Now that Verizon is deploying fiber in the Baltimore metro area, I've been asked a variety of questions about the technology, especially the speed. This essay is by no means complete; those interested in the gory details need only search the Web. I am also not concerned with precision here. The goal is to provide a general sense of the performance of fiber optic cable.

There are a variety of types of fiber having differing physical qualities. For the purposes of this essay, I choose to ignore this fact because it is very complex yet not particularly informative for most of us.

A single strand of glass fiber has an enormous data carrying capacity, or bandwidth, compared to current technologies based on copper wire. What's the highest capacity piece of wire you know about? Ethernet currently goes up to one gigabit per second (Gbps). The serial ATA (SATA) disk interface runs at 3Gbps. PCI Express version 2.0 is 5Gbps per "lane."

The theoretical capacity of a single glass strand is 25Tbps. That's terabits. That's 25,000Gbps, five thousand times more than that single PCI Express lane.

That's not the end of the story, though. Actual carrying capacity of a piece of fiber is based upon the various protocols and transmission methods employed. Let's discuss the characteristics of a single channel first.

The basic unit of capacity is designated OC-1. Its carrying capacity is 51.84 megabits per second (Mbps). This strange number comes from a multiple of units used in the telephone industry, which has been using fiber extensively for a long time. For example, the well-known T1 line has an aggregate capacity of 1.544Mbps. (If you've heard of "fractional" T1, each fraction is about 64Kbps; 24 fractions make up a full T1.)

The industry increased capacity in multiples of OC-1. Here's a small table in which I've rounded the capacity numbers for clarity.

Now these numbers are still a far cry from the huge carrying capacity of a single fiber. To increase capacity, a number of methods have been devised to transmit more than one channel simultaneously over the same piece of fiber. If 100 OC-192 channels could be placed on one fiber, the aggregate capacity would be one terabit, still just 1/25 the theoretical capacity of the strand.

To put these capacities into perspective, analog, non-HD cable TV based on coaxial cable required about 600Mbps of bandwidth to deliver roughly 96 channels. To deliver this same capacity over fiber would require one, just one, OC-12 channel.

FIOS Internet Speeds

In Maryland, Verizon's FIOS high-speed Internet service provides different capacities based upon price. The prices are subject to change so I won't include them here. The capacities are:

10 Mbps downstream, 2 Mbps upstream
20 Mbps downstream, 5 Mbps upstream
20 Mbps downstream, 20 Mbps upstream
50 Mbps downstream, 20 Mbps upstream

These capacities are a tiny fraction of what is possible on the single strand of fiber that Verizon will bring to your house. That's why the same piece of fiber can handle not only an Internet connection but also several phone lines and, much to Verizon's delight, high-definition cable TV.

The speeds listed above are available for both consumer (home) and business FIOS packages.