LambdaGain Blog
Introducing: DWDM Passive Line Systems
In a previous blog (link here) we looked at Pluggable Line Systems (PLS), which incorporate pluggable EDFA amplifiers to achieve many of the benefits of DWDM optical lines systems, while dramatically reducing the cost, complexity and energy consumption of fiber backhaul. In this blog, we are going to back up one step and introduce Passive Line Systems (PaSL), which use many of the same building blocks as Pluggable Line Systems, but are un-amplified. The result is a network which may be more limited in range, but is extremely robust, cost effective, and easy to deploy.
A proven backhaul solution
Passive Line Systems (PaLS) are widely used to backhaul 4G/5G small cells, remote OLTs, MAC/PHY devices in cable MSO DAA networks, and remote antennas in DAS systems. For operators, key benefits of Passive Line Systems include low cost, zero power requirements, extremely high reliability, and tremendous flexibility in adapting to application requirements and available infrastructure – deployed fiber, street cabinets, poles, etcetera.
Start with the basics
Here’s what most people think of when they imagine a wavelength-multiplexed access network:
This type of simple point-to-point connection is an excellent choice when operators want to bring substantial bandwidth to a single location while reducing fiber costs. A typical configuration would use 40 DWDM Channels, with each wavelength carrying 10Gbps, for a total capacity of 200Gbps for single fiber operation or 400Gbps on dual fiber. Of course, as pluggable optics increase in speed and range, this link could be upgraded with 25Gbps optics (potentially with some range limitations) and eventually 100Gbps optics (expensive today, but doable). Clearly, Passive DWDM systems can deliver massive backhaul capacity today, and offer plenty of headroom for future growth.
Beyond Point-to-Point
Many DWDM Passive Line Systems go beyond point-to-point connections, serving multiple remote nodes with architectures adapted to the specific application. Two of the key architectures for these types of access networks are explained in the following sections.
Passive DWDM Cascade
DWDM Passive Line Systems using a cascade architecture are widely deployed whenever the underlying infrastructure is linear in nature. That could be a series of 4G/5G small cells located along a major road, or DAS antennas located in a road, subway, or railway tunnel, or OLTs located along a country road as it passes through small communities. The primary benefit of the DWDM cascade architecture is that it uses the absolute minimum amount of fiber – typically a single fiber – to serve all nodes in the network. When this style of system is reinforced with SFP EDFAs, the Passive Line System evolves into a Pluggable Line System, which offers extended range at the cost of some additional complexity and investment. There is no fundamental difference in the approach, and the Passive Line System remains by far the most common type of deployment.
In terms of capacity, a high-capacity DWDM cascade might have a 40-channel mux/demux at the headend (as shown in our illustration), and present 8 or 10 channels at each remote node. A lower capacity network might only have an 8-channel mux/demux at the headend and drop a single pair of wavelengths at each remote node – ideal to serve a single DAS antenna, for example.
Passive DWDM Point-To-Multipoint Network
The other type of Passive Line System is the Point-to-Multipoint architecture, shown below:
The passive DWDM Point-To-Multipoint network is ideal to conserve distribution fiber from a headend to a central splitting point, then leverage fiber available in the service area to reach terminal nodes. In a typical use case, the central point could be a single macro base station site, which serves several small cell sites nearby; or a central splitting node that serves several remote OLT locations from a fiber distribution location.
Horses for Courses
The three basic passive DWDM architectures: Point-To-Point, Cascade, and Point-To-Multipoint can be adapted to a wide range of applications in the access network. Once an operator makes the decision to standardize on passive line systems, LambdaGain works with them to design a simple set of building blocks, adapted to their specific requirements and practices, to build a toolkit that network designers can use to implement specific projects. The choice of the best solution will depend on the available fiber resources, the physical infrastructure available (e.g. remote cabinets and shelters), and the distribution of the service points.
Next Steps
In forthcoming blogs, we’ll look in more detail at the mux/demux modules, band splitters, and other building blocks used in Passive Line Systems and Pluggable Line Systems; we’ll also look at the engineering required for passive line systems – stay tuned.
If you have an application where Passive Line System technology could be a fit, please contact us here. Our experts have worked with operators to save millions of dollars using Passive and Pluggable Line System solutions, and we’re ready to put that expertise to work on your projects.
