Provisioning multicast QoS for WDM-based optical wireless networks

Computer Communications 27 (2004) 1025–1035 www.elsevier.com/locate/comcom

Provisioning multicast QoS for WDM-based optical wireless networks Ko-Shung Chena,*, Chao-Ping Yub, Chiao Yua, Nen-Fu Huangb a

Department of Information Management, St John’s and St Mary’s Institute of Technology, Tamshui, Taipei, Taiwan, ROC b Department of Computer Science, National Tsing Hua University, Hsinchu, Taiwan, ROC

Abstract Wavelength division multiplexing (WDM)-based optical technique is becoming the right choice for the next-generation Internet infrastructure to transport high-speed Internet Protocol traffic. Today, wireless asynchronous transfer mode (WATM) network plays the leading role; undoubtedly it will be replaced with wireless WDM (WWDM) network to provide higher quality of service for mobile users. In addition many conventional and emerging applications, such as teleconferencing and distributed games, require multicasting increasingly. Many efforts to support multicasting for WATM networks have been offered, yet there are rarely addressed on WWDM networks. Moreover, conventional operations to set up and tear down optical connections in WDM networks are unable to apply directly to WWDM networks. User’s mobility may make the operating optical-multicast-tree to change into very inefficient. The tree may expand and consume excessive resources, and even lost connection due to violating the associated QoS constraints. In this paper, a Constrained Optical Tree Migration Scheme (COTMS) is proposed to support real-time multicast services for WWDM networks. COTMS efficiently deals with the constrained tree migration problem and adapts easily to the operations of WDM. Simulation results reveal that COTMS can markedly reduce the resources used by per optical tree, thus achieving both low handoff-dropping/join-blocking rate and high-resource utilization. Moreover, COTMS is also suitable for ad-hoc networks with multiple different frequencies for multicast routing. q 2004 Elsevier B.V. All rights reserved. Keywords: Optical wireless network; Light-tree migration; Wavelength conversion

1. Introduction Advances in networking architectures and protocols are driven by both new inventions in communications technologies and new requirements in applications. The explosive growth of the Internet and bandwidth-intensive applications, such as multimedia conferencing and distributed games, require high-bandwidth transport networks whose capacity is much beyond what current high-speed networks, such as asynchronous transfer mode (ATM) network, can provide. While the need of communication channels for high-bandwidth, low latency, and reasonable packet loss rate has been on the rise, a continuous demand for networks of high capacities at low costs is seen now. This can be achieved with the help of optical networks, as the optical fiber provides an excellent medium for transfer of huge amounts of data (nearly 50 terabit per second (Tbps)). Apart from providing such huge bandwidth, optical fiber has extremely low bit error rates, low signal attenuation, low signal distortion, low power requirement, low material use, * Corresponding author. Tel.: þ886-2-2801-3131; fax: þ 886-2-28013143. E-mail address: [email protected] (K.-S. Chen). 0140-3664/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2004.01.025

and small space requirement. In addition, optical fibers are more secure, compared to copper cables, from tapping and are immune to interference and crosstalk. Optical network employing wavelength division multiplexing (WDM) is seen as the technology for the next-generation Internet (NGI) networks to transport high-speed Internet Protocol (IP) traffic [1]. At the same time, the evolution of IP-based office applications with sufficient roaming capability has created a strong demand for public wireless broadband access technology. The demand requires more capacity far beyond current cellular systems. Recently, wireless LANs linked together with one backbone network is in overcoming the inherent limitations of wireless wide area networks, such as 3G cell phones [17]. The new integrated infrastructure of dense urban broadband wireless LANs provides wider bandwidths, even higher data rates, global availability, and quality approaching that of the traditional wireline networks. Future wireless systems are envisioned to provide high-capacity, ubiquitous wireless communication in a variety of environments, including indoor office, pedestrian, vehicle, and satellite. To fulfill the dream of making anywhere and anytime communications a reality, wireless

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service providers need cost-effective, scalable, and reliable alternatives in backbone networks to lease wireless core infrastructure and eliminate existent leased lines, such as T1/T3 and E1/E3. Meanwhile, the ubiquitous applications of the IP have led to the much-touted IP over WDM as the core architecture for the next-generation optical Internet [2]. These phenomena will force the world toward a realizable convergence under the wireless IP-based optical technology (e.g. wireless IP over WDM) in the near future. For providing expansive and ubiquitous wireless access, the core infrastructure employs optical fiber to provide large capacity and high-quality transport interface. Recently, wireless ATM (WATM) architecture has been deployed widely. Operations developed over WATM networks seem to be able to directly extend to new wireless WDM (WWDM) optical networks. For WWDM, however, current most efforts on connection management are all concentrated on wireless and optical networks separately, and very little attention devoted to the integration of wireless and optical networks. A unifying methodology to resolve the handoff problems of heterogeneous connections for mobile hosts has been proposed in Ref. [3], however, the studied environment is not focused on WWDM. Connection routing and maintaining over WWDM introduces a new set of challenges since both the constraint of wavelength-continuity in optical area and the mobility behavior of users in wireless domain should be considered together. Conventional routing and wavelength assignment schemes for WDM networks are not any more suitable for WWDM. This is because traditional methods just consider connections setting up and tearing down, but lack of the provisioning of handoff management. We know that the traffic pattern of mobile network is intensely dynamic. Roaming caused by any member of the communication group can result in severe traffic flow interruptions if the handoff operation is not performed properly. In the worst case, improper scheme may create routing loops and waste too much system resource. Previously, we have proposed a unifying and uniform constrained multicast tree (CMT) migration scheme for generic wireless systems, called Constrained Tree Migration Scheme (CTMS) [4]. CTMS is designed for wireless circuit-switched networks, and it is not completely suitable for wireless wavelength-routed WDM network due to a restriction known as the wavelength-continuity constraint. However, the basic operations can be extended and conformed to the requirements of WWDM architecture. To help the control system possessing, an efficient multicast capability in WWDM networks, a Constrained Optical Tree Migration Scheme (COTMS) for supporting real-time multicast services in optical wireless networks is presented herein. This scheme is inspired by the expected trend with regard to WDM networks. COTMS is a superset of CTMS and inherits its efficiencies entirely. Simulation results reveal that COTMS can significantly reduce the

resources occupied by per optical tree, thus achieving both low handoff-dropping/join-blocking rate and high-resource utilization. The rest of this paper is organized as follows. Section 2 describes WWDM network architecture and the related work. Section 3 defines and formulates the constrained optical tree migration problem. Meanwhile, Section 4 briefly reviews the incorporating scheme CTMS and presents the operations of COTMS. Finally, simulation results are given in Section 5, and are followed by conclusions.

2. Network architecture and related work Most current wireless network architectures are based on the structural integration of the wireless portions into the wired backbone of packet switching networks, such as the famous WATM [5,6]. Another emerging broadband-wireless-access (BWA) architecture has been proposed for providing many new approaches and platforms for highbandwidth wireless access and distribution networks [7]. This technology has created potential for increased capacity, and extended the fiber-based bandwidth and services to users via wireless. In their demonstration, a Free-Space Optical Wireless point-to-point link is employed to complement and extend the NGI wireless access capabilities for true gigabit-per-second data transport. In such architecture, Hybrid fiber radio, RF photonics, and radio on fiber technologies are appropriated to interconnect the access points to the backbone fiber network. As a result, based on WDM technology, the links are capable of transporting both high-speed digital and analog signals as well as multiple wireless services. WDM multiplexes a number of optical signals with different wavelengths into a single optical fiber. The previous technology, time division multiplexing, used only a single wavelength per optical fiber, while WDM increases the number of wavelengths handled to double or triple the data rate, just as if new optical fibers were installed. In addition, compared to the costs and time that would be involved in running new fibers, WDM is significantly cheaper. In support of the NGI reach network extension, this makes the possibility by utilizing the optical broadband backbone network integrated and combined with wireless technology. This approach leads to faster deployment and lower system cost for service providers through a single shared and integrated infrastructure. Based on the BWA concept and implementation techniques, a typical wavelength-routed WDM-based cellular network is shown in Fig. 1, in which the covered geographical area is partitioned into a set of disjoin clusters, each consisting of a set of microcells. Each microcell is equipped with a base station to serve the mobile hosts within the cell. The base stations in each cluster are linked locally by an access node attached to an optical switch that forms

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Fig. 1. A wavelength-routed WDM-based cellular network.

a wireless LAN. Meanwhile, optical backbone links are used to connect pairs of optical switches that constructs a wireless WAN. For high-speed transmission, in the WDM backbone networks, using lightpath to carry circuit-switched traffic enables data to be switched entirely in the optical domain without requiring conversion to electronics at the intermediate nodes [8]. For multicasting, a light-tree concept has been proposed to sufficiently utilize all of the wavelengths on all of the fiber links in the network [9]. The establishment of lightpath (light-tree) is referred to as path (tree) multiplexing for which the same wavelength must be assigned to each link on the path. This restriction is known as the wavelength-continuity constraint. In a wavelengthrouted network, the traffic can be either static or dynamic. In a static traffic pattern, developer may be able to design an optimal logical topology with regard to lightpaths (light-trees) by statistical measurement. In a dynamic traffic pattern, however, the optimal logical topology is difficultly obtained due to data traffic derives from somewhere indefinitely. Therefore, the on-demand lightpath (lighttree) establishment architecture will be appropriate for service providers to respond quickly and economically to customer demands. However, it is still difficult to design adaptive lightpath (light-tree) based architecture if wireless applications is involved therein. Many approaches have been proposed for fixed WDM networks to establish optical connections dynamically; however, the utilization of available wavelength is still low in such networks [10]. On the other hand, at interfaces where multiple WDM lines intersect, a technology is required that can extract only the precise single frequency from within the multiplexed signals, and convert it to another wavelength or transfer it elsewhere. If the intermediate optical switches through which the lightpath passes are equipped with wavelength converting

devices, the wavelength-continuity constraint can be relaxed. The establishment of lightpath (light-tree) is referred to as link multiplexing. Such a technique is feasible and networks with this capability are referred to as wavelength-convertible networks. Because all-optical tree on the WWDM network is difficultly preserved when the group size is large, a WWDM network that supports complete wavelength conversion functions at all-optical switches is considered herein. Such a network is functionally equivalent to a circuit-switched network, i.e. lightpath requests are blocked only when there is no available capacity on the path. Although wavelength conversion may improve the efficiency in the network by resolving the wavelength conflicts of the lightpaths, myriad of wavelength conversions produces a larger latency between sender and receiver. This phenomenon will be more serious in WWDM networks due to mobility subjects the topological connectivity to frequent and unpredictable changes. Moreover, real-time connection can be injured from the increasing delay jitter during the mobile host moves to different locations continuously. In order to the QoS constraint would not be violated, the number of wavelength conversion should be kept in as small as possible. WWDM system needs an efficient regime to support a new (existing) mobile user initially joins (sometimes roams, finally leaves) the group. In general, multicast applications often require users to send the same information to multiple recipients. The conventional means of enhancing the resource usage efficiency is to construct a multicast tree, which spans the group members in the session and along which session communication is conducted. A more detail review of existing approaches about mobility management in nextgeneration wireless systems can be caught in Ref. [13], and related multicast algorithms possibly used for wireless

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systems can be found in Ref. [4]. Detailed presentations relating to COTMS will be given later, but first the multicast optimization problem in WWDM networks will be formally defined.

3. Problem definition This section presents a modeling of the wavelengthconvertible WWDM network using a graph approach, and formulates the CMT optimization problem. A WWDM backbone network is represented by a weighted undirected graph G ¼ ðV; EÞ with a set of nodes V and a set of links E: The nodes represent optical switches and the links represent the fiber links between the optical switches. The bandwidth of each link is wavelength division de-multiplexed into a set of n wavelengths, L ¼ {l1 ; l2 ; …; ln }: A tree T rooted at s on G consists of a set of connected links, each of which occupies a portion of single dedicated wavelength (i.e. WDM) in the discrete wavelength set L: In WWDM network, mobile clients may dynamically and freely ask to join the multicast trees rooted at the servers. A connection has to be established linking the mobile terminal and the server to serve the join request. This path comprises a sequence of wavelength channels, one involved in each backbone link. As numerous paths are established for requesting the same service on a server, a multicast tree is constructed, with root at the cluster associated with the server. For each multicast tree, a link is a dedicated link if it is only included in one of the paths linking the root and leaves of the multicast tree. Otherwise, a link is called a shared link. Actually, each path linking a leaf node and the root consists of two subpaths, the first of which is a sequence of shared links (denoted as shared-subpath, SSP), while the second is a sequence of dedicated links (denoted as dedicatedsubpath, DSP). An example considered here to explain the terms of SSP and DSP, a multicast tree T shown in Fig. 1 for a server within v3 and two mobile hosts in clusters v4 and v7 ; respectively. The multicast tree includes the paths ðv1 ; v5 Þ; ðv1 ; v6 Þ; ðv3 ; v2 ; v6 ; v5 ; v4 Þ and ðv3 ; v2 ; v6 ; v7 Þ: The former path may be created due to the lack of available wavelength channels in the link ðv2 ; v4 Þ when it is established. Meanwhile, the subpath ðv3 ; v2 ; v6 Þ is an SSP shared by paths ðv3 ; v2 ; v6 ; v5 ; v4 Þ and ðv3 ; v2 ; v6 ; v7 Þ while the subpaths ðv6 ; v5 ; v4 Þ and ðv6 ; v7 Þ are DSP dedicated to the path ðv3 ; v2 ; v6 ; v5 ; v4 Þ and ðv3 ; v2 ; v6 ; v7 Þ; respectively. This multicast tree requires five wavelength channels (two shared links and three dedicated links), assuming one wavelength channel for each backbone link over the multicast tree. Some other multicast trees occupying the channels of link ðv2 ; v4 Þ may later be terminated, making the channels once again available. In this case, the path ðv3 ; v2 ; v6 ; v5 ; v4 Þ can be migrated to ðv3 ; v2 ; v4 Þ to save one wavelength channel (one shared link and three dedicated links).

Notably, ðv3 ; v2 ; v6 Þ is a SSP belonging to paths ðv3 ; v2 ; v6 ; v5 ; v4 Þ and ðv3 ; v2 ; v6 ; v7 Þ; and a split point occurred at node v6 : However, for node v6 ; path ðv3 ; v2 ; v6 Þ can be considered as a DSP linking node v6 and the root node, which can also be migrated to a shorter path, subject to availability, such as ðv3 ; v6 Þ: Additionally, T is a light-tree on wavelength l2 means that there has no wavelength conversion from source to all destinations. The path ðv3 ; v2 ; v6 ; v5 ; v4 Þ can be migrated to ðv3 ; v2 ; v4 Þ in terms of the efficiency of resource utilization; however, node v2 must perform a wavelength conversion function if wavelength l2 is not available on link ðv2 ; v4 Þ: If the delay constraint associated with the mobile host d1 is violated and it is caused by the wavelength conversion, then this path migration will fail and another alternative may be initiated, if any. Next, the optimization parameters will be defined explicitly. The link-cost costðeÞ function is a measure of the link utilization. The link-cost discriminates network links based on wavelength capacity. The total cost of tree costðTÞ is the sum of costðeÞ; for all e [ T: If there are two links assigned with different wavelength and concatenated by a node, then the wavelength conversion function should be involved in this node. However, this will introduce a larger latency. The node-delay function delayðvÞ is defined as 1 if a wavelength conversion function is performed prior the packet transmitted; otherwise 0. A path P from a leaf node d to the root node s on the multicast tree over a graph G ¼ ðV; EÞ comprises two parts, SSP and DSP, while the SSP can be considered as another DSP associated with a particular node on the path. Notably, all of the DSP associated with a particular node form a multicast tree and are disjoined each other. The cost of a DSP is defined as the sum of the cost of the edges along the DSP. P represents a set of the sequencing DSPs, denoted as DSPP : Hence, the cost of P is defined as the sum of the DSPP costs, that is X costðPÞ ¼ costðeÞ e[DSPP

The delay of a path P is defined as the sum of the delays along the edges of the path. X delayðeÞ delayðPÞ ¼ e[P

The path-delay delayðPÞ (i.e. conversion delay) is the number of wavelength conversion occurred on path P: A multicast tree T is a directed acyclic subgraph of graph G that spans outwards from a source s to a set of destinations D. T comprises a set of DSPP excluding the redundant edges. The cost of T is defined as the sum of the DSPP costs excluding the redundant edges. The definition of optimal constrained multicast tree (OCMT) for a modeled WWDM network is as following. Given a graph G ¼ ðV; EÞ with a link-cost function costðeÞ; a link-delay function delayðeÞ; a source s; a set of destinations D ¼ {d1 ; …dm } # V 2 {s}; and a QoS constraints Di for each di [ D; construct a CMT spanning

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D < {s}; such that Minimize costðTÞ ¼

X

costðeÞ

e[T

Subject to delayðpi Þ ¼

X

delayðvÞ # Di

;di [ D

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transformation technique, denoted as wavelength migration scheme (WMS). It is once again emphasized that COTMS is a superset of CTMS; however, COTMS is designed for wavelength-convertible WWDM systems. The enhanced features for WWDM networks are made obvious below.

v[Pi ðs;di Þ

As mentioned before, our first goal is maximizing the resource utilization. It is clear that this goal is same with CTMS [4]. COTMS can be easily obtained by a few enhancements from CTMS. COTMS is a superset of CTMS and employs CTMS for migrating inefficient dedicated subpaths, which will be illustrated later. Since we address problem in wavelength-convertible WWDM networks, the limitation of the wavelength-continuity constraint does not exist. However, the constraints delayðpi Þ # Di may not be achievable due to myriad wavelength conversions. This will cause higher connection-blocking rate and may incur the delay sensitive applications. Therefore, COTMS has another goal that is efficiently to reduce the connection-blocking rate (i.e. minimizing delayðpi Þ) by wavelength migration (reassignment) with which the QoS can be guaranteed as far as possible. The best case is delayðpi Þ ¼ 0 (i.e. all-optical) that is data transmission without any wavelength conversion occurred on path pi : However, by wavelength reassignment may change the routes of certain existing connections. To avoid disruptions existing connections to accommodate the new connection, algorithm reassigned the wavelength of connections has been proposed for alloptical wide area networks [11]. In Ref. [11], delayðpi Þ restricted to be 0 since only all-optical connections are allowed in the network (i.e. preserve the wavelengthcontinuity constraint). In order to avoid the disruptions of other existing connections, wavelength migration by COTMS is just performed in the migrating path itself without changing routing path. For minimizing delayðpi Þ; a definition of wavelength variation is stated as below and followed by the wavelength reassignment problem (WRP) on a path. Minimizing the number of wavelength conversion on path pi ; Minimizing delayðpi Þ: Definition. For every concatenate optical link-pair ða; bÞ on a path in which wavelength la is assigned to link a and wavelength lb is assigned to link b; let variationða; bÞ ¼ 1 if la – lb ; otherwise 0. Wavelength Reassignment Problem (WRP): Given an existing path pi over a sequence of links ða1 ; a2 ; …; an Þ; the available wavelengths on each link of the path is Lj # L; reassign the wavelength of each link on path p; such that X Minimize variationðaj ; ajþ1 Þ 1#j#n21

The OCMT is well-known NP-hard problem while WRP can be solved in a polynomial time by using graph

4. The COTMS 4.1. Procedures A comprehensive survey [13] discussed the problem of mobility management for WATM networks and pointed out that the central aim of mobility management is to enable a wireless network to locate roaming terminals for call delivery and maintain the connections as the terminals change their network access points. To guarantee that connections remain uninterrupted throughout their lifetime, several adaptive mechanisms have been proposed to allocate some of the network resources for potential intracluster and inter-cluster handoff connections [13]. The proposed COTMS, as mentioned earlier, is a superset of CTMS. For a new join request required to satisfy the QoS constraints, in COTMS, the shortest feasible path (SFP) heuristic is performed to find the SFP from the node at which the mobile user is attached to the multicast tree. A join request is rejected if the SFP algorithm cannot find a feasible path to the multicast tree, and rejection may occur if link overloaded or delay constraint violated. Resources occupied by the users that have departed will be released and may be allocated in advance to the handoff users, because serving the handoff users should be prioritized over the new incoming requests. A path extension scheme is employed by COTMS to serve the handoff requests to obtain more benefits in terms of fast handoff processing and cell packets sequencing. However, the associated paths on a multicast tree might be elongated due to user roaming. After a roaming sequence, each of the elongated paths may consume increasingly dedicated wavelength channels or violate the QoS constraints, especially the delay. This tendency may cause the multicast tree to consume unnecessary system resources or obstruct real-time communication. COTMS inherits from CTMS through a set of operations, which are responsible for migrating each of the inefficient subpaths (longer DSPs) on the multicast tree to better subpaths (shorter DSPs), wherever possible. Although a tolerable delay constraint also be maintained by CTMS, COTMS employs a new additional scheme, WMS, to fast recover the connection of which the delay constraint is violated. WMS is only operated on the path of which the accumulative delay exceeds the specified constraint, rather than finds a new route to the multicast tree, thus the extra overhead is comparative low than CTMS. However, the operations of CTMS will be performed if WMS is still not to solve the delay violation. On summary, COTMS is consists

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of WMS and CTMS. WMS provides the faster process to guarantee the QoS, while CTMS maintains the efficient multicast tree to save the system resource. CTMS had been proposed in Ref. [4] and then this paper focused on the operations of WMS. COTMS is triggered by a destination in two conditions, the cost of DSP is great than a threshold and the accumulative delay for the mobile host is great than the specified constraint. For maintaining a CMT in the WWDM system, the latter condition will be processed prior while the former will be considered when the DSP is immoderate inefficiency. A mobile host following a sequence of roaming, if the elongated DSP violates the QoS constraints then WMS is performed to migrate the wavelengths on the DSP such that the number of wavelength conversion minimized. After the migration, if the QoS is preserved then COTMS returns to ready state; otherwise, triggers CTMS to perform DSP migration for finding a satisfied new route. If CTMS can’t find such a route yet, then this connection will be blocked. On the other hand, if the elongated path consumes increasingly dedicated wavelength channels then the CTMS is also performed to shorten the path (i.e. reduces the occupation of wavelength channels). The state diagram of COTMS is displayed as shown in Fig. 2. Cooperating WMS with CTMS makes COTMS to hold a QoS guarantee and cost-effective optical-multicast-tree. Detailed presentations relating to WMS will be presented later, but a brief review of CTMS is described first. 4.2. Review of CTMS Multimedia applications are requiring multipoint communications on the increase. There are two main functions provided by a multicast routing protocol for wireless environments. One function is to acquire a multicast tree based on certain QoS constraints so that a multicast connection can be fast established while the other function is to handle problems associated with mobility of the multicast members. Existing multicast approaches can be

Fig. 2. The state diagram of COTMS.

classified into three categories: Minimum Steiner Tree (MST), Shortest Path Tree (SPT), and Constrained Multicast Tree (CMT). In MST, a minimum cost-spanning tree per multicast group has to be computed. The total cost of a multicast tree is defined as the sum of the cost associated with all its edges. MST algorithms attempt to minimize, however, they do not consider the end-to-end delay requirement. Meanwhile, constructing minimum-cost Steiner trees is a difficult and well-known NP-complete problem. While MST minimizes cost, SPT minimizes the end-toend delay. Many approaches, such as DVMRP, MOSPF, CBT, and PIM, focus on minimizing delay have been considered for use with the Internet. DVMRP and MOSPF are classified into source-based SPT protocols while CBT is an example of a non-source-based SPT. PIM combines both the earlier schemes to provide additional flexibility for applications that demand specific QoS requirements, and falls under the hybrid SPT category. In addition, the Resource Reservation Protocol concept is widely involved in reservation-based multicast routing protocols to vary and extend the feature of PIM. Although, these approaches attempt to minimize delay, they do not consider bandwidth sharing to minimize the total cost of the tree. Finally, CMT combines SPT and MST to consider both cost and delay during multicast tree constructions. This has been motivated by the need to support multipoint multimedia communications. CMT algorithms assume that sufficient information is available at the source node in order to construct for an appropriate multicast tree. CTMS extends from CMT approaches to maintain a shared and cost-effective multicast tree satisfying QoS constraints in mobile wireless networks. CTMS does not eagerly to derive an optimal tree when a new join request arrived, instead of to migrate the inefficient portion of the tree progressively. CTMS incorporates the delay bound parameter into the migration control message for migrating inefficient paths. The migration control message uses a cost value and a delay-bound value to search another efficient path. This value can be treated as a traveling credit grant and will be decreased once the migration control message reaches another node. The migration control message will continue to travel until a better path is found or the credit is used up. In order to demonstrate the efficiency of CTMS, an example is used to illustrate its operations; detailed scheme description can be found in Ref. [4]. Consider the modeled network topology shown in Fig. 3(a), and assume that a multicast tree Tk with identifier k rooted at v1 consists of two already existing paths p1k ¼ p41 ¼ ðv1 ; v2 ; v3 ; v4 Þ and p2k ¼ p71 ¼ ðv1 ; v6 ; v7 Þ; as shown in Fig. 3(b). This multicast tree occupies five channels. Assume two mobile hosts MH4 and MH7 are within clusters C4 and C7 ; respectively; assume the migration threshold of a DSP cost equals 2, and a delay constraint specified by MH4 is 4; and assume both links ðv4 ; v5 Þ and ðv4 ; v9 Þ still have available channels. Node v4 recognizes an event for which the DSP of p41

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Fig. 3. Operating example of the CTMS scheme: (a) graph representation; (b) multicast tree Tk before migration; (c) multicast tree Tk after migration.

(dedicated cost ¼ 3 . 2 ¼ migration threshold and dedicated delay ¼ 3 , 4 ¼ delay constraint) is a candidate DSP for migration, then issues a search message M ¼ ½S; k; p1k ; 3 2 1; 4 2 1; v4  to nodes v5 and v9 : The search message is one type of the migration message. The format of the migration message is detailed in Ref. [4]. Meanwhile, one channel in each of the links ðv4 ; v5 Þ and ðv4 ; v9 Þ is reserved for this path. When receiving this search message, node v5 updates the search message to ½S; k; p1k ; 2-1; 3-1; {v4 ; v5 } and then forwards it to node v7 provided channels are available on link ðv5 ; v7 Þ: Similarly, when node v9 receives the search message, an updated search message ½S; k; p1k ; 2-1; 3-1; {v4 ; v9 } is forwarded to node v8 provided link ðv9 ; v8 Þ is available. When node v7 (a node on Tk ) receives the search message ½S; k; p1k ; 1; 2; {v4 ; v5 }; it examines its routing table. Since the tree identifier k already exists in the table, the remained cost credit (i.e. 1) is positive and the remained delay credit (i.e. 2) is not less than the delay constraint from root to node v7 ; node v7 returns a confirm message ½C; k; p1k ; 1; 2; {v4 ; v5 ; v7 } to node v4 along path ðv7 ; v5 ; v4 Þ: Meanwhile, when node v8 (not on Tk ) receives the search message ½S; k; p1k ; 1; 2; {v4 ; v9 }; it returns a reject message ½RJ; k; p1k ; 1; 2; {v4 ; v9 ; v8 } to node v4 the path ðv8 ; v9 ; v4 Þ since the remained cost credit # 1 (no more benefit for continue forwarding). Meanwhile, a channel is released in link ðv9 ; v8 Þ when this event occurs. Finally, when node v4 receives the confirm message ½C; k; p1k ; 1; 2; {v4 ; v5 ; v7 }; it informs node v1 (the crossover switch of new and old paths) to initiate the Prune, Join, and Update operations. The old DSP ðv1 ; v2 ; v3 ; v4 Þ is pruned and the new DSP ðv7 ; v5 ; v4 Þ is joined. Meanwhile, the QoS guarantee about the delay constraint is still preserved. The tree Tk occupies just four channels after migration, as shown in Fig. 3(c). CTMS has a lot of salient features which include: (1) automatically recognizing the inefficiency of the multicast trees, then migrating them to better ones, while maintaining the QoS guarantees specified by mobile users; (2) conserving network resources by maintaining a low-cost multicast tree, thus accommodating more users; (3) operating efficiently in a truly distributed manner through event

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driven and diffusing computations, thus increasing the degree of scalability; (4) synchronizing data transmission flow for transparency during the tree migration, and thus providing seamless handoff control; and (5) handling the concurrent migration problem effectively within the wireless system, thus eliminating the oscillation paradox. However, it involved a large overhead due to the migration process in a broadcast manner. Although CTMS employs the cost and delay credits to limit the broadcast domain, it is unworthy to perform such a complex procedure immediately if the wireless system is a WWDM network. We know that delay and cost are two major factors to affect the construction of the CMT; in general, the former is more important than the latter since the connection disruption is unacceptable during the user roaming. In Section 4.3, an additional procedure (WMS) for wavelength-convertible WWDM networks is exploited to help CTMS for faster recovering the connection of a delayviolated path without path rerouting. 4.3. The WMS In wavelength-convertible WDM network, myriad of wavelength conversions produces a larger latency between sender and receiver. This phenomenon will be more serious in WWDM networks if a path extension approach is used for faster handoff handling. Therefore, the real-time connection with increasing and different wavelengths on the path must be injured from the increasing delay jitter during the mobile host moves to different locations continuously. WMS will be employed to enhance CTMS for faster recovering the connecting of a delay-violated path. It should be noted that WMS is to reassign the available wavelengths to the original delay-violated path rather than to perform path rerouting. As mentioned before, the optical tree optimization is regarded as dedicated optical subpath optimization independently. After a roaming sequence, the elongated optical DSP may violate the delay constraint and hurt the delay-sensitive applications. At the time, WMS is triggered in which perform the graph transformation of the optical DSP and then run a feasible shortest path (FSP) algorithm to minimize the number of wavelength conversion on the optical DSP. If the minimization process cannot recover the delay-violated DSP (i.e. the optical DSP is no longer satisfied) then the path rerouting process is triggered (i.e. CTMS is triggered). For illustrating the operations of WMS, an assistant example is shown in Fig. 4. Assume each optical link over the WWDM system is de-multiplexed into a set of wavelength l1 ; l2 ; l3 ; and l4 : Consider Fig. 4(a), there is a delay-violated DSP between crossover optical switch v1 and destination optical switch v7 : Considering a system snapshot, the available wavelength on the DSP is shown in Fig. 4(b), where four lines between pairs of nodes from top to down denoted the wavelength channel l1 ; l2 ; l3 ; and l4 ; respectively; the solid line denotes that such a wavelength is currently available for using while the dotted line indicates

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5. Simulation 5.1. Simulation model and assumptions To perform simulation, we adopted the Doar –Leslie model in order to reflect the reasonable mean degree of each node degree in the real network [14]. The graph generation starts from randomly distributing n nodes over a rectangular coordinate grid. Then, a link between any pair of two distinct nodes u and v is added according to the following probability function Pe ðu; vÞ ¼

Fig. 4. Minimizing the number of wavelength conversion on a delayviolated DSP by using graph transformation technique and FSP algorithm.

that it is occupied by existing connection. Fig. 4(b) is regarded as having multiple links between each pair of nodes. Fig. 4(b) is transformed to a graph associated with multiple wavelength planes as shown in Fig. 4(c), where the thin lines connected with optical switch over a wavelength plane denote data transmission without wavelength conversion while the thick line across different wavelength planes indicates the associated optical switch must perform a wavelength conversion function. In the transformation graph, thereby, assume the cost associated with the thick and thin lines are 1 and 0, respectively. According to the transformation graph, then WMS can perform the FSP algorithm to recover the connecting of the delay-violated DSP. Once the WMS succeeded, no more processes will be involved; otherwise, the CTMS will be launched in order to perform path migration. CTMS takes account of both delay and cost metrics simultaneously and can find a better DSP, if any. If CTMS also fails, in this case implies that the system has no sufficient resource to serve the mobile host, and then the connection is blocked. As shown by the above description, the enhanced hybrid migration scheme (COTMS) is suitable for mobile hosts with unicast or multicast connections. The scheme employs the same principle of partial re-establishments (i.e. DSP[s]) for heterogeneous-connection migrations. There is no difference in the scheme procedures for source and destination handoffs since DSP in both cases is independently considered between a crossover optical switch and an end optical switch. Because the same scheme procedures are employed for both unicastand multicast-connection migrations and the fact that crossover optical switch discovery is required during these migrations, we attain a uniform and unified migration methodology for mobile hosts in a WWDM network. It is worth to emphasize again, WMS addresses on supporting fast handoff management for real-time connections and keeps the migration overhead as small as possible. With WMS, the notable improvements of COTMS for WWDM networks are made evident in Section 5.

ke 2dðu; vÞ ; b exp n aL

where dðu; vÞ is the Euclidian distance from node u to v; L is the maximum possible distance between two nodes; a and b are parameters in the range (0,1) (larger values of b result in graphs with higher edge densities, while small values of a increase the density of short edges relative to longer ones); e is the desired average node degree; n is the number of nodes in the graph; and k is a constant that depends on a and b: This model is not fundamentally different than the original Waxman model (appears in Ref. [15]) since the parameter of the Waxman model can be chosen to be equivalent to any particular setting of parameters in the Doar – Leslie model. However, the addition of the scaling factor ke=n gives more directly control over the number of edges in the graph generation, provided k is known. Thereby, ensuring the mean degree of each node remains constant regardless of the network size. By following the mentioned rules, several connected graphs consisting of 32 WDM switches located at random points of 32 £ 32 coordinates are examined; a typical one is displayed in Fig. 5, for which k; e; a; and b have values of 16, 4, 0.25 and 0.2, respectively.

Fig. 5. Random generated network with 32 nodes.

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Besides, this simulation includes several assumptions as following: the capacity1 (i.e. DWDM channels) of each link between two optical switches is set as 170 of which 80% is resorted to carry data and the remaining is applied for the usage of control messages. The cost of each link is measured by the amount of bandwidth depending on communication requirements. A link can accept sessions and reserve bandwidth for them until its cost exceeds 85% of the link’s capacity then is saturated. This admission control policy allowed statistical multiplexing and efficient utilization of the available resources [16]. Mobile and constrained multicast sessions have tight delay requirements. Algorithms need to minimize the value of delay in order to allow the higher-level end-to-end protocols enough time to process the transmitted information without affecting the quality of the interaction. Assume the propagation speed through the links is taken to be two-thirds the speed of light, this is fast for transmission. However, the wavelength conversion performed by an optical switch is dominant for the purposes of our study. Thus, assume that the propagation-delay (PD) through a link is relatively less than the wavelength-conversion-delay (WD, i.e. queuing delay) across an optical switch when calculating the delay metric. For the sake of simplicity, WD and PD are assumed to be 2 and 1, respectively. Moreover, cost threshold for migration and the delay bound constraints specified by all mobile hosts are the same, and are set as 2 and 10, respectively. Based on Fig. 5, in this simulation, a constrained optical-multicast-tree is constructed under a given network with given link loads. We select five random nodes as the initial receiver members of a multicast group to join a randomly selected server; then some late joiners, leavers, and movers are involved by a random sequence. By adding nodes into the multicast group, we utilize the SFP algorithm to find the SFP from the node at which the mobile user is attached to the multicast tree. As a receiver wishing to join the group, it issues the join request over a network. All nodes over the tree can reply to the requests generated by the joining receiver. The acknowledge packets can follow the reverse path from the joiner to the nodes over the tree. After receiving multiple replies, the joiner determines which node it will attach to for making the branch of the tree. A join request is rejected if the SFP algorithm cannot find a feasible path to the multicast tree, and rejection may occur if link overloaded or delay constraint violated. The occupied wavelength channels will be recycled after the joined user departed. The SFP algorithm is also applied to extend the 1

Fujitsu Limited and Fujitsu Laboratories Ltd have developed a dense wavelength division multiplexing (DWDM) optical fiber amplifier and tunable wavelength transmitter—key components for achieving terabit transmission systems that will support high-speed Internet infrastructure in the future. These components will contribute to the development of a nextgeneration optical transmission system that supports 1.7 Tbps data transmission using 170 DWDM channels.

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elongating path from the old location node to the new one due to ensuring fast handoff and cell sequence continuity when a mobile changes its location. A roaming host either roams within the original cluster or roams to a randomly determined neighboring cluster. Besides, 10 sorts of multicast trees are examined whose group size are set from 6 to 50 increased by one. All of the numeric results for adequate traffic samples are calculated in average. The performance of a multicast algorithm was evaluated based on the quality of the multicast trees it creates and the algorithm’s efficiency in managing the network. The evaluation metrics to maintain an acceptable multicast tree for a given network with given link loads are defined as the following: † Overhead. It reveals that the transmission overhead of control messages for migrating the inefficient tree. This reflects the algorithm’s ability to maintain an efficient multicast tree using low overhead, less number of control messages. † Tree cost (TC). TC is the sum of each link-cost, and not the total cost of individual paths from the sender to the receivers. This reflects the algorithm’s ability to construct a multicast tree using low-cost, lightly utilized wavelengths. † Blocking rate. This is judged by monitoring how frequently that algorithm fails to handle an acceptable handoff for a mobile user. This reflects the algorithm’s ability for a network to provide more remaining resource and also to satisfy the delay bound imposed by the application. As above-mentioned rules, we evaluated the following three schemes for a multicast tree construction and migration: † Light-Tree Scheme (LTS). This scheme is regarded as an all-optical tree approach which routing and wavelength assignment without wavelength conversion. This is the ideal case for constrained-based applications since the WD is zero. However, it is extremely arduous to maintain a dynamic optical tree of which every link has the same wavelength. † Constrained Tree Migration Scheme (CTMS). On the contrary, this scheme considers tree’s routing and migrating with wavelength conversion. For a delayviolated path, find a new acceptable route to touch the associated tree. However, the signaling overhead involved is high to maintain the tree. † Constrained Optical Tree Migration Scheme (COTMS). As mentioned before, this scheme is similar to CTMS, but employs wavelength migration on the origin path first to recover the delay-violated path. It fast reflects on constrained-based applications, also utilizes CTMS to maintain the tree’s cost efficiency.

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5.2. Simulation results First, Fig. 6 shows the comparison of the different schemes for constructing an optical-multicast-tree over a WWDM network in terms of total control messages—the number of control messages versus system offered load. As shown in Fig. 6, as the system offered load grows, the total number of control messages also increases since to find feasible paths to join the tree is increasingly difficult in highload case. In this simulation, LTS has more testing steps with same wavelength assignment than that of CTMS, thus the latter shows better performance from the point of control messages view. On the other hand, COTMS not only inherits the advantage of CTMS, but also reveals somewhat superiority since the delay-violated connection can be recovered over the existing route via wavelength migration without rerouting. The severity of this phenomenon increases with the number of movers. For high-load case, the results of COTMS and CTMS is close each other. This is because there is not same wavelength enough for the joinpath constructing and/or the move-path migrating. Next, Fig. 7 indicates the average total link costs of tree—TC (the sum of links cost) versus system offered load. As shown in Fig. 7, the CTMS and COTMS approaches have less TCs when compared to the LTS. The former two schemes allow wavelength conversion over the shared delivery trees that have resulted in higher link-cost efficiency, while the LTS appears drawback because no shorter route with same wavelength can be obtained and longer route will be selected. Therefore, LTS results in making the higher TC and this phenomenon will be more serious when the multicast group size increases. The curve of LTS is decreased when the system offered load is increased. This is because there are more blocked users in high-load; the served connections will be terminated and omitted from calculating their cost. The COTMS performed a bit better than the CTMS because the delay-violated

Fig. 6. Total control messages for constructing a tree over the WWDM network.

Fig. 7. Average total link costs of tree.

portion can be recovered by the wavelength migration without path rerouting. Finally, Fig. 8 displays the comparison of the blocking rate after mobile hosts’ joining and migration—the blocking rate versus system offered load. Blocking occurrence includes the failure of joining and handoff. Undoubtedly, LTS makes the value of blocking rate increase rapidly when the system offered load increases due to the limitation of wavelength-continuity. Mobile wireless systems and wireline fixed systems differ mainly in their application movable. Wireless systems forbid a high handoff-dropping rate during the user movement. CTMS and COTMS have wavelength conversion function, thus relax the limitation. However, COTMS obtains more appropriate results than CTMS for a given system load due to the effect of wavelength migration. Restated, COTMS can successfully serve more handoff users under a given acceptable blocking rate. From this simulation, we can conclude that in the environment with average system offered load, the COTMS is suitable for maintaining constrained multicast optical trees in the viewpoints of number of control messages and TCs.

Fig. 8. The blocking rate measured after mobile hosts’ join and migration.

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6. Conclusions

References

Internet traffic has been roughly doubling in volume every year, and the pace seems likely to pick up [12]. As traffic increases, the only technology that boosts the optical backbone network is WDM. Today, almost of manufacturers in the optical network field are working on WDM. Without this technology, it would be impossible to promote broadband access and mobile Internet services. This study proposed and demonstrated an adaptive COTMS to serve the multicast real-time applications in wavelength-convertible WWDM systems. COTMS is a superset of CTMS, thus, is designed to operate efficiently in an asynchronous mobile wireless network environment, and is truly distributed and intelligent, being based on progressive computations and driven by events. COTMS significantly conserves network resources by operating in a branch-and-bound manner. The number of nodes participating in the multicast tree maintenance is minimized, thus increasing the degree of scalability. COTMS inherits the advantages of CTMS and additionally enhances the tolerance of delay increased by wavelength conversion. Finally, it is worthily emphasized here that COTMS is not only suitable for a network, which integrated wireless LANs with WDM-based wired architectures, but also for routing in fully mobile networks, such as ad-hoc networks. In ad-hoc network deployed with multiple frequencies for data communications, if the intermediate nodes have no capability to convert the transmission frequency that is similar to an all-optical WWDM network; otherwise, is a frequency-convertible network. If COTMS applied to such a network, the number of frequency conversion will be minimized as well as resource consumption will be reduced significantly.

[1] P. Green, Progress in optical networking, IEEE Commun. Mag. January (2001) 54–61. [2] N. Ghani, S. Dixit, T.S. Wang, On IP-over-WDM integration, IEEE Commun. Mag. March (2000) 72–84. [3] C.-K. Toh, A unifying methodology for handovers of heterogeneous connections in wireless ATM networks, ACM SIGCOMM Comput. Commun. Rev. 27 (1) (1997). [4] K.S. Chen, N.F. Huang, B. Li, CTMS: a novel constrained tree migration scheme for multicast services in generic wireless systems, IEEE JSAC 19 (10) (2001) 1998–2014. [5] D. Raychaudhuri, Wireless ATM networks: architecture, system design and prototyping, IEEE Pers. Commun. Mag. Aug. (1996) 42–49. [6] E. Ayaanoglu, K.Y. Eng, M.J. Karol, Wireless ATM: limits, challenges, and proposals, IEEE Pers. Commun. Mag. Aug. (1996) 18– 34. [7] H. Izadpanah, A millimeter-wave broadband wireless access technology demonstrator for the next-generation internet network reach extension, IEEE Commun. Mag. Sep. (2001) 140– 145. [8] B. Mukherjee, D. Banerjee, S. Ramamurthy, A. Mukherjee, Some principles for designing a wide-area optical network, IEEE/ACM Trans. Networking 4 (1996) 684–696. [9] L.H. Sahasrabuddhe, B. Mukherjee, Light-trees: optical multicasting for improved performance in wavelength-routed networks, IEEE Commun. Mag. Feb. (1999) 67–73. [10] C.S.R. Murthy, M. Gurusamy, WDM Optical Networks: Concepts, Design, and Algorithms, 1/e, Prentice-Hall PTR, Englewood Cliffs, NJ, 2002. [11] K.C. Lee, V.O.K. Li, A. Circuit, A circuit rerouting algorithm for alloptical wide-area networks, IEEE INFOCOM ’94, Toronto, Canada, 1994, pp. 954–961. [12] M. Kato, H. Yomogita, WDM, Optical Switches Bolster Network Backbones, Nikkei Electronics Asia, Cover Story, April 2001. [13] I.F. Akyildiz, J. McNair, J.S.M. Ho, H. Uzunalioglu, W. Wang, Mobility management in next generation wireless systems, Proc. IEEE 87 (8) (1999) 1347–1384. [14] B.M. Waxman, Routing of multipoint connections, IEEE JSAC 6 (9) (1988) 1617– 1622. [15] M. Doar, I. Leslie, How bad is naive multicast routing?, IEEE INFOCOM (1993) 82–89. [16] S. Rampal, D. Reeves, An evaluation of routing and admission control algorithms for multimedia traffic, Comput. Commun. 18 (10) (1995) 755–768. [17] B. Bing, R.V. Nee, V. Hayes, Wireless local area and home networks, IEEE Commun. Mag. 39 (11) (2001).