Fiber vs. Coax

This week, I'm digging into why the cable companies are looking at deploying passive optical networks, even though their public statements profess and undying belief in DOCSIS 3.0.  At it's root, there are two areas of concern about cable's existing hybrid fiber coax (HFC) plant:

• Limitations imposed by coaxial cable versus fiber
• Historical limtations imposed by the DOCSIS architecture

In today's post, I'll dig into the differences between fiber optic cable and coaxial cable, in terms of their data-carrying capabilities, and we'll leave the DOCSIS architecture piece for tomorrow's post.

In a perfect world, where cost was no object, and you would make a decision about what kind of cable to deploy based only on what works better, fiber would be the only choice.  Without the constraints of the particular technology used at either end of the cable, fiber optic cable beats coaxial cable, hands down.  To see why, check out the table below:

	Fiber	Coax
Theoretical Maximum Data Rate	~129 Terabits/sec per wavelength	8 Gbps (assuming 8 bits/Hz, and 1000 MHz RF spectrum)
Distance without regeneration	5-7 miles (BPON)	~1400 ft.
	up to 70 miles for long distance fiber technology
Sensitive to temperature	No	Yes
Sensitive to electro-magnetic fields	No	Yes
Sensitive to wind	No	Yes

The first advantage for fiber is the data-carrying capacity.  A single thread can theoretically carry 129 Tbps on a single wavelength...and depending on the sophistication of the equipment used on either end of the thread, each thread can carry multiple wavelengths, meaning the data carrying capacity of a single fiber optic cable is virtually limitless.  Compare that to a coaxial cable that carries 1000 MHz of RF spectrum (more than the more typical 870 MHz HFC systems carry today), where there is a theoretical maximum of about 8 Gbps in carrying capacity, assuming all of the spectrum is used for data and none of the spectrum is used to carry television signals. Even assuming only one wavelength, a single fiber has over 16,000 times the potential carrying capacity of coaxial cable. 

That's why fiber optic cable is often described as "future-proof".  Sure, a typical Verizon FiOS deployment today only provides about 622 Mbps downstream, and 155 Mbps upstream, per thread, but Verizon can upgrade the service to higher speeds in the future without replacing the fiber optic cable, and instead replacing only the equipment on either end of the cable.

Fiber also has an advantage in the distance it can carry a signal without having to re-generate or amplify the signal.  With today's passive optical networks, a fiber can carry signals five to seven miles without regeneration.  A coaxial cable may need to amplify the signal every 1400 feet or so.  The important thing to remember here is that amplification or regeneration translateds into operating cost, because you need to power the devices that do the amplification or regeneration.  So, fiber has a lower operating cost than coaxial cable over time.

Of course, passive optical networks based on fiber have the advantage that they are passive, meaning that much of the network does not need electrical power in order to keep operating.  Not only does this reduce operating cost for fiber networks versus coax networks, it increases reliability. 

Fiber is also immune to signal corruption from electro-magnetic fields, and is immune to changes in performance from temperature or wind.  Coaxial cable is not.

So, why doesn't everybody just deploy fiber these days?  The main advantage that coax enjoys is that the equipment used to send signals down a coax cable has historically been less expensive than the equipment used to send signals down a fiber optic cable.  However, that is changing with time as well, as volume manufacturing of passive optical network equipment is creating a drop in cost.  Equipment that drives fiber is still more expensive than equipment that drives coax, but the differences are not as large as they once were.

Cable companies are facing a choice:  do they continue to deploy coaxial cable in the last mile of their plant, or do they deploy a future-proof optical cable?  The answer depends on how quickly cable thinks their coax plant will really be at a competitive disadvantage to PON-based service providers like Verizon.  You could argue that this is already the case, and that Comcast is already at a disadvantage versus Verizon, and for the time being, that's true.  However, Comcast believes that their DOCSIS 3.0 rollout this year will bring them back to parity or better, and at much lower up-front cost than a fiber-based PON rollout. 

Someday, though, cable companies will no longer be able to keep pace with fiber deployments while operating an HFC plant.   How soon will that be, and is it soon enough to justify a PON deployment now?  To take an educated guess at that, tomorrow, we'll look at the biggest bottleneck in the DOCSIS architecture, to get a sense for how soon it might motivate a cable switch to fiber.

Comments

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very informative post bookmarked look forward to reading more.