DWDM Topology Design: How to Make it Right?

Network expansion spurs the demand for faster data transmission and higher capacity over the network. In this case, DWDM emerges as a cost-effective solution to handle these issues, working efficiently to combine multiple wavelengths together and sent them over one single fiber. With the ability to carry up to 140 channels theoretically, higher capacity can be achieved by DWDM technology. This article guides you through some basics of DWDM topology.

Common DWDM Topology Overview

DWDM networks are grouped into four major topological configurations: DWDM point-to-point with or without add-drop multiplexing network, fully connected mesh network, star network, and DWDM ring network with OADM nodes and a hub. The requirements of each DWDM topology differ, and based on various application, it may involve different optical components. Besides these four common DWDM topology, there also exists hybrid network topology, consisting of stars and/or rings that are interconnected with point-to-point links.

Configurations of DWDM Topology

This section illustrates the four basic DWDM topology configurations, help to understand the major differences and applications of them.

Point-To-Point Topology

Point-to-point topology is typically found in long-haul transport, which demands for ultra high speed (10-40Gb/s), ultra high aggregate bandwidth, high signal integrity, great reliability, and fast path restoration capability. The transmitter and receiver within this DWDM topology can be several hundred kilometers away, and the number of amplifiers between the two end points is generally less than 10. Together with add-drop multiplexing, point-to-point DWDM topology enables the system to drop and add channels along its path. A DWDM point-to-point system includes lasers, an optical multiplexer and demultiplexer, fibers, optical amplifiers, and an optical add-drop multiplexer.

Ring-Configuration Mesh and Star Networks

Basically, a DWDM ring network includes a fiber in a ring configuration that fully interconnects nodes. Two fiber rings are even presented in some systems for network protection. This ring DWDM topology is commonly adopted in a local or a metropolitan area which can span a few tens of kilometers. Many wavelength channels and nodes may be involved in DWDM ring system. One of the nodes in the ring is a hub station where all wavelengths are sourced, terminated, and managed, connectivity with other networks takes place at this hub station. Each node and the hub have optical add-drop multiplexers (OADM) to drop off and add one or more designated wavelength channels. As the number of OADMs increases, signal loss occurs and optical amplifier is needed here.

In the ring DWDM topology, a hub station works to manage channel assignment so that a fully connected network of nodes with OADM is accomplished. The hub also makes it possible to connect other networks. A DWDM mux/demux can be connected to an OADM node to multiplex several data sources. The following picture demonstrates a simple DWDM ring topology with a hub and two nodes (A and B).

Transmit and Receive Directions of DWDM Hub

In the previous part, we’ve mentioned DWDM hub, which serves as a very essential parts in a DWDM system. Here we further explain the transmit and receive direction of a DWDM hub, proving system solutions for your reference.

Transmit Direction

A DWDM hub accepts various electrical payloads, such as communications transport protoco/Internet Protocol (TCP/IP), asynchronous transfer mode (ATM), STM, and high-speed Ethernet (l Gb/s, 10 Gb/s). Each traffic type (channel) is sent to its corresponding physical interface, where a wavelength is assigned and is modulated at the electrical-to-optical converter. The optically modulated signals from each source are then optically multiplexed and launched into the fiber.

Receive Direction

When a hub receives a WDM signal, it optically demultiplexes it to its component wavelengths (channels) and converts each optically modulated signal to a digital electrical signal. Each digital signal then is routed to its corresponding electrical interface: TCPIIP, ATM, STM, and so on However, that each channel requires its own clock recovery circuitry because all channels may be at different bit rates.

Conclusion

The network topology of your DWDM system depends on various factors, including the number of nodes, maximum traffic capacity, scalability, number of fiber links between nodes and so on. Attentions also should be attached to the network components involved in the DWDM system. Hope this article could help to get more understanding towards DWDM technology.

Source: http://www.fiber-optic-solutions.com/dwdm-topology-design-make-right.html

Introduction to Optical Add-Drop Multiplexer (OADM)

There exist several different channel routing technologies in the field of optical communications. However, the evolution of single wavelength point-to-point transmission lines to wavelength division multiplexed optical networks has introduced a demand for wavelength selective optical add-drop multiplexers (OADM) to separate or route different wavelength channels. This article provides some fundamentals relevant to OADM.

What Is OADM?

Optical add-drop multiplexer (OADM) is a device used in wavelength-division multiplexing (WDM) systems. "Add" and "drop" is a capability device to add one or more new wavelength channels to an existing multi-wavelength WDM signal or to drop, which means to remove one or more channels, passing those signals to another network path. OADM can be used at different points along the optical link to insert, remove or route selected channels thus to increase the network flexibility. OADM is particularly important in metropolitan WDM light wave services where offices or sited can be connected by different add-drop channels.

A traditional OADM consists of three stages: an optical demultiplexer, an optical multiplexer, and between them a method of re-configuring the paths between the demultiplexer, the multiplexer and a set of ports for adding and dropping signals. The demultiplexer separates wavelengths in an input fiber onto ports. The reconfiguration can be achieved by a fiber patch panel or by optical switches which direct the wavelengths to the multiplexer or to drop ports. The multiplexer multiplexes the wavelength channels that are to continue on from demultiplexer ports with those from the add ports, onto a single output fiber.

The Functions of OADM

As we have mentioned above, the main function of an optical multiplexer is to couple two or more wavelengths into the same fiber. If place a demultiplexer and properly aligned it back to back with a multiplexer, there would exist two individual wavelength in the area between them. Then, this offers a chance for an enhanced function that individual wavelengths could be removed and inserted as well. The function would be called an optical wavelength drop and add demultiplexer/multiplexer—to make it briefly, optical add-drop multiplexer.

The model of an OADM is clearly shown in the picture below, where F1 signifies a filter selecting wavelength λ1 while passing through all other wavelength, and M1 signifies a multiplexer that multiplexes all wavelengths.

An even better view of OADM function is shown in the following picture. This function is often employed in WDM ring systems as well as in long-haul with drop-add features.

Types of OADM

There are two main types of OADM that can be used in WDM optical networks: fixed OADMs that are used to drop or add data signals on dedicated WDM channels, and re-configurable OADMs that have the ability to electronically alter the selected channel routing through the optical network.

The fixed optical add-drop multiplexer (FOADM) is a traditional wavelength arrangement scheme that can only input or output a single wavelength via the fixed port. FOADMs have pre-assigned channels at static nodes and allowed adding and dropping of individual or multiple wavelength channels from a DWDM.

The re-configurable optical add-drop multiplexer (ROADM), on the other hand, is a dynamic wavelength arrangement scheme, allows for dynamic wavelength arrangement scheme using a wavelength selective switch (WSS). The 8-dimensional cross-connect provided by the WSS enables quick service start-up, remote cross-connect and WDM mesh networking. The ROADM scheme can also achieve inputting/outputting a single wavelength or wavelength group via the fixed port. ROADM can add, block, pass or redirect modulated infrared (IR) and visible light beams of various wavelengths in a fiber optic network. Which featured by providing flexibility in rerouting optical streams, bypassing faulty connections, allowing minimal service disruption and the ability to adapt or upgrade the optical network to different WDM technologies.

Conclusion

To summarize, OADM plays a vital role in improving and optimizing the network performance and reliability. And it is fully compatible with both local area network (LAN) as well as long haul networks. Moreover, OADM also serves as an essential device to meet the requirement of the rapidly developed network.

First published: http://www.china-cable-suppliers.com/introduction-to-optical-add-drop-multiplexer-oadm.html