24-07-2014, 03:19 PM
WDM Optical Communication Networks
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INTRODUCTION
We are moving toward a society which requires that we have access to information at our fingertips when we need it, where we need it, and in whatever format we need it. The information is provided to us through our global mesh of communication networks, whose current implementations, e.g., today’s Internet and asynchronous transfer mode (ATM) networks do not have the capacity to support the foreseeable band- width demands.
Fiber-optic technology can be considered our savior for meeting our above-mentioned need because of its potentially limitless capabilities [1], [2]: huge bandwidth [nearly 50 terabits per second (Tb/s)], low signal attenuation (as low as 0.2dB/km), low signal distortion, low power requirement, low material usage, small space requirement, and low cost. Our challenge is to turn the promise of fiber optics to reality to meet our information networking demands of the next decade.
Thus, the basic premise of the subject on optical wavelength- division multiplexing (WDM) networks is that, as more and more users start to use our data networks, and as their usage pat- terns evolve to include more and more bandwidth-intensive net- working applications such as data browsing on the world wide web (WWW), java applications, video conferencing, etc., there emerges an acute need for very high-bandwidth transport net- work facilities, whose capabilities are much beyond those that current high-speed (ATM) networks can provide. There is just not enough bandwidth in our networks today to support the exponential growth in user traffic!
Fig. 1 shows the past and projected future growth of data and voice traffic reported by most telecom carriers [3]. Although voice traffic continues to experience a healthy growth of approximately 7% per year (which would be considered to be very strong growth in most business sectors), it is the data-traffic growth that is drawing people’s attention. Most carriers report that data traffic has just recently overtaken or will soon overtake the voice traffic in their fiber links and networks [4].
Given that a single-mode fiber’s potential bandwidth is nearly 50 Tb/s, which is nearly four orders of magnitude higher than electronic data rates of a few gigabits per second (Gb/s), every effort should be made to tap into this huge optoelectronic band- width mismatch. Realizing that the maximum rate at which an end-user which can be a workstation or a gateway that inter- faces with lower-speed sub-networks can access the network is limited by electronic speed (to a few Gb/s), the key in designing optical communication networks in order to exploit the fiber’s huge bandwidth is to introduce concurrency among multiple user transmissions into the network architectures and protocols. In an optical communication network, this concurrency may be provided according to wavelength or
WAVELENGTH-DIVISION MULTIPLEXING (WDM)
Wavelength-division multiplexing (WDM) is an approach that can exploit the huge optoelectronic bandwidth mismatch by requiring that each end-user’s equipment operate only at electronic rate, but multiple WDM channels from different end users may be multiplexed on the same fiber. Under WDM, the optical transmission spectrum (see Fig. 2.1) is carved up into a number of non-overlapping wavelength (or frequency) bands, with each wavelength supporting a single communication channel operating at whatever rate one desires, e.g., peak electronic speed. Thus, by allowing multiple WDM channels to coexist on a single fiber, one can tap into the huge fiber bandwidth, with the corresponding challenges being the design and development of appropriate network architectures, protocols, and algorithms. Also, WDM devices are easier to implement since, generally, all components in a WDM device need to operate only at electronic speed; as a result, several WDM devices are available in the marketplace today, and more are emerging.
Point-to-Point WDM Systems
WDM technology is being deployed by several telecommunication companies for point-to-point communications. This deployment is being driven by the increasing demands on communication bandwidth. When the demand exceeds the capacity in existing fibers, WDM is turning out to be a more cost-effective alternative compared to laying more fibers. A study compared the relative costs of upgrading the trans- mission capacity of a point-to-point transmission link fromOC-48 (2.5 Gb/s) to OC-192 (10 Gb/s) via the following three possible solutions. (The terminology OC is a widely used telecommunications jargon. “OC” stands for “optical channel” and it specifies electronic data rates. “OC-n” stands for a data rate of nx51.84 megabits per second (Mb/s) approximately; so OC-48 and OC-192 correspond to approximate data rates of 2.5 Gb/s and 10 Gb/s, respectively. OC-768 (40 Gb/s) is the next milestone in highest achievable electronic communication speed.)
Wavelength Add/Drop Multiplexer (WADM)
One form of a wavelength add/drop multiplexer (WADM) is shown in Fig.3.2. It consists of a de-mux, followed by a set of 2x2 switches—one switch per wavelength followed by a mux. The WADM can be essentially “inserted” on a physical fibre link. If all of the 2x2 switches are in the “bar” state, then all of the wavelengths flow through the WADM “undisturbed.” However, if one of the 2x2 switches is configured into the “cross” state (as is the case for the switch in Fig. 3.2) via electronic control (not shown in Fig.3.2), then the signal on the corresponding wavelength is “dropped” locally, and a new data stream can be “added” on to the same wavelength at this WADM location. More than one wavelength can be “dropped and added” if the WADM interface has the necessary hardware and processing capability
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.5.1. 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) multi hop. Also, note that, when a source transmits on a particular wavelength , more than one receiver can be tuned to wavelength, and all such receivers may pick up the information stream. Thus, the passive-star can support “multicast” services
3 A Sample WDM Networking Problem
As we have described in Subsection B, end-users in a fiber- based WDM backbone network may communicate with one another via all-optical (WDM) channels, which are referred to as light paths. A light path may span multiple fiber links, e.g., to provide a “circuit-switched” interconnection between two nodes which may have a heavy traffic flow between them and which may be located “far” from each other in the physical fiber net- work topology. Each intermediate node in the light path essentially provides an all-optical bypass facility to support the light- path.
In an N-node network, if each node is equipped with N-1 transceivers [transmitters (lasers) and receivers (filters)] and if there are enough wavelengths on all fiber links, then every node pair could be connected by an all-optical light path, and there is no networking problem to solve. However, it should be noted that the network size (N) should be scalable, transceivers are expensive so that each node may be equipped with only a few of them, and technological constraints dictate that the number of WDM channels that can be supported in a fiber be limited to (whose value is a few tens today, but is expected to improve with time and technological breakthroughs). Thus, only a limited number of light paths may be set up on the network
CARRIER STRATEGIES
In this section, we examine two major carriers’ strategies in developing WDM networks. Several other carriers are also developing similar strategies; however, the objective of this section is not to provide an exhaustive list of all carriers’ strategies. Instead, we focus on two major carriers whose approaches are representative of the industry trend and whose strategies were available.
Network Control and Management
In a wavelength-routed WDM network, a control mechanism is needed to set up and take down all-optical connections (i.e., light paths). Upon the arrival of a connection re- quest, this mechanism must be able to select a route, assign a wavelength to the connection, and configure the appropriate optical switches in the network. The mechanism must also be able to provide updates to reflect which wavelengths are currently being used on each fiber link so that nodes may make informed routing decisions. This control mechanism can either be centralized or distributed. Distributed systems are usually more robust than centralized systems; so they are generally more preferred. The objectives of various research efforts on this subject are to minimize 1) the blocking probability of connection requests, 2) the connection setup delays, and 3) the bandwidth used for control messages; as well as to maximize the scalability of such networks.
There are two distributed network control management schemes which have been examined in the literature. The first approach is the “link-state approach” because it routes connections in a link-state fashion. The second approach is the “distributed routing approach” because it utilizes the distributed Bellman–Ford routing algorithm. We describe the two approaches below.
IP Over WDM
The need for high bandwidth in today’s IP-based Internet, and the promise of WDM to provide this high capacity, is fueling the current investigations on IP-over-WDM networks. In an IP-over-WDM network, network nodes employ wavelength- routing switches (WRSs) and IP routers. Nodes are connected by fibers to form an arbitrary physical mesh topology. Any two IP routers in this network can be connected together by an all-optical WDM channel, called a light path, and the set of light paths that are set up form a virtual interconnection pattern.
A light path is a point-to-point all-optical wavelength channel that connects a transmitter at a source node to a receiver at a destination node. Using WRSs at intermediate nodes and via appropriate routing and wavelength assignment, a light path can create virtual (or logical) neighbors out of nodes that are geo- graphically far apart in the network; thus, a set of light paths embeds a virtual (or logical) topology on the network. In the virtual topology, a light path carries not only the direct traffic between the nodes it interconnects, but also traffic from nodes upstream of the source (including the source) to nodes downstream of the destination (including the destination). Nodes that are not connected directly in the virtual topology can still communicate with one another using the “multi-hop approach,” namely, by using electronic packet switching at the intermediate nodes in the virtual topology. This electronic packet-switching functionality can be provided by IP routers, ATM switches, etc., leading to an IP-over-WDM or an ATM-over-WDM network, respectively.
Traffic Grooming in WDM Ring Networks
Traffic grooming is a term used to describe how different (low-speed) traffic streams are packed into higher-speed streams. In a WDM/SONET ring network, each wavelength can carry several lower-rate traffic streams in TDM fashion. The traffic demand, which is an integer multiple of the timeslot capacity, between any two nodes is established on several TDM virtual connections. A virtual connection needs to be added and dropped only at the two end nodes of the connection; as a result, the electronic add/drop multiplexors (ADMs) at intermediate nodes (if there are any) will electronically bypass this timeslot. Instead of having an ADM on every wavelength at every node, it may be possible to have some nodes on some wavelength where no add/drop is needed on any time slot; thus, the total number of ADMs in the networks (and hence the network cost) can be reduced. Under the static traffic pattern, the savings can be maximized by carefully packing the virtual connections into wavelengths.
CONCLUSION
The objective of this case study was to provide an overview of the research and development work in the area of optical networking. Because of the large amount of activity on this subject over the past dozen years, capturing the important and most significant developments in a concise format is a challenging task. This report summarized the basic optical-networking approaches (focusing on functionalities of various devices and technologies rather than exact vendor implementations), reported on the WDM deployment strategies of two major U.S. carriers, discussed physical-layer impairments which may strongly influence network architectures, and outlined the current research and development trends on WDM optical networks, focusing mainly on the core (long-haul) wide-area network.