WDM Network Constructions

Broadcast-and-Select (Local) Optical WDM Network

A local WDM optical network may be constructed by connecting network nodes via two-way fibers to a passive star, as shown in Fig. 1.7. A node sends its transmission to the star on one available wavelength, using a laser which produces an optical information stream. The information streams from multiple sources are optically combined by the star and the signal power of each stream is equally split and forwarded to all of the nodes on their receive fibers. A node's receiver, using an optical filter, is tuned to only one of the wavelengths; hence it can receive the information stream. Communication between sources and receivers may follow one of two methods: (1) single-hop, or (2) multihop (which will be studied in Part II of this book). Also, note that, when a source transmits on a particular wavelength lambda_1, more than one receiver can be tuned to wavelength lambda_1, and all such receivers may pick up the information stream. Thus, the passive-star can support ``multicast'' services.

  
Figure 1.7: A passive-star-based local optical WDM network.

Passive-Star-Based Optical WDM LAN vs. Centralized, Nonblocking-Switch-Based LAN

Consider the passive-star-based optical WDM LAN in Fig. 1.7. If there are N nodes in the system and as many wavelengths as nodes, and also if the bit rate of each WDM channel (and hence of each electronic interface) is B bps, then the aggregate information-carrying capacity of the LAN is upper-bounded by N * B bps.

Now consider the same network topology as in Fig. 1.7, but the passive star is replaced by a centralized, nonblocking, space-division switch, where the notion of WDM does not exist. Each of the N nodal interfaces still operate at B bps, all on the ``same wavelength,'' so that the aggregate system capacity is still N * B bps, due to ``space-division multiplexing'' at the nonblocking switch. So, how does this architecture compare with the passive-star-based WDM LAN solution?

While the passive-star WDM solution cannot boast any capacity enhancement, it nevertheless has the following advantages:

1. In the space-division-switch solution, the ``switching intelligence'' is centralized. However, the passive star relegates the switching functions to the end nodes (in terms of wavelength tunability at the nodal transmitters and receivers). If a node is down, the rest of the network can still function. Hence, the passive-star solution enjoys the fault-tolerance advantage of any distributed switching solution, relative to the centralized-switch architecture, where the entire network goes down if the switch is down.

2. Another advantage of the WDM passive-star solution is that it allows multicasting ``for free.'' There are some processing requirements with respect to appropriately coordinating the nodal transmitters and receivers. Centralized coordination for supporting multicasting in a switch (also referred to as a ``copy'' facility) is expected to require more processing.

3. Also, the passive-star solution can be potentially much cheaper since it is purely glass with very little electronics.

Wavelength-Routed (Wide-Area) Optical Network

A wavelength-routed network (wide-area) optical WDM network is shown in Fig. 1.8. The network consists of a photonic switching fabric, comprising ``active switches'' connected by fiber links to form an arbitrary physical topology. Each end-user is connected to an active switch via a fiber link. The combination of an end-user and its corresponding switch is referred to as a network node.

  
Figure 1.8: A wavelength-routed (wide-area) optical WDM network.

Each node (at its access station) is equipped with a set of transmitters and receivers, both of which may be wavelength tunable. A transmitter at a node sends data into the network and a receiver receives data from the network.

The basic mechanism of communication in a wavelength-routed network is a lightpath. A lightpath is an all-optical communication channel between two nodes in the network, and it may span more than one fiber link. The intermediate nodes in the fiber path route the lightpath in the optical domain using their active switches. The end-nodes of the lightpath access the lightpath with transmitters and receivers that are tuned to the wavelength on which the lightpath operates. For example, in Fig. 1.8, lightpaths are established between nodes A and C on wavelength channel lambda_1, between B and F on wavelength channel lambda_2, and between H and G on wavelength channel lambda_1. The lightpath between nodes A and C is routed via active switches 1, 6, and 7. (Note the wavelength reuse for lambda_1.)

In the absence of any wavelength conversion device, a lightpath is required to be on the same wavelength channel throughout its path in the network; this requirement is referred to as the wavelength continuity property of the lightpath. This requirement may not be necessary if we also have wavelength converters in the network. For example, in Fig. 1.8, the lightpath between nodes D and E traverses the fiber link from node D to switch 10 on wavelength lambda_1, gets converted to wavelength lambda_2 at switch 10, traverses the fiber link between switch 10 and switch 9 on wavelength lambda_1, gets converted back to wavelength lambda_1 at switch 9, and traverses the fiber link from switch 9 to node E on wavelength lambda_1.

A fundamental requirement in a wavelength-routed optical network is that two or more lightpaths traversing the same fiber link must be on different wavelength channels so that they do not interfere with one another.