ATM-based wireless mobile networks,” Proc. IEEE GLOBECOM, pp.1136–1140, 1995. [9] S.J. Lee and D.K. Sung, “A fast handoff management scheme in ATM-based personal communication networks,” Wireless Personal Communication, vol.11, pp.231–245, 1999.

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[10] S.C. Chang and D.K. Sung, “A dynamic resource management scheme for fast handoff in wireless ATM networks,” IEICE Trans. Commun., vol.E82-B, no.6, pp.978–983, June 1999. [11] Y.B. Lin, S. Mohan, and A. Noerpel, “Queueing priority channel assignment strategies for PCS hand-off and initial access,” IEEE Trans. Veh. Technol., vol.43, no.3, pp.704– 712, Aug. 1994.

Appendix:

Exhaustion Probability of PSVCs to a New BS γk (hk )

If there are no available PSVCs between the current BS and a new BS, a fast handoff request fails. We derive the exhaustion probability of PSVCs connected to a new BS for the k-th type connections when hk PSVCs among M · dk PSVCs are exhausted. Let N (b, c, e) denote the number of all possible events when e PSVCs are exhausted if a BS has c bundles of b PSVCs.  1, e=0     0, e = 0 and b · c = 0   c    , N (b, c, e) = b b c        ··· δ(e − im ), o.w.   i1 =0

ic =0

m=1

(A· 1)

1, x = 0 and im is the number of 0, x = 0 PSVCs that are assigned for handoff calls to the m-th neighboring BS but not reserved with new SVCs. Let P (z|hk ) denote the probability that z boundaries among M boundaries have no available PSVCs when hk PSVCs among M · dk PSVCs are exhausted. Given hk , it is possible that the variable z can have up to [ hdkk ], where [x] is the largest integer less than or equal to x. P (z|hk ) is the ratio of the number of events that only z boundaries among M boundaries exhaust all the PSVCs of dk and that (M − z) boundaries have at least one PSVC if (hk − z · dk ) PVCs are exhausted, to the number of all the possible events.  M Cz ·N (dk −1,M −z,hk −z·dk ) 0 ≤ z ≤ [ hdkk ] N (dk ,M,hk ) . P (z|hk ) = 0 [ hdkk ] < z where δ(x) =

(A· 2) Then, γk (hk ) is given by: γk (hk ) =

M  z · P (z|hk ). M z=1

(A· 3)

Sung Cheol Chang received the B.S. degree in electronics engineering from KyungPook National University in 1992 and the M.S. and Ph.D. degrees in electrical and electronics engineering from the Korea Advanced Institute of Science and Technology (KAIST), Taejeon, Korea, in 1994 and 1999, respectively. Since July 1999, he has been with a Senior Member of Research Staff with the Electronics and Telecommunications Research Institute (ETRI), Taejeon, Korea. His research interests include radio resource management, admission control, traffic control, multiple access protocol, and CDMA system engineering in wireless mobile communication systems and development of radio network controller systems including IMT-2000.

Dan Keun Sung received the B.S. degree in Electronics Engineering from Seoul National University in 1975 and the M.S. and Ph.D. degrees in Electrical & Computer Engineering from the University of Texas at Austin, in 1982 and 1986, respectively. From May 1977 to July 1980, he was a research engineer at the Electronics & Telecommunications Research Institute (ETRI) where he had been engaged in various projects including the development of an electronic switching system. In 1986 he joined the faculty of KAIST where he is currently Professor at the department of Electrical Engineering and Computer Science. He was Director of the Satellite Technology Research Center(SaTReC) of KAIST from 1996 to 1999. He is Editor of IEEE Communication Magazine. He is also Editor of the Journal of Communications and Networks. He is currently Vice Chair of ICC 2002 Symposium on Global Service Portability & Infrastructure for Next Generation Virtual Home & Office Environments and Program Cochair of Globecom 2002 Symposium on Service Infrastructure for Virtual Enterprise Environments. His research interests include mobile communication systems & networks, high speed networks, next generation IP based networks, traffic control in wireless & wireline networks, signaling networks, intelligent networks, performance & reliability of communication systems, and microsatellites. He has published more than 250 papers in journals and conferences and has filed more than 80 patents or patents pending. He is a Senior Member of IEEE, and a Member of KICS. He is also a Member of Phi Kappa Phi and Tau Beta Pi.