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FTTX Fundamentals: Practical Fiber Infrastructure Considerations

FTTX Fundamentals: Practical Fiber Infrastructure Considerations

20 November 2018 | Reading Time: 4 minutes


Fiber offers considerable benefits to service providers, enabling very high bandwidth and low latency data transport, therefore making delays in data transport unnoticeable and allowing huge volumes of data to be carried. Fiber is also small and lightweight so it is easy to ship and install, with the additional benefits of being immune to electromagnetic interference, and posing minimal security risk (as it is difficult to “tap off” light without being noticed).

As we’ve previously outlined, fiber presents the solution for today’s and tomorrow’s connectivity challenges. But what practical considerations should be taken into account when it comes to fiber solutions? Here we explore some key fiber choices and their implications.

Why Light in Fiber?

As it is incredibly fast, it makes sense to transmit data using light – rather than electrical pulses, for example. In a vacuum, light travels at 300,000 kilometers per second and is just one third slower when travelling through a fiber-optic cable. Indeed, some coax cables can achieve speeds of even higher than 200,000 kilometers per second. However, coax transmission lines need many more amplifiers than optical fibers which makes optical fiber technology the fastest transmission solution for long distance data transport.

An optical fiber contains a glass core through which light travels. Around this there is another layer of glass called the ‘cladding’ which ensures that light doesn’t escape from the core. Total Internal Reflection keeps the light in the core while a protective polymer coating protects the glass cladding from moisture, dirt, and damage. The total diameter of an optical fiber is about 250 μm or 1/4th of a millimeter. Thin optical fibers are not robust enough to be handled and exposed to the outer world so the optical fiber is protected from mechanical strain by a tougher strengthening material (aramid yarns).

Single mode or multimode?

The ‘mode’ is the path a beam of light follows as it travels along the optical fiber. Multimode fiber allows light to travel along many different paths in the core of the fiber whereas single mode fiber – used in all long distance lines and FTTX deployments today – carries just one mode.

In a single mode optical fiber, the signal travels straight down the middle which makes it possible to transport signals over distances of up to 100 km and still be usable. In comparison, multimode fiber has a large core (typical diameter 50 μm) which makes it less costly to make connections and allows the use of VCEL light sources, which can be significantly less expensive than lasers. However, the distance over which data can be transmitted is much shorter than single mode. Some typical applications of single mode include telecom networks, campuses, TV cable and industrial estates. Multimode is more commonly found in short distance audio/video transmission and broadcast applications, Local Area Networks, and data centers.

Wavelengths

Like sound, light is made up of vibrating waves. Light can have different wavelengths and we perceive these as different colors in the visible spectrum – expressed in nanometers (nm). As light travels, it loses some of its intensity which is called ‘attenuation.’ The greater the attenuation, the weaker the signal at the end of the line. Having longer wavelengths means that less attenuation occurs – and therefore there is a better signal quality.

At around 1550 nm, when wavelengths in the infrared region (which is invisible to the human eye) are used, attenuation is relatively low in glass – so this wavelength is commonly used for long distance.

Multiplexing

Multiplexing allows a single fiber to be used to transport multiple signals and services – allowing an optical fiber to be shared by multiple customers. With Time-Domain-Multiplexing (TDM), services for different customers are sent and received as packages in specific ‘time-slots.’ TDM can be compared to a train with multiple carriages (each carrying a certain amount of information to each customer). The carriages travel in sequence over the information railway and then are separated and delivered to the correct customer at the end of the line.

TDM can be used in long haul point-to-point networks but also in FTTH Passive Optical Networks (PON). The multiplexing and demultiplexing is done in electric equipment such as the OLT (Optical Line Termination) in the central office and ONU (Optical Network Unit) at the subscriber.

What are the different types of multiplexing technologies?

Wavelength-Division-Multiplexing (WDM) transmits different services at different wavelengths so that these signals don’t interfere with each other. Many different wavelengths can be combined in a single fiber using a device called a ‘multiplexer’ (MUX). On the receiving end, the combined signal is ‘unscrambled’ by a demultiplexer (DEMUX). Thus, many different signals can be transmitted across a single fiber at the same time, increasing the cable’s capacity, while allowing you to send and receive multiple data streams.

 

 

 

 

 

 

 

 

Dense Wavelength Division Multiplexing (DWDM) refers to signals that are ‘multiplexed’ within a specific range of wavelengths, around 1550 nm. Erbium doped fiber amplifiers (EDFAs) are particularly effective for wavelengths between approximately 1525-1565 nm and 1570-1610. This enables large volumes of data to be received and transmitted in just one fiber over very long distances. Typically, 40 DWDM channels per fiber are used but it’s possible to go up to 128 channels. Adding channels instead of introducing more fiber and other network components can expand network capacity without the need to install new cables. Optical amplifiers that ‘boost’ the signal can be used to achieve distances of 1,000 km. Another variant is Coarse Wavelength Division Multiplexing (CWDM), which allows up to 18 channels per fiber.

Using WDM to increase capacity

When deploying multiple fiber services – and there is a need for additional capacity – you can simply add more fiber by rolling out new cables. However, in many cases the deployment cost is very high as well as disruptive – requiring streets to be dug up. On the other hand, using WDM with existing fiber, allows you to deliver additional separate services across a single fiber by separating them into different wavelength regions. Although DWDM electronics and passives require a significant upfront investment, typically, the overall cost is lower than rolling out new fiber. Furthermore, although WDM is frequently considered to be the point-to-point solution, there are also add-drop multiplexer solutions which allow you to multiplex eight different wavelengths at the source and then only pull off two of them at a given location, allowing the rest to travel on.

You can learn more about FTTX considerations here.


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