Contents

Introduction
Structure
Multi-mode vs Single Mode
Transmission Distance and Wavelength
Types of Optical Fiber Cable
Connector
GBIC / SFP
Optical Interface Readings

Introduction

Layer 1 (Physical Layer) is not included in the exam. As an network engineer, we should know some basic theory. Cable is an interesting topic. Copper cables and optical fiber cables are mainly used today. I am going to introduce the structure and standard of fiber cable in this article. Compared to copper cable, fiber cable is not influenced by electromagnetic field, has longer transmission distance and physically thinner,  softer and easier to implement.

Structure

Light goes straight but cable cannot be straight. So, light is reflected in the cable to move forward. From high school physics, we know that the total internal reflection angle can be calculated if we know the refractive index of the materials. It is applied to the fiber cables, it contains two different types of materials.

fiber-cable

A fiber cables consists of three parts: Core, Cladding and Coating.

fiber-cable

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Core

Core is the material with larger refractive index. The light beam is going through the core. The diameter of the core is between 8 to 62.5 µm which is nearly equal to the hair of human.

Cladding

Cladding is the material with smaller refractive index. It reflects the light. The diameter of cladding is 125 µm.

Coating / Jacket

Coating provides protection. To identify different types of fiber cables, different color of coating is used.

Two numbers are printed on the coating. In the following figure, 50/125 means that the core diameter is 50 µm and the cladding diameter is 125 µm.

fiber-cable

Multi-mode vs Single Mode

The core of Multi-mode Fiber (MMF) is thick, many light beams can go forward together at the same time. The price of MMF is cheaper but attenuation occurs since light beams interferer each other, thus MMF only suitable for a short distance(< 2 km).

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The core of Single Mode Fiber (SMF) is thin, only one light beam is allowed to pass through. But the transmission distance is much longer (10 km).

fiber-cable

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Transmission Distance and Wavelength

Although light is not influenced by electromagnetic field, it is attenuated after each reflection. It makes the maximum bandwidth decreases. 3 factors vary the bandwidth of an optical fiber:

Transmission Distance

Shorter transmission distance, less attenuation. Thus, larger bandwidth.

Wavelength

Base on the characteristic of light, larger wavelength can decrease attenuation, and bandwidth increases. The unit of wavelength is nm. 850 nm to 1300 nm is used in Multi-mode Fiber and 1310 nm to 1550 nm is used in Single Mode. Of course, no free lunch, devices that can generate light beam with large wavelength is more expensive.

Light Source

There are two types of light source, LED (Light-emitting Diode) and VCSEL (Vertical-Cavity Surface-Emitting Laser). Since the frequency of VCSEL is higher than that of LED, VCSEL can transmit more signal than LED at the same timeslot. In other words, higher bandwidth. LED can support maximum 40 Gbps only but VCSEL can support up to 100 Gbps. Only high grade optical fiber can support VCSEL, such as, OM3, OM4, SC1 and SC2. Devices and fibers that support VCSEL will be more expensive.

Types of Optical Fiber Cable

Let us talk about the standard. According to TIA/EIA (Telecommunications Industry Association and Electronic Industries Association), there are four types of Multi-mode, OM1, OM2, OM3 and OM4. Larger number means higher prevention to attenuation so that support higher bandwidth and longer transmission distance.

TypesCoating ColourDiameter (Core/Cladding)Longest distance (m) with 850 nm wavelength Longest distance (m) with 1300 nm wavelength
1Gbps10Gbps40Gbps100Gbps1Gbps10Gbps40Gbps100Gbps
OM1  Orange 62.5/125 µm 220 32 Not Support Not Support 550 Not Support Not Support Not Support
OM2  Orange 50/125 µm 550 82 Not Support Not Support 550 82 Not Support Not Support
OM3  Aqua 50/125 µm 550 300 100 100 1000 300 Not Support Not Support
OM4 Aqua 50/125 µm 550 550 125 125 1000 550 Not Support Not Support

Two types of Single Mode fiber are OS1 and OS2, the transmission distance of the two types are nearly the same. But OS2 use a technique that called Loose-tube to provide waterproof to make it good for outdoor usage. OS2 is higher in price.

TypeCoating ColourDiameter (Core/Cladding)Longest distance (km) with 1310 nm wavelengthLongest distance (km) with 1550 nm wavelength
1Gbps10Gbps40Gbps100Gbps1Gbps10Gbps40Gbps100Gbps
OS1 Yellow 9/125 µm 5 5 10 10 Not Support 40 40 40
OS2 Yellow 9/125 µm 5 5 10 10 Not Support 40 40 40

Connector

Beside fibers, we also have a number of choices for connector.

SC (Square Connector / Subscriber Connector)

Old style connector that used in old devices.

fiber-cable

LC (Little Connector / Lucent Connector)

Commonly used. Smaller form factor compared to SC. Less spaces is required for the devices.

fiber-cable

FC (Fiber Channel / Ferrule Connector)

Screw design to provide reliable connecting.

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ST (Straight Tip)

Commonly used in fiber patch panel.

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MTP (Multifiber Termination Push-on)

1 MTP connector can support 24 optical fiber cables. It is suitable for environment that requires a large amount of fiber cables..

fiber-cable

GBIC / SFP

So, there are many choices for connectors. Which connectors should a device provides? Factory let you choose by yourself! Generally, the network devices only provide an interface for you to plug the interface card that is suitable for your environment so that you can choose the connectors that you want.

GBIC

GBIC (Giga Bitrate Interface Converter),GBIC changes electrical signal to optical signal, commonly used with SC connector.

fiber-cable

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SFP

SFP (Small Form Pluggable) do the same thing as GBIC but the size is smaller. It also called Mini-GBIC and commonly works with LC.

fiber-cable

fiber-cable

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Optical Interface Readings

Use show interfaces <int> transceiver to check the interface readings. We should pay attention to Tx Power and Rx Power, Tx Power should be at least -10dBm and Rx Power should be more than -17dBm. If the value is too low, the optical fiber may be damaged. Actually, no singal is received if the value is lower than -30dBm.

SW#show interfaces gigabitEthernet 1/0/1 transceiver
ITU Channel not available (Wavelength not available),
Transceiver is internally calibrated.
If device is externally calibrated, only calibrated values are printed.
++ : high alarm, +  : high warning, -  : low warning, -- : low alarm.
NA or N/A: not applicable, Tx: transmit, Rx: receive.
mA: milliamperes, dBm: decibels (milliwatts).

                                           Optical   Optical
           Temperature  Voltage  Current   Tx Power  Rx Power
Port       (Celsius)    (Volts)  (mA)      (dBm)     (dBm)
---------  -----------  -------  --------  --------  --------
Gi1/0/1      38.7       3.29       6.1      -5.1      -4.4

If you found that following message while use the command, it means that the GBIC / SFP do not support monitoring so that no values can be read.

SW#show interfaces gigabitEthernet 1/0/2 transceiver
Diagnostic Monitoring is not implemented.