# HFC11.0 Fundamentals of HFC

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HFC11.0 Fundamentals of HFC

Agenda Basics – AC, DC, Ohm’s law, Units and measurements.
Log, Antilog, why use them loss, gain, power level units? dB, dBμV, dBmV, dBm and their conversions. Passives, types of passives, functions of different passives. What is network? Different network architectures. Homes passed, homes connected. What is HFC network. Fundamentals of fiber optics. Various losses in fiber link. Fiber optic hardware, cables, and accessories. Commonly used measurement techniques. Use of multi-meter and signal level meter (dB meter). Use of spectrum analyzer. Loss calculations and signal level estimation in network segments. Symbols used for Network hardware. Hands on practice. Field visits. Theory and practical test. Grade allocation.

Alternating current (AC), Direct current (DC)
AC voltage There are two types of voltages you will be coming across in the network. AC voltage continuously changes with time as shown in the figure. The power supplied by electricity board has a voltage which is AC. The voltage measured by a meter on the three pin power socket (between 230 to 240 volts) is called R.M.S. (Root Mean Square) voltage which is sort of an ‘average’ voltage. Voltage Amplitude Time

RMS explained For example: suppose the time samples are as shown in the diagram:   Values: Squares: 0,49,100,49,0,49,100,49   Sum of squares = 396 Average of squares = 396/8 = almost 50 Square root ~ 7 7 = 0.7 x 10 (peak voltage)   With more intervals the r.m.s. average turns out to be (peak value) / √2 = peak value/1.41 = x peak value Peak RMS = 0.707 x peak 325/1.41=230V RMS

Sine wave A Sine wave is a periodic signal.
A clock pendulum is an example of periodic motion. A Sine wave is also a fundamental signal. All wave forms encountered in real life are made of combination of number of sine waves. When signal goes through all possible variations and comes back to the next starting point, one cycle is completed. Number of cycles per second is called Frequency. It is measured in Hertz (Hz). Frequency of power is 50 Hz. As an example, starting from 6 a.m. today to 6 a.m. tomorrow is one day cycle. Similarly we can have a one year cycle. Sine wave 1 cycle 1 cycle

Voltages on 3 pin power socket
0 to 3 V 230 V Approx N L 230 V Approx Front view of mains socket Color code for wires Indian code L = Red N = Black E = Green European code L = Brown N = Blue E = Green + white L = Line terminal N = Neutral terminal E = Earth terminal

Top plug connections The picture shows Line, Neutral and Ground connections. There is a fuse in the line terminal for over current protection. Ground Line Neutral Back view of a top plug

Batteries and DC voltage
A source of DC voltage is a battery. Batteries are used in conjunction with a UPS (Un interruptible Power Supply unit) to maintain power in a network when electricity board power (Grid power) is unavailable due to a fault or load shedding. Batteries come in different voltages, sizes, shapes and in technologies. Batteries used in a CATV network can be of 12 or 24 volts, maintenance free or tubular which requires water topping periodically. The size of a battery depends on the time it can give a back up to maintain power in the event of power outage. It is specified in Ampere-Hours (AH). 65 and 100 AH batteries are most commonly used in a CATV network. Size of a battery will increase as AH rating increases.

Batteries and DC voltage
Battery symbol A Battery voltage is constant. It does not change with time like AC voltage. Voltage, however, will drop when batteries get discharged. Batteries can be connected in series to increase voltage or in parallel to increase AH. All electronic devices like lap top, mobile, TV, radio, music system work on DC voltage. We need a power adaptor for charging batteries of battery operated devices such as mobile or lap top. Devices working directly on 230 V AC have an internal power supply to convert AC voltage to DC voltage. Positive terminal Negative terminal Time Voltage 12 Volts 12 V, 65 AH battery Battery discharge Series connection: 48 V, 65 AH Battery bank Parallel connection: 24 V, 130 AH Battery bank

Converter/Inverter A Converter converts AC voltage to DC Voltage.
An Inverter converts DC voltage to AC Voltage. A UPS has both a converter and an inverter built into it. When grid power is available, it will convert AC to DC and charge the batteries while powering the network also. When grid power fails, it will take DC from batteries and converts to AC to continue powering the network. Typically the batteries will give a back-up of 2 to 3 hours, depending on battery condition. One end of AC supply and the negative terminal of DC supply are normally grounded in a CATV network as shown. The co-axial cable outer conductor is grounded and acts as a return conductor. + AC voltage DC voltage _ Converter + DC voltage AC voltage _ Inverter

Voltage, resistance, current, power and Ohm’s law
Open circuit, Very large resistance of air, No current. Current in Amperes = Voltage in Volts/ Resistance in Ohms I = V/R Power in watts = Voltage in Volts x Current in Amperes P = V x I Also P = (I x R) x R = I2 x R From Ohm’s law, we can have following equations: V = I x R R = V/I P = I2 x R P = V2/R 1 unit of electricity = 1 Kilowatt hours (KWH) 12 V battery Short circuit, Zero resistance Heavy current with sparks. Load connected, Current depends on load in watts If the bulb is of 30 watts, then 30 = 12 x I I = 30/12 = 2.5 Amperes. Current flowing through the bulb is 2.5 Amps.

Examples If V = 24 volts, R = 6 ohms then I = 24/6 (V/R) = 4 amps and P = (24)2/6 (V2/R) = 576/6 = 96 watts. If V = 24 volts, I = 4 amps then R = 24/4 (V/I) = 6 Ohms and P = 24 x 4 (V x I) = 96 watts. If I = 4 amps, R = 6 Ohms then V = 4 x 6 (I x R) = 24 volts and P = 42 x 6 ( I2 x R) = 16 x 6 = 96 watts.

μa, ma, A, μV, mV, V, μW, mW, W I μA = I Amp x 1000,000
I mA = I Amp x 1000 V μV = V Volt x 1000,000 V mV = V Volt x 1000 P μW = P Watt x 1000,000 P mW = P Watt x 1000 I Amp = I μA/1000,000 I Amp = I mA/1000 V Volt = V μV/1000,000 V Volt = V mV/1000 P Watt = P μW/1000,000 P Watt = P mw/1000

Logarithm (Log), Antilogarithm (Antilog)
100 = 1 101 = 10 102 = 100 103 = 1000 Log 1 = 0 Log 10 = 1 Log 100 = 2 Log 1000 = 3 Antilog 0 = 1 Antilog 1 = 10 Antilog 2 = 100 Antilog 3 = 1000 Find log Find anti-log

Multiplication, division of numbers
100 x 10 = 1000 Log 100 = 2 Log 10 = 1 Sum of Logs = = 3 Antilog 3 = 1000 1000/100 = 10 Log 1000 = 3 Log 100 = 2 Subtraction of logs 3 – 2 = 1 Antilog 1 = 10 Multiplication of two numbers is the sum of their Logs Division of two numbers is the subtraction of their Logs

Why Log, Antilog? There are 2 advantages in using logarithmic unit.
First, as we have seen, multiplication function gets converted to addition and division function gets converted to subtraction. Because of this conversion, as you will see later, we can add gain and subtract losses when we are estimating signal level at different points in network segments. The whole process becomes very simple. Second, large numbers get transformed to small numbers e.g. log of 100,000 is 5. A 6 digit number has transformed to 1 digit. In a cable network one can encounter signal levels from 100 μV to 1000,000 μV (1V). When converted to dBμV, they are 40 dBμV to 120 dBμV – manageable numbers.

Ratio and Decibel A ratio compares one number against another number.
If a son is weighing 40 Kg. and the father is weighing 80 Kg. then the father is 80/40 = 2 times heavier than the son. The number 2 does not indicate absolute weight of either the son or the father (40 and 80 Kg). If an amplifier input signal power is 10 mw and the amplified output power is 100 mw then output power is 100/10 = 10 times higher than the input power. The number 10 does not indicate absolute power at either the input or the output (10 and 100 mw). Similarly Decibel is a comparison of two signal power levels. This comparison can be against input power level or any reference power level we may choose.

Decibel (dB) Decibel is the Log of ratio of voltages across same value of resistor 20 x Log Vout/Vin If Vout is 100 mV and Vin is 10 mV Then dB = 20 x Log 100/10 = 20 x Log 10 = 20 x 1 = 20 dB Vout is 20 dB greater than Vin Vin can also be a reference voltage and is generally 1 μV or 1 mV. Decibel is the Log of ratio of Powers 10 x Log Pout/Pin If Pout is 100 mw and Pin is 10 mw Then dB = 10 x Log 100/10 = 10 x Log 10 = 10 x 1 = 10 dB Pout is 10 dB greater than Pin Pin can also be a reference power and is generally 1 mw.

dBμV, dBmV, dBm dBμV indicates that our reference voltage is 1 μV .
Thus, 1000 μV is 20 Log(1000 μV/1 μV) = 20 Log 1000 = 20 x 3 = 60 dBμV. 60 dBμV means that power delivered by 1000 μV in a resistor is 60 dB greater than power delivered by 1 μV in the same resistor. dBmV indicates that our reference voltage is 1 mV. Thus, to convert 1000 μV in to dBmV, first convert 1000 μV to mV. 1000 μV = 1 mV. 20 Log 1 mV/1 mV = 20 Log 1 = 20 x 0 = 0 dBmV. 0 dBmV means that that power delivered by 1mV (1000 μV) in a resistor is 0 dB greater than power delivered by 1 mV in the same resistor. Thus we can say that 0 dBmV = 60 dBμV.

dBmV dBμV dBm conversion
dBμV = dBmV + 60 30 dBmV = 90 dBμV 55 dBmV = 115 dBμV -10 dBmV = 50 dBμV -25 dBmV = 35 dBμV dBmV = dBμV – 60 60 dBμV = 0 dBmV 75 dBμV = 15 dBmV 40 dBμV = -20 dBmV 55 dBμV = -5 dBmV Similarly we can prove that for a 75 Ohm system: dBμV = dBm dBm = dBμV – dBmV = dBm dBm = dBmV – 48.75

Passives: what do they do?
Signal distribution on HFC network can be compared to water distribution in a city. We can say that our ultimate objective is to distribute water to all the households within certain pressure limits irrespective of their location. For this we need to have pipes of suitable diameters with valves and pressure regulators installed at correct locations. Each household needs only a small portion of water that is flowing in the main pipe. Similarly, each household requires only a small portion of a signal that is flowing in the trunk cable. Also the signal should be within certain strength (60 to 80 dBμV) irrespective of the locations of households. Clearly, we need some devices which will distribute and control the amount of signal to households. They are called passives.

Passives Passives are used in the HFC distribution network to manipulate and distribute signal in the desired direction and proportion (power level). The word ‘Passive’ signifies that the passive devices do not require power for their functioning. As opposed to a ‘Passive’, an ‘Active’ device such as an amplifier requires power for it to work. Directional coupler, splitter, tap-off, power inserter, diplex filter, noise blocker, attenuator, terminator (Dummy load) are the passives used in a distribution network. Passives are made by using inductors, capacitors, resistors and R.F. transformers wound on ferrite material. A Passive is specified for the frequency range it can work for and losses between its various ports. All passives are designed for Input/Output impedance (Zo) of 75 Ohms.

Directional coupler (DC)
A directional coupler (DC) divides the signal unequally - major portion going to the output port, and only a small portion (design value) going to the coupled port. It couples a desired (by design) amount of signal to its coupled port from its input port. It comes in 8 or 12 dB coupling loss. An 8 dB coupler will have the signal level at the coupled port 8 dB lower than the input port level. This is called as ‘Tap loss‘ or ‘Coupling Loss’. Loss between Input and Output ports is called ‘Insertion loss’ or ‘Through loss’. Loss between Output port and Coupled port is called ‘Isolation’. Power can be made to pass through all the ports. SSP-7N, SSP-9N and SSP-12N are examples of a DC which give 7, 9 and 12 dB loss between IN and TAP ports. SSP-7N

Directional coupler as a signal combiner
A directional coupler can also be used as a signal combiner. In the example shown, signal source1 is connected to the OUT port of a SSP-7N. Signal source2 is connected to the TAP port of the SSP-7N. Signal 1 will suffer insertion loss of 2 dB and signal2 will suffer tap loss of 7dB. After combining, signal2 will be 5dB lower than signal1. If you wish to have both, signal1 and signal2 levels to be the same, you need to have signal2 level at 105 dBμV so that at the IN port both signals will be 98 dBμV. Signal 1 + signal2 SSP-7N Signal source 1 In Out 98dBμV + 93dBμV Tap 100 dBμV Signal source 2 100 dBμV

Use of a DC Typically, a DC is used at the output of a node or an amplifier or on a trunk. When the network needs to be split to cover two different areas and where one cable jump is long and the other is short, a DC is used. 8 or 12 dB DC Long cable jump More signal Amplifier Short cable jump 8 or 12 dB less signal

Typical losses in a DC In all the Directional Couplers, as frequency increases, loss increases by 1 to 2 dB. As the DC value increases from 8 to 12 to 16, the insertion loss decreases.

Splitters A splitter divides the signal equally. The input signal is divided in to 2 or 3 outputs. 2 way splitter has 4 dB loss between their input and output ports. This is called as ‘Splitting Loss’ or ‘Insertion Loss’. Loss between the Output ports is called ‘isolation’ 3 way splitter has 2 ports with 8 dB loss while 1 port with 4 dB loss. Power can be made to pass through all the ports (outdoor type only). Indoor varieties can have 1 input and 2/4/6/8 output ports. SSP-636N SSP-3N

Use of a 2 way splitters Typically, a 2 way splitter is used at the output of a node or an amplifier or on a trunk. When the network needs to be split to cover two different areas and where both the cable jumps are approximately equal, a 2 way splitter is used. Approx. 2 equal cable jump 2 way splitter Amplifier

Use of 3 way splitters Typically, a 3 way splitter is used at the output of a node or an amplifier or on a trunk. When the network needs to be split to cover three different areas and where two cable jumps are approximately equal and short while the third jump is long, a 3 way splitter is used. Approx. 2 equal short cable jumps Long cable jump 3 way splitter Amplifier

Typical losses in 2 way and 3 way splitters
As the frequency increases, losses increase by 1 to 2 dB.

Splitter used as combiner
A splitter can work as a combiner, if it is used in reverse. It means that to make a 4 way combiner, one has to feed signals to the output ports of a 4 way splitter and get combined output in the ‘Input’ port. Indoor 4 way splitter

Tap-offs The picture shows a 2 way tap diagram.
A tap-off is a combination of a DC and a splitter. Signal loss between the Input port and the Output port is called ‘Insertion’ or ‘Through’ loss. Signal loss between Input and any tap port is called ‘Tap loss’. This loss is a designed loss and can be between 4 to 32 dB, in steps of 3 dB. We need different tap values because we need to feed equal amount of signal (60dBμV) to all the customers irrespective their location from the serving amplifier. Customers nearer the amplifier will be fed through high value taps while the ones away from the amplifier will be fed from low value taps. Symbol

Tap-offs Signal loss between Output port and any tap port is called ‘Isolation’ and is greater than 20 dB. Signal loss between any two tap ports is called ‘Tap to tap isolation’ and is greater than 20 dB. The picture shows a 4 way tap-off diagram. 2way, 4way, 8way tap-offs come in Outdoor and Indoor variety. In the Outdoor type, there is power passing facility between In and Out ports. But usually there is no power passing on the tap ports. However, recently tap-offs with power passing facility on tap ports with protection fuses are available. Generally, there is no power passing facility on any of the ports in the Indoor type. Symbol

Indoor tap verities

2, 4, 8 way outdoor tap-offs

Typical losses in 2 way Tap-offs
As the frequency increases, Insertion loss increases by 1 to 2 dB. As the tap value increases, Insertion loss decreases. 4 dB tap is a terminating tap – meaning the Output is terminated internally with 75 ohms and therefore, there is no external Output port.

Typical losses in 4 way Tap-offs
As the frequency increases, Insertion loss increases by 1 to 2 dB. As the tap value increases, Insertion loss decreases. 8 dB tap is a terminating tap.

Typical losses in 8 way Tap-offs
As the frequency increases, Insertion loss increases by 1 to 2 dB. As the tap value increases, Insertion loss decreases. 11 dB tap is a terminating tap. As the number of tap ports increase from 2 to 4 to 8, for the same tap value Insertion loss increases. A 14 dB 2 way tap has an insertion loss of 1.8 dB at 450 MHz, 4 way has 2.5 dB and 8 way has 4.1 dB.

Line power inserter (LPI)
Line power inserters are used for injecting 90 V A.C. voltage from a power supply into a trunk cable route to power a node and amplifiers. Signal and power are combined on such a cable. They normally come in outdoor type only. SSP-PIN UPS 230V

Power/signal filter in LPI
AC VOLTAGE BLOCKING CAPACITOR SIGNAL SIGNAL + AC 40~90 V RF AC 40~90 V RF BLOCKING CHOKE POWER

In all the outdoor passives there are two gaskets which fit between the unit and its base plate. Rubber gasket prevents rain water entry. Wire mesh gasket prevents ingress/egress. You should tighten base plate firmly by tightening the four bolts. Install tap-offs so that tap ports face the ground. This will prevent rain water entry in to tap ports. Tighten all check nuts to prevent rain water entry.

Diplex filter Plug in units for node and amplifiers Head end unit Diplex filters are used to either combine or to separate the downstream (54 –860 MHz) and upstream (5 – 40 MHz) signals. In a 2 way network a diplex filter is installed at every Output of a node and at the Input and every Output of an amplifier. A Diplex filter is also required at the Headend if Co-ax network is directly fed from the Headend. The specification of the split of the diplex filter must match with that of your cable network.

Attenuators (5 to 1000 MHz) 5 to 1000 MHz – gives known uniform loss
Attenuators are used to introduce known loss in the network. They introduce almost uniform loss for all the frequencies from 5 to 1000 MHz. Many a times, signal level is too high for a network or a customer device. Therefore, we need to reduce the signal level by a known amount. This is done by an attenuator. Attenuators generally have F male interface at one end and F female interface at the other end. They come in values from 5 to 25 dB in 5 dB steps or from 3 dB 24 dB in 3 dB steps or from 1 dB to 20 dB in 1 dB step depending on the manufacturer and price. 5 to 1000 MHz – gives known uniform loss

Return step attenuators (54 to 860 MHz pass)
We use highest tap value for the customers located nearest to the amplifier, gradually reducing the value as we go away from the feeding amplifier. We do this to equalize the forward path signal (to 60 dBμV) to all the subscribers irrespective of their location from the feeding amplifier. In the return path also we need to equalize the return signals generated by customer devices (modem) to the same level (105 to 110 dBμV) irrespective of their location from the amplifier. A Return step attenuator introduces known amount loss only in the customer’s Return path, while introducing only 1 dB loss to the Forward path signals. 5 to 40 MHz – gives known uniform loss (5,10,15 and 20 dB) 54 to 860 MHz – gives 1 dB uniform loss

High pass filter (54-860 MHz pass, 5-40 MHz block)
In a network catering to the TV and Internet services, there will be some customers who are taking TV + Internet services and some taking only TV services. Only TV customers don’t require return path. Noise generated within the homes of only TV customers will enter in the network. Noise from many only TV homes will accumulate at a node degrading Return SNR. High pass filters are fitted on the tap ports connected to the drop cables going to the only TV customers, not requiring Return Path. These high pass filters (54 –860 MHz pass) block the noise generated in homes, not using the Return Path.

A Terminator is a 75 ohm resistor fitted in to a metallic casing which can be fitted on an out door casing or F female connector. All open ports of nodes, amplifiers and passives must be terminated with a Terminator to maintain network integrity. If the Terminators are not installed in the network, signal will get reflected from open ports and also noise ingress will take place.

Homes passed and homes connected
8(3) 5(1) 6(2) 1(1) 8(0) 7(5) 20 (5) 30 (8) Cable run Homes passed tells us the number of homes (independent homes or apartments in a building) situated along a length of a cable. Homes passed indicates the number of customers which can be connected to a network for offering services. Homes passed indicates population density and the business potential of the area. It has a direct impact on the cost/connection. If homes passed/km high then cost per connection is low and makes a business case. Homes connected tells us the number of homes actually connected to a network. In the example above, Homes passed is 94 and Home connected is 26.

What is HFC? HFC stands for hybrid fiber Co-ax.
HFC network is built using optical fibers, co-axial cables and associated electronic hardware. Optical fiber is the medium in which communication signals are transmitted from one location to another in the form of light, guided through a thin fiber of glass. Co-axial cables, with their special construction, then guide the signal along the metallic conductors on the last mile to the destination.

What is a network? A network is the way fibers, co-axial cables and the associated hardware pieces are connected. The main considerations for fiber network are: Availability of fiber (owned or leased). Fiber susceptibility to breakage. Reliability and redundancy requirement. Cost of laying a fiber cable or leasing Vs business potential. Signal format – whether IP optical or Amplitude Modulated optical. Distances to be covered. Back-up power arrangements. The main considerations for Co-ax network are signal integrity in the last mile i.e. considerations for noise (C/N) and distortion components (CSO/CTB/XMod) and power outages. For this reason cascade of amplifiers is limited to 3 or 4 and UPS power back-up is always provided.

Why HFC? Prior to the emergence of fiber optic technology, networks were built using only co-ax cables. However, because of unique properties of optical fiber, it is coming ever nearer to home. It is the best combination for signal delivery in terms of cost, reach, reliability and bandwidth. Because fiber introduces very low loss to signal, large distances (20 to 50 km) are covered using a fiber to bring signal to a distribution point (Node). From the node signal is then distributed on co-ax cables to homes covering a radius of 1 km.

Basic fundas of fiber optics
To understand fiber fundas, we need to go back to school days. Remember pictures of bent straw in a water filled glass or mirage in a desert in your physics books? When a light ray passes from one medium to another having different densities (water and air), it changes its path. Density of material is indicated by refractive index – denoted as ‘n’.

Refractive index ‘n’ of a material is:
speed of light in vacuum speed of light in the material Light has highest speed in vacuum and slows down in other medium such as glass, plastic, water. The Refractive Index for all these materials is greater than 1. Water has R.I. of 1.3 and glass has R.I. of 1.5 approx. When light travels from thick (Higher ‘n’ such as water) to thin (Lower ‘n’ such as air) or vice versa, its path will bend causing an illusion similar to the ‘bent’ stick which you learnt in school physics.

How light travels within a fiber
Total internal reflection Optical fiber has a core and cladding. The core is made of higher Refractive Index than the cladding (outer). Light ray CC’ enters the cladding after bending. Light ray BB’ travels along the boundary (critical angle). Light ray AA’ is reflected internally in the core. This is because entry angles at the boundary are progressively increased. The core and cladding are usually very pure fused silica glass covered by a plastic coating, called ‘Buffer’. Core and cladding are not physically separable and the boundary can’t be identified. How light travels within a fiber

Frequency, period, wavelength, micrometer, nanometer
Frequency (f) is: Number of cycles/second. Example: Power frequency is 50cycles/second (Hz), Human audio frequency response range is 20 to 20,000 cycles/second (Hz). Period (T) is: Time taken to complete one cycle. T= 1/f Wavelength (λ) is: Distance traveled by signal in a time of period T. λ = C/f where C is speed of light in a medium in meters/second. Light signal is normally characterized in wavelength which is easy to write and talk about. Frequency characterization results in awkward and lengthy numbers. Micrometer = Micron = μM = Meter/1000,000 Nanometer = nm = Meter/1000,000,000

Multimode Vs Single mode fiber.
There are two basic types of optical fiber – multimode (used for short distance in LAN) and single mode (used for long distance in CATV / Telco). Multimode fiber means that light can travel many different paths (called Modes) through the core of the fiber, entering and leaving the fiber at various angles. In single mode, the core size is reduced (8 to 10 microns) causing all the light to travel in one mode (straight line).

Multi-Mode Single-Mode Modes of light Many One Distance Short (meters) Long (km) Bandwidth Low High Typical Application Access Metro, Core, CATV n2 n1 Cladding Core n2 n1 Cladding Core Multi-Mode: supports hundreds paths for light. Single-Mode: supports a single path for light

Multimode Dispersion Light rays are transmitted from the source at a variety of angles and arrive at the receiver at different times, thereby creating distortion at the output. This sets a limit on the distance covered in Multimode fiber. Source

1310 nm Vs 1550 nm These are the two most commonly used wavelengths in optical communication. 1310 nm wavelength has a loss of 0.35 dB/km. It is used up to a distance of 30 km. 1550 nm wavelength has a loss of 0.23 dB/km. It is used up to a distance of 50 km, because of lower loss. Single mode G.652 fiber (ITU) is the most widely deployed (90%) fiber in the world which accepts both 1310 and 1550 nm wavelengths.

Other losses in a fiber link
Apart from losses in the fiber, there are losses in splices and connectors. Typically, splice loss is 0.05 dB/splice and connector loss is 0.25 dB/connector. There is also sag loss if fiber is laid overhead. Sag loss is equivalent of adding 4% to the fiber length. Loss also occurs if fiber is sharply bent, depending on the bending radius. The bending loss is about 3 times for 1550 nm wavelength as compared to 1310 nm for the same bending radius. One needs to calculate and sum up all these losses to arrive at a link budget.

Re-generation RF Light Tx Rx Re-generation Using 1310 nm wavelength, to cover more distance, one has to do re-generation. This means light signal is converted to electrical signal using an optical receiver and then again converted to light signal using another optical transmitter. However, this process adds 3 dB noise and one should not do more than two re-generations.

Optical amplifier RF Light Tx EDFA For 1550 nm wavelength, re-generation is not necessary to increase distance. One can use optical amplifiers (also called EDFA – Erbium Doped Fiber Amplifier). EDFA amplifies weak light signal in to powerful one to cover further distance. 40 different wavelengths, from 1528 to 1561 nm, each carrying independent information can be multiplexed together on a single fiber and can be amplified by EDFA. This is known a Dense Wave length Division Multiplexing (DWDM). However, one shouldn’t have more than two EDFAs in cascade because of noise consideration.

Multiplexer / De-multiplexer
Wavelengths Wavelength Multiplexed Signals DWDM Mux DWDM Demux Wavelength Multiplexed Amplified Signals EDFA Wavelengths separated into individual ITU Specific lambdas EDFA accepts optical power anywhere from 0 to 6 dBm and delivers optical output power between 16 to 23 dBm depending on the model.

Fiber cable construction
Loose tube construction Single fiber/tube

Loose tube construction, multiple fibers/tube

Fiber protection Ultra violate curable lacquer coat at the time drawing Extruded colored buffer coat Gel Aramid yarn Kevlar coating Outer plastic jacket with embedded strength members. Steel wire armor in case of UG cable

Cable without steel armor is immune to high voltage. No effect of external electromagnetic interference. Almost no effect of moisture. Almost no effect of temperature variation. Light weight. Very large bandwidth with DWDM. Very affordable at today’s prices.

Introduction to hardware
The optical fiber link is made of: A light source (Laser: Light Amplification by Simulated Emission of Radiation) with support electronics, to receive and convert electrical information to optical, using a connector and alignment mechanism. Fiber with connectors / splices. Photo detector at the receiving end – which converts incoming light back to electrical signal, producing a copy of the original electrical input. The light source and the photo detector with necessary support electronics are called as transmitter and receiver.

The electrical input is RF signal from CMTS and video Head End.
Transmitters: A transmitter takes electrical input and converts it into light output. The electrical input is RF signal from CMTS and video Head End. It sits in at HE and/or Hub and can come as a stand alone 19”, 1U unit or can come as a plug-in unit to fit into a 3U chassis. Multiple transmitters and RPR can fit into a chassis having a common power supply. It comes in different wave lengths, 1310 and 1550 nm being the most common. It can come in ITU defined DWDM wavelengths too! It comes in different standard steps of power i.e. 3, 6, 8, 10, 12, 13, 15 and 16 dBm. First, we need to decide the wavelength, then calculate the link budget and decide transmitter power.

Transmitter power can’t be increased later.
Transmitter power is expensive, cost increasing with power. Therefore, correct link budget is necessary. 1550 transmitter is more expensive than 1310. 1550 transmitter comes in two verities: Direct modulation External modulation – these are more expensive DFB (Distributed Feed Back) laser is used in the (Forward) transmitters because of higher BW and power.

Optical input power to node should be between -2 to +1 dBm
A node receives light signal through a fiber and a Forward receiver in the node converts it into an electrical signal (RF). Optical input power to node should be between -2 to +1 dBm The RF signal is further amplified and sent on to a co-ax network. Return RF signal coming from modems in homes enters a node, is amplified and converted in to a light signal using a Return transmitter. Return transmitter power may be -4, 0 or +3 dBm. The return laser used is mostly Fabry-Perot (FP) but can be DFB in 0 and +3dBm power. Choice of type and power of a laser is left to the user. It is normally a plug-in device. DFB laser is more expensive than FP laser.

Diplex filter

Thus a node has one incoming (Forward) and one outgoing fiber (Return) and may have 2 or 3 RF cables (500 series) going out to a co-ax network. A node normally sits securely in a kiosk. A kiosk also accommodates battery bank, UPS and Raw power/UPS power modems. These modems indicate the failure of Raw power and how long the UPS back-up is ON, to a person in POP. 90V power from UPS/Power supply is also inserted in to a node to power the node and amplifiers in the co-ax network via 500 series cables – which also carries Forward and Return RF signals.

A RPR, sitting in at HE and/or Hub receives light signal from a node and converts it to RF signal. This Return RF signal then goes to Return port of a CMTS, thus establishing two way communication between CMTS and modems. Normally, RF Return signals of 2 or 3 nodes are combined before entering in to a Return port of CMTS.

HE/Hub Field CMTS Ana+Digi TV Forward RF Forward fiber Return fiber
TX Node Co-ax network RPR Return fiber To/From Co-ax network ReturnRF RPR From another node

Requires more number of fibers, but more secure.
Couplers Head end splitting Requires more number of fibers, but more secure. Couplers are used to split optical power in to two parts to feed to nodes sitting at two different locations in the field. Power is split in such a way that a node sitting at a larger distance gets more power compared to the node sitting nearer. Couplers can sit at POP (HE) as shown:

Requires only one fiber, but not so secure.
Field split Requires only one fiber, but not so secure. Or they can sit in the field as shown above. Couplers come in standard split from 50:50 to 95:5. These numbers indicate the percentage of input power going into two output legs of a coupler. 60:40 means that 60% of input power goes to one leg and 40% goes to other. The fibers will have stickers to indicate which is 60% and which is 40%.

Fiber management system (FMS)
FMS is a metal cabinet in which following facilities are incorporated: Openings at top and bottom for incoming and outgoing fiber cables. Secure clamping of cables. Splice trays. Patch panels. Provision for winding extra patch cords. Labeling All fiber cables and fibers coming from equipment terminate on patch panel. Patch cords are used to make optical connections. This arrangement introduces order, clarity and flexibility in connections. Changing connection can be done easily and quickly by interchanging connection on the patch panel using patch cords.

Patch panel Patch cord FMS

Connectors SC/APC – Snap Connector/Angled Physical Contact – Push type - Green color This is used almost on all equipment e.g. Tx, RPR, node. SC/PC - Snap Connector/ Physical Contact – Push type – Blue color This is used for data equipment like media converters, switches. FC/APC – Ferrule Connector/Angled Physical Contact - Push and thread tight type – Green color This is used on power meters and OTDR

Patch cord, pigtail Patch cord (mostly yellow color):
This is a flexible, protected single strand of fiber with connectors fitted at both ends, for indoor use. It can be purchased in different standard lengths like 3, 10,15 meters with a choice of connectors. Pigtail: When we cut a patch cord into two, we get two pig tails. A pig tail has a connector at one end and bare fiber at the other end. Note that G.652 fiber, nodes, couplers, connectors, patch cords and pigtails support both 1310 and 1550 nm wavelengths.

Splice enclosure Splice enclosure:
When a fiber cable is cut unintentionally or intentionally to take out some fibers for connection, all fibers are exposed. All the fibers have to be spliced to establish the connectivity. Fibers being fragile, splices and bare fibers are neatly put in a splice enclosure. They come in various sizes depending how many splices can be accommodated.

Optical coupler Splice enclosures

Power meter, OTDR Power meter is used for measuring optical power in dBm for both 1310 and 1550 nm. It is used for confirming estimated power and as a tool for fault finding. It costs few thousands of Rupees Optical Time Domain Reflectometer is a diagnostic tool with a LCD screen to locate the distance of fiber break, loss along the length of cable, reflection and poor connectorization. This is an expensive instrument, costing few lacs of Rupees.

Typical HFC architecture

Typical HFC architecture
Signal originates from the Headend. The ring connecting the Regional HE and Hub is the core network. It will have fiber redundancy. If the core network is within a metro, covering distances within 40 k. m. the signal format may be IP optical/STM or A.M.optical. If core network is encompassing multiple cities, the signal format will be IP optical/STM and bandwidth is taken from the carriers. A hub caters to an area of up to 5 to 40 k.m. radius. It is possible to do local program insertion at the hub level. The network beyond the hub to homes is called Access network.

Star architecture Fibers going from HE to every node. Local channel insertion not possible. No fiber redundancy. High fiber requirement.

Double star architecture
Single fiber to a hub. Re-generation at hub. Local program insertion possible. No redundancy.

Ring architecture Full automatic redundancy. High fiber requirement.

Shared ring/star architecture
This architecture provides for auto redundancy, local program insertion using minimum fibers.

Access network Return fiber Forward fiber Node Kiosk
BLE AMP. Access network Return fiber MB AMP. Forward fiber POWER Node Kiosk Spacing between amplifiers 300 to 325 Meters To other amplifiers

Use of a multi-meter A multi meter is used for measuring 1) AC voltage 2) DC voltage 3) DC current 3) Resistance 4) Continuity of a conductor. Black lead always connected to COM terminal of a meter. When measuring AC voltage, DC voltage, resistance and continuity the Red lead is connected to VΩmA terminal of a meter. When measuring DC current, shift the Red lead to 15A DC. Make sure that when measuring voltage the Red lead is not in the 15A DC. It may blow a fuse in the meter. When doing measurement, always connect Black lead to –ve of a DC source and neutral of an AC source. While measuring continuity, there will be an audible tone if continuity exists in the conductor being measured. DC voltage AC voltage DC current Resistance Continuity

Measurements – signal level meter (SLM)
A TV signal level meter (SLM) measures signal levels at desired points in the network. It is an inexpensive instrument for daily use for every field technician. It gives signal level in dBμV or in dBmV or in dBm. One can select a channel plan, edit it, select a channel type whether analog or digital. Measurements are possible in 4 modes – 1) level 2) scan 3) spectrum mode 4)Tilt mode.

SLM In level mode one can select channel by its number or by frequency. This is a mode most frequently used. The refresh time is short in this mode because meter does measurement of only one channel. Scan mode shows a bar graph of all channels and is most useful when you want find out variations in channel levels at one go. Also my moving marker you can go from one channel to another channel very quickly. The refresh time is long because meter does measurement of all the channels. Spectrum mode is useful for fault finding. In particular for finding out any unwanted signal generated within the HFC plant or aerial signal that is picked up. Refresh time increases with increase in the span selected. In Tilt mode one can configure lowest (E3), highest (U69) and some channels in between for tilt measurement. This measurement shows a bar graph of configured channels. Since lowest and highest channels are selected, it measures and displays the tilt directly. This is useful while doing alignment of amplifiers.

Measurements – spectrum analyzer
A spectrum analyzer gives a graph of signal amplitude (Y axis) in frequency domain (X axis). It is a precision instrument normally used for accurate measurements and difficult to find faults at the HE or in the field. Measurements that are normally done are level, C/N, CTB, CSO and interference etc. It comes with a tracking generator as an option. With this option, we can do response measurement of cables, amplifiers, taps, splitters etc. Analyzer tailor made for CATV requirements are also available, making measurements very quick and accurate.

Loss calculations and signal level estimation
30M/RG6 DC8 40M/500 series 100 dBμV @ 860 MHz 21dB 14 dB 11dB 30M/500 series 50M/500 series 60M/500 series 70M/500 series Find out signal reaching the home at 860 MHz? Loss in 30M 500 series cable = 7.7x0.3 = 2.3 dB Loss in 40M 500 series cable = 7.7x0.4 = 3.1 dB Loss in 50M 500 series cable = 7.7x0.5 = 3.9 dB Loss in 60M 500 series cable = 7.7x0.6 = 4.6 dB Loss in 70M 500 series cable = 7.7x0.7 = 5.4 dB Loss in 30M RG6 cable = 20x0.3 = 6.0 dB Total cable loss = =25.3 dB

Loss calculations and signal level estimation
30M/RG6 DC8 40M/500 series 100 dBμV @ 860 MHz 20dB 14 dB 11dB 30M/500 series 50M/500 series 60M/500 series 70M/500 series Passive loss: DC8, tap loss = 8.5 dB 2 way splitter, insertion loss = 5.0 dB 8 way tap, insertion loss = 2.4 dB 4 way tap, insertion loss = 3.0 dB 2 way tap, tap loss = 11.0 dB Total passive loss = = 29.9 dB

Loss calculations and signal level estimation
30M/RG6 DC8 40M/500 series 100 dBμV @ 860 MHz 20dB 14 dB 44.8 dBμV 11dB 30M/500 series 50M/500 series 60M/500 series 70M/500 series Cable loss + Passive loss: = 55.2 dB Signal level at the home = 100 – 55.2 = 44.8 dBμV

Symbols Directional coupler/one way tap 2 way tap 4 way tap 8 way tap
Terminating tap

Symbols LE with Return 2 way splitter MB with Return
3 way splitter unbalanced Dot shows high output leg Headend Optical node Optical transmitter Primary Hub

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