WDM Basics: Understanding Wavelength Division Multiplexing Technology

Wavelength division multiplexing (WDM), known as the classic technology that provides optimal solutions for transporting large amounts of data between sites. With the endless upgrades and improvements, WDM technology is no longer just adopted by carriers and service providers, but also applied for enterprises, organizations, and data centers, meeting the demands for both public and private network uses. So, what is WDM? What benefits will WDM networking bring to the users? How about the applications of WDM? This WDM tutorial will explain all your inquiries.

Scenario Intro

Suppose a building on one side of a district requires eight 10G fiber connections.

Option 1: connect all of the fibers to the switch in the central office several kilometers away. Running 8 pairs of fiber all the way across the district will be an extremely expensive proposition at the risk of fiber breakage at the same time.

Option 2: Adding WDM Mux and Demux and only requiring one pair of fibers to conduct and realize the transmission, this solution saves fiber resources substantially and offers a more cost-effective and reliable alternative compared with the abovementioned one obviously. How to achieve this? With the aid of WDM technology.

What Is WDM (Wavelength Division Multiplexing)?

Briefly speaking, WDM is a technique in fiber optic transmission for using multiple light wavelengths to send data over the same medium. Specifically speaking, this technology multiplexes a number of optical carrier signals onto a single optical fiber by using different optical wavelengths (i.e., colors) of laser light. Every wavelength carries an individual signal that does not interfere with the other wavelengths. The diagram below illustrates the working principle of WDM technology.

Benefits of Adopting WDM Technology

• Ultra-large Capacity Transmission: WDM is capable to transmit and receive high capacity data, which is a ready-made option for high-bandwidth transmission like 100G, 400G.

• Ultra-long Distance Transmission: EDFA (Erbium Doped Fiber Amplifier) is often adopted in the WDM system, which will help to further boost the intensity of optical signals for long-range transmission.

• Transparent Transmission: due to the physical properties of light, all the wavelengths are independent since the channels don't interfere with each other to ensure transmission transparency. Adopting the optoelectronic devices will help to guarantee the reliability of the WDM system.

• Flexible for Smooth Expansion: wavelength division multiplexing technique allows new channels to be connected as needed without disrupting the existing traffic services, making the upgrades easier.

• Saving Both Fiber Resources & Expenses: for the single-wavelength system, one SDH system requires a pair of fibers, while the entire multiplexing system only requires one pair of fibers regardless of the number of SDH subsystems. Fiber exhaust will be prevented accordingly. In this way, WDM not only maximizes the utilization of fibers but also helps to optimize overall network investments.

WDM Classifications: CWDM vs DWDM

What Is CWDM?

CWDM (Coarse Wavelength Division Multiplexing) is defined by wavelengths, which belongs to the ITU (International Telecommunication Union) in ITU-T G.694.2 spectral grids. It uses the wavelengths from 1270 nm to 1610 nm within a 20nm channel spacing. CWDM supports fewer channels than DWDM, which makes it the ideal solution for short-range communications as it's compact and cost-effective.

What Is DWDM?

DWDM (Dense Wavelength Division Multiplexing) is defined by frequencies, which belongs to ITU-T G.694.1 spectral grids. In practice, DWDM frequency is usually converted to wavelength. DWDM typically has the capability to transport up to 80 channels (wavelengths) in what is known as the Conventional band (C-band) spectrum, with all 80 channels in the 1550 nm region. Allowing huge amounts of data to travel in one single network link, DWDM is ideal for long-haul transmission as the wavelengths are tightly packed together.

The differences between CWDM vs DWDM lie in channel spacing, transmission distance, modulation laser, costs, etc., which has reflected in their practical applications.

WDM Systems: Single Fiber vs Dual Fiber

Single Fiber Transmission

Single fiber transmission method, namely, is a kind of bi-directional communication over one single fiber. This system utilizes two identical sets of wavelengths for both directions over a single fiber. Individual channels residing on the single fiber system may propagate in either direction.

Dual Fiber Transmission

The dual fiber transmission method is comprised of two single fibers—one fiber is used for the transmit direction and the other is used for the receive direction. In a dual fiber transmission system, the same wavelength is normally used in both the transmit and receive directions. The second fiber may serve as a backup fiber as in a redundant system, or it may provide an optical path in the opposite direction.

Common WDM System Components

The simplest form of the WDM systems consists of four main elements. The below sketch may picture a basic WDM structure:

Mux and Demux

WDM Mux and Demux is the key to optimize the use of the fiber. At the heart of the operation, the WDM multiplexer gathers all the data streams together to be transported simultaneously over a single fiber. At the other end of the fiber, the streams are demultiplexed, i.e. separated into different channels again. It's crucial to understand the Mux ports on a WDM Mux/Demux. Must-have ports are twofold: channel port & line port; functioning ports are basically covering expansion port, 1310nm port and 1550nm port, monitor port, etc. Understand Mux Ports On CWDM and DWDM Mux Demux will illustrate the different ports for you clearly.

Transceiver

The optical transceivers adopted in the WDM system are wavelength-specific lasers, corresponding to the CWDM and DWDM bands, which differ from the common modules with 850nm, 1310nm, 1550nm bands. It converts data signals from switches to optical signals that can be transmitted into the fiber. Each data stream is converted into a signal with a light wavelength that is a unique color.

Patch Cord

Briefly speaking, the fiber patch cable functions as the glue that joins the previous optical modules and multiplexers together to realize the transmission—connecting the output of the transceiver to the input on the multiplexer.

Dark Fiber

A prerequisite for any WDM solution is access to a dark fiber network. Adopting a fiber pair is considered as the common way to transport optical traffic. One of the fibers is used for transmitting the data and the other is used for receiving the data, which allows the maximum amount of traffic to be transported.

Additionally, optical add-drop multiplexer (OADM), optical amplifiers, EDFAs, DCMs, transponders may all be adopted to realize connectivity in some typical WDM topologies.

Typical WDM Topologies

Point-to-point Topology

Point-to-point topology, typically in long-range transport, can be implemented with or without OADM. This kind of P2P topology is characterized by high channel speeds, high signal integrity and reliability, and fast path restoration. A WDM point-to-point system normally includes lasers, optical multiplexers and demultiplexers, fibers, optical amplifiers, and optical add-drop multiplexers. In long-haul networks, the distance between transmitter and receiver can be several hundred kilometers, and the number of amplifiers required between endpoints is typically less than 10. In the MAN, amplifiers are often not needed.

Ring Topology

Ring topology, typically in metropolitan areas, can span a few tens of kilometers. A ring configuration may cover one hub station and one or more OADM nodes. At the hub node, where the traffic originates, the traffic is terminated and connected with other networks. At the OADM nodes, the selected wavelengths are dropped and added, while the others pass through transparently. The ring architecture allows nodes on the ring to provide access to network elements such as routers, switches, or servers by adding or dropping wavelength channels in the optical domain. With an increase in the number of OADMs, however, the signal is subject to lose and amplification will be required.

Mesh Topology

Mesh architecture seems to be the future of optical networks because it is considered as a robust topology. By beginning with point-to-point links, equipped with OADM nodes at the outset for flexibility, and subsequently interconnecting them, the network can evolve into a mesh without a complete redesign. Additionally, mesh and ring topologies can be joined by point-to-point links.

Actually, different WDM topologies can be deployed in different scenarios. Point-to-point topologies might be used for connecting enterprise locations, ring topologies might be more suitable for connecting inter-office facilities (IOFs) or for residential access, and mesh topologies might fit for inter-POP connections and connections to the long-haul backbone. In practice, there also exists hybrid network topology that consists of mesh and/or rings that are interconnected with point-to-point links.

Common WDM Applications and Future Predictions

WDM technology not only witnesses the changes and improvements in optical transmission but also gets involved in the revolution, which has been reflected in the deployment of OTN (Optical Transport Network). Based on WDM technology, OTN with ultra-huge transmission capacity has added SDH (Synchronous Digital Hierarchy) technology to realize powerful functionalities of operation, maintenance, and management. OTN networks with WDM technology has been widely deployed in various scenarios, especially for long-haul and metro networks, which helps to optimize the links in the system and guarantee high-reliability networks.

Technically speaking, the construction of large-scale optical transmission networks like 100G OTN backbone networks has taken effects. The industry is accelerating the R&D and industrialization of ultra-high-speed transmission technologies like 400G, 800G, and 1T, striving for greater breakthroughs in the optical transmission field. This pursuit will further promote wider applications with WDM technology in networking deployments. From the perspective of WDM products, the optical transmission network equipment product system is complete gradually with more and more WDM and optical access equipment adopted worldwide. The scale of the optical transmission industry has expanded accordingly, it is predictable that the demands for WDM applications will continue to swell.

WDM technology is a crucial part of accelerating the development of OTN networks towards higher speed, larger capacity, lower expense, plus more intelligent and environmentally-friendly in the future.

Appendix: WDM Glossary of Terminologies

• TDM: the abbreviation for "Time Division Multiplexing," an alternative to WDM. Instead of multiple channel wavelengths, TDM protocols define multiple time-slots in which different channels occupy each at the same wavelength.

• Wavelength: refers to the distance between two points of the same phase in consecutive cycles of a wave, measured in the direction of propagation. The wavelength λm of monochromatic light traveling in an optical fiber is expressed:

  λm = λ / n = v / f

  λ = optical wavelength in a vacuum

  n = the refractive index of the dielectric medium

  v = phase velocity, given by c / n

  c = the speed of light in a vacuum: 2.99792458 X 108 m/s

  f = the optical frequency




• Channel: in WDM systems, each input channel is assigned a unique wavelength (i.e. color of light), thus the channels can traverse the fiber "in parallel."

• Circuit: sometimes used to identify a group of WDM wavelengths that are combined and associated with a single fiber path or span. Ports on the WDM products will be labeled to identify the circuit associated with that port.

• Add/Drop: the add/drop terminology may refer to a single wavelength filter, or to a multi-channel WDM product. In the case of the filter, it is just another way to describe the bi-directional nature of the filter, in which a particular channel wavelength may be either added as in multiplexed; or dropped as in de-multiplexed.

• Insertion Loss/Return Loss: insertion loss is the attenuation caused by the insertion of WDM filter in an optical transmission system; return loss is the loss of power in the signal returned/reflected by a discontinuity in a transmission line or optical fiber of WDM systems.

• Polarization Dependent Loss (PDL): The loss exhibited by a WDM filter is dependent on the optical polarization of the light. PDL is the largest difference in maximum insertion loss occurring at all states of optical polarization.