POL Basics - Optical Fiber

One of the key distinguishing features of Passive Optical LAN (POL), compared to the traditional switch based LAN implementation is that almost all cabling in the LAN deployment is optical fibre.

The type of fibre that is being used for POL is Single Mode Fibre (SMF), standardised in ITU-T G.652. It's called that way as there is only one mode allowed, the so-called transverse mode. However, multiple wavelengths are possible within the same fibre at the same time (which is the case for POL).

The structure of a typical SMF as per the schematic above:

1. Core 8 – 9 μm diameter: this is where the optical signal is transmitted
2. Cladding 125 μm dia.: optical material to keep the optical signal within the core
3. Buffer 250 μm dia.
4. Jacket 900 μm dia.

Refraction and reflection

Due to the extreme small size of the core, the source of the optical signal for SMF is always a laser. Both core and cladding are optical material (basically made of glass), but with a different refraction index. This allows for the signal to remain within the core, the photons bounce off the cladding and are reflected back into the core. That is if the angle at which the signal hits the boundary between core and cladding is not too steep.

This means that bending an optical fiber beyond certain limits can create attenuation (more refraction and less reflection). This is referred to as bending-loss. ITU-T standard G.657 has defined the characteristics of a type of SMF that is highly insensitive to bending-loss.

Wavelenghts

Multiple wavelengths (or colours of light) can be transmitted over a SMF, at the same time. Wavelengths can be compared with frequencies over a copper cable. The wavelengths used in POL are in the Infrared spectrum, which means they are invisible to the human eye.

Connectors

The start and finish of an optical fiber will be a connector that will be inserted into a transceiver at the active equipment side. A transceiver, most typically in the form of an SFP (Small-Form-factor Pluggable unit) and contains both a transmitter (laser) and an optical receiver. SFPs typically are attuned to specific wavelengths.

The connectors used in POL are SC or Squared Connectors. Two types are used:

UPC: Ultra Polished Connector, Blue colour

APC: Angled Polished Connector. Green colour; due to the angle, it generate less return loss

Be aware of which of the two types is required on which end of the SMF before deployment so you end up with the right equipment.

There are two ways to connect 2 sections of fiber:

• connectors: used when the connection might have to be disconnected, which typically introduce a loss of around 0.3 dB
• splicing: used for permanent connections, the loss depends on the quality of the splice. With modern technique and done by an experienced installer, the loss is consider to be 0.1 dB (or less). With the small size of the core, the most critical issue with splicing is that the cores of both ends are aligned perfectly to prevent any additional loss.

Benefits

The following are the main benefits of optical fibre as compared to traditional copper based cabling:

1 - Extremely high bandwidth

When optical fiber was first introduced in the 1970s, the capacity was at 100Mbps, around 2000 we got up to 1 Tbps (Terabit per second, terabit = 10^12 bits per second) and the end is not yet in sight.

2 - Smaller and lighter cables

• SMF cable weighs around 4lb per 1Kft and a diameter of 2.9mm
• Cat6 cable weighs around 39lb per 1Kft and has a diameter of around 5.7mm

SMF cables for installation (for installation in risers) typically come as multicore, with 12 being the minimum amount. This makes the relative weight and diameter per core even more advantageous. This has a positive impact on the structural cost of laying fiber.

3 - No crosstalk between parallel fibers

Transmission of data on a copper cable is done by sending an electrical current over a copper wire. This creates an electro-magnetic field. One of the issues with having a bundle of copper cables is that the EM fields of the different cables interfere with each other, this is referred to as crosstalk. Optical fiber however is made of glass and the data transmission is nothing more than laser light been sent through the core. There is no electro-magnetic field generated.

4 - Immune to inductive interference

For the same reason as mentioned above, optical fiber is immune to external interference. This interference can come from power lines that run close to the data cables. In the case of fiber optical, there is no issue to have both data cables and power cables in the same trench.

5 - High quality transmission

On top of being immune to any interference, there is little to no attenuation on a SMF, roughly 0.4dB per km of fiber. This in sharp contrast to the strict relationship between attenuation and the length of copper cables (let's not forget the limit of 100m).

6 - Low installation and operating cost

With the vast amount of SMF being produced today, the price has gone down to a point where it is at the same level or less than Cat 6 cables.

Installation time for a POL deployment compared to traditional cabling is less time consuming, which reduced the labour cost significantly.

In order to transmit data over a SMF, a much smaller amount of energy is required. Based on research done at the University of Melbourne, POL consumes over 50% less power per Ethernet port than traditional LAN.

References and standards used

https://www.foa.org/ : The Fiber Optic Association Inc.

ITU-T G.652: geometrical, mechanical and transmission attributes of a single-mode optical fibre and cable https://www.itu.int/rec/T-REC-G.652

ITU-T G.657: Characteristics of a bending-loss insensitive single-mode optical fibre and cable. https://www.itu.int/rec/T-REC-G.657

https://en.wikipedia.org/wiki/Single-mode_optical_fiber

https://en.wikipedia.org/wiki/Snell%27s_law

Capacity Trends in Optical Networks: https://www.icg.isy.liu.se/courses/optical/Capacity_Trends.pdf

This article is part of a series called POL Basics. You can follow the series on my blog:

https://poldigitaltransformation.blogspot.com/