CWDM - Cost-Effective Alternative to Expand Network Capacity

Fiber optic cabling is highly valued in telecommunications for its exceptional data throughput and long-distance capabilities. However, deploying it universally is costly. Wavelength Division Multiplexing (WDM), which includes Coarse WDM (CWDM) and Dense WDM (DWDM), offers a cost-effective alternative by combining multiple signals onto one fiber using different light wavelengths. CWDM is ideal for enterprise networks and metropolitan short-distance transmissions, while DWDM is optimized for long-haul transmissions with greater channel capacity. In this context, the article delves into how WDM technology enhances modern networks and its varied applications.

CWDM Technology - Alternative for Increasing Transmission Capacity

Understanding what is CWDM (Coarse Wavelength Division Multiplexing) is crucial for appreciating its technological and practical advantages. CWDM was standardized by the ITU-T G.694.2 based on a grid or wavelength separation of 20 nm in the range of 1270-1610 nm. It can carry up to 18 CWDM wavelengths over one pair of fibers. Each signal is assigned to a different wavelength of light. Each wavelength does not affect another wavelength, so the signals do not interfere. Each channel is usually transparent to the speed and type of data, so any mix of SAN, WAN, Voice, and Video services can be transported simultaneously over a single fiber or fiber pair.

Figure 1 : CWDM System

CWDM is a cost-effective solution to provide a capacity boost in the access network. It can address traffic growth demands without overbuilding the infrastructure. For example, a typical 8-channel CWDM system offers 8 times the amount of bandwidth that can be achieved using a SONET/SDH system for a given transmission line speed with the same optical fibers. It is a perfect alternative for carriers looking to increase the capacity of their installed optical network without replacing existing equipment with higher bit rate transmission equipment, and without installing new fibers.

Key CWDM Network Component - CWDM Mux Demux

Figure 2 : CWDM Mux Demux

CWDM Mux Demux: Dual-Fiber VS. Single-Fiber

Dual-Fiber CWDM Mux Demux

A Dual-Fiber CWDM Mux Demux supports up to 18 channels, each capable of transmitting and receiving signals within the wavelength range of 1270 nm to 1610 nm. The CWDM transceiver used should match the wavelength of the Mux port to ensure proper signal transmission.

Single-Fiber CWDM Mux Demux

In contrast, a Single-Fiber CWDM Mux Demux transmits and receives signals using the same fiber, requiring different wavelengths for RX (receiving) and TX (transmitting) on the same port, making its operation more complex than the dual-fiber counterpart. These units, used in pairs, effectively manage integrated signal transmission and reception. Additionally, the design offers flexible equipment location, allowing network equipment to be placed in the same rack as the CWDM multiplexers or in remote locations connected to a CWDM multiplexer at a hub site, thus enabling adaptable network configurations and efficient use of space and resources.

Figure 3 : Dual-Fiber CWDM Mux Demux VS. Single-Fiber CWDM Mux Demux

CWDM Applications

CWDM is applied primarily in two broad areas: metro and access network, performing two functions - one is to use each optical channel to carry a distinct input signal at an individual rate, and another is to use CWDM to break down a high-speed signal into slower components that can be transmitted more economically, such as some 10 G transceivers.

CWDM in Metropolitan Area Network (MAN)

Metropolitan area network (MAN) refers to the network that covers the city and its suburbs, providing an integrated transmission platform for metropolitan areas. CWDM networks enable wavelength services to be provisioned over a large metro area, with the functional and economic benefits of full logical mesh connectivity, wavelength reuse, and low end-to-end latency. These features apply to the Inter-Office (CO-CO) and Fiber to the Building (FTTB) segments of the metro network. The low latency benefits of CWDM are especially attractive in ESCON and FICON/Fibre Channel-based SAN applications. The less space, low power, and cost benefits of CWDM also enable its deployment in the Outside Plant (OSP) or Remote Terminal (RT) segments of the metro market.

Figure 4 : CWDM in Metropolitan Area Network

CWDM in LAN and SAN Connection

CWDM has abundant network topology, such as point-to-point, ring, mesh, etc. The ring network can provide self-healing protection: the style of restoring includes link-breaking protection and node failure separation. CWDM rings and point-to-point links are well suited for interconnecting geographically dispersed LAN (local area network) and SAN (storage area network). Corporations can benefit from CWDM by integrating multiple Gigabit Ethernet, 10 Gigabit Ethernet, and Fibre Channel links over a single optical fiber for point-to-point applications or ring applications.

CWDM Integrated in 10 Gigabit Ethernet

With the benefits of low implementation cost, robustness, simplicity of installation, and maintenance, Ethernet has been used intensively in the metro/access system now. As the bandwidth increases, a higher data rate 10 Gigabit Ethernet was put forward. Ethernet integration with CWDM is one of the best-implementing methods. In one of the 10 Gigabit Ethernet standards in the IEEE 802.3ae is a four-channel, 1300 nm CWDM solution. However, if CWDM were based on 10 channels of 1 Gbps, then 200 nm of the wavelength spectrum would be used. Compared with TDM (transmission time division multiplexing), 10G CWDM technology may have a higher initial cost, but it can offer better scalability and flexibility than TDM.

CWDM in PON (Passive Optical Network)

PON is a point-to-multipoint optical network that uses existing fiber. It is an economical way to deliver bandwidth to the last mile. Its cost savings come from using passive devices in the form of couplers and splitters, rather than higher-cost active electronics. PON expands the number of endpoints and increases the capacity of the fiber. But PON is limited in the amount of bandwidth it can support. As CWDM can multiply the bandwidths cost-effectively, when combining them, each additional lambda becomes a virtual point-to-point connection from a central office to an end user. If one end user in the original PON deployment grows to the point where he needs his own fiber, adding CWDM to the PON fiber creates a virtual fiber for that user. Once the traffic is switched to the assigned lambda, the bandwidth taken from the PON is now available for other end users. So the access system can maximize fiber efficiency.

Figure 5 : CWDM in PON

CWDM VS DWDM Difference

Wavelength Spacing

CWDM can transport up to 16 wavelengths with a channel spacing of 20 nm in the spectrum grid from 1270 nm to 1610 nm. While DWDM can carry 40, 80, or up to 160 wavelengths with a narrower spacing of 0.8 nm, 0.4 nm, or 0.2 nm from the wavelengths of 1525 nm to 1565 nm (C band) or 1570 nm to 1610 nm (L band).

Figure 6 : CWDM Wavelength Grid

Transmission Distance

DWDM multiplexing system is capable of having a longer haul transmission by keeping the wavelengths tightly packed. It can transmit more data over a larger run of cable with less interference than the CWDM system. CWDM system cannot transmit data over long distances as the wavelengths are not amplified. Usually, CWDM can transmit data up to 100 miles (160 km).

Modulation Laser

The CWDM system uses the uncooled laser while the DWDM system uses the cooling laser. Laser cooling refers to a number of techniques in which atomic and molecular samples are cooled down to near absolute zero through the interaction with one or more laser fields. Cooling laser adopts temperature tuning that ensures better performance, higher safety, and longer life span of DWDM system. But it also consumes more power than the electronic tuning uncooled laser used by the CWDM system.

Price

The cost of DWDM is typically four to five times higher than that of CWDM, due to several key factors related to the lasers. DWDM lasers require manufacturing wavelength tolerances of about ±0.1 nm, compared to the looser ±2-3 nm tolerances for CWDM lasers, resulting in lower die yields and higher costs. Additionally, DWDM lasers need expensive packaging for temperature stabilization with a Peltier cooler and thermistor, unlike the uncooled packaging for CWDM lasers. Consequently, CWDM is a more cost-effective option for many access and enterprise applications, offering simpler network design, implementation, and operation with fewer parameters to optimize. Conversely, DWDM systems require complex calculations for power balance per channel, which become more intricate when channels are added or removed or in ring networks with optical amplifiers. The following table compares CWDM and DWDM.

Conclusion

CWDM is an attractive solution for carriers seeking to upgrade their networks to meet current and future traffic demands while minimizing the use of valuable fiber strands. FS's CWDM Mux Demux offer a versatile and cost-effective method for optimizing network performance. Supporting up to 18 channels, these solutions facilitate scalable network expansion and provide both single-fiber and dual-fiber configurations. This flexibility makes CWDM devices ideal for high-demand access and metro network environments. As the industry continues to drive down costs and increase capacity, FS's Mux Demux products are well-positioned to support the transition to more advanced, converged circuit networks, ensuring carriers can efficiently manage rising traffic demands.