Multicast

To provide high bandwidth network communication, several commercial protocols including synchronous digital hierarchy (SDH), synchronous optical network (SONET), asynchronous transfer mode (ATM), and internet protocol (IP), are investigated to implement them in WDM networks [74]. For example, in the protocol IP over WDM, the transmitted data will be packeted based upon IP protocols. To provide more flexibility of services, various existing protocols over WDM could be directly supported in wavelength channels.

Due to the attractive communication bandwidth in WDM networks, many new network applications (distributed databases [9], replicated file systems [51], resource allocation in distributed systems [26], distributed process management [13], distributed games [6], replicated procedure calls [15], and teleconferencing [55], and so on) are inspiring new communication models, among which multicast is an important communication of a point to multipoint to distribute multimedia content or data. In multicast, data will be sent from a single source (transmitter) to multiple destinations (receivers) and the route of transmitting the type of requests is tree-like structure called a multicast tree.

Multicast in traditional copper networks has been well studied since the 1990s.

Multicast Backbone (MBONE) [24], which can be seen as an overlay of the internet exploring applications of multicast over IP layer by using the reliable multicast transport protocol (RMTP) [57] for IP, is the first demonstrator. RMTP is used to reliability guarantee in application development. For example, distributed databases which need to be certain that all members of a multicast group agree on which packets have been received. The only service demanded by RMTP from the underlying network is the establishment of a multicast tree

from the sender to the receivers, where the multicast tree can be set up by multicast routing protocols (such as, DVMRP [61], PIM [66], or CBT[5]). The function of RMTP is to deliver packets from the sender to the receivers in sequence along the multicast tree, independent of how the tree is created and resources are allocated. Roca, et al. [77], gave an overview of most of the directions taken by research in multicast research. Mir [52] gave a survey of techniques, architectures, and algorithms for multicasting data in communication switch networks.

Nevertheless, a realistic demonstrator of multicast in WDM networks seems in development. For the protocol stack, multicast can be implemented at different layers; for example, WDM layer, SDH/SONET layer, ATM layer, or IP layer. Three schemes, IP multicast, Multiple-unicast (or IP multicast via WDM multicast), and WDM multicast for multicasting data in IP over WDM networks, were introduced by Qiao et al. [62]. In IP multicast, the multicast tree is constructed in the IP layer, and each node will make copies of data and transmit each copy to each successor. As shown in Figure 2-5 (a), v2 is a branch node to pass data to v3 and v4. Therefore, it is necessary to make a copy in v3, to send the copy to passing through v2 to v4. Because it requires OEO conversions of packing data at each router, this scheme is not only inefficient but also unaffordable. To avoid these OEO conversions, Multiple-unicast is proposed to construct a virtual topology consisting of a set of light-paths from the source to all destinations, where the number of light-paths may equal the number of destinations. Because data will only be copied in the source node, the transmission delay of OEO conversion is still required. Besides, if some link is shared by more than one light-path, each light-path would need a different wavelength for routing the data. As shown in Figure 2-5(b), two light-paths, v1-v2-v3 and v1-v2-v4, would need two different wavelengths λ1 and λ2

because the link between v1 and v2 is shared. If each light-path requires one specific wavelength, the wavelength consumption may become unaffordable. The WDM multicast

scheme is thus proposed to reduce wavelength consumption.

(a) IP multicast scheme (b) Multiple-unicast scheme (c) WDM multicast scheme Figure 2-5: Three multicast schemes

To avoid making copies in the source node and sending a separate copy to each receiver using different wavelength, light signals need to be duplicated using optical splitters [53] or tap [60] for providing multicasting in the WDM layer; that is, WDM multicast is implemented by using a multicast tree in the WDM layer, in which the root represents the multicast source.

Following the links in the multicast tree, the same data is transmitted only once on each link.

Nevertheless, the optical splitters make the switch architecture complex and also cause power loss that requires optical amplifiers, but no OEO conversion is required and wavelength consumption is thus saved.

In an optical network, a tap [60] is an optical device which taps a small amount of the power of the signal from an optical fiber, and allows the signal to continue with negligible power degradation. An n-way splitter [53] is an optical device which splits an input signal among n outputs, but the power of each output will be reduced to

n

1th of the original signal.

The ability to split the signal without the knowledge of the signal’s characteristics allows an optical network to realize multicasting without the need for buffering. Therefore, all-optical networks are much more powerful than electronic networks in which

store-and-forward is necessary to achieve multicasting. In general, a node implementing optical splitter is called an MC (multicast capable) node as introduced in [62]; otherwise, it is called an MI (multicast incapable) node. For a multicast tree, if each branch nodes with more than two outbound links to connect the other nodes is an MC node, the multicast tree will be a light-tree [67]. Because the split signals can be transmitted by links to other nodes concurrently, locating an MC node for routing data to several destinations would have significant wavelength saving over multiple-unicast. To describe the splitting capacity, the light splitting capacity of a node is used to indicate the maximum number of split signals at an output port.

The light splitting capacity of an MC node (respectively, MI node) is greater than (respectively, equal to) 1. As shown in Figure 2-5(c), since v2 is an MC node,only wavelength λ1 is required for routing data to v3 and v4 and wavelength λ2 can be saved.

Chapter 3 Preliminaries of Routing and Wavelength