LIGHT TREES

Today, there is a general consensus that, in the near future, wide area networks (WAN)(such as, a nation wide backbone network) will be based on Wavelength Division Multiplexed (WDM) optical networks. One of the main advantages of a WDM WAN over other optical technologies, such as, Time Division Multiplexed (TDM) optical networks, is that it allows us to exploit the enormous bandwidth of an optical fiber (up to 50 terabits bits per second) with requiring electronic devices, which operate at extremely high speeds.

The concept of light tree is introduced in a wavelength routed optical network, which employs wavelength -division multiplexing (WDM). Depending on the underlying physical topology networks can be classified into three generations:

First Generation: these networks do not employ fiber optic technology; instead they employ copper-based or microwave technology. E.g. Ethernet.

Second Generation: these networks use optical fibers for data transmission but switching is performed in electronic domain. E.g. FDDI.

Third Generation: in these networks both data transmission and switching is performed in optical domain. E.g. WDM.                                   

WDM wide area networks employ tunable lasers and filters at access nodes and optical/electronic switches at routing nodes. An access node may transmit signals on different wavelengths, which are coupled into the fiber using wavelength multiplexers. An optical signal passing through an optical wavelength-routing switch (WRS) may be routed from an output fiber without undergoing opto-electronic conversion.

LIGHT PATH

A light path is an all-optical channel, which may be used to carry circuit switched traffic, and it may span multiple fiber links. Assigning a particular wavelength to it sets these up. In the absence of wavelength converters, a light path would occupy the same wavelength continuity constraint.

A light path can create logical (or virtual) neighbors out of nodes that may be geographically far apart from each other. A light path carries not only the direct traffic between the nodes it interconnects, but also the traffic from nodes upstream of the source to nodes upstream of the destination. A major objective of light path communication is to reduce the number of hops a packet has to traverse.

Under light path communication, the network employs an equal number of transmitters and receivers because each light path operates on a point-to-point basis. However this approach is not able to fully utilize all of the wavelengths on all of the fiber links in the network, also it is not able to fully exploit all the switching capability of each WRS.

LIGHT TREES

Thus, incorporating an optical multicasting capability extends the light path concept. Multicasting is the ability of an application at a node to send a single message to the communication network and have it delivered to multiple recipients at different locations. We refer light tree as a point to multi point extension of light path. Today, many multicasting applications exist, such as, teleconferencing, software/file distribution including file replication on mirrored sites, distributed games, Inter net news distribution-mail mailing lists, etc., but the implementation of these applications is not necessarily efficient because today’s WANs were designed to support point-to-point (unicast) communication. In the future, as multicast applications become more popular and bandwidth intensive, there emerges a pressing need to provide multicasting support on WANs.

A light tree is a point to point multipoint all optical channel, which may span multiple fiber links. Hence, a light tree enables single-hop communication between a source node and a set of destination nodes. Thus, a light tree based virtual topology can significantly reduce the hop distance, thereby increasing the network throughput.

Figure 1a shows a light tree, which connects node UT to nodes TX, NE and IL. Thus, an optical signal transmitted by node UT travels down the light tree till it reaches node CO, where it is split by an optical splitter into two copies. One copy of the optical signal is routed to node TX, where it is terminated at a receiver. The other copy is routed towards node NE, where it is again split into two copies. At node NE, one copy of the optical signal is terminated at receiver, while the other copy is routed towards node IL. Finally, a copy of the optical signal reaches node IL, where it is terminated at a receiver. Thus the virtual topology induced by this light tree consists of three logical links.  

Let us assume that the bit rate of each light path is normalized to one unit, and node UT wants to send a certain amount of packet traffic to nodes TX, NE and IL. Let assume that we are allowed only one free wavelength on the links UT-CO, CO-NE, NE-IL and CO-TX. Then, a light path based solution would consist of the following four light paths:

   

From UT to CO

From CO to NE

From CO to TX

From NE to IL

     

Thus the light path based solution requires a switch at nodes CO and NE and a total of eight transceivers (one transmitter and one receiver per light path). On the other hand, a light tree based solution consists of a single light tree, which requires a total of four transceivers (one transmitter at UT and one receiver per node at TX, NE, and IL) and does not utilize the electronic switch at node CO or NE.

CONCLUSION

Recently, there has been a lot of interest in WDM based fiber optic networks. In fact, there is a general consensus that, in the near future, WANs will be based on WDM optical networks. So far, all architectures that have been proposed for WDM WANs have only considered the problem of providing unicast services. In addition to unicast services future WDM WANs need to provide multicast and broadcast services. A novel WDM WAN architecture based on light trees that is capable of supporting broadcasting and multicasting over a wide-area network by employing a minimum number of opto-electronic devices was discussed. Such WDMWAN can provide a very high bandwidth optical layer, which efficiently routes unicast, broadcast and multicast packet-switch traffic.

Each node in the WDM WAN consists of a multicast-capable wavelength routing switch (WRS), an “off –the-shelf  ” electronic packet switch, and a set of opto electronic converters. The problem of finding an optimum set of light-trees was formulated as a mixed integer linear problem. Preliminary results show that if we employ a set of light trees, then significant savings can be achieved in terms of the number of opto electronic devices that are required in the network.