For 40GBASE-LR4 QSFP+ Transceiver Link: CWDM or PSM?

Nowadays, the 40 Gigabit Ethernet (GbE) system comes as the popular deployment among some enterprises for their high-performance fiber optic networks. And for 40GbE system, fiber optic transceivers are the indispensable high-capacity modules for multi-lane communications, like 40GBASE-LR4 QSFP+ transceiver. It’s known that 40GBASE-LR4 QSFP+ transceiver has two link options: coarse wavelength division multiplexing (CWDM) and parallel single-mode fiber (PSM). How much do you know about them? Can you figure out the differences between them? Following this article and you will get something.

40GBASE-LR4 CWDM QSFP+ Transceiver Brief

Compliant to 40GBASE-LR4 (eg. QSFP-40G-LR4) of the IEEE P802.3ba standard, this 40GBASE-LR4 CWDM QSFP+ transceiver uses a duplex LC connector as the the optical interface, able to support transmission distance up to 10km over single-mode fiber (SMF) used to minimize the optical dispersion in the long-haul system.

This kind of 40GBASE-LR4 QSFP+ transceiver converts 4 inputs channels of 10G electrical data to 4 CWDM optical signals by a driven 4-wavelength distributed feedback (DFB) laser array, and then multiplexes them into a single channel for 40G optical transmission, propagating out of the transmitter module from the SMF. Reversely, the receiver module accepts the 40G CWDM optical signals input, and demultiplexes it into 4 individual 10G channels with different wavelengths. The central wavelengths of the 4 CWDM channels are 1271, 1291, 1311 and 1331 nm as members of the CWDM wavelength grid defined in ITU-T G694.2. Each wavelength channel is collected by a discrete photo diode and output as electric data after being amplified by a transimpedance amplifier (TIA).

CWDM QSFP+, 2 optical SMFs with a duplex LC connector

40GBASE-LR4 PSM QSFP+ Transceiver Brief

Differently, PSM QSFP+ is a parallel single-mode optical transceiver and uses a MTP/MPO fiber ribbon connector instead of LC. Similarly, PSM QSFP+ also offers 4 independent transmit and receive channels, each capable of 10G operation for an aggregate data rate of 40G with 10km reach over SMF.

In a PSM QSFP+, the transmitter module accepts electrical input signals, while he receiver module converts parallel optical input signals via a photo detector array into parallel electrical output signals. Both the input signals and output signals are compatible with common mode logic (CML) levels.

PSM QSFP+, 8 optical SMFs with a MTP/MPO fiber ribbon connector

CWDM vs. PSM

Allowing for the transceiver module structure, PSM seems more cost effective, since it uses a single uncooled CW laser which splits its output power into four integrated silicon modulators. Additionally, its array-fiber coupling to a MTP connector is relatively simple.

However, when taking the infrastructure into consideration, PSM would be more expensive when the link distance is long, because it uses 8 optical single-mode fibers while CWDM only uses 2 optical single-mode fibers. Besides, in the data center fiber infrastructure, the patch panel has to be changed to accommodate MTP cables, which would cost more than LC connectors and regular SMF cables. Besides, it’s a little difficult to clean MTP connectors. So CWDM is more ideal for 40GBASE-LR4 QSFP+ link.

Conclusion

For 40GBASE-LR4 QSFP+ transceiver link options, both CWDM QSFP+ and PSM QSFP+ support the maximum transmission distance of 10km. The former establishes 40G links over 2 optical SMFs with a duplex LC connector, and the latter achieves 40G links via 8 optical SMFs with a MTP/MPO fiber ribbon connector. Thus no change is required for migration from 10G infrastructure to 40G infrastructure, saving cost when CWDM QSFP+ is chosen. Fiberstore supplies a broad selection of 40GBASE-LR4 QSFP+ transceivers which are fully compatible with major brands, such as Finisar (FTL4C1QE1C). For more information about 40GBASE-LR4 QSFP+ transceivers, please visit Fiberstore.

Differences between CWDM and DWDM

In fiber-optic communications, WDM (wavelength-division multiplexing) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light. This technique enables bidirectional communications over one strand of fiber as well as multiplication of capacity. Generally, WDM technology is applied to an optical carrier which is typically described by its wavelength.

WDM system uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split the signals apart (see Figure 1). WDM system is very popular in the telecommunication industry because it allows the capacity of the network to be expanded without laying more fiber. By utilizing WDM and optical amplifiers, users can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. Moreover, the capacity of a given link can be expanded simply by upgrading the multiplexers and demultiplexers at each end.

WDM operating principle

Figure 1

WDM could be divided into CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). DWDM and CWDM are based on the same concept of using multiple wavelengths of light on a single fiber but differ in the spacing of the wavelengths, number of channels, and the ability to amplify the multiplexed signals in the optical space. Below part will introduce some differences between CWDM and DWDM system.

Wavelength Spacing

CWDM provides 8 channels with 8 wavelengths (from 1470nm through 1610nm) with a channel spacing of 20nm. While DWDM can accommodate 40, 80 or even 160 wavelengths with narrower wavelength spans which are as small as 0.8nm, 0.4nm or even 0.2nm (see Figure 2).

CWDM-VS-DWDM

Figure 2

Transmission Distance

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

Power Requirements

The power requirements for DWDM are significantly higher. For instance, DWDM lasers are temperature-stabilized with Peltier coolers integrated into their module package. The cooler along with associated monitor and control circuitry consumes around 4W per wavelength. Meanwhile, an uncooled CWDM laser transmitter uses about 0.5W of power.

Price

The DWDM price is typically four or five times higher than that of the CWDM counterparts. The higher cost of DWDM is attributed to the factors related to the lasers. The manufacturing wavelength tolerance of a DWDM laser die compared to a CWDM die is a key factor. Typical wavelength tolerances for DWDM lasers are on the order of ±0.1 nm, while tolerances for CWDM laser die are ±2-3 nm. Lower die yields also drive up the costs of DWDM lasers relative to CWDM lasers. Moreover, packaging DWDM laser die for temperature stabilization with a Peltier cooler and thermister in a butterfly package is more expensive than the uncooled CWDM coaxial laser packing.

To sum up, CWDM and DWDM have different features. Choosing CWDM or DWDM is a difficult decision. We should first understand the differences between them. Fiberstore has various kinds of WDM products, such as 10GBASE DWDM, 40 channel DWDM Mux, CWDM Mux/Demux module and so on. It is an excellent option for choosing CWDM and DWDM equipment.

Learn more details about CWDM and DWDM SFP+ transceivers at Everything You Need to Know Before Buying CWDM and DWDM SFP+ Transceivers

Related Article: The Advantages and Disadvantages of Multimode and Single-mode Fiber

CWDM vs DWDM: What’s the Difference?

Differences Between CWDM and DWDM

Wavelength division multiplexing (WDM) is a technology or technique modulating numerous data streams, i.e. optical carrier signals of varying wavelengths (colors) of laser light, onto a single optical fiber. The goal of WDM is to have a signal not to interfere with each other. It is usually used to make data transmission more efficiently. It has also been proven more cost effective in many applications, such as WDM network applications, broadband network application and fiber to the home (FTTH) applications and so on. According to channel spacing between neighbored wavelengths, there are two main types of WDM: Coarse WDM (CWDM) and Dense WDM (DWDM). Though both of them belong to WDM technology, they are quite different. We can differentiate them from the definition, data capacity, cable cost and transmission distance.

Definition
CWDM is defined by wavelengths and has wide-range channel spacing. DWDM is defined by frequencies and has narrow channel spacing.

  • CWDM is a method of combining multiple signals on laser beams at various wavelengths for transmission along fiber optic cables, such that the number of channels is fewer than in DWDM but more than in standard WDM. “Course” means the channel spacing is 20 nm with a working channel passband of +/-6.5 nm from the wavelengths center. From 1270 nm to 1610 nm, there are 18 individual wavelengths separated by 20nm spacing.
  • DWDM is a technology that puts data from different sources together on an optical fiber, with each signal carried at the same time on its own separate light wavelength. “Dense” refers to the very narrow channel spacing measured in Gigahertz (GHz) as opposed to nanometer (nm). DWDM typically uses channel spacing of 100 GHz with a working channel passband of +/-12.5 GHz from the wavelengths center. It uses 200GHz spacing essentially skipping every other channel in the DWDM grid. And it has also gone one step further using an Optical Interleaver to get down to 50GHz spacing doubling the channels’ capacity from 100GHz spacing.
Data Capacity

In fiber optic network system, DWDM system could fit more than 40 different data streams in the same amount of fiber used for two data streams in a CWDM system. In some cases, CWDM system can perform many of the same tasks compared to DWDM. Despite the lower transmission of data through a CWDM system, these are still viable options for fiber optic data transmission.

Cable Cost

CWDM system carries less data, but the cabling used to run them is less expensive and less complex. A DWDM system has much denser cabling and can carry a significantly larger amount of data, but it can be cost prohibitive, especially where there is necessary to have a large amount of cabling in an application.

Transmission Distance

DWDM system is used for a longer-haul transmission through keeping the wavelengths tightly packed. It can transmit more data over a significantly larger run of cable with less interference. However, CWDM system cannot travel long distances because the wavelengths are not amplified, and therefore CWDM is limited in its functionality over longer distances. If we need to transmit the data over a very long range, DWDM system solution may be the best choice in terms of functionality of the data transmission as well as the lessened interference over the longer distances that the wavelengths must travel. As far as cost is concerned, when required to provide signal amplification about 100 miles (160 km), CWDM system is the best solution for short runs.