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Optical Networking And Dense Wavelength Division Multiplexing (DWDM)
Abstract
This paper deals with the twin concepts of optical networking and dense wavelength division multiplexing. The paper talks about the various optical network architectures and the various components of an all-optical network like Optical Amplifiers, Optical Add/Drop Multiplexers, Optical Splitters etc. Important optical networking concepts like wavelength routing and wavelength conversion are explained in detail. Finally this paper deals with industry related issues like the gap between research and the industry, current and projected market for optical networking & DWDM equipment and future direction of research in this field.
Introduction
One of the major issues in the networking industry today is tremendous demand for
more and more bandwidth. Before the introduction of optical networks, the reduced availability of fibers became a big problem for the network providers. However, with the development of optical networks and the use of Dense Wavelength Division Multiplexing (DWDM) technology, a new and probably, a very crucial milestone is being reached in network evolution. The existing SONET/SDH network architecture is best suited for voice traffic rather than today’s high-speed data traffic. To upgrade the system to handle this kind of traffic is very expensive and hence the need for the development of an intelligent all-optical network. Such a network will bring intelligence and scalability to the optical domain by combining the intelligence and functional capability of SONET/SDH, the tremendous bandwidth of DWDM and innovative networking software to spawn a variety of optical transport, switching and management related products.
Optical Networking
Optical networks are high-capacity telecommunications networks based on optical
technologies and component that provide routing, grooming, and restoration at the
wavelength level as well as wavelength-based services. The origin of optical
networks is linked to Wavelength Division Multiplexing (WDM) which arose to
provide additional capacity on existing fibers. The optical layer, whose standards are being developed, will ideally be transparent to the SONET layer, providing
restoration, performance monitoring, and provisioning of individual wavelengths
instead of electrical SONET signals. So in essence a lot of network elements will be eliminated and there will be a reduction of electrical equipment.
It is possible to classify networks into three generations depending on the physicallevel technology employed. First generation networks use copper-based or microwave technologies e.g Ethernet, satellites etc. In second generation networks, these copper links or microwave links with optical fibers. However, these networks still perform the switching of data in the electronic domain though the transmission of data is done in the optical domain. Finally we have the third generation networks that employ Wavelength Division Multiplexing technology. They do both the transmission and the switching of data in the optical domain. This has resulted in the onset of tremendous amount of bandwidth availability. Further the use of non-overlapping channels allows each channel to operate at peak speeds.
Dense Wavelength Division Multiplexing (DWDM)
Dense Wavelength Division Multiplexing (DWDM) is a fiber-optic transmission
technique. It involves the process of multiplexing many different wavelength signals onto a single fiber. So each fiber has a set of parallel optical channels each using slightly different light wavelengths. It employs light wavelengths to transmit data parallel-by-bit or serial-by-character. DWDM is a very crucial component of optical networks that will allow the transmission of data: voice, video-IP, ATM and SONET/SDH respectively, over the optical layer.
Hence with the development of WDM technology, optical layer provides the only
means for carriers to integrate the diverse technologies of their existing networks into one physical infrastructure. For example, though a carrier might be operating both ATM and SONET networks, with the use of DWDM it is not necessary for the ATM signal to be multiplexed up to the SONET rate to be carried on the DWDM network. Hence carriers can quickly introduce ATM or IP without having to deploy an overlay network for multiplexing.
DWDM SYSTEM
As mentioned earlier, optical networks use Dense Wavelength Multiplexing as the
underlying carrier. The most important components of any DWDM system are
transmitters, receivers, Erbium-doped fiber Amplifiers, DWDM multiplexers and
DWDM demultiplexers. Fig 1 gives the structure of a typical DWDM system.
The concepts of optical fiber transmission, amplifiers, loss control, all optical header replacement, network topology, synchronization and physical layer security play a major role in deciding the throughput of the network. These factors have been discussed briefly in this sections that follow.
Optical Transmission Principles
The DWDM system has an important photonic layer, which is responsible for
transmission of the optical data through the network. Some basic principles,
concerning the optical transmission, are explained in this section. These are necessary for the proper operation of the system.
Channel Spacing
The minimum frequency separation between two different signals multiplexed in
known as the Channel spacing. Since the wavelength of operation is inversely
proportional to the frequency, a corresponding difference is introduced in the
wavelength of each signal. The factors controlling channel spacing are the optical
amplifier’s bandwidth and the capability of the receiver in identifying two close
wavelengths sets the lower bound on the channel spacing. Both factors ultimately
restrict the number of unique wavelengths passing through the amplifier.
Signal Direction
An optical fiber helps transmit signal in both directions. Based on this feature, a
DWDM system can be implemented in two ways:
Unidirectional: All wavelengths travel in the same direction within the fiber. It is
similar to a simplex case. This calls in for laying one another parallel fiber for
supporting transmission on the other side.
Bi-directional: The channels in the DWDM fiber are split into two separate bands,
one for each direction. This removes the need for the second fiber, but, in turn reduces the capacity or transmission bandwidth.
Signal Trace
The procedure of detecting if a signal reaches the correct destination at the other end. This helps follow the light signal through the whole network. It can be achieved by plugging in extra information on a wavelength, using an electrical receiver to extract if from the network and inspecting for errors. The receiver the reports the signal trace to the transmitter. Taking into consideration the above two factors, the international bodies have established a spacing of 100GHz to be the worldwide standard for DWDM. This means that the frequency of each signal is less than the rest by atleast 0.1THz.
Network classification
A network can be physically structured in the form of either a ring, a mesh, star based or linear bus based on the connection between the various nodes. Although the physical topology of a DWDM system might be that of a ring, the logical traffic distribution topology can be arbitrary. This is done through the use of different wavelengths to interconnect each node. Until the development of EDFAs the passive star configuration was the most popular configuration due to its superior power budget. However, with the advent of EDFAs, the ring network works out much better after overcoming its power budget problems. What makes the ring network better is its superior resilience. The Optical Cross Connect (OXC) help pass on traffic between each of the rings. A Path-in-Lambda architecture for connecting all-optical networks is under development.
Ring Topology vs Mesh Topology
A ring topology is preferable owing to many of its capabilities. Unlike a mesh
network, the expense of laying out the links is reduced in the ring, because the
number of links increases only as a linear progression. The rings also have better
resilience and restoration than meshes. The ring topology besides serving as a stand by page link helps share the load. The working segment (Refer to Fig.2) and the protection segment of the fiber together handle the large data burst of the computer network.
This reduces the load on the router and removes the need for buffering.
Single-Hop Networks vs Multi-hop Networks
Multi-wavelength networks can be also classified as single-hop networks and multihop networks. In single-hop networks, the data stream travels from source to
destination as a light stream. There is no conversion to electronic form in any of the intermediate nodes. Two examples of a single-hop networks are the broadcast-and select and the wavelength-routed architecture.
Broadcast-and-select networks: It is based on a passive star coupler device
connected to several nodes in a star topology. Basically a signal received on one port is split and broadcast to all ports. Networks are simple and have natural multicasting capabilities. Generally used in high speed LANs or MANs. Other elements in this type of network are tunable receivers and fixed transmitters or fixed receivers and tunable transmitters.
Wavelength routed networks: The key element here is the wavelength-selective
switching subsystem. There are again two types of wavelength switching. Wavelength path switching involves dynamic signal switching from one path to another by changing WDM routing while wavelength conversion the reuse of the same wavelength in some other part of the network as long as both light
