From TDM to WDM
Early digital transmission systems like SDH/SONET relied on . This involved sending bits from different channels one after another in rapid succession. Increasing the rate required faster electronic processing and shorter time slots. Some earlier TDM systems were described at around 40 Gb/s per fiber, but that figure was dependent on the generation and architecture rather than a universal limit.
Wavelength Division Multiplexing addresses this scaling problem by sending independent optical carriers simultaneously through the same fiber. Each carrier carries a separate data stream, so aggregate capacity can grow without raising every electronic stream to the same rate.
WDM Principle
is an optical equivalent of Frequency Division Multiplexing (FDM). A single fiber can carry several wavelength channels at once when their spectra, powers, and receiver filters are designed with the required isolation. Each wavelength acts as a separate communication channel.
CWDM and DWDM Channel Spacing
- CWDM (Coarse WDM): ITU-T G.694.2 specifies a 20 nm nominal wavelength spacing. The wide spacing permits less expensive, uncooled lasers and simpler components. Metropolitan and access deployments often use 8 or 16 standardized channel positions, while an 18-channel configuration depends on equipment and link budget.
- DWDM (Dense WDM): DWDM uses a much finer frequency grid. ITU-T G.694.1 anchors the grid at 193.1 THz and defines fixed spacings including 12.5, 25, 50, and 100 GHz, with wider options. Near 1550 nm, 100 GHz is approximately 0.8 nm and 50 GHz approximately 0.4 nm. Frequency is the normative quantity, and the wavelength equivalents vary with frequency. The 10/2020 edition is In force. Practical systems can use 40, 80, 160, or more channels in a selected band, depending on the band and equipment. The recommendation defines the frequency grid, while the selected band and equipment determine the channel count. It also defines a flexible grid with 6.25 GHz center-frequency granularity and 12.5 GHz slot-width granularity. The grid is specified in an recommendation. DWDM is used for long-haul, high-capacity transport, including backbone links whose aggregate data rate can reach the terabit-per-second range.
Anatomy of a DWDM Link
A typical long-haul DWDM line combines several components to launch, amplify, manage, and receive the multi-wavelength signal.
- Transponder: It accepts a client signal, for example from a router or SDH equipment, and maps it to an optical carrier at a wavelength or frequency on the DWDM grid. Equipment that supports 3R regeneration performs re-amplifying, re-shaping, and re-timing before transmission.
- Multiplexer (MUX): This device combines the individual grid-aligned optical carriers from multiple transponders into one multi-wavelength signal for transmission over a single fiber.
- Optical Amplifier (e.g., EDFA): Over long distances, the optical signal loses power (attenuation). An amplifies channels within its gain band simultaneously without converting them to electrical signals. EDFAs can be placed periodically along a long fiber link, subject to the link budget and channel plan. Amplifier spacing and launch power are selected from the optical power budget, nonlinear effects, and the required signal-to-noise ratio.
- Dispersion Compensating Fiber (DCF): A special fiber that can be inserted into a link to counteract chromatic dispersion. Dispersion makes optical pulses at different wavelengths travel with different group delays and can broaden the signal over distance.
- Demultiplexer (DEMUX): At the receiving end, this device separates the incoming multi-wavelength signal into individual grid channels and directs each one to its receiving transponder.
Network Architectures and Protection
DWDM is deployed in several network topologies, with protection mechanisms selected for the required availability and failure model.
- Point-to-Point and Ring Topologies: Point-to-point links connect two locations. Ring topologies connect multiple nodes and can support protection paths when the network design includes them.
- Flexible Nodes (OADM and OXC): Intermediate nodes can manage selected wavelengths without terminating every channel. An allows selected channels to be dropped at an intermediate site while others pass through. An OXC (Optical Cross-Connect) switches wavelength paths or complete fiber signals between multiple input and output fibers and can form a mesh transport node.
- Protection Schemes: To guard against failures like a fiber cut, DWDM networks use protection switching. In a 1+1 scheme, the signal is sent simultaneously over two separate fiber paths, and the receiver chooses the better one. In 1:N schemes, one backup path (or set of wavelengths) can protect multiple working paths.