Dense Wavelength Division Multiplexing (DWDM) is an optical networking technology that carries many separate channels of data over a single fiber pair by sending each channel on its own wavelength, or color, of light. The channels are spaced closely together, which lets one fiber pair carry far more traffic than it could with a single signal. DWDM is the technology underneath most long-haul and metro carrier networks and the wavelength services carriers sell.
At a glance
- DWDM puts many closely spaced wavelengths on one fiber pair, each acting as an independent channel.
- It is a member of the wavelength division multiplexing (WDM) family, alongside the lower-cost, lower-capacity coarse WDM (CWDM).
- Carriers use it to build backbone and metro networks and to sell wavelength services.
- Enterprises sometimes run their own DWDM over leased dark fiber between data centers.
- With optical amplifiers, DWDM systems can reach hundreds or thousands of kilometers, depending on the design.
What problem it solves
Laying new fiber is slow and expensive, and leasing extra fiber pairs adds cost every month. Without multiplexing, one fiber pair carries one signal, so more capacity means more fiber. DWDM breaks that link. By sending many wavelengths down the same fiber at once, it multiplies the capacity of glass that is already in the ground.
For carriers, that is what makes high-capacity backbones economical. For enterprises with dark fiber between sites, DWDM is a way to add new links, such as storage replication, a second data center interconnect or a separate network for a business unit, without leasing more fiber.
How it works
Transponders and optics. Each client signal, such as 10, 100 or 400 Gigabit Ethernet, is placed on a specific wavelength by a transponder or a tunable or fixed-wavelength optical transceiver. Coherent optics, now common at higher speeds, carry more data per wavelength over longer distances.
Multiplexers. A multiplexer combines the wavelengths onto one fiber; a demultiplexer at the far end separates them again. Channels sit on a standard grid defined by the ITU, with fixed spacing or, on newer systems, flexible spacing.
Amplification. Optical amplifiers boost all channels together along long routes, so a signal can travel much farther before it needs to be regenerated.
Add/drop. Reconfigurable optical add-drop multiplexers (ROADMs) let a network drop some wavelengths at intermediate sites and pass others through, and on many systems let operators reroute channels remotely.
Framing and management. Many carrier systems wrap traffic in Optical Transport Network (OTN) framing for monitoring and error correction.
DWDM generally runs over single-mode fiber (SMF). The fiber’s loss, age and type affect how far and how fast the system can run, so route testing comes before design.
DWDM vs. CWDM
Coarse wavelength division multiplexing (CWDM) uses wider channel spacing and typically up to 18 channels. Its optics are cheaper and typically use uncooled lasers, but most CWDM channels fall outside the range of common optical amplifiers, so CWDM suits shorter metro and campus links with modest channel counts. DWDM supports many more channels and longer distances at higher cost.
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When it matters for buyers
- Data center interconnect. Several high-speed links between two sites over one leased fiber pair.
- Comparing a wavelength service with dark fiber. DWDM equipment cost and operations are part of the dark fiber business case.
- Capacity planning. A system that can add channels later may avoid a new fiber lease.
- Evaluating carrier routes. The carrier’s DWDM design affects latency, protection options and upgrade speed on the services you buy.
Questions to ask vendors
- How many channels does the system support, at what speeds, and how much headroom remains for us?
- What distance can it reach on our fiber route without regeneration, based on actual route measurements?
- Is it a fixed or flexible grid, and does it use ROADMs?
- What optics are supported, and are third-party optics allowed?
- How is the system monitored, and who responds to optical alarms?
- What protection or restoration options are available if a fiber route is cut?
How it differs from a wavelength service
DWDM is a technology. A wavelength service is a product: a carrier sells you one channel on its DWDM network between two locations and manages the equipment, routes and repairs. Running your own DWDM over dark fiber gives you control over channel count and upgrades but makes you responsible for equipment and operations. Buying wavelengths keeps that with the carrier, priced per channel.