March 2017


With Inputs From Piyush Dedhia Of Optilink


Indian consumers receive their monthly Cable TV Service at an extremely low cost. The service includes more than 200 digitally encrypted TV Channels for a monthly fee that averages less than Rs. 250 per consumer, countrywide. While this is good news for consumers, Cable TV networks which deliver the service get a very low Average Return Per User (ARPU) on their investment. For years Cable TV networks and even DTH platforms have tried hard to induce consumers to pay more. However, there is a strong consumer resistance to higher monthly Cable TV revenues.


Interestingly, consumers will happily pay Rs. 600 per month or more for monthly Broadband connectivity. Clearly if a Cable network can also deliver Broadband to its subscribers, the network can triple its monthly (ARPU) revenues! As proof of this, one of India's largest MSOs - Hathway has indicated in its financials that more than 50% of its revenue comes from just 30% of its customers who subscribe to Hathway's Broadband service!

Clearly, Cable TV provides an excellent entry into consumers' homes. However Broadband provides the financial rewards.


Broadband can be delivered on an existing Cable TV network using either:


ii. Cable Modems

This article (Part 1) will provide an overview of the CWDM Technology and how it can be used to deliver Broadband on existing Cable TV Networks. Approximate prices of commercially available products are also indicated.


Most Indian Cable TV Networks already use Optical Fibre along their trunk line. Cable TV signals are delivered through Optic Fibre usually at 1310 nm or at 1550 nm wavelengths. If necessary, both wavelengths can be used simultaneously on the same fibre without interference. This is similar to the use of multiple frequencies in Analog Cable TV to deliver almost 100 TV channels simultaneously on the same coaxial cable. Use of 2 different wavelengths (e.g. 1310 nm and 1550 nm) simultaneously is referred to as "Multiplexing". Since the signal is divided over 2 or more wavelengths, it is referred to as Wavelength Division Multiplexing (WDM).

It is possible to carry not just 2, but many more wavelengths along the same Optic Fibre.


It is quite easy and inexpensive to carry multiple wavelengths separated by 20 nm from each other on the same fibre. When a spacing of 20nm is used, it is referred to as Coarse Wavelength Division Multiplexing (CWDM).

Table 1 indicates the 18 different wavelengths or carriers that can be used with CWDM.

CWDM equipment uses inexpensive lasers which can be manufactured in bulk.

As a result CWDM does not use Optical Amplifiers for extending the range. Wherever necessary, the CWDM signal is simply converted to an electrical signal and fed to another laser transmitter.


Figure 1 indicates a block diagram for CWDM system that can be added to an existing, Cable TV Optical Fibre network.


The existing Cable TV signals continue to run at either 1310 nm or 1550 nm.

If the total distance between the initial sending point and the final receiving point does not require Optical Repeaters (EDFA amplifiers), then 1310 nm can be used for Cable TV signals. 1310 nm of course provides an excellent, low cost solution but it is restricted to transmission over a distance of approximately 30-35 Kms.

On the other hand use of 1550 nm for Cable TV delivery, permits the use of Optical Amplifiers (EDFA) which can effectively extend the transmission range almost indefinitely. A 1550 nm system deploying EDFAs will also require a much more expensive externally modulated transmitter at the initial sending point.


As indicated in Figure 1, data can simply be added on to the existing system at a different wavelength.

Just as 2 or more RF TV Channels can be combined on a coaxial cable using a channel combiner, multiple optical wavelengths can be combined using a "Multiplexer" (also called "MUX") as shown in Figure 1.

After the multiple wavelengths are combined they can be transmitted simultaneously along a single optical fibre without interference.


If necessary, a third service such as an IP multicast signals can also be simultaneously delivered on the same optical fibre of the cable TV network. The IP multicast signal will require a separate wavelength. Since CWDM can accommodate upto 18 different wavelengths (See Table 1), 18 different services can be simultaneously carried on the Cable TV Network's optical fibre. If necessary, multiple wavelengths can be allocated for a much-in-demand service such as Broadband delivery.

If IP multicasting is not required, some of the 18 wavelengths need not be used or they could be reallocated for broadband delivery.


IP Multicasting can be used to transmit digital IP signals from one digital Headend to a single or multiple remote locations. As an example, an MSO who sets up a digital Headend in Mumbai can transmit the output of the entire digital Headend to any other location such as Pune & Nasik, using IP multicasting. At the sending point no additional equipment needs to be installed at the digital Headend as long as the digital Headend uses the IP format for all its digital interfacing. (An IP interface is used by almost all modern Digital Headends).

The IP (Internet Protocol) signal is simply added on to a CWDM System at designated wavelengths.

On the receiving end and Edge QAM Modulator receives the incoming IP signal and creates a complete Cable TV digital signal, ready for distribution to Last Mile Owners (LMOs) or even directly to end consumers.

Single Edge QAM Modulator will recreate 45- 60 digital Cable TV channels. Edge QAM Modulator prices very significantly but are typically available around Rs. 3 to 3.5 lakhs.

Hence using IP Multicasting and Edge QAM Modulators, an entire digital Headend can be recreated at any remote location.


As indicated in Figure 1, at the receiving end the optical fibre is terminated into a De-Multiplexer (DEMUX) which effectively separates the different optical wavelengths and redirects each wavelengths to its appropriate receiver.


The Multiplexer and De-Multiplexer are both passive devices. They do not amplify or alter the optical signals in anyway. As a result they have an extremely wide bandwidth. Most Multiplexers and De-Multiplexers will support 1 GB/s to 40 GB/s (1 GB/s = 1 Giga Bits Per Second).

An 8 channel Multiplexer costs approximately Rs. 27,000. A De-Multiplexer costs a similar amount. Hence Multiplexer + De-Multiplexer pair represents a total cost of Rs. 54,000.

If only 4 channels (4 wavelengths) are required a Multiplexer - De-multiplexer pair costs just Rs. 27,000.


Data is fed into the multiplexer using Small Form-factor Pluggable (SFP) Modules.

Each SFP is effectively an optical transmitter + receiver (Trans-Receiver). Each SFP will transmit data at any one of the wavelengths indicated in the Table 1. Simultaneously the SFP will also receive data any other wavelength indicated in Table 1. Since the wavelengths for transmitting and receiving are different, there is no interference between the incoming and outgoing data streams.

Each SFP is an active device. It requires power for its operation. The SFP is in turn connected to either a Fibre Switch or Media Converters.

Similar to media converters, SFPs are available based on the distance over which they will transmit and receive data.

♣ A 40 km SFP costs approximately Rs. 5,500 each

♣ A 80 km SFP costs approximately Rs. 8,000 each

♣ A 120 km SFP costs approximately Rs. 11,000 each

Keep in mind that SFPs need to be deployed in pairs. For every SFP transmitting at Location 1 (in Figure 1) must be supported by an SFP receiving in Location 2. Hence SFPs are usually bought in pairs and the cost of a pair twice the cost indicated above.

The costs indicated above are for SFPs supporting data speeds of upto 1GB/s.


No special CATV input/output devices are necessary. To send (or launch) the optical signal a CATV Optical Transmitters required. The transmitter will operate at either 1310 nm or 1550 nm as discussed earlier.

The CATV receiver will be a regular Cable TV Optical Node.


Multicasting IP transmissions can also be accommodated on the same system along with Cable TV and Data / Broadband Signals.

IP Encoders, typically used at all digital Headends are ideal IP I/O devices. While the price of IP Encoders is gradually falling, the current price for MPEG-2 encoders ranges from Rs. 1 lakhs to 1.5 lakhs for a unit containing 4 Video Encoders.


Figure 1 and the details indicated so far explain how Cable TV + Broadband/Data + IP Multicasting Signals can be simultaneously carried from Point A to Point B on the trunk of an existing Cable TV Optic Fibre Network. The Sending and Receiving points could be separated by distances of even 120 Kms, depending on the transmission capability of the SFPs & EDFAs used.

For Cable TV Applications, use of 1310 nm Optical Signals will limit the distance to approx. 35 km.

For larger distances, it will be necessary to optically amplify the CATV signal along the fibre using EDFA optical amplifiers. This is possible only for operation at 1550 nm and the use of an externally modulated (expensive) optical transmitter at the sending point.


Often it may be necessary to even supply signals at some point between Location A & Location B to provide for intermediate (in-between) distribution of signals. In such cases, an Optical Add/Drop Multiplexer (OADM) is deployed at the intermediate location.

Figure 2 indicates a system that provides not only point to point transmissions but also distribution of signals at an intermediate location C.


The OADM is different from a multiplexer. The OADM performs the following functions simultaneously: i. Receives a multiplexed optical signal consisting of CATV + Data + IP.

ii. Onward Transmits a multiplex signal of CATV + Data + IP.

iii. Permits tapping of any of the CATV, Data and IP Signals. Conceptually this is similar to a RF 'Tap Off' which takes out the small percentage of the main signal for use locally.

iv. If required, remove the CATV or Data or IP signals at location B.

v. Insertion of new CATV or Data or IP signals at the intermediate point.

If the cable TV optical signal needs to be amplified using an external EDFA amplifier, this can be easily done with an OADM as indicated in Figure 2. The OADM is used to remove the incoming CATV signal, feed it to an EDFA and reinsert it through the OADM.

The OADM is a passive device and therefore exhibits a wide (1 GB/s), similar bandwidth as multiplexers and de-multiplexers.


The OADM provides multiple options depending on requirements of the particular system. The OADM therefore has a modular construction. Specific modules can be added or removed depending upon the actual requirement of the system.

Based on its actual configuration, the price of a single OADM will typically vary from Rs. 25,000 to Rs. 50,000.


This article has provided an overview of the use of CWDM technology to simultaneously transfer internet/broadband bi-directional signals as well as even IP Multicast Streams if necessary, on an existing Cable TV Network's Optical Fibre.

Costs of various modules that can be deployed in such a system are fairly low.

The incremental cost incurred to relay data can quickly be recovered due to the substantially higher consumer revenues obtained for Internet Delivery.

BROADBAND/Internet Delivery provides much higher revenues for Cable TV networks compared to the basic delivery of TV channels on Cable.

Broadband delivery is clearly the future which will help sustain and grow our industry. Even the Government plans to relax Licensing requirements so that Cable TV networks can be used for rapidly providing High Speed Broadband throughout the country. Reliance Jio plans to deploy its 4G High Speed Broadband Service commencing March 2015. Airtel has already commenced 4G Broadband Delivery in Select Areas. 4G promises high speed wireless broadband at a much lower cost and almost 10 times the speed of 3G broadband. However, the Cable TV network can deliver far higher speeds than even 4G and at a lower cost. Fortunately, the cost of adding data delivery to Cable TV networks is not significant and Cable networks need to seize this opportunity and capture customers before the 4G onslaught.